Isotopic Composition of Interstitial Fluids in Sediment of the Nankai Trough, Deep Sea Drilling Project Leg 87

Abstract

The isotopic compositions of dissolved CO 2 and CH 4 in sediments of the Nankai Trough indicate that CH 4 is formed during early diagenesis by microbial reduction of CO 2. At the shallowest sampled depths, the CO 2 dissolved in the pore water is unusually enriched in 12 C (δ 13 C =-35.2‰), indicating contribution of CO 2 from oxidation of CH 4. The most intense microbiological activity appears to be confined to the uppermost 50 m of sediment, based on relative lack of change in the isotopic compositions below this depth. Gas hydrate probably is not present at these localities (Sites 582, 583) because of CH 4 concentrations that are insufficient to saturate the pore water with respect to gas hydrate stability.

Microbial Methane Generation and Gas Transport in Shallow Sediments of an Accretionary Complex, Southern Hydrate Ridge (ODP Leg 204), offshore Oregon, USA

“…At the depth where the dissolved sulfate supply is exhausted, the DIC δ 13 C has a minimum δ 13 C value then reverses and becomes heavier because carbon that is ~70‰ lighter than the DIC pool is being removed to make methane at a faster rate than DIC of -20‰ is being added (Claypool and Kaplan, 1974). If methane were being oxidized at the base of the sulfate reduction zone, the DIC δ 13 C values would be much more negative (from -60‰ to -30‰) as observed in sediments at other locations where AMO is apparently taking place (Claypool and Threlkeld, 1983;Claypool et al, 1985Claypool et al, , 2003Vuletich et al, 1989;Blair and Aller, 1995;Burns, 1998;Borowski et al, 2000).…”

Origin of fluids and related barite deposits at vent sites along the Peru convergent margin

“…The nature and origin of fluids in convergent margins have been investigated through geochemical and isotopic studies of venting fluids and pore fluids (Boulegue et al, 1987a;Elderfield et al, 1990;Kastner et al, 1990Kastner et al, , 1991. However, the extensive work done during the past ten years was dedicated mainly to active margins exhibiting well-developed accretionary prisms, including the Nankai (Claypool et al, 1986;Boulegue et al, 1987aBoulegue et al, , 1987b, Barbados (Westbrook and Smith, 1983;Westbrook et al, 1988), and Oregon-Cascadia (Carson et al, 1990) margins. Little work was done on the fluid regime of Andean-type convergent margins, where subduction-related erosion processes (Scholl et al, 1980) occur at depth.…”

Carbon Isotope Ratio of Total Inorganic Carbon in Pore Waters Associated with Diagenesis of Organic Material at Site 808, Nankai Trough

“…Such positive δ 13 C (ΣCO 2 ) values due to CH 4 generation were also observed at the DSDP Sites 582 and 583 (Claypool et al, 1986). As this 13 C-enriched ΣCO 2 is thought to migrate upward into the sulfate reduction zone along with the CH 4 , even more CH 4 oxidation is implied to maintain the low δ 13 C (XCO 2 ) value in the sulfate reduction zone.…”

“…The unusually 13 C-poor XCO 2 (δ 13 C = -35.2%o) was also observed at 3 mbsf at Site 582 in the western Nankai Trough (see Fig. 1) during DSDP Leg 87 (Claypool et al, 1986).…”