Award: OCE-1559215

We developed and deployed a new instrument to measure the dissolution rate of CaCO3 in the ocean’s water column. Our new measurements match those generated in the lab, confirming, for the first time, that the same mechanisms diagnosed for dissolution of carbonate minerals in freshwaters are at work in the ocean as well. There is a critical value of undersaturation, about 10% below equilibrium, where the surface mechanism of dissolution switches from step-edge retreat to defect assisted pit formation. This is true for all carbonates in the ocean, inorganic and biogenic and it greatly increases the rate, giving rise to the non-linear rate laws long observed by the rest of the community. However, our in situ rates are too slow for the attack of undersaturated seawater to generate the signal of excess alkalinity seen in the North Pacific water column. Instead, we find both solid phase and dissolved tracer evidence for carbonate dissolution driven by the acid generated from organic matter respiration. This intimate coupling of acid (CO2) and base (CaCO3) in the biologically produced carbonates of the ocean creates the opportunity for carbonate to dissolve even when the surrounding waters would favor precipitation instead. This new discovery of a shallow alkalinity cycle shortens thee timescale required for the ocean-atmosphere system to adjust to large inputs of new CO2 from either natural or anthropogenic sources. Our training of graduate students and post-docs, both in the lab and out at sea, has paved the way for a new generation of scientists to take up the questions of carbonate dissolution’s role in past climate changes, the modern carbon cycle, and as a route to CO2 removal at a global scale. Our new results confirm that ocean alkalinity addition is a viable pathway for creating a negative CO2 emissions economy by storing carbon as the harmless bicarbonate ion permanently in the ocean. Last Modified: 08/08/2020 Submitted by: Jess F Adkins