Description: Sediments from Sites 582 (11 samples), 583 (19 samples), 584 (31 samples), 294 (1 sample), 296 (9 samples), 297 (3 samples), 436 (11 samples), and 439 (3 samples) were analyzed by X-ray fluorescence and/or instrumental neutron activation analysis. Ten major elements and 24 minor and trace elements (including 7 rare earth elements) were determined with these methods. Geochemistry varies systematically with both the site location and sediment age. Such variations are explained in terms of changes in sedimentation processes caused by plate motion and changes in ocean currents.

Description: Carbonate concretions in Pliocene to Miocene diatomaceous sediments at Sites 438, 439, and 584 on the deep-sea terrace near the Japan Trench are composed of calcite, dolomite, and siderite with varying degrees of chemical substitution. d18O ranges from -0.81 to +5.15 per mil for calcite and +2.00 to +7.29 per mil for dolomite. d13C values vary widely between -36.96 and + 11.45 per mil for calcite and between -20.99 and +4.75 per mil for dolomite. Siderite has -1.90 per mil d18O and +0.03 per mil d13C. The depth of burial at which these carbonates precipitated is estimated using the "oxygen isotope thermometer" on the basis of d18O values of carbonates and the interstitial water, and the geothermal gradients measured at present. Based on the estimated depth of formation of carbonates, four stages of carbonate authigenesis are defined, Stage I (0-20 m sub-bottom depth, calcian dolomite), Stage IIa (20-250 m sub-bottom depth, magnesian calcite), Stage IIb (250-530 m sub-bottom depth, ferroan calcite), Stage III (530-590 m sub-bottom depth, ferroan dolomite), and Stage IV (590 m to total depth, siderite). The progressive changes in mineralogy and chemistry of authigenic carbonates are well matched with the variation of the chemical composition of the interstitial water with increasing depth. The fluctuation of d13C of carbonates reflects the changing origin of CO2; oxidation and sulfate reduction at Stages I and IIa; and methane fermentation at Stages IIb, III, and IV.

Description: In the present paper, we report preliminary results on the 18O/16O ratios of the interstitial waters of the DSDP cores taken from subduction-related trenches near Japan: Sites 582 and 583 at the Nankai Trough off southwestern Japan, and Site 584 at the Japan Trench off northern Honshu, where thick piles of young sediments have accumulated. Special attention was paid to any differences in isotopic behavior of interstitial waters with different surrounding lithoiogy, the details of isotopic variation of interstitial waters in young, unconsolidated sediments, and the effects of sedimentary structural disturbance on interstitial waters.

Description: Petrographic and mineralogical studies using optical and scanning electron microscopes and X-ray diffraction and X-ray microprobe analyzers on 137 samples of Pleistocene to Upper Cretaceous sediments at Sites 438 and 439 off northeast Honshu in northwest Pacific yielded the following major results: 1) Sites 438 and 439 are characterized by carbonate-free terrigenous and diatomaceous sediments throughout the Pleistocene to the Upper Cretaceous. However, carbonate concretions, thin limestone beds, and carbonate-cemented sandstones are sporadically intercalated. Moreover, terrigenous and diatomaceous sediments occasionally contain significant amounts of carbonates. 2) Carbonate minerals encountered include calcite, dolomite, ankerite, and siderite. Calcite and dolomite are either of authigenic or detrital origin. Biogenic calcite grains are not uncommon. Ankerite and siderite are of detrital origin. 3) Authigenic calcite ranges in chemical composition from high Mg calcite in the Pleistocene sediments to Fe calcite in the uppermost Oligocene sediments through FeMg calcite in the Miocene sediments. Composition is probably controlled by diagenetic alteration of initially precipitated high Mg calcite during burial. 4) Mg ions released from high Mg calcite during the transformation seem to cause dolomitization. 5) The transformation and dolomitization in the uppermost Oligocene sandstone were probably accelerated by the recharge of fresh or brackish water from the underlying subaerially formed conglomerate and/or from the nearby Oyashio ancient landmass.

Description: We detected authigenic clinoptilolites in two core samples of tuffaceous, siliceous mudstone in the lower Miocene section of Hole 439. They occur as prismatic and tabular crystals as long as 0.03 mm in various voids of dissolved glass shards, radiolarian shells, calcareous foraminifers, and calcareous algae. They are high in alkalies, especially Na, and in silica varieties. There is a slight difference in composition among them. The Si : (Al+ Fe3+) ratio is highest (4.65) in radiolarian voids, intermediate (4.34) in dissolved glass voids, and lowest (4.26) in voids of calcareous organisms. This difference corresponds to the association of authigenic silica minerals revealed by the scanning electron microscope: There are abundant opal-CT lepispheres in radiolarian voids, low cristobalite and some lepispheres in dissolved glass voids, and a lack of silica minerals in the voids of calcareous organisms. Although it contains some silica from biogenic opal and alkalies from trapped sea water, clinoptilolite derives principally from dissolved glass. Although they are scattered in core samples of Quaternary through lower Miocene diatomaceous and siliceous deposits, acidic glass fragments react with interstitial water to form clinoptilolite only at a sub-bottom depth of 935 meters at approximately 25°C. Analcimes occur in sand-sized clasts of altered acidic vitric tuff in the uppermost Oligocene sandstones. The analcimic tuff clasts were probably reworked from the Upper Cretaceous terrain adjacent to Site 439. Low cristobalite and opal-CT are found in tuffaceous, siliceous mudstone of the middle and lower Miocene sections at Sites 438 and 439. Low cristobalite derives from acidic volcanic glass and opal-CT from biogenic silica. Both siliceous organic remains and acidic glass fragments occur in sediments from the Quaternary through lower Miocene sections. However, the shallowest occurrence is at 700 meters subbottom in Hole 438A, where temperature is estimated to be 21°C. The d(101) spacing of opal-CT varies from 4.09 to 4.11 Å and that of low cristobalite from 4.04 to 4.06 Å. Some opal-CT lepispheres are precipitated onto clinoptilolites in the voids of radiolarian shells at a sub-bottom depth of 950 meters in Hole 439. Sandstone interlaminated with Upper Cretaceous shale is chlorite- calcite cemented and feldspathic. Sandstones in the uppermost Oligocene section are lithic graywacke and consist of large amounts of lithic clasts grouped into older sedimentary and weakly metamorphosed rocks, younger sedimentary rocks, and acidic volcanic rocks. The acidic volcanic clasts probably originated from the volcanic high, which supplied the basal conglomerate with dacite gravels. The older sedimentary and weakly metamorphosed rocks and green rock correspond to the lithologies of the lower Mesozoic to upper Paleozoic Sorachi Group, including the chert, limestone, and slate in south-central Hokkaido. However, the angular shape and coarseness of the clasts and the abundance of carbonate rock fragments indicate a nearby provenance, which is probably the southern offshore extension of the Sorachi Group. The younger sedimentary rocks, including mudstone, carbonaceous shale, and analcime-bearing tuff, correspond to the lithologies of the Upper Cretaceous strata in south-central Hokkaido. Their clasts were reworked from the southern offshore extension of the strata. Because of the discontinuity of the zeolite zoning due to burial diagenesis, an overburden several kilometers thick must have been denuded before the deposition of sediments in the early Oligocene.