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Sediment sequence and site formation processes at Cova de l’Arbreda, NE Iberia - Results of a high-resolution geochemical, granulometrical and micromorphological study.

Martin Kehl1, Eileen Eckmeier2, Sven Oliver Franz3, Joaquim Soler4, Narcis Soler4, Klaus Reicherter5, Gerd-Christian Weniger6
1 Institute of Geography, University of Cologne, Otto-Fischer-Str. 4, 50674 Cologne
2 INRES-Soil Science, University of Bonn, Nussallee 13, 53115 Bonn
3 Steinmann Institute for Geology, Mineralogy and Palaeontology, University of Bonn, Nussallee 8, 53115 Bonn
4 Institut de Recerca Historica, Universitat de Girona, Plaça Ferrater i Mora 1, 17071 Girona, Spain
5 Institute of Neotectonics & Natural Hazards, RWTH Aachen University, Lochnerstraße 4–20, 52056 Aachen, Germany
6 Neanderthal Museum, Talstr. 300, 40822 Mettmann

V 7.4 in Freie Themen

19.09.2012, 09:30-09:50, H8

Cova de l’Arbreda provides a detailed archaeological record of the Middle to Upper Palaeolithic and is a key site for studying cultural transitions in NE Iberia. The archaeological finds are embedded in a sequence of silty to clayey sediments, more than 6 m thick, including layers rich in travertine rock fragments. The mode of sediment accumulation and the types and degrees of post-depositional alteration have not yet been investigated in detail. A profound knowledge of these processes is a prerequisite for evaluating the stratigraphic integrity of the sequence and possible taphonomic changes. Recently, new evidence on climatic dry spells in NE Iberia during the Last Glacial has been derived from geochemical signatures of limnic sediments of Lake Banyoles (Höbig et al., in press) located about 4 km south of L’Arbreda cave. The L’Arbreda sequence may also hold considerable information on short- and long-term climate changes. These may be reflected in changes of Aeolian input and weathering degrees or in the presence of unconformities testifying breaks in sediment accumulation (hiatuses).

We conducted a high-resolution sedimentological and geochemical study on the sediment sequence in order to identify possible breaks in the stratigraphic record and to elucidate sediment accumulation and alteration processes. Samples were taken at the north profile (square E0) from a 20 cm wide sediment column at 2.5 cm equidistant sampling intervals. All measurements were conducted on the air-dried fine-earth fraction (< 2 mm in diameter). In addition, thin sections, 6 cm x 8 cm large, were prepared for selected parts of the sequence. Reflectance in the visible spectrum and color values (L*a*b*) were measured with a Konica Minolta CM-5 spectrophotometer. Total and inorganic carbon contents were determined on a subset of samples using elemental analysis (Vario EL of Elementar) and the gas volumetric method, respectively. Organic carbon contents were calculated by substracting inorganic from total carbon. A screening for volumetric magnetic susceptibility was accomplished by using a Bartington field spectrometer equipped with the MS2F-sensor. Major and trace elements were analyzed with X-ray fluorescence (Axios 3kW, PANanalytical GmbH) on powdered press pills. The granulometric composition was measured using a Beckman coulter laser diffractometer (LS 13320) for 116 grain size fractions covering the range from  <0.04 µm to 2000 µm in diameter and applying Mie theory for data processing.

 

As based on field and laboratory data, six major sediment units can be distinguished from top to the bottom. Unit A correlates with the “Terra Rossa” of Soler & Maroto (1987) and consists of reddish brown silty clay loam with many travertine rock fragments. Its sedimentological and geochemical properties are distinctly different from those of the underlying unit B1.1 (Figure 1). Units B1.1 and B1.2 contain several upper palaeolithic layers from Postsolutrian down to Evolved Aurignacian (archaeological levels A to G). Archaeological finds are locally concentrated in thin layers which often show peak values in magnetic susceptibility, probably owing to the presence of superparamagnetic minerals produced by fire, and which commonly correspond to peaks in luminance (L*), or darker color. Units B1.1 and B1.2 consist of light colored sandy silts with many travertine and few sandstone fragments. The fine sediment is moderately rich in CaO mainly derived from primary carbonates. These in combination with comparatively high Na2O/Al2O3 and Sr/Ba ratios suggest a lower weathering degree than in unit A. Unit B1.2 contains more P2O5 and heavy metal contents of Zn and Cu are increased in comparison to unit B1.1. Maximum concentrations of Cu, Pb and Ni are reached in the dark sandy silts of unit B1.3 which is rich in organic matter reflected by lower L* values. This unit disconformably rests on the yellowish sandy silts to silty sands of unit B2.1. Here, color values strongly change due to patches and bands of secondary phosphate and redoximorphic mottling. Besides strong enrichments with P2O5, a strong increase in Sr as well as concomitant rises of K2O and Zr are recorded, whereas the heavy metals Cu, Zn and Ni, and the magnetic susceptibility values have minimum values. Finally, unit B2.2 is slightly darker colored but still patchy and frequent concretions of Mn are found in the lower part which is overlying the bedrock. The heavy metal contents increase and Mn reaches a maximum (data not shown in Figure 1).

Grain size distributions are polymodal suggesting infiltration of fine sediment by hillwash rather than Aeolian additions of well sorted materials. Evidence for significant changes in granulometric and geochemical composition within layers B1.1 and B1.2 suggest a further lithological subdivision of these layers. Beside the major discontinuity in sedimentation observed in between Mousterian layers J and I, microscopic evidence of lamination at the contact between levels I and H was observed. This probably points to discontinuous sedimentation during the change from Middle Palaeolithic to Archaic Aurignacian techno-complexes. However, this break is not clearly reflected in granulometrical or geochemical data. Distinct geochemical and granulometric fingerprints are found for unit A. These reflect higher rates of weathering during the Holocene than during formation of the Upper Pleistocene layers, when fines show lower degrees of weathering. Units B1.2 and B1.3 are characterized by anthropogenic and/or zoogenic inputs of organic matter and phosphate. The major phosphate peak is, however, found in unit B2.1 where phosphate nodules and infillings indicate diagenetic transformation. The degree of weathering of the lower two units is difficult to estimate, since weathering indicators such as the Na2O/Al2O3 or the Ba/Zr ratios yield low values whereas the K2O/Rb and Sr/Ba ratios show opposite trends.

 

Overall, the analytical data derived from this multiproxy-study refines the macroscopic subdivision into major sediment units presented before (Soler & Maroto 1987) and adds to our understanding of the key sequence. Similar investigations may prove useful at other cave sites in the Iberian Peninsula.

 

 

References

Höbig, N., Weber, M.E., Kehl, M., Weniger, G.-C., Julià, R., Melles, M., Fülöp, R.-H., Vogel, H., Reicherter, K. (in press): Lake Banyoles (northeastern Spain): A last glacial to Holocene multi-proxy study with regard to environmental variability and human occupation. - Quaternary International, doi:10.1016/j.quaint.2012.05.036.

Soler, N., Maroto, J. (1987): L' estratigrafia de la cova de l'Arbreda (Serinyà, Girona). – Cypsela, VI: 53-66.

The north profile of square EO at Cova de l’Arbreda with selected results of sedimentological and geochemical analyses
The north profile of square EO at Cova de l’Arbreda with selected results of sedimentological and geochemical analyses



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Letzte Änderung 16.07.2012