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Sub-Boreal aggradation along the Apennine margin of the Central Po Plain: geomorphological and geoarchaeological aspects

L’aggradation au Subboréal le long de la marge apennine de la plaine centrale du Pô : aspects géomorphologiques et géoarchéologiques
Mauro Cremaschi et Cristiano Nicosia
p. 155-174

Résumés

Le marge des Apennins, à la limite sud de la plaine du Pô (Italie du Nord), est bordée par une bande de cônes alluviaux coalescents qui se développent vers le nord, en direction de la plaine. Leur aggradation a été contrôlée par les cycles glaciaires et interglaciaires du Pléistocène moyen et supérieur qui ont interagi avec les processus tectoniques. A la différence des plus anciennes unités, fortement érodées, les cônes alluviaux mis en place au cours de la dernière période glaciaire sont encore morphologiquement bien conservés. A la transition Pléistocène supérieur/Holocène, l’accroissement de la plupart d’entre eux s’arrête et leur surface est soumise à la pédogenèse pendant les périodes du Boréal et de l’Atlantique, entraînant la formation d’alfisols rubéfiés (sols bruns fersiallitiques). Depuis le début de la période subboréale, les marges des cônes alluviaux ont été recouvertes par des dépôts alluviaux fins intercalés entre des entisols, des inceptisols et des vertisols souvent associés à des sites archéologiques. La cause de ce nouveau processus sédimentaire doit se chercher dans la dégradation climatique qui caractérise la fin de la période postglaciaire et le début du Néoglaciaire. Les sols à la surface des cônes alluviaux et ceux intercalés dans les dépôts alluviaux sous-jacents ont été densément occupés par les communautés humaines du Néolithique jusqu’à l’Age du Bronze et témoignent d’un changement majeur de l’exploitation des sols au Subboréal.

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Notes de la rédaction

Article soumis le 2 février 2011, accepté le 31 août 2011.

Texte intégral

Introduction

1It is now widely accepted that the most recent climatic change of the Holocene, the present global warming, is mainly a consequence of human activities that have increased the concentration of greenhouse gases in the atmosphere. There is also a growing awareness that the influence of humans on climate predates the Anthropocene period (Crutzen, 2002) and may relate to the beginning of the Neolithic revolution at the very end of the Upper Pleistocene and at the beginning of the Holocene (Rudimann, 2003; Barker, 2006; Vavrus et al., 2008). Subsistence strategies adopted since this period, through a nonlinear trend of successes and collapses (Diamond, 2005), have resulted in steady demographic growth. This in turn has led to a progressively higher efficiency of human activities in driving geomorphological processes and to an increasingly visible human ecological footprint on the landscape, particularly at middle latitudes (Chesworth, 2010). However, the relationships between human activities and “natural” climatic changes still need more thorough investigation, particularly at a regional scale, to be understood. In this perspective, alluvial plains and their connected river systems are especially suitable as proxies for long-term changes in flood regime, soil erosion, flux of sediments and organic matter, and for the environmental and climatic causes behind them (Brown, 1997; Hoffmann et al., 2009). Furthermore, alluvial plains were densely settled since the early Holocene and were contexts where agriculture and pastoralism were practiced. These activities, through deforestation by fire, ploughing and irrigation, had significant impact on the soil and on geomorphological processes. Long-lasting human influence has been observed in several areas of temperate Europe (Hoffman et al., 2007; Houben et al., 2009; Berger, 2011). This paper presents a case study regarding the Holocene plain at the Apennine margin of the Central Po basin. Here, a change in pedosedimetary processes during the Middle Holocene (transition from Atlantic to Sub-Boreal period) is related to a main climatic fluctuation (Mayewski et al., 2004). This appears to coincide with new modes of land use and exploitation performed by the resident prehistoric communities. The data cited here were collected in the course of rescue archaeological projects directed by the Soprintendenza per i Beni Archeologici dell’Emilia Romagna, in which scientific investigation was carried out by a team of archaeologist and geoarchaeologists. Highway and railroad construction areas, gravel quarries and large excavations for building foundations provided wide exposures of soils embedded in alluvium. These were unearthed over broad surfaces and provided the unique opportunity to investigate not only the archaeological sites included in these soils, but also their surroundings. It was thus possible to gather information on soil use and agricultural practices, and to raise questions about the relationships between human activities and environmental change.

The geomorphological context of the central Po plain and the distal fringes of the alluvial fans at the Apennine margin

2The frontal moraines deposited by the Quaternary Alpine glaciers delimit the whole northern fringe of the Po plain. In its central part (fig. 1), outside the Last Glacial Maximum (LGM) moraines of the Garda Lake system, a large outwash plain, cited in the local geological literature as the “Livello fondamentale della pianura Lombarda” Main Level of the Lombardy Plain (Cremaschi, 1987; see also Ravazzi et al., this issue), extends homogeneously southward until it meets the Holocene alluvial plain. The Main Level of the Lombardy Plain was deeply dissected by the post-LGM valleys of the Alpine tributaries of the Po River (Marchetti, 2001). Its wide extension and the relative homogeneity are due to the subsurface “homocline” with slight tectonic disturbance gently dipping southward (fig. 1, cross section A-A’). The Holocene plain on which the bed of the Po River is located today corresponds to the area of maximum subsidence of the system. The present-day Po riverbed is surrounded by large meandering palaeochannels, which indicate instability of the course from the early Holocene through the Roman period. Towards the south, beyond the large depressed areas occupied by swamps during medieval times, the palaeochannels of the Apennine watercourses emerge from the flat alluvial plain and appear as alluvial ridges (Marchetti, 2001). These connect the fringe of the pede-apennine alluvial fans with the Po floodplain. The Apennine margin is rather complicated being tectonically active, as it coincides with a complex belt of folded thrust fronts trending to the N-NE, active throughout the Quaternary (Pieri and Groppi, 1981; Bartolini et al., 1982; Barbacini et al., 2002). This resulted in an uplift of the mountain area margin and the consequent lowering of the plain in front of it. One main geomorphological consequence of these processes was the formation of an apron of coalescent gravelly fans and of alluvial facies in front of them, trending to the north towards the alluvial deposits of the Po, the depocenter of the system.

3According to recent sedimentological models applied to the region context (Di Dio and Valloni, 1997; Valloni and Baio, 2009), the climate change associated with glacial and interglacial cycles of the Middle and Upper Pleistocene interacted with tectonic processes in shaping the alluvial fans and regulated the fans’ relations with the adjoining alluvial plain. To summarise, in the northern Apennines, the glacial periods enhanced the erosion in the mountain area where slopes were intensively degraded in a periglacial morphosystem. As a consequence, the discharge and sediment load of rivers increased, promoting the aggradation and progradation of alluvial fans in the piedmont area. The opposite took place during the interglacial periods. The drop of erosion in the mountain areas, resulting from the onset of a dense vegetation cover, induced a decrease in competence and in solid transport of streams, and stopped the aggradation of the fans. Their surface was consequently subject to weathering (Cremaschi and Marchetti, 1995). Each subsequent aggradational cycle implied incision in the apical part of the previous fan, while a new one prograded towards a more distal position. Therefore, telescopic systems of fans formed throughout the Quaternary, while their preservation was strongly limited by tectonic deformations of the Apennine margin.

Fig. 1 – Schematic geomorphological map of the central Po Plain
Fig. 1 – Carte géomorphologique schématique de la partie centrale de la plaine du Pô

Fig. 1 – Schematic geomorphological map of the central Po Plain Fig. 1 – Carte géomorphologique schématique de la partie centrale de la plaine du Pô

A: Middle Pleistocene terraces. B: LGM frontal moraines of the Garda Lakeand Rivoli systems; C: Main level of the Lombardy plain. D: Pede-Appennine alluvial fans. E: Finely textured alluvial plain. F: Depressed areas in the alluvial plain. G: Main palaeochannels. H: Fluvial ridges. I: A-A’ geological cross section (from Pieri and Groppi, 1981). Q: Quaternary deposits; Plm, Pls: upper and middle Pliocene deposits; M1, M2, M3: early, middle and upper Miocene deposits. Contour lines indicate elevation in m a.s.l.
A : Terrasses du Pléistocène moyen. B : Moraines datant de la dernière période glaciaire du système morainique du lac de Garda. C : Niveau principal de la plaine de Lombardie. D : Cônes alluviaux de la marge des Appennins. E : Plaine alluviale à sédiments fins. F : Zones basses de la plaine alluviale. G : Principaux paléochenaux. H : Bourrelets alluviaux. I : Section géologique A-A’. Q : sédiments quaternaires ; Plm, Pls : Pliocène supérieur et moyen ; M1, M2, M3 : Miocène inférieur, moyen et supérieur. Les courbes de niveau indiquent l’altitude en mètres au-dessus du niveau de la mer.

From Cremaschi, 1987; Castiglioni, 1997; Castiglioni and Pellegrini, 2001.
D’après Cremaschi, 1987; Castiglioni, 1997; Castiglioni et Pellegrini, 2001.

4In the area under examination, the surfaces of the terraced remnants of Middle Pleistocene fans are covered by thick polygenetic palaeosols and loess sheets (Cremaschi, 1987; Busacca and Cremaschi, 1998). A later generation of alluvial fans (fig. 2), ascribed by radiocarbon dating and archaeological evidence to the Upper Pleistocene (Cremaschi and Marchetti, 1995), is better preserved; the fans’ shape can be seen clearly in the microtopography revealed by contour lines with one meter spacing (Castiglioni, 1997). These fans are composed primarily of gravels, which are weathered into rubified Alfisols (Sols bruns fersiallitiques; Cremaschi, 1979), characterised by an A-Bt-Ck profile and preserved over wide surfaces. Such soils begun to develop in the beginning of the Holocene but were still forming during the late Atlantic period, since their characteristic processes (decarbonatation, clay translocation and rubefaction) also affected the Neolithic archaeological deposits associated with them (Cremaschi, 1983, 1990, in press). This observation is supported by the study of the Alfisols of the Razza di Campegine Neolithic site (Bernabò Brea et al., 2009) on top of the Enza Upper Pleistocene alluvial fan. These soils provided the radiocarbon dating 6490±90 a 14C BP (7570-7250 a cal. BP) on the humic fraction of the soil of the site, which displayed evidence of clay illuviation. The date 5940±40 a 14C BP (6400-6200 a cal. BP) was obtained on charcoal related to the Neolithic archaeological (Square Mouthed Pottery culture) site and contained in the soil, suggesting a slightly older age of soil formation with respect to the archaeological dwelling (Bernabò Brea et al., 2009).

Fig. 2 – Schematic geomorphological map of the investigated area
Fig. 2 – Carte géomorphologique schématique de la région étudiée

Fig. 2 – Schematic geomorphological map of the investigated area Fig. 2 – Carte géomorphologique schématique de la région étudiée

A: Middle Pleistocene dissected alluvial fans. B: Upper Pleistocene gravelly alluvial fans with “Bruns lessivés” soils and vertisols at the surface. C: Upper Pleistocene alluvial fans covered by alluvial deposits. D: Fine-textured alluvial plain. E: Main towns and villages. F: Main scarps. G: Fluvial ridges. H: Main palaeochannels. I: Depressed areas in the alluvial plain. Location of the studied sequences. 1: Rubiera, Secchia River and Corradini gravel pit; 2: Sant’Ilario/Taneto; 3: Cave Spalletti; 4: San Pancrazio; 5: Botteghino. Contour lines indicate elevation in m a.s.l.
A : Terraces alluviales du Pléistocène Moyen ; B : Cônes alluviaux du Pléistocène supérieur composés principalement de graviers et affectés en surface de
Sols Bruns Fersiallitiques ; C : Cônes alluviaux du Pléistocène supérieur recouverts par des alluvions ; D : Plaine alluviale à sédiments fins ; E : Villes et villages principaux ; F : Principaux escarpements ; G : Bourrelets alluviaux ; H : Principaux paléochenaux ; I : Zones basses de la plaine alluviale. Localisation des séquences étudiées. 1 : Rubiera, fleuve Secchia et carrière Corradini ; 2 : Sant’Ilario/Taneto ; 3 : Carrière Spalletti ; 4 : San Pancrazio ; 5 : Botteghino. Les courbes de niveau indiquent l’altitude en mètres au-dessus du niveau de la mer.

5Towards the plain, north of the Via Emilia national road line, the distal part of the Upper Pleistocene alluvial fans and the soils overlying it are covered by fine-grained sediments that grade smoothly northwards into the flat alluvial plain. The distal part of the fans, covered by alluvial deposits, displays sedimentological, geomorphological, pedological and geoarchaeological peculiarities. It consists of overbank deposits, locally with a slightly undulating surface as they blanket the underlying undulated morphology of the gravelly fans, intercalated to frequent weakly developed buried soils. These are often associated with archaeological sites ranging in age from the Neolithic to the early Bronze Age. Such characteristics were already noted by the archaeologists of the 19th c., especially during the investigation of the Sant’Ilario clay quarry (Chierici, 1881) which supplied an initial chronological framework for the sedimentation of the alluvial deposits at the distal margin of the fans. More radiocarbon datings have been obtained from archaeological investigations of sites exposed along riverbanks, in pits and in building foundations (Barfield et al., 1975; Alessio et al., 1981). Recently, the Regione Emilia Romagna carried out a radiocarbon dating program on the humic fraction of buried soils encountered in drillings in the area under examination (www.regione.emilia-romagna.it/geologia). These datings, along with others gathered from literature (Alessio et al., 1981; Cremaschi and Marchetti, 1995), provide a good chronological control and contribute to the interpretation of local changes in environment through the Holocene (tab. 6).

Material and methods

6This paper relies on the study of soil profiles recorded and described in the field according to the USDA pedological descriptive code (Soil Survey Staff, 2003). Soil micromorphology was employed as an integrative analytical tool for the study of buried soils (Salvioni, 2005-2006; Cremaschi et al., in press). Thin sections were manufactured according to the standard methods outlined in C.P. Murphy (1986) and described using the terminology of G. Stoops (2003). For the description and designation of pedogenic horizons and for the classification of soils, we referred to the French classification system (Duchaufour, 1983) and to Soil Taxonomy (Soil Survey Staff, 2003), well aware of the problems occurring in transposing nomenclature and taxonomy of present-day soils onto a palaeo-pedological context (Krasilnikov and Calderón, 2006; Zerboni et al., 2011). The chronological framework is derived from radiocarbon dates and archaeological contexts. These range from the Neolithic (Fiorano and Square Mouthed Pottery cultures: 7450-5500 a cal. BP; Pessina and Tiné, 2008), to the Chalcolithic (Brushed Pottery, Scale Decorated Coarse Ware, Bell-Beaker cultural facies; ca. 5450-4100 a cal. BP, Aspes et al., 1988) and to the Early Bronze Age (ca. 4100-3500 a cal. BP; Bernabò Brea and Cardarelli, 1997). All the datings, if not otherwise specified, were calibrated with a precision of 2 σ (95.4% probability; Blaauw, 2010) using the software OxCal v. 3.10 (Bronk Ramsey, 2001, 2005) with IntCal04 atmospheric data (northern hemisphere atmospheric; Reimer et al., 2004). Conventional radiocarbon ages are expressed as a ‘14C BP’, while once converted into calendar ages they are indicated as ‘a cal. BP’.

Description and micromorphology of the investigated sites

7The sites dealt with in this paper are located in the Holocene alluvial plain, on the distal fine-grained fringe of the pede-Apennine fans. From east to west, the studied sites are: Rubiera, on the alluvial fan of the Secchia River; Sant’Ilario/Taneto and Cave Spalletti, on the fan of the Enza River; San Pancrazio and Botteghino di Marano, in the outskirts of Parma, both on the system of the Parma and Baganza fans (fig. 2).

Rubiera (Reggio Emilia)

Description

8The pedo-stratigraphic complex described hereafter (50 m a.s.l.; 44°39’14’’N; 10°47’51’’E) was exposed on the western bank of the Secchia River and inside the “Corradini” gravel pit and is 6-m thick (Cremaschi 1997). It consists of fine-grained floodplain sediments encompassed between the gravels of the late Pleistocene/early Holocene alluvial fan at the base of the sequence, and the gravels filling a large river channel presumably of Roman age (Cremaschi, 1987, 1997). Three buried soils are intercalated in the fine-textured floodplain sediments (fig. 3 and tab. 1). All of the buried soils contained preserved tree stumps in living position, remains of ancient forests exposed during quarrying (Bertolani Marchetti and Forlani, 1972). The archaeological material and radiocarbon dates on wood and charcoal are the basis for the dating of the buried soils (Cremaschi, 1997). The upper, weakly developed buried soil (Ab) occurs at the depth of 130 cm from the top of the recent terrace corresponding to the riverbed active during the 1960s (fig. 3) and does not include archaeological material. However, a radiocarbon dating within the Early Bronze Age was obtained from charcoal fragments contained in it: 3540±50 a 14C BP (3980-3690 a cal. BP). The second buried soil (2ABb), occurring at the depth of 210 cm, is partially decarbonated and has a better developed soil structure. In the 1980s, a Bell-Beaker archaeological site lying on this soil was investigated nearby (Bermond Montanari et al., 1982). The site included hearths, postholes and a stone pavement. It was interpreted as the remains of a dwelling structure and charcoal was dated to 3835±90 a 14C BP (4550-3950 a cal. BP). The third buried soil (3ABb), occurring at the depth of 345 cm in the “Corradini“ gravel pit, had many rooted tree stumps in living position with charcoal patches at their base along the whole exposed stretch. The radiocarbon datings of a tree stump and of the charcoal at its base (Cremaschi, 1997) are from the late Neolithic/early Chalcolithic period: (tree stump) 4725±95 a 14C BP, 5700-5250 a cal. BP (93.3%), 5200-5050 a cal. BP (2.1%); (charcoal) 4585±115 a 14C BP, 5600-4850 a cal. BP.

Tab. 1 – Rubiera, Secchia riverbed. Profile description
Tab. 1 – Description de la séquence de Rubiera

Horizons

Description of the Rubiera sequence

0-120 cm; coarse clast-supported gravel and occasional sand lenses; few brick fragments; abrupt wavy boundary.

120-130 cm; silty-clay loam; 5Y 6/3 (pale olive); few fine mottles, 2.5Y 6/6 (olive yellow); massive; strongly calcareous; abrupt smooth boundary.

Ab

130-146 cm; silty-clay; 2.5Y 5/2 (grayish brown), strongly calcareous; weakly developed angular blocky; few and fine roots, common gasteropod shell fragments; gradual smooth boundary.

C

146-210 cm; clayed-silt to silty-clay loam; 2.5Y 7/2 (light grey) to 5Y 5/3 (olive), massive, 7.5Y 5/8 (strong brown) around rare tubular pores filled with tubular CaCO3 concretions; strongly calcareous; clear smooth boundary.

2 ABb

210-260 cm; silty-clay loam; 2.5Y 4/2 (dark grey); common mottles 2.5Y 4/4 (olive brown); massive, slightly calcareous; well developed angular blocky, few coarse pores, few charcoal fragments; clear smooth boundary.

2C

260- 345 cm; silty-clay; 2.5Y 5/4 (light olive brown); common mottles 2.5Y 4/4 (olive brown); massive; strongly calcareous; few coarse pores, few charcoal fragments; abrupt smooth boundary.

3 ABb

345-395 cm; silty-clay loam; 2.5Y 4/2 (dark grey), slightly calcareous; well developed angular blocky; few charcoal fragments; gradual smooth boundary.

3C

395-465 cm; silt loam; 2.5Y 5/4 (light olive brown); common mottles 2.5Y 5/2 (greyish brown), massive, strongly calcareous with common CaCO3 concretions, well developed angular blocky; abrupt wavy boundary.

4C

465-600 cm; heterometric gravel, clast-supported, slightly cemented; lower boundary not reached.

Fig. 3 – The Rubiera, Secchia riverbed and Corradini gravel quarry stratigraphic sequence
Fig. 3 – Coupe stratigraphique de Rubiera dans le lit du fleuve Secchia et dans la carrière Corradini

Fig. 3 – The Rubiera, Secchia riverbed and Corradini gravel quarry stratigraphic sequence Fig. 3 – Coupe stratigraphique de Rubiera dans le lit du fleuve Secchia et dans la carrière Corradini

1: embankment; 2: Roman age gravel; 3: fluvial deposits with tree trunks; 4: fine-grained alluvial deposits with intercalated buried soils and alluvial fan gravel at the base of the sequence; 5: archaeological sites (BK: Bell Beaker site; CHA: Chalcolithic site). Solid triangles indicate spots of burned soil; 6: standing tree stumps; 7: tree trunks.
1 : digue ; 2 : graviers d’âge romain ; 3 : sédiments fluviatiles contenant des troncs d’arbre ; 4 : dépôts alluviaux à sols intercalés et graviers de cône alluvial à la base ; 5 : sites archéologiques (BK : Culture du Vase campaniforme ; CHA : site chalcolithique). Le triangle noir indique une zone de terrain brûlé ; 6 : souches d’arbre en position de vie ; 7 : troncs d’arbre.

Soil Micromorphology

9Micromorphological analyses were performed on the whole sequence but in the present paper the discussion will focus only on the Bell-Beaker buried soil found at ca. 2 m depth (2ABb horizon) and on the late Neolithic/early Chalcolithic buried soil (horizon 3ABb) at 3.5 m depth (tab. 5). Horizon 2ABb is partially decarbonated as it has a non-calcareous groundmass but still contains detrital calcite grains in the coarse fraction. The concentration of calcite biospheroids, known to accumulate especially in the uppermost part of the soil (Canti, 2003), allows us to identify 2ABb as a topsoil. Finely comminuted (<100 µm) charcoal and charred vegetal fragments are dispersed in the horizon, while coarser deciduous wood fragments are very rare (fig. 6A). Few burned soil and burned bone fragments have also been observed. Indicators of topsoil disturbance, not recorded in the horizons above this soil and hence ascribed to the Bell-Beaker human dwelling, are present. They correspond to rare dusty clay coatings and intercalations. Given that trees were found still in living position at this site, this disturbance could have been caused by practices of secondary clearance (Macphail, 1992), which would account for the large quantities of fine charcoal, and possibly by cultivation (Courty et al., 1989; Macphail et al., 1990). This interpretation is reinforced by the characteristics reported from the coeval buried soil in the “Corradini” gravel pit. Here, a soil sample collected around preserved tree stumps contained abundant coarse deciduous wood charcoal fragments and reworked soil fragments, both burned and unburned, which were interpreted as the result of clearance and soil disruption (Cremaschi, 1997). Horizon 3ABb is completely decarbonated except for rare weathered calcite biospheroids, pointing to a decalcified topsoil horizon more developed than the Bell-Beaker buried soil described above. The horizon contains both very fine (50-200 µm) and coarse (up to 3.5 mm) charcoal fragments, occasionally identifiable as remains of deciduous wood. Dusty clay textural pedofeatures are rare in this horizon (Fe-depleted dusty clay coatings inside vughs) but increase significantly in the underlying horizon 3C. These indicators suggest that the soil was subject to slaking in response to human disturbance (e.g., deforestation, fire, cultivation).

Sant’Ilario/Taneto (Reggio Emilia)

Description

10At Sant’Ilario/Taneto (50 m a.s.l.; 44°45’50”N; 10°27’30”E), rescue archaeological excavations (Bernabò Brea et al., in press) exposed a 5.5 m-thick pedostratigraphic sequence. This is composed of fine-grained deposits with intercalated soils, lying on fluvial sands that constitute the top of the Upper Pleistocene fan at this location (fig. 4 and tab. 2). The gravels of the alluvial fan are 2 m deeper than the base of the sequence, as observed in drillings for water wells (Barfield et al., 1975; Valloni and Baio, 2009). Below the Ap horizon, an Iron Age pit cuts the top part of the sequence, which is composed of fine-grained sediments in which three weakly developed soil horizons are intercalated. Such soils are identifiable in the field mostly for their slightly darker colour compared to the alluvial materials in which they formed, for their weakly developed pedality, and by concentrations of gastropod shells. The uppermost of these buried soils (2Ab), found at 162 cm depth, is very weakly developed and devoid of archaeological material. A radiocarbon dating was obtained from charcoal extracted from a similar soil at a comparable depth in the “Cave Nuove” quarry, 500 m east of this profile (Alessio et al., 1981), and might be extended to our 2Ab horizon: 3900±60 a 14C BP, 4520-4470 a cal. BP (4.4%); 4450-4150 a cal. BP (91.0%). The second weakly developed buried soil (3Ab) occurs at a depth of 244 cm. Within it, archaeological features were excavated. These are associated with materials of the Scaled Coarse Ware Chalcolithic culture and consist of postholes, shallow pits and hearths (Bernabò Brea et al., in press). At the margins of the settled area, irregular sub-rounded patches composed of large charcoal pieces associated with burned soil lenses were exposed, ranging in diametre between 0.6 m and 1.5 m. The following radiocarbon date was obtained from charcoal: 4525±35 a 14C BP; 3360-3090 a cal. BC. The third weakly developed buried soil, 5Ab occurs at a depth of 330 cm and is similar to the one described immediately above (3Ab). An archaeological site of the Brushed Pottery Chalcolithic culture was exposed on it. The site included aligned postholes and pits and, at the margin of the settlement, irregular spots of burned clay. At this level a charcoal fragment was radiocarbon dated to 4920±50 a 14C BP, 5850-5750 a cal. BP (1.1%); 5750-5580 a cal. BP (94.3%). Below this soil the grain size of the sediments becomes coarser with a peak in a layer of laminated sandy loam resting on an erosional surface. A fourth soil (7ABb) is present at the depth of 430 cm. The moderately developed pedality and the strong depletion of carbonates indicate a stronger degree of weathering compared to the three weakly developed buried soils described above. Archaeological materials such as potsherds and flints associated with charcoal, probably related to an extra-site area of the middle Neolithic period (Square Mouthed Pottery culture), are included in it. The base of the soil grades into the coarse fluvial sands of the top of the Pleistocene alluvial fan. The stratigraphic terms of the Sant’Ilario/Taneto sequence are fairly common in the area. In addition to the one already described in the second half of the 19th c. (Chierici, 1881), a similar sequence was exposed five kilometers to the south, in the “Spalletti“ gravel pit, where investigations are still in progress (fig. 4). A slightly more upfan position of the latter is responsible for the differences between the sequences, namely the lower thickness of alluvial deposits (3.3 m instead of 5.5 m) and the fact that the Upper Pleistocene alluvial fan deposits are much coarser in texture. These consist of discontinuously planar-bedded gravels down to a depth of 20 m. Nevertheless, the “Spalletti” sequence is intercalated by the same thin weakly developed soils, the archaeological content of which allows for a perfect correlation with those of Sant’Ilario/Taneto. In the “Spalletti” sequence, the soil that developed on the fan gravel has characteristics that result from having a coarser parent material, such as a 40 cm-thick Bt horizon, and contains rare Neolithic flints and sherds in its top part.

Tab. 2 – Sant’Ilario/Taneto. Profile description
Tab. 2 – Description de la séquence de Sant’Ilario/Taneto

Horizons

Description of the Sant’Ilario/Taneto sequence

Ap

0-25 cm; silty-clay loam; 10YR 4/3 (brown); strongly calcareous; moderate fine angular blocky; very few very fine pores; few fine CaCO3 infillings; clear smooth boundary.

A1

Iron Age

pit

25-162 cm; silty-clay loam (with few charcoal fragments); 10YR 4/2 (dark grayish brown); moderately calcareous; weak fine crumb structure; common very fine pores; few very fine CaCO3 infillings; abrupt boundary.

2 Ab

162-200 cm; silty-clay loam; 10YR 6/3 (pale brown); moderately calcareous; weak fine angular blocky; common very fine pores; few very fine CaCO3 infillings; few gasteropod shells; gradual boundary.

2 C

200-244 cm; silt loam; 10YR 5/4 (yellowish brown); strongly calcareous; massive; common very fine pores; common very fine CaCO3 infillings; clear smooth boundary.

3 Ab

244-270 cm; silty-clay; 10YR 5/3 (brown); strongly calcareous; weak fine and medium angular blocky; common very fine pores; many fine CaCO3 infillings; clear smooth boundary.

3 C

270-290 cm; silty-clay loam; matrix 10YR 4/3 (brown); few fine depleted areas 10YR 6/3 (pale brown); few very fine mottles 10YR 5/8 (yellowish brown); strongly calcareous; massive; common very fine pores; common very fine CaCO3 infillings; clear smooth boundary.

4 C

290-330 cm; silt loam; matrix 10YR 3/4 (dark yellowish brown); many fine depleted areas 10YR 6/3 (pale brown); fine mottles 10YR 5/8 (yellowish brown); strongly calcareous; massive; common very fine pores; common very fine CaCO3 infillings; abrupt irregular boundary.

5 Ab

330-350 cm; silty-clay loam (few charcoal and baked soil fragments); 10YR 4/6 (dark yellowish brown); many fine Fe-depleted areas 10YR 5/2 (grayish brown); moderately to strongly calcareous; weak very coarse angular blocky; common very fine pores; common very fine CaCO3 infillings; many gasteropod shells; gradual smooth boundary.

5 C

350-390/408 cm sandy loam at the base grading to sandy clay loam at the top; 10 YR 5/4 (yellowish brown); fine mottles 10YR 5/6 (yellowish brown); few Fe-depleted areas 10YR 6/3 (pale brown); moderately to strongly calcareous; massive; few very fine pores; common very fine CaCO3 infillings; very few medium hard carbonate nodules; abrupt smooth boundary.

6 C

390/408-430 cm; silt loam; matrix 2.5Y 6/4 (light yellowish brown); many fine depleted areas 2.5Y 6/3 (light brownish gray); fine mottles 10YR 6/8 (brownish yellow); strongly calcareous; massive; common very fine pores; few very fine hard CaCO3 concretions; clear smooth boundary.

7 ABb

430-450 cm; clay loam; matrix 10YR 4/3 (brown); many depleted areas along pores 10YR 5/2 (grayish brown); moderately calcareous; weak coarse angular blocky; common very fine and fine pores; very few very fine CaCO3 concretions; common fine charcoal and few burned soil fragments; gradual irregular boundary.

7 Cg

450-470 cm; silt loam; matrix 10YR 3/2 (very dark grayish brown); abundant depleted areas 10YR 6/2 (light brownish gray); mottles 10YR 5/8 (yellowish brown); slightly calcareous; massive; very rare fine pores; very rare fine hard CaCO3 concretions, very rare very fine CaCO3 infillings; gradual irregular boundary.

8 Cg

470-500 cm; sandy clay; matrix 10YR 5/4 (yellowish brown); many depleted areas 10YR 6/3 (pale brown; mottles 10YR 5/8 (yellowish brown); moderately to strongly calcareous; massive; very rare fine pores; boundary not reached.

Fig. 4 – Sant’Ilario/Taneto and Cave Spalletti stratigraphic sequences
Fig. 4 – Coupe stratigraphique de Sant’Ilario/Taneto et de la carrière Spalletti

Fig. 4 – Sant’Ilario/Taneto and Cave Spalletti stratigraphic sequencesFig. 4 – Coupe stratigraphique de Sant’Ilario/Taneto et de la carrière Spalletti

1: gasteropod shells; 2: laminated sand; 3: iron mottles. IR: iron age pit fill; CHA: Chalcolithic sites; NEO: Neolithic sites. Solid triangles indicate spots of burned soil. Key for grain size: Cl: clay; Si: silt; Sa: sand; G: gravel.
1 : fragments de gastéropodes ; 2 : sables laminés ; 3 : précipitation localisée de fer ferrique. IR : remplissage d’une structure archéologique de l’Age du Fer ; CHA : sites chalcolithiques ; NEO : site néolithique. Le triangle noir indique une zone de terrain brûlé. Légende pour la granulométrie : C : argiles ; Si : limons ; Sa : sables ; G : graviers.

Soil Micromorphology

11In the late Chalcolithic buried soil (horizon 3Ab) of the Sant’Ilario/Taneto sequence, the strong biological activity, the presence of earthworm granules (Canti, 2003), and the large quantities of Fe-substituted organic matter allow us to identify an original topsoil horizon (tab. 5). The short period of exposure, resulting in a low degree of weathering, is attested by the very calcareous nature of the groundmass. Several micromorphological indicators point to clearance by fire and correlate to the large charcoal/burned soil patches exposed on the surface of this soil during excavation. Very finely (<50 µm) and finely (50-250 µm) comminuted charcoal and charred vegetal fragments are dispersed in the groundmass. Burned soil fragments indicate combustion in both reducing (very dark brown in PPL, parallel polariser) and oxidising conditions (light reddish brown in PPL), and appear as rounded medium sand-sized aggregates. The limits between the aggregates and the adjoining groundmass are sharp, suggesting reworking. Wood ash aggregates are rare and occasionally retain the pseudomorph morphology of the original deciduous wood tissue (fig. 6B). Anthropogenic inclusions, such as ashed excrements, pottery, burned and unburned bone fragments and dispersed faecal spherulites (Canti, 2003), have also been identified. These might derive from manuring practices involving the use of domestic wastes mixed with excrements (Bakels, 1997; Goldberg and Macphail, 2006). Dusty clay textural pedofeatures are very abundant, and correspond to non-laminated coatings with broad extinction lines. These pedofeatures indicate a severely disturbed, bare soil surface resulting from vegetation clearance and possibly from cultivation practices (Courty et al., 1989; Macphail et al., 1990). The middle Neolithic buried soil (horizon 7ABb) also corresponds to a former topsoil horizon (biogenic porosity, abundant Fe-substituted organic matter; earthworm granules). The groundmass appears decarbonated and the outer rim of earthworm granules is weathered. Abundant very finely (<50 μm) and finely (50-250 μm) comminuted charcoal and charred vegetal fragments are dispersed in the groundmass, together with rare large deciduous wood charcoal pieces. Dusty clay coatings, containing black and very dark brown microparticles, are rare and occur predominantly inside vughs (fig. 6C). The overall picture is that of a well developed, decarbonated topsoil horizon, in which large quantities of charcoal have been reworked, possibly as the result of vegetation clearance in this peripheral part of the Neolithic settlement. The presence of dusty clay pedofeatures points in fact to surface slaking processes, most likely resulting from the exposure of the soil surface after vegetation removal (De Ploey and Mücher, 1981; Courty et al., 1989; Macphail et al., 1990).

San Pancrazio (Parma)

Description

12The stratigraphic sequence exposed at this site (57 m a.s.l.; 44°48’46”N; 10°16’41”E) during rescue archaeological operations consists of about 7.5 m of alluvial sands and silts with intercalated soils (fig. 5A and tab. 3). Below the present-day horizon, which contains Roman and Iron Age pottery, four different buried soils were found at the depths of ca. 1 m, 3 m, 5 m and 7 m from the present-day surface. The first two show clearance features and will be discussed in detail below, whereas no archaeological evidence is associated with the deeper soils. Gravel deposits that might represent the top of the Upper Pleistocene alluvial fan were reached at the depth of 7.5 m. The upper buried soil consists of a well-developed Vertisol (2Ab) containing archaeological material predominantly of the early Bronze Age with minor amounts from the Chalcolithic period, and was excavated over an area of 7000 m2. Upon removal of a layer of white carbonatic silt loam that covered the soil, numerous pseudomorph traces of roots were exposed, filled with the same white silts that buried the soil. Wide charcoal patches were also observed to occur in association with the root traces on the soil. The root traces show that tree stumps were left in place, not uprooted and/or burned. After being buried by alluvium, the wood decomposed and the resulting pseudomorph hollows were filled with white carbonatic silts, whose colour contrasts with the adjoining dark-coloured soil. The charcoal patches are surrounded by very abundant comminute pottery fragments, concentrated at a depth of 10 cm within the buried soil. The positions of these potsherds were mapped through a detailed topographic survey using a total station and were seen to be concentrated along horizontal lines with trending N-S, the orientation of the local slope. This spatial distribution, together with their fragmented and worn state, has been interpreted as a result of ploughing (Bernabò Brea et al., in press). Radiocarbon datings performed on two different charcoal patches, both on top of the soil, yielded the following dates: sample 1, 3860±60 a 14C BP (4430-4090 a cal. BP); sample 2, 3890±50 a 14C BP (4440-4150 a cal. BP). A further, weakly developed soil (3Ab) was exposed at a depth of 330 cm. Subcircular and lobate burned soil patches were found on it. They range from 10 cm to l m in diametre, similar to those observed on the 3Ab and 5Ab buried soils of Sant’Ilario/Taneto. One of these patches ended in a long strip of charcoal and burned soil, interpreted as a collapsed tree trunk that burned on the ground. This soil was exposed by archaeological excavation over a wide area, and many spots of burned soil associated with charcoal were found, though no associated archaeological material was observed. No radiocarbon dating has yet been performed on the charcoal, but the 3Ab buried soil might be correlated with the one drilled by the Emilia Romagna Geological Service in the same location at comparable depth (.Regione.Emilia-Romagna.it/geologia). This soil gave a date of 4205±100 a 14C BP (5050-4400 a cal. BP) on its humic fraction.

Tab. 3 – San Pancrazio. Profile description
Tab. 3 – Description de la séquence de San Pancrazio

Horizons

Description of the San Pancrazio sequence

Ap

0-50 cm; silty-clay loam; 10YR 3/3 (dark brown); well developed angular blocky, slightly calcareous; Iron Age and Roman Age sherds; clear smooth boundary.

C

45-85 cm; silt loam to sand loam; massive to discontinuously laminated, very calcareous; 2.5Y 5/4 (light olive brown); abrupt smooth boundary.

2Ab

110-150 cm; clay; 10YR 3/2 (dark greyish brown); well developed angular blocky; few fine pores; discontinuous layer of charcoal fragment at the top, clusters of fine ceramic fragments; clear smooth boundary.

2 Bw

150-190 cm; clay; 10YR 3/3 (dark brown), well developed angular blocky; gradual wavy boundary.

2C

190-530 cm; silty-clay loam; 2.5Y 5/4 (light olive brown); massive; many calcareous infillings; many gasteropod shell fragments; clear smooth boundary.

3Ab

530-570 cm; silty-clay; 2.5 Y 5/2 (grayish brown), fine angular blocky; many fine and coarse pores; many gasteropod shell fragments; clear smooth boundary.

4C

570-670 cm; silt loam grading to silty-sand between 650-660cm; 2.5Y 6/4 (light yellowish brown) and 2.5Y 5/6 (light olive brown); massive; calcareous, clear smooth boundary.

5Ab

670-700 cm; clay loam; 10YR 3/3 (dark brown); few mottles 2.5Y 5/2 (grayish brown); well developed angular blocky; slightly calcareous, few fine pores; gradual boundary.

6C

700-725 cm; loam; 2.5Y 5/4 (light olive brown); massive; few fine and coarse pores; clear smooth boundary.

7R

725-740 cm; clast-supported gravel, lower boundary not reached.

Soil Micromorphology

13In the early Bronze Age buried soil (horizon 2Ab) at San Pancrazio (tab. 5), the very strong vertic processes that characterise this clay-textured horizon are markedly expressed. The groundmass is decarbonated and shows a strongly expressed cross-striated b-fabric, with common granostriations and porostriations. The churning effect of expandable clays can be also seen in the concentric morphology of the frequent Fe and Fe/Mn nodules, both orthic and anorthic. The effects of clearance by means of fire are visible primarily in the abundant charcoal fragments of all size classes strongly mixed and reworked in the groundmass, together with occasional dark brown burned soil fragments. Elongated phytoliths, small rounded pottery fragments, rare phosphatic aggregates (fig. 6D), and the formation of Fe-phosphatic nodules indicate that manuring practices employed a mixture of domestic waste and excrements (Goldberg and Macphail, 2006). No textural pedofeatures are present, except for a laminated dusty clay coating fragmented and deformed by vertic processes. This absence is probably related to the high aggregate stability of this clay-texured soil and to the strong degree of pedoturbation that might have “digested” the textural pedofeatures into the soil, hampering their preservation.

Fig. 5 – San Pancrazio and Botteghino stratigraphic sequences
Fig. 5 – Les coupes stratigraphiques de San Pancrazio et Botteghino

Fig. 5 – San Pancrazio and Botteghino stratigraphic sequencesFig. 5 – Les coupes stratigraphiques de San Pancrazio et Botteghino

1: gastropod shells; 2: iron mottles; RO: Roman age finds; IR: Iron Age finds; EBA: Early Bronze Age finds; CHA: Chalcolithic Age finds; NEO: Neolithic Age site. Solid triangles indicate spots of burned soil. Key for grain size: Cl: clay; Si: silt; Sa: Sand; G: gravel.
1 : fragments de gastéropodes ; 2 : précipitation localisée de fer ferrique. RO : matériel archéologique de l’Antiquité romaine ; IR : matériel archéologique de l’Age du Fer ; EBA : matériel archéologique de l’Age du Bronze ancien ; CHA : matériel archéologique du Chalcolithique ; NEO : sites néolithiques. Les triangles noirs indiquent la présence de zones de terrain brûlé. Légende pour la granulométrie : Cl : argiles ; Si : limons ; Sa : sables ; G : graviers.

Fig. 6 – Micromorphology photos
Fig. 6 – Photographies de micromorphologie

Fig. 6 – Micromorphology photosFig. 6 – Photographies de micromorphologie

A: Rubiera, horizon 2ABb (Bell-Beaker buried soil). Finely comminuted charcoal and charred vegetal fragments (OIL, 100x). B: Sant’Ilario/Taneto, horizon 3Ab (Chalcolithic buried soil). Wood ash fragment retaining the morphology of the original vegetal tissue (XPL, cross polariser, 100x). C: Sant’Ilario/Taneto, horizon 7ABb (mid Neolithic buried soil). Dusty clay coating related to topsoil disturbance (PPL, parallel polariser, 100x). D: San Pancrazio, horizon 2Ab (early Bronze Age Vertisol). Phosphatic aggregate, possibly associated with manuring practices (PPL, 200x). E: Botteghino, horizon 3ABb (Chalcolithic soil adjacent to a burned tree-hollow). Reworked Bt horizon fragment, stripped from theunderlying Sol Brun Fersiallitique (5Btb) during the uprooting process (XPL, 20x). F: Botteghino, uprooted tree pit fill. Limpid clay coatings inside a burned soil aggregate (optically isotropic; XPL, 20x).
A : Rubiera, horizon 2ABb (sol campaniforme). Charbons fins et matière végétale partiellement brûlée (OIL, 100x). B : Sant’Ilario/Taneto, horizon 3Ab (paléosol chalcolithique). Fragment de cendre de bois (LP, 100x). C : Sant’Ilario/Taneto, horizon 7ABb (sol du Néolithique moyen). Revêtement d’argiles poussiéreuses liées à la perturbation de la surface du sol (LN, 100x). D : San Pancrazio, horizon 2Ab (vertisol de l’Age du Bronze inférieur). Agrégat phosphaté peut-être associé au compostage (PPL, 200x). E : Botteghino, horizon 3ABb (sol chalcolithique à proximité d’une cavité d’essartage). Fragment d’horizon Bt remanié, dérivé d’un sol brun fersiallitique (5Btb) sous-jacent (LP, 20x). F : Botteghino, remplissage d’une cavité d’essartage. Revêtement d’argile limpide au sein d’un fragment de sol brûlé (LP, 20x).

Botteghino (Parma)

Description

14Rescue archaeological excavations at this site (86 m a.s.l.; 44°44’52”N; 10°21’37”E) provided archaeological evidence dating from the late Neolithic to the Roman period. The site is located upstream on the Upper Pleistocene alluvial fan fringe with respect to the other sites in the present study. Floodplain deposits are therefore less thick, and Upper Pleistocene gravels outcrop at 1-1.3 m depth (fig. 5 and tab. 4). Below the present-day Ap horizon there is a slightly developed buried soil (2Ab) devoid of any archaeological evidence. Meanwhile, the buried soil at 60 cm of depth (3ABb) contained a few Chalcolithic sherds, and presented some areas reddened due to exposure to fire on its surface. These correspond to irregular, sometimes vaguely rectangular patches reaching up to 15 m2. These patches are usually bordered by a very dark brown buffer made up by soil fragments burned in reducing conditions, whereas their fill is predominantly composed of reddish soil material, often broken up in angular fragments and indicating combustion in oxidising conditions. A further buried soil (4ABb) containing middle Neolithic (Square Mouth Pottery) sherds, was found at the depth of 120 cm, resting directly on top of the remnants of the soil (5Btb) that formed on the gravels of the Upper Pleistocene alluvial fan. This soil (5Btb) may be regarded as an eroded Sol brun fersiallitique (Cremaschi, 1979, 1987).

Tab. 4 – Botteghino. Profile description
Tab. 4 – Description de la séquence de Botteghino

Horizons

Description of the Botteghino sequence

Ap

0-23 cm; clay loam, 10YR 3/3 (dark brown); well developed coarse angular blocky, slightly calcareous; clear smooth boundary

2Ab

23-40 cm; silty-clay loam; 10YR 5/3 (brown); weakly separated coarse prismatic structure; few very fine pores, very few fine pores; many very fine CaCO3 pore infillings; common gasteropod shell fragments; few charcoal fragments; gradual boundary.

2C

40-60 cm; silty-clay loam; 10YR 7/4 (very pale brown); massive; common very fine pores; very few fine pores; few very fine CaCO3 pore infillings; clear smooth boundary.

3ABb

60-93 cm; silty clay loam; 2.5Y 4/4 (olive brown matrix); common fine mottles, 7.5YR 4/6 (strong brown); well developed medium and coarse subangular blocky structure; few very fine pores; very few fine pores; many pressure faces; common fine and medium CaCO3 pore infillings; abrupt smooth boundary.

3Cg

93-120 cm; silty-clay loam; 2.5Y 4/4 (olive brown); massive, calcareous, clear linear boundary.

4ABb

120-132 cm; silty-clay loam, 10YR 3/2 (very dark grayish brown); moderately developed angular blocky, slightly calcareous; clear undulated erosive boundary

5 Btb

132-140 cm; clay loam, common small weathered stones, 7.5 YR 4/4 (brown), well developed fine angular blocky, common pores, clear boundary.

5C

140-170 cm; sand-supported loose gravel, lower boundary not reached.

Soil Micromorphology

15Thin sections were studied from the Chalcolithic buried soil (3ABb) adjacent to the burned tree-hollow, from the central portion of the fill of the hollow and from the dark-coloured margin of the fill. A fourth thin section was studied from horizon 4ABb, the Neolithic soil lying directly on top of the Sol brun fersiallitique formed on the Upper Pleistocene alluvial fan (tab. 5). The Chalcolithic buried soil (3ABb) is clay-textured and its groundmass is almost completely decarbonated. Fine (50-250 µm) charcoal and charred vegetal fragments are dispersed in the fine matrix, together with very rare non-articulated elongated phytoliths, whereas no coarse charcoal has been observed. Reworked burned soil fragments are abundant: their colour in PPL testifies to various degrees of exposure to heat in both reducing and oxidising conditions. The reworking of underlying soil horizons is attested by the presence of occasional Bt horizon fragments deriving from the underlying Sol brun fersiallitique (fig. 6E). The dispersion and reworking of burned soil and of Bt horizon fragments suggests soil loosening and disruption caused by tree uprooting. The fill of the tree hollow is composed of a mixture of burned soil aggregates (constituting ca. 70% of the sample) surrounded by inwashed calcareous clays. Burned soil aggregates appear light orange/red in oblique incident light (OIL) and optically isotropic as a result of exposure to heat in oxidising conditions. The sample is characterised by a striking quantity of different superimposed and/or juxtaposed textural pedofeatures. These consist primarily of cappings and coatings of very dark brown (PPL) impure clay occurring within or on top of aggregates, followed by the inwashed calcareous clays. A phase of carbonate precipitation (micrite hypocoating and infillings in the fine inwashed material) and subsequent dissolution follows, before the last generation of clay coatings and infillings is formed. The latter consist of well-ordered speckled or limpid clays, occurring both in pores in the inwashed material and in intrapedal pores within burned aggregates (fig. 6F). The margin of the tree hollow is also composed of rounded and subrounded burned soil aggregates with inwashed calcareous clays in the interaggregate space. The aggregates are dark brown (PPL) with stipple-speckled b-fabric, suggesting combustion in reducing conditions. Extremely rare fine charcoal and charred vegetal fragments are present both inside aggregates and in the inwashed fine material. Similarly to the filling of the tree hollow, a complex palimpsest of different generations of textural and cryptocrystalline pedofeatures has been observed here. The more humic character for this portion of the tree hollow can be recognised in the Fe/Mn substitution for fine organic matter. All of the above-mentioned micromorphological features are precisely consistent with those reported elsewhere for the uprooting and subsequent burning of a tree (e.g., Courty et al., 1989, p. 127-129 and fig. 7.5B; Macphail, 1990, p. 189 and figures 127 and 128; Macphail et al., 1990, p. 54-55 and table 1; Goldberg and Macphail, 2006, p. 199-200 and fig. 9.8). Severe soil loosening and disruption appears to have taken place in response to the uprooting of the tree (which could have been natural or man-induced). The subsoil retained on the root plate burned in oxydising conditions, whereas the soil at the contact between the root plate and the walls of the hollow was burned in reducing conditions, resulting in the different colours of the aggregates. The wide array of superimposed textural pedofeatures observed testifies to strong slaking and sediment inwash between loose aggregates accompanying or immediately following the fall-back of burned soil into the hollow. Ash-induced dispersion (Courty et al., 1989) could also have played a role in the slaking of the soil (H. Huismann, personal communication 2010). The micromorphological characteristics of horizon 4ABb reflect the moderate to high degree of weathering of the soil on top of the Upper Pleistocene alluvial fan, dwelled upon by humans in the Neolithic. These correspond to the decarbonatation of the matrix, the highly separated microstructure, and the reddish colour of the groundmass (PPL), indicating the incipient rubefaction process. No limpid clay coatings have been observed, but the well-expressed stipple-speckled b-fabric suggests an incipient reorganisation of the fine mass (Bullock and Murphy, 1979). The human impact on this soil is attested by the presence of fine charcoal and charred vegetal fragments dispersed in the groundmass along with rounded, baked soil fragments. Dusty clay coatings observed in the different type of pores suggest processes of topsoil disturbance and slaking. Based on these indicators, also the Neolithic occupation phase of Botteghino appears to be characterised by practices of clearance by fire.

Tab. 5 – Micromorphological description of thin sections
Tab. 5 – Description micromorphologique des lames minces

Tab. 5 – Micromorphological description of thin sectionsTab. 5 – Description micromorphologique des lames minces

Abundance of fabric units expressed according to G. Stoops (2003): * Very rare (<5%); ** rare (5-15%); *** common (15-30%); **** frequent (30-50%). 1 Site: Rub: Rubiera; Sit: Sant’Ilario/Taneto; SP: San Pancrazio; Bot: Botteghino. 2 MS (microstructure): ch: channel; v: vughy; sb: subangular blocky; ab: angular blocky. 3 GM (groundmass): (a) b-fabric: cs: cross-striated; ps: porostriated; gs: granostriated; cr: crystallitic; ss: stipple-speckled; (b) related distribution pattern: ssp: single-spaced porphyric; op: open porphyric; b/cp: basic microstructure/closed porphyric (reworked aggregates in inwashed clays); dsp: double spaced porphyric. 4 Biogenic, geogenic, anthropogenic components: (c) fine (<100 μm) charcoal and charred vegetal fragments; (d) coarse (>100 μm) charcoal; (e) burned soil fragments; (f) dung; (g) phytoliths; (h) earthworm granules; (i) shell fragments. 5 Pedofeatures: (j) compound layered silt, silty clay and/or dusty clay coatings and infillings; (k) dusty clay intercalations; (l) dusty clay coatings and infillings; (m) fragmented/reworked textural pedofeatures; (n) limpid clay coatings; (o) Fe and Fe/Mn nodules (orthic and anorthic).
Abondance des unités de fabrique exprimée selon G. Stoops (2003) : * très rare (<5 %) ; ** rare (5-15 %) ; *** abondant (15-30 %) ; **** fréquent (30-50 %). 1 Sites : Rub : Rubiera ; Sit : Sant’Ilario/Taneto ; SP : San Pancrazio ; Bot : Botteghino. 2 MS (microstructure) : ch : à chenaux ; v : cavitaire ; sb : en blocs subangulaires ; ab : polyédrique angulaire. 3 GM (structure du sol) : (a) b-fabric : cs : à stries entrecroisées ; ps : porostrié ; gs : granostrié ; cr : crystallitique ; ss : en tâches isolées ; (b) distribution relative : ssp : porphyrique à espacement simple ; op : porphyrique serrée ; b/cp : microstructure basique/porphyrique serrée (agrégats remaniés dans une masse argileuse) ; dsp : porphyrique à espacement double. 4 Composants biogéniques, géogéniques et anthropogéniques : (c) charbons fins et matière végétale fine brûlée partiellement (<100 μm) ; (d) charbons grossiers (>100 μm) ; (e) agrégats de terrain brûlé ; (f) excréments ; (g) phytolithes ; (h) biosphérolithes de lombric ; (i) fragments de coquille. 5 Traits pédologiques : (j) revêtement et remplissage lité complexe par des limons, des limons argileux ou des argiles poussiéreuses ; (k) intercalations d’argiles poussiéreuses ; (l) revêtement et remplissage par des argiles poussiéreuses ; (m) traits pédologiques texturaux fragmentés et/ou remaniés ; (n) revêtement d’argile limpide ; (o) nodules de Fe et de Fe/Mn (orthiques et anorthiques).

Tab. 6 – The radiocarbon dates cited in the present study
Tab 6 Liste des datations radiocarbone utilisées dans la présente étude

Location

Coordinates

Heigth

(a.s.l.)

Lab. ID

Conventional radiocarbon age

(in a BP)

2  (95.4%)

Calibrated date

(in cal. a BP)

Razza di Campegine

(humic fraction of the Neolithic soil)1

lat. 44° 46' 6" N long. 10° 30' 58" E

38 m

GX-29088

6490±90

7570-7250

Razza di Campegine (charcoal)1

lat. 44° 46' 6" N long. 10° 30' 58" E

38 m

GX-29087-

AMS

5940±40

6400-6200

Rubiera, Secchia river bed2

lat. 44° 39’ 14’’N long. 10°47’51’’ E

50 m

R-703

3540±50

3980-3690

Rubiera, “Corradini” quarry3

lat. 44° 38' 32” N long. 10° 47' 29 " E

56 m

GX19225

4585±115

5600-4850

Rubiera, “Corradini” quarry3

lat. 44° 38' 32” N long. 10° 47' 29 " E

56 m

GX19226

3835±90

4550-3950

Rubiera, “Corradini” quarry3

lat. 44° 38' 32” N long. 10° 47' 29 " E

56 m

GX19220

4725±95

5700-5250 (93.3%)

5200-5050 (2.1%)

S. Pancrazio

(buried soil humic fraction)4

lat. 44° 48' 42” N long. 10° 16' 5” E

56 m

na

4205±100

5050-4400

S. Pancrazio (charcoal)5

lat. 44°48’46” N long. 10°16’41” E

57 m

GX2315

3860±60

4430-4090

S. Pancrazio (charcoal)5

lat. 44°48’46” N long. 10°16’41” E

57 m

GX32454

3890±50

4440-4150

S. Ilario “Cave Nuove” quarry2

lat. 44° 45' 47 " N long. 10° 25' 44" E

76 m

R-1242

3900±60

4520-4470 (4.4%)

4450-4150 (91.0%)

Sant’Ilario/Taneto (charcoal)5

lat. 44° 45’50” N long. 10° 27’ 30” E

50 m

Poz-27522

4920±50

5850-5750 (1.1%)

5750-5580 (94.3%)

Sant’Ilario/Taneto (charcoal)5

lat. 44° 45’50” N long. 10° 27’ 30” E

50 m

Poz-27523

4525±35

5310-5210 (33.7%)

5200-5040 (61.7%)

Calibration has been performed with OxCal software, v. 3.10 (Bronk Ramsey, 2001, 2005), with IntCal04 atmospheric data (northern hemisphere atmospheric; Reimer et al., 2004). BP corresponds to “before AD 1950”. Apex numbers indicate origin of the dates: 1 Bernabò Brea et al. (2009); 2 Alessio et al. (1981); 3 Cremaschi (1997); 4 Regione Emilia-Romagna Geological Survey (www.Regione.Emilia-Romagna.it/geologia); 5 Bernabò Brea et al. (in press).
La calibration a été effectuée avec OxCal software, v. 3.10 (Bronk Ramsey, 2001, 2005) avec les données atmosphériques IntCal04 atmospheric data (northern hemisphere atmospheric; Reimer et al., 2004). BP signifie « avant 1950 ». Les valeurs en exposant précisent la source des datations : 1 Bernabò Brea et al. (2008) ; 2 Alessio et al. (1980) ; 3 Cremaschi (1997) ; 4 Regione Emilia-Romagna Geological Survey (www.Regione.Emilia-Romagna.it/geologia) ; 5 Bernabò Brea et al. (in press).

Discussion

The pedo-sedimentary sequence

16The top of the alluvial fans in the profiles studied in this paper can be identified by the texture of the sediments, which is usually gravelly. Only in the Sant’Ilario/Taneto sequence the top of the alluvial fan is sandy, but deep corings revealed that gravels are present a few meters below the sands. The alluvial fans are overlain by deep soils with an argillic horizon (Alfisols, Sols bruns fersiallitiques). These are encountered both in natural contexts and at archaeological sites pertaining to the Neolithic period (Cremaschi, 1979, 1987, 1990). These soils exhibit moderate rubefaction when developed on gravelly parent material and pass in a catenary connection to eroded phases (Botteghino), or to brunified soils (Sant’Ilario/Taneto). They were formed through decarbonatation, slight mineral weathering, clay illuviation and iron oxidation, processes requiring a wood cover in a xeric pedoclimate (Duchaufour, 1983). These processes can be linked to a phase of stability following the end of aggradation of the alluvial fans at the Apennine fringe. Connection with Neolithic settlements and contextual radiocarbon datings (see above) place the date of their formation between the end of the last glaciation and the Atlantic period. The alluvial deposits that bury the weathered surfaces of the alluvial fans are predominantly fine-textured and organised in fining upward cycles culminating in a pedogenetic horizon. They have accumulated in an alluvial plain context in a proximal overbank environment. The fluvial channels from which they originated have not yet been identified. However, the deposition of these alluvial sediments does not indicate a state of quiescence of alluvial fan progradation. Rather, it represents the restart of fluvial sedimentation in a different sedimentary environment (lower competency), after the phase of geomorphological stability represented by soil formation on top of the fans. The soils intercalated in the alluvial deposits are often of limited thickness (10-30 cm), weak pedality and with incomplete decarbonatation. They have a moderate to low organic matter content, often increased by the proximity of an archaeological site, and are articulated in poorly differentiated profiles of the Ab-C type. Therefore, they can be broadly described as Fluvents (Soil Survey Staff, 2003) or Sols alluviaux (Duchaufour, 1983), except for the few cases in which stronger decarbonatation and well-developed pedality (indicating a tendency toward a B horizon formation) might be considered as intermediary terms towards Inceptisols. In S. Pancrazio the 2Ab soil, due to its well-expressed vertic proprieties, must be regarded as a Vertisol. In any case, these soils were formed on unstable surfaces of alluvial deposits exposed to pedogenesis for a short period before being flooded again. The systematic occurrence of terrestrial pulmonates of the Pomatia and Cepaea genus can be linked to soils covered by bushes and/or a sparse woodland (Girod, 2004) and fits well with the model of human land exploitation discussed below.

17Systematic association with archaeological evidence and several radiocarbon datings allows us to date precisely the floodplain deposits at the margins of the alluvial fans. The sedimentation began at about (or slightly before) 5750-5580 a cal. BP, and ceased slightly before 3500 a cal. BP. Archaeological sites of the middle Bronze Age (which begun at ca. 3500 a cal. BP; Bernabò Brea and Cardarelli, 1997) lie at the surface, indicating that, in the area under examination, the aggradation of the alluvial deposits came to an end in that period. The aggradation of the alluvial deposits and the intercalated soils date to the first part of the Sub-Boreal (Mangerud, 1982; Orombelli and Ravazzi, 1996). In contrast to the soils formed on top of the Upper Pleistocene alluvial fans, the alternating sequence of weakly developed soils and alluvial deposits indicates renewed river activity and increased frequency of floods. These processes at the plain’s margin have a precise correspondence with the increased slope instability recorded in the mountain area, as revealed by the high number of landslides dating back to this period (Bertolini et al., 2005; Bertolini, 2007). The peak of the landslides recorded in the high Apennines coincides with that of the buried soils at the margin of alluvial fans, indicating an increased alternation of short-lived soils and episodes of fluvial aggradation (fig. 7). These are two different aspects to the same process of geomorphological instability at the beginning of the Sub-Boreal chronozone. This, in turn, must be related to a climatic worsening, implying a general rise in precipitation mainly in mountain areas. It must be stressed that this is not an isolated episode and that it is coincident with traces of climatic degradation in the same period (ca. 5600 a cal. BP) in the alpine area. These include the increase of landslide events (Soldati et al., 2004), and rapid glacier advance (Porter and Orombelli, 1985) attested by the quick burial of the Chalcolithic alpine Ice Man (Baroni and Orombelli, 1996) and by the rise of Lake Constance and other Jurassian lakes (Magny et al., 2006). Similarly, the increase in precipitations and the climate worsening responsible for the early Sub-Boreal sedimentation at the fringe of the pede-Apennine fans appear to coincide with the mid-Holocene climate transition (onset of the Neoglaciation) recorded also at global scale (Mayewski et al., 2004; Magny et al., 2006).

Fig. 7 – Available radiocarbon datings of buried soils of the Emilia alluvial plain (from Alessio et al., 1981; Regione Emilia-Romagna Geological Survey, www.Regione.Emilia-Romagna.it/geologia; Cremaschi, 1997) compared with those from Apennine landslides (from Bertolini et al., 2005; Bertolini, 2007)
Fig. 7 Les datations au radiocarbone des sols enterrés de la plaine alluviale (d’après Alessio et al., 1980; Regione Emilia-Romagna Geological Survey, www.Regione.Emilia-Romagna.it/geologia; Cremaschi, 1997) comparées à celles des glissements de terrain de l’Apennin septentrionale (d’après Bertolini et al., 2005; Bertolini, 2007)

Fig. 7 – Available radiocarbon datings of buried soils of the Emilia alluvial plain (from Alessio et al., 1981; Regione Emilia-Romagna Geological Survey, www.Regione.Emilia-Romagna.it/geologia; Cremaschi, 1997) compared with those from Apennine landslides (from Bertolini et al., 2005; Bertolini, 2007)Fig. 7 – Les datations au radiocarbone des sols enterrés de la plaine alluviale (d’après Alessio et al., 1980; Regione Emilia-Romagna Geological Survey, www.Regione.Emilia-Romagna.it/geologia; Cremaschi, 1997) comparées à celles des glissements de terrain de l’Apennin septentrionale (d’après Bertolini et al., 2005; Bertolini, 2007)

Soil use during the Chalcolithic in the Sub-Boreal period

18From an archaeological standpoint, the main characteristics of the shallow Chalcolithic soils intercalated within the alluvial deposits are the frequent occurrence of patches of burned soil and the clusters of charcoal connected to standing tree stumps or to burned stubs. Their traces are not distributed in close proximity to archaeological settlements, but at their margins and also far from them. The patches of burned soil and the wide charcoal concentrations spread on top of the buried soils (and integrated within them) should be linked to the burning of the vegetal cover, and hence to the ethnographically well-known practice of “slash and burn” (Rowley-Conwy, 1981). Soil micromorphology has highlighted the fact that woodland clearance and secondary clearance by fire resulted in processes of soil loosening, severe disturbance and slaking in all of the examined Chalcolithic soils. Moreover, such soils contain large amounts of finely comminuted charcoal and charred vegetal fragments along with larger charcoal pieces and burned soil aggregates. Soils with traces of uprooting (Botteghino) contain also fragments stripped from underlying soil horizons, whereas soils that were probably manured contain phytoliths, dung, ashes, and pottery sherds (San Pancrazio and Taneto/Sant’Ilario). Compared to the examined Neolithic soils, the Chalcolithic ones (and in part the early Bronze Age soil at San Pancrazio) are richer in charcoal and burned soil fragments and appear more severely disturbed, as illustrated by the wider occurrence of dusty clay textural pedofeatures. This is striking, especially considering that these Chalcolithic soils are generally very calcareous and therefore more resistant to slaking (Jongerius, 1970; De Ploey and Mücher, 1981; Limbrey, 1993), and that they were exposed for a much shorter time period than their Neolithic counterparts.

19Two main procedures for Chalcolithic slash-and-burn have been documented at the studied sites. The first (Rubiera, San Pancrazio) consisted of burning the top part of the plants (cut or simply set on fire), leaving the tree stump in place. The second practice (Botteghino; Santi’Ilario/Taneto) consisted of setting on fire the root system, the soil torn up with it, and the remains of the uprooted tree (Schaetzl et al., 1989; Langohr, 1993). This evidence of extensive woodland clearance by means of fire is not limited to the piedmont area, but is also recorded in the mountain range in the upper reaches of the Apennines (Cremaschi et al., 1984). The widespread occurrence of white fir charcoal in mountain soils (Cusna area and Monte Cimone; Bertolani Marchetti, 1963) indicates large fires at high altitude. These are linked to the renewed human exploitation of the Emilian and Ligurian Apennines that followed the diffusion of transhumant pastoralism in the Chalcolithic (De Marinis, 1994; Maggi, 2004). The Neolithic soils (Cremaschi, in press; Cremaschi et al., in press) of the studied sites do not preserve spots of burned soil or large concentrations of charcoal. Nevertheless, woodland clearance through fire is suggested by soil micromorphology on account of the presence in the groundmass of fine and coarse charcoal and of baked soil fragments, and due to the dusty clay pedofeatures (late Neolithic soils of Sant’Ilario/Taneto and Rubiera; similar features have also been observed at the Neolithic site of Buco del Signore, SE of Reggio Emilia; Salvioni, 2005-2006). At the Neolithic site of Razza di Campegine, soil micromorphology revealed alternating phases of clearance and of re-forestation (Bernabò Brea et al., 2009). All of these indicators can be interpreted as evidence that fire clearance was limited both spatially and temporally and was linked to shifting agriculture (Cremaschi, 1990; Pessina and Tiné, 2008). This land management system allowed the regeneration of forest cover on the cultivated plots that were abandoned once soil nutrients were exhausted. The thin Sub-Boreal Chalcolithic soils in the alluvial plain deposits therefore mark a net change in land use. Unlike in the Neolithic, in the Chalcolithic wide surfaces were cleared with fire, a practice that extended well into the Apennine mountain range.

Conclusions

20The fine-grained deposits that accumulated on the distal part of the alluvial fans at the margin of the Apennine attest to a significant environmental change that occurred in the area at the Atlantic/Sub-Boreal transition. They also show the contemporaneous onset during the Chalcolithic period of massive and extensive woodland clearance through slash and burn, testifying to a rather different soil use compared to the Neolithic period. After a phase of geomorphologic stability, which led to the formation of deep rubified Sols bruns fersiallitiques (Alfisols) on top of the alluvial fans up to the Atlantic period, a substantial change in pedo-sedimentary processes took place at the distal margin of the alluvial fans beginning in the Sub-Boreal period. At this stage, the fans and the soils developed therein were buried (mainly in their distal and intermediate parts) by fine-textured alluvial deposits intercalated with weakly developed soils. This process is related to the end of the Post-Glacial Hypsithermal period (Orombelli and Ravazzi, 1996) and corresponds to a main climatic change in the Holocene (middle-Holocene climatic transition; Magny et al., 2006). The process described above is contemporaneous with slope erosion and enhanced geomorphological instability in the mountain areas, conditions that are further indicated by the acceleration of landslide phenomena in this period, both in the Alps and the Apennines. The soils overlying the alluvial fans and in the later alluvial deposits were densely occupied by human communities from the Neolithic to the Early Bronze Age. These soils witness a major change in land exploitation during the Atlantic to Sub-Boreal transition. While during the Neolithic (Atlantic period) agricultural practices affected the vegetal mantle only marginally and were limited to the floodplain areas (Accorsi et al., 1999), the Chalcolithic and Early Bronze Age human groups (Sub-Boreal period) adopted a different type of land use. This was primarily linked to the newly introduced transhumant pastoralism (Cremaschi et al., in press.). The deforestation, carried out by slash and burn practices, was very strong and pervasive and extended far beyond the boundaries of the plain, deep within the mountain range, thus impacting large parts of the Po Plain and the adjoining Apennines. The data presented and discussed in this paper highlight the occurrence of a significant environmental change connected to the climate deterioration at the Atlantic/Sub-Boreal transition, coupled with an important shift in land management and soil use. This consisted of an aggressive impact on the vegetation cover of theretofore-unseen magnitude practiced by Chalcolithic pastoralist communities. These two processes (changes in climate and in land use) should be regarded as complementary. This might indicate that anthropic impact amplified (at least on a regional scale) geomorphologic phenomena of accelerated erosion, already triggered by climate change.

The authors are deeply indebted to Dr. Maria Bernabò Brea, for allowing us to access the excavations under her direction, and to Dr. Cristina Anghinetti for the permission to study the Cave Spalletti sequence. The authors also thank two anomymous referees for suggestions and improvements to the first version of the paper. Dr. Emily B. Modrall reviewed the English of the manuscript.

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Soldati M., Corsini A., Pasuto A. (2004) – Landslide and climate change in the Italian Dolomites since the Late Glacial. Catena 55, 141-161.
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Stoops G. (2003) – Guidelines for analysis and description of soil and regolith thin sections. Soil Science Society of America, Madison, 179 p.

Valloni R., Baio M. (2009) – Sedimentazione altoquaternaria nel tratto emiliano del tracciato Alta Velocità. In Bernabò Brea M., Valloni R. (Eds.) Archeologia ad Alta Velocità in Emilia Romagna. Indagini archeologiche e geologiche lungo il tracciato dell’Alta Velocità. Quaderni di archeologia dell’Emilia Romagna, 22. All’insegna del giglio, Firenze, 21-39.

Vavrus S., Ruddiman W.F., Kutzbach J.E. (2008) – Climate model tests of the anthropogenic influence on greenhouse-induced climate change: the role of early human agriculture, industrialization, and vegetation feedbacks. Quaternary Science Reviews 27, 1410-1425.

Zerboni A., Trombino L., Cremaschi M. (2011) – Micromorphological approach to polycyclic pedogenesis on the Messak Settafet plateau (central Sahara): Formative processes and palaeoenvironmental significance. Geomorphology 125, 319-335.

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Annexe

Version abrégée en français

La marge des Apennins, située à la limite sud de la plaine du Pô (Italie du Nord), est bordée par une bande de cônes alluviaux coalescents qui se développent vers le nord, en direction de la plaine (Pieri et Groppi, 1981 ; Bartolini et al., 1982). Leur aggradation a été contrôlée par les cycles glaciaires et interglaciaires du Pléistocène moyen et supérieur qui ont interagi avec les processus tectoniques (Di Dio et Valloni, 1997 ; Valloni et Baio, 2009). A la différence des plus anciennes unités fortement érodées, les cônes alluviaux déposés au cours de la dernière période glaciaire sont encore morphologiquement bien conservés (Cremaschi, 1987 ; Busacca et Cremaschi, 1998). A la transition Pléistocène supérieur/Holocène, l’accroissement de la plupart d’entre eux s’arrête et leur surface est soumise à la pédogenèse pendant les périodes boréale et atlantique, entraînant la formation d’alfisols rubéfiés (sols bruns fersiallitiques ; Cremaschi, 1979). Depuis le début de la période subboréale, la partie distale des cônes alluviaux a été recouverte par des dépôts fins de plaine alluviale qui s’intercalent entre des sols enterrés de type entisol, inceptisol ou vertisol, souvent associés à des sites archéologiques (fig. 1). La cause de ce nouveau processus pédosédimentaire doit se chercher dans la dégradation climatique qui caractérise la fin de la période postglaciaire et le début du Néoglaciaire. Les sols à la surface des cônes alluviaux et ceux intercalés dans les dépôts alluviaux sous-jacents ont été densément occupés par les communautés humaines du Néolithique à l’Age du Bronze ; ils sont les témoins d’un changement majeur de l’exploitation des sols au Subboréal.

Le présent travail s’appuie sur l’analyse des séquences pédo-sédimentaires sur quatre sites étudiés lors de fouilles archéologiques de sauvetage. L’étude de terrain a été associée à l’analyse micromorphologique (fig. 6 et tab. 5) alors que le cadre chronologique a été assuré par des datations au radiocarbone et archéologiques. Les sites étudiés sont 1) Rubiera, 2) Sant’Ilario/Taneto et Cave Spalletti, 3) San Pancrazio et 4) Botteghino di Marano, respectivement liés aux cônes alluviaux des fleuves Secchia, Enza/Parme et Baganza (fig. 2) :

- A Rubiera (Reggio Emilia ; fig. 3 et tab. 1), la séquence sédimentaire étudiée atteint 6 m d’épaisseur et est composée de sédiments fins de plaine alluviale recouvrant des graviers de la fin du Pléistocène/Holocène. De type cône alluvial à la base, ils sont enseveli plus haut par des graviers d’un chenal d’époque romaine (Bermond Montanari et al., 1982 ; Cremaschi, 1987, 1997). Trois sols enterrés sont intercalés dans les sédiments fins de plaine d’inondation à texture fine, chacun renfermant des souches d’arbres conservés en position de vie (Bertolani Marchetti et Forlani, 1972). Les sols ont été datés respectivement de l’Age du Bronze ancien, à la Culture du Vase campaniforme et au premier Chalcolithique. Dans le dernier sol, les souches d’arbre sont entourées de charbons qui attestent le phénomène de défrichement par incendie, du reste observé aussi par lame mince à l’intérieur du sol d’âge campaniforme.

- A Sant’Ilario/Taneto (fig. 4 et tab. 2), les fouilles archéologiques (Bernabò Brea et al., sous presse) ont mis à jour 5,5 m de dépôts fins avec quatre sols enterrés intercalés recouvrant le sommet sableux du cône alluvial daté du Pléistocène supérieur. Le sol supérieur de la séquence (2Ab) est très peu développé et dépourvu de matériel archéologique mais une datation au radiocarbone (Alessio et al., 1981) nous a permis de le dater de la seconde moitié du 3e millénaire avant J.-C. Le second sol enterré (3Ab) est aussi faiblement développé et contient des structures archéologiques qui appartiennent à la culture de la céramique en écailles (Chalcolithique supérieur). En marge de la zone habitée, les fouilles ont exposé des zones irrégulières de terrain brûlé, rougi par le feu et contenant des fragments de charbon de bois. En lame mince, ce niveau archéologique montre les traces abondantes de défrichement associé à des inclusions anthropiques (excréments, cendres, charbons finement triés) qui peuvent être dû à de la fumure. Le troisième sol (5Ab) est tout a fait semblable à celui décrit ci-dessus et contient un site archéologique de la culture de la céramique brossée (Chalcolithique inférieur) associé à des tâches irrégulières de terrain brûlé. Le sol le plus profond (7ABb), légèrement plus évolué du point de vue pédologique, est associé à un site du Néolithique moyen et montre des traces de défrichement. Une séquence similaire est également visible dans la carrière voisine de Spalletti mais les dépôts alluviaux sont moins épais et ils s’appuient sur un sol brun fersiallitique évolué au sommet des graviers du Pléistocène supérieur.

- À San Pancrazio (fig. 5 et tab. 3), la séquence pédosédimentaire est composée d’environ 7,5 m de sables et de limons intercalés entre des sols enterrés à environ 1 m, 3 m, 5 m et 7 m sous la surface actuelle. Le sol supérieur est composé d’un vertisol (2AB) bien développé contenant du matériel archéologique principalement de l’âge du Bronze ancien et de la période chalcolithique. A sa surface, de nombreuses traces de souches d’arbre ont été observées, résultant de la décomposition des arbres laissés en place (non brûlés ou déracinés) et qui sont entourés de très abondants et très petits fragments de poterie broyés. Leur répartition spatiale ainsi que leur état fragmenté et usé ont été interprétés comme une preuve de labourage (Bernabò Brea et al., sous presse). Des traces de fumure ont été identifiées par lame mince dans ce sol.

- Au Botteghino (fig. 5 et tab. 4), la séquence pédosédimentaire est réduite en épaisseur car les graviers du Pléistocène supérieur se trouvent à un peu plus d’un mètre de profondeur au-dessous des dépôts alluviaux fins. Un sol chalcolithique situé à 60 cm de profondeur (3ABb) présente de grandes tâches irrégulières de terrain brûlé résultant de l’arrachage et la combustion d’arbres en place. Au dessous, un sol enterré et évolué (4ABb) sur colluvions, daté du Néolithique moyen, présente des traces de décarbonatation, d’orientation d’argiles et de perturbation anthropique liée au défrichement. Ce sol s’appuie directement sur un paléosol brun fersiallitique très érodé et développé sur les graviers situés à la base de la séquence.

En conclusion, les sols bruns fersiallitiques qui recouvrent les cônes alluviaux attestent une phase de stabilité géomorphologique qui se déroule de la période postglaciaire à l’Atlantique. Les alluvions fines qui les recouvrent témoignent de la reprise de la sédimentation fluviale et les sols alluviaux faiblement développés sont des indicateurs de l’instabilité de la topographie et de la répétition fréquente d’épisodes d’alluvionnement. Les nombreuses dates radiocarbone et archéologiques permettent de dater la reprise de la sédimentation fluviale après la stabilité atlantique, i.e. au début de la période subboréale. Dans le même temps, dans la partie interne de l’Apennin en amont de la région étudiée, on assiste à une phase d’instabilité des versants qui produit de nombreux glissements de terrain (Bertolini et al., 2005; Bertolini, 2007). Les processus sédimentaires et pédologiques étudiés sur la marge de la plaine ne sont donc pas seulement un phénomène local ; ils cadrent bien avec la crise climatique de l’Holocène moyen qui est, au début de la Néoglaciation, enregistrée à l’échelle régionale (Mayewski et al., 2004 ; Magny et al., 2006). La présente étude a également porté sur les changements drastiques de l’exploitation des sols entre l’Atlantique et le Subboréal. Au cours de la période atlantique, l’agriculture itinérante (shifting agriculture) pratiquée par les populations du Néolithique, qui s’est limitée à la plaine, ne touche que marginalement le manteau forestier. Au cours du Subboréal, les populations chalcolithiques qui introduisent le pastoralisme transhumant provoquent une déforestation par incendie à grande échelle non seulement dans la plaine mais aussi à l’intérieur du massif de l’Apennin. Une synergie entre le changement climatique et la gestion des sols doit donc être envisagée dans le façonnement du paysage à la transition Atlantique/Subboréal.

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Table des illustrations

Titre Fig. 1 – Schematic geomorphological map of the central Po Plain Fig. 1 – Carte géomorphologique schématique de la partie centrale de la plaine du Pô
Légende A: Middle Pleistocene terraces. B: LGM frontal moraines of the Garda Lakeand Rivoli systems; C: Main level of the Lombardy plain. D: Pede-Appennine alluvial fans. E: Finely textured alluvial plain. F: Depressed areas in the alluvial plain. G: Main palaeochannels. H: Fluvial ridges. I: A-A’ geological cross section (from Pieri and Groppi, 1981). Q: Quaternary deposits; Plm, Pls: upper and middle Pliocene deposits; M1, M2, M3: early, middle and upper Miocene deposits. Contour lines indicate elevation in m a.s.l.A : Terrasses du Pléistocène moyen. B : Moraines datant de la dernière période glaciaire du système morainique du lac de Garda. C : Niveau principal de la plaine de Lombardie. D : Cônes alluviaux de la marge des Appennins. E : Plaine alluviale à sédiments fins. F : Zones basses de la plaine alluviale. G : Principaux paléochenaux. H : Bourrelets alluviaux. I : Section géologique A-A’. Q : sédiments quaternaires ; Plm, Pls : Pliocène supérieur et moyen ; M1, M2, M3 : Miocène inférieur, moyen et supérieur. Les courbes de niveau indiquent l’altitude en mètres au-dessus du niveau de la mer.
Crédits From Cremaschi, 1987; Castiglioni, 1997; Castiglioni and Pellegrini, 2001.D’après Cremaschi, 1987; Castiglioni, 1997; Castiglioni et Pellegrini, 2001.
URL http://geomorphologie.revues.org/docannexe/image/9810/img-1.png
Fichier image/png, 818k
Titre Fig. 2 – Schematic geomorphological map of the investigated area Fig. 2 – Carte géomorphologique schématique de la région étudiée
Légende A: Middle Pleistocene dissected alluvial fans. B: Upper Pleistocene gravelly alluvial fans with “Bruns lessivés” soils and vertisols at the surface. C: Upper Pleistocene alluvial fans covered by alluvial deposits. D: Fine-textured alluvial plain. E: Main towns and villages. F: Main scarps. G: Fluvial ridges. H: Main palaeochannels. I: Depressed areas in the alluvial plain. Location of the studied sequences. 1: Rubiera, Secchia River and Corradini gravel pit; 2: Sant’Ilario/Taneto; 3: Cave Spalletti; 4: San Pancrazio; 5: Botteghino. Contour lines indicate elevation in m a.s.l.A : Terraces alluviales du Pléistocène Moyen ; B : Cônes alluviaux du Pléistocène supérieur composés principalement de graviers et affectés en surface de Sols Bruns Fersiallitiques ; C : Cônes alluviaux du Pléistocène supérieur recouverts par des alluvions ; D : Plaine alluviale à sédiments fins ; E : Villes et villages principaux ; F : Principaux escarpements ; G : Bourrelets alluviaux ; H : Principaux paléochenaux ; I : Zones basses de la plaine alluviale. Localisation des séquences étudiées. 1 : Rubiera, fleuve Secchia et carrière Corradini ; 2 : Sant’Ilario/Taneto ; 3 : Carrière Spalletti ; 4 : San Pancrazio ; 5 : Botteghino. Les courbes de niveau indiquent l’altitude en mètres au-dessus du niveau de la mer.
URL http://geomorphologie.revues.org/docannexe/image/9810/img-2.png
Fichier image/png, 1,5M
Titre Fig. 3 – The Rubiera, Secchia riverbed and Corradini gravel quarry stratigraphic sequence Fig. 3 – Coupe stratigraphique de Rubiera dans le lit du fleuve Secchia et dans la carrière Corradini
Légende 1: embankment; 2: Roman age gravel; 3: fluvial deposits with tree trunks; 4: fine-grained alluvial deposits with intercalated buried soils and alluvial fan gravel at the base of the sequence; 5: archaeological sites (BK: Bell Beaker site; CHA: Chalcolithic site). Solid triangles indicate spots of burned soil; 6: standing tree stumps; 7: tree trunks.1 : digue ; 2 : graviers d’âge romain ; 3 : sédiments fluviatiles contenant des troncs d’arbre ; 4 : dépôts alluviaux à sols intercalés et graviers de cône alluvial à la base ; 5 : sites archéologiques (BK : Culture du Vase campaniforme ; CHA : site chalcolithique). Le triangle noir indique une zone de terrain brûlé ; 6 : souches d’arbre en position de vie ; 7 : troncs d’arbre.
URL http://geomorphologie.revues.org/docannexe/image/9810/img-3.png
Fichier image/png, 277k
Titre Fig. 4 – Sant’Ilario/Taneto and Cave Spalletti stratigraphic sequencesFig. 4 – Coupe stratigraphique de Sant’Ilario/Taneto et de la carrière Spalletti
Légende 1: gasteropod shells; 2: laminated sand; 3: iron mottles. IR: iron age pit fill; CHA: Chalcolithic sites; NEO: Neolithic sites. Solid triangles indicate spots of burned soil. Key for grain size: Cl: clay; Si: silt; Sa: sand; G: gravel.1 : fragments de gastéropodes ; 2 : sables laminés ; 3 : précipitation localisée de fer ferrique. IR : remplissage d’une structure archéologique de l’Age du Fer ; CHA : sites chalcolithiques ; NEO : site néolithique. Le triangle noir indique une zone de terrain brûlé. Légende pour la granulométrie : C : argiles ; Si : limons ; Sa : sables ; G : graviers.
URL http://geomorphologie.revues.org/docannexe/image/9810/img-4.png
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Titre Fig. 5 – San Pancrazio and Botteghino stratigraphic sequencesFig. 5 – Les coupes stratigraphiques de San Pancrazio et Botteghino
Légende 1: gastropod shells; 2: iron mottles; RO: Roman age finds; IR: Iron Age finds; EBA: Early Bronze Age finds; CHA: Chalcolithic Age finds; NEO: Neolithic Age site. Solid triangles indicate spots of burned soil. Key for grain size: Cl: clay; Si: silt; Sa: Sand; G: gravel.1 : fragments de gastéropodes ; 2 : précipitation localisée de fer ferrique. RO : matériel archéologique de l’Antiquité romaine ; IR : matériel archéologique de l’Age du Fer ; EBA : matériel archéologique de l’Age du Bronze ancien ; CHA : matériel archéologique du Chalcolithique ; NEO : sites néolithiques. Les triangles noirs indiquent la présence de zones de terrain brûlé. Légende pour la granulométrie : Cl : argiles ; Si : limons ; Sa : sables ; G : graviers.
URL http://geomorphologie.revues.org/docannexe/image/9810/img-5.png
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Titre Fig. 6 – Micromorphology photosFig. 6 – Photographies de micromorphologie
Légende A: Rubiera, horizon 2ABb (Bell-Beaker buried soil). Finely comminuted charcoal and charred vegetal fragments (OIL, 100x). B: Sant’Ilario/Taneto, horizon 3Ab (Chalcolithic buried soil). Wood ash fragment retaining the morphology of the original vegetal tissue (XPL, cross polariser, 100x). C: Sant’Ilario/Taneto, horizon 7ABb (mid Neolithic buried soil). Dusty clay coating related to topsoil disturbance (PPL, parallel polariser, 100x). D: San Pancrazio, horizon 2Ab (early Bronze Age Vertisol). Phosphatic aggregate, possibly associated with manuring practices (PPL, 200x). E: Botteghino, horizon 3ABb (Chalcolithic soil adjacent to a burned tree-hollow). Reworked Bt horizon fragment, stripped from theunderlying Sol Brun Fersiallitique (5Btb) during the uprooting process (XPL, 20x). F: Botteghino, uprooted tree pit fill. Limpid clay coatings inside a burned soil aggregate (optically isotropic; XPL, 20x).A : Rubiera, horizon 2ABb (sol campaniforme). Charbons fins et matière végétale partiellement brûlée (OIL, 100x). B : Sant’Ilario/Taneto, horizon 3Ab (paléosol chalcolithique). Fragment de cendre de bois (LP, 100x). C : Sant’Ilario/Taneto, horizon 7ABb (sol du Néolithique moyen). Revêtement d’argiles poussiéreuses liées à la perturbation de la surface du sol (LN, 100x). D : San Pancrazio, horizon 2Ab (vertisol de l’Age du Bronze inférieur). Agrégat phosphaté peut-être associé au compostage (PPL, 200x). E : Botteghino, horizon 3ABb (sol chalcolithique à proximité d’une cavité d’essartage). Fragment d’horizon Bt remanié, dérivé d’un sol brun fersiallitique (5Btb) sous-jacent (LP, 20x). F : Botteghino, remplissage d’une cavité d’essartage. Revêtement d’argile limpide au sein d’un fragment de sol brûlé (LP, 20x).
URL http://geomorphologie.revues.org/docannexe/image/9810/img-6.png
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Titre Tab. 5 – Micromorphological description of thin sectionsTab. 5 – Description micromorphologique des lames minces
Légende Abundance of fabric units expressed according to G. Stoops (2003): * Very rare (<5%); ** rare (5-15%); *** common (15-30%); **** frequent (30-50%). 1 Site: Rub: Rubiera; Sit: Sant’Ilario/Taneto; SP: San Pancrazio; Bot: Botteghino. 2 MS (microstructure): ch: channel; v: vughy; sb: subangular blocky; ab: angular blocky. 3 GM (groundmass): (a) b-fabric: cs: cross-striated; ps: porostriated; gs: granostriated; cr: crystallitic; ss: stipple-speckled; (b) related distribution pattern: ssp: single-spaced porphyric; op: open porphyric; b/cp: basic microstructure/closed porphyric (reworked aggregates in inwashed clays); dsp: double spaced porphyric. 4 Biogenic, geogenic, anthropogenic components: (c) fine (<100 μm) charcoal and charred vegetal fragments; (d) coarse (>100 μm) charcoal; (e) burned soil fragments; (f) dung; (g) phytoliths; (h) earthworm granules; (i) shell fragments. 5 Pedofeatures: (j) compound layered silt, silty clay and/or dusty clay coatings and infillings; (k) dusty clay intercalations; (l) dusty clay coatings and infillings; (m) fragmented/reworked textural pedofeatures; (n) limpid clay coatings; (o) Fe and Fe/Mn nodules (orthic and anorthic).Abondance des unités de fabrique exprimée selon G. Stoops (2003) : * très rare (<5 %) ; ** rare (5-15 %) ; *** abondant (15-30 %) ; **** fréquent (30-50 %). 1 Sites : Rub : Rubiera ; Sit : Sant’Ilario/Taneto ; SP : San Pancrazio ; Bot : Botteghino. 2 MS (microstructure) : ch : à chenaux ; v : cavitaire ; sb : en blocs subangulaires ; ab : polyédrique angulaire. 3 GM (structure du sol) : (a) b-fabric : cs : à stries entrecroisées ; ps : porostrié ; gs : granostrié ; cr : crystallitique ; ss : en tâches isolées ; (b) distribution relative : ssp : porphyrique à espacement simple ; op : porphyrique serrée ; b/cp : microstructure basique/porphyrique serrée (agrégats remaniés dans une masse argileuse) ; dsp : porphyrique à espacement double. 4 Composants biogéniques, géogéniques et anthropogéniques : (c) charbons fins et matière végétale fine brûlée partiellement (<100 μm) ; (d) charbons grossiers (>100 μm) ; (e) agrégats de terrain brûlé ; (f) excréments ; (g) phytolithes ; (h) biosphérolithes de lombric ; (i) fragments de coquille. 5 Traits pédologiques : (j) revêtement et remplissage lité complexe par des limons, des limons argileux ou des argiles poussiéreuses ; (k) intercalations d’argiles poussiéreuses ; (l) revêtement et remplissage par des argiles poussiéreuses ; (m) traits pédologiques texturaux fragmentés et/ou remaniés ; (n) revêtement d’argile limpide ; (o) nodules de Fe et de Fe/Mn (orthiques et anorthiques).
URL http://geomorphologie.revues.org/docannexe/image/9810/img-7.png
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Titre Fig. 7 – Available radiocarbon datings of buried soils of the Emilia alluvial plain (from Alessio et al., 1981; Regione Emilia-Romagna Geological Survey, www.Regione.Emilia-Romagna.it/geologia; Cremaschi, 1997) compared with those from Apennine landslides (from Bertolini et al., 2005; Bertolini, 2007)Fig. 7 Les datations au radiocarbone des sols enterrés de la plaine alluviale (d’après Alessio et al., 1980; Regione Emilia-Romagna Geological Survey, www.Regione.Emilia-Romagna.it/geologia; Cremaschi, 1997) comparées à celles des glissements de terrain de l’Apennin septentrionale (d’après Bertolini et al., 2005; Bertolini, 2007)
URL http://geomorphologie.revues.org/docannexe/image/9810/img-8.png
Fichier image/png, 213k
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Mauro Cremaschi et Cristiano Nicosia, « Sub-Boreal aggradation along the Apennine margin of the Central Po Plain: geomorphological and geoarchaeological aspects », Géomorphologie : relief, processus, environnement, vol. 18 - n° 2 | 2012, 155-174.

Référence électronique

Mauro Cremaschi et Cristiano Nicosia, « Sub-Boreal aggradation along the Apennine margin of the Central Po Plain: geomorphological and geoarchaeological aspects », Géomorphologie : relief, processus, environnement [En ligne], vol. 18 - n° 2 | 2012, mis en ligne le 02 novembre 2014, consulté le 27 juin 2016. URL : http://geomorphologie.revues.org/9810 ; DOI : 10.4000/geomorphologie.9810

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Auteurs

Mauro Cremaschi

Department of Earth Sciences “A. Desio” – University of Milano – Via Mangiagalli 34 – 20133 Milano – Italy (mauro.cremaschi@unimi.it)

Cristiano Nicosia

Department of Earth Sciences “A. Desio” – University of Milano – Via Mangiagalli 34 – 20133 Milano – Italy (cristiano.nicosia@unimi.it)

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