Reduced upwelling of nutrient and carbon-rich water in the subarctic Pacific during the Mid-Pleistocene Transition

Abstract

Reduction in atmospheric pCO2 has been hypothesised as a causal mechanism for the Mid-Pleistocene Transition (MPT), which saw global cooling and increased duration of glacials between 0.6 and 1.2 Ma. Sea ice-modulated high latitude upwelling and ocean-atmospheric CO2 flux is considered a potential mechanism for pCO2 decline, although there are no long-term nutrient upwelling records from high latitude regions to test this hypothesis. Using nitrogen isotopes and opal mass accumulation rates from 0 to 1.2 Ma, we calculate a continuous high resolution nutrient upwelling index for the Bering Sea and assess possible changes to regional CO2 fluxes and to the relative control of sea ice, sea level and glacial North Pacific Intermediate Water (GNPIW) on deep mixing and nutrient upwelling in the region. We find nutrient upwelling in the Bering Sea correlates with global ice volume and air temperature throughout the study interval. From ~1 Ma, and particularly during the 900 ka event, suppressed nutrient upwelling would have lowered oceanic fluxes of CO2 to the atmosphere supporting a reduction in global pCO2 during the MPT. This timing is consistent with a pronounced increase in sea ice during the early Pleistocene and restriction of flow through the Bering Strait during glacials after ~900 ka, both of which would have acted to suppress upwelling. We suggest that sea-level modulated GNPIW expansion during glacials after 900 ka was the dominant control on subarctic Pacific upwelling strength during the mid-late Pleistocene, while sea ice variability played a secondary role.