Calcium Carbonate Dissolution Mechanisms in the Ocean

The rate at which calcium carbonate dissolves in seawater is an important piece of the ocean carbon cycle. It is a key factor in the oceans’ ability to absorb fossil fuel carbon dioxide. It is also important to understanding and predicting ocean acidification and may play a role in new technologies to curb climate change. Despite this importance, a great deal remains uncertain about the mechanism and rate of calcium carbonate dissolution. This project will conduct laboratory experiments using new methods to study calcium carbonate dissolution at several different conditions. The project will support two early-career investigators including a graduate student. Results will be shared with the public, including through support of an animation artist. In previous work, this team and their collaborators have made several improvements to our understanding of the carbonate dissolution rate law in the ocean. Using a novel carbon isotope tracer technique in the lab, and in situ in the water column, they can measure the dissolution rate 20-200x more sensitively than other techniques. This is especially true near equilibrium where rates are slow, but also where most of the ocean resides. Their previous work demonstrated that the rate law has two critical thresholds between three separate modes of surface dissolution mechanisms: near equilibrium, dissolution proceeds as the advance of step edges; with further undersaturation, the free energy in solution opens fast-dissolving etch pits at lattice defects in the solid; furthest from equilibrium, etch pits begin to form homogeneously on the 2-D solid surface leading to maximal dissolution rates. However, understanding of the full chemical control on the rate law, and its interplay with these surface energetic features has yet to be determined. This project will seek to experimentally and theoretically constrain the mechanism behind the rate law for marine carbonate dissolution. The saturation state will be systematically changed as a function of other aspects of the chemistry to test the elementary reactions that have been successfully used for years to describe dissolution in freshwaters. The saturation state will be varied over a range of sulfate concentrations, calcium/carbonate ratios, and magnesium/calcium ratios in independent experiments. Both the initial time dependence of the raw data, and SIMS microprobe profiles of the experimental solids will allow determination of the gross precipitation and gross dissolution rates in conjunction with the net rate constraints from slopes of dissolved carbon isotope ratios produced versus time. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.