Isotopic Composition of Cadmium across the Subtropical Convergence in the Southern Ocean

The almost linear relationship between cadmium (Cd) and phosphate (PO₄) in the oceans has been used extensively as a paleoproxy to investigate historical nutrient cycling (Boyle, 1992, Annu. Rev. Earth Planet. Sci. 20, 245). However, our limited understanding of oceanic Cd cycling and the mechanism of the Cd/PO₄ relationship limits the reliability of this paleoproxy (Elderfield and Rickaby, 2000, Nature. 405, 305), particularly in the surface waters of the Southern Ocean. Recent studies have shown that the biological uptake of Cd by phytoplankton can create significant (permil-level) isotopic shifts between the phytoplankton and surface ocean water (Ripperger and Rehkamper, 2007, Earth Planet. Sci. Lett. 261, 670) suggesting that Cd isotopes have the potential to provide more information about the marine cycling of Cd than concentration measurements alone. The subtropical convergence (STC) in the Southern Ocean has some of the world’s lowest Cd:PO₄ ratios (Frew and Hunter, 1995, Mar. Chem. 51, 223) and provides an ideal location to study the biogeochemical cycling of Cd in the marine environment. Utilizing double-spiking protocols combined with multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) (Ripperger and Rehkamper, 2007, Geochim. Cosmochim. Acta. 71, 631), we have determined the isotopic composition of Cd in seawater samples collected from the Otago transect off the southeast coast of New Zealand. This transect intersects the STC, passing through both subtropical (iron replete) and subantarctic (HNLC) water masses. Samples were collected seasonally from May 2008 through January 2010 to identify seasonal variability of isotopic composition and concentration due to changes in water mass chemistry and biological activity. This Cd isotope data will help determine the mechanisms controlling Cd uptake in the Southern Ocean and provide insight into the effectiveness of the current Cd/PO₄ paleoproxy helping to improve its application.