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41.
Atsushi Okamoto Taketo Kikuchi Noriyoshi Tsuchiya 《Contributions to Mineralogy and Petrology》2008,156(3):323-336
Pelitic schists of the Sanbagawa metamorphic belt contain several types of polymineralic veins that formed during the late
stages of exhumation. The vein mineral assemblages are quartz + albite + K-feldspar + chlorite ± calcite (Type I, II) and
quartz + albite + calcite (Type III). Type I and II veins contain quartz and albite with stretched-crystal and elongate-blocky
textures, respectively. The mineral species within Type I veins vary with compositional bands within the host rocks. Type
III veins are characterized by euhedral to subhedral quartz grains with concentric zoning and a homogeneous distribution along
the vein length. The vein textures vary depending on the crack aperture during multiple crack-seal events: <0.08 mm for Type
I, and 0.5–10 mm for Type III. Type II veins show intermediate features between Type I and III veins in terms of mineral distribution
(weak dependence on the host rock composition) and apparent crack aperture (less than 1–15 mm). These observations suggest
a transition in the dominant transport mechanism of vein components with increasing crack aperture, from diffusion from host
rocks to fluid advection along cracks. 相似文献
42.
Enhancement/reduction of biological pump depends on ocean circulation in the sea-ice reduction regions of the Arctic Ocean 总被引:1,自引:0,他引:1
Shigeto Nishino Takashi Kikuchi Michiyo Yamamoto-Kawai Yusuke Kawaguchi Toru Hirawake Motoyo Itoh 《Journal of Oceanography》2011,67(3):305-314
The biological pump is a central process in the ocean carbon cycle, and is a key factor controlling atmospheric carbon dioxide
(CO2). However, whether the Arctic biological pump is enhanced or reduced by the recent loss of sea ice is still unclear. We examined
if the effect was dependent on ocean circulation. Melting of sea ice can both enhance and reduce the biological pump in the
Arctic Ocean, depending on ocean circulation. The biological pump is reduced within the Beaufort Gyre in the Canada Basin
because freshwater accumulation within the gyre limits nutrient supply from deep layers and shelves hence inhibits the growth
of large-bodied phytoplankton. Conversely, the biological pump is enhanced outside the Beaufort Gyre in the western Arctic
Ocean because of nutrient supply from shelves and greater light penetration, enhancing photosynthesis, caused by the sea ice
loss. The biological pump could also be enhanced by sea ice loss in the Eurasian Basin, where uplifted isohaline surfaces
associated with the Transpolar Drift supply nutrients upwards from deep layers. New data on nitrate uptake rates are consistent
with the pattern of enhancement and reduction of the Arctic biological pump. Our estimates indicate that the enhanced biological
pump can be as large as that in other oceans when the sea ice disappears. Contrary to a recent conclusion based on data from
the Canada Basin alone, our study suggests that the biological CO2 drawdown is important for the Arctic Ocean carbon sink under ice-free conditions. 相似文献
43.
This paper presents results of our monitoring of BL Lac at the 2.56 m Nordic Optical Telescope in La Palma and at the 0.91 m telescope at National Astronomical Observatory in Dodaira, Japan. On La Palma we used aUBVRI-photopolarimeter which gives truly simultaneous observations in all five colour bands. At Dodaira we used a photopolarimeter which gives simultaneously photometry atB, V andR-bands and polarimetry atV-band. We have observations of BL Lac during flare behaviour and during these times we have observed a clear rotation of the polarization position angle. During these flares the polarization level has also been strongly variable and the object has lost its preferred direction of the polarization position angle. The reason for this behaviour remains still unclear, but there are two possible explanations: shocks in the helical magnetic field in the jet pointing nearly towards us or polarized synchrotron flares rotating within the accretion disk (seen nearly face on) of the supermassive central black hole. 相似文献
44.
J. B. Callegary C. P. Kikuchi J. C. Koch M. R. Lilly S. A. Leake 《Hydrogeology Journal》2013,21(1):25-39
Groundwater in the US state of Alaska is critical to both humans and ecosystems. Interactions among physiography, ecology, geology, and current and past climate have largely determined the location and properties of aquifers as well as the timing and magnitude of fluxes to, from, and within the groundwater system. The climate ranges from maritime in the southern portion of the state to continental in the Interior, and arctic on the North Slope. During the Quaternary period, topography and rock type have combined with glacial and periglacial processes to develop the unconsolidated alluvial aquifers of Alaska and have resulted in highly heterogeneous hydrofacies. In addition, the long persistence of frozen ground, whether seasonal or permanent, greatly affects the distribution of aquifer recharge and discharge. Because of high runoff, a high proportion of groundwater use, and highly variable permeability controlled in part by permafrost and seasonally frozen ground, understanding groundwater/surface-water interactions and the effects of climate change is critical for understanding groundwater availability and the movement of natural and anthropogenic contaminants. 相似文献
45.
Climatological fields of dissolved oxygen and phosphates in the Bering Sea during the spring, summer, and fall seasons were generated on the basis of an extensive dataset of hydrochemical observations (16,356 stations, beginning in 1928) and a novel 3D variational algorithm for interpolation of a passive ocean tracer. The resulting patterns comply with maps produced earlier using an optimal interpolation method, though they also provide more detail and contain no “missing data” regions. Vertical, spatial, and temporal variability of both parameters follow large-scale patterns of circulation, upper mixed layer depth, and phytoplankton productivity in the Bering Sea. 相似文献
46.
Sau Pin Kok Tomohiko Kikuchi Tatsuki Toda Norio Kurosawa 《Journal of Oceanography》2012,68(5):599-613
The diversity of protistan microplankton in Sagami Bay was revealed by 18S rRNA gene clone analysis using newly designed PCR primers. PCR amplification consisted of a first reaction targeting the V3–V5 region of the 18S rRNA gene, followed by a nested reaction targeting the V3–V4 region. In total, 629 clones consisting of 108 phylotypes were affiliated with a variety of protistan groups including dinoflagellates, diatoms, prymnesiomonada, chlorophyta, ciliophora, cercozoa, and heterokonta. The dinoflagellate group was detected most frequently and shared approximately 74?% of the total clones. Within this group, approximately half of the clones belonged to the parasitic dinoflagellate Syndiniales group I, which was first reported from Sagami Bay. The genera Woloszynskia, Gonyaulax, Neoceratium, and Karlodinium have not been reported from this bay until now. The second most frequent group was diatoms, which shared approximately 22?% of the total clones. Within this group, highly diverse Thalassiosira phylotypes were detected, and they shared approximately 70?% of the diatom clones. Therefore, highly diverse protists including some candidate groups were successfully detected, indicating that the designed primers and PCR protocol will be useful for molecular diversity analyses of protistan microplankton communities in aquatic environments. 相似文献
47.
Yusuke Kawaguchi Jennifer K. Hutchings Takashi Kikuchi James H. Morison Richard A. Krishfield 《Polar Science》2012,6(1):39-53
During the summer of 2010 ice concentration in the Eurasian Basin, Arctic Ocean was unusually low. This study examines the sea-ice reduction in the Eurasian Basin using ice-based autonomous buoy systems that collect temperature and salinity of seawater under the ice along the course of buoy drift. An array of GPS drifters was deployed with 10 miles radius around an ice-based profiler, enabling the quantitative discussion for mechanical ice divergence/convergence and its contribution to the sea-ice reduction. Oceanic heat fluxes to the ice estimated using buoy motion and mixed-layer (ML) temperature suggest significant spatial difference between fluxes under first-year and multi-year ice. In the former, the ML temperature reached 0.6 K above freezing temperature, providing >60–70 W m?2 of heat flux to the overlying ice, equivalent to about 1.5 m of ice melt over three months. In contrast, the multiyear ice region indicates nearly 40 W m?2 at most and cumulatively produced 0.8 m ice melt. The ice concentration was found to be reduced in association with an extensive low pressure system that persisted over the central Eurasian Basin. SSM/I indicates that ice concentration was reduced by 30–40% while the low pressure persisted. The low ice concentration persisted for 30 days even after the low dissipated. It appears that the wind-forced ice divergence led to enhanced absorption of incident solar energy in the expanded areas of open water and thus to increased ice melt. 相似文献
48.
DSDP Hole 504B was drilled into 6 Ma crust, about 200 km south of the Costa Rica Rift, Galapagos Spreading Center, penetrating 1.35 km into a section that can be divided into four zones—Zone I: oxic submarine weathering; Zone II: anoxic alteration; Zones III and IV: hydrothermal alteration to greenschist facies. In Zone III there is intense veining of pillow basalts. Zone IV consists of altered sheeted dikes. Isotopic geochemical signatures in relation to the alteration zones are recorded in Hole 504B, as follows:
Alteration temperatures are as low as 10°C in Zones I and II based on oxygen isotope fractionation. Strontium isotopic data indicate that a circulation of seawater is much more restricted in Zone II than in Zone I. Fluid inclusion measurements of vein quartz indicate the alteration temperature was mainly 300 ± 20°C in Zones III and IV, which is consistent with secondary mineral assemblages.The strontium, oxygen, and hydrogen isotopic compositions of hydrothermal fluids which were responsible for the greenschist facies alteration in Zones III and IV are estimated to be 0.7037, 2‰, and 3‰, respectively. Strontium and oxygen isotope data indicate that completely altered portions of greenstones and vein minerals were in equilibrium with modified seawater under low water/rock ratios (in weight) of about 1.6. This value is close to that of the end-member hydrothermal fluids issuing at 21°N EPR.Basement rocks are not completely hydrothermally altered. About 32% of the greenstones in Zones III and IV have escaped alteration. Thus 1 g of fresh basalt including the 32% unaltered portion are required in order to make 1 g of end-member solution from fresh seawater in water-rock reactions. 相似文献
Zone | Depth(m) | Average87Sr/86Sr | Average δ18O (%o) | Average δD (%o) |
I | 275–550 | 0.7032 | 7.3 | −63 |
II | 550–890 | 0.7029 | 6.5 | −45 |
III | 890–1050 | 0.7035 | 5.6 | −31 |
IV | 1050–1350 | 0.7032 | 5.5 | −36 |
Full-size table
49.
Step-like temperature fluctuations associated with inverted ramps in a stable surface layer 总被引:1,自引:0,他引:1
Ensemble averages of temperature before and after step-like temperature fluctuations reveal the presence of inverted ramps in a stable surface layer. Normalized frequency of upward steps increases with increasing stability, whereas normalized magnitude of the temperature step decreases with stability and becomes constant at about R
i
= 1. These results suggest that the significance of temperature steps increases as stability increases. In moderate stability, the temperature pattern shows a gradual decrease after an upward step, which can be called a time-inverted ramp. Descending air and large downward heat flux are observed in a time-inverted ramp, suggesting a contribution from an ordered motion in wind. On the other hand, the temperature steps are related to gravity waves in strong stability. 相似文献
50.
A method of body-wave inversion is developed in an attempt to extract the information about asperities or barriers in a fault zone. A sequence of point sources, each being characterized with the seismic moment, the onset time and the location, are iteratively derived from observed records at multi-stations, where the two-dimensional extent of the source location is taken into account. A modification is made of the iterative method of Kikuchi and Kanamori on the formulation of inversion procedure to facilitate the computation.Using this method, we analyse long period P waves of the Tokachi-Oki earthquake of 1968 (Mw = 8.2) and obtain several significant subevents with time durations of ~ 10 s. Their spatio-temporal distribution shows that the rupture process consists of three characteristic stages: (A) a stage of introductory rupture, (B) a stage of main rupture and (C) a stage of aftershocks. The main rupture takes place in the form of clustering around a few sites of the fault plane. The largest subevent occurs in the northwestern corner. The stress drop associated with this event is estimated to be ~ 200 bars, one order of magnitude higher than the stress drop averaged over the entire fault plane. The sum of the seismic moments of the individual subevents amounts to 2.3 × 1028 dyn. cm which approximately coincides with the one estimated from the analysis of long-period surface waves. This implies that the source of the Tokachi-Oki earthquake consists of several major subevents with time durations of ~ 10 s in addition to other minor subevents. 相似文献