Earth’s Oxygen Poor Oceans Challenged Early Evolution Of Life
March 19, 2013

Lack Of Oxygen In Earth’s ‘Boring Oceans’ Hindered Early Evolution Of Life

April Flowers for - Your Universe Online

An international research group led by biogeochemists at the University of California, Riverside has filled a billion-year gap in our understanding of conditions in the early ocean during a critical time in the history of life on Earth.

There is a general consensus in the scientific community currently that appreciable levels of oxygen first accumulated in the atmosphere around 2.4 to 2.3 billion years ago and that the build-up of oxygen in the ocean may have lagged behind the increase in the atmosphere by well over a billion years. The details of those conditions, however, have been elusive because of the patchiness of the ancient rock record.

Scientists are particularly interested in the period 1.8 to 0.8 billion years ago because it is the essential first chapter in the history of eukaryotes, which are single-celled or multicellular organisms with complex cellular structures compared to prokaryotes“¯such as bacteria. The rise of eurkaryotes was a milestone in the history of all life. That includes animals that first appeared around 0.6 to 0.7 billion years ago.

Despite the rise in oxygen and eukaryotes, the most interesting thing about that billion-year interval is that the first steps forward were small and remarkably unchanging over a very long period. Oxygen levels likely remained low in the atmosphere and oceans, and marine life remained dominated by simple bacteria rather than diverse and large populations of more complex eukaryotes. Scientists call this time the "boring billion" because the chemical and biological conditions in this middle age of Earth history were so static.

However, the extraordinary circumstances required to maintain such biological and chemical stasis for a billion years make the "boring billion" of great interest and worthy of closer study. This was the motivation behind the new study, which included scientists from Caltech, the University of Alberta, the University of Manitoba and the Université de Bretagne Occidentale, among others.

The researchers revealed an ancient ocean that was oxygen-free (anoxic) and iron-rich in the deepest waters and hydrogen sulfide-containing over limited regions around the ocean´s periphery. The team compiled data for metals with very specific and well-known chemical responses to oxygen conditions in the ocean, emphasizing marine sediments from this critical time interval from around the world.

"Oxygen, by contrast, was limited, perhaps at very low levels, to the surface layers of the ocean," said“¯Christopher T. Reinhard, a former UC Riverside graduate student. "What's most unique about our study, however, is that by applying numerical techniques to the data, we were able to place estimates, for the first time, on the full global extent of these conditions. Our results suggest that most of the deep ocean was likely anoxic, compared to something much less than 1 percent today."

"A new modeling approach we took allowed us to build on our past work, which was mostly limited to defining very localized conditions in the ancient ocean," Reinhard said. "The particular strength of the method lies in its ability to define chemical conditions on the seafloor that have long since been lost to plate tectonic recycling."

Enrichments of the elements chromium and molybdenum in ancient organic-rich sediment rocks actually track the amount of the metals present in ancient seawater, explained Reinhard, who is currently a postdoctoral fellow at Caltech and soon to be an assistant professor at Georgia Institute of Technology. Most critical to the study, those metal concentrations are fingerprints of global patterns in oceanic chemistry.

Molybdenum is also a bioessential element critical in the biological cycling of nitrogen as well as tracking oxygen levels in the early ocean.

"Molybdenum's abundance in our ancient rocks is also a direct measure of its availability to early life," said“¯Timothy W. Lyons, a professor of“¯biogeochemistry“¯at UCR. "Our recent results tell us that poor supplies of molybdenum and their impact on nitrogen availability may have limited the rise of oxygen in the ocean and atmosphere and the proliferation of eukaryotic life. There is more to do, certainly, but this is a very tantalizing new read of a chapter in Earth history that is anything but boring."

The results of the team´s study appeared online earlier this week in the journal Proceedings of the National Academy of Sciences.