Evidence for multiple 4.0–3.7 Ga impact events within the Apollo 16 collection     

Takafumi Niihara  Sky P. Beard  Timothy D. Swindle  Lillian A. Schaffer  Hideaki Miyamoto  David A. Kring 《Meteoritics & planetary science》2019,54(4):675-698

In a histogram of lunar impact ages from the Apollo 16 site, there is a spike circa 3.9 Ga that has been interpreted to represent either a large number of nearly synchronous events or an abundance of samples that were affected slightly differently by the event that produced the Imbrium basin. To further scrutinize those age relationships, we extracted six centimeter‐sized clasts of impact melt from ancient regolith breccia 60016 and performed petrological and geochronological (⁴⁰Ar‐³⁹Ar) analyses. Three clasts have similar poikilitic textures, while others have porphyritic, aphanitic, or intergranular textures. Compositions and abundances of relict minerals are different in all six clasts and variously imply Mg‐suite and ferroan anorthosite target sequences. Estimated bulk compositions of four clasts are similar to previously defined group 1 Apollo 16 impact melt rocks, while the other two have higher Al₂O₃ and lower FeO+MgO compositions. All six clasts have similar K₂O and P₂O₅ concentrations, which could have been derived from a KREEP‐bearing component among target sequences. Eighteen ⁴⁰Ar/³⁹Ar analyses of the six clasts produced an age range from 3823 ± 75 to 4000 ± 23 Ma, consistent with estimates for the proposed late heavy bombardment. Four clasts have multiple temperature steps that define plateau ages. These ages are distinct, so they cannot be explained by a single impact event, such as the one that produced the Imbrium impact basin. The conclusion that these represent distinct ages remains after considering the possibility of artifacts in defining plateaus.  相似文献   

Cluster analysis on the bulk elemental compositions of Antarctic stony meteorites          下载免费PDF全文

Hideaki Miyamoto  Takafumi Niihara  Takeshi Kuritani  Peng K. Hong  James M. Dohm  Seiji Sugita 《Meteoritics & planetary science》2016,51(5):906-919

Remote sensing observations by recent successful missions to small bodies have revealed the difficulty in classifying the materials which cover their surfaces into a conventional classification of meteorites. Although reflectance spectroscopy is a powerful tool for this purpose, it is influenced by many factors, such as space weathering, lighting conditions, and surface physical conditions (e.g., particle size and style of mixing). Thus, complementary information, such as elemental compositions, which can be obtained by X‐ray fluorescence (XRF) and gamma‐ray spectrometers (GRS), have been considered very important. However, classifying planetary materials solely based on elemental compositions has not been investigated extensively. In this study, we perform principal component and cluster analyses on 12 major and minor elements of the bulk compositions of 500 meteorites reported in the National Institute of Polar Research (NIPR), Japan database. Our unique approach, which includes using hierarchical cluster analysis, indicates that meteorites can be classified into about 10 groups purely by their bulk elemental compositions. We suggest that Si, Fe, Mg, Ca, and Na are the optimal set of elements, as this set has been used successfully to classify meteorites of the NIPR database with more than 94% accuracy. Principal components analysis indicates that elemental compositions of meteorites form eight clusters in the three‐dimensional space of the components. The three major principal components (PC1, PC2, and PC3) are interpreted as (1) degree of differentiations of the source body (i.e., primitive versus differentiated), (2) degree of thermal effects, and (3) degree of chemical fractionation, respectively.  相似文献   

Data‐driven taxonomy matching of asteroid and meteorite     

Yuki Saito  Peng K. Hong  Takafumi Niihara  Hideaki Miyamoto  Kenji Fukumizu 《Meteoritics & planetary science》2020,55(1):193-206

The matching of asteroids and meteorites is a significant step toward a better understanding of the origin, structure, and history of the solar system. We propose a data‐driven approach for investigating common taxonomic structure between asteroids and meteorites; C‐, S‐, and V‐type for the former, and carbonaceous chondrite, ordinary chondrite, and howardite‐eucrite‐diogenite (HED) meteorite for the latter. In the numerical experiments, by checking whether the taxonomy information of meteorites improves classification for asteroid data, we examine the existence of common structure over the two domains. For this purpose, we compare the resultant accuracies of two clustering methods which are with/without the guidance of meteorite data. We observe that the guidance of meteorite taxonomy improves the accuracy for classifying asteroids, either with the reflectance spectra or major chemical compositions of meteorites. This fact serves as a piece of evidence that there is a common taxonomic structure and links between meteorites and asteroids, supporting a long‐standing hypothesis.  相似文献   

Breaking of wind waves and the sea surface wind stress     

Yoshiaki Toba  Hideaki Kunishi 《Journal of Oceanography》1970,26(2):71-80

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Study on the currents in Lake Biwa (I)     

Norihisa Imasato  Seiichi Kanari  Hideaki Kunishi 《Journal of Oceanography》1975,31(1):15-24

Some numerical experiments by the barotropic nonlinear two dimensional models are performed to study the water circulations in Lake Biwa, especially to study the large anticlockwise gyre in the north basin. The wind fields used in the experiments have no rotational component. This gyre is induced by the southerly wind and is approximately on the geostrophic balance. The vorticity of this gyre is contributed from the vertical stretching of the vortex tube by the variable bottom topography and the inertia term. But the latter term does not become effective if the bottom profile of the basin is flat. Therefore the horizontal circulation in the barotropic closed basin is the “topographic gyres”. The bottom stress has a little contribution to the vorticity balance of the gyre.  相似文献   

Characteristics of a front formed by cooling of the sea surface and inflow of the fresh water     

Yukio Oonishi  Akira Harashima  Hideaki Kunishi 《Journal of Oceanography》1978,34(1):17-23

Two numerical studies (Endoh, 1977;Harashima et al., 1978) have been proposed on a front formed by a coupling effect of cooling of the sea surface and inflow of the fresh water in a vertical two-dimensional plane without the rotation of the earth. It is, however, not easy to interpret their numerical results. A simple interpretation will be proposed by an analytical study in this paper.It is found that local convection due to the density inversion, which is expressed by the convective adjustment of the vertical diffusion coefficient in the actual numerical calculations, plays an important role on the front formation.The characteristics of the front is also clarified in the case of steady state. Namely, simple functional dependences are obtained of the position and the width of the front, the horizontal and the vertical velocities and the distribution of the buoyancy and the salinity in the neighborhood of the front on the horizontal coordinate, the cooling rate, the eddy coefficients of diffusion and viscosity, the water depth and the vertically averaged horizontal fluxes of buoyancy and salinity.  相似文献   

Long-term changes in the fish community structure from the Tsushima warm current region of the Japan/East Sea with an emphasis on the impacts of fishing and climate regime shift over the last four decades     

Yongjun Tian  Hideaki Kidokoro  Tatsuro Watanabe 《Progress in Oceanography》2006,68(2-4):217

Japanese fisheries production in the Japan/East Sea between 1958 and 2003 increased to their peak (1.76 million tons) in the late 1980s and decreased abruptly with the collapse of Japanese sardine. Catch results for 58 fisheries and various environmental time-series data sets and community indices, including mean trophic level (MTL) and Simpson’s diversity index (DI), were used to investigate the impacts of fishing and climate changes on the structure of the fish community in the Tsushima warm current (TWC) region of the Japan/East Sea. The long-term trend in fisheries production was largely dependent on the Japanese sardine that, as a single species, contributed up to 60% of the total production in the Japanese waters of the Japan/East Sea during the late 1980s. Excluding Japanese sardine, production of the small pelagic species was higher during 1960s and 1990s but lower during 1970s and 1980s. This variation pattern generally corresponds with the trend in water temperature, warmer before early 1960s and after 1990s but colder during 1970s and 1980s. The warm-water, large predatory fishes and cold water demersal species show opposite responses to the water temperature in the TWC region, indicating the significant impact of oceanic conditions on fisheries production of the Japan/East Sea. Declines in demersal fishes and invertebrates during 1970s and 1980s suggested some impact of fishing. MTL and DI show a similar variation pattern: higher during 1960s and 1990s but lower during 1970s and 1980s. In particular, the sharp decline during the 1980s resulted from the abundant sardine catches, suggesting that dominant species have a large effect on the structure of the fish community in the Japan/East Sea. Principal component analysis for 58 time-series data sets of fisheries catches suggested that the fish community varied on inter-annual to inter-decadal scales; the abrupt changes that occurred in the mid-1970s and late 1980s seemed to correspond closely with the climatic regime shifts in the North Pacific. These results strongly suggest that the structure of the fish community in the Japan/East Sea was largely affected by climatic and oceanic regime shifts rather than by fishing. There is no evidence showing “fishing down food webs” in the Japan/East Sea. However, in addition to the impacts of abrupt shifts that occurred in the late 1980s, the large predatory and demersal fishes seem to be facing stronger fishing pressure with the collapse of the Japanese sardine.  相似文献   

The role of tidal vortices in material transport around straits     

Yoshio Takasugi  Akira Hoshika  Hideaki Noguchi  Terumi Tanimoto 《Journal of Oceanography》1994,50(1):65-80

The strong tidal current (tidal jet) in straits generates tidal vortices with a scale of several kilometers. The role of the vortices in material transport was investigated in the Neko Seto Sea, located in the western part of the Seto Inland Sea of Japan. A clockwise vortex with a diameter of about 0.8 km was observed in Nigata Bay (lying between two straits, the Neko Seto Strait and the Meneko Seto Strait). It was concluded that the clockwise vortex was the tidal vortex which was generated by the tidal jet in the Meneko Seto Strait. The vortex moved into the bay with the tide, but tended to stay on the sand bank in the bay. It was confirmed by current measurement with an ADCP and turbidity measurement that the secondary convergent flow was generated in the bottom layer of the vortex. This secondary flow seemed to contribute to the formation of the sand bank. It was suggested that tidal vortices may play an important role in the sediment transport and formation of topography in and around straits.  相似文献   

Satellite detection of red tide in Ariake Sound, 1998–2001     

Joji Ishizaka  Yasuhisa Kitaura  Yasuharu Touke  Hiroaki Sasaki  Akihiko Tanaka  Hiroshi Murakami  Toshikazu Suzuki  Kazumi Matsuoka  Hideaki Nakata 《Journal of Oceanography》2006,62(1):37-45

High resolution SeaWiFS data was used to detect red tide events that occurred in the Ariake Sound, Japan, a small embayment known as one of the most productive areas in Japan. SeaWiFS chlorophyll data clearly showed that a large red tide event, which damaged seaweed (Nori) cultures, started early in December 2000 in Isahaya Bay, expanded to the whole sound and persisted to the end of February 2001. The monthly average of SeaWiFS data from May 1998 to December 2001 indicated that the chlorophyll peaks appeared twice a year, in early summer and in fall, after the peaks of rain and river discharge. The SeaWiFS data showed that the red tide event during 2000–2001 winter was part of the fall bloom; however, it started later and continued significantly longer than other years. Satellite ocean color data is useful to detect the red tide; however the algorithms require improvement to accurately estimate chlorophyll in highly turbid water and in red tide areas.  相似文献   

Preliminary assessment of eutrophication by remotely sensed chlorophyll-a in Toyama Bay,the Sea of Japan     

Genki Terauchi  Ryo Tsujimoto  Joji Ishizaka  Hideaki Nakata 《Journal of Oceanography》2014,70(2):175-184

Time series of the chlorophyll-a concentration (Chl-a) observed by ocean color satellites from 1998 to 2009 were used to assess eutrophication in Toyama Bay, the Sea of Japan. An overall mean of Chl-a during the 12-year period was used to divide the study area into “high” or “low” Chl-a areas based on a reference condition of 5 mg m^?3. The annual maximum monthly mean Chl-a trend was estimated pixel-wise and its significance examined by the Sen slope test at a 90 % confidence level. By combining the level and trend of remotely sensed Chl-a, Toyama Bay was then classified into six eutrophication states: high-increasing, high-no trend, high-decreasing, low-increasing, low-no trend and low-increasing. Our study indicates that the combined use of both the level and trend of remotely sensed Chl-a can be an efficient method to preliminarily assess eutrophication of coastal waters after a quality screening process with level 2 flags and validation with in situ Chl-a data.  相似文献