Xie, Ruifang C , Rehkämper, Mark , Grasse, Patricia , van de Flierdt, Tina , Frank, Martin and Xue, Zichen (2019) Isotopic evidence for complex biogeochemical cycling of Cd in the eastern tropical South Pacific. Earth and Planetary Science Letters, 512 . pp. 134-146. DOI 10.1016/j.epsl.2019.02.001.

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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 4◦S to 14◦S in the eastern tropical South Pacific, where one of the world’s strongest
OMZs prevails.
The depth profiles of Cd isotopes display high δ114/110Cd at the surface and decreasing δ114/110Cd 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 δ114/110Cd 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 δ114/110Cd 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 δ114/110Cd. A mass balance estimate thus constrains
a δ114/110Cd 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 δ114/110Cd. 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 α114/110Cdseawater-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.

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