The global sulphur cycle and anthropogenic effects

We now turn to the cycling of the element sulphur, outlining the nature of the cycle prior to any major alteration by human industrial and urban activity and examining how these activities have impacted, in a very major way, on the contemporary sulphur cycle. 

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  

3 The deposition flux of sulphur from the atmosphere on to the oceans and land surfaces has increased by approximately 2 5 and 163%, respectively. Although this input has essentially no impact on the chemistry of seawater, due to its buffer capacity and the large amount of sulphate (SO -) it contains (see 

4 The amount of sulphur entering the oceans in river runoff has probably more than doubled due to human activities (compare the fluxes in Fig. 7.17a b). This has been caused in part by sulphur-rich wastewaters and agricultural fertilizers entering river and groundwaters and thence the sea, although another major factor is sulphur deposited directly into surface waters from the atmos-ph ere. The combined (atmospheric and runoff) effects of enhanced sulphur inputs to seawater cause an increase of sulphur (as SO)- in the oceans) of only about 10 5% per annum. This estimate is probably an upper limit, since it assumes that removal of seawater sulphur into ocean sediments (see Section 6.4.6) remains as previously and has not increased following the enhanced inputs from the atmosphere and rivers. 

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