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doi:10.2204/iodp.proc.311.206.2009

Results

A total of 160 pore fluid samples were analyzed for δ¹³C_DIC; isotopic data are listed in Table T1. These data provide insights into two major issues involving carbon metabolic pathways and methane inventories: methanogenesis and methane oxidation. The methane source for the majority of gas hydrates recovered from northern Cascadia is known to have a biogenic origin (Whiticar et al., 1995; Claypool et al., 2006; Pohlman et al., 2006); less known, however, are the rates of methane generation and consumption that may be addressed with these results. These rates are key parameters in numerical models involving gas hydrate dynamics (generation, consumption, and recharge of the reservoir). Pore fluid samples recovered from ~100 to 200 meters below seafloor (mbsf) along the transect of sites drilled in the Cascadia margin during Expedition 311 are compared with similar samples recovered during ODP Legs 146 and 204. The isotopic composition of DIC pool at these sites shows a distinct and progressive enrichment in the δ¹³C of the residual dissolved CO₂ from a reference site drilled west of the accretionary toe (Site 888, δ¹³C_DIC = –5‰) to Site U1329 (δ¹³C_DIC = +32‰) drilled closest to the shore (65 km) at the eastern limit of gas hydrate occurrence in the margin (Fig. F2). The organic carbon diagenesis leading to methane accumulation began at ~100 ka when these sediments were deposited on the incoming plate. The progressively heavier δ¹³C values of the DIC reflect the preferential consumption of ¹²C by methanogens acting on the residual CO₂. Modeling efforts are under way to generate estimates of the methanogenesis rates from the progressive ¹³C increases of the residual DIC. Preliminary results (Torres et al., 2007) suggest a good agreement between these estimates and those based on geochemical modeling (Claypool et al., 2006) and microbial culture (Colwell et al., 2008) studies.

Some of this microbially produced methane is incorporated into gas hydrate and some is consumed by the coupled microbial reaction of anaerobic oxidation of methane (AOM) and sulfate reduction at the sulfate-methane transition (SMT) zone. Because biogenic methane is strongly depleted in ¹³C (δ¹³C_CH4 = –85‰ to –65‰) (Whiticar et al., 1995; Claypool et al., 2006; Pohlman et al., 2006), the bicarbonate produced by AOM also has significantly more negative carbon isotopes than bicarbonate generated by decomposition of organic carbon. Samples recovered from the flanks of Hydrate Ridge (Sites 1244, 1245, and 1246) during Leg 204 clearly indicate that AOM does not play a significant role in sulfate consumption at these sites (Torres and Rugh, 2006), which is consistent with geochemical models (Claypool et al., 2006). Only at sites where active fluid seepage led to gas hydrate breaching at the seafloor (Sites 1248, 1249, and 1250) was there evidence of AOM. Similarly at Site U1328, where active methane seepage results in massive near-surface gas hydrate accumulation, there is clear indication of AOM with δ¹³C_DIC values as low as –57‰. At two of the four sites drilled on the east–west transect (Sites U1326 and U1327), there is also evidence for a significant contribution of AOM to carbon cycling in the upper 20 mbsf (Fig. F3). In contrast, Site U1325 shows no indication of AOM. There is no correspondence, however, between the depth of the SMT, the relative contribution of AOM versus organic matter degradation, and the thickness of the GHSZ. This observation has been also reported in samples collected from the Indian Ocean (Kastner et al., in press) and illustrates the complexity of issues involving methane generation, transport, and consumption by AOM. Although methane availability is indeed a key factor controlling the distribution and concentration of gas hydrates, other issues, particularly the grain size of the host sediment, seem to play a determinant role in subsurface methane hydrology and resulting gas hydrate distribution and abundance (Torres et al., 2008).

In addition to characterizing DIC sources (organic matter degradation versus methane oxidation) and transport mechanisms (advection of deep fluids at the ridge summit), δ¹³C_DIC in pore fluids is incorporated into authigenic carbonate phases, which constitute an integrated record of carbon cycling and fluid flow. Carbonate samples were collected during Expedition 311. A synthesis of δ¹³C_DIC and authigenic carbonates would be used to provide a broad framework to unravel the history of gas hydrate formation and destabilization on the Cascadia margin (B. Teichert, pers. comm., 2007).

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