Nick E. Rollins

Catalysis and chemical mechanisms of calcite dissolution in seawater

Significance The experimental system described here provides constraints on the relative balance of gross dissolution and precipitation fluxes contributing to the observed net dissolution rate of calcite in seawater. We show that our dissolution rates fit well within a framework that accounts for the geometry of the dissolving mineral surface. We further show that carbonic anhydrase (CA) catalyzes calcite dissolution, which implicates the hydration of aqueous CO2 as a rate-limiting step for calcite dissolution in seawater. The presence of carbonic anhydrase in carbonate-rich environments such as coral reefs or sinking marine particles is poorly understood. However, our findings suggest that CA activity would significantly enhance the rate at which alkalinity is cycled between solids and seawater in these environments. Near-equilibrium calcite dissolution in seawater contributes significantly to the regulation of atmospheric CO2 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 13C-labeled calcites in natural seawater. We show that the time-evolving enrichment of 𝜹13C 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 13C-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 CO2 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.

Meter-Scale Early Diagenesis of Organic Matter Buried Within Deep-Sea Sediments Beneath the Amazon River Plume

Gravity cores and multi-cores were collected from the Demerara Abyssal plain to examine meter-scale downcore features of early diagenesis in the sediments and relate them to the location of the Amazon River plume in the North Atlantic Ocean. At all sites, the oxygen penetration depth, inferred from nitrate and dissolved manganese profiles, was ~10-20 cm and nitrate was depleted within ~50 cm. However, most of the cores also had a secondary nitrate maximum (4-13µM) at ~50 cm, at a location where we observed changes in gradients of dissolved manganese, iron, and ammonium. Although there is spatial heterogeneity in the profile behavior across the study, we do find subtle diagenetic profile patterns that occur in sediments in relation to their position below the Amazon plume. Dissolved silica profiles show an initial increase downcore, but then all show a decrease to depths of 30-100 cm, thereafter concentrations increase. We suggest this zone of silica uptake is due to reverse weathering processes, possibly involving iron oxidation. A semi-lithified iron crust appeared at nearly all sites, and its position is relict, likely an indicator of the transition from glacial to interglacial sediments.