NOAA/WDS Paleoclimatology - West Pacific Warm Pool Fossil Planktic Foraminifer Trace Metal and Oxygen Isotope Data during the Pliocene

A controversial aspect of Pliocene (5.3-2.6 Ma) climate is whether El Niño-like (El Padre) conditions, characterized by a reduced trans-equatorial sea surface temperature (SST) gradient, prevailed across the Pacific. Evidence for El Padre is chiefly based on reconstructions of sea-surface conditions derived from the oxygen isotope (d18O) and Mg/Ca compositions of shells belonging to the planktic foraminifer Trilobatus sacculifer. However, fossil shells of this species are a mixture of multiple carbonate phases - pre-gametogenic, gametogenic (reproductive), and diagenetic calcites - that formed under different physiological and/or environmental conditions and are averaged in conventional whole-shell analyses. Through in situ measurements of micrometer-scale domains within Pliocene-aged shells of T. sacculifer from Ocean Drilling Program Site 806 in the western equatorial Pacific, we show that the d18O of gametogenic calcite is 0.6-0.8‰ higher than pre-gametogenic calcite, while the Mg/Ca ratios of these two phases are the same. Both the whole-shell and pre-gametogenic Mg/Ca records indicate that average early Pliocene SSTs were ~1°C warmer than modern, with present-day SSTs being established during the latest Pliocene and early Pleistocene (~3.0-2.0 Ma). The measurement of multiple calcite phases by whole-shell d18O analyses masks a late Pliocene to earliest Pleistocene (3.6-2.2 Ma) decrease in seawater d18O (d18Osw) values reconstructed from in situ pre-gametogenic d18O and Mg/Ca measurements. Our novel d18Osw record indicates that sea-surface salinities in the west Pacific warm pool were higher than modern prior to ~3.5 Ma, which is consistent with more arid conditions under an El Padre state.