73. Constraining net calcification/dissolution in the open ocean using high precision ID measurement of dissolved Ca/Mg ratios in seawater

Ellen V Cliff (1)*, Stephen M Eggins (1), Linda McMorrow (1), Leslie Kinsley (1)

1 Research School of Earth Sciences, The Australian National University, Canberra, 2601, Australia

Background
Laboratory and field studies indicate higher pCO2 waters can effect calcium carbonate shell formation by marine calcifiers such as pteropods, foraminifers and coccolithophores. However, the larger scale effects of changing ocean chemistry on calcium carbonate production and dissolution are less well understood. Changes in dissolved Ca content of seawater has the potential to be used, without ambiguity, to constrain amounts and rates of change of net calcium carbonate production (dissolution) in both the surface and deep oceans.

Methods
We have developed a straightforward, isotope-dilution method for determining dissolved Ca and Mg concentrations in seawater with high precision and accuracy. A single 43Ca and 25Mg enriched spike, designed to minimise weighing errors, is added to seawater samples, which are then diluted and analysed by MC-ICPMS. Measured Ca/Mg molar ratios are used to calculate the excess or deficiency in Ca relative to a source (reference) water composition, assuming Mg is conserved in open ocean samples. This Ca excess/deficiency constrains the net amount of calcification/dissolution that has occurred in the sample relative to source water.

Findings
We have commenced applying the approach to a North-South transect of the Australian sector of the Southern Ocean (SR03 and SAZ lines), in an effort to constrain changes in Ca/Mg relative to upper circumpolar deep water, in surface waters, Antarctic intermediate water and Sub-Antarctic mode waters. We aim to apply and integrate the technique with oceanographic and carbon system observations in an effort to estimate current rates of net calcification and dissolution in Southern Ocean and to search for changes that may have taken place over the past few decades.

Conclusions
We have developed a method for determining the amount and rate of net calcification/dissolution in the ocean that is based on highly precise and accurate analyses of dissolved Ca and Mg in seawater.