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1. 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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2. Pelagic calcium carbonate production and shallow dissolution in the North Pacific Ocean

   https://doi.org/10.1038/s41467-023-36177-w  

   Ziveri, Patrizia ; Gray, William Robert ; Anglada-Ortiz, Griselda ; Manno, Clara ; Grelaud, Michael ; Incarbona, Alessandro ; Rae, James William Buchanan ; Subhas, Adam V. ; Pallacks, Sven ; White, Angelicque ; et al ( February 2023 , Nature Communications)

   Planktonic calcifying organisms play a key role in regulating ocean carbonate chemistry and atmospheric CO₂. Surprisingly, references to the absolute and relative contribution of these organisms to calcium carbonate production are lacking. Here we report quantification of pelagic calcium carbonate production in the North Pacific, providing new insights on the contribution of the three main planktonic calcifying groups. Our results show that coccolithophores dominate the living calcium carbonate (CaCO₃) standing stock, with coccolithophore calcite comprising ~90% of total CaCO₃production, and pteropods and foraminifera playing a secondary role. We show that pelagic CaCO₃production is higher than the sinking flux of CaCO₃at 150 and 200 m at ocean stations ALOHA and PAPA, implying that a large portion of pelagic calcium carbonate is remineralised within the photic zone; this extensive shallow dissolution explains the apparent discrepancy between previous estimates of CaCO₃production derived from satellite observations/biogeochemical modeling versus estimates from shallow sediment traps. We suggest future changes in the CaCO₃cycle and its impact on atmospheric CO₂will largely depend on how the poorly-understood processes that determine whether CaCO₃is remineralised in the photic zone or exported to depth respond to anthropogenic warming and acidification.

    

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3. 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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4. The Dissolution Rate of CaCO 3 in the Ocean

   https://doi.org/10.1146/annurev-marine-041720-092514  

   Adkins, Jess F. ; Naviaux, John D. ; Subhas, Adam V. ; Dong, Sijia ; Berelson, William M. ( January 2021 , Annual Review of Marine Science)

   null (Ed.)

   The dissolution of CaCO 3 minerals in the ocean is a fundamental part of the marine alkalinity and carbon cycles. While there have been decades of work aimed at deriving the relationship between dissolution rate and mineral saturation state (a so-called rate law), no real consensus has been reached. There are disagreements between laboratory- and field-based studies and differences in rates for inorganic and biogenic materials. Rates based on measurements on suspended particles do not always agree with rates inferred from measurements made near the sediment–water interface of the actual ocean. By contrast, the freshwater dissolution rate of calcite has been well described by bulk rate measurements from a number of different laboratories, fit by basic kinetic theory, and well studied by atomic force microscopy and vertical scanning interferometry to document the processes at the atomic scale. In this review, we try to better unify our understanding of carbonate dissolution in the ocean via a relatively new, highly sensitive method we have developed combined with a theoretical framework guided by the success of the freshwater studies. We show that empirical curve fits of seawater data as a function of saturation state do not agree, largely because the curvature is itself a function of the thermodynamics. Instead, we show that models that consider both surface energetic theory and the complicated speciation of seawater and calcite surfaces in seawater are able to explain most of the most recent data.This new framework can also explain features of the historical data that have not been previously explained. The existence of a kink in the relationship between rate and saturation state, reflecting a change in dissolution mechanism, may be playing an important role in accelerating CaCO 3 dissolution in key sedimentary environments. 

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5. 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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