Identifying processes within the Earth System that have modulated atmospheric pCO2during 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 pCO2rise. 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/pCO2curve for the surface oceans. The pH disequilibrium/CO2outgassing was widespread throughout the last deglaciation, likely explained by upwelling of CO2from 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 CO2release. Our stack is likely biased toward upwelling/CO2source regions. Nevertheless, the composite pCO2curve provides robust evidence that various parts of the ocean were releasing CO2to the atmosphere over the last 25 kyr.
Lower atmospheric CO2concentrations during the Last Glacial Maximum (LGM; 23.0–18.0 ka) have been attributed to the sequestration of respired carbon in the ocean interior, yet the mechanism responsible for the release of this CO2during the deglaciation remains uncertain. Here we present calculations of vertical differences in oxygen and carbon isotopes (∆δ18O and ∆δ13C, respectively) from a depth transect of southwest Pacific Ocean sediment cores to reconstruct changes in water mass structure and CO2storage. During the Last Glacial Maximum, ∆δ18O indicates a more homogenous deep Pacific below 1,100 m, whereas regional ∆δ13C elucidates greater sequestration of CO2in two distinct layers: enhanced CO2storage at intermediate depths between ~940 and 1,400 m, and significantly more CO2at 1,600 m and below. This highlights an isolated glacial intermediate water mass and places the main geochemical divide at least 500 m shallower than the Holocene. During the initial stages of the deglaciation in Heinrich Stadial 1 (17.5–14.5 ka), restructuring of the upper ~2,000 m of the southwest Pacific water column coincided with sea‐ice retreat and rapid CO2release from intermediate depths, while CO2release from the deep ocean was earlier and more gradual than waters above it. These changes suggest that sea‐ice retreat and shifts in Southern Ocean frontal locations contributed to rapid CO2ventilation from the Southern Ocean's intermediate depths and gradual ventilation from the deep ocean during the early deglaciation.
more » « less- NSF-PAR ID:
- 10461375
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 34
- Issue:
- 5
- ISSN:
- 2572-4517
- Page Range / eLocation ID:
- p. 734-754
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The prevailing hypothesis to explain pCO2rise at the last glacial termination calls upon enhanced ventilation of excess respired carbon that accumulated in the deep sea during the glacial. Recent studies argue lower [O2] in the glacial ocean is indicative of increased carbon respiration. The magnitude of [O2] depletion was 100–140 µ mol/kg at the glacial maximum. Because respiration is coupled to
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