Sulfur surface reservoir

Of the major volatile elements described above, water, a variety of carbon compounds, nitrogen and sulfur are the volatile compounds which dominate in the modern Earth. In this section we review the modern-Earth geochemical cycles for water, carbon, nitrogen, and sulfur and look in some detail at volatile mass balances between the Earth s surface reservoirs and the deep Earth. Then, having established how the modern Earth works we seek to determine how these geochemical cycles might have operated in the early Earth. 

In terms of the isotopic composition of volatiles in different Earth reservoirs, efficient recycling between the surface and mantle would tend to minimize isotopic differences between reservoirs. This is the case for sulfur and carbon where isotopic differences between the mantle and surface reservoirs are at the level of <10%o. Isotopic differences for water between the mantle and surface (e.g., ocean water) are larger ( 70 to 100%o Poreda et al. 1986). However, there is a huge difference in °Ar/ Ar ratios between the (upper) mantle (>40,000) and the atmosphere (296). This is consistent with very inefficient recycling of Ar into the deeper mantle, and supports the notion of a subduction barrier for argon (Staudacher and Allegre 1988). 

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. 

Table 13-1 includes many of the key naturally occurring molecular species of sulfur, subdivided by oxidation state and reservoir. The most reduced forms, S( — II), are seen to exist in all except the aerosol form, in spite of presence of free O2 in the atmosphere, ocean and surface waters. With the exception of H2S in oxygenated water, these species are oxidized very slowly by O2. The exception is due to the dissociation in water of H2S into H + HS . Since HS reacts quickly with O2, aerobic waters may contain, and be a source to the atmosphere of, RSH, RSR etc. but not of H2S itself. Anaerobic waters, as in swamps or intertidal mudflats, can contain H2S and can, therefore, be sources of H2S to the air. 

The definition of turnover time is total burden within a reservoir divided by the flux out of that reservoir - in symbols, t = M/S (see Chapter 4). A typical value for the flux of non-seasalt sulfate (nss-SOl"") to the ocean surface via rain is 0.11 g S/m per year (Galloway, 1985). Using this value, we may consider the residence time of nss-S04 itself and of total non-seasalt sulfur over the world oceans. Appropriate vertical column burdens (derived from the data review of Toon et ai, 1987) are 460 fxg S/m for nss-801 and 1700 jig S/m for the sum of DMS, SO2, and nss-S04. These numbers yield residence times of about 1.5 days for nss-S04 and 5.6 days for total non-seasalt sulfur. We might infer that the oxidation process is frequently... 

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. 

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. 

Equations (4.1) through (4.18) are supplemented in each cell of the spatial division of the ocean surface with initial conditions (Table 4.3). The boundary conditions for Equations (4.11) through (4.18) are zero. The calculation procedure to estimate sulfur concentration consists of two stages. First, at each time moment th for all cells Qiy, Equations (4.1)-(4.18) are solved by the quasi-linearization method, and all reservoirs of sulfur are estimated for ti+x = tf + At, where time step At is chosen from the convergence state of the calculation procedure. Then, at moment t(+1 using the climate unit of the global model these estimates are specified with account of the atmospheric transport and ocean currents over time At. 

Flux Ci4 relates to the surface and water reservoirs of sulfur. Let a be the share of the river system area on land and d3 the proportion coefficient, then CM = d3 JV S04L T (Ljj T L)2)e7. 

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. 

To begin the discussion, we will present briefly a view of the modern carbon cycle, with emphasis on processes, fluxes, reservoirs, and the "CO2 problem". In Chapter 4 we introduced this "problem" here it is developed further. We will then investigate the rock cycle and the sedimentary cycles of those elements most intimately involved with carbon. Weathering processes and source minerals, basalt-seawater reactions, and present-day sinks and oceanic balances of Ca, Mg, and C will be emphasized. The modern cycles of organic carbon, phosphorus, nitrogen, sulfur, and strontium are presented, and in Chapter 10 linked to those of Ca, Mg, and inorganic C. In conclusion in Chapter 10, aspects of the historical geochemistry of the carbon cycle are discussed, and tied to the evolution of Earth s surface environment. 

Sulfur is a prime candidate for the principal light element in the core. It has strong affinity for iron, reduces density and surface tension of iron, preferentially partitions into the liquid phase upon freezing, and dissolves into solid iron under high pressure and temperature. Until recently, the only strong objection for sulfur came from geochemical considerations (Dreibus and Palme, 1995 see Chapter 2.15). Theoretical studies indicate that the sulfur contents in liquid and solid iron under core pressure may be too similar to satisfy the density deficits in both reservoirs. This issue can be resolved by experimental studies in the near future. 

Figure 15.18. Comparison of global reservoirs and their residence times (t in years) (Example 15.3). The reservoirs of the atmosphere, of surface fresh waters, and of living biomass are significantly smaller than the reservoirs of sediment and marine waters and are thus more susceptible to distuibance. For example, the combustion of fossil fuel (from the reservoir of organic carbon in sediments) will have an impact on the smaller reservoirs CO2 in the atmosphere will be markedly enlarged. This combustion also fixes some N2 to NO and NO2 sulfur, associated with the organic carbon, introduces CO2 into the atmosphere. These nitrogen and sulfur compounds are washed out relatively rapidly into soil and aquatic ecosystems. The total groundwater reservoir may be twice that of surface fresh water but, however, is less accessible. (From Stumm, 1986.)...

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. 

At higher temperatures (65-120 °C) elemental sulfur is also soluble in compressed gases hke nitrogen, methane, carbon dioxide, and hydrogen sulfide, a fact which is of tremendous technical importance for the gas industry since many natural gas reservoirs also contain H2S and elemental sulfur. During production of the gas the sulfur is partly transported to the surface and precipitates on decompression and/or cooling of the gas mixture at the well-head [160-165]. Clogging of pipelines may then result [166]. 

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). 

Sulfate anions are retained only weakly by soils, but the retention increases with soil acidity. Sulfate anions are absorbed readily by plants and incorporated into biomass. Hence, biomass and SOM constitute large sulfur reservoirs at the earth s surface. The C/S mass ratio in soil organic matter is typically about 100/1. The sulfate content of soils increases with aridity and with salt accumulation. 

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