Closure of the Bering Strait caused Mid-Pleistocene Transition cooling

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.

Benthic foraminifera indicate Glacial North Pacific Intermediate Water and reduced primary productivity over Bowers Ridge, Bering Sea, since the Mid-Brunhes Transition

“…Of our most dominant species, C. laevigata, Uvigerina peregrina, and species from the genera Bulimina, Bolivina, Stainforthia and Globobulimina are known denitrifiers able to survive in anoxic water in the modern ocean by respiring nitrate (Piña-Ochoa et al, 2010). We interpret the assemblages described here as generally tolerant of low oxygen, even though the majority of (Knudson and Ravelo, 2015) against the global benthic δ 18 O composite record (Lisiecki and Raymo, 2005). (b) Site U1342 magnetic susceptibility record (Expedition 323 Scientists, 2011).…”

“…The most likely reason is the presence of a different, less corrosive glacial water mass over Bowers Ridge, which has been identified as GNPIW from offsets in oxygen and carbon isotopes at U1342 (Knudson and Ravelo, 2015) and other sites (Cook et al, 2016;Kender et al, 2018;Worne et al, 2019), and neodymium isotope changes within the Bering Sea (Horikawa et al, 2010). Positive benthic δ 13 C and depleted benthic δ 18 O during glacials at U1342 were interpreted as the export of surface waters to depth via sea ice brine rejection and intermediate water formation (Knudson and Ravelo, 2015). Surface waters were likely depleted in 12 C, CO 2 , and DIC compared with aged deeper NPDW due to primary productivity, which could have led to an increased carbonate ion concentration and better preservation of CaCO 3 .…”

“…The age-depth tie points used to plot our data against age are derived from detailed benthic foraminiferal oxygen isotope (δ 18 O) analyses carried out on Uvigerina peregrina in the same samples by Knudson and Ravelo (2015). The isotope stratigraphy of U1342 has a correlation coefficient of 0.64 with the global reference stack LR04 (Lisiecki and Raymo, 2005) after a three-point running mean was applied.…”

“…Studies have shown that sea ice was present in the Bering Sea through at least the last 2.5 Myr, became more prevalent during the mid-Pleistocene (Stroynowski et al, 2015(Stroynowski et al, , 2017Detlef et al, 2018), and likely resulted in GNPIW expansion (Horikawa et al, 2010;Knudson and Ravelo, 2015;Kender et al, 2018;Worne et al, 2019) and a reduced OMZ. However, high-resolution Bering Sea records of organic carbon flux and OMZ presence have not yet been produced over the MBT.…”

Data constraints on ocean-carbon cycle feedbacks at the mid-Pleistocene transition

“…Yet all three pathways differ in their timing, and these scenarios are not necessarily exhaustive. For example, Kender et al (2018) argue for enhanced stratification in the bering Sea after 950 kyr bP (Fig. 1, white triangle), which may have also amplified oceanic CO 2 drawdown.…”