Sulfur Cycle - Sulfur Sources and Sinks

Sulfur Sources and Sinks

Sulfur is found in oxidation states ranging from +6 in SO₄ to -2 in sulfides. Thus elemental sulfur can either give or receive electrons depending on its environment. Igneous rocks such as pyrite (FeS₂) comprised the original pool of sulfur on earth. Owing to the sulfur cycle, the amount of mobile sulfur has been continuously increasing through volcanic activity as well as weathering of the crust in an oxygenated atmosphere. Earth’s main sulfur sink is the oceans as SO₂, where it is the major oxidizing agent.

When SO₄ is assimilated by organisms, it is reduced and converted to organic sulfur, which is an essential component of proteins. However, the biosphere does not act as a major sink for sulfur, instead the majority of sulfur is found in seawater or sedimentary rocks especially pyrite rich shales and evaporite rocks (anhydrite and baryte). The amount of sulfate in the oceans is controlled by three major processes:

1. input from rivers

2. sulfate reduction and sulfide reoxidation on continental shelves and slopes

3. burial of anhydrite and pyrite in the oceanic crust.

There is no significant amount of sulfur held in the atmosphere with all of it coming from either sea spray or windblown sulfur rich dust, neither of which is long lived in the atmosphere. In recent times the large annual input of sulfur from the burning of coal and other fossil fuels adds a substantial amount SO₂ which acts as an air pollutant. In the geologic past, igneous intrusions into coal measures have caused large scale burning of these measures, and consequential release of sulfur to the atmosphere. This has led to substantial disruption to the climate system, and is one of the proposed causes of the great dying.

Dimethylsulfide is produced by the decomposition of dimethylsulfoniopropionate DMSP) from dying phytoplankton cells in the shallow levels of the ocean, and is the major biogenic gas emitted from the sea, where it is responsible for the distinctive “smell of the sea” along coastlines. DMS is the largest natural source of sulfur gas, but still only has a residence time of about one day in the atmosphere and a majority of it is redeposited in the oceans rather than making it to land. However, it is a significant factor in the climate system, as it is involved in the formation of clouds.