SUSTAINED IMBALANCE IN THE CALCIUM CYCLE AND CLIMATE-OCEAN CHEMISTRY INTERACTIONS OVER THE EOCENE-OLIGOCENE TRANSITION FROM THE GEOCHEMISTRY OF LARGE BENTHIC FORAMINIFERA

The Eocene-Oligocene Transition (EOT) arguably represents the most dramatic permanent shift in Earth’s climate during the Cenozoic, marking the onset of an Antarctic ice sheet similar in size to modern. The transition is associated with a large drop in the carbonate compensation depth (CCD) and rapid weathering of shallow carbonate platforms as a result of a ~100 m sea level fall. Here, we explore whether these global changes resulted in a concommitant shift in the seawater calcium concentration ([Ca_sw]). Such reconstructions not only have the potential to inform us of changes in the processes governing the calcium cycle over the EOT, but may also have implications for the accuracy of temperature and ice volume reconstructions based on trace element systems if seawater chemistry shifted. We simultaneously reconstruct temperature and seawater chemistry based on two novel proxies: coupled Mg/Ca-clumped isotopes from shallow-dwelling large benthic foraminifera from the Tanzania Drilling Project, and the Na/Ca ratio of benthic foraminifera from DSDP Site 522 and IODP Site U1333. These data demonstrate a globally-coherent signal of calcium decrease over the transition, implying that calcium burial out-paced supply despite the weathering of carbonate platforms. A decrease in [Ca_sw] would also serve to further draw-down CO₂ across the transition, requiring a [CO₃^2-] increase to maintain an equivalent or deeper CCD. Furthermore, our clumped-isotope temperatures highlight a substantially larger magnitude of sea surface temperature cooling compared to published Mg/Ca records. The discrepancy possibly results from a bias in the trace element data due to the Mg/Ca_sw shift, indicating that ice volume and temperature reconstructions based on Mg/Ca may be systematically biased.