The Gunflint chert (1.88 billion years old) is a group of banded iron formation rocks found in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. This rock formation is important for studying ancient life, as it contains signs of microbial life from the Paleoproterozoic era. The Gunflint chert is made up of biogenic stromatolites, which are layered rock structures formed by living organisms. When scientists first discovered the Gunflint chert in the 1950s, it was the oldest known evidence of life and the earliest proof of photosynthesis found in scientific records. The black layers in the rock contain microfossils that are 1.9 to 2.3 billion years old. In Ontario, stromatolite colonies made by cyanobacteria have turned into jasper. The banded ironstone formation has alternating layers of iron oxide-rich material, such as hematite or magnetite, and silica-rich zones, which are mostly cryptocrystalline quartz found in chert or jasper, along with some smaller amounts of other silicate minerals.
The Gunflint Iron Formation, which is exposed as the Gunflint Range, is found in northwestern Ontario and northern Minnesota along the shores of Lake Superior. The specific location where this formation was first identified is near Schreiber, Ontario, close to Lake Superior’s Thunder Bay.
In 1953, geologist Stanley A. Tyler studied the area and noticed red-colored stromatolites. He also collected samples from a jet-black chert layer. When examined using special tools, these samples showed tiny, lifelike shapes such as spheres, rods, and filaments smaller than 10 micrometers. Elso Barghoorn, a paleobotanist at Harvard University, later studied the same samples and concluded they were structurally preserved unicellular organisms. In 1965, Tyler and Barghoorn published their groundbreaking discovery and named the first type of Gunflint flora. This finding sparked widespread scientific interest in studying Precambrian microfossils from similar Proterozoic environments. Although older microfossils have been found since, the Gunflint microfauna remains one of the most well-preserved and diverse collections of microfossils from the Precambrian era.
Stratigraphy
The Gunflint Iron Formation is a type of banded iron formation made mostly of dense chert and slate layers that are layered together with ankerite carbonate layers. The chert layers can be divided into black layers (which have organic material and pyrite), red layers (which have hematite), and green layers (which have siderite). The Gunflint Iron Formation is part of the Animike Group and can be divided into four sections: Lower Cherty, Lower Slaty, Upper Cherty, and Upper Slaty. Microfossils are found in the stromatolitic chert layers. These include cyanobacteria, algal filaments, spore-like spheroids, and organic-rich ooids.
History
In 1953, geologist Stanley A. Tyler first studied the Gunflint Range. He observed red iron banded formations and black chert and noticed possible stromatolites, but he did not publish his findings until a decade later. In 1956, A. M. Goodwin examined the rock layers of the Gunflint Iron Formation. His report was one of the first scientific studies on the region, but it did not mention microscopic life. The first scientific papers about the importance of life in the Gunflint Chert were published in 1965. These papers, titled "Microorganisms from the Gunflint Chert" by Stanley Tyler and Elso Barghoorn, and "Significance of the Gunflint (Precambrian) Microflora" by Preston Cloud (University of California at Santa Barbara), appeared in the journal Science. Both papers were important because they introduced the idea that life existed during the Precambrian period. Each paper focused on different aspects: Barghoorn and Tyler described the types and shapes of microorganisms found in the Gunflint Chert, while Cloud discussed the larger importance of finding life from the Precambrian period and its impact on the study of ancient life. These two papers led to many more studies about Precambrian microfossils in similar Proterozoic environments.
Age
The Gunflint chert microfauna is from the mid- to late-Paleoproterozoic era, approximately 1.878 billion years ago, as determined by Uranium-Lead dating methods. This age has changed slightly as dating methods have improved over time. Early whole-rock Rubidium-Strontium and Potassium-Argon dating suggested the Gunflint Iron Formation was between 1.56 and 1.63 billion years old. Later, whole-rock Neodymium-Samarium dating placed its age between 2.08 and 2.11 billion years. Finally, dating of ash layers found within the Gunflint Iron Formation gave ages between 1.86 and 1.99 billion years, which are closest to the current accepted age of 1.878 billion years. When the Gunflint Chert was discovered, the oldest known evidence of life was the Ediacaran fauna, which lived between 635 and 541 million years ago. This Precambrian group is much younger than the Gunflint microorganisms.
Microfaunal diversity
The most common organisms found in Gunflint are thin, thread-like structures called filaments, which are often found in layered rock formations. These filaments are usually between 0.5 and 6.0 micrometers wide and can be up to several hundred micrometers long. Scientists classify the Gunflint microfauna into two main groups: filaments and spheroids. In a major study published in 1965 by Barghoorn and Tyler, three new genera and four new species of filamentous cyanobacteria were discovered in Gunflint chert. Since then, many additional genera and species have been identified, with some named after Barghoorn, Tyler, and Cloud to honor their early work in studying Gunflint microorganisms.
Filamentous microorganisms in Gunflint chert include a mix of photosynthetic cyanobacteria and bacteria that use iron for energy. On a large scale, these filaments form layered domes that can be seen along the Gunflint Iron Formation. Examples of newly identified filamentous genera and species include Gunflintia and Animikiea septate, Entosphaeroides amplus, and Archaeorestis schreiberensis.
Spherical, spore-like structures in Gunflint chert are found throughout the Gunflint Iron Formation and range in size from 1 to 16 micrometers. These structures can be round or oval in shape and are often covered by a membrane that varies in thickness and structure. Scientists have proposed several possible identities for these spheroids, including single-celled cyanobacteria, bacterial endospores, free-swimming dinoflagellates, or fungal spores. Examples of newly identified spheroidal genera and species include Huroniospora, Eoasatrion, and Eosphaera tyleri.
Preservation of microfauna
Various ways that scientists believe helped preserve the tiny ancient life forms in the Gunflint Chert are called taphonomic models. These include organic residue preservation, fine-grained pyrite preservation, coarse-grained pyrite preservation, carbonate association, and hematite preservation. In organic residue preservation, a thin layer of dark brown material covers microorganisms, acting like a stain and protecting parts such as filaments, spore-like shapes, and carbonate structures inside the chert. Fine-grained pyrite preservation is the most common way microorganisms are preserved in the Gunflint Chert. This happens when tiny pyrite particles (measured in micrometers) mix with organic material, keeping the shapes of thread-like and round microorganisms. Coarse-grained pyrite preservation occurs when larger pyrite minerals (measured in millimeters) replace organic material in the chert, preserving the shapes of microorganisms. In carbonate association, filaments, spore-like shapes, and other organic structures are preserved when very small carbonate minerals (less than 1 micrometer in size) form inside the chert. These carbonate minerals can appear as solid shapes or as thin layers outlining ancient cyanobacterial remains. Carbonate minerals are often found near pyrite crystals. Hematite preservation is a less common method, but it sometimes happens where black stromatolitic chert meets red jasper. In this process, very small hematite filaments (less than 1 micrometer in size) surround or replace thread-like fossils, and these filaments are often covered by thin layers of carbon and pyrite grains. Because of the excellent preservation of microorganisms through these methods, the Gunflint Chert is sometimes called the first Precambrian lagerstätte, a term for a place where fossils are exceptionally well preserved.
Significance and paleoenvironmental implications
In the 1950s and 1960s, scientists did not fully understand the composition of Earth's Precambrian atmosphere. The discovery of the Gunflint microbiota showed that photosynthesis (or a similar process that produces its own food) was happening 1.8 billion years ago. This indicated that Earth's atmosphere had enough oxygen to support microbial life. The types of minerals found in the Gunflint banded iron formation show how oxygen levels and chemical reactions changed throughout the formation. The presence of different iron forms in the Gunflint Formation suggests that the atmosphere was mostly oxygen-rich, but some areas had lower oxygen levels, allowing iron to move in a form that could dissolve in water.
Although the Gunflint microfauna is no longer the oldest known life on Earth, its discovery at the time extended the estimated age of photosynthesis and the beginning of life by over one billion years. This finding encouraged many scientists to study ancient atmospheric oxygen levels and chemical conditions, and to keep searching for even older signs of microbial life.