Heavy Down Below

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Science  28 Aug 2009:
Vol. 325, Issue 5944, pp. 1048
DOI: 10.1126/science.325_1048b

Many chemical and biological processes skew, or fractionate, atomic isotope ratios in products relative to their initial relative abundances. For example, most of the iron in Earth's crust exists as 56Fe, but iron that undergoes reduction/oxidation (redox) reactions is on average slightly lighter (that is, enriched in 54Fe—even by just a few parts per thousand).

To determine the extent of contemporary Fe redox cycling in different marine settings, Homoky et al. compared Fe isotope signatures from pore water collected on a shallow continental shelf to those from pore water associated with sediments as deep as 4222 m below sea level, at the bottom of the Southern Ocean. The significant fractionation of 56Fe isotopes in shallow sediments indicated extensive Fe reduction by active communities of bacteria; in contrast, the Fe isotopes in the deep sediments showed almost no fractionation, thereby suggesting that deep marine sediments undergo very little redox cycling. Although limiting organic matter is expected to restrict the activity of iron-reducing bacteria at depth, the degree of fractionation is also dependent upon the reactivity of Fe-oxide minerals. Defining the controls on stable isotope fractionation in sediments will not only aid in understanding modern biogeochemical processes, but will also allow for a more accurate interpretation of Earth's history based on isotopic signatures found in the rock record.

Geology 37, 751 (2009).

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