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1. Constraining CaCO 3 Export and Dissolution With an Ocean Alkalinity Inverse Model

   https://doi.org/10.1029/2022GB007535  

   Liang, Hengdi ; Lunstrum, Abby M. ; Dong, Sijia ; Berelson, William M. ; John, Seth G. ( February 2023 , Global Biogeochemical Cycles)

   Ocean alkalinity plays a fundamental role in the apportionment of CO₂between the atmosphere and the ocean. The primary driver of the ocean's vertical alkalinity distribution is the formation of calcium carbonate (CaCO₃) by organisms at the ocean surface and its dissolution at depth. This so‐called “CaCO₃counterpump” is poorly constrained, however, both in terms of how much CaCO₃is exported from the surface ocean, and at what depth it dissolves. Here, we created a steady‐state model of global ocean alkalinity using Ocean Circulation Inverse Model transport, biogeochemical cycling, and field‐tested calcite and aragonite dissolution kinetics. We find that limiting CaCO₃dissolution to below the aragonite and calcite saturation horizons cannot explain excess alkalinity in the upper ocean, and that models allowing dissolution above the saturation horizons best match observations. Linking dissolution to organic matter respiration, or imposing a constant dissolution rate both produce good model fits. Our best performing models require export between 1.1 and 1.8 Gt PIC y⁻¹(from 73 m), but all converge to 1.0 Gt PIC y⁻¹export at 279 m, indicating that both high‐ and low‐export scenarios can match observations, as long as high export is coupled to high dissolution in the upper ocean. These results demonstrate that dissolution is not a simple function of seawater CaCO₃saturation (Ω) and calcite or aragonite solubility, and that other mechanisms, likely related to the biology and ecology of calcifiers, must drive significant dissolution throughout the water column.

    

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2. Shallow Calcium Carbonate Cycling in the North Pacific Ocean

   https://doi.org/10.1029/2022GB007388  

   Subhas, Adam V. ; Dong, Sijia ; Naviaux, John D. ; Rollins, Nick E. ; Ziveri, Patrizia ; Gray, William ; Rae, James W. B. ; Liu, Xuewu ; Byrne, Robert H. ; Chen, Sang ; et al ( May 2022 , Global Biogeochemical Cycles)

   The cycling of biologically produced calcium carbonate (CaCO₃) in the ocean is a fundamental component of the global carbon cycle. Here, we present experimental determinations of in situ coccolith and foraminiferal calcite dissolution rates. We combine these rates with solid phase fluxes, dissolved tracers, and historical data to constrain the alkalinity cycle in the shallow North Pacific Ocean. The in situ dissolution rates of coccolithophores demonstrate a nonlinear dependence on saturation state. Dissolution rates of all three major calcifying groups (coccoliths, foraminifera, and aragonitic pteropods) are too slow to explain the patterns of both CaCO₃sinking flux and alkalinity regeneration in the North Pacific. Using a combination of dissolved and solid‐phase tracers, we document a significant dissolution signal in seawater supersaturated for calcite. Driving CaCO₃dissolution with a combination of ambient saturation state and oxygen consumption simultaneously explains solid‐phase CaCO₃flux profiles and patterns of alkalinity regeneration across the entire N. Pacific basin. We do not need to invoke the presence of carbonate phases with higher solubilities. Instead, biomineralization and metabolic processes intimately associate the acid (CO₂) and the base (CaCO₃) in the same particles, driving the coupled shallow remineralization of organic carbon and CaCO₃. The linkage of these processes likely occurs through a combination of dissolution due to zooplankton grazing and microbial aerobic respiration within degrading particle aggregates. The coupling of these cycles acts as a major filter on the export of both organic and inorganic carbon to the deep ocean.

    

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3. Organic carbon inputs, common ions and degassing: rethinking mixing dissolution in coastal eogenetic carbonate aquifers

   https://doi.org/10.1002/esp.3975  

   Gulley, Jason D. ; B.Martin, Jonathan ; Brown, Amy ( July 2016 , Earth Surface Processes and Landforms)

   Caves deliver freshwater from coastal carbonate landscapes to estuaries but how these caves form and grow remains poorly understood. Models suggest fresh and salt water mixing drives dissolution in eogenetic limestone, but have rarely been validated through sampling of mixing waters. Here we assess controls on carbonate mineral saturation states using new and legacy geochemical data that were collected in vertical profiles through three cenotes and one borehole in the Yucatan Peninsula. Results suggest saturation states are primarily controlled by carbon fluxes rather than mixing. Undersaturation predicted by mixing models that rely on idealized end members is diminished or eliminated when end members are collected from above and below actual mixing zones. Undersaturation due to mixing is limited by CO₂degassing from fresh water in karst windows, which results in calcite supersaturation. With respect to saline groundwater, controls on capacity for mixing dissolution were more varied. Oxidation of organic carbon increased pCO₂of saline groundwater in caves (pCO₂ = 10^–2.06to 10^–0.96 atm) relative to matrix porosity (10^–2.39 atm) and local seawater (10^–3.12 atm). The impact of increased pCO₂on saturation state, however, depended on the geochemical composition of the saline water and the magnitude of organic carbon oxidation. Carbonate undersaturation due to mixing was limited where gypsum dissolution (Cenote Angelita) or sulfate reduction (Cenote Calica) increased concentrations of common ions (Ca²⁺or HCO₃⁻, respectively). Maximum undersaturation was found to occur in mixtures including saline water that had ion concentrations and ratios similar to seawater, but with moderately elevated pCO₂(Cenote Eden). Undersaturation, however, was dominated by the initial undersaturation of the saline end member, mixing was irrelevant. Our results add to a growing body of literature that suggests oxidation of organic carbon, and not mixing dissolution, is the dominant control on cave formation and enlargement in coastal eogenetic karst aquifers. Copyright © 2016 John Wiley & Sons, Ltd.

    

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4. The carbonic anhydrase activity of sinking and suspended particles in the North Pacific Ocean

   https://doi.org/10.1002/lno.11332  

   Subhas, Adam V. ; Adkins, Jess F. ; Dong, Sijia ; Rollins, Nick E. ; Berelson, William M. ( October 2019 , Limnology and Oceanography)

   The enzyme carbonic anhydrase (CA) is crucial to many physiological processes involvingCO₂, from photosynthesis and respiration, to calcification andCaCO₃dissolution. We present new measurements of CA activity along a North Pacific transect, on samples from in situ pumps, sediment traps, discreet plankton samples from the ship's underway seawater line, plankton tows, and surface sediment samples from multicores. CA activity is highest in the surface ocean and decreases with depth, both in suspended and sinking particles. Subpolar gyre surface particles exhibit 10× higher CA activity per liter of seawater compared to subtropical gyre surface particles. Activity persists to 4700 m in the subpolar gyre, but only to 1000 m in the subtropics. All sinking CA activity normalized to particulate organic carbon (POC) follows a single relationship (CA/POC = 1.9 ± 0.2 × 10⁻⁷mol mol⁻¹). This relationship is consistent with CA/POC values in subpolar plankton tow material, suspended particles, and core top sediments. We hypothesize that most subpolar CA activity is associated with rapidly sinking diatom blooms, consistent with a large mat of diatomaceous material identified on the seafloor. Compared to the basin‐wide sinking CA/POC relationship, a lower subtropical CA/POC suggests that the inventory of subtropical biomass is different in composition from exported material. Pteropods also demonstrate substantial CA activity. Scaled to the volume within pteropod shells, first‐orderCO₂hydration rate constants are elevated ≥ 1000× above background. This kinetic enhancement is large enough to catalyze carbonate dissolution within microenvironments, providing observational evidence for CA‐catalyzed, respiration‐drivenCaCO₃dissolution in the shallow North Pacific.

    

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5. Novel device to collect deep‐sea porewater in situ: A focus on benthic carbonate chemistry

   https://doi.org/10.1002/lom3.10530  

   Cetiner, Jaclyn E. P. ; Berelson, William M. ; Rollins, Nick E. ; Barnhart, Holly A. ; Liu, Xuewu ; Dong, Sijia ; Byrne, Robert H. ; Adkins, Jess F. ( December 2022 , Limnology and Oceanography: Methods)

   We have designed, built, tested, and deployed a novel device to extract porewater from deep‐sea sediments in situ, constructed to work with a standard multicorer. Despite the importance of porewater measurements for numerous applications, many sampling artifacts can bias data and interpretation during traditional porewater processing from shipboard‐processed cores. A well‐documented artifact occurs in deep‐sea porewater when carbonate precipitates during core recovery as a function of temperature and pressure changes, while porewater is in contact with sediment grains before filtration, thereby lowering porewater alkalinity and dissolved inorganic carbon (DIC). Here, we present a novel device built to obviate these sampling artifacts by filtering porewater in situ on the seafloor, with a focus near the sediment–water interface on cm‐scale resolution, to obtain accurate porewater profiles. We document 1–10% alkalinity loss in shipboard‐processed sediment cores compared to porewater filtered in situ, at depths of 1600–3200 m. We also show that alkalinity loss is a function of both weight % sedimentary CaCO₃and water column depth. The average ratio of alkalinity loss to DIC loss in shipboard‐processed sediment cores relative to in situ porewater is 2.2, consistent with the signal expected from carbonate precipitation. In addition to collecting porewater for defining natural profiles, we also conducted the first in situ dissolution experiments within the sediment column using isotopically labeled calcite. We present evidence of successful deployments of this device on and adjacent to the Cocos Ridge in the Eastern Equatorial Pacific across a range of depths and calcite saturation states.

    

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