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

VISSCHER, PIETER; FARIAS, MARIA E.; CONTRERAS MANUEL; NOVOA-CORTEZ, FERNANDO; RASUK, CECILLIA; PATTERSON, MOLLY M; PHILIPPOT, PASCAL; SFORNA, MARIE C; GALLAGHER, KIMBERLEY L.

Vancouver

Congreso; 2014 GSA Annual Meeting in Vancouver, British Columbia (19?22 October 2014); 2014

The biogeochemistry of arsenic cycling has been extensively investigated in modern sediments and a potential role for arsenicmetabolism in the Precambrian has been proposed. Specifically, arsenite oxidation and arsenate reduction may have played animportant 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 theAtacama desert in northern Chile. Gelatinous mats containing thin layers of carbonate precipitates photosynthesized but did notproduce oxygen. Instead, sulfide and arsenite oxidation were stimulated in the light. Sulfate and arsenate reduction were majorrespiration pathways in these anoxic mats. Calculations have shown that arsenic and sulfur cycling both played a potential role inmicrobialite formation (i.e., calcium carbonate precipitation). In another study using X-ray fluorescence in conjunction with asynchrotron beamline, we found evidence for arsenic cycling in 2.7 Ga stromatolites (Tumbiana formation, Pilbara, WesternAustralia). Micrometer-size element maps indicated that not iron but more likely arsenic cycling was associated with the formationof organic carbon. Although we cannot draw firm conclusions at this point, a careful and more detailed evaluation of the role ofarsenic cycling in microbialite formation seems warranted.