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1. Using DIC‐δ ¹³ C Pair to Constrain Anthropogenic Carbon Increase in the Southeastern Atlantic Ocean Over the Most Recent Decade (2010–2020)

   https://doi.org/10.1029/2024JC021586  

   Gao, Hui; Jin, Meibing; Zhao, Hui; Hussain, Najid; Cai, Wei‐Jun (November 2024, Journal of Geophysical Research: Oceans)

   Abstract The southeastern Atlantic Ocean is a crucial yet understudied region for the ocean absorption of anthropogenic carbon (C_anth). Data from the A12 (2020) and A13.5 (2010) cruises offer an opportunity to examine changes in dissolved inorganic carbon (DIC), its stable isotope (δ¹³C), and C_anthover the past decade within a limited region (1∼3°E, 32∼42°S). For the decade of 2010–2020, C_anthinvasion was observed from the sea surface down to 1,200 m based on both DIC and δ¹³C data. The mean C_anthincrease rate (1.08 ± 0.26 mol m⁻² yr⁻¹) during this period accelerated from 0.87 ± 0.05 mol m⁻² yr⁻¹during the previous period (1983/84–2010). The δ¹³C‐based C_anthincrease closely matches the DIC‐based estimation below 500 m but is 26% higher in the upper ocean. This discrepancy is likely due to δ¹³C's longer air‐sea exchange timescale, seasonal variability in the upper ocean, and the chosen ratio of anthropogenically induced changes in δ¹³C and DIC. Finally, column inventory changes based on the two methods also exhibit very similar mean C_anthuptake rates. The paired DIC concentration and stable isotope dataset may enhance our ability to constrain C_anthaccumulation and its controlling mechanisms in the ocean. 

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2. Orbital Influences on Deep Ocean Oxygen Concentrations and Respired Carbon Storage

   https://doi.org/10.1029/2025GB008503  

   Jacobel, A W; Pallone, C T; Costa, K M; Anderson, R F; McManus, J F (June 2025, Global Biogeochemical Cycles)

   Quantitative records of bottom water oxygen (BWO) are critical for understanding deep ocean change through time. Because of the stoichiometric relationship between oxygen and carbon, BWO records provide insight into the physical and biogeochemical processes that control the air‐sea partitioning of both gases with important implications for climate over Quaternary glacial‐interglacial cycles. Here, we present new geochemical data sets from Ocean Discovery Program Site 1240 in the eastern equatorial Pacific to constrain paleoproductivity (Ba_xsflux) and BWO using a multiproxy approach (aU, Mn/Al, Δδ¹³C, and U/Ba). This combination of approaches allows us to quantitatively identify changes in BWO and to parse local and basin‐wide contributions to the signal. We find that upwelling, not dust input, is responsible for driving productivity changes at the site. Changes in local carbon export are not the primary driver of changes in BWO, which instead reflect basin‐wide changes driven by processes in the Southern Ocean. Our BWO results provide direct evidence for the role of orbital precession and obliquity in driving deep sea respired carbon and oxygen concentrations. We find variations in BWO on the order of ∼50 μmol/kg that occur with ∼23 kyr periodicity during the substages of Marine Isotope Stage 5, and variations of ∼100 μmol/kg on glacial‐interglacial timescales. These findings have important implications for the role of insolation in driving deep ocean respired oxygen and carbon concentrations, and point to physical and biogeochemical changes in the Southern Ocean as key drivers of planetary‐scale carbon change. 

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3. 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)

   Abstract 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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4. Offset Between Profiling Float and Shipboard Oxygen Observations at Depth Imparts Bias on Float pH and Derived p CO ₂

   https://doi.org/10.1029/2024GB008185  

   Bushinsky, Seth M; Nachod, Zachary; Fassbender, Andrea J; Tamsitt, Veronica; Takeshita, Yuichiro; Williams, Nancy (May 2025, Global Biogeochemical Cycles)

   Abstract Profiles of oxygen measurements from Argo profiling floats now vastly outnumber shipboard profiles. To correct for drift, float oxygen data are often initially adjusted to deployment casts, ship‐based climatologies, or, recently, measurements of atmospheric oxygen for in situ calibration. Air calibration enables accurate measurements in the upper ocean but may not provide similar accuracy at depth. Using a quality controlled shipboard data set, we find that the entire Argo oxygen data set is offset relative to shipboard measurements (float minus ship) at pressures of 1,450–2,000 db by a median of −1.9 μmol kg⁻¹(mean ± SD of −1.9 ± 3.9, 95% confidence interval around the mean of {−2.2, −1.6}) and air‐calibrated floats are offset by −2.7 μmol kg⁻¹(−3.0 ± 3.4 (CI_95%{−3.7, −2.4}). The difference between float and shipboard oxygen is likely due to offsets in the float oxygen data and not oxygen changes at depth or biases in the shipboard data set. In addition to complicating the calculation of long‐term ocean oxygen changes, these float oxygen offsets impact the adjustment of float nitrate and pH measurements, therefore biasing important derived quantities such as the partial pressure of CO₂(pCO₂) and dissolved inorganic carbon. Correcting floats with air‐calibrated oxygen sensors for the float‐ship oxygen offsets alters float pH by a median of 3.0 mpH (3.1 ± 3.7) and float‐derived surfacepCO₂by −3.2 μatm (−3.2 ± 3.9). This adjustment to floatpCO₂represents half, or more, of the bias in float‐derivedpCO₂reported in studies comparing floatpCO₂to shipboardpCO₂measurements. 

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5. Dissolved and Particulate Barium Distributions Along the US GEOTRACES North Atlantic and East Pacific Zonal Transects (GA03 and GP16): Global Implications for the Marine Barium Cycle

   https://doi.org/10.1029/2022GB007330  

   Rahman, Shaily; Shiller, Alan M; Anderson, Robert F; Charette, Matthew A; Hayes, Christopher T; Gilbert, Melissa; Grissom, Karen R; Lam, Phoebe J; Ohnemus, Daniel C; Pavia, Frank J; et al (June 2022, Global Biogeochemical Cycles)

   Abstract Processes controlling dissolved barium (dBa) were investigated along the GEOTRACES GA03 North Atlantic and GP16 Eastern Tropical Pacific transects, which traversed similar physical and biogeochemical provinces. Dissolved Ba concentrations are lowest in surface waters (∼35–50 nmol kg⁻¹) and increase to 70–80 and 140–150 nmol kg⁻¹in deep waters of the Atlantic and Pacific transects, respectively. Using water mass mixing models, we estimate conservative mixing that accounts for most of dBa variability in both transects. To examine nonconservative processes, particulate excess Ba (pBa_xs) formation and dissolution rates were tracked by normalizing particulate excess²³⁰Th activities. Th‐normalized pBa_xsfluxes, with barite as the likely phase, have subsurface maxima in the top 1,000 m (∼100–200 μmol m⁻² year⁻¹average) in both basins. Barite precipitation depletes dBa within oxygen minimum zones from concentrations predicted by water mass mixing, whereas inputs from continental margins, particle dissolution in the water column, and benthic diffusive flux raise dBa above predications. Average pBa_xsburial efficiencies along GA03 and GP16 are ∼37% and 17%–100%, respectively, and do not seem to be predicated on barite saturation indices in the overlying water column. Using published values, we reevaluate the global freshwater dBa river input as 6.6 ± 3.9 Gmol year⁻¹. Estuarine mixing processes may add another 3–13 Gmol year⁻¹. Dissolved Ba inputs from broad shallow continental margins, previously unaccounted for in global marine summaries, are substantial (∼17 Gmol year⁻¹), exceeding terrestrial freshwater inputs. Revising river and shelf dBa inputs may help bring the marine Ba isotope budget more into balance. 

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