Abstract
The Mid-Pleistocene Transition (MPT) is characterised by cooling and lengthening glacial cycles from 600–1200 ka, thought to be driven by reductions in glacial CO2 in particular from ~900 ka onwards. Reduced high latitude upwelling, a process that retains CO2 within the deep ocean over glacials, could have aided drawdown but has so far not been constrained in either hemisphere over the MPT. Here, we find that reduced nutrient upwelling in the Bering Sea, and North Pacific Intermediate Water expansion, coincided with the MPT and became more persistent at ~900 ka. We propose reduced upwelling was controlled by expanding sea ice and North Pacific Intermediate Water formation, which may have been enhanced by closure of the Bering Strait. The regional extent of North Pacific Intermediate Water across the subarctic northwest Pacific would have contributed to lower atmospheric CO2 and global cooling during the MPT.
Highlights
The Mid-Pleistocene Transition (MPT) is characterised by cooling and lengthening glacial cycles from 600–1200 ka, thought to be driven by reductions in glacial CO2 in particular from ~900 ka onwards
To sequester CO2 in the deep ocean likely requires stifled nutrient and CO2-rich deep water upwelling at high latitudes to reduce outgassing, as proposed for the last glacial;[13,14] recent records of Southern Ocean biogenic opal suggest glacial reductions in productivity occurred throughout the Pleistocene[15], there are so far no direct records of high latitude upwelling from either hemisphere with which to evaluate this potentially important mechanism for the MPT glacial CO2 reduction
Our study focuses on the time period ~900 ka (MIS 21–29), which occurs within the MPT when sea levels fell, glacials began to lengthen, and productivity across the Bering Sea and North Pacific fell (Fig. 2a–c; Fig. 3)
Summary
The Mid-Pleistocene Transition (MPT) is characterised by cooling and lengthening glacial cycles from 600–1200 ka, thought to be driven by reductions in glacial CO2 in particular from ~900 ka onwards. A secular decline in atmospheric CO2 concentrations has been suggested as a central cause[5,6,9,10,11], and a slowdown in Atlantic meridional overturning circulation and North Atlantic Deep Water (NADW) production has led to the idea that there was enhanced CO2 transfer from the surface ocean to the deep at ~900 ka[12]. These processes are not mutually exclusive, and could each be associated with a build-up of ice on North America for which there is ample evidence[4]. Bering Sea, is a high-nutrient, low-chlorophyll (HNLC) region limited by iron, and is in this respect similar to the subantarctic
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