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.
During the last deglaciation Earth’s climate experienced strong and abrupt variations, resulting in major changes in global temperature, sea level, and ocean circulation. Although proxy records have significantly improved our understanding of climate during this period, questions remain regarding the connection between ocean circulation evolution and resulting geotracer distributions, including those of deep waters in the Pacific. Here we use the C‐iTRACE simulation, a transient ocean‐only, isotope‐enabled version of the Community Earth System Model, to better understand deglacial deep Pacific radiocarbon evolution in the context of circulation and reservoir age changes. Throughout the deglaciation, the Pacific Ocean circulation in C‐iTRACE responds strongly to glacial meltwater forcing, leading to large changes in deep Pacific Δ14C age. A multi‐millennial weakening of the overturning circulation from 20 to 15 ka BP leads to increases in deep Pacific Δ14C ages, but from 20 to 18 ka BP, nearly half (40%–60%) of this aging is controlled by changing surface reservoir age, corroborating previous studies showing that Δ14C is not solely a circulation age tracer. As the deglaciation proceeds, circulation change controls progressively more of the Δ14C age, accounting for more than 75% of it across the deep Pacific from 15 to 8 ka BP.
more » « less- Award ID(s):
- 1566432
- NSF-PAR ID:
- 10370454
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 37
- Issue:
- 8
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The Southern Hemisphere westerly winds influence deep ocean circulation and carbon storage. While the westerlies are hypothesized to play a key role in regulating atmospheric CO2over glacial‐interglacial cycles, past changes in their position and strength remain poorly constrained. Here, we use a compilation of planktic foraminiferal δ18O from across the Southern Ocean and emergent relationships within an ensemble of climate models to reconstruct changes in the Southern Hemisphere surface westerlies over the last deglaciation. We infer a 4.8° (2.9–7.1°, 95% confidence interval) equatorward shift and about a 25% weakening of the westerlies during the Last Glacial Maximum (20 ka) relative to the mid‐Holocene (6.5 ka). Climate models from the Palaeoclimate Modeling Intercomparison Project substantially underestimate this inferred equatorward wind shift. According to our reconstruction, the poleward shift in the westerlies over deglaciation closely mirrors the rise in atmospheric CO2(
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