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41.
Miyuki Tahata Yusuke Sawaki Yuichiro Ueno Manabu Nishizawa Naohiro Yoshida Toshikazu Ebisuzaki Tsuyoshi Komiya Shigenori Maruyama 《地学前缘(英文版)》2015,6(1):121-136
Important ecological changes of the Earth(oxidization of the atmosphere and the ocean) increase in nutrient supply due to the break-up of the super continent(Rodinia) and the appearance of multi-cellular organisms(macroscopic algae and metazoan) took place in the Ediacaran period,priming the Cambrian explosion.The strong perturbations in carbon cycles in the ocean are recorded as excursions in carbonate and organic carbon isotope ratio(δ~(13)C_(carb) and δ~(13)C_(org)) from the Ediacaran through early Cambrian periods.The Ediacaran-early Cambrian sediment records of δ~(13)C_(carb) and δ~(13)C_(org),obtained from the drill-core samples in Three Gorges in South China,are compared with the results of numerical simulation of a simple one-zone model of the carbon cycle of the ocean,which has two reservoirs(i.e.,dissolved organic carbon(DOC) and dissolved inorganic carbon(DIC).The fluxes from the reservoirs are assumed to be proportional to the mass of the carbon reservoirs.We constructed a model,referred to here as the Best Fit Model(BFM),which reproduce δ~(13)C_(carb) and δ~(13)C_(org) records in the Ediacaran-early Cambrian period noted above.BFM reveals that the Shuram excursion is related to three major changes in the carbon cycle or the global ecological system of the Earth:(1) an increase in the coefficient of remineralization by a factor of ca.100,possibly corresponding to a change in the dominant metabolism from anaerobic respiration to aerobic respiration,(2) an increase of carbon fractionation index from 25‰ to 33‰,possibly corresponding to the change in the primary producer from rock-living cyanobacteria to free-living macro algae,and(3) an increase in the coefficient of the organic carbon burial by a factor of ca.100,possibly corresponding to the onset of a biological pump driven by the flourishing metazoan and zooplankton.The former two changes took place at the start of the Shuram excursion,while the third occurred at the end of the Shuram excursion.The other two excursions are explained by the tentative decrease in primary production due to cold periods,which correspond to the Gaskiers(ca.580 Ma) and Bikonor(ca.542 Ma) glaciations. 相似文献
42.
Seismic observations have shown structural variation near the base of the mantle transition zone(MTZ)where subducted cold slabs,as visualized with high seismic speed anomalies(HSSAs),flatten to form stagnant slabs or sink further into the lower mantle.The different slab behaviors were also accompanied by variation of the "660 km" discontinuity depths and low viscosity layers(LVLs) beneath the MTZ that are suggested by geoid inversion studies.We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics.A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ.We found that the viscosity reduction of subducted crustal material leads to a separation of crustal material from the slab main body and its transient stagnation in the MTZ.The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20-30 My from the start of the plate subduction.The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model.Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals,dehydration should occur at the phase transformation depth,~660 km.The variation of the discontinuity depths and highly localized low seismic speed anomaly(LSSA) zones observed from seismic P waveforms in a relatively high frequency band(~1 Hz) support the hypothesis of dehydration from hydrous slabs at the phase boundary.The LSSAs which correspond to dehydration induced fluids are likely to be very local,given very small hydrogen(H~+) diffusivity associated with subducted slabs.The image of such local LSSA zones embedded in HSSAs may not be necessarily captured in tomography studies.The high electrical conductivity in the MTZ beneath the northwestern Pacific subduction zone does not necessarily require a broad range of high water content homogeneously. 相似文献
43.
Masaki Yoshida 《地学前缘(英文版)》2015,6(1):1-2
<正>About three decades after the establishment of the plate tectonics theory in the late 1960s,Maruyama(1994)proposed the"plume tectonics"theory based on whole-mantle seismic tomography image(Fukao,1992;Fukao et al.,1994).According to this theory,the earth's interior is divided into three regimes:the earth's surface region governed by lateral motions of tectonic plates,the mantle governed by vertical motions of"superplumes"(i.e.,large- 相似文献
44.
Robert G.Strom Renu Malhotra Zhi-Yong Xiao Takashi Ito Fumi Yoshida Lillian R Ostrach 《天文和天体物理学研究(英文版)》2015,(3):407-434
We review previously published and newly obtained crater size-frequency distributions in the inner solar system. These data indicate that the Moon and the terrestrial planets have been bombarded by two populations of objects. Population 1,dominating at early times, had nearly the same size distribution as the present-day asteroid belt, and produced heavily cratered surfaces with a complex, multi-sloped crater size-frequency distribution. Population 2, dominating since about 3.8–3.7 Gyr,had the same size distribution as near-Earth objects(NEOs) and a much lower impact flux, and produced a crater size distribution characterized by a differential –3single-slope power law in the crater diameter range 0.02 km to 100 km. Taken together with the results from a large body of work on age-dating of lunar and meteorite samples and theoretical work in solar system dynamics, a plausible interpretation of these data is as follows. The NEO population is the source of Population 2 and it has been in near-steady state over the past ~ 3.7–3.8 Gyr; these objects are derived from the main asteroid belt by size-dependent non-gravitational effects that favor the ejection of smaller asteroids. However, Population 1 was composed of main belt asteroids ejected from their source region in a size-independent manner, possibly by means of gravitational resonance sweeping during orbit migration of giant planets;this caused the so-called Late Heavy Bombardment(LHB). The LHB began some time before ~3.9 Gyr, peaked and declined rapidly over the next ~ 100 to 300 Myr,and possibly more slowly from about 3.8–3.7 Gyr to ~2 Gyr. A third crater population(Population S) consisted of secondary impact craters that can dominate the cratering record at small diameters. 相似文献
45.
Low-inclination near-earth asteroid(NEA)(297274) 1996 SK,which is also classified as a potentially hazardous asteroid,has a highly eccentric orbit.It was studied by multi-wavelength photometry within the framework of an NEA color survey at Lulin Observatory.Here,we report the finding of large color variation across the surface of(297274) 1996 SK within one asteroidal rotation period of4.656 ± 0.122 hours and classify it as an S-type asteroid according to its average colors of B- V = 0.767 ± 0.033,V- R = 0.482 ± 0.021,V- I = 0.801 ± 0.025and the corresponding relative reflectance spectrum.These results might be indicative of differential space weathering or compositional inhomogeneity in the surface materials. 相似文献
46.
Haruo Yoshida 《Celestial Mechanics and Dynamical Astronomy》1987,40(1):51-66
The straight-line collision solution in the anisotropic Kepler problem is extended to a periodic solution by means of Sundman's analytic continuation. It is shown that this collision periodic solution is always exponentially unstable. 相似文献
47.
Haruo Yoshida 《Celestial Mechanics and Dynamical Astronomy》1988,44(4):313-316
In Hamiltonian systems with a homogeneous rational potential and two degrees of freedom, the presence of imaginary Kowalevski exponents implies the non-existence of an additional analytic integral, i.e., non-integrability of the system. 相似文献
48.
Yoshida A Nomura H Toyoda K Nishino T Seo Y Yamada M Nishimura M Wada M Okamoto K Shibata A Takada H Kogure K Ohwada K 《Marine pollution bulletin》2006,52(1):89-95
Microbial responses to the addition of oil with or without a chemical dispersant were examined in mesocosm and microcosm experiments by using denaturing gradient gel electrophoresis of bacterial ribosomal DNA and direct cell counting. When a water-soluble fraction of oil was added to seawater, increases in cell density were observed in the first 24h, followed by a decrease in abundance and a change in bacterial species composition. After addition of an oil-dispersant mixture, increases in cell density and changes in community structure coincided, and the amount of bacteria remained high. These phenomena also occurred in response to addition of only dispersant. Our results suggest that the chemical dispersant may be used as a nutrient source by some bacterial groups and may directly or indirectly prevent the growth of other bacterial groups. 相似文献
49.
There are wide areas of granitic rocks in the Japanese orogenic belt. These granitic bodies inevitably contain fracture and fault systems associated with alteration zones. However, relatively little attention has been given to the possible influence of such widely distributed alteration zones on the migration of radionuclides from any radioactive waste repository that might in future be sited within granitic rock. In particular, the influences of alteration products and micro-fractures, due to chemical sorption and/or physical retardation require further consideration. In order to understand the retardation capacity of the altered deep granitic rocks, detailed geometrical characterization of pores, geochemical analysis, and batch sorption and flow-through experiments have been carried out. Those results show that the altered granite has a large volume of accessible pores, particularly in potassium-feldspar grains, which would influence nuclide retardation more than the accessible porosity in other minerals present, such as biotite. The distribution coefficients, Kd estimated from batch sorption tests and flow-through experiments suggest that altered granite has a high capability to retard the migration of nuclides. The retardation would probably be due to sorption on altered minerals such as sericite and iron hydroxides formed along grain boundaries and in pores created by dissolution, in addition to sorption on primary sorptive minerals. These results provide confidence that even altered and fractured parts of any granitic rock that might be encountered in a site for the disposal of high level radioactive waste may still retard radionuclide migration and thereby help the geosphere to function as a barrier. 相似文献
50.
Biogeochemistry of nitrous oxide in groundwater in a forested ecosystem elucidated by nitrous oxide isotopomer measurements 总被引:3,自引:0,他引:3
K. Koba K. Osaka Y. Tobari S. Toyoda N. Ohte M. Katsuyama N. Suzuki M. Itoh H. Yamagishi M. Kawasaki S.J. Kim N. Yoshida T. Nakajima 《Geochimica et cosmochimica acta》2009,73(11):3115-1037
The biological and physical controls on microbial processes that produce and consume N2O in soils are highly complex. Isotopomer ratios of N2O, with abundance of 14N15N16O, 15N14N16O, and 14N14N18O relative to 14N14N16O, are promising for elucidation of N2O biogeochemistry in an intact ecosystem. Site preference, the nitrogen isotope ratio of the central nitrogen atom minus that of the terminal nitrogen atom, is useful to distinguish between N2O via hydroxylamine oxidation and N2O via nitrite reduction.We applied this isotopomer analysis to a groundwater system in a temperate coniferous-forested ecosystem. Results of a previous study at this location showed that the N2O concentration in groundwater varied greatly according to groundwater chemistry, i.e. NO3−, DOC, and DO, although apportionment of N2O production to nitrification or denitrification was ambiguous. Our isotopic analysis (δ15N and δ18O) of NO3− and N2O implies that denitrification is the dominant production process of N2O, but definitive information is not derived from δ15N and δ18O analysis because of large variations in isotopic fractionations during production and consumption of N2O. However, the N2O site preference and the difference in δ15N between NO3− and N2O indicate that nitrification contributes to total N2O production and that most measured N2O has been subjected to further N2O reduction to N2. The implications of N2O biogeochemistry derived from isotope and isotopomer data differ entirely from those derived from conventional concentration data of DO, NO3−, and N2O. That difference underscores the need to reconsider our understanding of the N cycle in the oxic-anoxic interface. 相似文献