Paleoproductivity in the northwestern Pacific Ocean during the Pliocene‐Pleistocene climate transition (3.0–1.8 Ma)

Abstract

Alkenone mass accumulation rates (MARs) provide a proxy for export productivity in the northwestern Pacific (Ocean Drilling Program Site 1208) spanning the late Pliocene through early Pleistocene (3.0-1.8 Ma). We investigate changes in productivity associated with global cooling during the onset and expansion of Northern Hemisphere glaciation (NHG). Alkenone MARs vary on obliquity time-scales throughout, but the amplitude increases at 2.75 Ma concurrent with the intensification of NHG and cooling of the sea surface by 3 °C. The obliquity-scale variations in alkenone MARs parallel shipboard measurements of sediment color reflectance (%) with higher MARs significantly correlated (>95%) with darker (opal-rich) intervals. Variations in both lead benthic foraminiferal δ18O values by 1.5-2 kyr, suggesting that export productivity may be a contributing factor, rather than a response, to the extent of continental glaciation. The biological pump is therefore a plausible mechanism for transferring atmospheric CO2 into the deep ocean during the onset of NHG and the ensuing obliquity dominated climate regime. Obliquity-scale correlation between productivity and magnetic susceptibility is consistent with a link via westerly winds delivering terrigenous sediments and mixing the upper water column. Alkenone MARs also contain a ~400 kyr modulation. Because this periodicity is a multiple of the residence time of carbon in the ocean, it may reflect inputs of new nutrients associated with eccentricity forced changes in the terrestrial biosphere and weathering. We ascribe these findings to interactions between the East Asian winter monsoon and productivity in the North Pacific Ocean, perhaps contributing to Plio-Pleistocene climate change.