Jun-ichi Uchida

Physiographical and sedimentological characteristics of submarine canyons developed upon an active forearc slope: The Kushiro Submarine Canyon, northern Japan

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 Pacific. 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 profile with a gradual down-canyon increase in relief between the thalweg and the canyon rim. Although an infill 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 flow events. The down-canyon increase in flow 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 confluence 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 reflects 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.

Stratigraphic relationships between the last occurrence of Neogloboquadrina inglei and marine isotope stages in the northwest Pacific, D/V Chikyu Expedition 902, Hole C9001C

The stratigraphic relationships between the last occurrence (LO) of the planktic foraminiferan Neogloboquadrina inglei in the middle Pleistocene and established marine isotope stages (MIS) was investigated using a 365-m-long sediment core from a continental slope in the northwest Pacific near the Shimokita Peninsula, Japan. Two tephra layers (Shikotsu-Daiichi and Aso-4 tephras) and two nannofossil datum planes (first occurrence of Emiliania huxleyi and LO of Pseudoemiliania lacunosa) were used as age- control points, and the oxygen isotope stratigraphy of Hole C9001C was established by correlating the oxygen isotope values of the benthic foraminiferan Uvigerina akitaensis with the standard oxygen isotope curve LR04. Hole C9001C provides the first continuous, high-sedimentation-rate (20-90 cm/kyr) record from MIS 18 to present in the northwestern Pacific near Japan. The stratigraphic position of the LO of N. inglei is in late MIS 16 or near the MIS 16/15 boundary.