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1. Evaluating the Glacial-Deglacial Carbon Respiration and Ventilation Change Hypothesis as a Mechanism for Changing Atmospheric CO2

   Stott, Lowell Shao ( January 2021 , Geophysical research letters)

   null (Ed.)

   The prevailing hypothesis to explain pCO2 rise 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 δ13C of dissolved inorganic carbon (DIC), [O2] depletion of 100–140 μ mol/kg from carbon respiration would lower deep water δ13CDIC by ∼1‰ relative to surface water. Prolonged sequestration of respired carbon would also lower the amount of 14C in the deep sea. We show that Pacific Deep Water δ13CDIC did 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 in δ13CDIC, Δ14C, and ε14C that are not recorded in observations. 

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2. 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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3. Southwest Pacific Vertical Structure Influences on Oceanic Carbon Storage Since the Last Glacial Maximum

   https://doi.org/10.1029/2018PA003501  

   Clementi, Vincent J. ; Sikes, Elisabeth L. ( May 2019 , Paleoceanography and Paleoclimatology)

   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.

    

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4. Simultaneous onboard analysis of seawater dissolved inorganic carbon (DIC) concentration and stable isotope ratio ( δ 13 C‐DIC)

   https://doi.org/10.1002/lom3.10642  

   Sun, Zhentao ; Li, Xinyu ; Ouyang, Zhangxian ; Featherstone, Charles ; Atekwana, Eliot A ; Hussain, Najid ; Cai, Wei‐Jun ( August 2024 , Limnology and Oceanography: Methods)

   Dissolved inorganic carbon (DIC) and its stable carbon isotope (δ¹³C‐DIC) are valuable parameters for studying the aquatic carbon cycle and quantifying ocean anthropogenic carbon accumulation rates. However, the potential of this coupled pair is underexploited as only 15% or less of cruise samples have been analyzed forδ¹³C‐DIC because the traditional isotope analysis is labor‐intensive and restricted to onshore laboratories. Here, we improved the analytical precision and reported the protocol of an automated, efficient, and high‐precision method for ship‐based DIC andδ¹³C‐DIC analysis based on cavity ring‐down spectroscopy (CRDS). We also introduced a set of stable in‐house standards to ensure accurate and consistent DIC andδ¹³C‐DIC measurements, especially on prolonged cruises. With this method, we analyzed over 1600 discrete seawater samples over a 40‐d cruise along the North American eastern ocean margin in summer 2022, representing the first effort to collect a large dataset ofδ¹³C‐DIC onboard of any oceanographic expedition. We evaluated the method's uncertainty, which was 1.2 μmol kg⁻¹for the DIC concentration and 0.03‰ for theδ¹³C‐DIC value (1σ). An interlaboratory comparison of onboard DIC concentration analysis revealed an average offset of 2.0 ± 3.8 μmol kg⁻¹between CRDS and the coulometry‐based results. The cross‐validation ofδ¹³C‐DIC in the deep‐ocean data exhibited a mean difference of only −0.03‰ ± 0.07‰, emphasizing the consistency with historical data. Potential applications in aquatic biogeochemistry are discussed.

    

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5. Carbon and carbon-13 in the preindustrial and glacial ocean

   https://doi.org/10.1371/journal.pclm.0000434  

   Schmittner, Andreas ; Fillman, Nathaniel J ( July 2024 , PLOS Climate)

   Carré, Matthieu (Ed.)

   Despite their importance for Earth’s climate and paleoceanography, the cycles of carbon (C) and its isotope¹³C in the ocean are not well understood. Models typically do not decompose C and¹³C storage caused by different physical, biological, and chemical processes, which makes interpreting results difficult. Consequently, basic observed features, such as the decreased carbon isotopic signature (δ¹³C_DIC) of the glacial ocean remain unexplained. Here, we review recent progress in decomposing Dissolved Inorganic Carbon (DIC) into preformed and regenerated components, extend a precise and complete decomposition to δ¹³C_DIC, and apply it to data-constrained model simulations of the Preindustrial (PI) and Last Glacial Maximum (LGM) oceans. Regenerated components, from respired soft-tissue organic matter and dissolved biogenic calcium carbonate, are reduced in the LGM, indicating a decrease in the active part of the biological pump. Preformed components increase carbon storage and decrease δ¹³C_DICby 0.55 ‰ in the LGM. We separate preformed into saturation and disequilibrium components, each of which have biological and physical contributions. Whereas the physical disequilibrium in the PI is negative for both DIC and δ¹³C_DIC, and changes little between climate states, the biological disequilibrium is positive for DIC but negative for δ¹³C_DIC, a pattern that is magnified in the LGM. The biological disequilibrium is the dominant driver of the increase in glacial ocean C and the decrease in δ¹³C_DIC, indicating a reduced sink of biological carbon. Overall, in the LGM, biological processes increase the ocean’s DIC inventory by 355 Pg more than in the PI, reduce its mean δ¹³C_DICby an additional 0.52 ‰, and contribute 60 ppm to the lowering of atmospheric CO₂. Spatial distributions of the δ¹³C_DICcomponents are presented. Commonly used approximations based on apparent oxygen utilization and phosphate are evaluated and shown to have large errors.

    

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