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Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric CO 2 on 1,000-y timescales. Despite many studies on far-from-equilibrium dissolution, little is known about the detailed mechanisms responsible for calcite dissolution in seawater. In this paper, we dissolve 13 C-labeled calcites in natural seawater. We show that the timeevolving enrichment of δ 13 C in solution is a direct measure of both dissolution and precipitation reactions across a large range of saturation states. Secondary Ion Mass Spectrometer profiles into the 13 C-labeled solids confirm the presence of precipitated material even in undersaturated conditions. The close balance of precipitation and dissolution near equilibrium can alter the chemical composition of calcite deeper than one monolayer into the crystal. This balance of dissolution-precipitation shifts significantly toward a dissolution-dominated mechanism below about Ω = 0.7. Finally, we show that the enzyme carbonic anhydrase (CA) increases the dissolution rate across all saturation states, and the effect is most pronounced close to equilibrium. This finding suggests that the rate of hydration of CO 2 is a rate-limiting step for calcite dissolution in seawater. We then interpret our dissolution data in a framework that incorporates both solution chemistry and geometric constraints on the calcite solid. Near equilibrium, this framework demonstrates a lowered free energy barrier at the solid-solution interface in the presence of CA. This framework also indicates a significant change in dissolution mechanism at Ω = 0.7, which we interpret as the onset of homogeneous etch pit nucleation.mineral dissolution | isotope geochemistry | oceanography | catalysis T he production and dissolution of calcium carbonate minerals provide a crucial link between the marine carbon and alkalinity cycles. The ocean has absorbed about 25 to 30% of anthropogenic CO2 emissions, dropping mean surface ocean pH since the industrial era (1). As ocean pH decreases, sedimentary carbonate minerals will dissolve to compensate for the loss of buffering capacity, eventually restoring atmospheric pCO2 to about its preindustrial level (2, 3). This reaction will mostly take place in the deep ocean, where the calcite satura-Most of the deep ocean is only mildly undersaturated such that pelagic dissolution is primarily a near-equilibrium phenomenon. However, attempts to quantify the relationship between calcite dissolution rate and Ω are highly variable between different studies, both in functional form and absolute value (4-9).With the exception of very early work by Berner and Morse (7), few studies have attempted to unpack the chemical species responsible for calcite dissolution in seawater. Many studies choose instead to derive empirical relationships between saturation state and dissolution rate (5, 6, 10, 11). In contrast, freshwater and dilute solution dissolution studies have made large advances in identifying key chemical species responsible for observed dissolution...

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Quantifying variability in the ocean carbon sink remains problematic due to sparse observations and spatiotemporal variability in surface ocean pCO 2. To address this challenge, we have updated and improved ECCO-Darwin, a global ocean biogeochemistry model that assimilates both physical and biogeochemical observations. The model consists of an adjoint-based ocean circulation estimate from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium and an ecosystem model developed by the Massachusetts Institute of Technology Darwin Project. In addition to the data-constrained ECCO physics, a Green's function approach is used to optimize the biogeochemistry by adjusting initial conditions and six biogeochemical parameters. Over seasonal to multidecadal timescales (1995-2017), ECCO-Darwin exhibits broad-scale consistency with observed surface ocean pCO 2 and air-sea CO 2 flux reconstructions in most biomes, particularly in the subtropical and equatorial regions. The largest differences between CO 2 uptake occur in subpolar seasonally stratified biomes, where ECCO-Darwin results in stronger winter uptake. Compared to the Global Carbon Project OBMs, ECCO-Darwin has a time-mean global ocean CO 2 sink (2.47 ± 0.50 Pg C year −1) and interannual variability that are more consistent with interpolation-based products. Compared to interpolation-based methods, ECCO-Darwin is less sensitive to sparse and irregularly sampled observations. Thus, ECCO-Darwin provides a basis for identifying and predicting the consequences of natural and anthropogenic perturbations to the ocean carbon cycle, as well as the climate-related sensitivity of marine ecosystems. Our study further highlights the importance of physically consistent, property-conserving reconstructions, as are provided by ECCO, for ocean biogeochemistry studies. Plain Language Summary Data-driven estimates of how much carbon dioxide the ocean is absorbing (the so-called "ocean carbon sink") have improved substantially in recent years. However, computational ocean models that include biogeochemistry continue to play a critical role as they allow us to isolate and understand the individual processes that control ocean carbon sequestration. The ideal scenario is a combination of the above two methods, where data are ingested and then used to improve a model's fit to the observed ocean, also known as, data assimilation. While the physical oceanographic community has made great progress in developing data assimilation systems, for example, the Estimating the Circulation and Climate of the Ocean (ECCO) consortium, the biogeochemical community has generally lagged behind. The ECCO-Darwin model presented in this paper represents an important technological step forward as it is the first global ocean biogeochemistry model that (1) ingests both physical and biogeochemical observations into the model in a realistic manner and (2) considers how the nature of the ocean carbon sink has changed over multiple decades. As the ECCO ocean circulation

Knowledge of calcite dissolution kinetics in seawater is a critical component of our understanding of the changing global carbon budget. Towards this goal, we provide the first measurements of the temperature dependence of calcite dissolution kinetics in seawater. We measured the dissolution rates of 13 C-labeled calcite in seawater at 5, 12, 21, and 37°C across the full range of saturation states (0 < Ω = < 1). We show that the dissolution rate is non-linearly dependent on Ω and that the degree of non-linearity both increases with temperature, and changes abruptly at "critical" saturation states (Ωcrit). The traditional exponential rate law most often utilized in the oceanographic community, R=k(1-Ω) n , requires different fits to k and n depending upon the degree of undersaturation. Though we