Sulfur hydrothermal flux

Sano and Williams (1996) calculated present-day volcanic carbon flux from subduction zones to be 3.1 x 10 mol/year based on He and C isotopes and C02/ He ratios of volcanic gases and fumaroles in circum-Pacific volcanic regions. Williams et al. (1992) and Brantley and Koepenich (1995) reported that the global CO2 flux by subaerial volcanoes is (0.5-2.0) x lO mol/m.y. and (2-3) x 10 mol/m.y. (maximum value), respectively. Le Guern (1982) has compiled several measurements from terrestrial individual volcanoes to derive a CO2 flux of ca. 2 x 10 mol/m.y. Le Cloarec and Marty (1991) and Marty and Jambon (1987) estimated a volcanic gas carbon flux of 3.3 X 10 mol/m.y. based on C/S ratio of volcanic gas and sulfur flux. Gerlach (1991) estimated about 1.8 x 10 mol/m.y. based on an extrapolation of measured flux. Thus, from previous estimates it is considered that the volcanic gas carbon flux from subduction zones is similar to or lower than that of hydrothermal solution from back-arc basins. 

We speculate from the above argument that primordial sulfur degasses from midoceanic ridges even at present time as well as He, because subduction flux to mantle seems to be small. However, we need more detailed study on long-term S cycle including hydrothermal S flux to evaluate this speculation. 

The hydrothermal-vent and cold-seep communities are dramatically different from the ecosystems typical of the abyssal plains. First, they are sites of high productivity supported by the abundant reduced chemicals in the hydrothermal fluids. Thus, these communities are independent of the skimpy flux of POM created in the surface waters that survives to settle on the seafloor. On the other hand, these communities have had to adapt to survival in hydrothermal systems that are ephemeral, disjunct, and characterized by extreme conditions, such as high temperatimes, high concentrations of reduced metals and sulfur, and low pH. As a result, vent communities have high rates of endemism. Of the 712 recorded vent species, 71% are found in no other setting ... 

Hydrothermal reactions between seawater and young oceanic crust have been proposed as an influence on atmospheric O2 (Walker, 1986 Carpenter and Lohmann, 1999 Hansen and Wallmann, 2002). While specific periods of oceanic anoxia may be associated with accelerated hydrothermal release of mantle sulfide (i.e., the Mid-Cretaceous, see Sinninghe-Damste and Koster, 1998), long-term sulfur and carbon isotope mass balance precludes substantial inputs of mantle sulfur to the Earth s surface of a different net oxidation state and mass flux than what is subducted at convergent margins (Petsch, 1999 Holland, 2002). 

Deep-sea vents have only been studied in the late twentieth century, so there is still much to learn in terms of their global contribution because of their inaccessibility. However, they can affect global fluxes, and some estimates would suggest that warm ridge-flank sites may remove each year, as much as 35% of the riverine flux of sulfur to the oceans (Wheat and Mottl, 2000). The hydrothermal vents are locally important sources of sulfide-containing materials. The black smokers yield polymetal sulfides, that will... 

Sulfur is also returned to the mantle. This may follow one of two routes. The sulfur that is mobilized in the oceanic crust during high-temperature hydrothermal activity is a mix of ocean crust sulfide and seawater sulfate. Some of the seawater sulfate is converted into sulfide and fixed in the ocean crust and subsequently subducted into the mantle. In addition sulfur is removed from the oceans to form pyritized sediments and returned to the mantle during sediment subduction. Estimates of the fluxes are given in Canfield (2004) and are shown in Fig. 5.5. 

Biota that dwell at or below the sea floor along the mid-ocean ridges obtain their energy principally from oxidation-reduction reactions involving reduced hydrothermal emanations (e.g. Jannasch and Wirsen 1979). Accordingly, their primary productivity cannot exceed the flux of reduced species from thermal sources. The principal reduced species include reduced sulfur, H2 (derived from water-rock reactions), and Fe. Today this total flux, expressed as O2 equivalents, is in the range (0.2 to 2.1) x 10 mol yr (Elderfield and Schultz 1996). 

Arsenic(As) in ocean is mainly removed by formation of pyrite in marine sediments. The production rate of sulfur in pyrite is 3.3 X 10 mol my (2.5 X 10 ° g my ) (Holland 1978). As/S ratio of pyrite in sediments previously reported is (8.7 3) x 10" (Huerta-Diaz and Morse 1992). Thus, As sink by pyrite is (1.7-3.9) x 10 mol my . This flux seems to be not different from As input to ocean ((1.6-8.1) x lO mol my (Table 5.3). As concentration of ocean is considered to be controlled by hydrothermal input, riverine input and pyrite output. Fluxes by volcanic gas from atmosphere and by weathering of ocean-floor basalt are small, compared with hydrothermal, riverine and pyrite As fluxes. Residence time of As in seawater is estimated as the amount of As in seawater (4.2 x 10 g) divided by As input to seawater (1.6-8.1) X 10 mol my which is equal to (1.7-3.8) x 10" year. This is shorter than previously estimated one (10 year by Holland 1978). Subducting sulfur flux is estimated to be 6.1 x 10 g my from S contents of altered basalt and sediments ( 0.1 wt%) (Kawahata and Shikazono 1988) and subducting rates of... 

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