Deepwater Expansion and Enhanced Remineralization in the Eastern Equatorial Pacific During the Last Glacial Maximum

Abstract

Published estimates of the radiocarbon content of middepth waters suggest a decrease in ventilation in multiple locations during the last glacial maximum (LGM; 24.0–18.1 ka). Reduced glacial ventilation would have allowed respired carbon to accumulate in those waters. A subsequent deglacial release of this respired carbon reservoir to the atmosphere could then account for the observed increases in atmospheric CO2 and decline in atmospheric radiocarbon content. However, age model error and a release of 14C‐depleted mantle carbon have also been cited as possible explanations for the observed middepth radiocarbon depletions, calling into question the deep ocean's role in storing respired carbon during the LGM. Joint measurements of benthic foraminiferal carbon isotope values (δ13C) and cadmium/calcium (Cd/Ca) ratios provide a method for isolating the air‐sea component of a water mass from changes in remineralization. Here we use benthic foraminiferal δ13C and Cd/Ca records from the eastern equatorial Pacific to constrain changes in remineralization and water‐mass mixing over the last glacial‐interglacial transition. These records are complemented with elemental measurements of the authigenic coatings of foraminifera to monitor postdepositional changes in bottom water properties. Our results suggest an increase of deep waters at midwater depths consistent with a shoaling of the boundary between the upper and lower branches of Southern Ocean overturning circulation. Additionally, our records demonstrate increased organic matter remineralization in middepth waters during the LGM, suggesting that respired carbon did accumulate in middepth waters under periods of reduced ventilation.

Proxy‐Based Preformed Phosphate Estimates Point to Increased Biological Pump Efficiency as Primary Cause of Last Glacial Maximum CO2 Drawdown

“…Data is available through Boyle (1992), Boyle et al (1995, Gottschalk, Vázquez Riveiros et al (2016), Hoogakker et al (2015, Marchitto et al (2002), Marchitto and Broecker (2006), Martin et al (2005), McKay et al (2016, Mojtahid et al (2017), Roberts et al (2016), Rosenthal et al (1997), Schmiedl and Mackensen (2006), Skinner et al (2003), and Umling et al (2018).…”

Evaluating the Biological Pump Efficiency of the Last Glacial Maximum Ocean using δ<sup>13</sup>C

“…2). The global mean efficiency of the biological pump ̅̅̅̅̅̅̅̅ decreases from 38 % in the PI simulation to 33 % in the LGM simulation, as opposed to reconstructions which infer an increased regenerated signature in the interior ocean (Jaccard et al, 2009;Umling et al, 2018;Freeman et al, 2016). The simulated increase in preformed phosphate (Fig.…”

Changes in productivity and intermediate circulation in the northern Indian Ocean since the last deglaciation: new insights from benthic foraminiferal Cd / Ca records and benthic assemblage analyses

“…CC BY 4.0 License. Umling et al, 2018;Valley et al, 2017). Unfortunately, the resolution of both intermediate Cd w and benthic δ 13 C from core MD77-176 (northeastern BoB) are very low for the HS1 and YD events, making it difficult to extract reliable information.…”

“…Pacific and Atlantic Oceans (benthic Cd w , Valley et al, 2017;Umling et al, 2018) at intermediate water depths.…”