Sulfur major reservoirs

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. 

There are 5 major reservoirs in the Earth system atmosphere, biosphere (vegetation, animals), soils, hydrosphere (oceans, lakes, rivers, groundwater), and the lithosphere (Earth crust). Elemental cycles of carbon, oxygen, nitrogen, sulfur, phosphorus, and other elements interact with the different reservoirs of the Earth system. The carbon cycle has important aspects in tropical forests due to the large amount of carbon stored in the tropical forests and the high rate of tropical deforestation 0acob 1999)-... 

Figure 15 Major reservoirs and burdens of sulfur as Tg(S) (source Charlson et aL, 1992).

Figure 22.1 depicts the major reservoirs in the biogeochemical cycle of sulfur, with estimated quantities [in Tg(S)] in each reservoir. Directions of fluxes between the reservoirs are indicated by arrows. The major pathways of sulfur compounds in the atmosphere are depicted in Figure 22.2. The numbers on each arrow refer to the description of the process given in the caption to the figure (not to fluxes). Note the small amount of sulfur in the atmosphere relative to that in the other reservoirs. Also note the significant amount of sulfur in the marine atmosphere this is the result of dimethyl sulfide (DMS) emissions from the sea.

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... 

THE LIGHT STABLE isotopes of hydrogen, carbon, nitrogen, oxygen, and sulfur (HCNOS) are natural tracers of geologic/planetary processes. These elements and their isotopes are major constituents of common compounds that are foimd in gas, liquid, and solid form. As such, they make excellent tracers of interactions between major reservoirs such as the atmosphere, hydrosphere, lithosphere, and asthenosphere of a planetary bo(. Their isotopic ratios are readily measured by mass spectrometric methods. 

The vast majority of sulfur at any given time is in the lithosphere. The atmosphere, hydrosphere, and biosphere, on the other hand, are where most transfer of sulfur takes place. The role of the biosphere often involves reactions that result in the movement of sulfur from one reservoir to another. The burning of coal by humans (which oxidizes fossilized sulfur to SO2 gas) and the reduction of seawater sulfate by phytoplankton which can lead to the creation of another gas, dimethyl sulfide (CH3SCH3), are examples of such processes. 

Characteristics often ascribed to MVT deposits include temperatures generally <200°C and deposition from externally derived fluids, possibly basinal brines. Sulfur isotope valnes from MVT deposits suggest two major sulfide reservoirs, one between -5 and +15%c and one greater than +20%c (Seal 2006). Both sulfide reservoirs can be related, however, to a common sea water sulfate source that has undergone different sulfur fractionation processes. Reduction of sulfate occurs either bacterially or by abiotic thermochemical reduction. High 5 S-values should reflect minimal fractionations associated with thermochemical reduction of sea water sulfate (Jones et al. 1996). 

While the sulfuric acid is key nucleation precursor in the low troposphere, its contribution to the polar stratospheric chemistry is a lot more modest. Another strong acid-nitric-plays a major role as the dominant reservoir for ozone destroying odd nitrogen radicals (NOj) in the lower and middle polar stratosphere. Nitric acid is an extremely detrimental component in the polar stratosphere clouds (PSCs), where nitric acid and water are the main constituents, whose presence significantly increases the rate of the ozone depletion by halogen radicals. Gas-phase hydrates of the nitric acid that condense and crystallize in the stratosphere play an important role in the physics and chemistry of polar stratospheric clouds (PSCs) related directly to the ozone depletion in Arctic and Antarctic. 

The contents of some trace elements in the continental crust, shales, soils, bituminous coals and plankton are given in Table 1.1 to provide some perspective when considering other aspects of these elements. In each of these situations, organic matter is associated with the elements to a greater or a lesser degree. This is not usually very marked with crustal rocks except shales, but may be a major factor for some elements in surface soils and coals. The data in Table 1.1 show that, for some elements, e.g. beryllium, cadmium, cobalt and molybdenum, the contents of the various reservoirs are similar, while for others, there may be enrichments relative to the crust, e.g. boron and sulfur in many shales, soils and coals, mercury, nickel and selenium in many shales, and germanium in some coals. 

In comparison to other spheres, the sulfur content of the atmosphere is small, about 1.8 Tg compared with 1.3 X 10 Tg for hydrosphere (Table 6.4.1). However, in terms of the global cycle of sulfur, the atmosphere plays a complex and critical role (Fig. 6.4.1). The residence time for sulfur in the atmosphere is considered to be a few days with wide variations dependent upon meteorological and other factors (Kellogg et al., 1972). This contrasts with the case of the lithosphere, for example, which although by far the largest sulfur reservoir, has a turnover time in the order of millions of years (Holser and Kaplan, 1966). The atmosphere is also the recipient of the majority of anthropogenic sulfur. 

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