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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. Plain Language Summary: Burning of fossil fuels has increased since the industrial revolution, resulting in the addition of CO2, a gas which retains heat, to the atmosphere. Understanding how the earth's oceans work to regulate levels of atmospheric CO2 is key to understanding the effects of increased atmospheric CO2 on the earth's climate. The carbonate system in the ocean, of which CO2 is a component, is complex. Carbonate saturation state (ΔCO3) is a measure of how close the carbonate ion concentration is to the level at which calcite (which makes up the shell or test of some single‐cell organisms) will dissolve. It is related to the level of ocean storage of CO2. Boron to calcium ratios (B/Ca) found in test calcite have been used as a proxy for past ocean ΔCO3, but these records are limited. Here, to increase presently available ΔCO3 proxy records, we explore using strontium and sulfur to calcium (Sr/Ca, S/Ca) as additional proxies for ΔCO3. Unlike S/Ca, Sr/Ca shows no temperature influence, so makes a useable proxy for change in ΔCO3. Sr/Ca records can be used to infer the progressive release of oceanic CO2 to the atmospheric at three terminations, periods in the past when atmospheric CO2 increased dramatically. Key Points: Sr/Ca ratios in P. wuellerstorfi show no detectable influence of temperature and are useful for reconstructing changes in ΔCO3S/Ca ratios in P. wuellerstorfi may potentially be used to reconstruct a long‐term (e.g., Cenozoic) record of sulfate in the oceanSr/Ca derived ΔCO3 records suggest CO2 release from upper water and then from progressively deeper water during the last three terminations [ABSTRACT FROM AUTHOR]