Sulfur Biogeochemistry—Past and Present

Microbially mediated sulfur-redox 29reactions 7.1–7.8 listed in Table 7B are also plotted as a functionof reaction number in Figure 5. Note in this figure that, withthe exception of reactions 7.2 and 7.6, in which CO 2serves asthe oxidant, all reactions are exergonic. However, the amount ofenergy released varies tremendously from only ~21 kJ/mol S 0 forreaction 7.4 at Acque Calde 2 to almost 750 kJ/mol S 0 for reaction7.5 at Grip, Acque Calde 2, and Pozzo Istmo. Not surprisingly,reactions in which O 2serves as the TEA are the most exergonic,yielding 171 ± 8 kJ/mol S 0 for reaction 7.1 and 710 ± 34 kJ/molS 0 for reaction 7.5. Again, per mole of electrons transferred, thesetwo reactions yield comparable amounts of energy (82–89 kJ forreaction 7.1 and 85–93 kJ for reaction 7.5).A COMPARISON OF DEEP AND SHALLOW MARINEVENTSMcCollom and Shock (1997) showed that at temperatures>40 °C in the deep-sea hydrothermal system at 21°N on the EastPacific Rise, sulfate-reduction (reaction 4.1) and S 0 -reduction(reaction 5.1) with H 2as the electron donor are exergonic, butS 0 -oxidation (reaction 6.1) and sulfide-oxidation (reaction 7.5)with O 2as the TEA are endergonic. By comparison, these fourreactions are all exergonic at the seven Vulcano sites investigatedhere. At temperatures between 50 and 100 °C, values of ∆G rforsulfate- and S 0 -reduction at the deep-sea site yield ~130 and ~30kJ/mol, respectively. The energetics of these reactions at Vulcanoare similar, yielding between 90 and 140 kJ/mol sulfate (except atPozzo Istmo) and between 16 and 38 kJ/mol S 0 . At temperaturesbetween 50 and 100 °C, the oxidation of S 0 and sulfide consumesFigure 5. ∆G r(kJ/mol) of the sulfide-oxidation reactions listed inTable 7A (data given in Table 7B) plotted against reaction number.Symbols are as in Figure 2. Equilibrium (∆G r= 0) is indicated by asolid horizontal line.