What Precambrian iron formations tell us about the ancient biosphere

Iron formations (IF) are iron rich (~20-40% Fe) and siliceous (~40-50% SiO2) sedimentary deposits that precipitated throughout much of the Precambrian. Their trace element composition is now being used as a proxy for ancient seawater chemistry, with the view of better understanding nutrient availability for the ancient marine biosphere. Three examples are provided here. First, it has been proposed that low P concentrations in Archean and Paleoproterozoic IF indicate limited marine phosphorous availability at that time, which would have reduced levels of photosynthesis and carbon burial, thereby inhibiting long-term oxygen production on the early Earth1-2. A subsequent increase in P content of IF in the Neoproterozoic, following Snowball Earth deglaciations, may then have led to enhanced cyanobacterial photosynthesis, which in turn, produced enough oxygen to facilitate the evolution of animal life3. Second, it has been shown that the nickel content in IF has changed dramatically over time, and that a drop in Ni availability in the oceans around 2.7 billion years ago would have had profound consequences for microorganisms that depended on it, that being methane-producing bacteria called methanogens4. These bacteria have a unique Ni requirement for their methane-producing enzymes, and crucially, these bacteria have been implicated in controlling oxygen levels on the ancient Earth as the methane they produced was reactive with oxygen and kept atmospheric oxygen levels low. It is possible that a Ni famine eventually led to a cascade of events that began with reduced methane production, the expansion of cyanobacteria into shallow-water settings previously occupied by methanogens, and ultimately increased oxygenic photosynthesis that tipped the atmospheric balance in favour of oxygen, the so-called Great Oxidation Event (GOE) at 2.5 Gyr. Third, a recent compilation of Cr enrichment in IF shows a profound enrichment coincident with the GOE5. Given the insolubility of Cr minerals, its mobilization and incorporation into IF indicates enhanced chemical weathering at that time, most likely associated with the evolution of aerobic continental pyrite oxidation. If we accept that IF can serve as useful proxies for the composition of ancient seawater, the question then becomes what do other trace elements in IF tell us about the ancient biosphere? Prospectives for the future include evaluating marine trace element evolution as recorded by IF in relation to trace element records from other lithologies, such as black shales or carbonates, as well as in relation to other biological proxies, such as stable isotope records. 1. Bjerrum, C.J. & Canfield, D.E. Ocean productivity before about 1.9 Gyr limited by phosphorous adsorption onto iron oxides. Nature 417, 159-162 (2002). 2. Konhauser, K.O., Lalonde, S., Amskold, L., and Holland, H.D. Was there really an Archean phosphate crisis? Science 315, 1234 (2007). 3. Planavsky, N.J., Rouxel, O., Bekker, A., Lalonde, S.V., Konhauser, K.O., Reinhard, C.T., and Lyons, T., 2010. The evolution of the marine phosphate reservoir. Nature, 467:1088-1090. 4. Konhauser, K.O., Pecoits, E., Lalonde, S.V., Papineau, D., Nisbet, E.G., Barley, M.A., Arndt, N.T., Zahnle, K., and Kamber, B.S. Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event. Nature 458, 750-753 (2009). 5. Konhauser, K.O., Lalonde, S.V., Planavsky, N., Pecoits, E., Lyons, T., Mojzsis, S., Rouxel, O.J., Barley, M., Rosiere, C., Fralick, P.W., Kump, L.R., and Bekker, A. Chromium enrichment in iron formations record Earth’s first acid rock drainage during the Great Oxidation Event. Nature, in press (2011).