Evaluating a primary carbonate pathway for manganese enrichments in reducing environments

Chad Wittkop, Elizabeth D. Swanner, Ashley Grengs, Nicholas Lambrecht, Mojtaba Fakhraee, Amy Myrbo, Andrew W. Bray, Simon W. Poulton, Sergei Katsev

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27 Scopus citations

Abstract

Most manganese (Mn) enrichments in the sedimentary rock record are hosted in carbonate minerals, which are assumed to have formed by diagenetic reduction of precursor Mn-oxides, and are considered diagnostic of strongly oxidizing conditions. Here we explore an alternative model where Mn-carbonates form in redox-stratified water columns linked to calcium carbonate dissolution. In ferruginous Brownie Lake in Minnesota, USA, we document Mn-carbonates as an HCl-extractable phase present in sediment traps and in reducing portions of the water column. Mn-carbonate becomes supersaturated in the Brownie Lake chemocline where dissolved oxygen concentrations fall below 5 μM, and Mn-oxide reduction increases the dissolved Mn concentration. Supersaturation is enhanced when calcite originating from surface waters dissolves in more acidic waters at the chemocline. In the same zone, sulfate reduction and microaerobic methane oxidation add dissolved inorganic carbon (DIC) with negative δ13C. These observations demonstrate that sedimentary Mn enrichments may 1) develop from primary carbonate phases, and 2) can occur in environments with dissolved oxygen concentrations <5 μM. Primary Mn-carbonates are likely to originate in environments with high concentrations of dissolved Mn (>200 μM), and where Mn and Fe are partitioned by S cycling, photoferrotrophy, or microaerophilic Fe-oxidation. A shallow lysocline enhances Mn-carbonate production by providing additional DIC and nucleation sites for crystal growth. This carbonate model for Mn-enrichments is expected to be viable in both euxinic and ferruginous environments, and provides a more nuanced view of the relationships between Mn and carbon cycling, with applications throughout the rock record.

Original languageEnglish (US)
Article number116201
JournalEarth and Planetary Science Letters
Volume538
DOIs
StatePublished - May 15 2020

Bibliographical note

Funding Information:
We thank Andrey Bekker, Louis Derry, Frantz Ossa Ossa, and an anonymous reviewer for suggestions that significantly improved this contribution. J. Barry Maynard generously shared his compilation of manganese ore data. This study was supported by National Science Foundation (NSF) awards ( EAR-1660691 , EAR-1660761 , EAR-1660873 ) to ES, CW, and SK, and by NSF-1338322 to AM and others. ES's contribution benefited from support of the Iowa Space Grant Consortium under NASA Award No. NNX16AL88H . We thank Duncan Widman, Paige Bauer, Raisa Islam, and Gabrielle Ledesma for field and laboratory assistance.

Keywords

  • carbon isotopes
  • carbonates
  • ferruginous lake
  • manganese
  • redox-stratified

Continental Scientific Drilling Facility tags

  • SWAN

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