Ferromanganese crusts as archives of deep water Cd isotope compositions

Horner, T. J. and Schönbächler, M. and Rehkämper, M. and Nielsen, S. G. and Williams, H. M. and Halliday, A. N. and Xue, Z. and Hein, J. R. (2010) Ferromanganese crusts as archives of deep water Cd isotope compositions. Geochemistry, Geophysics, Geosystems, 11 (4). ISSN 15252027 DOI 10.1029/2009GC002987

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Abstract

The geochemistry of Cd in seawater has attracted significant attention owing to the nutrient-like properties of this element. Recent culturing studies have demonstrated that Cd is a biologically important trace metal that plays a role in the sequestration of inorganic carbon. This conclusion is supported by recent isotope data for Cd dissolved in seawater and incorporated in cultured phytoplankton. These results show that plankton features isotopically light Cd while Cd-depleted surface waters typically exhibit complimentary heavy Cd isotope compositions. Seawater samples from below 900 m depth display a uniform and intermediate isotope composition of ε114/110Cd = +3.3 ± 0.5. This study investigates whether ferromanganese (Fe-Mn) crusts are robust archives of deep water Cd isotope compositions. To this end, Cd isotope data were obtained for the recent growth surfaces of 15 Fe-Mn crusts from the Atlantic, Pacific, Indian, and Southern oceans and two USGS Fe-Mn reference nodules using double spike multiple collector inductively coupled plasma mass spectrometry. The Fe-Mn crusts yield a mean ε114/110Cd of +3.2 ± 0.4 (2 SE, n = 14). Data for all but one of the samples are identical, within the analytical uncertainty of ±1.1ε114/110Cd (2 SD), to the mean deep water Cd isotope value. This indicates that Fe-Mn crusts record seawater Cd isotope compositions without significant isotope fractionation. A single sample from the Southern Ocean exhibits a light Cd isotope composition of ε114/110Cd = 0.2 ± 1.1. The origin of this signature is unclear, but it may reflect variations in deep water Cd isotope compositions related to differences in surface water Cd utilization or long-term changes in seawater ε114/110Cd. The results suggest that time series analyses of Fe-Mn crusts may be utilized to study changes in marine Cd utilization.

Item Type: Article
Uncontrolled Keywords: NILAREP
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
08 - Green Open Access
Journal or Publication Title: Geochemistry, Geophysics, Geosystems
Volume: 11
Identification Number: 10.1029/2009GC002987
Depositing User: Sarah Humbert
Date Deposited: 20 Jul 2016 13:34
Last Modified: 21 Jul 2016 09:35
URI: http://eprints.esc.cam.ac.uk/id/eprint/3693

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