The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages, and a few new episodes were posted. Now, the blog/podcast is on a weekly schedule with diverse topics, and the Facebook Page showcases photos on Mineral Monday and Fossil Friday. Thanks for your interest!

Saturday, January 18, 2014

January 18. Gunflint Chert

By Richard I. Gibson

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When I was in college, the Gunflint Chert was a hot topic because back then it was the first assemblage of microfossils ever found in the Precambrian, and that discovery had just been reported in 1965. And the fact it was from Minnesota and Ontario helped, since I was learning geology in nearby Michigan and Indiana. The discovery of real fossils in rocks so old that no one thought fossils were possible was a big deal, and it touched off intense interest in early life that continues to this day.

The chert, a microcrystalline form of silica, is usually black or sometimes red. Red chert is called jasper. It holds tiny spheres, rods, and filaments that most likely represent the cyanobacteria that made stromatolite mats. It’s possible that the fossils may include some algae and fungi as well.

Stanley Tyler at the University of Wisconsin discovered and worked on these fossils beginning in 1954, and working with and Elso Barghoorn at Harvard published a report in 1965 calling them the oldest fossils ever seen. One organism, Eosphaera tyleri, named for Tyler, is a near-perfect sphere about 20 micrometers across – that’s just two hundredths of a millimeter. It may have been a floating bacterium or alga. It would take 100 of them to reach across a typical grain of sand. Image above or at right from J. W. Schopf, 2000, Proceedings of the National Academy of Sciences (PNAS), 97:6947-6953. This image is made available for teaching purposes by PNAS and I am using it on that basis. It is a copyrighted image, ©PNAS. 10 micrometers (μm) is one-one hundredth of a millimeter.

So how old are they? In 2002, Philip Fralick and colleagues, at Ontario’s Lakehead University and the Royal Ontario Museum reported a well-constrained age date for the Gunflint flora of 1 billion 878 million years, plus or minus 1.3 million, based on uranium-lead radioactive dating. That’s really very accurate, and has become accepted as the age of these fossils. We have older fossils now, but it was the Gunflint Chert that touched off the scientific gold rush to seek older and older microscopic life on earth.

The Gunflint Chert is part of a package of rocks called banded iron formation that we’ll talk more about in a couple days.

Photomicrograph at left of thin section from the Gunflint Chert. The clear areas are chert; yellowish are siderite (iron carbonate), and black is iron oxide, probably magnetite. The largest crystal, labeled M, is about ¼ millimeter across. It would take about 250 individual Eosphaeras to span this crystal. (From R.D. Irving and C.R. Van Hise, 1890, The Penokee iron-bearing series of Michigan and Wisconsin: USGS Tenth Annual Report.)

Further reading:
Source of Schopf image
The age dating
More about Stanley Tyler

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