CONTROLS ON IRON- AND MANGANESE-MINERAL SOLUBILITY IN FERRUGINOUS LAKES

Identifying iron and manganese particulate phases formed in oxidizing shallow photic waters that survive settling though a strongly reducing deep water column is a longstanding challenge to our understanding of the Precambrian oxidation of the oceans. Analyses of major ion, nutrient, and inorganic carbon geochemistry of waters from two newly documented ferruginous lakes in the north-central United States provide insight into the relationships between mineral solubility and biogeochemistry of these environments. Both study sites feature stable stratification (meromixis), with persistently high levels (e.g. hundreds of μM) of dissolved iron in deep waters. Nutrient-rich Brownie Lake in Minnesota features a shallow (~4.5 m) iron chemocline that hosts a community of photoferrotrophs. Oligotrophic Canyon Lake in Michigan is light-limited, but features an extended suboxic transition, which separates a shallower oxycline from a deeper iron chemocline, and hosts a prominent methane cycle. Solubility index calculations through the water column of each lake demonstrate that iron and manganese oxides are well represented in oxic waters, but become undersaturated in anoxic waters. Major solubility transitions correspond with those predicted by the classic redox ladder through the water column, with manganese phases experienceing solubility fluctuations in shallower water than iron phases. In deep ferruginous waters, iron and manganese solubility is governed by carbonate, phosphate (vivianite), and silicate (greenalite) phases in both lakes. Iron carbonate and phosphate minerals are documented within some lacustrine sediments, but occurrences of greenalite in modern sediments are not widely noted. Cryptic sulfur cycling may also locally control iron solubility, particularly in Brownie Lake, where seasonal water monitoring demonstrates that sulfate levels increase through summer, leading to significant sulfide precipitation events. The role of green rust phases as particulate shuttles is also being considered. Initial particulate analysis employing SEM-EDS instrumentation will be used to refine interpretations derived from solubility calculations as we advance to more detailed microgeochemical work on anoxically collected particulate samples.