Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate
Hardisty, Dalton S.
Diamond, Charles W.
Gill, Benjamin C.
Knoll, Andrew H.
Planavsky, Noah J.
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KeywordProterozoic oxygen; Shuram isotope anomaly; Carbonate diagenesis; Bahamas; Iodine; Metazoan evolution
The Proterozoic Eon hosted the emergence and initial recorded diversification of eukaryotes. Oxygen levels in the shallow marine settings critical to these events were lower than today’s, although how much lower is debated. Here, we use concentrations of iodate (the oxidized iodine species) in shallow-marine limestones and dolostones to generate the first comprehensive record of Proterozoic near-surface marine redox conditions. The iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters. To assess the validity of our approach, Neogene-Quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate-iodine contents. Despite the potential for diagenetic loss, maximum Proterozoic carbonate iodine levels are elevated relative to those of the Archean, particularly during the Lomagundi and Shuram carbon isotope excursions of the Paleo- and Neoproterozoic, respectively. For the Shuram anomaly, comparisons to Neogene-Quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends. The baseline low iodine levels in Proterozoic carbonates, relative to the Phanerozoic, are linked to a shallow oxic-anoxic interface. Oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes. However, the diagnostically low iodine data from mid-Proterozoic shallow-water carbonates, relative to those of the bracketing time intervals, are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans. This redox instability may have challenged early eukaryotic diversification and expansion, creating an evolutionary landscape unfavorable for the emergence of animals.
© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 463 (2017): 159-170, doi:10.1016/j.epsl.2017.01.032.
Suggested CitationPreprint: Hardisty, Dalton S., Lu, Zunli, Bekker, Andrey, Diamond, Charles W., Gill, Benjamin C., Jiang, Ganqing, Kah, Linda, Knoll, Andrew H., Loyd, Sean, Osburn, Magdalena, Planavsky, Noah J., "Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate", 2017-01, https://doi.org/10.1016/j.epsl.2017.01.032, https://hdl.handle.net/1912/8872
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