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

Abstract

Upwelling deep waters in the Southern Ocean release biologically sequestered carbon into the atmosphere, contributing to the relatively high atmospheric CO2 levels during interglacial climate periods. Paleoceanographic evidence suggests this “CO2 leak” was lessened during the last glacial maximum (LGM), potentially due to increased stratification, weaker and equatorward‐shifted winds, and/or enhanced biological carbon export. The collective influences of these mechanisms on the ocean's biological pump efficiency and amount of atmospheric CO2 can be quantified by determining preformed phosphate of deep waters. We quantify preformed PO4 (Ppre,AOU) and preformed δ13normalC ${\delta }^{13}\mathrm{C}$ (δ13Cnormalpnormalrnormale,normalAnormalOnormalU ${\delta }^{13}{\mathrm{C}}_{\mathrm{p}\mathrm{r}\mathrm{e},\mathrm{A}\mathrm{O}\mathrm{U}}$) of LGM bottom waters using a compilation of published paleo‐temperature, nutrient and oxygen estimates from benthic foraminifera. Our results show that preformed phosphate of the Pacific and Indian deep oceans was reduced by about −0.53 ± 0.13 μM and suggest that much (64 ± 28 ppmv) of the Glacial‐Interglacial CO2 drawdown resulted from changes in the ocean's biological pump efficiency. Once carbonate compensation is accounted for, this can explain the entire CO2 drawdown (87 ± 40 ppmv). Preformed δ13normalC ${\delta }^{13}\mathrm{C}$ shows similar results. The reconstructed LGM Ppre,AOU and oxygen are qualitatively consistent with the changes produced by a suite of numerical sensitivity experiments that roughly simulate three proposed mechanisms for an increase in LGM biological pump efficiency: an increase in biological activity, a decrease in wind‐driven upwelling, and an increase in stratification in the Southern Ocean.

Ocean Oxygen, Preformed Nutrients, and the Cause of the Lower Carbon Dioxide Concentration in the Atmosphere of the Last Glacial Maximum

“…Combining the findings of Vollmer et al. (2022) with atmospheric CO 2 δ 13 C reconstructions (e.g., Eggleston et al., 2016) suggests that the Holocene and LGM had similar differences between the ocean's preformed δ 13 C of DIC and atmospheric CO 2 δ 13 C. It is too early to say which set of the mechanisms is favored by this finding. In a scenario of air‐sea exchange limitation for the concentration or δ 13 C of CO 2 that still allowed for equilibration of oxygen (Figures 1c), Vollmer et al.…”

“…In a scenario of air‐sea exchange limitation for the concentration or δ 13 C of CO 2 that still allowed for equilibration of oxygen (Figures 1c), Vollmer et al. (2022) should have reconstructed a low preformed DIC δ 13 C, in conflict with their actual calculations. However, if air‐sea exchange of oxygen was also far from equilibrium during the LGM (Cliff et al., 2021), then the approach of Vollmer et al.…”

“…In contrast, if Subantarctic iron fertilization and/or a reduction in Antarctic surface/subsurface exchange were responsible for reduced Southern Ocean CO 2 leakage during the ice ages, then the preformed δ 13 C of DIC sourced from the Southern Ocean surface should have more closely approached the air-sea equilibrium value or could have been higher than it, likely making it higher than the Holocene value (Figure 1b). Combining the findings of Vollmer et al (2022) with atmospheric CO 2 δ 13 C reconstructions (e.g., Eggleston et al, 2016) suggests that the Holocene and LGM had similar differences between the ocean's preformed δ 13 C of DIC and atmospheric CO 2 δ 13 C. It is too early to say which set of the mechanisms is favored by this finding. In a scenario of air-sea exchange limitation for the concentration or δ 13 C of CO 2 that still allowed for equilibration of oxygen (Figures 1c), Vollmer et al (2022) should have reconstructed a low preformed DIC δ 13 C, in conflict with their actual calculations.…”

“…Combining the findings of Vollmer et al (2022) with atmospheric CO 2 δ 13 C reconstructions (e.g., Eggleston et al, 2016) suggests that the Holocene and LGM had similar differences between the ocean's preformed δ 13 C of DIC and atmospheric CO 2 δ 13 C. It is too early to say which set of the mechanisms is favored by this finding. In a scenario of air-sea exchange limitation for the concentration or δ 13 C of CO 2 that still allowed for equilibration of oxygen (Figures 1c), Vollmer et al (2022) should have reconstructed a low preformed DIC δ 13 C, in conflict with their actual calculations. However, if air-sea exchange of oxygen was also far from equilibrium during the LGM (Cliff et al, 2021), then the approach of Vollmer et al (2022) would have overestimated the preformed DIC δ 13 C (Figure 1d), complicating the interpretation.…”

“…So they could be used to test whether the findings of Vollmer et al (2022) are more consistent with ice age mechanisms for lowering atmospheric CO 2 that place the strongest CO 2 gradient between the surface ocean and the underlying ocean interior (Figure 1b) or between the surface ocean and the atmosphere (Figures 1c and 1d). Vollmer et al (2022) have brought paleoceanographic data on the concentrations of oxygen and nutrients in the ocean interior into a quantitative framework for estimating the role of the global ocean's biological pump in the decline of atmospheric CO 2 during the LGM, and they find that it can explain essentially all of it. Their findings should encourage researchers to continue with this approach, with carbon isotopes and other proxies, to determine the specific mechanisms that strengthened the ice age ocean's biological pump.…”