n-Alkane, fatty acid and hopanoic acid concentrations of ODP Site 190-1178 samples

Ocean Drilling Program (ODP) Leg 190 was programmed to investigate deformational, diagenetic, and hydrologic processes and their interactions in the Nankai Trough accretionary prism. Site 1178 is the northernmost site in the Muroto Transect. Slope sediments and the underlying landward-dipping reflector zone were successfully cored. Temperature measurements and Cl concentrations in pore water indirectly indicate the presence of gas hydrate between 120 and 400 meters below seafloor (mbsf) at Site 1178, with the highest concentrations between 150 and 200 mbsf (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.190.109.2001). Sedimentary structures show a broad range of deformation structures rich in fractures, suggesting active fluid circulation in the Nankai Trough prism. One of the objectives of Leg 190 was to clarify the interplay of various fundamental processes taking place in the Nankai Trough accretionary prism. Bacteria or prokaryotes in deep subsurface sediment play an important role for material transformation and circulation in an accretionary prism. Significant amounts of bacteria are detected in many of the samples examined (Shipboard Scientific Party, 2001, doi:10.2973/odp.proc.ir.190.109.2001). The type of organic matter in sediments is an important factor related to bacterial activity. To assist investigations on material circulation in deep subsurface sediments, the samples from Site 1178 were analyzed for geolipids (extractable organic matter). The basic data set is preliminarily compiled in the present report to show the types of organic matter and their concentrations in sediments from Site 1178.

Sedimentological, petrographical and biostratigraphical characteristics of ODP Leg 190 volcanic ashes and siliceous claystones

Leg 190 was the first of a two-leg program across the Nankai accretionary prism and Trough, offshore Japan, aiming to evaluate existing models for prism evolution and to constrain syntectonic sedimentation, deformation styles, mechanical properties, and prism hydrology (Moore, Taira, Klaus, et al., 2001; Moore et al., 2001). More than 400 volcanic ash and siliceous claystone (altered ash) layers were penetrated and sampled during drilling of the six sites from two transects across the accretionary prism (Sites 1173-1178). In sites from the subducting Shikoku Basin (Sites 1173 and 1177) and in the trench axis (Site 1174), recognition of ash layers and diagenetically altered ashes was initially important in defining major lithostratigraphic units. However, it is clear that understanding the diagenesis of the volcanic ashes has considerable implications for prism evolution, mechanical properties, prism hydrology, geochemistry, and fluid flow in the accretionary prism and associated subducting sediments (cf. Masuda et al., 1996, doi 10.1346/CCMN.1996.0440402). Particle size, chemical composition, temperature, depth of burial, and time are all thought to be factors that may affect volcanic ash diagenesis and preservation (Kuramoto et al., 1992, doi:10.2973/odp.proc.sr.127128-2.235.1992; Underwood et al., 1993, doi:10.2973/odp.proc.sr.131.137.1993). The overall aim of this research is to evaluate factors influencing volcanic ash diagenesis in the Nankai Trough area. This data report presents just the results of the sedimentological and petrographic analysis of the volcanic ashes and siliceous claystones from Sites 1173, 1174, and 1177. It is anticipated that when the results of additional geochemical analysis of these lithologies is available a more meaningful evaluation of factors influencing volcanic ash alteration will be possible.

(Table T1) Mineral composition of turbidite sands and sandstones from ODP Leg 190 sites

During Ocean Drilling Program Leg 190 several turbidite successions in the Nankai Trough were drilled through including Pleistocene trench fill (Sites 1173 and 1174), Pleistocene-Pliocene slope basin deposits and underlying trench fill (Sites 1175 and 1176), Miocene Shikoku Basin deposits (Site 1177), and upper Miocene trench fill (Site 1178). Sands from the Pleistocene trench-fill succession of the Nankai Trough are of mixed derivation with significant monomineralic components (quartz and feldspar) and mafic to intermediate volcanic rock fragments, in addition to sedimentary and less abundant metamorphic detritus. They have a source in the Izu collision zone in central Honshu. Sands from the slope and accreted trench fill at Sites 1175 and 1176 are dominated by quartz with less abundant feldspar, sedimentary rock fragments, and only minor volcanic and metamorphic rock fragments. In contrast to the trench turbidites of Sites 1173 and 1174, these sands are very quartzose with characteristic radiolarian chert fragments. Volcanic rock fragments are mainly of silicic composition. Potential sources of these sands are uplifted subduction complexes of southwest Japan. Sands from the accreted trench turbidites at Site 1178 have clast types similar to those at Sites 1175 and 1176. In contrast, however, framework detrital modes are distinctive, with Site 1178 sands having substantially lower total quartz contents and more abundant fine-grained sedimentary rock fragments. These sands were also probably derived from the island of Shikoku, but their composition indicates that sedimentary rocks were abundant in the source area and these may have been Miocene forearc basin successions that were largely removed by erosion. Erosional remnants of Miocene forearc basin deposits are present on the Kii Peninsula east-northeast of Shikoku. Erosion followed a phase of exhumation of the Shimanto Belt indicated by apatite fission track ages at ~10 Ma. Sand in the lower-upper Miocene turbidites of the lower Shikoku Basin section at Site 1177 is more varied in composition, with the upper part of the unit similar to Site 1178 (i.e., rich in sedimentary rock fragments) and the lower part similar to those at Sites 1175 and 1176 (i.e., rich in quartz with some silicic volcanic rock fragments). Sands from the lower part of the Miocene turbidite unit were derived from a continental source with plutonic and volcanic rocks, possibly the inner zone of southwest Japan.

(Table T1) Carbon contents, alkenone concentrations, alkenone unsaturation index, and SST in sediments of ODP Leg 190 sites

We have preliminarily generated the downcore records of total organic carbon (TOC) content, total alkenone concentration, alkenone unsaturation index, and the estimated sea-surface temperature (SST) in the northern three sites (Sites 1175, 1176, and 1178) of the Muroto Transect, Nankai Trough. The TOC content will be used for the evaluation of the burial of organic matter, which plays a role in the generation of natural gas and the formation of gas hydrate in this region. The downcore records of alkenone SST will benefit studies for the paleoceanography of the northwestern Pacific. Because those sites are located in the main path of the Kuroshio Current, the records provide the temperature change of the Kuroshio water, which is an end-member water mass in the northwestern Pacific.

(Table T1) Mineral composition of oriented clay aggregates from ODP Leg 190 sites

Three sites were cored on the landward slope of the Nankai margin of southwest Japan during Leg 190 of the Ocean Drilling Program. Sites 1175 and 1176 are located in a trench-slope basin that was constructed during the early Pleistocene (~1 Ma) by frontal offscraping of coarse-grained trench-wedge deposits. Rapid uplift elevated the substrate above the calcite compensation depth and rerouted a transverse canyon-channel system that had delivered most of the trench sediment during the late Pliocene (1.06-1.95 Ma). The basin's depth is now ~3000 to 3020 m below sea level. Clay-sized detritus (<2 µm) did not change significantly in composition during the transition from trench-floor to slope-basin environment. Relative mineral abundances for the two slope-basin sites average 36-37 wt% illite, 25 wt% smectite, 22-24 wt% chlorite, and 15-16 wt% quartz. Site 1178 is located higher up the landward slope at a water depth of 1741 m, ~70 km from the present-day deformation front. There is a pronounced discontinuity ~200 m below seafloor between muddy slope-apron deposits (Quaternary-late Miocene) and sandier trench-wedge deposits (late Miocene; 6.8-9.63 Ma). Clay minerals change downsection from an illite-chlorite assemblage (similar to Sites 1175 and 1176) to one that contains substantial amounts of smectite (average = 45 wt% of the clay-sized fraction; maximum = 76 wt%). Mixing in the water column homogenizes fine-grained suspended sediment eroded from the Izu-Bonin volcanic arc, the Izu-Honshu collision zone, and the Outer Zone of Kyushu and Shikoku, but the spatial balance among those contributors has shifted through time. Closure of the Central America Seaway at ~3 Ma was particularly important because it triggered intensification of the Kuroshio Current. With stronger and deeper flow of surface water toward the northeast, the flux of smectite from the Izu-Bonin volcanic arc was dampened and more detrital illite and chlorite were transported into the Shikoku-Nankai system from the Outer Zone of Japan.