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

Abstract

During the termination of the last ice age, atmospheric CO2 rose ~80 ppm, but the origin of this carbon has not been fully resolved. Here we present novel constraints on the patterns and processes of deglacial CO2 release using three marine sediment cores from the southwest Pacific. Carbon isotopes (δ13C) and boron to calcium ratios (B/Ca) of benthic foraminiferal calcite provide records of the δ13C of total dissolved inorganic carbon (DIC) and carbonate ion concentrations ([CO32−]) in seawater, respectively. Together these properties indicate enhanced storage of respired CO2 between 1.2‐ and 2.5‐km water depth during the Last Glacial Maximum (19–23 thousand years ago, ka). The first major rise in atmospheric CO2 during the last deglaciation, at the time of Heinrich Stadial 1, was accompanied by increases in δ13C and [CO32−] at all core depths. The initial increases could be attributed to southward shifted westerly winds driving increased upwelling in the Southern Ocean, sending a signal of enhanced ventilation northward into the Pacific. Our results confirm that southern Pacific interior water masses served as an important reservoir for CO2 during the last glacial period, likely extracted from the atmosphere via the biologic pump. Some abrupt changes in Pacific carbon storage coincide with changes in Southern Ocean pH (Rae et al., 2018, https://doi.org/10.1038/s41586-018-0614-0), upwelling indicators (Anderson et al., 2009, https://doi.org/10.1126/science.1167441), and pCO2 (Monnin et al., 2001, https://doi.org/10.1126/science.291.5501.112), indicating that portions of the deep Pacific carbon pool can be ventilated rapidly to the atmosphere via the Southern Ocean.

Radiocarbon as a Dating Tool and Tracer in Paleoceanography

“…Similarly, if geological CO 2 was supplied in the ocean via deep sedimentary/metamorphic seeps in the south Pacific (Stott et al, 2019), with vertical fluid flow occurring over several hundred meters or more, one might also expect geological CO 2 to be neutralized by carbonate dissolution occurring deep in the sediment column, far below the sea floor. This workaround could potentially reconcile the radiocarbon observations from Galapagos (Bova et al, 2018;Stott et al, 2009) and the Gulf of California (Rafter et al, 2018(Rafter et al, , 2019 (and maybe the South Pacific [Ronge et al, 2016]) with records of deglacial carbonate chemistry and carbonate preservation (Allen et al, 2015(Allen et al, , 2020Cartapanis et al, 2018;Marchitto et al, 2005;Yu et al, 2013). However, it would also render them somewhat irrelevant to deglacial atmospheric CO 2 change (Rafter et al, 2019).…”

“…However, there is no evidence for significant carbonate dissolution at any of the eastern Pacific sites (Lindsay et al., 2015; Marchitto et al., 2007; Ortiz et al., 2004; Rafter et al., 2019; Stott et al., 2009). Globally, and across the Pacific, carbonate mass accumulation rates actually increased during the last deglaciation (Cartapanis et al., 2018; Figure 17a), as did carbonate ion concentrations in the deep Pacific (Allen et al., 2015, 2020; Marchitto et al., 2005; Yu et al., 2013). This would mean that significant and widespread geological CO 2 neutralization, either in the water column or at the sediment/sea‐water interface, probably did not occur during deglaciation.…”

Evidence for late glacial oceanic carbon redistribution and discharge from the Pacific Southern Ocean

“…20 mol kg −1 increase during the early deglacial, indicating subsequent DIC release to the atmosphere 24 . However, because the B/Ca method requires mono-speci c epifaunal benthic foraminifera for measurements, which suitable Paci c and Southern Ocean locations are often lacking, [CO 3 2− ] reconstructions to date are restricted to a few sites shallower than UCDW 24,25 .…”

Trace Metal Evidence for Deglacial Ventilation of the Abyssal Pacific and Southern Oceans

“…While there is growing evidence for the glacial storage of respired CO 2 in the deep ocean, the release of respired CO 2 from the deep ocean during deglaciations has been difficult to prove. Reconstructions of carbon release based on benthic δ 13 C and B/Ca exist from the South Pacific at depths from 1 to 2.5 km and suggest rapid deglacial ocean degassing of respired CO 2 (Allen et al, 2015(Allen et al, , 2020, but do not yet extend deeper (>2,500 m) into the most voluminous waters of the Pacific basin. Additionally, reconstructions of glacial-interglacial oceanic-atmosphere carbon exchange typically do not extend back to the earliest glacial cycles of the last 1 Ma.…”