New research led by EPS Postdoctoral Fellow Sarah Slotznick documents magnetic evidence of oxygenation in a lake during the Proterozoic Eon. In the early history of life on Earth, oxygenated terrestrial lakes may have represented refuges for the evolution of eukaryotic life, given that marine environments, particularly deep waters, were likely low in oxygen. The 1.1 billion-year-old Nonesuch Formation, on the shores of Lake Superior, has been interpreted as being deposited in a predominantly anoxic lake. EPS Miller Postdoc Sarah P. Slotznick, EPS Professor Nick Swanson-Hysell and their colleague Erik Sperling from Stanford analyzed two drill cores from the Nonesuch Formation spanning various water depths within the lake. The authors used magnetic experiments to explore the mineralogy of the drill cores to determine the mineral phases of iron, with different phases representing different degrees of oxygenation. Three distinct assemblages of minerals emerged, suggesting a gradient of oxygenation according to water depth. Reduced iron oxides in the deepest waters indicated anoxic conditions, but mid-depth waters contained magnetite and hematite, suggesting low oxygenation, whereas the shallowest waters contained only the most oxidized iron mineral, hematite. These results indicate the presence of at least some oxygen in most of the water column, and throughout most of the history, of the 1.1 billion-year-old paleolake.
Slotznick, S.P., Swanson-Hysell, N.L., and Sperling, E.A. (2018) Oxygenated Mesoproterozoic lake revealed through magnetic mineralogy, Proceedings of the National Academy of Sciences. doi: 10.1073/pnas.1813493115. html