Palaeoenvironmental reconstruction of a Pleistocene lacustrine sequence from faunal assemblages and ostracode shell geochemistry, Baza Basin, SE Spain

Abstract

We have reconstructed the Early Pleistocene palaeoenvironmental record for a shallow lacustrine sequence from the Baza Basin (Southern Spain), using faunal assemblages and the shell chemistry of two ostracode species ( Candona sp. and Cyprideis torosa). In the NE basin sector (Orce area), a Lower Pleistocene lacustrine sequence, up to 10 m thick, contains alternating phases of two fossil assemblages which differ in their salinity requirements. The faunal assemblages record phases of (1) slightly saline, Ca+bicarbonate-rich water, when freshwater organisms predominated, and (2) saline, NaCl-dominant water in which marine-like organisms lived. The ostracode shells from intervals with a saline fauna have higher δ 18O values than those from intervals with a freshwater fauna. This feature corresponds to a parallel alternation of positive and negative hydrologic balances. δ 18O and δ 13C values from ostracode calcite display a covariant trend which indicates that the ostracodes lived in a closed lacustrine system. The isotopic data show a better agreement with the inferences from faunal assemblages than with trace element trends. The trace element data, although locally fitting well with the isotope and faunal interpretations, suggest overall that trace element contents in ostracodes are not consistent indicators of salinity and/or temperature variations, but depend on a set of complex factors, which may not be easily deciphered. The alternation of concentration/dilution phases recorded in the studied section can be correlated with climatic cycles described from synchronous ocean basin records from the Late Matuyama chron.