FE CYCLING IN MODERN AND ANCIENT ANOXIC ENVIRONMENTS AND FUTURE DIRECTIONS FOR THE FE ISOTOPE PALEOPROXY

Newly published data examining Fe isotope variations in ancient anoxic sedimentary deposits (Rouxel et al., Science, 2005) have revealed systematic trends in the geological record that broadly follow the first order divisions observed for S isotopes over the same interval. These intriguing new data highlight the potential of Fe isotopes as a paleoproxy to constrain Fe cycling and hence ocean-atmosphere-biosphere interactions in the early Earth. However, interpretations of Fe isotope variations in ancient sedimentary systems are complicated by the complexity of the multiple biotic and abiotic fractionation mechanisms and their expressions in natural systems.

Despite this complexity, the Archean sulfide data could be explained through variability in the principal flux terms, including iron oxide and sulfide precipitation and hydrothermal and continental weathering inputs, and their relationships to a large Archean dissolved Fe reservoir. The broad range of isotope values thus reflects the potential for large and variable fractionations in reservoirs with varying levels of exchange with the open ocean. The smaller spread in the Proterozoic and Phanerozoic would then follow straightforwardly from mass balance constraints because of the declining size of the ocean Fe reservoir after the rise of atmospheric oxygen. This interpretation is similar (although in the opposite sense) to that invoked to explain the long-term temporal trends in S isotope data: large variability corresponds to deposition from a large ocean reservoir, and vice-versa.

The ultimate utility of the Fe isotope paleoproxy hinges on a better understanding of the fractionations associated with Fe mineralization processes within a variety of modern marine, oxygen-deficient settings. To this end, we are exploring Fe isotope systematics on the California continental margin and within the euxinic Black Sea. We are using an integrated approach that combines well established tracers of seawater chemistry, such as S isotope composition and Fe-based redox indicators, with measurements of Fe isotope compositions.