WARC DISCOVERIES

Isotopic evidence that microbial cycling of carbon, sulfur, and iron developed at different times in the first three billion years of Earth’s history

Development of photosynthetic fixation of carbon has been long regarded as one of the most important biological innovations on the early Earth, and this set the stage for heterotrophic respiration pathways such as bacterial sulfate and iron reduction. In a major review paper in Annual Reviews in Earth and Planetary Sciences, WARC researchers Clark Johnson, Brian Beard, and Eric Roden compare the isotopic record for carbon, sulfur, and iron in 3.8 to 1.5 billion-year-old marine sedimentary rocks, and they find that expansion of bacterial iron reduction occurred after the carbon isotope record indicates development of photosynthesis, but before the major expansion of bacterial sulfate reduction. A well-known major excursion in carbon isotope compositions at about 2.7 billion years ago coincides with a major excursion in iron isotope compositions, which is thought to reflect development of a complex ecosystem on Earth that included photosynthesis, bacterial iron reduction, and methanotrophy, possibly reflecting sporadic increases in atmospheric oxygen contents before the major increase in atmospheric oxygen at about 2.4 billion years ago. Previous studies have shown that an increase in seawater sulfate contents after about 2.4 billion years ago is accompanied by an expansion of bacterial sulfate reduction, as recorded in sulfur isotope compositions in marine sedimentary rocks, and this was accompanied by a decrease in the iron isotope fingerprint of bacterial iron reduction; this is interpreted to record a contraction of bacterial iron reduction from large expanses of the oceans to restricted environments. WARC researchers will be continuing studies of the carbon, sulfur, and iron isotope records in these ancient rocks, focusing on the scale of individual basins so that the ecology of these ancient microbial systems may be understood.

Citation:
Johnson, CM, Beard, BL, and Roden, EE (2008) The iron isotope fingerprints of redox biogeochemical cycling in modern and ancient Earth. Annual Reviews in Earth and Planetary Sciences 36:457-493. A PDF copy of the paper can be found here.

Posted August 19, 2008