Sulfur Biogeochemistry—Past and Present

Microbially mediated sulfur-redox 23MethodsSampling and Chemical AnalysesFor a detailed discussion of sampling procedures and waterand gas analyses, see Amend et al. (2003b). Briefly, watersamples from several submarine vents, sediment seeps, and geothermalwells on Vulcano were analyzed in the field or preservedfor subsequent analysis. Temperature, pH, and conductivitywere measured in situ with hand-held meters and probes. Theredox-sensitive compounds Fe 2+ , NO 3−, NH 4+, H 2S, and dissolvedoxygen were analyzed by spectrophotometry in the field.Concentrations of major inorganic cations and anions as well ascarboxylate anions were determined on an ion chromatographequipped with an electrochemical detector. The chemical compositionof free gases was determined by gas chromatography usinghot wire and flame ionization detectors placed in series.Dissolved Organic CarbonWater samples for dissolved organic carbon (DOC) werecollected in all-glass bottles, immediately poisoned with an aliquotof HgCl 2(0.2 µM final concentration) to kill all microorganisms,and transported to the geochemistry laboratory at theMarcello Carapezza Center on Vulcano. There, samples werefiltered under low vacuum in glass filtration units (Millipore)with GF/F filters. Each filtered sample was collected in a secondall-glass bottle, refrigerated, and shipped cold (~4 °C) tothe United States, where each sample was frozen (−20 °C) untilanalysis. All handling of samples, containers, and gear for DOCanalysis was carried out with gloved hands. All glassware wasmuffled for 6 h at 500 °C; Whatman GF/F filters were muffledfor 6 h at 450 °C.Samples were analyzed with a nondispersive infrared detectoron an Apollo 9000HS combustion TOC Analyzer (Tekmar).Prior to analysis, samples were acidified (pH 80 °C; at Grip andPozzo Istmo, the temperature is ~55 °C. The pH is acid at allseven sites, but ranges considerably from 1.98 at Pozzo Vascato 5.84 at Pozzo Istmo. It can be deduced from the conductivitymeasurements (Table 2) and the concentrations of major cationsand anions (Amend et al., 2003b) that four of the hydrothermalsolutions (at Stinky Surf Rock, Grip, Acque Calde 2, and PozzoIstmo) are dominated by a marine end member; the other threesolutions (Punto 1, Punto 7, Pozzo Vasca) are characterized by asubstantially larger contribution from a fresh water end member.Particularly large variations among the sites are seen in concentrationsof Fe 2+ , NO 3−, and H 2. For example, Fe 2+ ranges from0.02 ppm at Pozzo Istmo to ~300 ppm at Punto 7 and PozzoVasca. Nitrate concentrations vary from