Quartz and polymineral luminescence dating of Japanese loess over the last 0.6 Ma: Comparison with an independent chronology     

Takuya Watanuki  Andrew S. Murray  Sumiko Tsukamoto 《Earth and Planetary Science Letters》2005,240(3-4):774-789

There are only a few luminescence dating studies of loess sediments in Japan, but interleaved with these deposits are many well-described tephras of known age based on ¹⁴C and fission track analysis; these independent age controls provide an opportunity to test the reliability of loess luminescence ages. This study provides such a comparison at two sites in central Honshu, the largest island in the Japanese archipelago. Samples were collected from sequences of interleaved volcanic tephra and loess deposited on a Middle Pleistocene river terrace in the Niigata Prefecture and on an Early to Middle Pleistocene dissected fluvial surface at in the Tochigi Prefecture, Honshu. Equivalent doses (D_e) were estimated from fine grains (4–11 μm) using both polymineral IR-OSL and (post-IR) blue-OSL, and quartz blue-OSL. The blue-stimulated luminescence signals could be represented by up to three exponentially decaying components; only the most light sensitive of these components was used in the final D_e estimation. Almost all the estimates of D_e from polymineral IR-OSL are smaller than those from polymineral (post-IR) blue-OSL and quartz blue-OSL, whereas the latter two are in good agreement. The blue-light stimulated luminescence ages using the most light sensitive component are shown to be in good agreement with the independent control, up to 500 ka. Although the IR-OSL signals underestimate the known age, a simple laboratory fading test is found empirically to correct for this underestimation. We conclude that the most rapidly blue-stimulated luminescence signals from quartz extracted from our loess samples give reliable ages, and that future loess dating studies should concentrate on these signals.  相似文献   

Imaging subduction from the trench to 300 km depth beneath the central North Island, New Zealand, with Vp and Vp/Vs     

Martin Reyners  Donna Eberhart-Phillips  Graham Stuart  Yuichi Nishimura 《Geophysical Journal International》2006,165(2):565-583

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Long-term high-frequency measurements of dibromomethane in the atmosphere at algae-rich and algae-poor coastal sites     

Yoko?YokouchiEmail author  Takuya?Saito  Hitoshi?Mukai 《Journal of Atmospheric Chemistry》2018,75(2):171-180

Dibromomethane (CH₂Br₂), a natural stratospheric ozone depleting substance, is mostly emitted from the ocean, but the relative importance of coastal (or macroalgae) and open ocean emissions is unknown. We made long-term high-frequency measurements of CH₂Br₂ concentrations at two remote coastal sites in Japan, on the subtropical Hateruma Island (poor in macroalgae) and at Cape Ochiishi (rich in macroalgae). CH₂Br₂ concentrations at Hateruma showed prominent seasonal variation, being lower in summer (around 0.94 ppt) than in winter (around 1.23 ppt). In contrast, CH₂Br₂ concentrations at Ochiishi were highly variable, often exceeding 2 ppt in the summer but with minimum baseline concentrations close to those from Hateruma; in the winter the concentrations were almost constant at about 1.3 ppt. Analysis of the data suggested that (1) emissions from macroalgae were not likely to extend offshore, but instead were localized near the shore, (2) strong macroalgal emissions of CH₂Br₂ were almost limited to the summer, but it was not reflected in the seasonality of the baseline concentrations of CH₂Br₂ in the atmosphere, and therefore (3) macroalgal or coastal emissions of CH₂Br₂ in the temperate zone might have a rather limited contribution to the global CH₂Br₂ sources. These findings are especially important for the understanding of the tropospheric and stratospheric bromine budget.  相似文献   

Contribution of shallow groundwater rapid fluctuation to soil salinization under arid and semiarid climate   总被引：4，自引：0，他引：4  

Mohamed Khaled Ibrahimi  Tsuyoshi Miyazaki  Taku Nishimura  Hiromi Imoto 《Arabian Journal of Geosciences》2014,7(9):3901-3911

Rising saline shallow groundwater and associated soil salinization problems are widespread especially in arid and semiarid areas. There have been numerous studies on groundwater-associated salinity, but more information is required on the effects of groundwater frequent and high fluctuations on soil salinization. In the present study, laboratory experiments and numerical simulations using HYDRUS-1D model were carried out for this purpose. The experimental and modeling results showed that groundwater fluctuation caused not only the accumulation of more salt in the soil profile compared to stable groundwater, but also an enhancement of the mechanism. Water table fluctuation induced a much greater spreading of the bromide (Br) tracer within the column than the constant water table. The Br content was on average five orders of magnitude greater under a fluctuating water table than under a constant one. Further, the numerical simulations showed that an increase in the groundwater fluctuation frequency brought about an increase in soil surface salinization under the same evaporation boundary conditions. Additional simulations with HYDRUS-1D were used to study the effects of various management strategies on soil salinization induced by shallow groundwater. Hence, by reducing the evaporation rate through the application of surface mulching, a significant reduction of salt concentration at the soil surface was observed. Moreover, frequent irrigations with small quantities were effective to reduce soil surface salt accumulation induced by saline shallow groundwater.  相似文献   

Deposits of the 1983 and 1993 tsunamis on the coast of Primorye     

L. A. Ganzey  N. G. Razjigaeva  Yu. Nishimura  T. A. Grebennikova  A. O. Gorbunov  V. M. Kaistrenko  Yu. A. Naumov  I. I. Lebedev 《Oceanology》2017,57(4):568-579

Deposits of the two strongest tsunamis of the 20th century have been found on the eastern coast of Primorye. The tsunamis had epicenters in the Sea of Japan west of the coast of Hokkaido. The distribution and preservation of deposits in bays of different geomorphological structure have been analyzed. The best defined sedimentary covers occur in the upper part of sections in low-lying areas of bay shores, where the wave runup was more than 3 m. The best preserved deposits have been observed in bays attributed to loworder streams. Variations of the structural composition of tsunami deposits formed by traction processes associated with the tsunamis have been analyzed depending on distance from the shoreline; the sources of material have been identified. Tsunami waves transported sand not only from beaches, ancient storm ridges, and terraces, but also from the underwater coastal slope; waves also grabbed material from estuarine lagoons and lakes located in the shore inundation zone. Deposits include marine diatoms with dominant sublittoral planktonic and benthic species, which suggests that the material was transported from a depth no more than 15 m. Deep-sea species of diatoms and their fragments have been encountered. Among freshwater diatoms are species with different ecological identities, indicating erosion and redeposition of material transported from various sources.  相似文献