A 5 million year comparison of Mg/Ca and alkenone paleothermometers

Abstract

Geochemical sea surface temperature (SST) proxies such as the magnesium to calcium ratio (Mg/Ca) in foraminifera and the alkenone unsaturation index (U K′ 37 ) are becoming widely used in pre‐Pleistocene climate records. This study quantitatively compares previously published Mg/Ca and U K′ 37 data from Ocean Drilling Program (ODP) site 847 in the eastern equatorial Pacific to assess the utility of these proxies to reconstruct tropical SST over the last 5 Ma. Foraminiferal Mg/Ca–SST calibrations that include a dissolution correction are most appropriate at this location because they provide SST estimates for the youngest sample that are close to modern mean annual SST. The long‐term trends in the two records are remarkably similar and confirm a ∼3.5°C cooling trend from the early Pliocene warm period to the late Pleistocene noted in previous work. Absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate SST from Mg/Ca are taken into account. Comparing the two SST records at ODP site 847 to other records in the region shows that the eastern equatorial Pacific was 2–4°C warmer during the early Pliocene compared to today.