8.14 The Global Sulfur Cycle P. Brimblecombe University of East Anglia, Norwich, UK 8.14.1 ELEMENTARY ISSUES 646 8.14.1.1 History 646 8.14.1.2 Isotopes 646 8.14.1.3 Allotropes 647 8.14.1.4 Vapor Pressure 647 8.14.1.5 Chemistry 647 8.14.2 ABUNDANCE OF SULFUR AND EARLY HISTORY 650 8.14.2.1 Sulfur in the Cosmos 650 8.14.2.2 Condensation, Accretion, and Evolution 651 8.14.2.3 Sulfur on the Early Earth 651 8.14.3 OCCURRENCE OF SULFUR 653 8.14.3.1 Elemental Sulfur 653 8.14.3.2 Sulfides 653 8.14.3.3 Evaporites 653 8.14.3.4 The Geological History of Sulfur 653 8.14.3.5 Utilization and Extraction of Sulfur Minerals 654 8.14.4 CHEMISTRY OF VOLCANOGENIC SULFUR 655 8.14.4.1 Deep-sea Vents 655 8.14.4.2 Aerial Emissions 656 8.14.4.3 Fumaroles 656 8.14.4.4 Crater Lakes 656 8.14.4.5 Impacts of Emissions on Local Environments 657 8.14.5 BIOCHEMISTRY OF SULFUR 657 8.14.5.1 Origin of Life 657 8.14.5.2 Sulfur Biomolecules 658 8.14.5.3 Uptake of Sulfur 659 8.14.6 SULFUR IN SEAWATER 659 8.14.6.1 Sulfate 659 8.14.6.2 Hydrogen Sulfide 659 8.14.6.3 OCS and Carbon Disulfide 660 8.14.6.4 Organosulfides 660 8.14.6.5 Coastal Marshes 662 8.14.7 SURFACE AND GROUNDWATERS 662 8.14.8 MARINE SEDIMENTS 663 8.14.9 SOILS AND VEGETATION 664 8.14.10 TROPOSPHERE 665 8.14.10.1 Atmospheric Budget of Sulfur Compounds 665 8.14.10.2 Hydrogen Sulfide 665 8.14.10.3 Carbonyl Sulfide 666 8.14.10.4 Carbon Disulfide 667 8.14.10.5 Dimethyl Sulfide 667 8.14.10.6 Dimethylsulfoxide and Methanesulfonic Acid 668 8.14.10.7 Sulfur 669 8.14.10.8 Sulfur Dioxide 669 8.14.10.9 Aerosol Sulfates and Climate 671 8.14.10.10 Deposition 671 645 646 The Global Sulfur Cycle 8.14.11 ANTHROPOGENIC IMPACTS ON THE SULFUR CYCLE 672 8.14.11.1 Combustion Emissions 672 8.14.11.2 Organosulfur Gases 673 8.14.11.3 Acid Rain 673 8.14.11.4 Water and Soil Pollutants 674 8.14.11.5 Coastal Pollution 675 8.14.12 SULFUR IN UPPER ATMOSPHERES 675 8.14.12.1 Radiation Balance and Sulfate Particles 675 8.14.12.2 Ozone 676 8.14.12.3 Aircraft 676 8.14.13 PLANETS AND MOONS 677 8.14.13.1 Venus 677 8.14.13.2 Jupiter 677 8.14.13.3 Io 677 8.14.13.4 Europa 677 8.14.14 CONCLUSIONS 677 REFERENCES 679 8.14.1 ELEMENTARY ISSUES gave the “story of sulfur” in an address on the 8.14.1.1 History concentration and circulation of the elements (Lindgren, 1923). The story is essentially a sulfur Sulfur is one of the elements from among the cycle with the notion of the volcanic release of small group of elements known from ancient reduced sulfur and oxidation and the transport as times. Homer described it as a disinfectant, and in soluble sulfates to the sea. He recognized that In Fasti (IV, 739–740) Ovid wrote: “caerulei sulfate in the oceans would rapidly have domi- fiant puro de sulpure fumi, tactaque fumanti nated over chloride were it not for biologically sulpure balet ovis…,” which explains how the mediated reduction. Later Conway (1942) specu- blue smoke from burning pure sulfur made the lated on the importance of oceanic hydrogen sheep bleat. Less poetical Roman writers gave sulfide as a source to the sulfur cycle. The idea more detail and distinguished among the many of elemental cycles formed a central part of forms of elemental sulfur, which could be mined Rankama and Sahama’s (1950) Geochemistry. from volcanic regions. It was used in trade and There were many early sulfur cycles drawn up craft activities such as cleaning of wool. In including those of Robinson and Robins, Granat addition to the native form, sulfur is also widely et al., Kellogg et al., and Friend et al. that found as sulfate and sulfide minerals. It was formed the basis of more recent cycles (e.g., important to alchemists, because they were able to Brimblecombe et al., 1989; Rodhe, 1999) produce sulfuric acid by heating the mineral, (Figure 1). green vitriol, FeSO4·7H2O and then condensing the acid from the vapor: FeSO4·7H2OðsÞ ! H2SO4ðlÞ þ FeOðsÞ þ 6H2OðgÞ 8.14.1.2 Isotopes Sulfuric acid rose to become such an important There are four stable isotopes of sulfur as listed industrial chemical that the demand for sulfur in in Table 1. The isotopic abundances vary slightly the production of the acid has sometimes been and this is frequently used to distinguish the considered an indicator of national wealth. source of the element. Because measurement of Early geochemists, such as Victor Moritz absolute isotope abundance is difficult, relative Goldschmidt (1888–1947), recognized the diffi- isotopic ratios are measured by comparison with culties associated with the geochemistry of a the abundance of the natural isotopes in a standard highly mobile and biologically active element sample. The Canyon Diablo meteorite has been such as sulfur. Fortunately, the composition of used as a standard for sulfur isotopes. various reservoirs was known by the beginning of There are nine known radioactive isotopes and the twentieth century and carefully arranged six are listed in Table 2. Sulfur-35 has the longest analyses in terms of reservoirs: the crust, waters, half-life and is produced by cosmogenic synthesis and the atmosphere. The collected data were in the upper atmosphere: cosmogenic S-35 compiled and clearly arranged by Frank W. Clarke (Tanaka and Turekian, 1991) is sufficiently long in his US Geological Survey (USGS) Bulletins: lived to be useful in determining overall removal The Data of Geochemistry from 1908. Soon after and transformation rates of SO2 from the atmos- Waldemar Lindgren, who had also been with the phere and an estimated dry deposition flux to total USGS and became head of the Massachusetts flux ratio is ,0.20 in the eastern US (Turekian and Institute of Technology’s Department of Geology, Tanaka, 1992). Elementary Issues 647 Figure 1 A generalized geochemical cycle for sulfur of the early 1970s. Note the large emissions of hydrogen sulfide from the land and oceans and that volcanic sulfur emissions are neglected (units: Tg (s) a2 1). Table 1 Stable isotopes of sulfur. atoms, which are essentially equivalent. This is not true of the S form, which is found in four Isotope Mass Abundance 7 crystalline modifications. Here the interatomic (%) distances vary between 199.3 pm and 218.1 pm. S-32 31.97207 94.93 This latter distance is exceptionally large com- S-33 32.971456 0.76 pared to the 2.037–2.066 pm typical of other sulfur S-34 33.967886 4.29 rings. S-36 35.96708 0.02 8.14.1.4 Vapor Pressure Table 2 Radioactive isotopes of sulfur. Sulfur vapor is a relatively volatile element Isotope Half-life Principal radiation and the vapor contains polyatomic species over the (MeV) range S2 –S10, with S7 the main form at high temperatures (see Figure 2). The strong sulfur– S-29 0.19 s p 21 þ þ sulfur double bond in S (422 kJ mol ) means S-30 1.4 s b 5.09, b 4.2, g 0.687 2 S-31 2.7 s bþ 4.42, g 1.27 that monatomic sulfur is found only at very high S-35 88 d b2 0.167 temperatures (.2,200 8C). S-37 5.06 min b2 4.7, b2 1.6, g 3.09 Molten sulfur is known from volcanic lakes S-38 2.87 h b2 3.0, b2 1.1, g 1.27 (Oppenheimer and Stevenson, 1989). The ele- mental liquid is a complex material. Elemental sulfur melts at ,160 8C giving a yellow liquid, 8.14.1.3 Allotropes which becomes brown and increasingly viscous as the temperature rises in the range 160–195 8C, In 1772 Lavoisier proved that the sulfur is an which is interpreted as a product of polymeri- elementary substance, which was not necessarily zation into forms that can contain more than obvious given that it is characterized by a number 2 £ 105 sulfur atoms. As temperature increases of allotropes (i.e., different forms). Sulfur has above this, the chain length (and thus viscosity) more allotropes than any element because of the decreases to ,100 by 600 8C. If molten sulfur readiness to form S—S bonds (Table 3). These is cooled rapidly by pouring into water, it bonds can be varied both in terms of length and condenses into plastic sulfur that can be stretched angle, so open and cyclic allotropes of Sn are as long fibers, which appear to be helical chains known where n ranges between 2 and 20. The of sulfur atoms with ,3.5 atoms for each turn of familiar form of yellow orthorhombic sulfur the helix (Cotton et al., 1999). (a-sulfur) is a cyclic crown S8 ring. Two other S8 ring forms are known the b-orthorhombic and the 8.14.1.5 Chemistry g-orthorhombic found at higher temperatures. Engel in 1891 prepared a rhombohedral form Sulfur is very reactive and will combine directly 1-sulfur, which was ultimately shown to be an S6 with many elements.