Sulphur cycle changes

Farquhar et al. (2000) investigated mass-independent fractionation in sulphur isotopes from across the period when oxygen levels are thought to have risen. They found a major change somewhere between 2090 and 2450 Ma ago. In rocks older than this, gas-phase atmospheric reactions may have influenced the sulphur cycle, playing a role in determining the oxidation state of sulphur. This would imply that atmospheric oxygen had low partial pressures, and microbial oxidation and reduction of sulphur were... 

Comparison of the global sulphur cycle as it is thought to have been prior to any major anthropogenic influence (Fig. 7.17a) with the cycle as it was in the mid 1980s (Fig. 7.17b) reveals some interesting apparent changes in the sizes of some inter-reservoir fluxes. There are also, however, some fluxes for which there is little or no evidence of change, and these are discussed first. 

Parts of the sulphur cycle which are thought to have changed significantly as a result of human activities include the following ... 

Fig. 4.17. The biological sulphur cycle the oxidations of the sulphur atoms are shown as tick arrows, reductions as thin arrows, and reactions involving no valence change as... 

Andreae, M. O. (1991) Biomass burning Its history, use, and distribution and its impact on environmental quality and global climate. In Levine (1991) pp. 3-27 Andreae, M. O. and W. Jaeschke (1992) Exchange of sulphur between biosphere and atmosphere over temperate and tropical regions. In Sulphur cycling on the continents (Eds. R. W. Howarth, J. W. B. Stewart and M. V. Ivanov). John WUey Sons, New York, pp. 27-61 Andreae, M. O. (1995) Climatic effects of changing atmospheric aerosol levels. In World survey of climatology. Vol. 16 Future climates of the world (Ed. A. Henderson-Sellers). Elsevier, Amsterdam, pp. 341-392... 

The cycle of changes proceeds until all the sulphur dioxide and oxygen are consumed ... 

Sulphur isotopes (32,33,34,36S) fractionate strongly in the earth s crust because (1) the element occurs in different oxidation states with differential preference for heavy isotopes, (2) the existence of volatile and easily soluble compounds favors kinetic separations, and (3) it is involved in biogenic cycles where the oxidation state is easily changed and kinetic processes are important. From theoretical calculations of Bigeleisen (1961) and data on the isotopic properties of sulphur compounds by Sakai (1957, 1968), the amount of S isotope fractionation and its temperature dependence is known. The information on experimental inorganic isotope fractionation in coexisting sulphide minerals which occur naturally was summarized by Thode (1970), who also discussed the application of S isotopes from sulphides for geo thermometry (cf. also Sakai, 1971). Analytical work on all types of sulphur compounds which occur in nature has been reviewed by Nielsen (1973). 

It is clear from the comparisons above that human activities have substantially changed the cycling of sulphur between the atmosphere, ocean and land surface. This alteration is arguably even greater than that described earlier for human impact on the carbon cycle (Section 7.2.3), and its impact locally and regionally is certainly more apparent, as described below. 

Any change in enviromnental conditions such as light, temperature, water and nutrient availability may modulate the photochemical reactions of photosynthesis to a different extent than the biochemical reactions involved in carbon reduction cycle, photorespiration, and nitrogen and sulphur assimilation. Consequently, these environmental changes will modulate excitation pressure. Excitation pressure reflects an imbalance between energy absorbed through photochemistry and energy utilized... 

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