Identification of microbial ecosystems in the stromatolites of the Biwabik Iron Formation (Paleoproterozoic, Minnesota) using mineralogy and facies analysis

Prior to the Great Oxygenation Event (GOE) at 2.4-2.3 Ga the world’s oceans were anoxic and drastically different from the oxygen stratified oceans of today. Previous work on defining the GOE has consisted of geochemical data including sulfur isotopic data, iron isotopic data, carbon isotopic data, and rare earth elements to address oceanic conditions. Because biomarkers and carbon isotopes are rarely preserved in iron-formations, the use of fossil evidence for the transition from anoxic bottom waters to suboxic shallow water conditions is poorly represented by rare stromatolites, and even more rare microfossils. Previous geochemical analysis led to the conclusion that Fe-oxidizing bacteria formed the stromatolites preserved in the 1.89 Ga Gunflint and Biwabik iron formations (Ontario and Minnesota) and other iron-formations of this post-GOE interval. This thesis uses a geological approach to define the ecological constraints on the formation of stromatolites in the Biwabik Iron Formation, specifically. Detailed facies analysis along with primary mineralization estimates shows that the primary deposition for the stromatolite layers was a high energy, shallow water environment. 400 million years after the GOE, oceans would have been stratified with regards to oxygen, and the shallow waters, while being suboxic, would have contained enough oxygen to limit the amount of non-reduced iron needed for Fe-oxidizing bacteria to utilize for energy synthesis. These factors suggest that cyanobacteria were the stromatolite builders in the Paleoproterozoic Biwabik Iron Formation.