(Tables 1, 3) Component composition of organic particles in samples from ODP Site 131-808

Analysis of the palynofacies and miospore thermal alteration indices (TAI) of sediments from ODP Site 808 in the Nankai Trough was undertaken to determine (1) the source, depositional environment, and diagenesis of organic matter in the accreted sediments, and (2) the thermal structure and history of the prism and its relationship to fluid flow. Using the Hartax classification system, two palynofacies were recognized in the sedimentary sequence. Facies 1 occurs within the upper 600 m of trench-wedge turbidites (sedimentation rate > 1 km/m.y.) and contains >50% inertite particles. The rest of the assemblage is dominated by well-preserved phytoclasts and contains small amounts of poorly preserved phytoclasts and well-preserved scleratoclasts. Facies 2 occurs within the Shikoku Basin hemipelagites (600-1300 m below seafloor; sedimentation rate <150 m/m.y.) and contains over two-thirds inertite particles. The rest of the assemblage is dominated by poorly preserved phytoclasts. Miospores and marine phytoplankton compose only a small percentage of both palynofacies. Degraded organic matter is most noticeable in Facies 2, whereas its presence in Facies 1 is overshadowed by the high influx of well-preserved primary organic matter. Most of the degraded organic matter and inertite is interpreted to be reworked. Some of the degraded organic matter may be primary, and may have experienced more biodegradation and thermal alteration in Facies 2 than in Facies 1. TAI values indicate an immature stage of organic maturation (< 2) down to about 900 mbsf. Below this, samples show an increase with depth to a mature stage, reaching peak levels of about 3 just above basement. Samples from within the thrust fault and decollement zones do not show levels of maturity significantly greater than those of surrounding samples, leaving uncertain whether hot fluids have migrated along these fault boundaries in the past.

(Table 1) I, Br, B, d11B and Mn concentrations in ODP Site 131-808

This study of the interstitial water concentration-depth distributions of iodide, bromide, boron, d11B, and dissolved organic carbon, as represented by absorbance at 325 nm (yellow substance: YS) and laser-induced fluorescence (LIF), is a follow-up of the extensive shipboard program of interstitial water analysis during ODP Leg 131. Most of the components studied are associated with processes involving the diagenesis of organic matter in these sediments. Three zones of the sediment column are discussed separately because of the different processes involved in causing concentration changes: 1. The upper few hundreds of meters: In this zone, characterized by very high sedimentation rates (>1200 m/m.y.), interstitial waters show very sharp increases in alkalinity, ammonia, iodide, bromide, YS, and LIF, mainly as a result of the diagenesis of organic carbon; 2. Whereas below 200 mbsf concentration gradients all show a decreasing trend, the zone at ~ 365 mbsf is characterized by concentration reversals, mainly due to the recent emplacement of deeper sediments above this depth as a result of thrust-faulting; 3. The décollement zone (945-964 mbsf) is characterized by concentration anomalies in various constituents (bromide, boron, d11B, manganese, LIF). These data are interpreted as resulting from an advective input of fluids along the zone of décollement as recent as ~ 200 ka. Possibly periodic inputs of anomalous fluids still seem to occur along this décollement zone.

Light hydrocarbons in gas pockets and sediments of ODP Site 131-808

A suite of gas samples obtained from gas pockets and sediments of the Nankai accretionary prism (Site 808) has been analyzed for their gas composition and carbon and hydrogen isotope ratios. Gases collected from gas pockets between 10 and 555 mbsf consist of CH4 and CO2. Stable carbon isotope ratios of these two components point to a bacterial formation of methane via CO2-reduction that is also supported by D/H ratios of methane. Methane desorbed from sediments by a vacuum/acid treatment is of bacterial and thermal origin. Mixing between these gas types is indicated by molecular composition and carbon isotope ratios. Diagenetic processes at low temperatures can explain ethane to pentane concentrations from 0 to 850 mbsf. Between 850 mbsf and the basaltic basement hydrocarbon occurrences are related to catagenetic processes at elevated temperatures. Thermal alteration of organic matter is reflected through different gas parameters. Propane carbon isotope values of a sample from the zone of the frontal thrust indicate that the gas likely migrated from sediments of a higher maturity into the immature sediments at 366 mbsf.

Clay mineralogy of ODP Site 131-808 sediments

Drill core recovered at Ocean Drilling Program Site 808 (Leg 131) proves that the wedge of trench sediment within the central region of the Nankai Trough comprises approximately 600 m of hemipelagic mud, sandy turbidites, and silty turbidites. The stratigraphic succession thickens and coarsens upward, with hemipelagic muds and volcanic-ash layers of the Shikoku Basin overlain by silty and sandy trench-wedge deposits. Past investigations of clay mineralogy and sand petrography within this region have led to the hypothesis that most of the detritus in the Nankai Trough was derived from the Izu-Honshu collision zone and transported southwestward via axial turbidity currents. Shipboard analyses of paleocurrent indicators, on the other hand, show that most of the ripple cross-laminae within silty turbidites of the outer marginal trench-wedge facies are inclined to the north and northwest; thus, many of the turbidity currents reflected off the seaward slope of the trench rather than moving straight down the trench axis. Shore-based analyses of detrital clay minerals demonstrate that the hemipelagic muds and matrix materials within sandy and silty turbidites are all enriched in illite; chlorite is the second-most abundant clay mineral, followed by smectite. In general, the relative mineral percentages change relatively little as a function of depth, and the hemipelagic clay-mineral population is virtually identical to the turbidite-matrix population. Comparisons between different size fractions (<2 µm and 2-6 µm) show modest amounts of mineral partitioning, with chlorite content increasing in the coarser fraction and smectite increasing in the finer fraction. Values of illite crystallinity index are consistent with conditions of advanced anchimetamorphism and epimetamorphism within the source region. Of the three mica polytypes detected, the 2M1 variety dominates over the 1M and 1Md polytypes; these data are consistent with values of illite crystallinity. Measurements of mica bo lattice spacing show that the detrital illite particles were eroded from a zone of intermediate-pressure metamorphism. Collectively, these data provide an excellent match with the lithologic and metamorphic character of the Izu-Honshu collision zone. Data from Leg 131, therefore, confirm the earlier interpretations of detrital provenance. The regional pattern of sediment dispersal is dominated by a combination of southwest-directed axial turbidity currents, radial expansion of the axial flows, oblique movement of suspended clouds onto and beyond the seaward slope of the Nankai Trough, and flow reflection back toward the trench axis.

Cation exchange capacity and water content of ODP sites from Nankai

Pore fluid chlorinity lower than seawater is often observed in accretionary wedges and one of the possible causes of pore water freshening is the smectite to illite reaction. This reaction occurs during diagenesis in the 80-150°C temperature range. Low chlorinity anomalies observed at the toe of accretionary wedges have thus been interpreted as evidence for lateral fluid migration from inner parts of the wedge and the seismogenic zone. However, temperature conditions in Nankai Trough are locally high enough for the smectite to illite transition to occur in situ. Cation exchange capacity is here used as a proxy for smectite content in the sediment and the amount of interlayer water released during the smectite to illite reaction represents in average 12 water molecules per cation charge. Water and chloride budget calculations show that there is enough smectite to explain the chlorinity anomalies by in situ reactions. The shape of the pore fluid chlorinity profiles can be explained if compaction is also taken into account in the model. Lateral flow is not needed. This argument, based solely on chloride concentration, does not imply that lateral flow is absent. However, previous estimations of lateral fluid fluxes, and of the duration of transient flow events along the de.collement, should be reconsidered.