[1] Recent geophysical observations on basaltic composite volcanoes in Izu-Bonin arc reveal the process of long-distance lateral magma transport within arc crust. To clarify a long-distance magma transport system of the basaltic arc volcano from geological and petrological aspects, we investigated 20-km-long submarine volcanic chains (Hachijo NW chain and Hachijo-kojima chain) and cones on the northeastern slope (NE edifices) as well as subaerial satellite cones nested Hachijo Nishiyama volcano in the northern Izu arc front. Basalts from Hachijo NW chain have more primitive composition than those from other edifices. The composition of the Hachijo NW chain basalts is controlled by fractional crystallization, while plagioclase accumulation occurred in NE edifices and subaerial satellite cones. Trace element and isotopic characteristics indicate that the same basaltic primary magma is involved in all sections of the volcano. This leads us to consider that magma was transported long distances between the Nishiyama volcano and the Hachijo NW chain. Primitive magma was laterally transported NNW for at least 20 km in the middle to lower crust (10-20 km deep) from Nishiyama volcano with only minimal crustal level modifications and formed Hachijo NW chain. On the other hand, magmas experienced crystal fractionation and accumulation at shallow magma chamber beneath Nishiyama volcano seems to have been transported in a short distance (<5 km) and formed NE trending edifices and subaerial satellite cones. The long-distance magma transport seems to be controlled by a regional extensional stress regime, while short-distance transport may be controlled by local stress regime affected by load of main volcanic edifice.

Comprehensive geological surveys have revealed the physiographical and sedimentological characteristics of the Kushiro Submarine Canyon, one of the largest submarine canyons around Japan. The canyon indents the outer shelf along a generally straight, deeply excavated course of more than 230 km in length upon the active forearc slope of the Kuril Trench in the Northwest Pacifi c. The forearc slope has a convex-upward geometry that can be divided into upper and lower parts separated by an outer-arc high (3200-3500 m water depth). The upper slope consists of gently folded forearc sediments, and the lower slope is underlain by sedimentary rocks deformed by subduction-related processes. The upper reaches of the canyon (~3250 m of thalweg water depth) are developed on the upper slope, showing a weakly concave-upward longitudinal profi le with a gradual down-canyon increase in relief between the thalweg and the canyon rim. Although an infi ll of hemipelagic mud and the absence of turbidite deposits indicates that the upper part of the upper reaches of the canyon (~900 m thalweg water depth) is inactive, the lower part of the upper reaches (900-3250 m thalweg water depth) is considered to be an active conduit to the lower reaches, as determined from voluminous turbidites recovered in sediment cores (~76-yr intervals) and rockfalls observed in the canyon bottom by deep-sea camera. A number of gullies developed upon the northern slope of the lower part of the upper reaches might well provide a frequent supply of turbidity currents, giving rise to a down-canyon increase in the frequency of fl ow events. The down-canyon increase in fl ow occurrence is related to a gradual decrease in gradient, demonstrating an inverse power-law relationship between slope and drainage area. In contrast, the lower reaches of the canyon (3250-7000 m thalweg water depth) are characterized by a gradual decrease in relief, a high gradient, and extremely low sinuosity. The limited increase in drainage area down-canyon of the confl uence with the Hiroo Submarine Channel, which is the largest tributary of the main canyon, indicates that the erosional force of turbidity currents decreases down-canyon. The gradient of the lower reaches largely refl ects the morphology of the forearc slope along the canyon, which has been deformed by subduction-related tectonics. The lack of an inverse power-law relationship between gradient and drainage area in the lower canyon supports the hypothesis that the topography of the lower reaches is dominated by subduction-related tectonic deformation of the substrate rather than canyon erosion. Interrelationships between canyon erosion by currents and tectonic processes along the forearc slope are important in the development of the physiography of submarine canyons upon active forearc margins.

[1] A previous study interpreted extremely 13 C-depleted excursions of planktonic and benthic foraminifera in last glacial sediments (17,500 to 25,400 cal years B.P.) of the core retrieved from off Shimokita Peninsula and off Hokkaido, Japan, as evidence for periodic releases of methane, arising from the dissociation of methane hydrate. To better understand the formation process of the 13 C-depleted excursions, we conducted high-resolution natural radiocarbon measurements and biogeochemical analyses. We found highly depleted 13 C excursions ranging from À10.2% to À1.6% and À6.8% to À1.6% in planktonic and benthic foraminifera, respectively. Most of the foraminiferal tests in these horizons were brown, most likely as a result of postdepositional alteration, reflecting the formation of authigenic carbonate on the surface of tests. These alterations were also supported by high levels of Mg-calcite and the acid-leaching test for anomalous foraminifera. To evaluate the carbon sources in the altered foraminifera tests, we quantified the relative contributions of 14 C-free methane-derived carbon sources to the formation of authigenic carbonates in foraminifera with depleted 13 C excursions using a coupled mass balance isotopic model ( 14 C/C and 13 C/ 12 C). The radiocarbon ages of both planktonic and benthic 13 Cdepleted foraminifera were approximately 600 to 2000 years older than those of normal tests from nearby horizons. The relative contributions of authigenic carbonates derived from the methane oxidizing process reached to $22 wt% for planktonic foraminifera and $15 wt% for benthic foraminifera. The d13 C values of methane calculated from the mass balance model were between À29% and À68% for planktonic foraminifera and between À40% and À108% for benthic foraminifera, consistent with d13 C values reported for thermogenic and abiogenic methane in global methane hydrate reservoirs. These data consistently suggest that methane-related drastic environmental change occurred in the horizons that included d 13 C anomalies. This study provides important information for interpreting geological records of the methane hydrate instability associated with climate.