Isotopic analysis of Cd in the mixing zone of Siberian rivers with the Arctic Ocean—New constraints on marine Cd cycling and the isotope composition of riverine Cd Myriam Lambelet a,n , Mark Rehk¨ amper a , Tina van de Flierdt a , Zichen Xue a , Katharina Kreissig a , Barry Coles a , Don Porcelli b , Per Andersson c a Department of Earth Science and Engineering, Imperial College London SW7 2AZ, UK b Department of Earth Sciences, University of Oxford, UK c Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Sweden article info Article history: Received 16 January 2012 Received in revised form 19 October 2012 Accepted 20 November 2012 Editor: G. Henderson Available online 20 December 2012 Keywords: marine geochemistry cadmium isotopes cadmium concentration Siberian Shelf Arctic rivers mixing abstract The Cd concentrations and isotopic compositions were determined for 19 water samples which cover the mixing zones of four major rivers with the coastal seas of the Siberian Shelf. The waters span salinities from about 1–32, with Cd concentrations of about 0.02–0.46 nmol/kg and small but resolvable Cd isotope fractionations, with e 114/110 Cd values of between þ1.4 and þ5.7. The data for the majority of the samples are in accord with the systematics expected for quasi-binary mixing of Arctic seawater (e 114/110 Cd E5.5 70.5 and Cd concentration between 0.1 and 0.25 nmol/kg) with river waters characterized by e 114/110 Cd E þ2 71 and low pristine Cd contents of about 0.02–0.06 nmol/kg. The river values are similar to the inferred composition of the continental crust, which implies that weathering produces no or only limited Cd isotope fractionation. The results for five samples provide clear evidence for non-conservative behaviour of Cd, as the element is released from suspended riverine particles during mixing with seawater. The isotopic data furthermore show that the desorbed Cd is characterized by e 114/110 Cd E þ3, in accord with a natural origin. This implies that the (natural) net riverine Cd fluxes of the Siberian rivers to seawater are also likely to be characterized by e 114/ 110 Cd E þ2 71, a value that is either identical to or intermediate between the composition of the continental crust and marine deep waters. Additional data for the boreal Kalix River in Sweden contrasts with the results obtained for the Siberian rivers, as the former exhibits a much lighter Cd isotope composition of e 114/110 Cd ¼–3.8 coupled with a much higher Cd content of 0.24 nmol/kg. These characteristics appear to be a consequence of the distinct hydro-geological setting of the Kalix drainage basin, which suggests that the riverine input of Cd isotopes to the oceans might display significant regional variability. In summary, our study underlines the important role that stable isotope analyses can play in biogeochemical investigations of trace metals. Here, the Cd isotope results provide important constraints, which are not available from concentration data alone, on the cycling of Cd in riverine and shelf environments. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Cadmium is an enigmatic element as it is highly toxic but in the oceans it displays a distribution akin to the macronutrient phosphate (Boyle et al., 1976; Bruland, 1980). Both Cd and phosphate abundances are commonly depleted in the surface ocean compared to deep water, presumably due to uptake by organisms. This conclusion is supported by work that has shown that Cd can substitute for Zn in the enzyme carbonic anhydrase, which plays an important role in the process of inorganic carbon acquisition. Cadmium is hence generally considered to be a micronutrient element (Price and Morel, 1990; Cullen et al., 1999; Lane et al., 2005; Xu et al., 2008). The marine Cd concen- tration rises rapidly in deeper waters due to regeneration from sinking biological debris, and reaches a maximum at about 1 km depth. The same open-ocean seawater concentration profile can be observed for phosphate (Boyle et al., 1976; Bruland, 1980). As a result, there exists a well-defined global correlation between Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/epsl Earth and Planetary Science Letters 0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2012.11.034 n Corresponding author. Tel.: þ44 20 7594 7137; fax: þ44 20 7594 7444. E-mail addresses: m.lambelet-salazar-serrudo09@imperial.ac.uk (M. Lambelet), markrehk@imperial.ac.uk (M. Rehk ¨ amper), tina.vandeflierdt@imperial.ac.uk (T. van de Flierdt), z.xue07@imperial.ac.uk (Z. Xue), k.kreissig@imperial.ac.uk (K. Kreissig), b.coles@imperial.ac.uk (B. Coles), don.porcelli@earth.ox.ac.uk (D. Porcelli), per.andersson@nrm.se (P. Andersson). Earth and Planetary Science Letters 361 (2013) 64–73