Eastern Equatorial Pacific productivity and related-CO2 changes since the last glacial period

Understanding oceanic processes, both physical and biological, that control atmospheric CO2 is vital for predicting their influence during the past and into the future. The Eastern Equatorial Pacific (EEP) is thought to have exerted a strong control over glacial/interglacial CO2 variations through its link to circulation and nutrient-related changes in the Southern Ocean, the primary region of the world oceans where CO2-enriched deep water is upwelled to the surface ocean and comes into contact with the atmosphere. Here we present a multiproxy record of surface ocean productivity, dust inputs, and thermocline conditions for the EEP over the last 40,000 y. This allows us to detect changes in phytoplankton productivity and composition associated with increases in equatorial upwelling intensity and influence of Si-rich waters of sub-Antarctic origin. Our evidence indicates that diatoms outcompeted coccolithophores at times when the influence of Si-rich Southern Ocean intermediate waters was greatest. This shift from calcareous to noncalcareous phytoplankton would cause a lowering in atmospheric CO2 through a reduced carbonate pump, as hypothesized by the Silicic Acid Leakage Hypothesis. However, this change does not seem to have been crucial in controlling atmospheric CO2, as it took place during the deglaciation, when atmospheric CO2 concentrations had already started to rise. Instead, the concomitant intensification of Antarctic upwelling brought large quantities of deep CO2-rich waters to the ocean surface. This process very likely dominated any biologically mediated CO2 sequestration and probably accounts for most of the deglacial rise in atmospheric CO2.