Acetate and hydrogen concentrations in pore fluids were measured from samples taken at seven sites from SHR. Figures F2, F3, F4, F5, F6, F7, and F8 show plots of acetate ion and hydrogen concentration with depth for each site.
Acetate concentrations ranged from 3.17 to 2515 然 (Table T1). Acetate concentrations are typically lowest within the upper 10 mbsf, above the sulfate/methane interface (SMI), and highest at various depths downhole, suggesting that in these shallow sediments, sulfate-reducing bacteria may consume acetate.
Within the gas hydrate stability zone, there are discrete excursions to high acetate concentrations at all sites. Hydrate distribution was constrained by Tr嶭u et al. (2004) using a multiproxy approach; however, given the spatial variability of the various proxies, here we correlate the acetate concentration with gas hydrate abundance in pore space primarily derived from dissolved chloride anomalies indicative of hydrate presence. Both chloride and acetate data were obtained in the same IW samples at similar intervals. A comparison between the acetate maxima and the discrete spikes in chloride concentration reveals that there is a general correspondence of higher concentrations of acetate. Hydrogen concentrations were measured in too few places to make any correlations; however, the build up of both acetate and hydrogen below the SMI suggests a buildup of microbial energy sources.
Acetate concentration maxima occurred at or near the BSR at Sites 1245, 1250, and 1251, and high concentrations were also observed at Sites 1244, 1246, and 1252. It is perhaps significant that the best defined maxima at the BSR occur at Sites 1250 and 1251, where the enhanced acetate content is observed in several samples, and at Site 1251 reaches values of 2600 然. Based on PCS data, it is apparent that there is free gas underneath the BSR at these locations, although at Site 1251 it is only a few percent of the available pore space (Tr嶭u, Bohrmann, Rack, Torres, et al., 2003). Hydrogen concentrations are noted by the format (number, letter) in Figures F2, F3, F4, F5, F6, F7, and F8, indicating the average concentration of that number of samples, while the letter designates the hole (e.g., 1244E). Hydrogen concentrations in PCS gases ranged from 16.45 to 1036 ppmv and are listed in Table T2. Hydrogen concentrations measured while degassing the PCS cannot be directly related to in situ concentration in pore water because of the sampling method; thus, the results given in ppmv should be considered as relative, semiquantitative concentrations. Hydrogen concentration measurements were attempted on ~20 headspace samples (i.e., approximately a 5-cm3 sediment plug). For each of these samples, hydrogen concentration was below the detection limit of about 10 ppmv, corresponding to concentrations of less than ~4 nM.
In general, pore water acetate concentrations in samples from Leg 204 were high in comparison to the non-gas hydrate–bearing Southern Ocean region, which range from 0 to 110 然 (Wellsbury et al., 2001). High acetate concentrations in this region were associated with the presence of localized diatom-rich laminae in the sediments at Site 1093 (Gersonde, Hodell, Blum, et al., 1999). In contrast, data from Leg 164 showed acetate concentrations exceeding 15,000 然 at ~700 mbsf in Site 997 (Egeberg and Barth, 1998; Wellsbury et al., 2000). These sediments were recovered from biogeochemically active zones (Wellsbury et al., 2000) with extensive gas hydrate deposits (Dickens et al., 1997). Drilling conducted during Leg 201 also showed high acetate concentrations at the Peru slope hydrate site (Site 1230). Here the acetate concentration reached 230 然 at 145 mbsf and corresponded to the presence of gas hydrate at 142 mbsf (Shipboard Scientific Party, 2002). The results from each of these legs suggest that the high acetate concentrations in the pore waters of gas-hydrated sediments or sediments containing unusually high concentrations of methane are unique and not the general situation in deep marine sediments. A similar conclusion was reached by Wellsbury et al. (2001).
Acetogenesis rates may increase with rising temperatures, resulting in an increase in acetate concentrations with increasing depth. Such an increase with depth was observed by Wellsbury et al. (1997) at Blake Ridge, offshore the southeastern Atlantic coast of the United States. They concluded that the acetate concentration buildup was indeed due to acetogenesis brought on by early diagenesis of organic matter and that the acetate helped fuel the generation of methane by methanogens utilizing acetate via the acetate fermentation pathway. A plot of Leg 204 acetate ion concentration vs. temperature (Fig. F9) reveals the lack of any general relationship that would suggest that acetogenesis via degradation of organic matter or early pyrolysis takes place at these sites.