Isotopic evidence for complex biogeochemical cycling of Cd in the eastern tropical South Pacific

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Title: Isotopic evidence for complex biogeochemical cycling of Cd in the eastern tropical South Pacific
Authors: Xie, RC
Rehkamper, M
Grasse, P
Van de Flierdt, T
Frank, M
Xue, Z
Item Type: Journal Article
Abstract: Over the past decades, observations have confirmed decreasing oxygen levels and shoaling of oxygen minimum zones (OMZs) in the tropical oceans. Such changes impact the biogeochemical cycling of micronutrients such as Cd, but the potential consequences are only poorly constrained. Here, we present seawater Cd concentrations and isotope compositions for 12 depth profiles at coastal, nearshore and offshore stations from 4ºS to 14ºS in the eastern tropical South Pacific, where one of the world’s strongest OMZs prevails. The depth profiles of Cd isotopes display high δ114/110 Cd at the surface and decreasing δ114/110 Cd with increasing water depth, consistent with preferential utilization of lighter Cd isotopes during biological uptake in the euphotic zone and subsequent remineralization of the sinking biomass. In the surface and subsurface ocean, seawater displays similar δ114/110 Cd signatures of 0.47 ±0.23‰ to 0.82±0.05‰ across the entire eastern tropical South Pacific despite highly variable Cd concentrations between 0.01 and 0.84 nmol/kg. This observation, best explained by an open system steady-state fractionation model, contrasts with previous studies of the South Atlantic and South Pacific Oceans, where only Cd-deficient waters have a relatively constant Cd isotope signature. For the subsurface to about 500 m depth, the variability of seawater Cd isotope compositions can be modeled by mixing of remineralized Cd with subsurface water from the base of the mixed layer. In the intermediate and deep eastern tropical South Pacific (>500 m), seawater [Cd] and δ114/110 Cd appear to follow the distribution and mixing of major water masses. We identified modified AAIW of the ETSP to be more enriched in [Cd] than AAIW from the source region, whilst both water masses have similar δ114/110 Cd. A mass balance estimate thus constrains a δ114/110 Cd of between 0.38‰ and 0.56‰ for the accumulated remineralized Cd in the ETSP. Nearly all samples show a tight coupling of Cd and PO4 concentrations, whereby surface and deeper waters define two distinct linear trends. However, seawater at a coastal station located within a pronounced plume of H2S, is depleted in [Cd] and features significantly higher δ114/110 Cd. This signature is attributed to the formation of authigenic CdS with preferential incorporation of lighter Cd isotopes. The process follows a Rayleigh fractionation model with a fractionation factor of α114/110 Cdseawater-CdS = 1.00029. Further deviations from the deep Cd-PO4 trend were observed for samples with O2 < 10 µmol/kg and are best explained by in situ CdS precipitation within the decaying organic matter even though dissolved H2S was not detectable in ambient seawater.
Issue Date: 15-Apr-2019
Date of Acceptance: 1-Feb-2019
DOI: 10.1016/j.epsl.2019.02.001
ISSN: 0012-821X
Publisher: Elsevier
Start Page: 134
End Page: 146
Journal / Book Title: Earth and Planetary Science Letters
Volume: 512
Copyright Statement: © 2019 Elsevier Ltd. All rights reserved. This manuscript is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International Licence
Sponsor/Funder: Natural Environment Research Council (NERC)
Natural Environment Research Council (NERC)
Funder's Grant Number: NE/H005390/1
Keywords: Science & Technology
Physical Sciences
Geochemistry & Geophysics
dissolved Cd stable isotopes
eastern tropical South Pacific
oxygen minimum zone
steady state fractionation model
Cd depletion
02 Physical Sciences
04 Earth Sciences
Geochemistry & Geophysics
Publication Status: Published
Embargo Date: 2020-02-19
Online Publication Date: 2019-02-19
Appears in Collections:Faculty of Engineering
Earth Science and Engineering

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