A variety of recent work suggests that biogenic calcium carbonate (CaCO3) is dissolving at shallower depths in the water column than had been conventionally thought. It has been speculated that this is occurring as a result of biologically mediated processes and/or from the dissolution of aragonite. Moreover, the oceanic uptake of anthropogenic CO2 has resulted in the shoaling of the aragonite and calcite saturation horizons compared to pre-industrial depths, implying that the dissolution of carbonate particles will increase over time. This has significant implications for our understanding of the non-steady state marine carbon cycle.
In this study researchers at the California State University at San Marcos, Scripps Institution of Oceanography, and the University of South Florida will measure dissolution rates of fresh biogenic carbonates directly in natural seawater under simulated in situ conditions of temperature and pressure. They will also investigate some of the processes - both biologically-mediated ones and those simply dependent on bulk CO2 concentrations - that may influence their dissolution within the upper water column (<1500 m). The experiments will use CaCO3 produced by coccolithophores (single-celled algae) that secrete calcite plates (coccoliths) that interlock to form a covering on the cell surface and by euthecosomatous pteropods (molluscs) that produce protective shells of aragonite. These groups of organisms account for nearly all of the export flux of CaCO3 in our North Pacific study area.
The interdisciplinary approach will build on the team members individual knowledge of both carbonate biology and chemistry and will employ a combination of methods/measurements involving radioactive tracers, CO2 chemistry including a high-pressure pH cell constructed especially for this work, and SEM to document and quantify dissolution. In addition to a series of rigorously controlled laboratory experiments, the team will conduct field studies on the CLIVAR repeat line 16N and on three annual cruises on the Canadian Line P time-series in the subarctic Pacific, performing experiments on freshly collected samples that parallel our laboratory studies. The at-sea dissolution measurement system will have state-of-the-art capabilities with respect to precision, sample size, and a capability for observation at in situ saturation conditions.
The use of fresh biogenic carbonates with no prior dissolution history or degradation of organic protective coatings coupled with two highly sensitive methods (spectrophotometric pH determinations and radioactive 45Ca) should enable these researchers to make high-quality measurements of dissolution rates at near-equilibrium conditions that occur in the upper ocean.
In terms of broader impacts, the project would involve international collaboration with Canadian colleagues. It would also make provision for the training and support of graduate and undergraduate (RUI) students.
