As
sulfur cycles through Earth’s atmosphere, oceans, and land, it undergoes
chemical changes that are often coupled to changes in other such elements as carbon
and oxygen. Although this affects the concentration of free oxygen, sulfur has
traditionally been portrayed as a secondary factor in regulating atmospheric
oxygen, with most of the heavy lifting done by carbon. However, new findings that appeared in Science
suggest that sulfur’s role may have been underestimated.
Itay Halevy of the Weizmann
Institute’s Environmental Science and Energy Research Department (Faculty of
Chemistry) and Shanan Peters of the University of Wisconsin and Woodward
Fischer of the California Institute of Technology were interested in better
understanding the global sulfur cycle over the last 550 million years—roughly
the period in which oxygen has been at its present atmospheric level of around
20%. They used a database developed and maintained by Peters at the University
of Wisconsin, called Macrostrat, which contains detailed information on
thousands of rock units in North America and beyond.
The
researchers used the database to trace one of the ways in which sulfur exits
ocean water into the underlying sediments—the formation of so-called sulfate
evaporite minerals. These sulfur-bearing minerals, such as gypsum, settle to
the bottom of shallow seas as seawater evaporates. The team found that the
formation and burial of sulfate evaporites were highly variable over the last
550 million years, due to changes in shallow sea area, the latitude of ancient
continents and sea level. More surprising to Halevy and colleagues was the
discovery that only a relatively small fraction of the sulfur cycling through
the oceans has exited seawater in this way. Their research showed that the
formation and burial of a second sulfur-bearing mineral—pyrite—has apparently
been much more important.
Pyrite
is an iron-sulfur mineral (also known as fools’ gold), which forms when
microbes in seafloor sediments use the sulfur dissolved in seawater to digest
organic matter. The microbes take up sulfur in the form of sulfate (bound to
four oxygen atoms) and release it as sulfide (with no oxygen). Oxygen is
released during this process, thus making it a source of oxygen in the air. But
because this part of the sulfur cycle was thought be minor in comparison to
sulfate evaporite burial (which does not release oxygen), its effect on oxygen
levels was also thought to be unimportant.
In
testing various theoretical models of the sulfur cycle against the Macrostrat
data, the team realized that the production and burial of pyrite has been much
more significant than previously thought, accounting for more than 80% of all
sulfur removed from the ocean (rather than the 30 to 40% in prior estimates).
As opposed to the variability they saw for sulfate evaporite burial, pyrite
burial has been relatively stable throughout the period. The analysis also
revealed that most of the sulfur entering the ocean washed in from the
weathering of pyrite exposed on land. In other words, there is a balance
between pyrite formation and burial, which releases oxygen, and the weathering
of pyrite on land, which consumes it. The implication of these findings is that
the sulfur cycle regulates the atmospheric concentration of oxygen more
strongly than previously appreciated.
“This
is the first use of Macrostrat to quantify chemical fluxes in the Earth
system,” said Peters. “I met my coauthors at a lecture I gave at Caltech, and
we immediately began discussing how we might apply Macrostrat to understanding
biogeochemical cycling. I think this study will open the door to many more uses
of Macrostrat for constraining biogeochemical cycles.”
“For
me, the truly surprising result is that pyrite weathering and burial appear to
be such important processes in the sulfur cycle throughout all of Earth’s
history. The carbon cycle is recognized as the central hub controlling redox
processes on Earth, but our work suggests that nearly as many electrons are
shuttled through the sulfur cycle,” said Fischer.
Halevy: “These findings, in addition to shedding new light on the role of sulfur in
regulating oxygen levels in the atmosphere, represent an important step forward
in developing a quantitative, mechanistic understanding of the processes
governing the global sulfur cycle.”
Source: Weizmann Institute of Science
