Magnetostratigraphy of the late Cenozoic of the Boso Peninsula, central Japan

Marine Pliocene and Pleistocene beds of the Boso Peninsula are more than 3 km thick and contain abundant fauna and flora. Almost continuous deposition occurred at a high sedimentation rate from the early part of the last reversed geomagnetic polarity epoch to the middle part of the last normal polarity epoch. Several microbiostratigraphic marker horizons enable correlation with deep-sea sediments fully independent of magnetostratigraphic zonation. Globorotalia truncatulinoides first occurs below the normal polarity event BO-B-2, which is correlated with the Olduvai Event in deep-sea sediments, whereas the disappearance of discoasters and the first occurrence of Gephyrocapsa caribbeanica are recognized at the top of BO-B-2, and the first occurrence of Gephyrocapsa oceanica is recognized at the top of BO-B-1. Based on the correlation of the Boso section with Mediterranean stage stratotype sections, the transition from the Pliocene to Pleistocene is estimated to be in the lower part of the Kazusa Group.     

Process of the Tsushima Current formation revealed by ADCP measurements in summer

In order to clarify detailed current structures over the continental shelf margin in the East China Sea, ADCP measurements were carried out in summers in 1991 and 1994 by the quadrireciprocal method (Katoh, 1988) for removing diurnal and semidiurnal tidal flows from observed flows, together with CTD measurements. We discussed the process of the Tsushima Current formation in the East China Sea. The Tsushima Current with a volume transport of 2 Sv (1 Sv=10⁶ m³s^–1) was found north of 31°N. A current with a volume transport of 0.4 Sv was clearly found along the 100 m isobath. Between the Kuroshio and the current along the 100 m isobath, southeastward component of velocity was dominant compared to northwestward one. Four eastward to southeastward currents were found over the sea bed shallower than 90 m depth. Total volume transport of these four currents was 1 Sv, and they seemed to be originated from the Taiwan Strait. Intrusion of offshore water into the inner shelf northwest of Amami Oshima was estimated to have a volume transport of 0.6 Sv. It is concluded that the Tsushima Current is the confluence of these currents over the continental shelf margin with the offshore water intruding northwest of Amami Oshima.     

U–Pb zircon ages of the Nakanogawa Group in the Hidaka Belt,northern Japan: Implications for its provenance and the protolith of the Hidaka metamorphic rocks

Zircon U–Pb ages of two acidic tuff and two turbidite sandstone samples from the Nakanogawa Group, Hidaka Belt, were measured to estimate its depositional age and the development of the Hokkaido Central Belt, northeast Japan. In the northern unit, homogeneous zircons from pelagic acidic tuff from a basal horizon dated to 58–57 Ma, zircons from sandstone from the upper part of the unit dated to 56–54 Ma, and zircons from acidic tuff from the uppermost part dated to 60–56 Ma and 69–63 Ma. Both of the tuff U–Pb ages are significantly older than the youngest radiolarian fossil age (66–48 Ma). Therefore, the maximum depositional age of the turbidite facies in the northern unit is 58 Ma and the younger age limit, estimated from the fossil age, is 48 Ma. In the southern unit, homogeneous zircons from turbidite sandstone dated to 58–57 Ma. Thus the depositional age of this turbidite facies was interpreted to be 66–56 Ma from the fossil age, probably close to 57 Ma. Most of the zircon U–Pb ages from the Nakanogawa Group are younger than 80 Ma, with a major peak at 60 Ma. This result implies that around Hokkaido volcanic activity occurred mainly after 80 Ma. Older zircon ages (120–80 Ma, 180–140 Ma, 340–220 Ma, 1.9 Ga, 2.2 Ga, and 2.7 Ga) give information about the provenance of other rocks in the Hidaka Belt. It is inferred that the Nakanogawa Group comprises protoliths of the upper sequence of the Hidaka Metamorphic Zone, which therefore has the same depositional age as the Nakanogawa Group (66–48 Ma). The depositional ages of the lower sequence of the Hidaka Metamorphic Zone and the Nakanogawa Group are probably the same.     

日本掀起对活断层定义和地震预报的热烈讨论

１９９７年１１月由日本地震学会宣传委员会发起的英文简称为ｎｆｍｌ的活动至１９９８年２月初已２个多月了。截至１９９８年２月５日,已有１４５名参与者。他们就地震、防灾、科学教育和媒体的作用进行了内容广泛的讨论。参与者中不仅有地震学专家,还有宣传、教育、行...     

Enrichment of alkanes within a phytoplankton bloom during an in situ iron enrichment experiment in the western subarctic Pacific

During the Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study ΙΙ (SEEDS-II), we monitored variations in the concentrations of non-methane hydrocarbons (NMHCs), CH₃Cl, N₂O, and CH₄ within a phytoplankton bloom. Stable isotopic compositions were also determined to evaluate the sources of the variations. Although there was little variation in either the concentrations or the stable isotopic compositions of alkenes, CH₃Cl, N₂O, and CH₄ during the 23-day observation period, alkane concentrations increased substantially as the phytoplankton bloomed. The column-integrated quantities of alkanes increased to 3 times pre-bloom levels for C₂H₆, 5 times for C₃H₈, and 20 times for n-C₄H₁₀. The δ¹³C values of both C₂H₆ and C₃H₈ remained almost constant while concentrations increased, whereas that of n-C₄H₁₀ increased by about 12‰. To evaluate the sources of the alkanes produced during the bloom, we compared their δ¹³C values with those of alkanes produced in axenic phytoplankton cultures in our laboratory. We concluded that during the SEEDS-ΙΙ experiment the major portions of C₂H₆ and C₃H₈ were produced during the autolysis of diatoms cells, whereas n-C₄H₁₀ was produced during autolysis of other phytoplankton cells such as cryptophytes and dinoflagellates.     

The stemflow of trees in a Bornean lowland tropical forest

Stemflow volume generation in lowland tropical forests was measured over a 1‐year period in the Malaysian state of Sarawak. The stemflow volume generated by 66 free‐standing trees with a diameter at breast height (DBH) over 1 cm and a tree height over 1 m were measured daily in a representative 10 m × 10 m plot of the forest. Throughfall in the plot was also measured using 20 gauges in a fixed position. Of the 2292 mm of total rainfall observed during the year‐long period, stemflow accounted for 3·5%, throughfall for 82% and there was an interception loss of 14·5%. Understory trees (DBH < 10 cm) played an important role in stemflow generation, producing 77% of the overall stemflow volume and 90% during storms with less than 20 mm of rainfall. Also, owing to their efficiency at funneling rainfall or throughfall water received by their crowns, some understory trees noticeably reduced the catches of the throughfall gauges situated under the reach of their crown areas. During storms producing greater than 20 mm of rainfall, 80% of the total stemflow occurred; trees with a large DBH or height and for which the ratio between crown's diameter and depth is less than 1, tended to generate more stemflow volume in these storms. Mean areal stemflow as a fraction of rainfall in this lowland tropical forest was 3·4%, but may range from 1–10% depending upon the proportion of trees that are high or poor stemflow yielders. Trees with DBH greater than 10 cm were likely to contribute less than 1% of the 3·4% mean areal stemflow in the forest. Copyright © 2004 John Wiley & Sons, Ltd.     

The topological association of Hα structures and magnetic fields

The topological associations between H structures and magnetic fields are examined for an active region observed on two different dates. The structures seen in the on and off band of H filtergrams are compared with the contour maps of magnetic fields at the level of magnetogram observations. Similar comparisons are made also with the configurations of force-free magnetic lines of force at various heights evaluated with the use of formulations developed previously by Nakagawa and Raadu (1972).Among the results of significance, we may note that (1) H plages could be identified with regions of magnetic field larger than ±80 G, (2) the network of bright dots seen in H -1 Å filtergrams follow closely ±80 G contours. (3) stable prominences lie along either neutral lines or valleys of magnetic fields, (4) the configuration of magnetic lines of force shows discrete domain structures suggesting bipolar nature of local magnetic fields, and (5) within a domain the configuration is governed apparently by evalutional consequences. Details of analyses are described with discussions on the limitations and possible future improvements.The National Center for Atmospheric Research is sponsored by the National Science Foundation.Operated by Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

On axisymmetric hydromagnetic equilibria

The physical characteristics of possible axisymmetric equilibria are examined on the basis of the integrals of hydromagnetic equations. It is shown for nearly spherical configurations that a surface differential rotation is possible only in the absence of a meridional circulation with either purely toroidal or purely poloidal magnetic field. In the presence of a meridional circulation, it is shown that no surface rotation or constant rotation is possible if the magnetic field is purely toroidal, and that no rotation is possible if the magnetic field is purely poloidal. A brief discussion is given on the possible solutions including the case of stellar winds with force-free magnetic fields.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

On periodic vortices over a concave semicircular wall fixed in a rotating tank

Abstract

New periodic vortices were observed during a rotating tank experiment, to be described. The peculiarities and the formation mechanism of the present vortices are based on observations. The vortices reach the complete form in stages, viz. laminar boundary layer growth, sinuous motion and rolling oscillation. After being formed, the vortices are shed periodically and the diameter of the vortices grows as they advance.

The vortices appear when the flow over the concave semicircular wall, fixed in the rotating tank, is maintained by the shear stresses at the inner surface of the rotating tank. The vortices and the flow were visualized with thymol blue dye.