Abstract
High latitude deep water upwelling has the potential to control global climate over glacial timescales through the biological pump and ocean-atmosphere CO2 exchange. However, there is currently a lack of continuous long nutrient upwelling records with which to assess this mechanism. Here we present geochemical proxy records for nutrient upwelling and glacial North Pacific Intermediate Water (GNPIW) formation in the Bering Sea over the past 850 kyr, which demonstrates that glacial periods were characterised by reduced nutrient upwelling, when global atmospheric CO2 and temperature were also lowered. We suggest that glacial expansion of sea ice in the Bering Sea, and the simultaneous expansion of low nutrient GNPIW, inhibited vertical mixing and nutrient supply across the subarctic Pacific Ocean. Our findings lend support to the suggestion that high latitude sea ice and the resultant intermediate water formation, modulated deep water upwelling and ocean-atmosphere CO2 exchange on glacial-interglacial timescales.
Highlights
Recent studies have shown a sharp increase in both surface ocean subarctic Pacific productivity and CO2 ventilation during the last deglaciation (Galbraith et al, 2007; Gray et al, 2018), and suggested that the equivalent of ~30 ppm atmospheric CO2 was released into the atmosphere due to increased overturning (Rae et al, 2014)
Δδ18O849 -U1343 values are significantly higher in glacial periods than interglacial periods, with values gradually increasing as glacial periods developed and became more intense, suggesting an increasing influence of glacial North Pacific Intermediate Water (GNPIW) in the Bering Sea slope region
During deglacial periods GNPIW collapses, and the Δδ18O849-U1343 becomes more negative during interglacials, as δ18OU1343 is more similar to the deep water record from the eastern equatorial Pacific Site 849, suggesting North Pacific Deep Water (NPDW) influence at the 2 km water depth at Bering Sea slope Site U1343
Summary
Recent studies have shown a sharp increase in both surface ocean subarctic Pacific productivity and CO2 ventilation during the last deglaciation (Galbraith et al, 2007; Gray et al, 2018), and suggested that the equivalent of ~30 ppm atmospheric CO2 was released into the atmosphere due to increased overturning (Rae et al, 2014). This was proposed to coincide with a collapse of glacial North Pacific Intermediate Water (GNPIW), allowing overturning to bring nutrient and CO2-rich deep water to the surface (Gray et al, 2018). Surface waters in the Bering Sea are fed by the subarctic Pacific Alaskan Stream, which enters through passes and straits in the Aleutian arc and circulates northward around the basin, forming the shelf-adjacent Bering Slope Current (BSC) (Stabeno et al, 1999) (Fig. 1)
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