Mid-Holocene buried paleo-Gypsisol in the semiarid Ebro Basin (NE Spain)

PÉREZ-LAMBÁN, F.; POCH, R.M.; BADIA, D.; PICAZO, J. V.; PEÑA MONNÉ, J.L.; SAMPIETRO VATTUONE, M.M.

México

Conferencia; International Conference on Soil micromorphology; 2016

UNAM

Paleosols formed under past environmental conditions are an important proxy for ancient landscape reconstruction. Central Ebro Basin (NE Spain) is a semiarid region with a Miocene sedimentary infill build up by easily erodible rocks (from bottom to top: lutites, gypsum, calcilutites and limestone). As a consequence of these two factors, it is a region very sensitive to erosive processes, especially when enhanced by anthropogenic intervention in the landscape. Therefore, there have been major landform transformations and soil destruction during the Holocene. As a consequence, prehistoric contexts in this region are difficult to find, and landscape reconstruction is challenging. Up to date, the only well-known Mesolithic primary archaeological context in the region is Cabezo de la Cruz hut-camp (6 km NW from the buried paleosol in this study). Depicting Mesolithic and Early Neolithic landscape and its subsequent evolution is one of the main objectives of the geoarchaeological project that frames this study. Buried paleosols are considered a capital source of environmental data for this scope.There are cumulative geomorphic contexts that favour the preservation of buried soils: relicts of slope deposits and flat bottom valley infills. Beneath a 4 m alluvial infilling in La Poza gully (Zaragoza), there is a buried paleosol developed on a former slope accumulation. The paleosol dates back to the 8th-9th millennia cal. BP. The aim of this study is to provide a clear environmental framework in which this paleosol could have been formed and evolved. In this way, the results of the study can bring some light to the precise environmental conditions of the Mid Holocene in the Iberian Peninsula and a landscape context for the Mesolithic and Early Neolithic human communities in the Ebro Valley.A 1,5 m thick six-horizon soil sequum has been described for this buried paleosol: A ? ABwk ? Bwy ? By ? BCy? Cy ? (R). The paleosol itself has been classified as a Calcic Gypsisol, according to the WBR (2014). The A horizon is characterized by a relatively high organic matter content and a consequent darker colour. The ABwk and underlying horizons shows weathered queras (residues of elongated infillings of calcite surrounded by calcite-depleted hypocoatings) and fragments of weathered compound nodules of calcite crystals that could be biogenic. In more depth secondary gypsum (infillings of lenticular gypsum in channels) is found. The C horizon is formed almost exclusively by weathered gypsum from the underlying Miocene gypsum rocks (R). The micromorphologic study of six thin sections shows relevant information of the pedogenic processes and environmental conditions. There is evident biogenic microstructure, which highlights the palaeosol nature of the horizons under study. Queras (biogenic calcite surrounded by a decarbonated hypocoating) are severely weathered: biosparite is almost completely micritized. Also, there are evidences of the presence of herbivores: in some horizons, calcium carbonate spherulites have been recorded. In the whole profile, and in particularin the Cy horizon the coexistence of large gypsum crystals with recrystalization evidences and microcrystalline gypsum suggest a succession of several gypsum genesis generations in different climatic conditions.