Controls on Sr/Ca in benthic foraminifera and implications for seawater Sr/Ca during the late Pleistocene

Abstract

Changes in the Sr to Ca ratio of sea water have important implications for the interpretation of past climate. It has proven difficult to interpret Sr/Ca of foraminiferal calcite as a measure of seawater Sr/Ca or as reflecting the influence of deep water carbonate ion saturation (Δ[CO32−]) on the incorporation of Sr into benthic foraminiferal carbonate. Here, we address this issue by measurements of paired benthic foraminiferal Sr/Ca and B/Ca (a proxy for deep water Δ[CO32−]) for core-tops from the global ocean and three down cores at different settings during the Last Glacial–interglacial cycle. These new data suggest a significant control of deep water Δ[CO32−] on benthic foraminiferal Sr/Ca, and that down-core shell Sr/Ca variations can be largely accounted for by past deep water Δ[CO32−] changes. We conclude that seawater Sr/Ca has likely remained near-constant on glacial–interglacial timescales during the late Pleistocene, in agreement with model results. With due caution, benthic Sr/Ca may be used as an auxiliary proxy for deep water Δ[CO32−] if seawater Sr/Ca is constant.