Redox Chemistry and Molybdenum Burial in a Mesoproterozoic Lake

Abstract

While marine sediments have been used to constrain a history of redox chemistry throughout the Precambrian, far fewer data have been generated from lakes. With major biological innovations thought to have occurred in Proterozoic lakes, understanding their chemistry is critical for understanding the evolution of eukaryotic life. We use sediment geochemistry to characterize the redox conditions of the Nonesuch Formation (~1.1 Ga) and a modern analogue for the Proterozoic: the Middle Island Sinkhole in Lake Huron (USA). Iron speciation, Mo contents, and Mo‐U covariation demonstrate oxic and anoxic—not euxinic—environments, with no clear indicators of enhanced biological productivity in the Nonesuch Formation. Moderate Mo enrichments observed in the Nonesuch Formation are not attributed to euxinia, but instead to an authigenic particulate shuttle. We suggest that the Fe and Mo sediment geochemistry of these lacustrine systems reflect only local water column and sediment burial conditions and not atmospheric oxygenation.

Associations between redox‐sensitive trace metals and microbial communities in a Proterozoic ocean analogue

“…Additionally, Mn‐oxyhydroxides can also be transported through reducing waters to the sediment–water interface via a “particulate shuttle” (Algeo & Tribovillard, 2009). Such a mechanism in MIS and LH is corroborated by Mo vs. U covariation in these sediments (Rico et al, 2019). However, few correlations between metals and Mn in MIS sediments, and no correlations between metals and Mn in the MIS mat, indicate that Mn‐oxyhydroxide deposition is not a key trace metal transport in these systems (Table S1).…”

“…Macronutrient and Fe data from Rico and Sheldon (2019). Mo and U data from Rico et al (2019). Relative abundances are from Kinsman‐Costello et al (2017)…”

“…Therefore, covariation between Mo and C org ( r = .41; p < .01) must be explained by something other than thiomolybdate formation or complexation with sulfide minerals. Using Mo‐U covariation, Rico et al (2019) found that Mo burial in MIS may follow the “particulate shuttle” pathway—wherein authigenic Mo is adsorbed to Mn‐ and Fe‐(oxy)hydroxides, transported through the water column, and buried in sediments— as opposed to a burial pathway associated with euxinia.…”

“…In comparison, a fully oxygenated Lake Huron control site (LH) of comparable depth does not have a microbial mat at the sediment–water interface; instead, LH sediments have a different microbial community structure than MIS (e.g., a lack of cyanobacteria that dominate MIS microbial mats; Kinsman‐Costello et al, 2017). Sediment geochemistry of these localities also varies, in that MIS sediments are enriched in macronutrients such as C, N, and P, as well as metals such as Fe, Mn, and Mo, relative to LH sediments (Kinsman‐Costello et al, 2017; Nold et al, 2013; Rico & Sheldon, 2019; Rico, Sheldon, Gallagher, & Chappaz, 2019). Differences in chemistry extend to the pore waters, with inorganic nutrient concentrations of MIS pore waters higher than that of LH pore waters, and evidence for variable oxygen penetration and sulfide accumulation in MIS pore waters (Kinsman‐Costello et al, 2017).…”

“…Sediment geochemistry of these localities also varies, in that MIS sediments are enriched in macronutrients such as C, N, and P, as well as metals such as Fe, Mn, and Mo, relative to LH sediments (Kinsman-Costello et al, 2017;Nold et al, 2013;Rico, Sheldon, Gallagher, & Chappaz, 2019). Differences in chemistry extend to the pore waters, with inorganic nutrient concentrations of MIS pore waters higher than that of LH pore waters, and evidence for variable oxygen penetration and sulfide accumulation in MIS pore waters (Kinsman-Costello et al, 2017).…”