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.

There is no evidence that volcanic emissions of sulphur (mainly as sulphur dioxide, SO2) have changed significantly during the last 150 years or so (i.e. the time period between parts (a) and (b) of Fig. 7.17) for either terrestrial or marine volcanoes. Similarly, there is no evidence for significant change in the sea-to-air fluxes of either sea-salt sulphate (coming from sea spray arising from wave breaking and bubble bursting at the sea surface) or volatile sulphur, or of emissions of sulphur gases from the terrestrial biosphere. It is important to note that these gaseous fluxes are major components in the cycling of sulphur. The geochemical budget of the element cannot be balanced without them and the total emissions from marine and terrestrial sources is about 70% of the amount of sulphur put into the atmosphere by fossil-fuel burning. The principal component of the marine emissions of volatile sulphur is a gas called dimethyl sulphide (DMS; see also Section 3.4.2 and Fig. 3.4a), produced by phytoplankton (see Fig. 6.10a) and seaweeds that live in the near-surface waters of the oceans. These marine algae also produce lesser amounts of carbonyl sulphide (OCS), carbon disulphide (CS2) and possibly some hydrogen sulphide (H2S). Land plants produce a similar suite of gases, but with H2S playing a major, possibly the dominant, role.

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

1 Aeolian emissions of sulphur-containing soil dust particles are thought to have increased by a factor of about two, from 10 to 20Tgsulphuryr-1. This is largely as a result of human-induced changes in farming and agricultural practice, particularly through pasturing, ploughing and irrigation.

2 By far the most significant impact on the system has been the input of sulphur (largely as SO2) direct to the atmosphere from the burning of fossil fuels, metal smelting and other industrial/urban activities. Such emissions have increased approximately 20-fold over the last 120 years. It is not certain that this upward trend will continue indefinitely, since there are ongoing moves in the most advanced industrial nations to restrict emissions by, for example, burning sulphur-poor fuels and removal of SO2 from power-station stack gases. By contrast, sulphur emissions from the developing nations of the world are likely to increase in the future as they become more industrialized but without the resources to minimize sulphur emitted to the atmosphere. Because of the large magnitude of the fossil-fuel sulphur emissions in relation to other flows in the natural sulphur cycle, this input has substantial impacts on other parts of the cycle, some of which are discussed below.

3 The deposition flux of sulphur from the atmosphere on to the oceans and land surfaces has increased by approximately 25 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 (SO4-) it contains (see

Deposition over continents

Net flow 10

Deposition over oceans

Aeolian Volcanic emission emission

Volatile sulphur emission from continents ▲

Volatile sulphur emission from

Volatile sulphur emission from continents ▲

oceans

oceans

Deposition over continents

Net flow 61

Deposition over oceans

vn vn

Aeolian Volcanic emission emission

Anthropogenic emission

Volatile sulphur emission from

Volatile sulphur emission from oceans ▲

Sea-salt Volcanic sulphate emission

Aeolian Volcanic emission emission

Anthropogenic emission

Volatile sulphur emission from

Volatile sulphur emission from oceans ▲

Sea-salt Volcanic sulphate emission

Fig. 7.17 Simplified version of the sulphur cycle (after Brimblecombe et al. 1989). (a) Sulphur cycle as it is thought to have been prior to any major anthropogenic influence. (b) Sulphur cycle as it was in the mid-1980s. Units for inter-reservoir flows are in TgSyr-1 (i.e. 1012gSyr-1). With permission from the Scientific Community on Problems of the Environment—SCOPE, John Wiley & Sons Ltd.

Fig. 7.17 Simplified version of the sulphur cycle (after Brimblecombe et al. 1989). (a) Sulphur cycle as it is thought to have been prior to any major anthropogenic influence. (b) Sulphur cycle as it was in the mid-1980s. Units for inter-reservoir flows are in TgSyr-1 (i.e. 1012gSyr-1). With permission from the Scientific Community on Problems of the Environment—SCOPE, John Wiley & Sons Ltd.

Table 6.1), it can have a profound impact on poorly buffered soils and fresh waters, as discussed in Section 5.4.1.

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 atmosphere. The combined (atmospheric and runoff) effects of enhanced sulphur inputs to seawater cause an increase of sulphur (as SO4- 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.

5 A final difference highlighted in Fig. 7.17 is in the balance of sulphur flows between the continental and marine atmospheres. In the unperturbed cycle (Fig. 7.17 a) there is a small net flow of sulphur from the continental to the marine atmosphere (10Tgsulphuryr-1). Today this balance is substantially altered, with about six times greater net flow of sulphur in air flowing seawards (61 Tg sulphur yr-1) compared with the unperturbed situation.

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.

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