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Gunflint Chert

The Gunflint Chert is a sequence of banded iron formation rocks belonging to the Gunflint Iron Formation in south central Canada. The Gunflint Chert is of paleontological significance, as it contains evidence of microbial life from the Paleoproterozoic. The Gunflint Chert is composed of biogenic stromatolites, or fossilized colonies of cyanobacteria dating back 1.88 billion years). At the time of its discovery in the 1950s, it was the earliest form of life discovered and described in scientific literature, as well as the earliest evidence for photosynthesis. 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 can be found at Schreiber, ON near Lake Superior’s Thunder Bay. 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.

Stratigraphy

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). The Gunflint Iron Formation belongs to the Animike Group, and can be broken up into four stratigraphic sections, the Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty sections. Microfossils can be found in the stromatolitic chert layers, comprising of cyanobacteria, algal filaments, spore-like spheroids, and organic-rich ooids.

History

Stanley A. Tyler examined the Gunflint Range in 1953 and observed the 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, but his report is devoid of any mention of microscopic life. The first publications noting the geobiological significance of the Gunflint Chert came in 1956 when two scientific papers highlighting the Gunflint microfauna were published in the preeminent journal ‘Science’. Stanley Tyler and Elso Barghoorn of Harvard University published ‘Microorganisms from the Gunflint Chert’ within months of Preston Cloud’s (University of California at Santa Barbara) ‘Significance of the Gunflint (Precambrian) Microflora’. While published at nearly the same time, both papers served as landmark publications introducing the idea of life occurring during the Precambrian. Each paper had markedly different foci: while Barghoorn and Tyler 1965 aimed to characterize the individual microorganisms that comprise the Gunflint chert from a taxonomical and morphological standpoint, Cloud 1965 focused on the larger-scale significance of the prospect of life existing during the Precambrian period, and its implications for the field of Precambrian paleontology. The publication of these two seminal papers opened the floodgates to a vast array of paleontological and geochemical studies to explore Precambrian microfossils from similar Proterozoic environments.

Age

The Gunflint Chert microfauna is mid- to late-Paleoproterozoic in age (approximately 1.878 Ga ± 1.3 Ma, as determined by Uranium-Lead dating techniques. This age has fluctuated through time as dating techniques have become more accurate and precise. Initial whole-rock Rubidium-Strontium and Potassium-Argon dating placed the age of the Gunflint Iron Formation at 1.56-163 Ga. Whole-rock Neodymium-Samarium 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 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 over half the age of the Gunflint microorganisms.

Microfaunal Diversity

The most abundant organisms in Gunflint are filaments found in stromatolitic fabrics, and typically range from 0.5 to 6.0μm in diameter and up to several hundred microns in length. The Gunflint microfauna can be split into two broad categories: filaments and spheroids. In the groundbreaking 1965 Barghoorn and Tyler paper, three new genera and four new species of filamentous cyanobacteria were discovered, since then various new genera and species have been identified, and some have been named after Barghoorn, Tyler, and Cloud in acknowledgement of their early contributions in defining the Gunflint microbial assemblages.  

Filamentous Microorganisms

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.

Spheroidal Microorganisms

Spheroidal spore-like bodies within the Gunflint Chert are found irregularly distributed throughout the Gunflint Iron Formation, and range from 1 to 16 μm in diameter. Despite their name, the spheroidal bodies range from spherical to ellipsoidal in morphology. They are typically encased in membrane, which can vary in wall thickness and morphology. The spheroidal bodies have been hypothesized to be various things, such as unicellular cyanobacteria, endogenously produced endospores of bacterial origin, free-swimming dinoflagellates, and fungus spores. Examples of newly identified spheroidal genera and species within the Gunflint Chert include novel the genera Huroniospora and Eoasatrion, as well as the species Eosphaera tyleri.

Preservation of Microfauna

Various predominant taphonomic modes have been suggested as mechanisms that resulted in the exceptional preservation of the Gunflint Chert microfauna. Examples of these taphonomic modes include organic residue preservation, fine-grain pyritization, coarse-grain pyritization, carbonate association, and hematite preservation. In organic residue preservation, a film of light-to-dark brown organic material outlines microorganisms, acting as a stain and preserving filaments, spore-like bodies, and carbonate rhombs within chert. Fine—grain pyritization is the most common type of preservation in the Gunflint Cherts, in which association of fine-grained (μm scale) pyrite with organic matter preserves the morphology of filamentous and spheroid microorganisms. 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. Carbonate minerals can form as continuous bodies or as a series of lenses outlining filamentous cyanobacterial remains. Carbonate mineralization is often seen trailing pyrite crystals. Hematite preservation is a less common taphonomic mode, but is occasionally found at the interface between black stromatolitic cherts and red jasper. In this preservational method, hematite filaments <1μm in diameter encase (and occasionally replace) filamentous fossils, and are often outlined by carbonaceous films and pyrite grains. As a result of the remarkable preservation of microorganisms given the taphonomic modes described above, the Gunflint Chert is sometimes described as a the first Precambrian lagerstätte, or exceptionally preserved fossil assemblage.

Significance and Paleoenvironmental Implications

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 the Gunflint banded iron formation reveal that there was 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 represent the oldest life discovered on Earth, and the time of discovery it had pushed back the presumptive age of photosynthesis and the origin of life boundary by over one billion years. This discovery opened the floodgates for generations of paleontologists and geomicrobiologists to contemplate ancient atmospheric oxygen conditions and redox states, and to continue searching for older microbial life.