![]() ![]() Thus, further work is required to constrain the cycling of Cd and its isotopes in the ocean. However, deviations from the global ocean Cd/PO 4 relationship are observed in oxygen deficient zones (ODZ) of the modern ocean (e.g., van Geen et al., 1995 Janssen et al., 2014 Middag et al., 2018), deviations that are in some cases associated with Cd isotope fractionation (e.g., Conway and John 2015a). Nevertheless, dissolved Cd has a spatial distribution akin to the macronutrient phosphate (PO 4) (e.g., Boyle et al., 1976 Bruland, 1980), and Cd/Ca ratios in calcareous microfossils have been utilized as a proxy for past oceanic P distributions (e.g., Marchitto and Broecker, 2006).Ĭadmium isotopes in marine carbonates and organic-rich sediments are emerging as a potential tracer of past ocean productivity ( Georgiev et al., 2015 John et al., 2017 Hohl et al., 2017, 2019), contingent on the observation that biological uptake of Cd in the surface ocean is associated with preferential uptake of the light isotope (e.g., Lacan et al., 2006 Ripperger et al., 2007). The reason for its nutrient-type behavior is not fully understood to date, only one biological function of Cd has been identified, in which it replaces Zn in the enzyme carbonic anhydrase ( Price and Morel, 1990 Lane et al., 2005 Xu et al., 2008). ![]() These multiple possible sources of isotopically light Cd to sediments, along with evidence for complex particle cycling of Cd in the water column, bring into question the straightforward application of Cd isotopes as a paleoproductivity proxy.ĭissolved cadmium (Cd) displays typical nutrient-type behavior in the oceans (e.g., Bruland et al., 2014), despite its known toxicity (e.g., Brand et al., 1986 Waldron and Robinson, 2009). We also observe notably light Cd isotope compositions associated with elevated solid phase Fe concentrations, suggesting that scavenging of Cd by Fe oxide phases may contribute to the light Cd isotope composition of sediments. ![]() However, water column CdS precipitation is also predicted to preferentially sequester light Cd isotopes from the water column, which could obfuscate Cd as a tracer. Non-quantitative biological uptake of light Cd by phytoplankton is one possible means of supplying light Cd to the sediment, which would imply that Cd isotopes could be used as a tracer of past ocean productivity. Therefore, the light Cd isotope composition of organic-rich sediments is likely to reflect an isotopically light source of Cd. However, isotope fractionation during diagenesis in continental margin settings appears to be small. Cadmium in organic-rich sediments is isotopically light ( δ 114/110Cd NIST-3108 = +0.02 ± 0.14‰, n = 26 2 SD) compared to deep seawater (+0.3 ± 0.1‰). We calculate a net accumulation rate of Cd in margin settings of between 2.6 to 12.0 × 10 7 mol/yr, higher than previous estimates, but at the low end of a recently published estimate for the magnitude of the marine sink due to water column CdS precipitation. These sediments all have enriched Cd concentrations relative to crustal material. We present Cd concentrations and Cd isotope compositions in organic-rich sediments deposited at several upwelling sites along the northeast Pacific continental margin. This decoupling has been suggested to result from cadmium sulphide (CdS) precipitation in reducing microenvironments within sinking organic matter. However, deviations from the global mean dissolved Cd/PO 4 relationship are present in oxygen deficient zones, where Cd is depleted relative to phosphate. 5Bigelow Laboratory for Ocean Sciences, East Boothbay, ME, United StatesĬadmium is a trace metal of interest in the ocean partly because its concentration mimics that of phosphate.4Department of Marine and Coastal Science, Rutgers University, New Brunswick, NJ, United States.3Department of Earth Science and Engineering, Imperial College London, London, United Kingdom.2Department of Earth Sciences, University College London, London, United Kingdom.1School of Earth and Environment, University of Leeds, Leeds, United Kingdom.Little 2,3 Katharina Kreissig 3 Silke Severmann 4 James McManus 5 ![]()
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