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1. 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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2. Evaluating the Glacial‐Deglacial Carbon Respiration and Ventilation Change Hypothesis as a Mechanism for Changing Atmospheric CO 2

   https://doi.org/10.1029/2020GL091296  

   Stott, Lowell D. ; Shao, Jun ; Yu, Jimin ; Harazin, Kathleen M. ( February 2021 , Geophysical Research Letters)

   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 toδ¹³C of dissolved inorganic carbon (DIC), [O₂] depletion of 100–140 µ mol/kg from carbon respiration would lower deep waterδ¹³C_DICby ∼1‰ relative to surface water. Prolonged sequestration of respired carbon would also lower the amount of¹⁴C in the deep sea. We show that Pacific Deep Waterδ¹³C_DICdid not decrease relative to the surface ocean and Δ¹⁴C was only ∼50‰ lower during the late glacial. Model simulations of the hypothesized ventilation change during deglaciation lead to large increases inδ¹³C_DIC, Δ¹⁴C, andε¹⁴C that are not recorded in observations.

    

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3. A Next Generation Ocean Carbon Isotope Model for Climate Studies I: Steady State Controls on Ocean 13 C

   https://doi.org/10.1029/2020GB006757  

   Claret, Mariona ; Sonnerup, Rolf E. ; Quay, Paul D. ( April 2021 , Global Biogeochemical Cycles)

   The¹³C/¹²C of dissolved inorganic carbon (δ¹³C_DIC) carries valuable information on ocean biological C‐cycling, air‐sea CO₂exchange, and circulation. Paleo‐reconstructions of oceanic¹³C from sediment cores provide key insights into past as changes in these three drivers. As a step toward full inclusion of¹³C in the next generation of Earth system models, we implemented¹³C‐cycling in a 1° lateral resolution ocean‐ice‐biogeochemistry Geophysical Fluid Dynamics Laboratory (GFDL) model driven by Common Ocean Reference Experiment perpetual year forcing. The model improved the mean of modernδ¹³C_DICover coarser resolution GFDL‐model implementations, capturing the Southern Ocean decline in surfaceδ¹³C_DICthat propagates to the deep sea via deep water formation. Controls onδ¹³C_DICof the deep‐sea are quantified using both observations and model output. The biological control is estimated from the relationship between deep‐sea Pacificδ¹³C_DICand phosphate (PO₄). Theδ¹³C_DIC:PO₄slope from observations is revised to a value of 1.01 ± 0.02‰ (μmol kg⁻¹)⁻¹, consistent with a carbon to phosphate ratio of organic matter (C:P_org) of 124 ± 10. Model output yields a lowerδ¹³C_DIC:PO₄than observed due to too low C:P_org. The ocean circulation impacts deep modernδ¹³C_DICin two ways, via the relative proportion of Southern Ocean and North Atlantic deep water masses, and via the preindustrialδ¹³C_DICof these water mass endmembers. Theδ¹³C_DICof the endmembers ventilating the deep sea are shown to be highly sensitive to the wind speed dependence of air‐sea CO₂gas exchange. Reducing the coefficient for air‐sea gas exchange following OMIP‐CMIP6 protocols improves significantly surfaceδ¹³C_DICrelative to previous gas exchange parameterizations.

    

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4. Origin and Accumulation of an Anthropogenic CO 2 and 13 C Suess Effect in the Arctic Ocean

   https://doi.org/10.1029/2019GB006423  

   Ko, Young Ho ; Quay, Paul D. ( February 2020 , Global Biogeochemical Cycles)

   We determined the impact of anthropogenic CO₂(C_ant) accumulation on the δ¹³C of dissolved inorganic carbon in the Arctic Ocean (i.e., the¹³C Suess effect) based on δ¹³C measurements during a GEOTRACES cruise in 2015. The δ¹³C decrease was estimated from the amount of C_antchange derived by the transit time distribution approach and the ratio of the anthropogenic δ¹³C/dissolved inorganic carbon change (RC). A significant C_antincrease (up to 45 μmol kg⁻¹) and δ¹³C decrease (up to −0.9‰) extends to ~2,000 m in the Canada and Makarov Basin. We find distinctly different RC values for the intermediate water (300–2,000 m) and upper halocline water (<200 m) of −0.020 and −0.012‰ (μmol kg⁻¹)⁻¹, respectively, which identifies two sources of C_antaccumulation from North Atlantic and North Pacific. Furthermore, estimated RC for intermediate waters is the same as the RC observed in the Greenland Sea and the rate of anthropogenic dissolved inorganic carbon increase estimated for intermediate waters at 0.9 μmol kg⁻¹yr⁻¹is identical to the estimated rate in the Iceland Sea. These observations indicate that the high rate of C_antaccumulation and δ¹³C decrease in the Arctic Ocean is primarily a result of the input of C_ant, via ventilation of intermediate waters, from the Nordic Sea rather than local anthropogenic CO₂uptake within the Arctic Basin. We determine the preindustrial δ¹³C (δ¹³C_PI) distributions and find distinct δ¹³C_PIsignatures of the intermediate and upper halocline waters that reflect the difference in δ¹³C_PI–PO₄relationship of Atlantic and Pacific source water.

    

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5. Carbon cycle dynamics and ecology revealed by the carbon isotopic composition of single organic microfossils during the Late Devonian Biotic Crisis

   https://doi.org/10.1111/gbi.12482  

   Cohen, Phoebe A. ; Junium, Christopher K. ; King Phillips, Ezekiel ; Uveges, Benjamin T. ( December 2021 , Geobiology)

   We apply a new approach for the δ¹³C analysis of single organic‐walled microfossils (OWM) to three sites in the Appalachian Basin of New York (AB) that span the Late Devonian Biotic Crisis (LDBC). Our data provide new insights into the nature of the Frasnian–Famennian carbon cycle in the AB and also provide possible constraints on the paleoecology of enigmatic OWM ubiquitous in Paleozoic shale successions. The carbon isotope compositions of OWM are consistent with normal marine organic matter of autochthonous origins and range from −32 to −17‰, but average −25‰ across all samples and are consistently¹³C‐enriched compared to bulk sediments (δ¹³C_bulk) by ~0–10‰. We observe no difference between the δ¹³C_OWMof leiospheres (smooth‐walled) and acanthomorphic (spinose) acritarch OWM, indicating that our data are driven by ecological rather than taxonomic signals. We hypothesize that the offset between δ¹³C_OWMand δ¹³C_bulkis in part due to a large δ¹³C gradient in the AB water column where OWM utilized relatively¹³C‐enriched dissolved inorganic carbon near the surface. Thus, the organisms producing the balance of the total organic carbon were assimilating¹³C‐depleted C sources, including but not limited to respired organic carbon or byproducts of fermentation. We also observe a systematic decrease in both δ¹³C_OWMand δ¹³C_bulkof 3‰ from shoreward to open‐ocean facies that may reflect the effect of¹³C‐enriched dissolved inorganic carbon (DIC) derived from riverine sources in the relatively enclosed AB. The hypothesized steep carbon isotope gradient in the AB could be due to a strong biological pump; this in turn may have contributed to low oxygen bottom water conditions during the LDBC. This is the first time single‐microfossil δ¹³C_organalyses of eukaryotes have been directly compared to bulk δ¹³C_orgin the deep‐time fossil record.

    

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