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561.
Takako Masuda Ken Furuya Naoko Kohashi Mitsuhide Sato Shigenobu Takeda Makoto Uchiyama Naho Horimoto Takashi Ishimaru 《Journal of Oceanography》2010,66(6):801-813
Phytoplankton dynamics in the lower euphotic zone were observed by tracking a subsurface water released at 20-m depth from
Takumi, an artificial upwelling device. Takumi continually discharged seawater pumped up from a depth of 205 m: this water was mixed with 5-m depth water to adjust the
density to that of 20-m depth water of Sagami Bay, Japan. The discharged water was pulse-labeled at Takumi with uranine and tracked for 63.9 h with a drifting buoy equipped with a drogue at 20-m depth. We present a simple model
to estimate in situ phytoplankton net growth rates from temporal changes in phytoplankton abundance in the discharged water with correction for
the influence of water exchange between the discharged water and neighboring layers. Lagrangian observation showed active
growth of pico- and nanophytoplankton, especially cryptophytes and Synechococcus (Cyanobacteria), in the subsurface layer. In contrast, diatoms grew little in spite of micromolar concentrations of nutrients.
The active growth of pico- and nanophytoplankton was in good agreement with shipboard serial dilution culture experiments.
The low growth activity of diatoms was suggested to be related to low light availability in the subsurface layer. 相似文献
562.
Lawrence Patrick C. Bernardo Kazuo Nadaoka Takashi Nakamura Atsushi Watanabe 《Ocean Dynamics》2017,67(11):1369-1384
While widely known for their destructive power, typhoon events can also bring benefit to coral reef ecosystems through typhoon-induced cooling which can mitigate against thermally stressful conditions causing coral bleaching. Sensor deployments in Sekisei Lagoon, Japan’s largest coral reef area, during the summer months of 2013, 2014, and 2015 were able to capture local hydrodynamic features of numerous typhoon passages. In particular, typhoons 2015-13 and 2015-15 featured steep drops in near-bottom temperature of 5 °C or more in the north and south sides of Sekisei Lagoon, respectively, indicating local cooling patterns which appeared to depend on the track and intensity of the passing typhoon. This was further investigated using Regional Ocean Modeling System (ROMS) numerical simulations conducted for the summer of 2015. The modeling results showed a cooling trend to the north of the Yaeyama Islands during the passage of typhoon 2015-13, and a cooling trend that moved clockwise from north to south of the islands during the passage of typhoon 2015-15. These local cooling events may have been initiated by the Yaeyama Islands acting as an obstacle to a strong typhoon-generated flow which was modulated and led to prominent cooling of waters on the leeward sides. These lower temperature waters from offshore may then be transported to the shallower inner parts of the lagoon area, which may partly be due to density-driven currents generated by the offshore-inner area temperature difference. 相似文献
563.
Michael E. Zolensky Atsushi Takenouchi Takashi Mikouchi Timothy Gregory Kunihiko Nishiizumi Marc W. Caffee Michael A. Velbel Daniel K. Ross Andrew Zolensky Loan Le Naoya Imae Akira Yamaguchi 《Meteoritics & planetary science》2021,56(1):49-55
Cosmic ray exposure (CRE) ages of CM chondrites have been found to have multiple peaks (as many as four), in stark contrast to other groups of chondrites (Nishiizumi and Caffee 2012; Herzog and Caffee 2014). In this study, we sought correlations between the CRE ages and petrography of CM chondrites, and we conclude that the degree of aqueous alteration does appear to vary with the CRE ages—the CMs displaying the most aqueous alteration all have relatively short exposure ages. However, some CMs with low degrees of alteration also have short exposure ages—thus, this apparent correlation is not exclusive. We also found a definite inverse relation between the number of distinctive lithologies in a CM and its exposure age, which could indicate different responses of homogeneous and heterogeneous meteoroids to the space environment between their onset of exposure (exhumation and ejection from the parent body) and arrival at Earth. Breccias have more internal surfaces of lithologic discontinuity, possibly resulting in weaker meteoroids that disintegrate more readily than their more homogeneous counterparts. Our results suggest that CM chondrite regoliths consist of numerous genomict lithologies in a breccia with millimeter‐ to decimeter‐scale clasts, with varying degree of heating/metamorphism. 相似文献
564.
Takashi Oguchi 《地球表面变化过程与地形》1997,22(2):107-120
Previous studies in Japanese mountains have shown that drainage density (D) correlates negatively with relief (R) and the erosion rate (E), whereas elsewhere both R and E correlate positively with D. To investigate the inconsistency, this paper compares two types of D–R relations for eight mountain river basins in central Japan. R is computed from a digital elevation model for 1109 morphometric samples of area 0·5 km × 0·5 km. Drainage networks in these cells were first constructed by map criteria applied previously in Japan — deeply notched V-shaped contours with an angle <53°. The resulting D correlates negatively with R, confirming preceding studies. When drainage lines along shallower hollows were added, however, the calculated D essentially constant. These relations arise from active landsliding in high-relief terrains, which has eroded steep channel banks into gentle ones. The decline of channel banks with increasing R is accelerated in terrains underlain by soft rocks, because of rapid erosion. The constant D for all the drainage lines indicates a uniform frequency or spacing of ridges and hollows on hillslopes in rugged humid mountains. Because the D–R and D–E relations for Japan reflect a uniquely Japanese physiographic setting characterized by frequent landsliding, they differ from those relations for other regions where channelization by gullying predominates. © 1997 by John Wiley & Sons, Ltd. 相似文献
565.
Tomoharu Senjyu Noriko Asano Masaji Matsuyama Takashi Ishimaru 《Journal of Oceanography》1998,54(1):29-44
Intermediate intrusion of low salinity water (LSW) into Sagami Bay was investigated on the basis of CTD data taken in Sagami
Bay and off the Boso Peninsula in 1993–1994. In October 1993, water of low temperature (<7.0°C), low salinity (<34.20 psu)
and high dissolved oxygen concentration (>3.5 ml I−1) intruded along the isopycnal surface of {ie29-1} at depths of 320–500 m from the Oshima East Channel to the center of the
bay. On the other hand, the LSW was absent in Sagami Bay in the period of September–November 1994, though it was always found
to the south off the Boso Peninsula. Salinity and dissolved oxygen distributions on relevant isopycnal surfaces and water
characteristics of LSW cores revealed that the LSW intruded from the south off the Boso Peninsula to Sagami Bay through the
Oshima East Channel. The LSW cores were distributed on the continental slope along 500–1000 m isobaths and its onshore-offshore
scales were two to three times the internal deformation radius. Initial phosphate concentrations in the LSW revealed its origin
in the northern seas. These facts suggest that the observed LSW is the submerged Oyashio Water and it flows southwestward
along the continental slope as a density current in the rotating fluid. The variation of the LSW near the center of Sagami
Bay is closely related to the Kuroshio flow path. The duration of LSW in Sagami Bay is 0.5 to 1.5 months. 相似文献
566.
Midori Kawabe Hiroshi Kohno Takashi IshimaruOsamu Baba Naho HorimotoReiko Ikeda Jota KandaTakafumi Kudo Masaji MatsuyamaMasato Moteki Yayoi OshimaTsuyoshi Sasaki 《Marine Policy》2009
This paper reviews the initial phase of a coastal education for sustainable development program for Edomae, the innermost reaches of Tokyo Bay. The program has been steered by a working group of Tokyo University of Marine Science and Technology faculty members from different academic backgrounds. Although the process began with conventional educational ideas, the ESD practice framework evolved to include more interactive activities. The overall goal is to pursue discussions of a plan for the sustainable use of Tokyo Bay in the coastal communities through a university–community partnership by developing Edomae ESD leaders in the coastal community. 相似文献
567.
Katherine H. Joy Channon Visscher Michael E. Zolensky Takashi Mikouchi Kenji Hagiya Kazumasa Ohsumi David A. Kring 《Meteoritics & planetary science》2015,50(7):1157-1172
Lunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe‐oxide phase with disseminated domains of troilite. Fragments of plagioclase (An95‐97), pyroxene (En74‐75, Fs21‐22,Wo3‐4), and olivine (Fo66‐67) are distributed in and adjacent to the Fe‐oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X‐ray absorption near edge spectroscopy (XANES) and X‐ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe3+/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe‐magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization‐driven mineral replacement of Fe‐metal or desulphurisation from Fe‐sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H2O steam/liquid or CO2 gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet impact. 相似文献
568.
569.
Takashi Murakami Bulusu Sreenivas Subrata Das Sharma 《Geochimica et cosmochimica acta》2011,75(14):3982-4004
The increase in atmospheric oxygen during the Precambrian is a key to understand the co-evolution of life and environment and has remained as a debatable topic. Among various proxies for the estimation of atmospheric oxygen levels, paleosols, ancient weathering profiles, can provide a quantitative pattern of atmospheric oxygen increase during the Precambrian period of Earth history. We have re-evaluated the chemical compositions of paleosols, and presented a new method of applying Fe2+ oxidation kinetics to the Fe2+ and Fe3+ concentrations in paleosols to decipher the quantitative partial pressure of atmospheric oxygen (PO2) between 2.5 and 2.0 Ga. We first estimated the compaction factor (CF, the fraction of original thickness) using the immobile elements such as Ti, Al and Zr on equal volume basis, which was then used to calculate retention fractions (MR), a mass ratio of paleosol to parent rock, of redox-sensitive elements. The CF and FeR values were evaluated for factors such as homogeneity of immobile elements, erosion, and formation time of weathering. FeR increased gradually within the time window of ∼2.5-2.1 Ga and remained close to 1.0 since ∼2.1 Ga onwards. MnR also increased gradually similar to FeR but at a slower rate and near complete retention was observed ∼1.85 Ga, suggesting an almost continuous increase in the oxidation of Fe2+ and Mn2+ in paleosols ranging in age between ∼2.5 and 1.9 Ga.We have modeled PO2 variations during the Paleoproterozoic by applying Fe2+ oxidation kinetics to the Fe2+ and Fe3+ concentrations in paleosols, which enabled us to derive an Fe2+ oxidation term referred to as ψ. Possible changes in temperature and PCO2 during this time window and their effects on resulting models of PO2 evolution have been also considered. We assumed four cases for the calculations of PO2 variations between 2.5 and 2.0 Ga: no change in either temperature or PCO2, long-term change in only PCO2, long-term changes in both temperature and PCO2, and short-term fluctuations of both temperature and PCO2 during the possible, multiple global-scale glaciations. The calculations indicate that PO2 increased gradually, linearly on the logarithmic scale, from <∼10−6 to >∼10−3 atm between 2.5 and 2.0 Ga. Our calculations show that the PO2 levels would have fluctuated significantly, if intense, global glaciation(s) followed by period(s) of high temperature occurred during the Paleoproterozoic. This gradual rise model proposes a distinct, quantitative pattern for the first atmospheric oxygen rise with important implications for the evolution of life. 相似文献
570.
Primary data on the organic geochemistry of the Tetori Group provide basic information about depositional environments and thermal maturation of organic matter through two geological sections in the Hokuriku region, central Japan. The thermal maturity of organic matter was evaluated by the methylphenanthrene index‐1. The maturity progressively increases stratigraphically down through the Izumi section in Fukui Prefecture. The estimated vitrinite reflectance equivalent is <1.35% near the top of the section, gradually increasing to >2.0% near the base. The thermal maturation process can account for stratigraphic changes in abundance of PAHs through this section. However, the occasional occurrence of coronene in the middle of the Izumi section is attributed to possible paleo‐wildfires that supplied more coronene to the depositional site. The stratigraphic distribution of PAHs through the Tateyama section, Toyama Prefecture, also can be explained by the same scenario as envisaged for the Izumi section, but weathering and/or other secondary factors may have partly modified primary signals for this section. Polycyclic aromatic sulfur compounds were observed in nearly all samples from both sections, even in samples for which a freshwater paleoenvironment is surmised. Most plausibly, some reduced sulfur was re‐oxidized to elemental sulfur, which persisted in the sediments and may have contributed to the formation of aromatic sulfur compounds. Hence, polycyclic aromatic sulfur compounds do not provide an index to separate marine environments from the freshwater settings for the Tetori Group. 相似文献