Manganese and iron are redox-sensitive elements that yield clues about biogeochemistry and redox conditions both in modern environments and in the geologic past. Here, we investigated Mn and Fe-bearing minerals preserved in basin sediments underlying Fayetteville Green Lake, a redox-stratified lake that serves as a geochemical analogue for Paleoproterozoic oceans. Synchrotron-source microprobe techniques (μXRF, μXANES, and μXRD) and bulk geochemical analyses were used to examine the microscale distribution and speciation of Mn, Fe, and S as a function of depth in the top 48 cm of anoxic lake sediments. Manganese was primarily associated with calcite grains as a manganese-rich carbonate that precipitated in the chemocline of the water column and settled through the euxinic basin to collect in lake sediments. Iron was preserved in framboidal iron sulfides that precipitated in euxinic bottom waters and underwent transformation to pyrite and marcasite in the sediments. Previous studies attribute the formation of manganese-rich carbonates to the diagenetic alteration of manganese oxides deposited in basins underlying oxygenated water. Our study challenges this paradigm by providing evidence that Mn-bearing carbonates form in the water column and accumulate in sediments below anoxic waters. Consequently, manganoan carbonates preserved in the rock record do not necessarily denote the presence of oxygenated bottom waters in ocean basins.
Bibliographical noteFunding Information:
This research used resources of the Advanced Photon Source, a DOE Office of Science User Facility operated by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. We acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by NSF Earth Sciences (EAR-1128799), and DOE Geosciences (DE-FG02-94ER14466). Portions of this work were supported by NASA Astrobiology Institute Cooperative Agreement NNA09DA76A. The authors thank Elizabeth Swanner, Jena Johnson, Vincent Noël, and Thilo Behrends for contributing XANES reference spectra. We also thank Hamilton College students Thomas Blanchard, Jonah Boucher, Jenna Crawford, and Eric Nieminen for their assistance during sample collection and processing, the Hamilton College Dean of Faculty for providing student summer research stipends, and Bruce Wegter for operating the Hamilton College research vessel. Additionally, we thank the Green Lakes State Park staff and the New York State Department of Parks for providing access to Green Lake. The authors would also like to acknowledge the reviewers whose comments helped to improve and clarify this work. Appendix A
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