RECORDS OF LIFE IN A FERRUGINOUS OCEAN: LESSONS FROM MEROMICTIC LAKES (Invited Presentation)

Meromictic ferruginous lakes are increasingly utilized as analogs for chemical and biological processes that might have occurred in ferruginous oceans, which characterized much of the Precambrian. Several such lakes have recently been characterized in the Midwest, USA. We have the opportunity to test how microbial processes contribute to sedimentation, and what indicators of microbial activity and redox conditions the sediments preserve. For instance, light carbon isotopes may be recorded in past ferruginous sediments in association with Fe- and Mn-bearing carbonate minerals. In lakes, however, light carbon isotopes may not reflect diagenetic formation from organic carbon and oxides in the sediments, but primary signatures of water column formation. Ferruginous deep waters degrade much exported organic carbon by methanogenesis, and oxidation of methane in the water column generates isotopically light inorganic carbon. In combination with dissolution of Ca-carbonate at the redoxcline due to pH changes, Ca-Mn-Fe carbonates become supersaturated and incorporate isotopically light carbon generated from methane oxidation. The presence of varying amounts of Fe(II)/Fe(III) in the particulate flux from the water column and in the sediment pile, especially with constraints on primary productivity and organic loadings, is also useful to provide depositional constraints on the formation of mixed-valence banded iron formations. Documentation of additional ferruginous lake sites expands our opportunity to test these and other ideas concerning mineralization processes in ancient ferruginous environments.