HR: 15:25h
AN: PP33D-08    [Abstracts]
TI: Cadmium Isotope Fractionation in Seawater - A Signature of Nutrient Utilization
AU: * Wichtlhuber, S
EM: wichtlhuber@erdw.ethz.ch
AF: Institute of Isotope Geology and Mineral Resources, ETH Zuerich, Sonneggstrasse 5, Zurich, 8032 Switzerland
AU: Rehkaemper, M
EM: markrehk@imperial.ac.uk
AF: Department of Earth Science and Engineering, Imperial College, South Kensington Campus, London, SW7 2AZ United Kingdom
AU: Halliday, A N
EM: alex.halliday@earth.ox.ac.uk
AF: Department of Earth Sciences, University of Oxford, Parks Road, Oxford, OX1 3PR United Kingdom
AB: Cadmium displays a nutrient-like distribution akin to phosphorous in the oceans. This has been attributed to the assimilation of Cd by phytoplankton in surface waters and re-mineralization at depth. If biological uptake is associated with kinetic isotopic fractionation, as recently suggested by Lacan et al. (2005), then the Cd-depleted surface waters of the oceans (with Cd contents of < 0.08 nmol/kg) should be depleted in the "light" isotopes of Cd, relative to the bottom waters, which typically have Cd concentrations of 0.2 to 1 nmol/kg. Previous investigations were, however, unable to identify any significant Cd isotope effects in either seawater samples or sedimentary rocks (Wombacher et. al, 2003; Lacan et al., 2005). In this study, we have extended the search for Cd isotope variations in the oceans with analyses of two depth profiles and various additional seawater samples from the North Pacific, the Arctic, and the Southern Ocean. The Cd isotope measurements utilized a double spike technique in conjunction with multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS), to achieve a precision and accuracy of about ± 0.8 to 1.5 &eps;^114/110Cd. This precision is about a factor of 3 to 4 better than that of previous studies, which did not utilize a double spike. The data collected for the samples display a clear co-variation of Cd isotope compositions with Cd concentrations. The most Cd-rich water samples (with ~1 nmol/kg Cd) display the "lightest" Cd isotope compositions with &eps;^114/110Cd ~ +3, akin to results previously obtained for crustal and mantle rocks (Wombacher et. al, 2003). In contrast, samples from the upper water column of the North Pacific (with < 0.02 nmol/kg Cd) have the heaviest Cd isotope compositions with &eps;^114/110Cd values of up to +35. To a first order, the Cd isotope and concentration data can be accounted for with a simple, single-stage Rayleigh fractionation model that applies a fractionation factor of about 1.0002 to 1.0005. The isotopic variations observed for seawater are likely to reflect isotope fractionation during uptake of Cd by phytoplankton, as recently reported by Lacan et al. (2005), because inorganic geological processes (other than evaporation/condensation) do not appear to generate isotope effects as large as those observed in the present study (Wombacher et. al, 2003). These preliminary results suggest that Cd isotopes have the potential to become a useful proxy of nutrient utilization, which could supplement the Cd/Ca and δ¹³C records of previous studies, if suitable sedimentary archives can be identified that preserve the Cd isotope signatures of past seawater.
References:
Lacan F., Francois R., Ji Y. and Sherrell R., 2005. Does oceanic productivity production lead to a cadmium isotope fractionation? Geophys. Res. Abstr. 7, 07657.
Wombacher F., Rehkämper M., Mezger K. and Münker C., 2003. Stable isotope compositions of cadmium in geological materials and meteorites determined by multiple collector-ICPMS. Geochim. Cosmochim. Acta, 67, 4639-4654.
DE: 1050 Marine geochemistry (4835, 4845, 4850)
DE: 4845 Nutrients and nutrient cycling (0470, 1050)
DE: 4875 Trace elements (0489)
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005