PP013-01
Examining the role of Fe-cycling in microbial mat lithification and stromatolite abundance through Earth history
Tuesday, 8 December 2020: 16:00
Virtual
Frank A Corsetti1, William Berelson1, Aaron J Celestian2, Prof Seth John1, Victoria A Petryshyn3, Kalen Rasmussen4, John R Spear4, Bradley S. Stevenson5 and Shun-Chung Yang1, (1)University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States, (2)Natural History Museum of Los Angeles, Los Angeles, United States, (3)University of Southern California, Environmental Studies Program, Los Angeles, CA, United States, (4)Colorado School of Mines, Civil and Environmental Engineering, Golden, CO, United States, (5)University of Oklahoma Norman Campus, Norman, OK, United States
Abstract:
Stromatolites reached a form diversity and abundance peak ~1.0 billion years ago, declined through the Neoproterozoic, and became comparatively rare throughout the remaining 600 million years of Earth history, but the mechanisms behind the pattern remain unsolved. For example, the evolution of widespread burrowing/grazing metazoans that might disrupt microbial mats occurs too late to explain stromatolite decline. Additionally, the two most common microbial metabolisms attributed to stromatolite formation/lithification, oxygenic photosynthesis and sulfate reduction, are ubiquitous today, yet modern stromatolites are rare. Here, we investigate a possible role for iron oxidation/reduction in microbial mat lithification and suggest that changes in dissolved marine iron concentration through time may link to the decline in stromatolite abundance well before the evolution of animals.
Before widespread marine oxygenation, the oxygen produced locally by cyanobacteria during sunlight hours could induce iron oxide precipitation in microbial mats (a process that we have observed in modern stromatolite-forming mats from hot springs). At night, when the mat would be anoxic, iron-reducing bacteria could exploit the iron oxides to respire, releasing iron into pore waters. During microbial iron reduction, the boost of alkalinity can shift the local geochemical environment in favor of calcium carbonate precipitation (more than any other common metabolism, ~10-fold alkalinity increase vs. sulfate reduction). Three lines of evidence are consistent with a previously unrecognized role of iron cycling in stromatolite formation and distribution through time: 1) Preliminary micro-elemental and mineralogical examination of some ancient stromatolites demonstrates enrichment of iron in light vs. dark laminae consistent with the predicted iron cycling, 2) iron-isotope compositions from modern hot spring stromatolites vs. spring water are consistent with the Fe-redox cycling in the mat, 3) the decline of stromatolites tracks the loss of ferruginous oceans as indicated by Mo and Cr proxies, lending credence to the hypothesis and linking it to a larger Earth-system process (the oxygenation of the atmosphere and oceans).
