Deep‐Sea Oxygen Depletion and Ocean Carbon Sequestration During the Last Ice Age

Abstract

Enhanced ocean carbon storage during the Pleistocene ice ages lowered atmospheric CO2 concentrations by 80 to 100 ppm relative to interglacial levels. Leading hypotheses to explain this phenomenon invoke a greater efficiency of the ocean's biological pump, in which case carbon storage in the deep sea would have been accompanied by a corresponding reduction in dissolved oxygen. We exploit the sensitivity of organic matter preservation in marine sediments to bottom water oxygen concentration to constrain the level of dissolved oxygen in the deep central equatorial Pacific Ocean during the last glacial period (18,000–28,000 years BP) to have been within the range of 20–50 μmol/kg, much less than the modern value of ~168 μmol/kg. We further demonstrate that reduced oxygen levels characterized the water column below a depth of ~1,000 m. Converting the ice age oxygen level to an equivalent concentration of respiratory CO2, and extrapolating globally, we estimate that deep‐sea CO2 storage during the last ice age exceeded modern values by as much as 850 Pg C, sufficient to balance the loss of carbon from the atmosphere (~200 Pg C) and from the terrestrial biosphere (~300–600 Pg C). In addition, recognizing the enhanced preservation of organic matter in ice age sediments of the deep Pacific Ocean helps reconcile previously unexplained inconsistencies among different geochemical and micropaleontological proxy records used to assess past changes in biological productivity of the ocean.

230Th Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

“…Over 50 years' (1966–2019) worth of data have been compiled to create the global thorium database ( n = 1,167) presented here (Adkins et al, ; Anderson et al, , Anderson et al, , Anderson et al, , Anderson et al, ; Bausch, ; Böhm et al, ; Bohrmann, ; Borole, ; Bradtmiller et al, , , ; Broecker, ; Broecker et al, ; Brunelle et al, , ; Causse & Hillaire‐Marcel, ; Chase et al, , ; Chong et al, ; Costa, McManus, & Anderson, ; Costa & McManus, ; Crusius et al, ; Dekov, ; Denis et al, ; Dezileau et al, , ; Durand et al, ; Fagel et al, ; Francois et al, , Francois et al, ; Frank, Eisenhauer, Bonn, et al, , Frank, Eisenhauer, Kubik, et al, , ; Fukuda et al, ; Galbraith et al, ; Geibert et al, ; Gherardi et al, , ; Gottschalk et al, ; Hickey, ; Hillaire‐Marcel et al, ; Hoffmann et al, , ; Jaccard et al, , ; Jacobel et al, ; Jonkers et al, ; Kienast et al, ; Ku & Broecker, ; Kumar et al, ; Lam et al, ; Lamy et al, ; Lao et al, ; Lippold et al, , Lippold et al, , Lippold et al, , Lippold et al, ; Loubere et al, ; Loveley et al, ; Lund et al, ; Mangini & Dominik, ; Marcantonio et al, , Marcantonio et al, ...…”

Rapid Loss of CO2 From the South Pacific Ocean During the Last Glacial Termination

“…2− ] was lower by 10, 16, and 19 μmol/kg, respectively, and these Holocene and LGM means are significantly different at 89% (1.2 km) and >99% confidence levels (1.6 and 2.5 km; two-tailed Student's t test). Given that glacial surface ocean [CO 3 2− ] was likely elevated~50 μmol/kg relative to the Holocene (Barker & Elderfield, 2002;Lea et al, 1999), the surface-tointerior ocean differences in this region must have been 60-70 µmol/ kg greater during the LGM than during the Holocene, consistent with the idea of higher biologic pump efficiency during the glacial period (Allen et al, 2015;Anderson et al, 2019;Hertzberg et al, 2016;Sikes, Elmore, et al, 2016).…”

“…Although the overall trend in the relationship between δ 13 C and [CO 3 2− ] during the LGM is consistent with greater storage of respiratory CO 2 in deep water, sufficient variability exists within the records to allow for imprints from other processes, such as air‐sea gas exchange. Additional reconstructions for dissolved nutrients (Yu et al, ) and/or oxygen (Anderson et al, ) are needed to quantify specific carbon contributions from carbonate dissolution, organic matter respiration, and air‐sea gas exchange.…”

“…The Pacific Ocean fills the world's largest ocean basin and acts as a major reservoir of carbon that can be readily exchanged with the atmosphere (Key et al, ). Many marine geochemical paleorecords indicate that during the Last Glacial Period, the Pacific Ocean held more carbon than it does today, accounting for a substantial portion of glacial atmospheric CO 2 drawdown (Anderson et al, ; Basak et al, ; Bradtmiller et al, ; Hoogakker et al, ; Ronge et al, ; Sikes et al, ; Skinner et al, ). However, the amounts, specific locations, and mechanisms of glacial carbon storage and subsequent release have not been fully resolved.…”

“…Past [CO 3 2 − ] and δ 13 C DIC may be reconstructed from the boron to calcium ratio (B/Ca) and δ 13 C, respectively, of benthic foraminiferal calcite preserved in marine sediments. Reconstructed values for bottom water oxygen may also be combined with [CO 3 2 − ] to estimate past alkalinity and DIC (Anderson et al, ).…”