Great Oxygenation Event Caused Ice Age

Geologists may have uncovered the answer to an age-old question – an ice-age-old question, that is. It appears that Earth's earliest ice ages may have been due to the rise of oxygen in Earth's atmosphere, which consumed atmospheric greenhouse gases and chilled the earth.

Alan J. Kaufman, professor of geology at the University of Maryland, Maryland geology colleague James Farquhar, and a team of scientists from Germany, South Africa, Canada, and the U.S.A., uncovered evidence that the oxygenation of Earth's atmosphere – generally known as the Great Oxygenation Event – coincided with the first widespread ice age on the planet.

"We can now put our hands on the rock library that preserves evidence of irreversible atmospheric change," said Kaufman. "This singular event had a profound effect on the climate, and also on life."

Using sulfur isotopes to determine the oxygen content of ~2.3 billion year-old rocks in the Transvaal Supergroup in South Africa, they found evidence of a sudden increase in atmospheric oxygen that broadly coincided with physical evidence of glacial debris, and geochemical evidence of a new world-order for the carbon cycle.

"The sulfur isotope change we recorded coincided with the first known anomaly in the carbon cycle. This may have resulted from the diversification of photosynthetic life that produced the oxygen that changed the atmosphere," Kaufman said.

Two and a half billion years ago, before the Earth's atmosphere contained appreciable oxygen, photosynthetic bacteria gave off oxygen that first likely oxygenated the surface of the ocean, and only later the atmosphere. The first formed oxygen reacted with iron in the oceans, creating iron oxides that settled to the ocean floor in sediments called banded iron-formations – layered deposits of red-brown rock that accumulated in ocean basins worldwide. Later, once the iron was used up, oxygen escaped from the oceans and started filling up the atmosphere.

Once oxygen made it into the atmosphere, Kaufman's team suggests that it reacted with methane, a powerful greenhouse gas, to form carbon dioxide, which is 62 times less effective at warming the surface of the planet. "With less warming potential, surface temperatures may have plummeted, resulting in globe-encompassing glaciers and sea ice" said Kaufman.

In addition to its affect on climate, the rise in oxygen stimulated the rise in stratospheric ozone, our global sunscreen. This gas layer, which lies between 12 and 30 miles above the surface, decreased the amount of damaging ultraviolet sunrays reaching the oceans, allowing photosynthetic organisms that previously lived deeper down, to move up to the surface, and hence increase their output of oxygen, further building up stratospheric ozone.

"New oxygen in the atmosphere would also have stimulated weathering processes, delivering more nutrients to the seas, and may have also pushed biological evolution towards eukaryotes, which require free oxygen for important biosynthetic pathways," said Kaufman.

Also keep in mind that the sun wasn't quite as bright back then.

Abstract:

Reconstructing Earth's surface oxidation across the Archean-Proterozoic transition
Qingjun Guo1,2, Harald Strauss2, Alan J. Kaufman2,3, Stefan Schröder4,5, Jens Gutzmer4,6, Boswell Wing7, Margaret A. Baker8, Andrey Bekker9, Qusheng Jin10, Sang-Tae Kim8 and James Farquhar3

1State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China
2Geologisch-Paläontologisches Institut der Westfälischen Wilhelms-Universität Münster, Corrensstraße 24, 48149 Münster, Germany
3Department of Geology and Earth System Science Interdisciplinary Center, University of Maryland, College Park, Maryland 20742, USA
4Paleoproterozoic Mineralization Research Group, Department of Geology, University of Johannesburg, Auckland Park, 2006 Johannesburg, South Africa
5Total E&P, Ave Larribau, F-64018 Pau, France
6Institut für Mineralogie, TU Bergakademie Freiberg, 09596 Freiberg, Germany
7Department of Earth and Planetary Sciences and GEOTOP UQAM-McGill, McGill University, Montreal, Quebec H3A 2A7, Canada
8Department of Geology, University of Maryland, College Park, Maryland 20742, USA
9Department of Geological Sciences, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
10Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA

*E-mail: guoqingjun@vip.skleg.cn.

The Archean-Proterozoic transition is characterized by the widespread deposition of organic-rich shale, sedimentary iron formation, glacial diamictite, and marine carbonates recording profound carbon isotope anomalies, but notably lacks bedded evaporites. All deposits reflect environmental changes in oceanic and atmospheric redox states, in part associated with Earth's earliest ice ages. Time-series data for multiple sulfur isotopes from carbonate-associated sulfate as well as sulfides in sediments of the Transvaal Supergroup, South Africa, capture the concomitant buildup of sulfate in the ocean and the loss of atmospheric mass-independent sulfur isotope fractionation. In phase with sulfur is the earliest recorded positive carbon isotope anomaly, convincingly linking these environmental perturbations to the Great Oxidation Event (ca. 2.3 Ga).

Link to the Paper.