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

Abstract

Reduction in atmosphericpCO2has 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-modulatedhigh latitude upwelling and ocean-atmospheric CO2flux is considered a potential mechanism forpCO2decline,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 highresolution nutrient upwelling index for the Bering Sea and assess possible changes to regional CO2fluxes and tothe relative control of sea ice, sea level and glacial North Pacific Intermediate Water (GNPIW) on deep mixingand nutrient upwelling in the region. Wefind nutrient upwelling in the Bering Sea correlates with global icevolume and air temperature throughout the study interval. From ~1 Ma, and particularly during the 900 kaevent, suppressed nutrient upwelling would have lowered oceanicfluxes of CO2to the atmosphere supporting areduction in globalpCO2during the MPT. This timing is consistent with a pronounced increase in sea ice duringthe early Pleistocene and restriction offlow through the Bering Strait during glacials after ~900 ka, both ofwhich would have acted to suppress upwelling. We suggest that sea-level modulated GNPIW expansion duringglacials after 900 ka was the dominant control on subarctic Pacific upwelling strength during the mid-latePleistocene, while sea ice variability played a secondary role.