Nutrient availability in the North Pacific region not primarily driven by climate through the Quaternary

The subarctic North Pacific Ocean is a relatively understudied region in terms of palaeoclimate, limiting our understanding of how the region has both driven and responded to palaeoenvironmental events. Today, the subarctic North Pacific Ocean is marked by a year round stratified water column with a halocline at c. 300 m water depth. Previous studies at ODP Site 882 in the Northwest Pacific have suggested the stratified water column system developed at the onset of major Northern Hemisphere Glaciation (2.73 Ma). In addition to limiting the upwelling of carbon-rich deep waters and associated ventilation of CO2 to the atmosphere, the shift to a stratified state fundamentally altered oceanographic conditions and biogeochemical cycling across the region. Key questions remain over whether the region was permanently stratified for all of the Quaternary, or whether the changes in stratification/biogeochemical cycling altered over major climatic transitions such as the Mid-Pleistocene Transition (MPT), a process that would alter regional ocean-atmospheric carbon exchanges. We present new silicon and oxygen isotope data from diatoms (δ30Sidiatom and δ18Odiatom), alongside previously published data in order to test the mechanisms of biogeochemical cycling in the subarctic North Pacific Ocean between 2.85 Ma and 0.06 Ma, including influences from the wider region such as Glacial North Pacific Intermediate Water (GNPIW) originating in the Bering Sea. This has enabled us to reconstruct temporal changes in photic zone nutrient utilisation and silicic acid supply in the northwest subarctic Pacific Ocean through the progressive intensification of glacial-interglacial cycles through the Quaternary and over the MPT. We show that prior to the MPT climate does not appear to be a primary controller of nutrient availability in the North Pacific region, but that following the MPT, it has a greater influence, shown by the interrelationship with the upwelling index from the Bering Sea.