Importance of Cadmium Sulfides for Biogeochemical Cycling of Cd and Its Isotopes in Oxygen Deficient Zones—A Case Study of the Angola Basin

Abstract

Understanding oceanic cadmium (Cd) cycling is paramount due to its micronutrient‐like behavior in seawater, which has been inferred from its similarity to phosphate (PO4) cycling. Cadmium concentrations follow a nutrient‐like consumption‐regeneration cycle in the top of the water column and are mainly controlled by water mass mixing and circulation in the deep ocean. However, an additional scavenging mechanism through cadmium sulfide (CdS) precipitates, occurring within sinking biogenic particles in oxygen deficient zones (ODZ), has been proposed. In this study, we report Cd stable isotope and concentration data for seven vertical seawater profiles sampled during GEOTRACES cruise GA08 in the northern Cape and Angola Basins, which feature a significant ODZ along their eastern margins. Outside the ODZ, Cd cycling is similar to that previously reported for the South Atlantic. While water mass mixing largely controls deep ocean Cd isotope signatures, Cd isotope fractionation in surface waters can be modeled as an open system at steady state buffered by organic ligand complexation. In the ODZ, stronger Cd depletion relative to PO4 is associated with a shift in δ114Cd toward heavier values, which is indicative of CdS precipitation. Our interpretation is supported by experimental CdS precipitation data and a size‐resolved particle model involving bacterial sulfate reduction as a precursor of CdS. Our estimates of the CdS flux to the seafloor (107 to 109 mol/yr) indicate that CdS precipitation is a significant process of Cd removal and constitutes a nonnegligible Cd sink that needs to be better quantified by Cd isotope analyses of marine sediments.

Cadmium Isotope Fractionation during Adsorption and Substitution with Iron (Oxyhydr)oxides

“…The cadmium isotope composition reflects Cd geochemical cycling and helps track Cd anthropogenic sources. ,,, However, based on previous research with other metals, interactions with minerals are likely to affect Cd isotope compositions in environmental systems. To the best of our knowledge, the present study is the first to investigate Cd isotope fractionation during adsorption on and isomorphous substitution in Fe (oxyhydr)­oxides.…”

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES‐Era Data

“…Some metals, such as Fe, may have large sources in OMZs associated with the reductive dissolution of particulate Fe–Mn oxides. Other metals may experience enhanced removal in OMZs via precipitation into sulfide minerals (e.g., Zn, Cu, Cd, Mo, Ni, Ag; Bianchi et al., 2018; Helz et al., 1996; Guinoiseau et al., 2019; Janssen et al., 2014; Kramer et al., 2011; McKay & Pedersen, 2008; Vance et al., 2016). The net effect of these non‐POC‐associated transformations is that processes other than POC cycling may redistribute trace metals, but not C, within the ocean interior.…”

Global Contrasts Between Oceanic Cycling of Cadmium and Phosphate

“…Dissolved Cd observations were compiled from the GEOTRACES Intermediate Data Product (Abouchami et al., 2011, 2014; Baars et al., 2014; Conway & John, 2015; George et al., 2019; Guinoiseau et al., 2019; John et al., 2018; Mawji et al., 2015; Middag et al., 2018; Roshan et al., 2017; Schlitzer et al., 2018; Sieber et al., 2019; Wu & Roshan, 2015), plus other non‐GEOTRACES and historical publications (Abe, 2001, 2002a, 2002b; Boyle et al., 1976; Bruland, 1980; Cullen, 2006; Fitzwater et al., 2000; Frew & Hunter, 1992; Janssen et al., 2020; Knauer & Martin, 1981; Martin et al., 1993, 1989; Nolting & De Baar, 1994; Nolting et al., 1991; Saager et al., 1992, 1997). The geographical locations of the dissolved Cd observations are shown in Figure S1a.…”