Gunflint chert

Geologic formation in Minnesota and Ontario
The Gunflint Iron Formation (exposed as the Gunflint Range) spans northwestern Ontario and northern Minnesota along the shores of Lake Superior. The type locality of the Gunflint Iron Formation is at Schreiber, ON near Lake Superior’s Thunder Bay. Geologist Stanley A. Tyler first examined the area in 1953 and noticed its red colored stromatolites. He also sampled a jet black chert layer which, when observed petrographically, revealed some lifelike small spheres, rods and filaments less than 10 micrometres in size. Elso Barghoorn, a paleobotanist at Harvard, subsequently looked at these same samples and concluded that "they were indeed structurally preserved unicellular organisms." In 1965 the two scientists published their landmark finding and named the first variety of Gunflint flora. This created an academic "stampede" to explore Precambrian microfossils from similar Proterozoic environments. While older microfossils have since been described, the Gunflint microfauna is a historic geologic discovery and remains one of the most robust and diverse microfaunal fossil assemblages from the Precambrian.

The Gunflint Iron Formation is a banded iron formation, composed predominantly of dense chert and slate layers interbedded with ankerite carbonate layers. The chert layers can be subdivided into black layers (containing organic material and pyrite), red layers (containing hematite), and green layers (containing siderite). Microfossils can be found in the stromatolitic chert layers, consisting of cyanobacteria, algal filaments, spore like spheroids, and organic rich ooids.

Geologist Stanley A. Tyler first examined the Gunflint Range in 1953 and observed red iron banded formations and black chert, noting probable stromatolites, though he would not go on to publish his observations for another decade. A. M. Goodwin later examined the geologic facies of the Gunflint Iron Formation in 1956, resulting in one of the first science publications on the region, Whole rock Neodymium Samarium dating later placed the age between 2.08 and 2.11 Ga. Finally, dating of interbedded ash layers within the Gunflint Iron Formation yielded ages between 1.86 and 1.99 Ga, which are most similar to the current consensus age of 1.878 Ga ± 1.3 Ma. At the time of discovery of the Gunflint Chert, the oldest evidence of life known was the Ediacaran fauna (635 541 Ma), a late Precambrian assemblage less than half the age of the Gunflint microorganisms.

The most abundant organisms in Gunflint are filaments found in stromatolitic fabrics, and typically range from 0.5 6.0 μm in diameter and up to several hundred microns in length.

Filamentous microorganisms within the Gunflint Chert represent a mixed population of photosynthetic cyanobacteria and iron oxidizing bacteria. On the outcrop scale, the filamentous Gunflint cyanobacteria form meter scale stromatolitic domes, which are discernible along the Gunflint Iron Formation stratigraphic section. Examples of newly identified filamentous genera and species within the Gunflint Chert include the genus Gunflintia and the species Animikiea septate, Entosphaeroides amplus, and Archaeorestis schreiberensis. Coarse grained pyritization occurs when millimeter scale pyrite minerals replace organic matter in cherts, preserving microorganism morphology. In carbonate association, filaments, spore like bodies, and other organic structures can be preserved by carbonate mineralization (<1μm in diameter) imbedded in a chert matrix.

In the 1950s and 1960s, the state of the Precambrian atmosphere was not well characterized. The discovery of the Gunflint microbiota revealed that photosynthesis (or an ancient autotrophic precursor modality) was occurring 1.8 billion years ago, and that the atmosphere was oxygenated enough to sustain microbial life. The mineralogy of the Gunflint banded iron formation reveals a complex relationship between these redox conditions throughout the Gunflint Formation. Multiple iron species in the Gunflint formation provides evidence for a highly oxidative atmosphere, with some localized reducing conditions which allowed for the transport of large quantities of iron in a soluble ferrous state. While the Gunflint microfauna no longer represents the oldest life discovered on Earth, at the time of discovery it pushed back the presumptive age of photosynthesis and the origin of life boundary by over one billion years. This discovery spurred generations of paleontologists and geomicrobiologists to contemplate ancient atmospheric oxygen conditions and redox states, and to continue searching for older microbial life.