Summary of Scientific Results: Gas Hydrates | Table of Contents


Geochemical Gradients, Muroto Transect
The shipboard geochemical data provide insight on the origin of fluids and the depth intervals and paths of recent flow. Sediment composition, physical properties, and both fluid chemistry and flow are associated. In addition, abiogenic and microbially mediated diagenetic reactions that have modified fluid composition have been characterized and quantified.

The most interesting and pronounced feature of the pore fluid concentration-depth profiles in the Muroto Transect from Site 1173 through Site 1174 to Site 808 is the ~350-m-broad low-Cl zone situated in the lower Shikoku Basin unit (Fig. 38). It has a clear concentration minimum ~140 m below the décollement. At Sites 1173 and 1174, this low-Cl zone decreases in intensity gradually upsection to the sediments overlying the upper Shikoku Basin facies. If the profile is considered a one-dimensional diffusive profile, its length suggests that this prominent feature has developed within the first ~250 k.y. after burial of the Shikoku Basin sediment by the trench wedge sediment. Additionally, the extent of Cl dilution relative to seawater Cl concentration systematically differs among the sites; it has evolved from 8%—9% at reference Site 1173 to 16%—17% at intermediate Site 1174 to 20%—21% at adjacent (<2 km) Site 808. Based on the very low smectite content of this sediment section, most of the freshening may not be due to local smectite dehydration but may result from horizontal transport. Note, however, that the original smectite concentration is not known. Some of the low-Cl concentrations most likely reflect the uptake of Cl by deep-seated hydrous silicate reactions; for example, serpentine, chlorite, talc, or amphibole incorporate considerable amounts of Cl in their structure. These reactions occur at temperatures of >250° or 300° to ~450°C, typical of the modeled temperatures at downdips of seismogenic zones (~350°C at Western Nankai). Thus, the broad low-Cl zone most probably carries a mixed signal of dilution via clay dehydration reactions plus Cl uptake by high temperature reactions at the seismogenic zone. The relative contributions of these two processes can be resolved by shore-based Cl, Br, and F concentration and isotope analyses.

The slightly higher Cl concentration within the décollement zone, observed at Sites 1174 and 808, may simply result from subtle differences in physical properties within and outside the décollement.

Other potential fluid flow horizons characterized by sharp changes in downnhole geochemical profile are

1. At Sites 1173, 1174, and 808, the boundary between the trench-wedge and upper Shikoku Basin sediments. The rather sharp reversal of the Cl gradient at this boundary may be maintained by flow of a slightly more saline fluid than seawater or by in situ hydration reactions that are faster than diffusion.

2. Along the protothrust (~470 mbsf) at Site 1174, as particularly indicated by the Cl, Na, Ca, and K concentration profiles.

3. At Site 1176, the Cl, Na, Ca, and K concentration profiles suggest communication with a deep fluid source, most likely associated with the out-of-sequence thrust.
It is interesting to note that the chemical characteristics of the protothrust and the deep source associated with the out-of-sequence thrust fluids are similar to the characteristics of the fluid in the low-Cl zone centered below the décollement. The composition of the fluid along the trench wedge/Shikoku Basin boundary is, however, distinct.

Another distinct characteristic of the Muroto Nankai Transect, not observed at any of the other drilled DSDP and ODP subduction zone sites, is the elevated (up to 10 mM) dissolved sulfate zone found at depth. It is beneath the near surface sulfate reduction zone, and prevails from the boundary between the upper and lower Shikoku Basin facies to oceanic basement and probably deeper. The fact that microbial activity has not reduced it over the past 0.5 m.y. indicates that the amount of labile organic matter available for microbial activity (for sulfate reducers and/or methane oxidizers) above the proto-décollement and décollement zones, where temperatures do not limit bacterial activity, is extremely low. Thus, much of the dissolved sulfate may persist into the seismogenic zone; it can only get reduced inorganically at least at 250°—300°C. The presence of dissolved sulfate in an anaerobic environment effects the oxidation state of the system and should influence sediment magnetic properties as well as inorganic reactions with transition metals, such as Fe and Mn.

The dominant diagenetic processes are ash alteration to clays and zeolites and silicate (mostly clay) reactions at the deep water sites and carbonate reactions at the shallow water sites; carbonate diagenesis, however, also occurs at the deep water sites. Opal-A dissolution controls the Si concentration profiles at each of the sites in the top few hundreds of meters, and other silicate reactions control it deeper in the sections.

Summary of Scientific Results: Gas Hydrates | Table of Contents