THE ROLE OF ARSENIC CYCLING IN CARBONATE PRECIPITATION IN MICROBIALITES THROUGH TIME

The biogeochemistry of arsenic cycling has been extensively investigated in modern sediments and a potential role for arsenic metabolism in the Precambrian has been proposed. Specifically, arsenite oxidation and arsenate reduction may have played an important ecological role in microbial mats and their lithified counterparts (microbialites) before oxygenation of the atmosphere. Here we present evidence for extensive arsenic cycling coupled to calcium carbonate precipitation in microbial mats of the Atacama desert in northern Chile. Gelatinous mats containing thin layers of carbonate precipitates photosynthesized but did not produce oxygen. Instead, sulfide and arsenite oxidation were stimulated in the light. Sulfate and arsenate reduction were major respiration pathways in these anoxic mats. Calculations have shown that arsenic and sulfur cycling both played a potential role in microbialite formation (i.e., calcium carbonate precipitation). In another study using X-ray fluorescence in conjunction with a synchrotron beamline, we found evidence for arsenic cycling in 2.7 Ga stromatolites (Tumbiana formation, Pilbara, Western Australia). Micrometer-size element maps indicated that not iron but more likely arsenic cycling was associated with the formation of organic carbon. Although we cannot draw firm conclusions at this point, a careful and more detailed evaluation of the role of arsenic cycling in microbialite formation seems warranted.