Space weathered rims found on the surfaces of the Itokawa dust particles     

Takaaki Noguchi  Makoto Kimura  Takahito Hashimoto  Mitsuru Konno  Tomoki Nakamura  Michael E. Zolensky  Ryuji Okazaki  Masahiko Tanaka  Akira Tsuchiyama  Aiko Nakato  Toshinori Ogami  Hatsumi Ishida  Ryosuke Sagae  Shinichi Tsujimoto  Toru Matsumoto  Junya Matsuno  Akio Fujimura  Masanao Abe  Toru Yada  Toshifumi Mukai  Munetaka Ueno  Tatsuaki Okada  Kei Shirai  Yukihiro Ishibashi 《Meteoritics & planetary science》2014,49(2):188-214

On the basis of observations using Cs‐corrected STEM, we identified three types of surface modification probably formed by space weathering on the surfaces of Itokawa particles. They are (1) redeposition rims (2–3 nm), (2) composite rims (30–60 nm), and (3) composite vesicular rims (60–80 nm). These rims are characterized by a combination of three zones. Zone I occupies the outermost part of the surface modification, which contains elements that are not included in the unchanged substrate minerals, suggesting that this zone is composed of sputter deposits and/or impact vapor deposits originating from the surrounding minerals. Redeposition rims are composed only of Zone I and directly attaches to the unchanged minerals (Zone III). Zone I of composite and composite vesicular rims often contains nanophase (Fe,Mg)S. The composite rims and the composite vesicular rims have a two‐layered structure: a combination of Zone I and Zone II, below which Zone III exists. Zone II is the partially amorphized zone. Zone II of ferromagnesian silicates contains abundant nanophase Fe. Radiation‐induced segregation and in situ reduction are the most plausible mechanisms to form nanophase Fe in Zone II. Their lattice fringes indicate that they contain metallic iron, which probably causes the reddening of the reflectance spectra of Itokawa. Zone II of the composite vesicular rims contains vesicles. The vesicles in Zone II were probably formed by segregation of solar wind He implanted in this zone. The textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modification and space weathering on Itokawa.  相似文献   

Mineral chemistry of MUSES‐C Regio inferred from analysis of dust particles collected from the first‐ and second‐touchdown sites on asteroid Itokawa     

Tomoki Nakamura  Aiko Nakato  Hatsumi Ishida  Shigeru Wakita  Takaaki Noguchi  Michael E. Zolensky  Masahiko Tanaka  Makoto Kimura  Akira Tshuchiyama  Toshihiro Ogami  Takahito Hashimoto  Mitsuru Konno  Masayuki Uesugi  Toru Yada  Kei Shirai  Akio Fujimura  Ryuji Okazaki  Scott. A. Sandford  Yukihiro Ishibashi  Masanao Abe  Tatsuaki Okada  Munetaka Ueno  Junichiro Kawaguchi 《Meteoritics & planetary science》2014,49(2):215-227

The mineralogy and mineral chemistry of Itokawa dust particles captured during the first and second touchdowns on the MUSES‐C Regio were characterized by synchrotron‐radiation X‐ray diffraction and field‐emission electron microprobe analysis. Olivine and low‐ and high‐Ca pyroxene, plagioclase, and merrillite compositions of the first‐touchdown particles are similar to those of the second‐touchdown particles. The two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that MUSES‐C Regio is covered with dust particles of uniform mineral chemistry of LL chondrites. Quantitative compositional properties of 48 dust particles, including both first‐ and second‐touchdown samples, indicate that dust particles of MUSES‐C Regio have experienced prolonged thermal metamorphism, but they are not fully equilibrated in terms of chemical composition. This suggests that MUSES‐C particles were heated in a single asteroid at different temperatures. During slow cooling from a peak temperature of approximately 800 °C, chemical compositions of plagioclase and K‐feldspar seem to have been modified: Ab and Or contents changed during cooling, but An did not. This compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized Itokawa parent asteroid. After cooling, some particles have been heavily impacted and heated, which resulted in heterogeneous distributions of Na and K within plagioclase crystals. Impact‐induced chemical modification of plagioclase was verified by a comparison to a shock vein in the Kilabo LL6 ordinary chondrite where Na‐K distributions of plagioclase have been disturbed.  相似文献   

Winter precipitation isotope slopes of the contiguous USA and their relationship to the Pacific/North American (PNA) pattern   总被引：1，自引：0，他引：1  

Zhongfang Liu  Gabriel J. Bowen  Jeffrey M. Welker  Kei Yoshimura 《Climate Dynamics》2013,41(2):403-420

This study compares the synoptic-dynamic relationship between two phases of the Pacific/North American (PNA) pattern and winter precipitation isotopes at 73 sites across the contiguous USA. We use the spatial pattern of isotope slope—the rate of changes in precipitation isotope ratios with distance—to identify features in the seasonal precipitation isotope fields related to climatic patterns, PNA positive and PNA negative. Our results show relationships between zones of high isotope slopes and the spatial position of the polar jet stream and juxtaposition of air masses associated with the PNA pattern. During a positive PNA winter, zones of high isotope slope in the eastern USA shift southward. This change is coincident with a southward displacement of the polar jet stream in this region, which leads to a greater frequency of polar air masses and ¹⁸O-depleted isotope values of precipitation in the region. In the western USA, zones of high slope shift eastward during the positive PNA winter, associated with more frequent penetration of tropical air masses that bring ¹⁸O-enriched precipitation to the region. Differences in δ¹⁸O/temperature relationships between the PNA-positive and -negative winters and contrasting δ¹⁸O/temperature behaviors in the eastern and western USA provide support for the role of variation in moisture source and transport as a control on the isotopic patterns. These findings highlight the importance of synoptic climate driven by PNA pattern in determining the spatial patterns of precipitation isotopes and provide constraints on paleo-water isotope interpretation and modern isotope hydrological processes.  相似文献   

Development of a new operational system for monitoring and forecasting coastal and open-ocean states around Japan     

Hirose  Nariaki  Usui  Norihisa  Sakamoto  Kei  Tsujino  Hiroyuki  Yamanaka  Goro  Nakano  Hideyuki  Urakawa  Shogo  Toyoda  Takahiro  Fujii  Yosuke  Kohno  Nadao 《Ocean Dynamics》2019,69(11):1333-1357

We developed a new system to monitor and forecast coastal and open-ocean states around Japan for operational use by the Japan Meteorological Agency. The system consists of an eddy-resolving analysis model based on four-dimensional variational assimilation and a high (2-km) resolution forecast model covering Japanese coastal areas that incorporates an initialization scheme with temporal and spatial filtering. Assimilation and forecast experiments were performed for 2008 to 2017, and the results were validated against various observation datasets. The assimilation results captured well the observed variability in sea surface temperature, coastal sea level, volume transport, and sea ice. Furthermore, the volume budget for the Japan Sea was significantly improved by the use of the 2-km resolution forecast model compared with the 10-km resolution analysis model. The forecast results indicate that this system has a predictive limit longer than 1 month in many areas, including in the Kuroshio current area south of Japan and the southern Japan Sea. In the forecast results of case studies, the 2017 Kuroshio large meander was well predicted, and warm water intrusions accompanying Kuroshio path variations south of Japan were also successfully reproduced. Sea ice forecasts for the Sea of Okhotsk largely captured the evolution of sea ice in late winter, but sea ice in early winter included relatively large errors. This system has high potential to meet operational requirements for monitoring and forecasting ocean phenomena at both meso- and coastal scales.

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Does suspended matter drained from the Isahaya freshwater reservoir cause organic enrichment in the northern Ariake Bay?     

Kenji Yoshino  Katsumasa Yamada  Kei Kimura 《Journal of Oceanography》2018,74(6):619-628

The inner part of Isahaya Bay was converted to a freshwater reservoir following the closure of the land claim dike in 1997. Turbid water drains into Isahaya Bay when water levels increase. We investigated whether particulate organic matter (POM) from the reservoir in Isahaya Bay has caused bottom organic enrichment in the northern part of Ariake Bay. Using potential end-members from before to after the rainy seasons, during which a frequent discharge from the reservoir was expected, stable isotope analyses were performed on sediments collected from Isahaya Bay and northern Ariake Bay. Each end-member was isotopically differentiated by δ¹³C and δ¹⁵N (riverine POM: ?28.5 to ?27.2‰ and 3.3–4.6‰; reservoir POM: ?25.7 to ?25.3‰ and 7.4–8.4‰; marine POM: ?21.8 to ?19.7‰ and 6.7–7.6‰; microphytobenthos estimated from consumers: ?16.1 to ?15.9‰ and 5.2–6.1‰, respectively). Sediment isotopic signatures fell within the mixing space defined by the signatures of the end-members. Marine POM contributed greatly to bottom sediments in both seasons in Isahaya Bay and Ariake Bay, ranging from ca. 60–70 and 40–60%, respectively. Reservoir POM contributed around 10% to bottom sediments. This percentage slightly increased in the sediment of Isahaya Bay after the rainy season, but decreased in the sediment of Ariake Bay. Thus, most of the POM discharged from the reservoir would not reach the northern part of Ariake Bay and would not be a major contributor to organic enrichment. This study is the first to quantitatively describe the contribution of drained reservoir POM outside Isahaya Bay.  相似文献   

Identification of the fronts from the Kuroshio Extension to the Subarctic Current using absolute dynamic topographies in satellite altimetry products     

Hideyuki Nakano  Hiroyuki Tsujino  Kei Sakamoto  Shogo Urakawa  Takahiro Toyoda  Goro Yamanaka 《Journal of Oceanography》2018,74(4):393-420

The climatologies and variabilities of the fronts associated with the Kuroshio Extension (KE), the Kuroshio Extension Northern Branch (KENB), the flow along the Subarctic Boundary (SAB), and the Subarctic Current (SAC) are identified by a new method based on the absolute dynamic height product from the archiving, validation, and interpretation of satellite oceanographic data (the AVISO product). The fronts are detected by examining the specific contour values of the absolute dynamic topography (ADT) in the AVISO product. The (time-varying) specific contour values are decided from the local maxima of the averaged surface geostrophic velocity along the ADT. Assuming the specific contours as the front locations, we obtain an occurrence frequency map of the four front locations, and determine the monthly variability of the fronts over the 1993–2015 period. The results are validated by hydrographic observations. The KE and KENB east of the Shatsky Rise migrate southward several times at a speed of ~ 0.2 cm s^?1, while the SAB and SAC are mostly stationary.  相似文献