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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.

The sedimentary pyrite sulfur isotope (δ34S) record is an archive of ancient microbial sulfur cycling and environmental conditions. Interpretations of pyrite δ34S signatures in sediments deposited in microbial mat ecosystems are based on studies of modern microbial mat porewater sulfide δ34S geochemistry. Pyrite δ34S values often capture δ34S signatures of porewater sulfide at the location of pyrite formation. However, microbial mats are dynamic environments in which biogeochemical cycling shifts vertically on diurnal cycles. Therefore, there is a need to study how the location of pyrite formation impacts pyrite δ34S patterns in these dynamic systems. Here, we present diurnal porewater sulfide δ34S trends and δ34S values of pyrite and iron monosulfides from Middle Island Sinkhole, Lake Huron. The sediment–water interface of this sinkhole hosts a low‐oxygen cyanobacterial mat ecosystem, which serves as a useful location to explore preservation of sedimentary pyrite δ34S signatures in early Earth environments. Porewater sulfide δ34S values vary by up to ~25‰ throughout the day due to light‐driven changes in surface microbial community activity that propagate downwards, affecting porewater geochemistry as deep as 7.5 cm in the sediment. Progressive consumption of the sulfate reservoir drives δ34S variability, instead of variations in average cell‐specific sulfate reduction rates and/or sulfide oxidation at different depths in the sediment. The δ34S values of pyrite are similar to porewater sulfide δ34S values near the mat surface. We suggest that oxidative sulfur cycling and other microbial activity promote pyrite formation in and immediately adjacent to the microbial mat and that iron geochemistry limits further pyrite formation with depth in the sediment. These results imply that primary δ34S signatures of pyrite deposited in organic‐rich, iron‐poor microbial mat environments capture information about microbial sulfur cycling and environmental conditions at the mat surface and are only minimally affected by deeper sedimentary processes during early diagenesis.

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