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 є114=110Cd data with a precision
of about ± 0:5є (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 ± 5є. The new methodology was applied to investigate Cd isotope variations in about 150
seawater samples from the North Atlantic Ocean, the Southern Ocean HNLC (high nutrient
low chlorophyll) region, and the Peruvian oxygen minimum zone in the Eastern Pacific
Ocean. The samples exhibited variable but highly systematic Cd isotope variations that
were comprehensively interpreted in the context of previously published oceanographic
and biogeochemical data.