Snowball Earth: Life Before The Oxygen Atmosphere

How do we begin to understand what early life was like on Earth about 700 million years ago as our planet shifted from an oxygen-free and probably ice-covered realm to the oxygen-rich world that we know today?

2.1 billion year old rock with black-band ironstone, evidence of oxygen generation/Wikipedia

Banded iron formations (BIF) are sedimentary rocks of layered iron oxide and iron-poor chert. In the presence of oxygen, iron naturally rusts and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of the Earth's atmosphere during the Paleoproterozoic era, when dissolved iron in the ocean came in contact with photosynthetically-produced oxygen and precipitated out as iron oxide.

The bands were produced at the tipping point between an anoxic and an oxygenated ocean. Since today's atmosphere is oxygen rich (nearly 21 percent by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Paleoproterozoic (after 1.8 billion years ago) are associated with Cryogenian glacial deposits.

One geochemist who decodes the early record of life on Earth has found a method featuring a combination of chemical analyses for a significantly clearer picture of this dynamic environment. Alison Olcott Marshall of the University of Kansas presented her findings today at the Goldschmidt Conference in Knoxville, Tenn. The conference is attended by several thousand geochemists and features new scientific discoveries regarding the Earth, energy and the environment. It is hosted by the University of Tennessee, Knoxville, and Oak Ridge National Laboratory.

Marshall is particularly interested in the time called Snowball Earth, a period at the end of the Precambrian Era when geochemists speculate that the world was covered from pole to pole with glacial ice and the existing organisms lived exclusively in water. At that time life was still primarily single-cell organisms. So Marshall looks at chemical fossils to recreate the environment. The chemical complexes left from the cell walls of these organisms are more abundant and more easily classified than body fossils within the samples.

Marshall's research carries her to southeast Brazil, where there is stable sedimentary rock from the late Precambrian era. Her samples come from exploratory drilling that reached eight hundred meters down into the core of black shale that was at the bottom of a sea 700 million years ago.

"The one caveat with biomarkers (chemical fossils) is that there is always a danger of contamination," Marshall said. Her initial tests using an instrument that looks at chemical compounds by molecular weight often had questionable results due to the possibility of contamination from material of a later period. However, by using another type of high resolution analysis called Raman spectroscopy, she also measured the subtle nuances of vibration that occur at the molecular level. Her high resolution results revealed two previously undetected distinctions in time generations.

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The Goldschmidt Conference, held the week of June 13-19 in Knoxville, Tenn., is sponsored by a number of international geochemical societies and named for Victor Goldschmidt (1888-1947), the Swiss-Norwegian scientist and father of geochemistry. University of Tennessee at Knoxville