EXAMINING FE-CYCLING IN MICROBIAL MATS AS A LITHIFICATION MECHANISM

Stromatolites are abundant in Archean/Proterozoic successions, reached a form diversity and abundance peak ~1.0 billion years ago, declined through the Neoproterozoic, and became comparatively rare throughout the remaining 500 million years of Earth history. The mechanisms behind stromatolite formation and their abundance pattern through time remain unsolved. Most hypotheses focus on the destruction of stromatolite-building microbial mats to explain the decline (e.g., advent of meiofauna or burrowing metazoans). However, it is curious that microbial mats are abundant in modern systems and include microbes attributed to stromatolite formation (e.g., cyanobacteria, sulfate reducers, etc.), yet modern mats typically do not form stromatolites. Here, rather than focus on the destruction of mats to explain the stromatolite decline, we investigate the loss of an underappreciated lithification pathway (via microbial iron reduction) over geologic time to help explain the stromatolite decline versus mechanical destruction alone.

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 hot spring mats). 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 alkalinity boost can shift the local geochemical environment in favor of calcium carbonate precipitation (~10-fold alkalinity increase vs. sulfate reduction). In order to test this hypotheses, light/dark incubation experiments were conducted on actively growing silicious stromatolites in Obsidian Pool Prime Hot Spring (Yellowstone National Park). Iron concentrations and isotope data are consistent with the Fe-redox cycling hypothesis ([Fe²⁺] rose and d⁵⁶Fe fell in dark vs. light incubations). Metagenomic data reveal the presence of organisms that possess the metabolic pathways for iron cycling. Finally, micro-elemental examination of ancient stromatolites demonstrates enrichment of iron in dark vs. light laminae consistent with the predicted iron cycling. These results lead us to suggest that the history of iron in marine settings may be linked to stromatolite decline.