Keiji Takemura

Regulation of the monsoon climate by two different orbital rhythms and forcing mechanisms

The East Asian monsoon is responsible for transferring huge amounts of heat and moisture between the land and the adjacent ocean. Significant changes in its capacity to do this will have direct impacts on regional climatic gradients and global atmospheric circulation. Determining the mechanisms that force long-term variation in monsoon behavior is therefore important for understanding global climate change. Competing theories vary in the degree of importance attached to glacial forcing, other orbital rhythms, and internal feedback mechanisms as primary drivers of change. There is, however, no convincing explanation as to why different proxy records from closely neighboring regions are tuned to different orbital rhythms. Here we present quantitative climatic reconstructions for the past 450 k.y. based on a long pollen record from Lake Biwa in Japan. The data suggest that continental and oceanic air mass temperatures respond predominantly to the 100 k.y. orbital rhythm, whereas the land-ocean temperature gradient and monsoon vigor oscillate mainly at the 23 k.y. insolation cycle. We suggest that the mechanisms for this behavior lie in the differential response of land and ocean to solar forcing, and conclude that the 100 k.y. signal dominates monsoon intensity only when the amplitude of solar forcing falls below a threshold level.

Tectonic and climatic record of the Lake Biwa, Japan, region, provided by the sediments deposited since pliocene times

Abstract The sediments of Lake Biwa, Japan, contain a potential record of eastern Asian paleoclimates that date from the early Pliocene. A continuous, drilled core of lake sediments with a recovery of 70% was obtained in 1982–1983 to allow study of this record. Dating of core samples by a combination of tephrochronology, magnetostratigraphy, and fission-track methods has revealed that parts of the sedimentary record apparently are missing, even though obvious discontinuities cannot readily be detected from lithostratigraphic and seismic data. Tectonically induced migration of the basin center from its former more southerly location may be responsible for several hiatuses in the sedimentary record, which are more common in its earlier part. The record during the Brunhes epoch appears to be relatively complete; sediments indicate warm-cold changes in land vegetation which may record glacial cycles. Deeper sediments contain evidence of warmer-climate late Neogene flora. Tectonic events can be inferred from changes in sediment type and in sedimentation rates.

History of basin formation and tectonic evolution at the termination of a large transcurrent fault system: deformation mode of central Kyushu, Japana

Abstract The central part of the Kyushu Island is a locus of active arc-volcanism associated with subduction of the Philippine Sea plate. Being located at a bend in the boundary between the Philippine Sea and Eurasian plates, a complicated lateral displacement has generated half-grabens, a zone of strike-slip, and rhomboidal basins filled with volcanic material in central Kyushu. Initial northward subduction in latest Miocene time activated the N-S-trending Kokura-Tagawa Tectonic Line (KTL) as a left-lateral slip fault that bounds the western margin of the area of volcano-tectonic depression, the Hohi volcanic zone, which was initially formed about 6 Ma ago. The depocenter, which is now called Kuju basin, in the Hohi volcanic zone was at the corner of the KTL and the E-W-trending Median Tectonic Line (MTL). Relative convergence direction of the Philippine Sea plate shifted counterclockwise at about 1.5 Ma, and active motion on the KTL declined. At the same time, west-northwestward subduction enhanced right slip on the MTL. Geologic, gravimetric, and seismic data indicate that the MTL has shifted its active trace northward in central Kyushu as far as 10 km. As a result, the depocenter adjacent to the transcurrent fault migrated northeastward in the Hohi volcanic zone, specifically, from Kuju basin (Pliocene) via Shonai basin (early Quaternary) to Beppu Bay basin (late Quaternary). The latest depocenter of the Beppu Bay is surrounded by active faults that clearly delineate a rhomboidal basin on the MTL. Central Kyushu exemplifies the basin forming history and tectonic evolution at the termination of a large transcurrent fault system.

Preliminary results of the first scientific drilling on Lake Baikal, Buguldeika Site, Southeastern Siberia

The Baikal Drilling Project (BDP) is a multinational effort to investigate the paleoclimatic history and tectonic evolution of the Baikal sedimentary basin during the Late Neogene. In March 1993 the Baikal drilling system was successfuly deployed from a barge frozen into position over a topographic high, termed the Buguldeika saddle, in the southern basin of Lake Baikal. The BDP-93 scientific team, made up of Russian, American and Japanese scientists, successfully recovered the first long (>100 m) hydraulic piston cores from two holes in 354 m of water. High quality cores of 98 m (Hole 1) and 102 m (Hole 2), representing sedimentation over the last 500,000 years, were collected in 78 mm diameter plastic liners with an average recovery of 72% and 90%, respectively. Magnetic susceptibility logging reveals an excellent hole-to-hole correlation. In this report the scientific team describes the preliminary analytical results from BDP-93 hole 1 cores. Radiocarbon dating by accelerator mass spectrometry provides an accurate chronology for the upper portion of Hole 1. Detailed lithologic characteristics, rock magnetic properties and inorganic element distributions show a significant change to the depositional environment occuring at 50 m subbottom depth, approximately 250,000 BP. This change may be due to uplift and rotation of the horst block in the Buguldeika saddle. The sedimentary section above 50 m is pelitic with varve-like laminae, whereas the section below 50 m contains a high proportion of sand and gravel horizons often organized into turbidite sequences. Accordingly, high resolution seismic records reveal a change in sonic velocity at this depth. It is inferred that sedimentation prior to 250 ka BP was from the west via the Buguldeika river system. After 250 ka BP the Bugnldeika saddle reflects an increase in hemipelagic sediments admixed with free-grained material from the Selenga River drainage basin, east of Lake Baikal. Variations in the spore-pollen assemblage, diatoms, biogenic silica content, rock magnetic properties, clay mineralogy and organic carbon in the upper 50 m of BDP-93-1 reveal a detailed record of climate change over approximately the last 250,000 years. These variables alternate in a pattern characteristic of glacial/interglacial climatic fluctuations. The present age model suggests that the climate signal recorded in Lake Baikal sediments is similar to Late Quaternary signals recorded in Chinese loess sections and in marine sediments. Copyright

Geomorphology, kinematic history, and earthquake behavior of the active Kuwana wedge thrust anticline, central Japan

[1] We combine surface mapping of fault and fold scarps that deform late Quaternary alluvial strata with interpretation of a high-resolution seismic reflection profile to develop a kinematic model and determine fault slip rates for an active blind wedge thrust system that underlies Kuwana anticline in central Japan. Surface fold scarps on Kuwana anticline are closely correlated with narrow fold limbs and angular hinges on the seismic profile that suggest at least ∼1.3 km of fault slip completely consumed by folding in the upper 4 km of the crust. The close coincidence and kinematic link between folded terraces and the underlying thrust geometry indicate that Kuwana anticline has accommodated slip at an average rate of 2.2 ± 0.5 mm/yr on a 27°, west dipping thrust fault since early-middle Pleistocene time. In contrast to classical fault bend folds the fault slip budget in the stacked wedge thrusts also indicates that (1) the fault tip propagated upward at a low rate relative to the accrual of fault slip and (2) fault slip is partly absorbed by numerous bedding plane flexural-slip faults above the tips of wedge thrusts. An historic earthquake that occurred on the Kuwana blind thrust system possibly in A.D. 1586 is shown to have produced coseismic surface deformation above the doubly vergent wedge tip. Structural analyses of Kuwana anticline coupled with tectonic geomorphology at 10 3 -10 5 years timescales illustrate the significance of active folds as indicators of slip on underlying blind thrust faults and thus their otherwise inaccessible seismic hazards.

Stratigraphy of multiple piston-core sediments for the last 30,000 years from Lake Biwa, Japan

Analysis of seven piston cores obtained from three sites in Lake Biwa provide a detailed stratigraphy of the lake sediments for the last 30 kyrs. Description of lithology and measurement of magnetic susceptibility reveal occurrence of volcanic ashes at ten horizons, that were visually observed or microscopically found from clayey sediments. Most ash layers are observed in multiple cores from the three sites and also in earlier cores, establishing correlation and age assignment of the core sediments. This result demonstrates usefulness of magnetic susceptibility data in finding dispersed volcanic ashes. The age-depth curves deduced from published tephra ages show that significant difference in sedimentation rates occurs in Lake Biwa. The central part of the depression is characterized by homogeneous sediments deposited with a high sedimentation rate. The 250-m-thick lacustrine clay in this sub-basin has great potential for high-resolution study of the past environment and paleomagnetism for the last 0.5 Myr.

Environmental magnetic record and paleosecular variation data for the last 40 kyrs from the Lake Biwa sediments, Central Japan

We have conducted paleomagnetic and environmental magnetic analysis of a sediment piston core recovered from Lake Biwa, central Japan. Tephrochronology and AMS radiocarbon dating showed that this core covers the time period since about 40 kyr BP. The variation of paleomagnetic direction shows a good agreement with the PSV record for the last 10 kyrs from the deeper water site (BIWA SV-3; Ali et al., 1999), although the amplitudes are subdued probably due to the relatively lower accumulation rate at the shallower site. Inclination lows of the pre-Holocene interval are correlated to PSV records reported from the marine sediments off Shikoku and in the Japan Sea. In addition, the variation of magnetic mineral concentration reflects environmental changes during the last glacial period. It is suggested that the flux of fine-grained magnetite, probably associated with greater precipitation, was increased during interstadial periods. The variation of anhysteretic remanent magnetization is likely correlated to the Dansgaard-Oeschger (D-O) cycles recorded in Greenland ice cores. An apparent swing of the PSV curve is recognized at about 27 ka, but evidence for the Mono Lake excursion at 32 ka around the D-O events 6 and 7 is unclear. Combination of the detailed paleomagnetic record and the sub-Milankovitch climate cycles thus provides better resolution for understanding geomagnetic secular variation and polarity excursions in space and time.

Quaternary changes in delivery and accumulation of organic matter in sediments of Lake Biwa, Japan

A 911-m-long sediment core from Lake Biwa, Japan, provides a record of organic matter delivery and accumulation in this large lake during a succession of tectonic and climatic changes dating back to the latest Pliocene. Sediments deposited since 430 ky are profundal; older sediments vary in setting between shallow-water and fluviodeltaic conditions, with occasional deep-water intervals. C/N ratios identify algal production as the dominant source of organic matter throughout the core, although the proportion of land-derived contributions episodically increases in the fluviodeltaic and shallow-water sediments. Rates of organic matter delivery and burial in lake sediments change in response to glacial-interglacial climate changes over the past 430 ky. Sediments deposited during interglacial intervals have organic carbon mass accumulation rates up to 9 times greater than those from glacial intervals, reflecting interglacial climates that were wetter than glacial climates. Algal production of organic matter increased during interglacial times because of greater wash-in of soil nutrients, and organic matter preservation was enhanced because of faster sedimentation rates.

Restoration of the depression structure at the eastern part of central Kyushu, Japan by means of dislocation modeling

Abstract We have attempted to restore a subsurface structure in the eastern part of central Kyushu, Japan, by combining fault motions which were modeled as dislocation planes embedded in an elastic isotropic half space. The simulated crustal deformation pattern was compared with the subsurface structures estimated from gravity anomalies and/or seismic prospecting. The modeling procedure successfully restored all tectonic basins in the area without any need for motive forces for uplift or subsidence. The results also suggest that two major tectonic events have occurred in this region. Those are (1) the formation of half-graben caused by north–south extension, and (2) the formation of the pull-apart basin caused by east–west compression.

Caldera geometry determined by the depth of the magma chamber

The depth of the magma chamber is shown to be an important factor governing the initial type, scale, and collapse of a caldera. The collapse of the magma chamber is approximated by the contraction of a sphere in an elastic medium, and the distribution of plastic and/or rupturing area on the surface is calculated using the Coulomb failure criterion under the assumption of an elastic-perfectly plastic material. It is found that the necessary contraction for the formation of a caldera is described by fifth-power polynomial expression of the depth of the magma chamber, and that the radius and geometry of the caldera are dependent on the depth of the magma chamber.