Soil-stratigraphy of the Middle Pleistocene sedimentary units in the Elefante, Galeria and Gran Dolina Cave entrance deposits (Sierra de Atapuerca, Spain)

Abstract

In the cave entrance deposits of the Trinchera del Ferrocarril (Sierra de Atapuerca) there are sediments derived from soils and palesoils. Paleosol studies provide observations to characterize paleoclimates, rates of sedimentation or fix hierarchies on stratigraphic discontinuities, among others. In this contribution, we present the description and classification of soil-horizons through field and microscopic features in order to propose a preliminary soil stratigraphy of the Middle Pleistocene lithostratigraphical units of the Elefante, Galeria and Gran Dolina sites. The description of soil-horizon forming processes is based on the association of relict features and are distinguished from disturbed and inherited (resedimented) pedological features. The indirect evidences available about paleoclimates and climate change in the Middle Pleistocene record of the Trinchera del Ferrocarril are mainly based on the abundant biostratigraphic record. This biostratigraphic record illustrates the dominance of temperate habitats according to the stable ecological composition of the mammalian record. The climatic-related significance and temporal span of the soil-forming processes can be added to the evidences devoted to interpret the Paleoecology in the Sierra de Atapuerca project. Trinchera del Ferrocarril soil-horizons of Middle Pleistocene age can be grouped according to the dominant nature and number of phases in the soil forming processes recognized. Sediments derived from upslope soils recorded in the lithostratigraphic units can be characterised by their mineral composition and colour as yellow calcitic, red yellowish carbonated and brown carbonaceous. The calcitic and carbonated sediments shows monophased pedological features of carbonatation and criogenic soil forming processes. Also, carbonated sediments may contain polyphased carbonatation and cryptocristalline enrichments and depletions. Carbonaceous sediments contain monophased carbonatation and iron depletion. During the Middle Pleistocene, the cave entrances of the Trinchera del Ferrocarril are located in the karstified middle slope of the Sierra de Atapuerca. The cave entrance depositional environment is a product of short episodes of sedimentation, when sediment gravity flows and gravitational collapses on high slope surfaces occur, separated by large episodes of no deposition or erosion. This depositional environment, similar to colluvial or debris flow dominated alluvial fan sedimentary model, point out to the habitat stability suggested by biostratigraphic assemblages. The habitat stability in the cave entrance deposits Trinchera del Ferrocarril is likely related to sedimentary dominance of the redyellowish carbonated sediments (terra rossa). However, petrographic and micromorphological observations allowed us to distinguish different kinds of soil horizons in order to systematize a local paleoclimatic evolution in the cave entrance setting. The ability to trace chronostratigraphic correlations between cave entrance soil stratigraphies helps us to suggest a regional biome for the paleoclimatic interpretation. Also, the temporal scale of the depositional recurrence may be considered in the chronological frame of the marine isotopic stages and other paleoclimatic events (stadials and interstadials, Bond cycles, etc.).