Cd Isotope Fractionation During Adsorption Varies with Salinity

Because its marine concentration profile is very similar to that of phosphate [1], Cd is considered to have potential as a paleophosphate or paleonutrient proxy in the geologic record. Previous work [2,3] has established that lighter isotopes of Cd are preferentially assimilated by phytoplankton, leaving surface waters isotopically heavy. Another recent study [4] suggests that analysis of Cd isotope variations in transects of ferromanganese crusts could reveal past variations in the extent to which Cd, and thus phosphate, has been depleted over time. This idea presumes that the extent of consumption of Cd by phytoplankton is reflected in the isotopic composition of seawater and that the Cd isotopic composition of seawater is in turn faithfully recorded in ferromanganese crusts. To test the latter assumption, Rehkämper et al. [4] measured the Cd isotopic composition of 15 Fe-Mn crusts from various ocean basins and found that 13 of those samples were within analytical error of the Cd isotopic composition of deep seawater from [3], indicating that Cd often does not fractionate appreciably during incorporation into ferromanganese crusts. Other studies [5,6] have likewise revealed little or no variation in Cd isotopic compositions among various terrestrial rocks and carbonaceous chondrites, suggesting that few earth processes significantly fractionate Cd isotopes. To test this conclusion experimentally, we performed adsorption experiments in which aqueous Cd was allowed to adsorb to synthetic birnessite (Mn oxyhydroxide). Stock solutions of dissolved Cd and birnessite suspension were mixed and agitated from 1 to 48 hours at room temperature. Some experiments had 0.1m KNO3 as background electrolyte, while others had 0.3m NaCl + 0.1m KNO3. After filtration, both the fluid with remaining dissolved Cd and solids with adsorbed Cd were purified with anion exchange chemistry. Column yields and proportions of dissolved and adsorbed Cd were determined by ICP-MS, and isotope compositions were measured using MC-ICP-MS with a Ag spike for mass bias correction. The fractionation between dissolved and adsorbed Cd was ~0.3‰ in terms of δ114/110Cd (relative to our ICP standard solution) for the KNO3-only experiments (adsorbed Cd is lighter) and very near zero for the NaCl-rich experiments. Our NaCl-rich result is consistent with the results of [4] and [6]. In addition, our KNO3-only result suggests that adsorption of Cd to mineral surfaces in low salinity waters, such as riverine water or groundwater, could be one of the few abiotic processes that can significantly fractionate Cd isotopes. We hypothesize that NaCl concentration affects Cd fractionation in this system by changing Cd speciation. Cd likely complexes with Cl in predominantly tetrahedral molecules, in contrast to the octahedral hydrated Cd cations present in low-Cl solutions. [1] Boyle et al. (1976) Nature 263, 42. [2] Lacan et al. (2006) GCA 70, 5104. [3] Ripperger et al. (2007) EPSL 261, 670. [4] Rehkämper et al. (2008) GCA 72 Suppl., A392. [5] Wombacher et al. (2003) GCA 67, 4639. [6] Schmitt et al. GCA 71 Suppl., A898.