Taqumi TuZino

Radiocarbon‐based carbon source quantification of anomalous isotopic foraminifera in last glacial sediments in the western North Pacific

A previous study interpreted extremely 13C-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 13C-depleted excursions, we conducted high-resolution natural radiocarbon measurements and biogeochemical analyses. We found highly depleted 13C excursions ranging from −10.2‰ to −1.6‰ and −6.8‰ to −1.6‰ 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 14C-free methane-derived carbon sources to the formation of authigenic carbonates in foraminifera with depleted 13C excursions using a coupled mass balance isotopic model (14C/C and 13C/12C). The radiocarbon ages of both planktonic and benthic 13C-depleted 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 δ13C values of methane calculated from the mass balance model were between −29‰ and −68‰ for planktonic foraminifera and between −40‰ and −108‰ for benthic foraminifera, consistent with δ13C 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 δ13C anomalies. This study provides important information for interpreting geological records of the methane hydrate instability associated with climate.

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.

Exceptional Preservation Within Pleistocene Lacustrine Sediments of Shiobara, Japan

Abstract A new conservation Lagerstätte is described from the middle Pleistocene Shiobara Group of central Japan. The biota includes mammals, birds, amphibians, fish, insects, arachnids, flowers, abundant leaves, fungi, and bacteria preserved within laminites that were deposited in a lacustrine environment. Comminuted plant material in medium-grained, massive sandstones was deposited from high-density flows. This fragmentation of plant material probably indicates that it had decayed prior to transport. Plant and animal remains are largely nonfragmented and were thus transported prior to decay. The laminites are composed of rhythmic, millimeter-scale alternations of clay-to-silt-grade clastics with siliceous, diatom-rich layers. In the western part of the basin the diatoms are preserved as opal-A, but in the eastern part, where soft-part preservation is most common, they have been altered to opal-CT and form thin, white, porcelaneous layers with a lepispheric texture. Soft parts are preserved as carbon residues and microbial films, and although siliceous laminae enclose the fossils, permineralization of tissues is infrequent. Soft-part preservation was promoted by the self-sedimentation of aggregated mats of diatoms that shrouded the biota on the lakebed. This stabilized the carcasses and prevented them from being disturbed. It also prevented the diffusion of both the incoming nutrients and outgoing metabolic by-products between carcasses and surrounding water and may thus have promoted soft-part preservation. Silica cementation also inhibited the destruction of fossils by the intense weathering in the humid Japanese climate.

Deep-Sea Turbidite Evidence on the Recurrence of Large Earthquakes Off Shakotan Peninsula, Northeastern Japan Sea

Twelve deep-sea turbidites were identified in the Holocene sequence of a core collected from the western foot of the Kaiyo Seamount, located near the fault area of the 1940 Shakotan-oki earthquake, northern Okushiri Ridge, eastern margin of the northern Japan Sea. On the basis of the geological, tectonic, and topographical settings of the site, the turbidites are interpreted as of earthquake origin (seismoturbidite). Age determinations on radiolarian remains in the sediments suggested that the uppermost sediments, younger than 3 ky BP, were lost during the coring. Four clear faunal boundaries, at 4, 6, 8.5, and 10.5 ka, were recognized. The turbidite recurrence interval in the deep-sea basin indicated an average recurrence interval for large earthquakes of 500–1,000 years around the Kaiyo Seamount during the early- to middle Holocene.