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HARVARD GAZETTE ARCHIVES
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| Photomicrographs illustrating early diagenetic origin of pyrite. A. spheroids overgrown by fibrous pyrite in pyrite matrix; B. laminae consisting of clay minerals, organic matter, and silt-sized quartz grains bend around pyrite spheroid indicating pyrite growth before compaction; C. microbial mat rip-ups mineralized by pyrite; D. wavy and crinkly mineralized microbial mat structures in organic-rich shale. (Photomicrographs by Andry Bekker) |
Researchers close in on date of critical rise in Earth's oxygen
Harvard News Office
Harvard researchers examining black shale from more than a kilometer beneath South Africa's grasslands have shed new light on the first critical rise of oxygen in the Earth's atmosphere, finding that oxygen was present 2.32 billion years ago.
The finding narrows the range of possible dates for that critical change in the Earth's atmosphere. Scientists had previously believed oxygen first appeared sometime between 2.45 billion and 2.22 billion years ago, a span of about 230 million years. The new findings narrow that window dramatically - to about 130 million years.
"It's a fascinating transition, from a physical, chemical, and biological point of view," said Heinrich D. Holland, Harry C. Dudley Research Professor of Economic Geology, who, with postdoctoral fellow Andrey Bekker and colleagues from other institutions, conducted the research. Defining when, how, and why atmospheric oxygen rose has been the main theme of Holland's research for decades.
The initial rise in oxygen is thought to be the first of two major periods of change in the Earth's atmosphere that ultimately led to the air we breathe today. The first change occurred in the Paleoproterozoic Era at a time when life was thought to have been limited to photosynthetic algae and primitive bacteria.
The appearance of oxygen began the atmosphere's transformation from a mix that contained no oxygen, but probably contained higher levels of carbon dioxide and methane than found in today's air. That ancient atmosphere probably also featured a gentle rain of powdery yellow particles of sulfur, which in today's atmosphere is bound up in compounds with oxygen.
The increase in oxygen allowed the development of early oxygen-using creatures and sowed the seeds for the eventual development of large land animals, an event scientists believe occurred after a second large increase in oxygen more than a billion years later.
The timing and the cause of the appearance of oxygen in Earth's atmosphere have been subjects of scientific inquiry and speculation for some time. Though science is zeroing in on the date, the cause of the rising oxygen levels is still being debated.
Whatever the cause, Bekker, Holland, and their colleagues, writing in the Jan. 8 issue of Nature, detailed their investigation of a black shale, a sedimentary rock thought to have been deposited 2.32 billion years ago in a shallow marine river delta. Researchers examined rock cores drilled by mining companies exploring South Africa's rich mineral resources.
Bekker, a postdoctoral fellow in the Earth and Planetary Sciences Department until December, when he moved to the Carnegie Institution in Washington, D.C., and Holland were joined on the project by colleagues from the Carnegie Institution, Colorado State University, and Rand Afrikaans University in South Africa.
Bekker, Holland, and their colleagues used the chemistry of the South African black shales to determine the composition of the atmosphere at the time when those shales were formed. Specifically, they looked at the composition of sulfur isotopes in pyrite, a combination of iron and sulfur also known as fool's gold.
They examined the isotopes for evidence of a process called mass independent fractionation, whose effect would be present in oxygen-free skies, but which would be much reduced in skies with more than what Holland termed "a sniff" of oxygen. Their examination showed no evidence of mass independent fraction and indicated that the river delta in which the shales formed lay under oxygenated skies.
While their examination indicated that oxygen was present in the atmosphere 2.32 billion years ago, it couldn't tell how much there was above an extremely low threshold, 1/100,000th of the levels of oxygen in today's atmosphere.
Now that their research has narrowed the range of dates for the first oxygen rise, Bekker said efforts are continuing to pinpoint it more precisely. The difficulty, he said, is finding appropriate rock samples of the right type and age.
"We're still looking and hoping," Bekker said.
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