37. Toward improved modelling of CaCO3 dissolution in seawater

Jim E. Greenwood (1)*, Victor W. Truesdale (2)

1 CSIRO Oceans and Atmosphere, Perth, WA, 6014, Australia.
2 School of Life Sciences, Oxford-Brookes University, Oxford, OX3OBP, UK.

Background
Accurate representation of the dissolution kinetics of calcium carbonate in seawater is needed to reduce uncertainty in ocean carbon models. The dissolution mechanism is known to be complex, and despite having been studied over several decades, substantial disagreements remain as to the correct form, and accompanying parameter values, of the necessary rate equation. The use of a number of different experimental approaches to control the chemical background of the dissolving solution, and the application of several different models, has added to the confusion.

Methods
We apply a commonly used model of calcium carbonate dissolution to the Hawaiian Ocean Time-series site in the North Pacific, and test the sensitivity of calcite export to the suggested parameter values. We also present some new laboratory results for the dissolution of calcite and compare them against previous studies.

Findings
We find that large uncertainties in the export of CaCO3 can arise from the use of the most commonly used model of CaCO3 dissolution in the ocean, depending on the choice of parameter values reported by different workers. Meanwhile, our laboratory experiments confirm that the dissolution of calcite is non-ideal with respect to existing models. A new model that takes account of changes in surface reactivity is tested.

Conclusions
The activity of determining empirical rates for fieldwork purposes from arbitrary curve fittings to laboratory plots of rate versus concentration data has limited value. Instead, much more detailed experimentation is needed. We recommend that this follows strict guidelines recommended for chemical kinetics study.