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Redox fluctuations, trace metal enrichment and phosphogenesis in the ~2.0 Ga Zaonega Formation

Kipp, Michael A. and Lepland, Aivo and Buick, Roger (2020) Redox fluctuations, trace metal enrichment and phosphogenesis in the ~2.0 Ga Zaonega Formation. Precambrian Research, 343 . Art. No. 105716. ISSN 0301-9268.

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The ~2.0 Ga Zaonega Formation (ZF) holds one of the oldest phosphorites in the geologic record, reaching >15% P2O5. Understanding the depositional conditions that enabled sedimentary phosphorus enrichment in this unit will thus help us to interpret the significance of the temporal distribution of phosphorites in Earth’s early history. Here we use an array of major and trace element data to constrain the redox conditions in the water column and extent of basinal restriction during deposition of the ZF. We also present new selenium (Se) abundance and isotopic data to provide firmer constraints on fluctuations across high redox potentials, which might be critical for phosphogenesis. We find that Se isotope ratios shift over a range of ~3‰ in the ZF, with the earliest stratigraphically-resolved negative Se isotope excursion in the geologic record, implying at least temporarily suboxic waters in the basin. Furthermore, we find that redox-sensitive element (RSE) enrichments coincide with episodes of P enrichment, thereby implicating a common set of environmental controls on these processes. Together, our dataset implies deposition under a predominantly anoxic water column with periodic fluctuations to more oxidizing conditions because of connections to a large oxic reservoir containing Se oxyanions (and other RSE’s, as well as sulfate) in the open ocean. This is broadly consistent with the depositional setting of many modern and recent phosphorites, thereby tying these ancient deposits to a common depositional mechanism. In light of these data, we propose that the broader prevalence of phosphogenesis in the Paleoproterozoic Era was driven by growth of the seawater oxidant reservoir (namely sulfate), thus enabling diagenetic apatite precipitation in basins with high rates of export production, particularly by facilitating the activity of sulfide-oxidizing bacteria. This suggests that the muted authigenic P burial observed in marginal, marine siliciclastic sedimentary rocks during other intervals of the Precambrian was not merely a result of low dissolved P levels in the global deep ocean, but also was influenced by sulfate scarcity and strongly reducing bottom-water conditions.

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Additional Information:© 2020 Elsevier B.V. Received 1 October 2019, Revised 10 February 2020, Accepted 20 March 2020, Available online 24 March 2020. We thank Melanie Mesli and the Norwegian Geological Survey for help with core sampling, as well as Timmu Kreitsmann for help with access to outcrop samples. We also thank Brett Smith, Andy Schauer and Scott Kuehner for technical assistance. MAK acknowledges support from NSF Graduate Research Fellowship DGE-1256082. Funding for this work was provided by NASA Exobiology grant NNX16AI37G to RB. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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NSF Graduate Research FellowshipDGE-1256082
Record Number:CaltechAUTHORS:20200528-145710865
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Official Citation:Michael A. Kipp, Aivo Lepland, Roger Buick, Redox fluctuations, trace metal enrichment and phosphogenesis in the ~2.0 Ga Zaonega Formation, Precambrian Research, Volume 343, 2020, 105716, ISSN 0301-9268, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103530
Deposited By: Tony Diaz
Deposited On:28 May 2020 22:08
Last Modified:28 May 2020 22:08

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