Geochemistry

Siderite in Time

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Science  28 Jun 2013:
Vol. 340, Issue 6140, pp. 1501
DOI: 10.1126/science.340.6140.1501-a

Banded iron formations are some of the oldest existing rocks that provide direct evidence of ancient sedimentary processes before Earth's atmosphere oxidized. The alternating layers of iron-rich minerals probably reflect changes in Precambrian seawater chemistry, but the numerous abiotic and potentially biotic reactions that resulted in their formation remain difficult to decipher. Based on high-pressure and high-temperature experiments, Köhler et al. show that the iron-carbonate mineral siderite (FeCO3) forms as a consequence of the concomitant burial of iron oxyhydroxide minerals and organic matter. Because the reactions depend on the amount of sedimentary organic matter, which was probably microbial in origin, these spheroidal siderite grains may serve as an indicator of ancient microbial metabolisms. In a series of related laboratory experiments, Kim et al. show that siderite in the Precambrian water column could photo-oxidize to form molecular hydrogen and iron oxide minerals that served as building blocks of banded iron formations. If global in scale, this abiotic process could have contributed to the early oxidation of Earth's atmosphere and provided an energy source for anaerobic microorganisms.

CREDIT: I. KÖHLER ET AL., NAUTRE COMMUNICATIONS 4 (23 APRIL 2013) © 2013 NATURE PUBLISHING GROUP

Nat. Comm. 4, 1741 (2013); Proc. Natl. Acad. Sci. U.S.A. 110, 10.1073/pnas.1308958110 (2013).

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