Geochemical analyses of sedimentary barites (barium sulfates) in the geological record have yielded fundamental insights into the chemistry of the Archean environment and evolutionary origin of microbial metabolisms. However, the question of how barites were able to precipitate from a contemporary ocean that contained only trace amounts of sulfate remains controversial. Here we report dissolved and particulate multi-element and barium-isotopic data from Lake Superior that evidence pelagic barite precipitation at micromolar ambient sulfate. These pelagic barites likely precipitate within particle-associated microenvironments supplied with additional barium and sulfate ions derived from heterotrophic remineralization of organic matter. If active during the Archean, pelagic precipitation and subsequent sedimentation may account for the genesis of enigmatic barite deposits. Indeed, barium-isotopic analyses of barites from the Paleoarchean Dresser Formation are consistent with a pelagic mechanism of precipitation, which altogether offers a new paradigm for interpreting the temporal occurrence of barites in the geological record.
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Tremendous thanks go to the Captain and Crew of the R/V Blue Heron; J. Alberts for facilitating the UNOLS Chief Scientist Training Program; BH15-11 Chief Scientists N.A. D’souza, C.T. Filstrup, K.R. Salk, and A.D. Thaler; P.J. Lam for sharing samples from MV1101; H.R. Marschall for SEM access; and P. Martin for discussions. We acknowledge the use of the NSF-supported WHOI Plasma Facility, and thank G. Swarr and J. Blusztajn for their assistance. This research was made possible with support from the National Science Foundation Division of Ocean Sciences (OCE-PRF 1421196, OCE-1430015, and OCE-1443577), The Andrew W. Mellon Foundation Endowed Fund for Innovative Research, and the Agouron Institute Geobiology Postdoctoral Fellowship Program.
© 2017 The Author(s).