Living in Their Heyday: Iron-Oxidizing Bacteria Bloomed in Shallow-Marine, Subtidal Environments at ca. 1.88 Ga

Abstract

The majority of large iron formations (IFs) were deposited leading up to Earth's great oxidation episode (GOE). Following the GOE, IF deposition decreased for almost 500 Myr. Subsequently, around 1.88 Ga, there was widespread deposition of shallow-water granular iron formations (GIF) within a geologically short time interval, which has been linked to enhanced iron (Fe) supply to seawater from submarine hydrothermal venting associated with the emplacement of large igneous provinces. Previous studies of Fe-rich, microfossil-bearing stromatolites from the ca. 1.88 Ga Gunflint Formation on the Superior craton suggested direct microbial oxidation of seawater Fe²⁺_(aq) by microaerophilic, Fe-oxidizing bacteria (FeOB), as a driver of GIF deposition. Although Fe-rich, microfossil-bearing stromatolites are common in 1.88 Ga GIF deposits on several cratons, combined paleontological and geochemical studies have been applied only to the Gunflint Formation. Here, we present new paleontological and geochemical observations for the ca. 1.89 Ga Gibraltar Formation GIFs from the East Arm of the Great Slave Lake, Northwest Territories, Canada. Fossil morphology, Rare Earth element (REE) concentrations, and Fe isotopic compositions support Fe oxidation by FeOB at a redoxcline poised above the fair-weather wave base. Small positive Eu anomalies and positive ε_Nd (1.89 Ga) values suggest upwelling of deep, Fe-rich, hydrothermally influenced seawater. While high [Fe²⁺_(aq)] combined with low atmospheric pO₂ in the late Paleoproterozoic would have provided optimal conditions in shallow oceans for FeOB to precipitate Fe oxyhydroxide, these redox conditions were likely toxic to cyanobacteria. As long as local O₂ production by cyanobacteria was strongly diminished, FeOB would have had to rely on an atmospheric O₂ supply by diffusion to shallow seawater to oxidize Fe²⁺_(aq). Using a 1-D reaction dispersion model, we calculate [O_2(aq)] sufficient to deplete an upwelling Fe²⁺_(aq) source. Our results for GIF deposition are consistent with late Paleoproterozoic pO₂ estimates of ~1%–10% PAL and constraints for metabolic [O_2(aq)] requirements for modern FeOB. Widespread GIF deposition at ca. 1.88 Ga appears to mark a temporally restricted episode of optimal biogeochemical conditions in Earth's history when increased hydrothermal Fe²⁺_(aq) sourced from the deep oceans, in combination with low mid-Paleoproterozoic atmospheric pO₂, globally satisfied FeOB metabolic Fe²⁺_(aq) and O_2(aq) requirements in shallow-marine subtidal environments above the fair-weather wave base.