Cadmium is toxic to most life forms, yet there are some diatoms that substitute Cd for Zn in their carbonic anhydrase enzyme [1]. Diatoms fractionate Cd isotopes [2]; thus Cd isotopes might serve as a proxy for Cd usage in the past, if we can find where isotopic signals are preserved. Horner et al. (2010) measured Cd isotopes in 15 ferromanganese crusts from the Atlantic, Pacific, Indian, and Southern Oceans and reported that all values were within error of a few analyses of deep water, suggesting that Cd isotopes preserved in ferromanganese sediments could serve as a proxy for biological use of Cd in ancient oceans. Before we can apply such a Cd isotope proxy, we must first understand the process of Cd incorporation into marine sediment to verify that Mn crusts faithfully record the Cd isotopic composition of the ocean. To address this question, we conducted two sets of Cd adsorption experiments at low and high ionic strength. Particles of synthetic birnessite (~MnO2) were mixed with varying concentrations of Cd-bearing solution for 24 hours. The dissolved and adsorbed Cd were separated by filtration. After purification via column chromatography, samples were analyzed with multicollector ICP-MS. Our experiments revealed fractionation during adsorption, with lighter isotopes preferentially adsorbed. Fractionation at low ionic strength displayed a clear equilibrium effect. At high ionic strength, the magnitude of fractionation varied with the proportion of Cd adsorbed, suggesting a kinetic or Rayleigh effect. The difference between low and high ionic strength results may result from different speciation of Cd in solution; Cd makes chloro-complexes in artificial seawater. These results indicate the need to better comprehend abiotic fractionation of cadmium if its isotopes are to provide a reliable paleoproxy. [1] Price & Morel (1990) Nature 344, 658.[2] Lacan et al. (2006) Geochim. Cosmochim. Acta. 70, 5104.

Isotopic fractionation of cadmium during adsorption to manganese oxide

246th National Meeting (2013)

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