Controls on Sr/Ca, S/Ca, and Mg/Ca in Benthic Foraminifera: Implications for the Carbonate Chemistry of the Pacific Ocean Over the Last 350 ky

Abstract

Boron to calcium (B/Ca) records in benthic foraminifera, used for reconstructing the carbonate ion saturation state (ΔCO₃) of the deep ocean, suggest that carbon sequestration in the Southern Pacific contributed to lowering atmospheric CO₂ during the last glacial interval. However, the spatial and temporal extent of this storage is debated due to limited ΔCO₃ records. To increase available ΔCO₃ records, we explored using strontium and sulfur to calcium (Sr/Ca, S/Ca) in Planulina wuellerstorfi as additional proxies for ΔCO₃ based on comparison with paired B/Ca down-core records from Pacific Sites U1486 (1,332 m depth) and U1487 (874 m depth) cored during the International Ocean Discovery Program Expedition 363. The Sr/Ca and S/Ca records from P. wuellerstorfi closely covary with the B/Ca-derived ΔCO₃ records. Temperature, reconstructed using Uvigerina peregrina magnesium to calcium (Mg/Ca), has no discernible effect on Sr/Ca, whereas S/Ca also varies with Mg/Ca in both U. peregrina and P. wuellerstorfi, suggesting an additional temperature effect. Mg/Ca records from P. wuellerstorfi are affected by both temperature and ΔCO₃. We assess calibrations of Sr/Ca to ΔCO₃ for the Atlantic, Pacific, and Indian Oceans and recommend using the down-core rather than core-top calibrations as they yield consistent sensitivity, though with offsets, in all ocean basins. Reconstructing Pacific ΔCO₃ records from sites U1486, U1487, and DSDP 593, we demonstrate the benefit of using Sr/Ca as an additional ΔCO₃ proxy to assess the contribution of the Southern Pacific to the increase of atmospheric CO₂ at glacial terminations.