Oxygen-Free Oceans Wrought Changes on Early Earth

Geologists at the University of California, Riverside, have discovered chemical evidence that indicates Earth’s ancient oceans were not only oxygen-free, but also contained large quantities of hydrogen sulphide in some areas.

“We are the first to show that ample hydrogen sulfide in the ocean was possible this early in Earth’s history,” said Timothy Lyons, a professor of biogeochemistry and the senior investigator in the study, which appears in the February issue of Geology. “This surprising finding adds to growing evidence showing that ancient ocean chemistry was far more complex than previously imagined and likely influenced life’s evolution on Earth in unexpected ways – such as, by delaying the appearance and proliferation of some key groups of organisms.”

Hydrogen sulphide traditionally was believed to arise as a result of a process which saw oxygen weathering rocks, resulting in sulphate, which in turn was washed into the oceans where bacteria then convert the sulphate into hydrogen sulphide.

How then, did an almost oxygen-free environment create large quantities of hydrogen sulphide in the oceans, as the 2.6 million year old rocks which were studied by the UCR geologists suggested.

The answer is volcanoes.

With the right amount of volcanic activity and sources, the sulphate delivery which is currently the result of oxygen weathering can take place.

“Our previous work showed evidence for hydrogen sulfide in the ocean more than 100 million years before the first appreciable accumulation of oxygen in the atmosphere at the Great Oxidation Event,” Lyons said. “The data pointing to this 2.5 billion-year-old hydrogen sulfide are fingerprints of incipient atmospheric oxygenation. Now, in contrast, our evidence for abundant 2.6 billion-year-old hydrogen sulfide in the ocean – that is, another 100 million years earlier – shows that oxygen wasn’t a prerequisite. The important implication is that hydrogen sulfide was potentially common for a billion or more years before the Great Oxidation Event, and that kind of ocean chemistry has key implications for the evolution of early life.”

Research such as this plays an important part in helping us understand the evolution of life on Earth.

“Our research has important implications for the evolutionary history of life on Earth,” added Clint Scott, the first author of the research paper and a former graduate student in Lyons’s lab, “because biological evolution both initiated and responded to changes in ocean chemistry. We are trying to piece together the cause-and-effect relationships that resulted, billions of years later, in the evolution of animals and, ultimately, humans. This is really the story of how we got here.”

Source: University of California, Riverside

Image Source: Nogwater

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