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Project Progress

Fractionation of Transition Metal Isotopes (dm)

The focus of this research is to determine if fractionations of transition metal isotopes can be used as biosignatures. We are also interested in the possible use of such fractionations to study changes in metal geochemical cycling, which could provide insight into environmental change and/or biochemical evolution. The latter applications are pursued jointly with the Harvard Team.

Following our demonstration of Fe isotope fractionation by nonbiological processes, we conducted studies to elucidate the fractionation mechanism in our experiments. We found strong evidence of an equilibrium isotope effect and no evidence of a kinetic isotope effect, although the expression of equilibrium fractionation can be kinetically inhibited under certain conditions. We also extended our work to natural samples in which biological influences are unlikely. In particular, we examined the isotopic composition of Fe in hydrothermal fluids from the deep-sea Juan de Fuca ridge. These 350°C fluids, sampled in situ, are sterile. We observed small (< 1 �) differences in isotopic composition between fluid and mineral Fe which we attribute to nonbiological fractionation. This is good evidence that chemical processes such as identified in the laboratory are also important in nature.

On the biological front, we are looking into Fe isotope fractionation during leaching from minerals in the presence and absence of microbes and of metal chelating ligands. Preliminary results suggest fractionation during biogenic leaching associated with chelation, indicating that paleosols could preserve biosignature information. We also continue to explore fractionation between biogenic siderite and magnetite and between non-biogenic mineral phases.

Finally, we have begun to explore metal isotope fractionation from a theoretical standpoint, to be able to predict the magnitudes of equilibrium effects.