MIS11–19 pollen stratigraphy from the 250-m Choshi core, northeast Boso Peninsula, central Japan: Implications for the early/mid-Brunhes (400–780 ka) climate signals

Abstract   The 250-m Choshi core (CHOSHI-1), drilled from hemipelagic muds of the Inubo Group, has been physically, geochemically and tephrochronologically analyzed back to 1 Ma. We provide pollen results for the 19–169 m section of the core (400–780 ka) bracketed by the marker tephra Ty1 (equivalent to J4) and the Brunhes–Matuyama paleomagnetic boundary. The results show good agreement with the corresponding oxygen isotope (δ¹⁸O) profile, with high δ¹⁸O intervals dominated by boreal conifers Picea , Abies , Pinus (subgen. Haploxylon ) and Tsuga ( diversifolia ), whereas low δ¹⁸O intervals are dominated by temperate conifers Cryptomeria , Taxaceae-Cephalotaxaceae-Cupressaceae and Sciadopitys . In order to confirm pollen-climate relations for the relevant taxa, a modern surface pollen dataset for the Japanese archipelago was consulted. In this analysis, the ratios of Cryptomeria / Picea and temperate/boreal conifers serve as proxies for the 100-kyr glacial/interglacial cycle during the Middle Pleistocene. Distinct signals for marine isotope stages (MIS) 11, 12, 13–15, 16, 17 and 18–19 are recognized in accordance with the tephrochronology and δ¹⁸O of the same core. Application of the criteria to an independent pollen record from Lake Biwa provides an integrated pollen stratigraphy for mid-latitude Japan during the past 800 ky. Some degree of uncertainty remains in the chronology of the MIS13–15 interval, relating to the uncertainty in the eruption age of widespread tephra Ks11.     

Possible Coseismic Large-scale Landslide off the Northern Coast of Papua New Guinea in July 1998: Geophysical and Geological Results from SOS Cruises

— The epicentral area of the 1998 Papua New Guinea Earthquake and Tsunami off Sissano Lagoon, northern coast of Papua New Guinea (M=7.1; 17 July, 1998) was surveyed by the Research Vessel Kairei in January 1999. Precise bathymetric survey by use of SEABEAM-2112 and other geophysical surveys (bottom reflectivity, sub-bottom profiling, surface ship gravity and geomagnetic surveys) were carried out and four piston core samples were collected during the nine days' survey. The area was also surveyed by use of the Japanese deep sea research ROV Dolphin-3K and research submersible Shinkai-2000 after the regional geophysical survey, in order to locate the possible seismic faults and/or underwater landslides as the source of tsunami and to study the process which took place off Sissano Lagoon and the driving force of the event. The study area is characterised by substantial fan sediment supply from Sissano Lagoon. Straight small-scale submarine canyons and valleys are eroding the shelf slope constructed by the fan sediment. Topographic features of arcuate slumps caused by landslides are recognised at numerous sites of the study area. Most of them are old and the most recent is located 25 km northeast off the Sissano Lagoon. Two major topographic depressions on the shelf were located off Sissano Lagoon. The western is a depression of about 10-km width and the eastern is a meandering deep-sea canyon. Amphitheatre topographic features with slumps caused by landslides that were discovered by Kairei's cruise were extensively surveyed in the study area. Six dives by Dolphin-3K and seven dives by Shinkai-2000 revealed that a fresh crack of about 15 km in total length is located along the slope of the amphitheatre. The crack is apparently the upper slope of a large-scale slumping on the amphitheatre, characterised by tensional stress on it. The easternmost part of the fresh crack is accompanied by living chemosynthetic organisms such as mussels and tube worms. The chemosynthetic community is apparently associated with cold seepage along the crack suggesting that the crack was constructed very recently. The areas other than the crack were rather old with bioturbation.     

Source mechanisms of volcanic explosion earthquakes: single force and implosive sources

Volcanic explosion earthquakes accompanying explosive eruptions are viewed as representing an abrupt release of pressurized magma beneath a volcano through a vent. I examine the source mechanisms of explosion earthquakes assuming two extreme cases of magma property. When magma is assumed to be a perfect gas which migrates upward isentropically through a vent acting as a nozzle, the seismic source is expressed as a single force. On the other hand, when magma flow is assumed to be incompressible, an implosive source is dominant. Both source models predict that the seismic magnitude is proportional to the cross-sectional area of the vent. I investigate observed seismic magnitudes for different volcanoes based on data in published papers and reports. The results show that the seismic magnitude of the largest event for each volcano is essentially proportional to the cross-sectional area of the vent for vent radii ranging from 10 to 600 m. The consistency found between the theoretical prediction and the observed relation suggests that the vent area plays a substantial role in the magnitude of explosion earthquakes. I further estimate that initial pressures in the reservoir are of the order of a few MPa, with a one order of magnitude uncertainty based on the single force model. I also apply the implosive source model to the observed seismic magnitudes and estimate the product of bulk modulus of magma and flow velocity to be 10¹⁰–10¹¹ J/m² s.     

Interplate Coupling in the Kanto District, Central Japan, and the Boso Peninsula Silent Earthquake in May 1996

— I studied crustal deformation in the Kanto district, central Japan, based on continuous GPS data. Horizontal as well as vertical displacement rate demonstrate significant interaction between the landward Kanto block and the Philippine Sea plate. Although the subduction effect of the Pacific plate is not apparent, it is reasonable to consider the entire Kanto district is displaced westward due to the interaction with the Pacific plate. The GPS velocity data were inverted to estimate the slip deficit distribution on the Sagami Trough subduction zone. The result delineates a strongly coupled region on the plate interface, part of which corresponds to the 1923 Kanto earthquake. The strongly coupled region is located shallower than 20 km. In addition, the plate interaction is laterally heterogeneous even in the same depth range, implying thermal structure is not the only factor controlling interplate coupling. The GPS data also detected a silent earthquake event on the interface of the Philippine Sea slab east of the Boso Peninsula in the middle of May, 1996. The silent rupture propagated over a 50 km * 50 km wide area during about a week. The maximum slip was approximately 50 mm and the released seismic moment was 4.7*10¹⁸Nm (M _w 6.4). There was a small seismicity triggered by this silent event. The silent slip was located in the peripheral of the strongly coupled area, suggesting that frictional properties and/or stress conditions are inhomogeneous on the plate boundary interface.     

大井锡多金属矿床矿化元素分布特征研究

通过主要剖面、平面、长纵剖面矿化元素的空间分布、矿石多元素相关分析得出 :Cu、Sn矿化既有关、又有分离 ,二者并非总是同时空富集 ,存在独立的锡矿体和铜矿体 ;总体上Sn矿化与Cu -Ag矿化关系密切、局部与Zn矿化有关 ;Ag矿化与Cu矿化、Pb -Zn矿化均有关 ;Cu与Pb、Zn的相关性差 ,空间上经常分离。从NWW到SEE矿化元素存在Ag -Cu -Sn -Pb -Zn混合矿化→Cu -Sn -Ag矿化→Pb -Zn -Ag矿化的纵向分布。成矿地球化学阶段可划分为 :Sn +As+Co、Cu +Sn +Ag±Au±Co、弱Ag +Cu±Au、Zn +Pb +Ag±Cu四个阶段。推断NE向断裂F2 -1及F2 可能是导矿构造。F1可能是另一个类似于F2 -1的导矿构造。     

Geothermometry of spinel lherzolites

Three methods of geothermometry, currently used for spinel lherzolites, are refined based on new experiments on subsolidus phase equilibria of olivine, pyroxenes and spinel in CaO-MgO-Al₂O₃-SiO₂ and natural rock systems at 16 kb and 1200 °C. Although quasi-thermodynamic modelling is employed, the methods are essentially based on the pyroxene solvus, alumina contents in clinopyroxene and orthopyroxene. Increasing alumina contents in pyroxenes reduce enstatite and diopside components in clinopyroxene and orthopyroxene, respectively. Thus, neglect of alumina in pyroxenes causes underestimates of temperatures by the solvus method.The three geothermometers were tested by applying them to homogeneous spinel lherzolites which were especially selected for this purpose. Coincidence of the three temperatures thus estimated gives confidence in the effectiveness of the geothermometers.They were also applied to spinel lherzolite nodules in basalts and intrusive lherzolites described in the literature. It was found that equilibration temperature of the nodules varies from 1000 °C to 1300 °C, i.e., temperatures somewhat higher than have been generally thought. In contrast to the nodules, the intrusive spinel lherzolites show extensive disequilibrium, which is probably due to retrogressive metamorphism suffered by the intrusives.     

Paleointensity determination by a modified thellier method

It has been frequently observed that the ferromagnetic minerals in volcanic rocks may undergo some physical/chemical changes when they are heated in a laboratory. For paleointensity determinations, it is therefore desirable to use the Thellier method, in which internal consistency of the data assures that such undesirable changes did not take place in a particular sample. The most serious drawback of this method is that it requires a considerable number of heatings and coolings and measurements of the remanent magnetization. Following a suggestion of Kono, a modified version of Thellier's technique was developed and applied to a historical lava flow of Hawaii and Oligocene rhyolites and welded tuffs of San Juan volcanic field, Colorado, U.S.A. Samples were heated only once to each temperature and cooled under a magnetic field applied perpendicular to the NRM directions. Results were compared to those obtained by the conventional Thelliers' technique. It was found that the modified method gives results as satisfactory as the original method. With the selection of appropriate samples, the modified method will cut down the time required for the experiment to almost half of what is required in the original method, without losing the merits of Thelliers' technique.     

Recent advances in aquaculture in Japan

Conclusions During the last ten years, aquaculture, especially mariculture has undergone remarkable expansion to counter-balance the gap between the supply and demand of fishery products in Japan. Its expansion has also been supported by the traditional preference of the Japanese people for seafood. Mariculture is now being forced to change its structure due to adverse economic circumstances such as the rise in the cost of energy, feeds and other materials for culture, low market prices, deterioration of environmental conditions, and many other factors. Before these constraints can be resolved, advanced scientific knowledge and technologies must be integrated to develop new types of aquaculture. Expansion of maricultural activities to more open-sea and off shore areas would be one route to follow. Technologies for the development of formulated diets not only for young but also for adults and larvae, establishment of feeding standards, prevention of epizootic diseases must support the establishment of modern aquaculture.The culture-based fisheries, or marine ranching, is becoming a subject for further development in Japan. Much research work has been conducted with a great degree of governmental support. Establishment of a net work of seedstock production farms has made it possible to carry out releasing experiments on a large scale. A newly integrated restoration system could be developed on the basis of these experiments, even though the profits resulting from the artificial restoration of stocks is not thouroughly verified at present. The commencement of a national scheme for culture-based fisheries would be a manifestation of the enthusiasm of the Japanese fish industry.