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301.
During our monitoring observations of comet Hale-Bopp, we found several sporadic ejections of dust from the nucleus. The most
prominent ejection was observed on May 6–9, 1997, in the post-perihelion phase of the apparition. In this paper, we report
preliminary analysis of this event, in which the total mass of the dust cloud is estimated to be 1.6 × 1011 g.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
302.
S. Yamamoto 《Environmental Geology》1996,27(2):118-119
There are numerous springs in Japan. The largest and most famous ones are volcanic and karstic springs. Springs in the areas of Mount Fuji, Mount Aso, and Akiyoshidai are discussed here as sources of water used for drinking, irrigation, fish cultivation, industrial water, and sightseeing. 相似文献
303.
304.
Masa-Yuki Yamamoto Masayuki Toda Yoshihiro Higa Kouji Maeda Jun-Ichi Watanabe 《Earth, Moon, and Planets》2004,95(1-4):279-287
Observations of persistent meteor trains are limited because of the extreme rarity of the phenomenon. The altitudinal distribution
of persistent trains has previously been investigated via limited instances of simultaneous observation from multiple sites,
however, a statistical study of persistent trains has yet to be realized. The meteor train observation (METRO) campaign was
established in Japan in 1998 to obtain images of persistent trains. From 1998 to 2002, the METRO campaign, involving Japanese
amateur collaborators, captured more than 400 image sequences of persistent trains, resulting in 53 simultaneous multi-site
observations. Several Japanese observers were involved in imaging persistent trains prior to the METRO campaign, producing
6 simultaneous observations over the period 1988–1997. In this paper, simultaneous multi-site observations of high spatial
and temporal resolution are used to determine, via triangulation, the altitudinal distribution of persistent trains for 20
events. The altitudinal range of 2 Orionid trains was slightly higher than that of 18 Leonid trains. The Leonid train data
reveal no clear dependence of upper and lower altitude on the brightness of the associated fireball. The upper altitude of
the 18 observed Leonid trains were almost invariant with respect to local time (LT) of observation, however, a possible dependence
of lower altitude on LT of observation was also found, indicating a near-constant penetrating path-length in the upper atmosphere
for each train. The average upper altitude of persistent Leonid trains was 99.8 km, while the average central altitude was
93.0 km. These trends are probably determined by atmospheric conditions such as the abundance of O and O3. 相似文献
305.
Noble gas abundances in basaltic glasses from ocean islands (OIBs) are generally lower than those of mid-oceanic ridge basalts (MORBs), contrary to most geodynamic models which usually require that the source of OIBs is less degassed (resulting in higher primordial noble gas abundances) and more trace element enriched (resulting in higher radiogenic noble gas abundances) than the MORB source. Therefore, noble gas abundances in OIBs are often thought to have been reduced by extensive gas loss from the magma before eruption.The extent of magmatic degassing can be tested as it will cause characteristic changes in the composition of the volatiles; notably the 4He/40Ar* ratio (where 40Ar* is 40Ar corrected for atmospheric contamination) will increase in residual volatiles due to the higher solubility of He relative to Ar. The degree of He-Ar fractionation for a given fraction of gas loss depends on the ratio of the solubilities, SHe/SAr, which is sensitive to (among other things) the CO2 and H2O content of the basalt at the time of degassing.From a global database of OIB and MORB glasses, we show that 4He/40Ar* ratios of MORB glasses are broadly consistent with degassing of a magma with an initial 40Ar of ≈1.5 × 10−5 ccSTP/g, i.e., similar to that of the “popping rock.” However, OIB glasses generally have lower 40Ar* concentration for a given 4He/40Ar*. While this would appear to require lower 40Ar* abundances in the undegassed OIB magmas, the higher volatile contents of OIBs will reduce SHe/SAr (relative to MORBs) during degassing. By modeling SHe/SAr in OIBs, it is possible to show that extensive degassing of OIBs can occur without dramatically increasing the 4He/40Ar* ratio. We show that undegassed 40Ar concentrations of OIB magmas were probably similar to those of MORBs. 相似文献
306.
Abstract The structure, paleomagnetism and biostratigraphy of the Nishizaki and Kagamigaura formations on the southern Boso Peninsula, central Japan, were investigated to determine the chronographic constraints on the accretion, post-Late Miocene rotation and regional tectonics in the Izu–Bonin island arc collision zone. The geological structures on the southern Boso Peninsula are characterized by an east–west trending and south-verging fold and thrust belt that curves toward the northwest–southeast in the northwest extent of the Nishizaki Formation. Two stages of tectonic rotation were revealed by paleomagnetic and structural studies. The first is believed to have occurred after the accretion of the Nishizaki Formation and before the deposition of the Kagamigaura Formation, while the second is confidently correlated with the 1 Ma Izu block collision. The northwest extent of the Nishizaki Formation was rotated clockwise by approximately 65–80°, whereas the rotation was only 25–30° in the east, and 11–13° in the overlying Kagamigaura Formation. Radiolarian biostratigraphy suggests a depositional age of 9.9–6.8 Ma (Upper Miocene period) for the Nishizaki Formation and 4.19-3.75 Ma (Pliocene period) for the lower Kagamigaura Formation. These results indicate that the age of accretion and first-stage rotation of the Nishizaki Formation can be constrained to the interval of 6.80–3.75 Ma. This structure most likely represents the northward bending caused by collisions of the Tanzawa and Izu blocks with the Honshu island arc, and suggests rapid processes of accretion, collision, uplift and the formation of new sedimentary basins within a relatively short period of time (2.61–3.05 my). 相似文献
307.
Abstract River incision into bedrock results in the decrease of burial depths, which can be of critical importance, for example, in the safe long‐term storage of high‐level radioactive waste. For the long‐term prediction of river erosion, it is essential to know the rate of incision during the Late Quaternary period. In the Abukuma Mountains on the forearc side of northeast Japan, a low‐relief peneplain that was uplifted in the Pliocene period is widely developed. Degradational fluvial terraces that are discontinuously distributed along draining rivers are scattered in the study area. The fluvial sediments were mainly transported from the summit regions as debris flows or hyperconcentrated flows. The terrace deposits are capped by a series of eolian veneers containing key tephra beds. From the oldest to the youngest, these tephra beds are the 150–125 ka Iizuna–Kamitaru tephra, the 135–125 ka Hiuchigatake–Tagashira tephra, the 120 ka Adatara–Dake tephra, the 70–80 ka Bandai–Hayama‐2 tephra, the 45 ka Numazawa–Mizunuma tephra and the 42 ka Bandai–Hayama‐1 tephra. Using tephrochronological data, the terraces are divided into three groups: higher, middle and lower. The ages of formation of the higher, middle and lower terraces are estimated to be within marine isotope stage (MIS) 6, MIS 5.4–5.2 and MIS 3 to MIS 2, respectively. The incision rate, calculated from the relative height between the terrace surface and present‐day valley floor fill, is 1.5–0.8 m/10 000 years in the elevations from 350 to 700 m. The calculated rate does not show significant differences between the higher, middle and lower terraces. All the relative heights decrease with increasing elevation, because the erosional rates of streams in the upper reaches are lower than those in downstream reaches where the discharge rates are higher. This value can be regarded as an estimate of the rate of incision in granitic mountains where there is no volcanic or distinct tectonic activity. 相似文献
308.
Hitoshi Yusa Masaki Akaogi Nagayoshi Sata Hiroshi Kojitani Ryo Yamamoto Yasuo Ohishi 《Physics and Chemistry of Minerals》2006,33(3):217-226
In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO3. ZnGeO3 ilmenite transformed into perovskite at 30.0 GPa and 1300±150 K in a laser-heated diamond anvil cell. After releasing the pressure, the lithium niobate phase was recovered as a quenched product. The perovskite was also obtained by recompression of the lithium niobate phase at room temperature under a lower pressure than the equilibrium phase boundary of the ilmenite–perovskite transition. Bulk moduli of ilmenite, lithium niobate, and perovskite phases were calculated on the basis of the refined X-ray diffraction data. The structural relations among these phases are considered in terms of the rotation of GeO6 octahedra. A slight rotation of the octahedra plays an important role for the transition from lithium niobate to perovskite at ambient temperature. On the other hand, high temperature is needed to rearrange GeO6 octahedra in the ilmenite–perovskite transition. The correlation of quenchability with rotation angle of GeO6 octahedra for other germanate perovskites is also discussed. 相似文献
309.
Lignin and fatty acid records in Lake Baikal sediments over the last 130 kyr: A comparison with pollen records 总被引:1,自引:0,他引:1
A 10 m sediment core from Academician Ridge in Lake Baikal was analyzed for its molecular composition using on-line TMAH (tetramethylammomium hydroxide) thermochemolysis. Major products are lignin phenols, n-C14 to C30 fatty acids (alkanoic acids), cutin acids, hydroxy acids and aliphatic dicarboxylic acids. Lignin phenols are abundant in warmer periods (the interglacial: marine isotope stage (MIS) 5e and MIS 1), but extremely low in the other (colder) periods. This result coincides well with pollen records reported for a core near the present site, where an expansion of coniferous forests in sub-stage 5e and MIS 1 was implied. Normal C24–C30 alkanoic acids, important components of plant wax esters, are abundant in 5e and MIS 1 and are present in significant amounts in the other (colder) periods, unlike the lignin phenols. A high abundance of n-C24 to C30 alkanoic acids relative to lignin phenols in the Bølling–Allerød warm period suggests an enhanced development of herbs.It is implied from comparison of the sedimentary lignin phenol record with fossil pollen records and lignin phenol analysis of modern pollen that the ratio of cinnamyl phenols to vanillyl phenols may serve as an indicator of pollen contribution to sedimentary organic matter. 相似文献
310.
It has been thought that granitic crust,having been formed on the surface,must have survived through the Earth’s evolution because of its buoyancy.At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction,subduction erosion, and continental subduction.Granitic rocks,the major constituent of the continental crust,are lighter than the mantle at depths shallower than 270 km,but we show here,based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone,and thus can be subducted in the depth range of 270-660 km.This suggests that there can be two reservoirs of granitic material in the Earth,one on the surface and the other at the base of the mantle transition zone(MTZ).The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust.Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle.This could explain the anomalous seismic-wave velocities observed around 660 km depth.The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation,chemical discontinuities between granitic material and the surrounding mantle,or a combination thereof. 相似文献