ESTABLISHMENT OF EARLY OCEAN ANALOGS IN MIDWESTERN FERRUGINOUS LAKES: KEYS TO THE EVOLUTION OF BIOGEOCHEMICAL (IRON) CYCLING (Invited Presentation)

Lake Matano in Indonesia, Lake Pavin in France, Lake La Cruz in Spain, Kabuno Bay of Lake Kivu in East Africa – these are all examples of permanently stratified lakes with anoxic and Fe(II)-rich (“ferruginous”) bottom waters. As such, they are wonderful analogs to the low-sulfate oceans of Archean and early Proterozoic times, which were characterized by pervasively ferruginous bottom waters. However, to American scientists these sites are a world away.

We report on our initial investigations of two permanently stratified lakes with deep, ferruginous waters in the midwestern United States. Fourteen-meter-deep Brownie Lake is part of Minneapolis’ Chain of Lakes, and has been stratified since the middle of the 20^th century. Oxygen disappears below 5 m depth, corresponding to an increase in dissolved Fe²⁺, reaching over 600 μM at depth, with non-limiting N and P at all depths. We will interpret depth-dependent trends in carbon isotopes and significant measured methane fluxes from the lake in terms of potential (photosynthetic) microbial activity and biogeochemical cycling in the lake based on our initial culturing and DNA sequencing work.

Canyon Lake is located within the Huron Mountains of the Upper Peninsula of Michigan. At 21 m deep, the chemocline between oxygen and dissolved Fe²⁺occurs much deeper than at Brownie Lake, about 14.5 m, and total Fe at depth reaches nearly 1 mM. Canyon Lake is extremely N-limited, but P is available throughout the water column. Fluorescent pigment analyses hint at a potential photosynthetic community living at the chemocline, under light-limited conditions.

Brownie and Canyon Lake offer complementary laboratories to investigate the biogeochemical consequences of different nutrient and physical regimes within ferruginous water columns, which will undoubtedly be useful for testing hypotheses regarding the activity of major functional microbial classes during Earth’s Iron Age.