Paleoenvironmental evolution of the Pliocene Villarroya Lake, northern Spain, from stable isotopes and trace-element geochemistry of ostracods and molluscs

Abstract

Stable isotopes and trace-element content of calcite ostracod valves and aragonite mollusc shells from the Pliocene lacustrine succession of Villarroya allow depicting the geochemical record of environmental changes and to compare our data to the paleoenvironmental reconstruction obtained from other proxies. The lower sequences (A and B) are characterized by relatively high isotopic and Me/Ca values in the biogenic carbonates. The recorded large variations of δ18O in these carbonates mainly reflect variations in the δ18Ow due to precipitation–evaporation processes and, to a lesser extent, variations in temperature of calcification. The δ13C data inform about changes in DIC although they are probably biased by the vital effects of the studied taxa. Minor and trace element contents in ostracod (Mg/Ca, Sr/Ca) and mollusc shells (Sr/Ca) are mainly linked to the Me/Ca of the lake water (Me/Caw), and to a lesser extent to temperature and to uptake kinetic effects. Several possible mechanisms may make the Me/Caw to vary: long periods of Ca-carbonate (calcite, aragonite) deposition after charophyte development, and different inputs for Ca and Mg to the lake due to changes in drainage area configurations through time, including the changes in saline inputs (Na-Cl type) to the lake. The stable isotopes and the calculated Sr/Caw and Mg/Caw from sequence C display lower values than those from sequences A and B. The isotopic values from biogenic carbonates of unit C indicate isotopically diluted waters in a hydrologically open lacustrine environment. Distinct δ13C and δ18O plots for molluscs from unit C reflect the different biotopes and metabolism type. For several intervals of the Villarroya succession there is no direct relationships among: (i) salinity changes inferred from invertebrate paleoecology, (ii) paleoenvironmental slices based on isotopic signatures and Me/Caw calculations (from biogenic carbonate geochemistry) and (iii) climate in the hinterland deduced from pollen data. Lakes where geochemical behaviour is constrained by sporadic saline inputs and/or relative depletion in Ca due to long periods of Ca-carbonate precipitation or biomineralization, like the Pliocene Villarroya lake was, do not show clear correlation patterns between geochemical signals and climate proxies. In these lakes only major environmental trends display unambiguous geochemical signatures, and only some main shifts in the geochemical signature profiles may be correlated with significant global and/or regional environmental changes that have been reported from other paleoenvironmental records. Electronic supplementary material The online version of this article (doi:10.1007/s10933-007-9121-2) contains supplementary material, which is available to authorized users.