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Sulfur cycle



According to Wikipedia:

Sulfur is one of the constituents of many proteins, vitamins and hormones. It recycles as in other biogeochemical cycles. The essential steps of the sulfur cycle are: Mineralization of organic sulfur to the inorganic form, hydrogen sulfide: H 2SH_2S. Oxidation of sulfide and elemental sulfur (S) and related compounds to sulfate (SO42–).Reduction of sulfate to sulfide. Microbial immobilization of the sulfur compounds and subsequent incorporation into the organic form of sulfur.


So sulfur is needed for the creation of living organisms as described by Dr. Stanislaw Kopriva at The John Innes Centre (an Institute of the BBSRC) in UK:

Sulfur is essential for life as a constituent of the amino acids cysteine and methionine and of coenzymes, such as iron-sulfur centres, lipoic acid, or thiamine. In addition, plants synthesise a great variety of secondary metabolites containing sulfur in the oxidised form of sulfate. The best characterised group of such compounds is the glucosinolates, important for plant defence against pathogens, but also of great nutritional value. Sulfation of the desulfo- precursors of the secondary metabolites is catalysed by sulfotransferases that utilise phosphoadenosine phosphosulfate (PAPS) as a sulfate donor. PAPS is synthesised by phosphorylation of adenosine phosphosulfate (APS), a key intermediate in primary sulfate assimilation to cysteine, by APS kinase. Thus, primary and secondary sulfur metabolism is connected by a single enzymatic reaction catalysed by APS kinase


Canfield and Farqhuar on the history - with the fraction of the total sulfur leaving the oceans as pyrite? given by ff

Graph showing key aspects of the history of the sulfur cycle. (Top) Isotopic composition of sedimentary sulfides (diamonds) and sulfate (indicated by the 2 red parallel lines) through time. A complete list of data can be found in the Dataset S1. (Middle) The proportion of total sulfur buried as pyrite through time. This is calculated using Eq. 1, using data from the isotope record. Through the Phanerozoic, the data has been binned from individual Periods, and in the Precambrian, the data were binned in the time intervals: 542–580 Ma, 580–636 Ma, 636–660 Ma, 660–700 Ma, 750–805 Ma, 805 to 1000 Ma, and into 300 Ma bins hereafter. Note that between 636 and 700 Ma (between the Sturtian and Marinoan glaciations) the f ratios are off scale (2.9 and 9.3 in the 2 time bins in this interval) for reasons that are not well understood (1). (Bottom) A diagram representing our best estimate for the history of seawater sulfate concentrations. See text for details. The vertical dotted line at 2.4 Ga represents the “Great Oxidation Event” (15) and the initial rise in the concentration of atmospheric oxygen (see text for details). The line at 0.542 Ga represents the Cambrian–Precambrian boundary.

Modeling and results

Modeling by Canfrield and Farquar shows:

In summary, the evolution of bioturbating organisms had a profound impact on the concentrations of sulfate in the oceans. Indeed, we propose that the oxidation of sedimentary sulfides associated with sediment stirring by benthic animals has resulted in a severalfold increase in seawater sulfate concentrations, initiating the widespread deposition of gypsum evaporites. The cessation of bioturbation associated with major extinctions would have also influenced sulfate concentrations and left an imprint in the isotopic composition of seawater sulfate.


Abstract We use a coupled climate–carbon cycle model of intermediate complexity to investigate scenarios of stratospheric sulfur injections as a measure to compensate for CO2-induced global warming. The baseline scenario includes the burning of 5,000 GtC of fossil fuels. A full compensation of CO2-induced warming requires a load of about 13 MtS in the stratosphere at the peak of atmospheric CO2 concentration. Keeping global warming below 2 ◦ C reduces this load to 9 MtS.

Compensation of CO2 forcing by stratospheric aerosols leads to a global reduction in precipitation, warmer winters in the high northern latitudes and cooler summers over northern hemisphere landmasses. The average surface ocean pH decreases by 0.7, reducing the calcifying ability of marine organisms. Because of the millennial persistence of the fossil fuel CO2 in the atmosphere, high levels of stratospheric aerosol loading would have to continue for thousands of years until CO2 was removed from the atmosphere. A termination of stratospheric aerosol loading results in abrupt global warming of up to 5 ◦ C within several decades, a vulnerability of the Earth system to technological failure.

category: earth science