Oyster shells, bulk carbonate sediment, and meteoric calcite cement as recorders of oceanic and radiogenic 87Sr/86Sr in mixed heterozoan carbonates and terrigenous sediment

Abstract

ABSTRACT Well-preserved oyster shells from mixed heterozoan carbonates and terrigenous sediments of Miocene age in the La Chanata area (southern Spain) show varying luminescence and covarying δ18O and δ13C compositions > –2‰ VPDB. Temperature and salinity modeling based on the δ18O give temperatures ranging from 13 to 33 °C (average 22.4 °C) and normal-marine salinities (34–37 PSU). The isotopic compositions likely reflect environmental conditions during shell secretion; moreover, their identical intra-shell 87Sr/86Sr compositions (equal to or less than the analytical precision of 20 × 10–6) support a primary origin. Additional information from δ18O and 87Sr/86Sr compositions of Heterostegina foraminifera and sediment and cement samples show that: a) bulk sediment and cement samples that have significantly lower δ18O and δ13C values but 87Sr/86Sr values similar to the oyster shells likely derived their isotopic compositions from intraformational dissolution of high-Mg calcite and aragonite allochems in meteoric groundwaters, b) cement samples that have the lowest recorded δ18O and δ13C compositions (< –6.5‰ VPDB) and highest 87Sr/86Sr ratios (> 0.709198) record meteoric diagenesis where the strontium was derived mainly from weathering of the uplifted Alpujárride bedrock, whereas c) bulk sediment and Heterostegina foraminifera with intermediate 87Sr/86Sr values likely received their strontium from weathering of the surrounding bedrock as well as dissolution of intraformational carbonates and possibly overlying Messinian marls (now eroded). Oysters with intermediate 87Sr/86Sr ratios likely secreted their shell parts during transient salinity drops (down to 16 PSU) when rivers and submarine groundwater discharge supplied radiogenic strontium to the nearshore environment. Numerical age determination of sediment samples and oyster shells that have similar (within the analytical precision) 87Sr/86Sr ratios indicate a late Tortonian age (8.1 Ma) of the La Chanata deposits.