Sulfur cycle reservoirs

Comparison of Figs 13-6a and 13-6b clearly demonstrates the degree to which human activity has modified the cycle of sulfur, largely via an atmospheric pathway. The influence of this perturbation can be inferred, and in some cases measured, in reservoirs that are very distant from industrial activity. Ivanov (1983) estimates that the flux of sulfur down the Earth s rivers to the ocean has roughly doubled due to human activity. Included in Table 13-2 and Fig. 13-6 are fluxes to the hydrosphere and lithosphere, which leads us to these other important parts of the sulfur cycle. 

Additional material on this subject is provided in the supplemental information for Chapter 25.4 that is available online at http //elsevierdirect.eom/companions/9780120885305. Key topics covered are the role of tectonism in the geologic carbon cycle and how the evolution of pelagic calcifiers in the Phanerozoic led to the development of feedbacks, some stabilizing and some destabilizing, that act on the atmospheric COj reservoir. Also included is a short summary of how the global carbon cycle interacts with the atmospheric O2 and sulfur cycles. 

The biochemical reduction of sulfate to sulfide by bacteria of the genus Desulfovibrio in anoxic waters is a significant process in terms of the chemistry of natural waters since sulfide participates in precipitation and redox reactions with other elements. Examples of these reactions are discussed later in this paper. It is appropriate now, however, to mention the enrichment of heavy isotopes of sulfur in lakes. Deevey and Nakai (13) observed a dramatic demonstration of the isotope effect in Green Lake, a meromictic lake near Syracuse, N. Y. Because the sulfur cycle in such a lake cannot be completed, depletion of 32S04, with respect to 34S04, continues without interruption, and 32S sulfide is never returned to the sulfate reservoir in the monimolimnion. Deevey and Nakai compared the lake to a reflux system. H2S-enriched 32S diffuses to the surface waters and is washed out of the lake, leaving a sulfur reservoir depleted in 32S. The result is an 34S value of +57.5% in the monimolimnion. 

The surface part of the sulfur cycle is connected with the functioning of the atmosphere-vegetation-soil system. Plants adsorb sulfur from the atmosphere in the form of S02 (fluxes C7 and C22) and assimilate sulfur from the soil in the form of SO4 (flux C15). In the hierarchy of soil processes, two levels can be selected defining the sulfur reservoirs as dead organics and S04 in soil . The transitions between them are described by flux C16 = b2STL, where the coefficient b2 = b2, b2 2 reflects the rate b2 of transition of sulfur contained in dead organics into the form assimilated by vegetation The coefficient b2>2 indicates the content of sulfur in dead plants. 

The reduction is typically limited by the availability of organic carbon and often occurs in shallow waters at continental margins. Thus, global sulfide production would be dependent on the availability of biological productive areas over geological time. Sulfur-isotope data can be used to constrain simple models of the sulfur cycle over geological time and establish the size of the reservoirs as shown in Figure 5(b). 

The Earth s sulfur cycle (Figure 22) transfers enormous amounts of this biologically important element through various reservoirs each year. Sulfur has a wide range of oxidation states and shows the ability to form a large number of oxides and oxyanions, many of which are found in the environment. It also has the potential to form polymeric species with a significant number of... 

Hence the Archaean sulfur cycle (Fig. 5.5) would comprise inputs into the atmosphere and oceans from volcanic gases and into the oceans from hydrothermal activity but not river-borne sulfate. In addition, in the anoxic oceans, the oxidative alteration of the ocean floor would not take place. Thus the surface sulfur reservoir would have been small and most sulfur recycled back into the mantle as sulfide minerals. The sulfate part of the sulfur cycle is unlikely to have been fully operational until the late Proterozoic (Canfield, 2004). 

Ivanov, M. V., V. A. Grinenko, and A. P. Rabinovich (1983). The sulfur cycle in continental Reservoirs, Part II. Sulfur flux from continents to ocean. In The Global Biogeochemical Sulphur Cycle (M. V. Ivanov and J. R. Freney, eds.), Scope 19, 331-356. 

Fig. 10.10 Main reservoirs of the sedimentary sulfur cycle (adopted from Strauss 1997).

With the many compounds of sulfur found in the atmosphere, in aquatic environments, and in soils and minerals, sulfur cycles through the biosphere in much the same way that nitrogen does. However, unlike the relative abundances of nitrogen—for which the atmosphere is the major reservoir—the relative abundance of sulfur in the atmosphere is small compared with its abundance in other environments. 

The carbon, oxygen, and sulfur cycles are strongly coupled. Study of this coupling provides independent information on how closed (or open) the crustal cycle is over periods of several hundred milhon years (see Refs. [14,16]). The O2 from photosynthesis reacted with sulfide to form sulfate that is now a major reservoir for free oxygen. The net effect of photosynthesis is the idealized Reaction (2) with... 

Fig. 5.9 Holser and Kaplan s (1966) representation of the geochemical cycle of sulfur. Masses are in metric tons. Most material above the dashed line is oxidized to sulfate, most below this line is reduced to sulfide above the solid line heavy sulfur predominates (5 > +5 %o). Long-term (dark) and short-term fiuxes of sulfur between reservoirs are indicated on the basis of 100 for the longterm component of fresh water sulfate flowing to the sea (Holland 1984)...

Long-term sulfur cycle between reservoirs is shown in Fig. 5.9. Important processes for long-term sulfur cycle include formation of sulfur-cOTitaining compounds (e.g., pytrite in sedimentary rocks, sulfates in evaporite), oxidatimi of sulhdes in terrestrial environment, subduction of plate, emissions of volcanic gas and hydrothermal... 

A simplified diagram representing the various reservoirs and transport mechanisms and pathways involved in the cycles of nutrient elements at and above the surface of the Earth is given in Eigure 1. The processes are those considered to be the most important in the context of this article, but others of lesser significance can be postulated. Eor some of the elements, notably carbon, sulfur, chlorine, and nitrogen, considerable research has been done to evaluate (quantitatively) the amount of the various elements in the reservoirs and the rates of transfer. 

The ocean plays a central role in the hydro-spheric cycling of sulfur since the major reservoirs of sulfur on the Earth s surface are related to various oceanic depositional processes. In this section we consider the reservoirs and the fluxes focusing on the cycling of sulfur through this oceanic node. 

Accepting these relative proportions from evaporites (2/3) and sulfides (1/3), the characteristic times, T of cycling of the evaporite sulfur and sulfide sulfur reservoirs can be estimated from the reservoir sizes (R,) in Table 13-3, and the river flux of sulfur. For evaporites ... 

Because Thiobacillus feed on sulfur-containing compounds, they require other carbon-containing fuel to survive and multiply. Having a scaffold that can serve as a reservoir for nutrients could be an advantage, as opposed to the feed-and-starve cycle typically used. The team decided that the packing materials listed in Table 1.1 were not sufficient for the new biofilter design. 

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