WERE MICROAEROPHILIC IRON-OXIDIZING BACTERIA RESPONSIBLE FOR THE DEPOSITION OF CA. 1.88 GA GRANULAR IRON FORMATIONS?

Granular Iron Formations (GIFs) are iron-rich sedimentary rocks, predominantly Proterozoic in age and deposited in shallow-marine, oxygenated nearshore environments where anoxic, ferruginous deep-waters upwelled. At ca. 1.88 Ga, GIFs were deposited on several active continental margins of paleogeographically distinct cratons. The synchronous deposition of these sediments suggests a global driving mechanism. Previous arguments have related these GIFs to either increased iron flux from large igneous provinces (LIPs) or change in seawater composition and redox. Iron-oxyhydroxide precipitation at the redoxcline in these basins is thought to be due to either oxygen production via photosynthesis or iron oxidation by microaerophilic iron-oxidizing bacteria (IOB). Microfossils identified in the ca. 1.88 Ga Gunflint GIF of the Superior craton have been linked to these biological processes. In the ca. 1.88 Ga Gibraltar GIFs from the Slave craton, we found microfossils in stromatolites exhibiting twisted-stalk morphology like that observed in modern IOB. Iron isotope composition of these GIFs supports oxidation at the redoxcline, and weak positive Eu anomalies might indicate high submarine hydrothermal activity associated with contemporaneous LIPs. REE data exhibit a significant MREE-arch indicating an influence of riverine waters capable of overprinting typical hydrothermal REE signatures. Therefore, rather than invoking a non-hydrothermal iron source, we relate this signature to deposition distally to, but in connection with, the global ocean in an estuary-like setting. The Gibraltar and other contemporaneous GIFs indicate that global redox conditions and deep-ocean iron supply to shallow-marine settings at ca. 1.88 Ga generated an ideal habitat for IOB to thrive in and play an essential role in deposition of GIFs.