Biogeochemical cycling of cadmium isotopes along a high-resolution section through the North Atlantic Ocean

Cadmium (Cd) is a bioactive trace element in the oceans, with a nutrient-like distribution that closely matches dissolved phosphate. Seawater-dissolved stable Cd isotope ratios (delta Cd-114) are a relatively new parameter, which show much promise for furthering our understanding of the biogeochemical cycling of Cd in the oceans. Here we present a high-resolution paired section of dissolved Cd concentrations and dissolved delta Cd-114 from 21 open-ocean stations along the US GEOTRACES GA03 transect through the North Atlantic Ocean. Dissolved Cd concentrations along the section are strongly influenced by water-mass distribution and the cycling of Cd. The highest dissolved Cd concentrations (400-540 pmol kg(-1)) are associated with Antarctic-sourced water masses, whilst biological uptake in the surface ocean results in a strong vertical gradient in dissolved Cd towards the surface, reaching as low as 0.03 pmol kg(-1) in western surface waters. Dissolved delta Cd-114 is also characterized by a vertical gradient from similar to+0.2 parts per thousand in the deep ocean to +2 parts per thousand to +5 parts per thousand in the Cd-depleted surface ocean (relative to NIST SRM 3108). This variability in delta Cd-114 can be ascribed to mixing of Antarctic and North Atlantic water masses, together with fractionation due to in situ biological uptake of light Cd in the very surface ocean. Subtle deviations from this overall pattern of dissolved Cd concentration and dissolved delta Cd-114 are observed within low-oxygen waters off North Africa, where a dissolved Cd deficit relative to phosphate is associated with higher dissolved delta Cd-114 values. Together with elevated particulate Cd and Ba, this suggests that Cd sulfide precipitation is occurring within the water column of the North Atlantic, constituting a potentially important sink for isotopically light Cd. Additionally, the first measurements of dissolved delta Cd-114 within a hydrothermal plume at the Mid-Atlantic Ridge show that Cd is scavenged from the dissolved phase, leaving the remnant dissolved Cd isotopically heavier. Constraining the significance of these marine sinks for dissolved Cd is important, not only for our understanding of the marine biogeochemical cycling of Cd in the modern oceans, but also for the successful application of the microfossil Cd/Ca proxy and the development of delta Cd-114 as a tracer for past-ocean biogeochemical cycling. (C) 2014 Elsevier Ltd. All rights reserved.