Coupled Mg/Ca and Clumped Isotope Measurements Indicate Lack of Substantial Mixed Layer Cooling in the Western Pacific Warm Pool During the Last ∼5 Million Years

Abstract

The Indo-Pacific Warm Pool (IPWP) plays a crucial role in influencing climate dynamics both in the tropics and globally. Yet, there is an ongoing controversy concerning the evolution of surface temperatures in the IPWP since the Pliocene, which is fueled by contradictory proxy evidence. Temperature reconstructions using TEX86 indicate a gradual cooling by ∼2°C from the Pliocene to today while Mg/Ca-based studies using planktonic foraminifera do not report any long-term trends. A bias in Mg/Ca records due to seawater chemistry changes has been suggested as an explanation for this proxy mismatch. Here, we present data from two independent foraminifera-based temperature proxies, Mg/Ca and clumped isotopes (Δ47), measured on the same samples from IODP Site U1488 in the IPWP. We reconstructed mixed layer and subsurface temperatures and find very good agreement among Mg/Ca and Δ47 when applying a minor correction for changing Mg/Ca ratios of seawater. Diagenetic effects could influence Δ47 but the evaluation of foraminifera preservation at Site U1488 suggests that this effect is unlikely to have masked a long-term trend in the data. While remaining uncertainties prevent us from fully ruling out particular hypotheses, our study adds evidence that mixed layer temperatures likely did not cool substantially, while subsurface temperatures cooled more strongly since the Pliocene. The substantial Pleistocene cooling previously observed in TEX86 data is consistent with this finding when interpreting it as a combined surface and subsurface signal