13C values of methane and C1/(C2+C3) ratios show that gas below 400 mbsf is composed of thermogenic gas or mixed gas rather than microbial gas (Fig. F4) (Wiese and Kvenvolden, 1993).
The chemical and isotopic compositions of pore fluids from Site 1146 show that the interstitial water and headspace gas between 0 and 400 mbsf are of a marine origin. Below 400 mbsf, a slight decrease in chlorine concentrations and a sharp increase in lithium concentrations correspond to a considerable increase in methane concentration and the presence of ethane and propane in the headspace gas samples. In particular, chlorine concentrations begin to decrease, whereas methane, ethane, and propane concentrations reach the maximum values at ~560 mbsf.
Possible explanations for the low-chlorine anomalies include phase transformations of minerals (e.g., dewatering of clay minerals and opal/quartz transformation), dissociation of gas hydrates, migration of diluted liquids from other places, and influx of meteoric water from the nearby continent or from deeper waters. Phase transformation in clay minerals was not observed at 560 mbsf from the shipboard XRD data (Shipboard Scientific Party, 2000b). Dewatering of clay minerals and opal/quartz transformation cannot explain the increase in the hydrocarbon concentrations. Influx of continental meteoric waters can be excluded, judging from the oxygen and hydrogen isotopic compositions of interstitial waters at Site 1146.
High methane contents might originate from in situ microbial methanogenesis, migration from a deep thermogenic gas reservoir, or dissociation of gas hydrate. The 13C values of methane and C1/(C2+C3) ratios show that gas below 400 mbsf is composed of thermogenic gas or mixed gas rather than microbial gas (Fig. F4) (Wiese and Kvenvolden, 1993).
It is well known that the crystallization of gas hydrate excludes ions of dissolved salts from the crystal structure (Hesse and Harrison, 1981). When gas hydrate decomposes, it releases freshwater into the pore space and decreases the salinity of interstitial water. In this case, the concentrations of chlorine and sodium slightly decrease. During this process, a large amount of hydrocarbon is also released into the pore space, which would lead to increased methane, ethane, and propane concentrations. Therefore, the most likely explanation for these geochemical anomalies at Site 1146 is decomposition of gas hydrates, which dilutes pore water and increases methane levels. The bottom boundary of the gas GHSZ is predicted to be ~268 mbsf at Site 1146 based on the thermal gradient of 59°C/km and a bottom-water temperature of 2.9°C (Shipboard Scientific Party, 2000b). It is, therefore, impossible that the geochemical anomalies at 560 mbsf (Site 1146) are caused by in situ decomposition of gas hydrate. More likely, pore fluids with low salinity and high hydrocarbon concentrations were released from decomposition of nearby gas hydrate and migrated laterally to Site 1146 through faults or bedded planes. However, migration of pore fluids with high hydrocarbon contents and low salinity from deeper levels cannot be excluded. Preliminary results of testing by the Shipboard Scientific Party indicate that reflector T2 at 430 mbsf (Site 1146) is equivalent to the middle/upper Miocene boundary. Reflector T4 at 520-530 mbsf, near the middle/lower Miocene boundary, could be linked with a fault extending to ~1 nmi north-northwest of Site 1146 (Shipboard Scientific Party, 2000a, 2000b). The pore fluids with low salinity and high hydrocarbons from decomposition of gas hydrate could thus migrate laterally to Site 1146 along reflector T2 or T4 and the linked fault. Authigenic siderite is commonly associated with gas hydrate-bearing sediments (Matsumoto, 1989), and authigenic rhodochrosite concretions related to the dissociation of gas hydrate were also detected at Site 503 of the Deep Sea Drilling Project Leg 68 in the Guatemala Basin (Morad and Al-Aasm, 1997). Authigenic carbonates with high 18O values have been discovered in nearly all gas hydrate sites, indicating that carbonate formation was related to the decomposition of gas hydrate, which releases water enriched in 18O (Davidson et al., 1983). Therefore, 18O enrichment in the authigenic siderite concretions at Site 1146 is probably the result of gas hydrate decomposition. Furthermore, the low 13C values of these carbonates indicate that the occurrence of CO2 was not produced by in situ bacterial fermentation.
BSRs have been found near Site 1146 (Fig. F1) and at Site 1144 (Song et al., 2001). The multichannel seismic survey of Guangzhou Marine Geological Survey in 2001 again proved that BSRs are present in the areas near Site 1146 (Huang, unpubl. data). Furthermore, a relatively thin sulfate reduction zone and linear sulfate gradient may indicate gas hydrate (Borowski et al., 1996, 1999). The sulfate reduction zone is ~11 m at Site 1144 (Wang et al., 2000) and ~65 m at Site 1146, and the sulfate gradient is linear through the sulfate reduction zone. Gas hydrate could, therefore, be present in the areas near Site 1146.
