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1. 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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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. The Subtropical Ocean's Biological Carbon Pump Determined From O 2 and DIC/DI 13 C Tracers

   https://doi.org/10.1029/2018GL081239  

   Yang, Bo ; Emerson, Steven R. ; Quay, Paul D. ( May 2019 , Geophysical Research Letters)

   The ocean's biological organic carbon pump regulates thepCO₂of the atmosphere and helps maintain the oxygen distributions in the ocean. Global models of this flux are poorly verified with observations. We used upper‐ocean budgets of O₂and the¹³C/¹²C of dissolved inorganic carbon (DIC) to estimate the biological pump in the subtropical gyres. These two tracers yield, within errors, similar result (~2.0 mol C·m⁻²·year⁻¹) at three Northern Hemisphere subtropical locations. Values for three Southern Hemisphere subtropical regions are lower and more variable determined by the O₂mass balance than by the DI¹³C method (−0.5 to 0.8 mol C·m⁻²·year⁻¹and 0.9 to 1.3 mol C·m⁻²·year⁻¹, respectively). Both methods suggest that the subtropical ocean is, on the whole, autotropic. The gas exchange residence times of O₂and dissolved inorganic carbon result in different spatial and temporal averaging creating complementary tracers for biological pump model verification.

    

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4. Evaluation of the moderate DI 13 C isotope enrichment method for measuring photochemical mineralization of marine dissolved organic carbon

   https://doi.org/10.1002/lom3.10450  

   Lu, Xi ; Beaupré, Steven ( August 2021 , Limnology and Oceanography: Methods)

   The moderate DI¹³C isotope enrichment (MoDIE) method by Powers et al. (2017) is a promising method to precisely measure the photochemical mineralization of dissolved organic carbon (DOC) in water samples without dramatically altering a sample's pH or organic carbon pool. Here, we evaluated the analytical uncertainties of the MoDIE method and used Monte Carlo simulations to optimize the experimental design for the most precise measurements of dissolved inorganic carbon (DIC) that is produced photochemically (DIC_hν). Analytically, we recommend calculating yields of DIC_hvwith an exact expression of conservation of mass that intrinsically reduces error and uncertainty. Methodologically, the overall uncertainty and detection limit of the MoDIE method can be significantly reduced by partially stripping away the original DIC pool, enriching the residual DIC with more DI¹³C, and increasing the yields of DIC_hvvia longer irradiation. Instrumentally, more precise measurements of enriched δ¹³C values before and after irradiation are needed to further improve the precision of DIC_hνconcentration determinations. Higher precision DIC_hvmeasurements via the optimized MoDIE method can improve our understanding of the photochemical mineralization of DOC and thus the budget of marine DOC. The optimizations and detection limits reported here will become more refined as measurements and associated uncertainties from future MoDIE experiments become available.

    

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5. 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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