Mechanistic Descriptions of Calcite Dissolution in Seawater

Author: Barnhart, Holly Ann

Year: 2027

Degree: Dissertation (Ph.D.)

Advisor: Adkins, Jess F.

Committee Members: Eiler, John M.; Berelson, William M.; Zakem, Emily J.; Adkins, Jess F.

Option: Geochemistry

Abstract

Calcium carbonate minerals in the ocean regulate atmospheric CO₂ through carbonate compensation. Excess atmospheric CO₂ taken up by the ocean drives a decrease in pH, promoting the dissolution of carbonate minerals, primarily at the seafloor. Carbonate mineral dissolution increases ocean alkalinity and subsequently rebalances pH, allowing for further CO₂ absorption. This process regulates Earth's climate on glacial-interglacial timescales and is likely important in the modern ocean as anthropogenic emissions drive a rapid increase in atmospheric CO₂.

The canonical timescale for carbonate compensation is roughly 6,000 years and is set largely by ocean circulation, bioturbation and the dissolution kinetics of carbonate minerals. Disagreements about the kinetics of calcite dissolution drive some of the largest uncertainties in the estimate of this timescale. Recent advancements have improved our mechanistic understanding of calcite dissolution kinetics, helping to reconcile disagreements in the field, but the picture is far from complete. In particular, there is debate around the degree of saturation dependence for rates close to equilibrium, where much of the ocean sits. Likewise, processes controlling dissolution rates in marine sediments are poorly understood. From a mechanistic perspective, there has been little work exploring how the major ion composition of any specific species in seawater conspires to set dissolution rates. The work presented here aims to fill these critical gaps in the description of calcite dissolution kinetics.

First, I revisit the experiments of Naviaux et al. (2019), that measured the temperature dependence of calcite dissolution kinetics at very high resolution using an isotope labeling technique. This method has faced criticisms due to its lack of sensitivity in differentiating net rates from gross rates, sometimes termed "isotope exchange". I reinterpret the Naviaux et al. (2019) dataset using a model, reformulated from Subhas et al. (2019), that explicitly tracks gross dissolution rates and the size of the mineral surface reservoir. I illustrate how the nonlinear signal of dissolution can be parsed into gross dissolution and gross precipitation rates, capturing the “isotope exchange” signal while still producing net dissolution rates. I show that the Naviaux et al. (2019) method for calculating net rates is highly sensitive to the duration of experiments and whether they achieve isotopic steady state. The model also provides insights about the undersaturation dependence of gross rates and the size of the reactive surface reservoir.

Next, I modify the model for application in interpreting novel calcite dissolution experiments performed in situ in marine porewaters. These experiments represent the first direct measurements of calcite dissolution rates in marine sediments. At some sites, 2-10 cm below the sediment water interface, we measured undersaturation values and dissolution rates much higher than predicted from the overlying bottom water. Evidence from chemical profiles of alkalinity, DIC, pH and Mn in the same porewaters suggest that high dissolution rates can be driven by the products of aerobic respiration of organic matter and/or the products of reduced matter oxidation. Both processes lead to a build up of acidic porewater conditions and can drive undersaturation in sediments below their bottom water values. We also note that the porewater dissolution rates close to equilibrium are faster than the rates observed in benchtop experiments. We invoke a previously noted kinetic pressure effect to explain these elevated rates. Regardless of offset, the porewater dissolution rates exhibit a strong, nonlinear dependence on undersaturation that is largely consistent with the behavior of laboratory and water column dissolution experiments.

Finally, I studied the influence of sulfate on the rate of calcite dissolution in seawater. Compared to seawater with modern sulfate concentrations, calcite in sulfate-free seawater dissolves faster at high undersaturations but slower near equilibrium. The change in sign arises from a higher sensitivity to undersaturation far from equilibrium that corresponds to a higher free energy for forming etch pits. In parallel, we interpret the rate behavior in terms of the relative stability of the chemical species formed on the mineral surface. The presence or absence of sulfate modulates the surface stability differently close to and far from equilibrium in a manner consistent with our rate observations.