INVESTIGATING FE AND MN GEOCHEMISTRY IN SEDIMENTS OF A REDOX-STRATIFIED LAKE

Manganese and iron are biologically important and redox-sensitive elements that have been used to constrain the redox state of the oceans through earth history. Here, we investigated Mn and Fe geochemistry in lake sediments of the meromictic Fayetteville Green Lake (New York, USA) that serves as a proxy for redox-stratified Proterozoic oceans. It has been proposed that biologically mediated cycling of Mn and Fe in the chemocline of the water column influences mineral precipitation and transfer from the water column to the sediments. Our objective was to identify the assemblage of Mn- and Fe-bearing minerals preserved in lake sediments using benchtop and synchrotron-source microprobe techniques, i.e., scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDS), X-ray fluorescence (µXRF), X-ray absorption near edge structure spectroscopy (µXANES), and X-ray diffraction (µXRD). Lake sediments were dominated by carbonate minerals intermixed with gypsum, detrital aluminosilicates, organic matter, and sulfides. Manganese was primarily found in Mn carbonate minerals (e.g., rhodochrosite, kuntahorite) that formed coatings around calcite grains, while Fe was present in iron sulfides (e.g., griegite, mackinawite, pyrite) and detrital aluminosilicates. We propose that Mn and Fe precipitated as manganese carbonate and iron sulfide minerals, respectively, in euxinic bottom waters following Mn-oxide and Fe-oxide reduction and solubilization in the chemocline. Manganese-rich coatings that formed around calcite grains may protect them from dissolution as they settle through the monimolimnion, which is undersaturated with respect to calcite. Manganese and Fe were subsequently sequestered in sediments as these particulates settled through the water column to the lake bottom. Investigating the speciation and mineralogy of solid-phase Fe and Mn addresses a knowledge gap that currently limits understanding of how these metals were cycled in the stratified oceans that characterized Earth’s early history.