Cadmium Isotope Variations in the Oceans

A number of previous studies have identified considerable mass dependent variations in the Cd isotope compositions of both terrestrial and extraterrestrial samples. On Earth, stable isotope effects for Cd are particularly prominent in the oceans, and the largest natural terrestrial Cd isotope fractionations of about 4‰ have been reported for Cd-depleted surface seawater. These effects have generally been attributed to reflect isotope fractionation of Cd that occurs during biological uptake and utilization of dissolved seawater Cd. This finding confirms studies, which identified Cd as an essential marine micronutrient. This was first inferred from the phosphate-like distribution of the metal in the oceans and more recently demonstrated by work, which confirmed that Cd can act as catalytic metal ion in carbonic anhydrase, an enzyme which plays a central role in inorganic carbon acquisition of phytoplankton in the oceans. The marine Cd isotope fractionations are thus of interest, as they can be used to study the cycling of the micronutrient Cd as well as its impact on ocean productivity and the global carbon cycle. As part of this PhD project, I have developed a new procedure for Cd isotope analyses of seawater, which is suitable for samples as large as 20 L and Cd concentrations as low as 1 pmol/L. The procedure involves use of a 111Cd-113Cd double spike, co-precipitation of Cd from seawater with Al(OH)3 Cd purification by column chromatography, and subsequent isotope analysis by MC-ICP-MS (multiple collector inductively coupled plasma mass spectrometry). The methodology can routinely provide Cd data with a precision of about ±0.5 (2sd) when at least 20-30 ng of natural Cd are available for analysis. However, even seawater samples with Cd contents of only 1-3 ng can be analysed with a reproducibility of about ±3 to ±5 .