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Project Progress

The field of Fe isotope geochemistry, which the UW-Madison group has been largely responsible for developing, has matured greatly in the last year. Over 2,500 Fe isotope analyses are now available, as compared to less than two dozen in 1999, and these data constrain the biologic and abiologic mechanisms for producing Fe isotope fractionations in the field and the laboratory, as well as provide a rapidly emerging picture of Fe biogeochemical cycling over the last 3 billion years of Earth history. This explosion in our understanding of this new isotope system contrasts dramatically with that at the beginning of the five-year grant, when it was not yet known if isotopic analyses of Fe could even be done.

The major focus of our group has been on development of Fe isotopes as a biosignature for detecting metabolic processing of Fe. For example, we have quantified the Fe isotope fractionations that are produced by anaerobic photosynthetic Fe oxidation, which appears to be unique as compared to equilibrium Fe isotope fractionations in abiologic systems. However, it is possible that kinetic isotope effects that occur during rapid oxidation in abiologic systems may produce similar isotopic effects, which would limit the effectiveness of Fe isotopes to detect this form of Fe metabolism in the ancient rock record on Earth or other planetary bodies. More distinct Fe isotope fractionations that are uniquely attributed to biology appear to occur during dissimilatory Fe(III) reduction, and a major effort in Year 5 has been to quantify these fractionations during formation of biogenic magnetite and Fe carbonates. These results show that the Fe isotope compositions of Archean Banded Iron Formations likely were produced by dissimilatory Fe(III) reduction, and the Fe isotope biosignature for this metabolic process may be traced back at least 3 billion years on Earth (Figure 1).

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