Publication Date:
2022-05-25
Description:
© 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.
Description:
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.
Description:
TL, ZL, and DH thank NSF EAR-1349252. ZL further thanks OCE-1232620. DH, ZL, and TL
acknowledge further funding from a NASA Early Career Collaboration Award. TL, AB, NP, DH,
and AK thank the NASA Astrobiology Institute. TL and NP received support from the Earth-Life
Transitions Program of the NSF. AB acknowledges support from NSF grant EAR-05-45484 and
an NSERC Discovery and Accelerator Grants. CW acknowledges support from NSFC grant
40972021.
Keywords:
Proterozoic oxygen
;
Shuram isotope anomaly
;
Carbonate diagenesis
;
Bahamas
;
Iodine
;
Metazoan evolution
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
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