Seawater carbonate chemistry regulates ocean-atmosphere carbon exchange and stabilizes Earth’s climate, yet its evolution remains challenging to reconstruct. In this talk, we present the stable strontium isotope compositions (δ^88/86Sr) of multiple benthic foraminiferal taxa from core-top samples across multiple ocean basins with diverse bottom-water carbonate chemistries. The δ^88/86Sr values for two of these taxa, the epifaunal C. wuellerstorfi and surficial C. pachyderma, robustly correlate with the degree of seawater carbonate saturation (Δ[CO₃^2-]). Given that Δ[CO₃^2-] exerts a first-order control on calcification rate, these results are consistent with a precipitation rate-dependent kinetic isotope fractionation. Building from these δ^88/86Sr observations, we reconstructed bottom-water [CO₃^2-] over the past 20,000 years by applying the C. pachyderma calibration to a tropical Atlantic core. The record closely matches a complementary B/Ca-based reconstruction. We propose that benthic foraminiferal δ^88/86Sr values provide a robust, independent proxy for reconstructing past seawater carbonate chemistry.

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