| Gunflint chert | |
|---|---|
| Stratigraphic range: 1.88Ga[1] | |
 Microfossils of microbes similar tocyanobacteria, Gunflint Formation, north shore of Lake Superior, 1.9 billion years old | |
| Type | Geological formation |
| Lithology | |
| Primary | Banded iron formation |
| Location | |
| Region |  Minnesota Ontario |
| Type section | |
| Named for | Gunflint Range |
TheGunflintchert (1.88Ga[1]) is a sequence ofbanded iron formation rocks that are exposed in theGunflint Range of northernMinnesota andnorthwestern Ontario along the north shore ofLake Superior. The Gunflint Chert is ofpaleontological significance, as it contains evidence ofmicrobial life from thePaleoproterozoic.[2] The Gunflint Chert is composed of biogenicstromatolites.[3] 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 forphotosynthesis.[4] The black layers in the sequence containmicrofossils that are 1.9 to 2.3 billion years in age.Stromatolitecolonies ofcyanobacteria that have converted tojasper are found in Ontario. The bandedironstone formation consists of alternatingstrata ofiron oxide-rich layers interbedded withsilica-rich zones. The iron oxides are typicallyhematite ormagnetite withilmenite, while the silicates are predominantlycryptocrystallinequartz aschert orjasper, along with some minor silicate minerals.
The Gunflint Iron Formation (exposed as theGunflint Range) spans northwestern Ontario and northern Minnesota along the shores of Lake Superior. Thetype locality of the Gunflint Iron Formation is atSchreiber, ON near Lake Superior’sThunder Bay.[5]
GeologistStanley A. Tyler first examined the area in 1953 and noticed its red-colored stromatolites. He also sampled a jet-black chert layer which, when observedpetrographically, revealed some lifelike small spheres, rods and filaments less than 10micrometres in size.Elso Barghoorn, apaleobotanist atHarvard, subsequently looked at these same samples and concluded that "they were indeed structurally preservedunicellularorganisms."[6] In 1965 the two scientists published their landmark finding and named the first variety ofGunflint flora.[2] This created an academic "stampede" to explorePrecambrian microfossils from similarProterozoic 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 microfaunalfossil assemblages from the Precambrian.
The Gunflint Iron Formation is abanded iron formation, composed predominantly of densechert andslate layers interbedded withankeritecarbonate layers. The chert layers can be subdivided into black layers (containing organic material andpyrite), red layers (containinghematite), and green layers (containingsiderite).[5] The Gunflint Iron Formation belongs to theAnimike Group and can be broken up into fourstratigraphic sections, the Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty sections.[7]Microfossils can be found in thestromatolitic chert layers, consisting ofcyanobacteria, algal filaments, spore-like spheroids, and organic-richooids.
Geologist Stanley A. Tyler first examined theGunflint Range in 1953 and observed red iron banded formations and black chert, noting probablestromatolites, though he would not go on to publish his observations for another decade. A. M. Goodwin later examined the geologicfacies of the Gunflint Iron Formation in 1956, resulting in one of the first science publications on the region,[5] but his report is devoid of any mention of microscopic life. The first publications noting the geobiological significance of the Gunflint Chert came in 1965 when two scientific papers highlighting the Gunflintmicrofauna were published in the preeminent journalScience. These papers were Stanley Tyler andElso Barghoorn's ‘Microorganisms from the Gunflint Chert’[2] andPreston Cloud’s (University of California at Santa Barbara) ‘Significance of the Gunflint (Precambrian) Microflora’.[4] 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 aimed to characterize the individual microorganisms that comprise the Gunflint chert from ataxonomical andmorphological standpoint, Cloud focused on the larger-scale significance of the prospect of life existing during the Precambrian period and its implications for the field of Precambrianpaleontology. The publication of these two seminal papers opened the floodgates to a vast array of paleontological andgeochemical studies to explore Precambrianmicrofossils from similar Proterozoic environments.
The Gunflint chert microfauna is mid- to late-Paleoproterozoic in age (approximately 1.878 Ga ± 1.3Ma, as determined byUranium-Lead dating techniques).[1] This age has fluctuated as dating techniques have become more accurate and precise. Initial whole-rockRubidium-Strontium andPotassium-Argon dating placed the age of the Gunflint Iron Formation at 1.56-163Ga.[8][9][10][11] Whole-rockNeodymium-Samarium dating later placed the age between 2.08 and 2.11 Ga.[12][13] Finally, dating of interbeddedash layers within the Gunflint Iron Formation yielded ages between 1.86 and 1.99 Ga,[14] 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 theEdiacaran fauna (635-541 Ma),[15] a late Precambrian assemblage less than half the age of the Gunflint microorganisms.
The most abundant organisms in Gunflint arefilaments found instromatolitic fabrics, and typically range from 0.5-6.0μm in diameter and up to several hundredmicrons in length.[3] The Gunflintmicrofauna can be split into two broad categories: filaments andspheroids. In the groundbreaking 1965 Barghoorn and Tyler paper, three newgenera and four newspecies of filamentouscyanobacteria were discovered from Gunflint chert.[2] Since then various new genera and species have been identified, some named after Barghoorn, Tyler, and Cloud in acknowledgement of their early contributions in defining the Gunflintmicrobialassemblages.[3][7][16][17]
Filamentousmicroorganisms within the GunflintChert represent a mixed population ofphotosyntheticcyanobacteria andiron oxidizing bacteria. On theoutcrop scale, the filamentous Gunflint cyanobacteria form meter-scalestromatolitic domes, which are discernible along the Gunflint Iron Formationstratigraphic section. Examples of newly identified filamentous genera and species within the Gunflint Chert include the genusGunflintia and the speciesAnimikieaseptate,Entosphaeroides amplus, andArchaeorestis schreiberensis.[2]
Spheroidalspore-like bodies within the GunflintChert are found irregularly distributed throughout the Gunflint Iron Formation, and range from 1 to 16μm in diameter. The spheroidal bodies range from spherical to ellipsoidal inmorphology. They are typically encased in a membrane which can vary in wall thickness and morphology. The spheroidal bodies have been hypothesized to be various things, such asunicellularcyanobacteria,endogenously producedendospores ofbacterial origin, free-swimmingdinoflagellates, andfungusspores.[2] Examples of newly identified spheroidal genera and species within the Gunflint Chert include the generaHuroniospora andEoasatrion, as well as the speciesEosphaera tyleri.[3][17]
Various predominanttaphonomic models have been suggested as mechanisms for the exceptional preservation of the Gunflint Chertmicrofauna. Examples of these taphonomic models include organic residue preservation, fine-grainpyritization, coarse-grain pyritization,carbonate association, andhematite preservation.[2] In organic residue preservation, a film of light-to-dark brown organic material outlinesmicroorganisms, acting as a stain and preserving filaments, spore-like bodies, and carbonaterhombs withinchert. Fine-grain pyritization is the most common type of preservation in the Gunflint Cherts, in which association of fine-grained (micrometer scale)pyrite with organic matter preserves themorphology of filamentous and spheroidal microorganisms.[18] 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 chertmatrix.[18] 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 blackstromatolitic cherts and redjasper. 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.[16] As a result of the remarkable preservation of microorganisms given the taphonomic modes described above, the Gunflint Chert is sometimes described as the first Precambrianlagerstätte, or exceptionally preserved fossil assemblage.[19]
In the 1950s and 1960s, the state of the Precambrian atmosphere was not well characterized. The discovery of the Gunflintmicrobiota revealed thatphotosynthesis (or an ancientautotrophic precursor modality) was occurring 1.8 billion years ago, and that the atmosphere was oxygenated enough to sustain microbial life.[4] The mineralogy of the Gunflint banded iron formation reveals a complex relationship between theseredox conditions throughout the Gunflint Formation.[4] 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.[4]
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 ofpaleontologists andgeomicrobiologists to contemplate ancient atmospheric oxygen conditions and redox states, and to continue searching for older microbial life.
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