The sulfur cycle: not what you thought

In an article published in the journal Science, Dr. Itai Halevi from the Department of Environmental Sciences and Energy Research in the Faculty of Chemistry at the Weizmann Institute, together with Dr. Shannon Peters from the University of Wisconsin and Dr. and Edward Fisher from the California Institute of Technology, sought to understand the global sulfur cycle over the past 550 million years

The sulfur cycle in nature moves through the Earth's atmosphere, seas and continents. While moving, the sulfur undergoes chemical changes, and these are sometimes related to the chemistry of other elements, such as carbon and oxygen. So, for example, some single-celled organisms use sulfur compounds to digest their food, similar to how other organisms use oxygen. This activity does affect the concentration of free oxygen in the air, but sulfur is generally considered a secondary factor in the process of controlling oxygen levels in the atmosphere, while carbon is always considered the "main player". New findings recently published in the scientific journal Science indicate that the role of sulfur is greater than we thought.

Dr. Itai Halevi from the Department of Environmental Sciences and Energy Research in the Faculty of Chemistry at the Weizmann Institute, together with Dr. Shanan Peters from the University of Wisconsin and Dr. Edward Fisher from the California Institute of Technology, sought to understand the global sulfur cycle over the past 550 million years - during which the oxygen levels in the atmosphere were close to those found in it today - about 20%. For this purpose, they used a database called "Macrostrat", developed by Dr. Peters. The database contains detailed information on the geometry, age and mineralogical properties of thousands of rock units in North America and abroad.

The researchers used the database to trace one of the ways in which sulfur dissolved in ocean water forms sediments at the bottom and thus leaves the oceans: through the formation of minerals called sulfur evaporites. These sulphur-bearing minerals, such as, for example, natural gypsum, sink to the bottom of shallow seas as a result of the evaporation of seawater. The team found that the rates of production and deposition of sulfuric evaporites changed relatively frequently over the past 550 million years. These changes are related to changes in the area of ​​the shallow seas, the geographic latitudes where the ancient continents were found, and the sea level. But the discovery that surprised Dr. Halevi and his colleagues was that only a relatively small percentage of the sulfur that reaches the ocean waters leaves them in this way. Their findings pointed to a much more important way to precipitate sulfur - through another mineral, called pyrite.

Pyrite (also known as "fools' gold") is a compound of iron and sulfur, which is formed when single-celled creatures living in sediments at the bottom of the sea use sulfur dissolved in seawater to digest organic matter. These creatures absorb sulfur in the form of sulfate (that is, bound to four oxygen atoms) and release it as sulfide (without oxygen). During the process, four oxygen atoms are released, so it is considered a source of oxygen in the air. However, this part of the sulfur cycle is always considered secondary - in relation to the deposition of sulfur evaporites (which does not involve the release of oxygen), and therefore its effect on the oxygen levels in the air is considered negligible.

By checking theoretical models of different sulfur cycles against the information in the database, the team realized that the production and deposition of pyrite is actually much more significant than it is common to think: over 80% of the sulfur dissolved in the ocean water comes out of them this way (compared to previous estimates of 30-40%) Compared to the high variability in evaporite deposition throughout the period, pyrite deposition maintained relative stability. The analysis also showed that the source of most of the sulfur entering the sea is the weathering of pyrite from the land. In other words, it was discovered that there is a balance between the production and deposition of pyrite - in the process that releases oxygen, and the weathering of pyrite from the land , while consuming oxygen. The findings point to the great importance of the sulfur cycle in controlling the concentration of oxygen in the atmosphere.

Dr. Fisher: "The most surprising result is the fact that the weathering and deposition of the pyrite were, apparently, very important processes, throughout the history of the earth. The carbon cycle is known as a central player, which controls the chemistry of oxygen. However, according to our findings, a similar number of electrons move through the sulfur circuit."

Dr. Halevi: These findings not only shed new light on the role of sulfur in controlling oxygen levels in the atmosphere, but are an important step in developing a quantitative understanding of the processes that control the global sulfur cycle."

for the scientific article