13C values; most of the other carbonates sampled reflect a DIC source that originated from the remineralization of organic matter (B.M.A. Teichert, pers. comm., 2006).
One of the objectives of Leg 204 was to identify and calibrate proxies for the presence of gas hydrate in the past. Several studies addressed this issue. A second, fainter negative polarity reflection that is observed ~30 m beneath the western flank of SHR has been interpreted to result from gas trapped in impermeable sediments after shallowing of the BSR in response to ocean warming (Bangs et al., 2005). The presence of magnetic minerals that can be associated with gas hydrate supports this interpretation (Musgrave et al., 2006; Larrasoaņa et al., this volume). However, this observation is also compatible with the presence of a small amount of Structure II hydrate that contains higher-order hydrocarbons in the interval between the primary and secondary BSRs (Claypool et al., this volume). Although no direct or indirect evidence for gas hydrate was found in this interval, gas chemistry allows the possibility of this alternative explanation.
Because carbonate deposits are closely associated with the history of methane discharge and of the processes that transform the carbon in methane to carbonate, these mineral phases contain valuable information to reconstruct the history and processes driving gas hydrate formation and dissociation. Isotopic analyses of carbonates from Leg 204 indicate that only carbonates sampled in Holes 1248B, 1249C, and 1249K are related to anaerobic methane oxidation, as reflected in their 13C values; most of the other carbonates sampled reflect a DIC source that originated from the remineralization of organic matter (B.M.A. Teichert, pers. comm., 2006).
In addition to documenting gas hydrate history, authigenic carbonate formation also alters pore water chemistry. Carbonate precipitation contributes to a decrease in the 18O of the pore water at all sites (Tomaru et al., this volume); however, 18O is also affected by hydrate formation and decomposition, clay mineral dehydration, and alteration of oceanic crust, and separation of these effects requires further modeling. Authigenic carbonate precipitation and dissolution may also impact dissolved fluoride concentrations (Dickens et al., this volume). However, because of a lack of data on the concentration of halogens in the various carbonate phases, this idea remains untested.
