Reconstructing Pliocene West Pacific Warm Pool Hydroclimate Using In Situ Microanalyses on Fossil Planktic Foraminifer Shells. Issue 7 (26th June 2020) - Wycech, J.
Abstract: 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 (δ 18 O) 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 δ 18 O 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 δ 18 O analyses masks a late Pliocene to earliest Pleistocene (3.6–2.2 Ma) decrease in seawater δ 18 O (δ 18 Osw ) values reconstructed from in situ pre‐gametogenic δ 18 O and Mg/Ca measurements. Our novel δ 18 Osw 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. Plain Language Summary: The Pliocene (5.3–2.6 million years ago) is the most recent time in Earth history when atmospheric carbon dioxide concentrations may have been modern‐like and the continents were in their current configuration. Thus, Pliocene climate reconstructions are considered a useful indicator of future conditions. Several Pliocene reconstructions from the equatorial Pacific argue for a sea‐surface temperature distribution that resembles modern El Niño events. Relevant to this study, modern El Niño events also produce drier conditions in the western equatorial Pacific. We investigate Pliocene permanent El Niño, or El Padre, in the west Pacific by using paired oxygen isotope and Mg/Ca measurements of marine microfossils (foraminifera) to reconstruct the oxygen isotope composition of surface water (δ 18 Osw ), which tracks sea‐surface salinity. In situ analyses were used to measure tiny spots within individual shells of Trilobatus sacculifer to target only domains formed near the sea surface. These measurements indicate that the early Pliocene west Pacific was drier than today, which is similar to modern El Niño events and consistent with El Padre. The in situ‐based δ 18 Osw trend toward modern sea‐surface salinities is not reconstructed by whole‐shell measurements primarily due to the life cycle of T. sacculifer, which produces large oxygen isotope variability within individual shells. Key Points: The western equatorial Pacific was more saline than modern during the early Pliocene (5–3.6 Ma), consistent with an El Padre state In situ analyses of foraminifers improve the accuracy of salinity (δ 18 Osw ) reconstruction Trends in the in situ δ 18 Osw record are not apparent in the whole‐shell record due to dissolution of primary foraminifer calcite