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Project Progress

Building upon earlier experimental work in relatively simple systems, we have been exploring Fe isotope fractionations in laboratory experiments using pure cultures and parallel abiologic control experiments that are analogs to ancient seawater, specifically the high levels of dissolved silica that must have existed in the Archean. Silica was chosen as the first component to explore because chert is a major component of ancient Fe-rich rocks such as banded iron formations (BIFs). We find that abiologic exchange of Fe isotopes may occur in presence of dissolved silica. Moreover, production of Fe(II) by dissimilatory iron reduction (DIR) is not inhibited by the presence of dissolved silica, and such Fe(II) has low ⁵⁶Fe/⁵⁴Fe ratios, indicating that Fe-reducing bacteria could have produced the low-⁵⁶Fe/⁵⁴Fe ratios seen Archean marine sedimentary rocks.

We determined iron inventories and their isotopic compositions in a modern field site (Keswick Reservoir, northern California, USA) where rapid deposition of amorphous iron oxides and organic carbon support a DIR microbial community that has produced large quantities of low-⁵⁶Fe/⁵⁴Fe aqueous Fe(II). Parallel laboratory incubation experiments using the same reservoir sediment and microbial community revealed similar linkages between DIR and Fe isotope fractionation. The very high rates of DIR and low rates of bacterial sulfate reduction (BSR), make the Keswick Reservoir site an excellent analog site for BIF deposition in the ancient earth relative to modern marine sediments. These results provide the first evidence for production of low-⁵⁶Fe/⁵⁴Fe ratios by DIR in a modern environment whose iron pathways were similar to those that existed on the ancient earth, providing strong support for using iron isotopes to trace the interplay of microbial metabolisms over Earth’s history.