Synthesis of In Situ Marine Calcium Carbonate Dissolution Kinetic Measurements in the Water Column

Abstract

Calcium carbonate (CaCO₃) dissolution is an integral part of the ocean's carbon cycle. However, laboratory measurements and ocean alkalinity budgets disagree on the rate and loci of dissolution. In situ dissolution studies can help to bridge this gap, but so far published studies have not been utilized as a whole because they have not previously been compiled into one data set and lack carbonate system data to compare between studies. Here, we compile all published measurements of CaCO₃ dissolution rates in the water column (11 studies, 752 data points). Combining World Ocean Atlas data (temperature, salinity) with the neural network CANYON-B (carbonate system variables), we estimate seawater saturation state (Ω) for each rate measurement. We find that dissolution rates at the same Ω vary by 2 orders of magnitude. Using a machine learning approach, we show that while Ω is the main driver of dissolution rate, most variability can be attributed to differences in experimental design, above all bias due to (diffusive) transport and the synthetic or biogenic nature of CaCO₃. The compiled data set supports previous findings of a change in the mechanism driving dissolution at Ω_crit = 0.8 that separates two distinct dissolution regimes: r_slow = 0.29 · (1 − Ω)^0.68(±0.16) mass% day⁻¹ and r_fast = 2.95 · (1 − Ω)^2.2(±0.2) mass% day⁻¹. Above the saturation horizon, one study shows significant dissolution that cannot solely be explained by established theories such as zooplankton grazing and organic matter degradation. This suggests that other, non-biological factors may play a role in shallow dissolution.