New record of occurrence ofSergia lncens (Hansen) (crustacea,sergestidae) off Tung-kang,Taiwan, with special reference to phylogeny and distribution of the species

The commercially important sergestid shrimp,Sergia lucens (Hansen), has long been considered an endemic species of Japan because it had been found only in Suruga Bay and neighbouring waters. Recently, however, a considerable amount of a similar shrimp was caught by trawl nets off Tung-kang, southwestern Taiwan. This shrimp is distributed at depths of 100–300 m on the continental slope, around a deep submarine canyon adjacent to the mouth of the Kao-ping River. A morphological comparison with specimens from Suruga Bay reveals that the shrimp is identical toS. lucens. However, a slight difference in the patterns of water-soluble proteins was observed in a thin layer isoelectrophoretic examination. A difference was also observed in the spawning season of the two populations, indicating sexual and geographic isolation. Considering the distribution and phylogeny of the family Sergestidae, based on a possible evolutionary development from a benthic neritic organism to a pelagic oceanic one along the generaSicyonella-Sergestes-Sergia, it is assumed thatS. lucens entered a lower epipelagic habitat in the coastal waters from the warm oceanic mesopelagic habitat of the original stock. A hypothesis is proposed that speciation ofS. lucens from the original stock occurred when it was trapped in a semi-enclosed inlet (the paleo-East China Sea Gulf) that existed at the present Okinawa Trough during the late Pliocene to early Pleistocene. The inlet was deep, but had a neritic environment due to drainage from ancient large river systems, including the paleo-Yangtze River. The species expanded its distribution to the neighbouring waters during the warm interglacial period. However, a rapid rise in sea level after 14,000 years significantly changed the environmental conditions in the distributional area and the species could not expand into a neritic environment, which was too shallow for survival. Accordingly,S. lucens populations remained only in Suruga Bay and Tung-kang waters, where the environment has remained stable for the last 17,000 years or more. The two areas have the following common characteristics:

1. A large amount of fresh water is discharged into the deep submarine canyon adjacent to the river mouth.
2. The northeastern part is surrounded by land so that a direct inflow of boreal water into the environment is blocked.
3. Warm Kuroshio extension water intrudes and mixes with water from the rivers, and for Suruga Bay there is a current system that keeps distribution of the eggs and larvae restricted to the local habitats.

Taiwan annually exported some tens of metric tons of the driedS. lucens to Japan for the last few years, but the standing stock of Tung-kang waters is probably not as large as that in Suruga Bay.     

Evaluation of Sea Water Quality in the Coastal Zone of Primor'e Using the Method of Microbial Indication

The response of marine microorganism community to variations in the concentrations of heavy metals in the environment is investigated for the coastal waters of Peter the Great Bay and the northern part of Primor'e. The fact of the existence and sensitivity of the individual response of plankton community microorganisms to variations in the concentrations of metals in the water under the conditions of water contamination with a complex of metals are confirmed on the basis of microbiological data and the results of their comparison to the data of chemical analysis. Microbial indication is shown to serve as an operative method of monitoring and short-term forecasting of changes in the environmental conditions of sea coastal waters. This method allows obtaining preliminary information at the stage of selecting environmental quality observation stations.     

Neodymium isotopic variations in Northwest Pacific waters

Four vertical profiles of the concentration and isotopic composition of Nd in seawater were obtained in the western North Pacific. Two profiles from the Kuroshio Current regime showed congruently that although the Nd concentration increases gradually with depth, its isotopic composition varies significantly with depth depending upon the water mass occupying the water column. The high-salinity Kuroshio waters originating from the North Pacific Tropical Water (NPTW) carry the least radiogenic Nd (?_Nd = −7.4 to −8.7) to this region at ∼250 m from the western margin continental shelves, most likely from the East China Sea. The Nd isotopic compositions in the North Pacific Intermediate Water (NPIW) that occurs at 600 to 1000 m in the subtropical region are fairly uniform at ?_Nd = −3.7. The profile data from the ∼38° to 40°N Kuroshio/Oyashio mixed water region off Sanriku of Honshu, Japan, also suggest that the newest NPIW with ?_Nd = −3.2 is formed there by the mixing of various source waters, and the radiogenic component of Nd is derived mainly from the Oyashio waters.In the Pacific Deep Water (PDW) below ∼1000 m, the Nd isotopic composition is neither vertically nor horizontally homogeneous, suggesting that it serves as a useful tracer for sluggish deep water circulation as well. Two profiles from the Izu-Ogasawara Trench showed a minimum ?_Nd value at ∼2000 m, suggesting that there exists a horizontal advective flow in the vicinity of Honshu, Japan. There is some evidence from other chemical properties to support this observation. The waters below 4000 m including those within the trench in the subtropical region have ?_Nd values of around −5, suggesting that the deep waters are fed from the south along the western boundary, ultimately from the Antarctic Bottom Water (AABW) in the South Pacific. This extends up to ∼40°N along the Japanese Islands. In the subarctic region (>∼42°N), the waters have more radiogenic Nd with ?_Nd > −4.0 throughout the water column, presumably due to the supply of Nd by weathering in such igneous provinces as the Kuril-Kamchatska-Aleutian Island chain. The lateral inhomogeneity of the Nd isotopic composition in PDW suggests that there may be different circulation and mixing regimes in the North Pacific Basin.     

Dynamic slope stability analyses with a non-linear finite element method

A new technique is presented with which to investigate slope stability during strong earthquake motion. This technique is based on a non-linear finite element method that uses a joint element to express non-linear behaviour and the progressive failure of a slope. Joint elements are arranged at every interface between soil elements. Accordingly, each soil element is allowed to move in directions parallel, perpendicular and rotational to neighbouring elements; consequently, they express the sliding and separation at any interface between the soil elements. The method was used to investigate the stability of an existing slope during strong earthquake motions. Preliminary static analyses were made, and their results were compared with results obtained with Janbu's method in order to check the validity of our proposed method. The dynamic analyses also took into account the material non-linearity of the soil. The process of progressive failure was examined for a slope whose material constants are known. The influence of input excitations on slope stability is discussed in detail. The method also has been used to assess the effectiveness of a countermeasure used to prevent slope failure.     

Origin of high-magnesian andesites in the Setouchi volcanic belt,southwest Japan,I. Petrographical and chemical characteristics

High-magnesian andesites of middle Miocene age occur in southwest Japan, forming an obvious volcanic belt. These andesites have low FeO*/MgO ratios (0.546–0.931), and are rich in Ni (101–312 ppm), Co (30.0–45.1 ppm), and Cr (208–756 ppm). They are relatively aphyric (phenocrysts <10 vol.%), and the phenocrysts of magnesian olivine (～Fo₈₈) are in equilibrium with the host high-magnesian andesite magmas on the basis of the Fe-Mg exchange partitioning. These features suggest that the high-magnesian andesites are not differentiated or accumulative; they appear to represent primary andesites generated in the upper mantle. These southwest Japanese high-magnesian andesites are rich in incompatible elements, and show light rare earth enrichment relative to boninites, suggesting that the former is derived from a less depleted mantle source than the latter.     

On-going orogeny in the outer-arc of the Timor–Tanimbar region, eastern Indonesia

The Timor–Tanimbar islands of eastern Indonesia form a non-volcanic arc in front of a 7 km deep fore-arc basin that separates it from a volcanic inner arc. The Timor–Tanimbar Islands expose one of the youngest high P/T metamorphic belts in the world, providing us with an excellent opportunity to study the inception of orogenic processes, undisturbed by later tectonic events.Structural and petrological studies of the high P/T metamorphic belt show that both deformation and metamorphic grade increase towards the centre of the 1 km thick crystalline belt. Kinematic indicators exhibit top-to-the-north sense of shear along the subhorizontal upper boundaries and top-to-the-south sense in the bottom boundaries of the high P/T metamorphic belt. Overall configuration suggests that the high P/T metamorphic rocks extruded as a thin sheet into a space between overlying ophiolites and underlying continental shelf sediments. Petrological study further illustrates that the central crystalline unit underwent a Barrovian-type overprint of the original high P/T metamorphic assemblages during wedge extrusion, and the metamorphic grade ranged from pumpellyite-actinolite to upper amphibolite facies.Quaternary uplift, marked by elevation of recent reefs, was estimated to be about 1260 m in Timor in the west and decreases toward Tanimbar in the east. In contrast, radiometric ages for the high P/T metamorphic rocks suggest that the exhumation of the high P/T metamorphic belt started in western Timor in Late Miocene time and migrated toward the east. Thus, the tectonic evolution of this region is diachronous and youngs to the east. We conclude that the deep-seated high P/T metamorphic belt extrudes into shallow crustal levels as a first step, followed by doming at a later stage. The so-called ‘mountain building’ process is restricted to the second stage. We attribute this Quaternary rapid uplift to rebound of the subducting Australian continental crust beneath Timor after it achieved positive buoyancy, due to break-off of the oceanic slab fringing the continental crust. In contrast, Tanimbar in the east has not yet been affected by later doming. A wide spectrum of processes, starting from extrusion of the high P/T metamorphic rocks and ending with the later doming due to slab break-off, can be observed in the Timor–Tanimbar region.     

Feasibility of dumping fossil-fuel CO2 into the deep ocean

An idea has been proposed to recover and dispose of the fossil-fuel CO₂ generated by stationary power plants into the deep sea, thereby reducing the CO₂ emission to the atmosphere. The assessment of its environmental effects and risks is essential for developing new technology for the recovery/disposal system, which has recently received growing attention in Japan.The simple calculation made here did not yield any decisively significant environmental risks so far. However, a further detailed study is needed for the consequences of lowering local pH and the abrupt dissolution of CaCO₃ in the deep-sea sediments. It is also shown that, on longer term, the atmospheric concentration of CO₂ may be influenced by the degree at which the reaction between the disposed fossil-fuel CO₂ and the sedimentary CaCO₃ takes place under such conditions. At any rate, the problem should be brought to serious scientific attention much like the case of ocean dumping of radioactive wastes.     

Tectonics and sedimentation around Kashinosaki Knoll: A subducting basement high in the eastern Nankai Trough

Abstract   When seamounts and other topographic highs on an oceanic plate are subducted, they cause significant deformation of the overriding plate and may act as asperities deeper in the seismogenic zone. Kashinosaki Knoll (KK) is an isolated basement high of volcanic origin on the subducting Philippine Sea Plate that will soon be subducted at the eastern Nankai Trough. Seismic reflection imaging reveals a thick accumulation of sediments (∼1200 m) over and around the knoll. The lower portion of the sedimentary section has a package of high-amplitude, continuous reflections, interpreted as turbidites, that lap onto steep basement slopes but are parallel to the gentler basement slopes. Total sediment thickness on the western and northern slopes is approximately 40–50% more than on the summit and southeastern slopes of KK. These characteristics imply that the basal sedimentary section northwest of KK was deposited by infrequent high-energy turbidity currents, whereas the area southeast of KK was dominated by hemipelagic sedimentation over asymmetric basement relief. From the sediment structure and magnetic anomalies, we estimate that the knoll likely formed near the spreading center of the Shikoku Basin in the early Miocene. Its origin differs from that of nearby Zenisu Ridge, which is a piece of the Shikoku Basin crust uplifted along a thrust fault related to the collision of the Izu–Bonin arc and Honshu. KK has been carried into the margin of the Nankai Trough, and its high topography is deflecting Quaternary trench turbidites to the south. When KK collides with the accretionary prism in about 1 My, the associated variations in sediment type and thickness around the knoll will likely result in complex local variations in prism deformation.