Geologists have long considered sulfate, a common salt dissolved in seawater, as the key to determining how and when life evolved. On the ancient Earth, acquiring enough ocean sulfate measurements to accurately define the ecological conditions during evolution has been a serious challenge. Now, a novel method for extracting sulfate from ancient rocks has enabled a research team including University of Missouri-Columbia geological science professor Tim Lyons to uncover new evidence for sulfate levels in prehistoric oceans.
Lyons and his collaborators, whose findings are published in the scientific journal Nature, say their results are important because the amount of sulfate in seawater tracks the amount of oxygen present at that same time. Scientists want to know when and how fast oxygen accumulated in the prehistoric oceans and atmosphere because many forms of life on Earth, particularly multicellular organisms, could not flourish without it.
In their report, Lyons and his colleagues say they were able to confirm a prior suspicion that the rise in ocean sulfate levels, and therefore the oxygenation of the atmosphere, was a protracted process that extended 1 billion to 2 billion years after the first accumulation of oxygen in the atmosphere 2.3 billion years ago. The new estimates suggest that during the time period from roughly 2.3 billion to 1.2 billion years ago, the amount of sulfate grew from less than 1 percent to no more than 15 percent of today's value.
"If the increase in oceanic sulfate and atmospheric oxygen indeed extended over more than a billion years, that undoubtedly affected how and when many forms of life evolved," Lyons said.
The researchers conducted their research by analyzing 1.7 billion-year-old and 1.2 billion-year-old ocean sediments. Some of the measurements came from gypsum, a sulfate-containing mineral, from the arctic region of Canada. They also extracted sulfate from ancient limestone, which is more abundant than gypsum, using a method they helped pioneer.
To estimate levels of sulfate in ancient seawater, the team first measured the ratio of sulfur isotopes within the sulfate. Isotopes are atoms of the same element with different numbers of neutrons in their nucleus. In recent times, the isotopic composition of sulfate has varied little, which is consistent with the high concentrations of sulfate in modern seawater.
“We saw something really different,” Lyons said. “We saw very rapid isotopic variability, which suggests there wasn't much sulfate in the early ocean and that oxygen in the atmosphere remained comparatively low for more than 80 percent of Earth's history."
Joining Lyons in the research were Linda Kah from the University of Tennessee-Knoxville and Tracy Frank of the University of Nebraska.
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