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 Geochemistry, Geophysics, Geosystems

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.

底栖有孔虫（benthic foraminifera）中的硼钙比（B/Ca）记录可用于重建深海碳酸根离子饱和状态（ΔCO3），相关研究显示，末次冰期时段南太平洋的碳固存（carbon sequestration）过程有助于降低大气CO₂浓度。然而，由于ΔCO3记录较为有限，该碳固存过程的空间与时间分布范围仍存在争议。
为扩充可用的ΔCO3记录数据集，本研究基于太平洋U1486（水深1332米）与U1487（水深874米）站点（由国际大洋发现计划（International Ocean Discovery Program，简称IODP）第363航次钻探获取）的成对硼钙比岩芯垂向记录（down-core records），探索采用武氏平旋虫（Planulina wuellerstorfi）中的锶钙比（Sr/Ca）与硫钙比（S/Ca）作为ΔCO3的额外代用指标。
武氏平旋虫的锶钙比与硫钙比记录，与基于硼钙比重建的ΔCO3记录高度协变。采用优格陵虫（Uvigerina peregrina）的镁钙比（Mg/Ca）重建的海水温度，对锶钙比无显著影响；而硫钙比在优格陵虫与武氏平旋虫中均随镁钙比变化，提示存在额外的温度效应。武氏平旋虫的镁钙比记录同时受温度与ΔCO3的影响。
本研究评估了大西洋、太平洋与印度洋中锶钙比与ΔCO3的校准关系，并建议采用岩芯垂向校准而非表层岩芯校准（core-top calibrations）——尽管不同大洋盆地均存在偏移，但前者可得到一致的灵敏度。
通过对U1486、U1487与DSDP 593站点的太平洋ΔCO3记录进行重建，我们验证了将锶钙比作为ΔCO3额外代用指标的优势，可用于评估南太平洋对冰消期（glacial terminations）大气CO₂升高的贡献。