Graduate School of NOC PhD Project Description

Eric Achterberg, Toby Tyrrell, David Hydes: New Approaches to Determine CaCO3 Saturation in Seawater

Suitable for graduates with degrees in: Oceanography/Marine Science, Chemistry, Physics or Engineering

Keywords: Carbonate chemistry, saturation state, ocean acidification

Rationale

Experiments in artificial solutions show that dissolution/precipitation of inorganic CaCO3 is controlled by saturation state: Ω = [Ca2+][CO32-]/Ksp where Ksp is the solubility product, [Ca2+] the concentration of calcium, [CO32-] of carbonate. Ω predicts CaCO3 dynamics well in artificial solutions, but less well in real seawater. Ω is >1 (between 1.5 and 7) in surface oceans around the world, suggesting strong supersaturation, but yet spontaneous inorganic precipitation (‘whitings’) rarely occurs. Also, the depth below which CaCO3 starts to disappear from seafloor sediments due to dissolution (‘the lysocline’) lies below the depth at which Ω=1. Some of these discrepancies may be attributed to kinetics, but other factors are also important: magnesium, organic coatings on biogenic CaCO3, and DOM. Together they diminish the usefulness of Ω in elucidating real-world CaCO3 dynamics. Such inaccuracies are especially important as we study ocean acidification

Methodology

In this PhD project a combination of chemical laboratory and fieldwork experiments, with subsequent model simulations will be undertaken. In the laboratory, new techniques for more accurate determinations of the carbonate saturation state of seawater will be developed. These techniques will subsequently be applied in natural seawaters during oceanic cruises (e.g Portsmouth-Santander Ferrybox cruises; Extended Ellett Line cruises). Water column measurements obtained using the novel techniques will be compared with carbonate system measurements and Ω calculations using conventional approaches. The outcomes of the novel CaCO3 saturation state measurements will be used as input to model simulations of current and future ocean saturation states.

Training

You will be trained in methods and techniques for the analysis of dissolved inorganic carbon concentrations in seawater. These will include measurements of pH, pCO2, DIC and alkalinity, the latter two on a VINDTA system. You will gain hands-on experience with the FerryBox team on the ‘Pride of Bilbao’, which you will use for initial sea tests of the technique. In this established programme carbonate system parameters are measured underway in the English Channel and Bay of Biscay during monthly manned crossings. You will be trained in techniques for carbonate modelling using supplied box models. By the end of the PhD you will have been trained in and become familiar with work practices both in state-of-the-art laboratories and on research vessels.

Wider implications

By allowing more accurate determinations of the degree of saturation of seawater with respect to calcium carbonate, your work will lead to major improvements in our understanding of calcium carbonate cycling in the oceans, which will in turn lead to advances in models of the ocean carbon cycle. It will also contribute to elucidation of whether coccolithophores, coral reefs, etc will become scarce in future low pH oceans. Your work will help improve our understanding of the probable fate of chalk-shelled organisms in the increasingly acidic future ocean.

Background reading

Pilson MEQ (1998) An Introduction to the Chemistry of the Sea. Prentice-Hall Inc. (see section 7.3)
Broecker WS (2003) The Oceanic CaCO3 Cycle. In: Treatise on Geochemistry, Volume 6. Editor: Henry Elderfield. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 625. Elsevier, p.529-549
Peterson, L.C., Deep-sea sediment: Calcium carbonates. In: Steele J. H., Thorpe S. A. and Turekian K.K. (Eds.), Encyclopedia of Ocean Sciences, vol. 1, Academic Press, London: 359-368.
Zeebe Re & Wolf-Gladrow D (2001) CO2 in Seawater: Equilibrium, Kinetics, Isotopes. Elsevier

More information

This project is associated with NOCS research in Ocean Biogeochemistry & Ecosystems

eric@noc.soton.ac.uk Pages served at NOCS