THE BIOGEOCHEMISTRY OF FERRUGINOUS LAKES, AND PAST FERRUGINOUS OCEANS (Invited Presentation)

Anoxic and ferruginous (Fe(II)-rich) systems are generally thought to be rare on modern Earth, but pervasive on Precambrian Earth, in the form of redox-stratified oceans. Our work seeks to utilize permanently-stratified lakes in the Midwest, USA, as analogues to past oceans. From this perspective I will share several examples of microbial processes influencing element cycling and potential biosignatures under ferruginous conditions from Brownie Lake in MN and Canyon Lake in MI. These include microbial production and consumption of methane within the sediments and water column, and biotic vs. abiotic controls on methane emissions from ferruginous systems. A perennial question that can be addressed in these systems is determining the suite of physical and chemical conditions that control whether anoxygenic or oxygenic photosynthesis predominates in carbon fixation as a whole or at discrete depths within these lakes. Pigment data, DNA sequencing, and ¹³C-fixation measurements will be presented to inform this question. Our work is also addressing what mineral, isotopic, and organic biosignatures are produced in ferruginous water columns, reflecting the aforementioned microbial processes, and how these signals are recorded in the sediments. Finally, I will touch on the role of ferruginous systems in modern global biogeochemical cycles, by considering how common ferruginous and/or permanently-stratified lakes are in the world, and the geological controls on the distribution of these systems.