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1. Atmosphere‐Ocean CO 2 Exchange Across the Last Deglaciation From the Boron Isotope Proxy

   https://doi.org/10.1029/2018PA003498  

   Shao, Jun ; Stott, Lowell D. ; Gray, William R. ; Greenop, Rosanna ; Pecher, Ingo ; Neil, Helen L. ; Coffin, Richard B. ; Davy, Bryan ; Rae, James W. B. ( October 2019 , Paleoceanography and Paleoclimatology)

   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.

    

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2. Ocean Oxygen, Preformed Nutrients, and the Cause of the Lower Carbon Dioxide Concentration in the Atmosphere of the Last Glacial Maximum

   https://doi.org/10.1029/2023PA004775  

   Sigman, Daniel M. ; Hain, Mathis P. ( January 2024 , Paleoceanography and Paleoclimatology)

   All else equal, if the ocean's “biological [carbon] pump” strengthens, the dissolved oxygen (O₂) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO₂) concentrations of the ice ages. Vollmer et al. (2022,https://doi.org/10.1029/2021PA004339) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were “regenerated” (transported as sinking organic matter from the ocean surface to the interior) rather than “preformed” (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed¹³C/¹²C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO₂storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange.

    

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

   https://doi.org/10.1029/2021PA004226  

   Pavia, Frank J. ; Wang, Shouyi ; Middleton, Jennifer ; Murray, Richard W. ; Anderson, Robert F. ( August 2021 , Paleoceanography and Paleoclimatology)

   The deep ocean has long been recognized as the reservoir that stores the carbon dioxide (CO₂) removed from the atmosphere during Pleistocene glacial periods. The removal of glacial atmospheric CO₂into the ocean is likely modulated by an increase in the degree of utilization of macronutrients at the sea surface and enhanced storage of respired CO₂in the deep ocean, known as enhanced efficiency of the biological pump. Enhanced biological pump efficiency during glacial periods is most easily documented in the deep ocean using proxies for oxygen concentrations, which are directly linked to respiratory CO₂levels. We document the enhanced storage of respired CO₂during the Last Glacial Maximum (LGM) in the Pacific Southern Ocean and deepest Equatorial Pacific using records of deglacial authigenic manganese, which form as relict peaks during increases in bottom water oxygen (BWO) concentration. These peaks are found at depths and regions where other oxygenation histories have been ambiguous, due to diagenetic alteration of authigenic uranium, another proxy for BWO. Our results require that the entirety of the abyssal Pacific below approximately 1,000 m was enriched in respired CO₂and depleted in oxygen during the LGM. The presence of authigenic Mn enrichment in the deep Equatorial Pacific for each of the last five deglaciations suggests that the storage of respired CO₂in the deep ocean is a ubiquitous feature of late‐Pleistocene ice ages.

    

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4. ASSESSING THE STRATIGRAPHIC INTEGRITY OF PLANKTIC AND BENTHIC 14 C RECORDS IN THE WESTERN PACIFIC FOR Δ 14 C RECONSTRUCTIONS AT THE LAST GLACIAL TERMINATION

   https://doi.org/10.1017/RDC.2020.82  

   Stott, Lowell D ( October 2020 , Radiocarbon)

   null (Ed.)

   ABSTRACT There is a growing database of radiocarbon ( 14 C) reconstructions from biogenic carbonate taken from marine sediment cores being used to investigate changing ocean circulation and carbon cycling at the end of the last great ice age. Reported here are 14 C results from a marine core taken in the Makassar Straits of the western equatorial Pacific that was intended to test whether there was evidence of geologic carbon release to the ocean during the glacial termination. A thorough investigation of planktic and benthic 14 C ages with stable isotopes and CT-scans revealed extensive burrowing in the upper 2 m of the core that displaced younger sediments downward by more than half a meter into the glacial section of the core. The vertical displacement is evident in both planktic and benthic fossils. However, the extent of displacement and the stratigraphic disturbance became evident only after multiple measurements of different species and genera. A CT-scan prior to sampling would be an effective screening tool to avoid sampling problem cores such as this. 

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5. An abrupt temperature and hydroclimate transition in eastern Africa during Termination V

   Ramirez, B. ; Castañeda, I.S. ; Salacup, J.M. ; Johnson, T.C. ( July 2023 , XXI INQUA Congress)

   Over the last few million years, Africa’s climate exhibits a long-term drying trend with episodes of high climate variability coinciding with the intensification of glacial-interglacial cycles. Of particular interest, is a shift to drier and more variable conditions noted in the Olorgesailie Formation (Kenya) between 500 and 300 thousand years ago (ka) in which Potts et al. (2018) observed a turnover of ~85% of large-body mammalian fauna to smaller-body related taxa and suggested that the shift was an evolutionary response to better adapt to the changing climate. However, an erosional gap in the Olorgesailie record during this time interval means that the cause of this faunal shift is still an outstanding question. To understand East African climate variability during the Mid-Pleistocene, we analyze Lake Malawi drill core MAL 05–1 (~11ºS, 34ºE) to investigate if a specific climatic event stands out as a possible driver of the dramatic change observed in the East African mammal community. We use organic geochemical proxies including branched glycerol diaklyl glycerol tetraethers (brGDGTs; the MBT′5ME index) andleaf wax carbon and deuterium isotopes to develop high-resolution temperature, vegetation, and precipitation records, respectively, between 600 and 200 ka. Results show an abrupt temperature increase of ~9°C occurring in less than 3000 years during Glacial Termination V, which is the Marine Isotope Stage (MIS) 12 to MIS 11 transition at ~330 ka. Preliminary leaf wax deuterium isotopic values show an enrichment that coincides with deglacial warmings suggesting a shift to more arid conditions during interglacial than in glacial periods. This change from a cold/wet glacial to a warm/dry interglacial contrast with the cool/dry pattern of the Last Glacial Maximum (LGM) in East Africa which transitioned to a warm/wet Holocene. Leaf wax carbon isotopes are presently being analyzed to understand past shifts in C3 vs C4 vegetation type, which can be related to climatic conditions. We propose that the major warming and drying during Termination V in East Africa represents a significant abrupt change in the climate of eastern Africa and was a likely driver of the major faunal turnover noted in the region. 

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