Atmosphere‐Ocean CO₂ Exchange Across the Last Deglaciation From the Boron Isotope Proxy

Abstract

Identifying processes within the Earth System that have modulated atmospheric pCO2 during each glacial cycle of the late Pleistocene stands as one of the grand challenges in climate science. The growing array of surface ocean pH estimates from the boron isotope proxy across the last glacial termination may reveal regions of the ocean that influenced the timing and magnitude of pCO2 rise. Here we present two new boron isotope records from the subtropical‐subpolar transition zone of the Southwest Pacific that span the last 20 kyr, as well as new radiocarbon data from the same cores. The new data suggest this region was a source of carbon to the atmosphere rather than a moderate sink as it is today. Significantly higher outgassing is observed between ~16.5 and 14 kyr BP, associated with increasing δ13C and [CO3]2− at depth, suggesting loss of carbon from the intermediate ocean to the atmosphere. We use these new boron isotope records together with existing records to build a composite pH/pCO2 curve for the surface oceans. The pH disequilibrium/CO2 outgassing was widespread throughout the last deglaciation, likely explained by upwelling of CO2 from the deep/intermediate ocean. During the Holocene, a smaller outgassing peak is observed at a time of relatively stable atmospheric CO2, which may be explained by regrowth of the terrestrial biosphere countering ocean CO2 release. Our stack is likely biased toward upwelling/CO2 source regions. Nevertheless, the composite pCO2 curve provides robust evidence that various parts of the ocean were releasing CO2 to the atmosphere over the last 25 kyr.

CO2 Release From Pockmarks on the Chatham Rise‐Bounty Trough at the Glacial Termination

“…For radiocarbon isotope analysis, background radiocarbon shipboard wipe tests were conducted to determine levels of radiocarbon present in laboratories, work, and storage areas and in the portable lab van (Coffin, Rose, et al, ). The Piston Core PC75 Δ 14 C and stable isotope data are from Shao et al (). The bulk sediment 14 C ages for Piston Cores PC45 and PC54 were generated at the Rafter AMS lab in New Zealand (supporting information Table S1).…”

“…We did this so that all of the core chronologies are derived using the same calibration curve (MARINE13). For the late glacial and early deglacial sections, we adopted their surface reservoir ages because they are within ~300 years of that used by Shao et al (). There is one exception; in Ronge et al's “tuned” age model for the PS75‐104‐1 core, the authors estimated a surface reservoir age of 243 years at 96 cm (15,435 14 C age).…”

“…Stable carbon isotope analyses were combined with radiocarbon data to assess if there has been methane oxidation (Coffin et al, 2015). Stable oxygen and carbon isotope data from foraminifers from piston core PC75 are from Shao et al (2019).…”

“…waters in the southwest Pacific (Rose et al, 2010;Shao et al, 2019;Skinner et al, 2015). Further studies will now be necessary to evaluate whether there were changes in regional carbonate chemistry close to and away from the sites of carbon release.…”

Boron Isotopes Provide Insights into Biomineralization, Seawater pH, and Ancient Atmospheric CO2

“…White diamonds = Cibicidoides wuellerstorfi benthic foraminiferal δ 11 B (Rae et al, 2011). (26,,000 years ago), evidence that CO 2 is stored in the deep ocean during glacial periods (Figure 2), while planktic δ 11 B (Shao et al, 2019) provides evidence of widespread outgassing of CO 2 via the surface ocean during the last deglaciation.…”