54 sediments were sampled at 5 sites, 1244, 1245, 1246,1250 and 1251 from ODPLeg 204, Hydrate Ridge, Cascadia Margin. The stable carbon and nitrogen isotopic composition,δ^(13)C of organic matter and δ^(15)N, have been used to characterize the depositional variationof sediments with gas hydrate bearing. Values of δ^(13)C of bulk organic matter range between-22.37 per thousand and -24.72 per thousand PDB, with a mean value of -23.54 ± 1.17 per thousandPDB; δ^(15)N of the same sedimentary samples range from 3.30 per thousand to 5.68 per thousand,with a mean value of 4.49 ± 1.19 per thousand; Values of C/N ratio range from 4.53 to 9.26, with amean value of 6.90 ± 2.37. The occurrence of gas hydrate changed the diagenetic evolution trend ofthe carbon and nitrogen in organic matter. The δ^(15)N and C/N ratio depletion suggest that thenitrogen fixation related with the bacteria/archaea maybe occurred in the sediments near thehydrate. The stable carbon and nitrogen isotopic composition could be used to evaluate thedepositional environment in the study area.
Experiments were made on 58 sediment samples from four sites (1244, 1245, 1250 and 1251) of ODP204 at five temperature points (25, 35, 45, 55 and 65°C) to simulate methane production from hydrate-bearing sediments. Simulation results from site 1244 show that the gas components consist mainly of methane and carbon dioxide, and heavy hydrocarbons more than C2 + cannot be detected. This site also gives results, similar to those from the other three, that the methane production is controlled by experimental temperatures, generally reaching the maximum gas yields per gram sediment or TOC under lower temperatures (25 and 35°C). In other words, the methane amount could be related to the buried depth of sediments, given the close relation between the depth and temperature. Sediments less than 1200 m below seafloor are inferred to still act as a biogenic gas producer to pour methane into the present hydrate zone, while sedimentary layers more than 1200 m below seafloor have become too biogenically exhausted to offer any biogas, but instead they produce thermogenic gas to give additional supply to the hydrate formation in the study area.
GONG JianMingCAO ZhiMinCHEN JianWenZHANG MinLI JinYANG GuiFang
