Characterization of total suspended solid dynamics in a large shallow lake using long‐term daily satellite images     

Keisuke Hoshikawa  Yoichi Fujihara  Sokly Siev  Seiya Arai  Takashi Nakamura  Hideto Fujii  Ty Sok  Chihiro Yoshimura 《水文研究》2019,33(21):2745-2758

Large, shallow‐water lakes located on floodplains play an important role in creating highly productive ecosystems and are prone to high concentrations of suspended solids due to sediment resuspension. In this study, the aim was to determine the dominant processes governing the total suspended solid (TSS) concentration at the water surface in Tonle Sap Lake, Cambodia, which is a large, shallow‐water lake. Satellite remotely sensed daily reflectance data from 2003 to 2017 were used. Seasonal changes in TSS concentration indicated that bottom sediment resuspension during dry seasons was mostly caused by wind and the TSS concentration was closely correlated with the water depth of the lake. The TSS concentration during flood periods was controlled by both wind and inflow currents from the Tonle Sap River. Additionally, we confirmed that surface/subsurface flow with a low TSS concentration from forests on the floodplain lowered the TSS concentration year round, except during August and September. This fact implied that the floodplain forest area decrease may increase the lake TSS concentration. An analysis of the long‐term changes in TSS indicated that a decrease in the water level during flood periods resulted in the high TSS concentrations observed during the subsequent dry periods. Therefore, climate change and water resource development, which are likely to cause water level reductions in the Mekong River during flood periods, may increase the TSS concentration in Tonle Sap Lake, particularly during the dry season.  相似文献   

Human impacts on large benthic foraminifers near a densely populated area of Majuro Atoll, Marshall Islands     

Yoko Osawa  Yu Umezawa  Yoichi Ide  Toshihiro Miyajima 《Marine pollution bulletin》2010,60(8):1279-1287

Human impacts on sand-producing, large benthic foraminifers were investigated on ocean reef flats at the northeast Majuro Atoll, Marshall Islands, along a human population gradient. The densities of dominant foraminifers Calcarina and Amphistegina declined with distance from densely populated islands. Macrophyte composition on ocean reef flats differed between locations near sparsely or densely populated islands. Nutrient concentrations in reef-flat seawater and groundwater were high near or on densely populated islands. δ¹⁵N values in macroalgal tissues indicated that macroalgae in nearshore lagoons assimilate wastewater-derived nitrogen, whereas those on nearshore ocean reef flats assimilate nitrogen from other sources. These results suggest that increases in the human population result in high nutrient loading in groundwater and possibly into nearshore waters. High nutrient inputs into ambient seawater may have both direct and indirect negative effects on sand-producing foraminifers through habitat changes and/or the collapse of algal symbiosis.  相似文献   

Rifting of Kyushu, Japan, based on the fault-controlled concurrent eruption of oceanic island basalt-type and island arc-type lavas     

Itsuro Kita  Yoshitada Asakawa  Tsutomu Yuri  Mitsuhiro Yasui  Yoichi Shimoike  Masatsugu Yamamoto  Hidenao Hasegawa  Sachihiro Taguchi  Hirochika Sumino 《Bulletin of Volcanology》2012,74(5):1121-1139

The tectonic environment of Kyushu, Japan is affected both by the subduction of the Philippine Sea plate and by the extensional tectonics related to rifting of Okinawa Trough at the eastern margin of the Eurasia Plate. We found that the Sendai fault zone acts as a channel for concurrent eruption of oceanic island basalt (OIB)-type and island arc (IA)-type basaltic rocks, propagating west to east in the Sendai region of southern Kyushu. The location of the Sendai fault zone is likely to correspond to the left-lateral shear zone in southern Kyushu as inferred by GPS Earth Observation Network. A similar magmatic association is present in the Beppu–Shimabara (BS) graben system in central Kyushu. The associate magmas of OIB-type rocks in Kyushu can be classified into typical, EM II-like and their intermediate OIB-type magmas in addition to MORB-like OIB-type magma in ⁸⁷Sr/⁸⁶Sr–Nb/Y systematics. Typical OIB-type and intermediate OIB-type magmas are erupted within the Sendai fault zone and BS graben system, respectively. The former is characterized by highest Nb/Y but low ⁸⁷Sr/⁸⁶Sr similar to MORB-like OIB-type magma erupted in northern Kyushu and the latter has intermediate Nb/Y and ⁸⁷Sr/⁸⁶Sr between typical and EM II-like OIB-type magmas. Almost all the IA-type rocks within the Sendai fault zone are generated from parental IA-type magma in Kyushu and characterized by weak crustal assimilation, having the lowest ⁸⁷Sr/⁸⁶Sr similar to typical OIB-type magma but the highest ¹⁴³Nd/¹⁴⁴Nd of arc magmas in Kyushu. The ages of both types of basaltic rocks within the Sendai fault zone range from 1.6 to <0.01?Ma clearly younger than those of andesitic rocks on northern and southern outsides of the fault zone and become younger from west to east. Initial formation of the fault zone has been induced by the counterclockwise rotation of southern Kyushu during the last 2?Ma as well as the BS graben system. Kyushu has continued to be split into three parts by the Sendai fault zone and BS graben during the Quaternary; northern, central, and southern zones. Their initial formation ages are likely to be linked to the initial rifting age of the middle Okinawa Trough back-arc basin.  相似文献   

Formation mechanism of the Weddell Sea Polynya and the impact on the global abyssal ocean     

Mikitoshi Hirabara  Hiroyuki Tsujino  Hideyuki Nakano  Goro Yamanaka 《Journal of Oceanography》2012,68(5):771-796

An experiment using a global ocean–ice model with an interannual forcing data set was conducted to understand the variability in the Southern Ocean. A winter-persisting polynya in the Weddell Sea (the Weddell Polynya, WP) was simulated. The process of WP breaking out after no-WP years was explored using the successive WPs found in the late 1950s. The results suggested that the anomalously warm deep water, saline surface layer, and a cyclonic wind stress over the Maud polynya region in early winter are essential for the surface layer to be dense enough to trigger deep convections which maintain a winter-persisting polynya; also, the reanalyzed surface air temperature (SAT) over the observed polynya region is too high for an ocean–ice model’s bulk formula to yield sufficient upward heat fluxes to induce WP formation. Therefore the Weddell Polynya, a series of WPs observed from satellite in the mid-1970s, is reproduced by replacing the SAT with a climatological one. Subsequent to the successive WP events, density anomalies excited in the Weddell Sea propagate northward in the Atlantic deep basins. The Antarctic Circumpolar Current (ACC) is enhanced through the increased meridional density gradient. The enhanced ACC and its meandering over the abyssal ridges excite buoyancy anomalies near the bottom at the southwestern end of the South Pacific basin. The buoyancy signals propagate northward and eventually arrive in the northern North Pacific.  相似文献   

Field measurements of absolute gravity in East Antarctica     

Koichiro Doi  Hideaki Hayakawa  Takahito Kazama  Toshihiro Higashi  Shingo Osono  Yoichi Fukuda  Jun Nishijima  Yuichi Aoyama  Junichi Ueda 《极地研究(英文版)》2013,(4):339-343

Abstract This paper reports the results of field-based absolute gravity measurements aimed at detecting gravity change and crustal displacement caused by glacial isostatic adjustment. The project was initiated within the framework of the 53rd Japanese Antarctic Research Expedition （JARE53）. Absolute gravity measurements, together with GPS measurements, were planned at several outcrops along the Prince Olav Coast and S6ya Coast of East Antarctica, including at Syowa Station. Since the icebreaker Shirase （AGB 5003） was unable to moor alongside Syowa Station, operations were somewhat restricted during JARE53. However, despite this setback, we were able to complete measurements at two sites： Syowa Station and Langhovde. The absolute gravity value at the Syowa Station IAGBN （A） site, observed using an FG-5 absolute gravimeter （serial number 210; FG-5 #210）, was 982 524 322.7＋0.1 ktGal, and the gravity change rate at the beginning of 2012 was -0.26 gGal.a-1. An absolute gravity value of 982 535 584.2~0.7 ktGal was obtained using a portable A-10 absolute gravimeter （serial number 017; A-10 #017） at the newly located site AGS01 in Langhovde.  相似文献   

Three‐dimensional structures and elemental distributions of Stardust impact tracks using synchrotron microtomography and X‐ray fluorescence analysis     

A. Tsuchiyama  T. Nakamura  T. Okazaki  K. Uesugi  T. Nakano  K. Sakamoto  T. Akaki  Y. Iida  T. Kadono  K. Jogo  Y. Suzuki 《Meteoritics & planetary science》2009,44(8):1203-1224

Abstract— Three‐dimensional structures and elemental abundances of four impact tracks in silica aerogel keystones of Stardust samples from comet 81P/Wild 2 (bulbous track 67 and carrot‐type tracks 46, 47, and 68) were examined non‐destructively by synchrotron radiation‐based microtomography and X‐ray fluorescence analysis. Track features, such as lengths, volumes and width as a function of track depth, were obtained quantitatively by tomography. A bulbous portion was present near the track entrance even in carrot‐type tracks. Each impact of a cometary dust particle results in the particle disaggregated into small pieces that were widely distributed on the track walls as well as at its terminal. Fe, S, Ca, Ni, and eight minor elements are concentrated in the bulbous portion of track 68 as well as in terminal grains. It was confirmed that bulbous portions and thin tracks were formed by disaggregation of very fine fragile materials and relatively coarse crystalline particles, respectively. The almost constant ratio of whole Fe mass to track volume indicates that the track volume is almost proportional to the impact kinetic energy. The size of the original impactor was estimated from the absolute Fe mass by assuming its Fe content (CI) and bulk density. Relations between the track sizes normalized by the impactor size and impact conditions are roughly consistent with those of previous hypervelocity impact experiments.  相似文献   

Four-dimensional variational ocean reanalysis: a 30-year high-resolution dataset in the western North Pacific (FORA-WNP30)     

Norihisa Usui  Tsuyoshi Wakamatsu  Yusuke Tanaka  Nariaki Hirose  Takahiro Toyoda  Shiro Nishikawa  Yosuke Fujii  Yasushi Takatsuki  Hiromichi Igarashi  Haruka Nishikawa  Yoichi Ishikawa  Tsurane Kuragano  Masafumi Kamachi 《Journal of Oceanography》2017,73(2):205-233

We produced a four-dimensional variational ocean re-analysis for the Western North Pacific over 30 years (FORA-WNP30). It is the first-ever dataset covering the western North Pacific over 3 decades at eddy-resolving resolution. The four-dimensional variational analysis scheme version of the Meteorological Research Institute Multivariate Ocean Variational Estimation system (MOVE-4DVAR) is employed to conduct a long-term reanalysis experiment during 1982–2012. After evaluating the basic performance of FORA-WNP30, the interannual to decadal variability is analyzed. Overall, FORA-WNP30 reproduces basic features in the western North Pacific well. One of outstanding features in FORA-WNP30 is that anomalous events such as the Kuroshio large meander and anomalous intrusion of the Oyashio in the 1980s, when there were no altimeter data, are successfully reproduced. FORA-WNP30 is therefore a valuable dataset for a variety of oceanographic research topics and potentially for related fields such as climate study, meteorology and fisheries.  相似文献   

Use of similarity solutions for the problem of a wetting front — a question of unique representation     

Yoshisuke Nakano 《Advances in water resources》1982,5(3):156-166

The use of similarity solutions for the problem of horizontal infiltration of water into a semi-infinite, dry and homogeneous porous medium is studied based upon some recent results of functional analysis. It is found that the so-called non-unique representation of reported experimental moisture profiles for this problem is not necessarily evidence against the validity of the extended Darcy's law for unsaturated flow through porous media.  相似文献   

Particular solutions to the problem of horizontal flow of water and air through porous media near a water table     

Yoshisuke Nakano 《Advances in water resources》1980,3(1):19-24

Particular solutions to the problem of horizontal flow of water and air through porous media near a water table are derived and regularity properties of the solutions are presented. It is found that a singularity occurs in the solutions at the water table and the water table can be interpreted as an acceleration wave of the nth order in terms of either air or water flow where n is a positive integer. Effects of air flow on water flow are discussed.  相似文献   

Chemical variations of magmas at Izu-Oshima volcano,Japan: plagioclase-controlled and differentiated magmas     

Shun Nakano  Takahiro Yamamoto 《Bulletin of Volcanology》1991,53(2):112-120

Systematic analyses of the major-element chemistry of products of several eruptions during syn-and post-caldera stages of Izu-Oshima volcano were compiled. Comparisons of the products of large-scale eruptions in 1338?, 1421? and 1777–1778, of intermediate-scale eruptions in 1950–1951 and 1986, and of small-scale eruptions in 1954, 1964 and 1974 clearly show the existence of two types of magmas. One is “plagioclase-controlled” and the other is “differentiated” magma (multimineral-controlled); i.e. the bulk chemistry of the first magma type is controlled by plagioclase addition or removal, while that of the second type is controlled by fractionation of plagioclase, orthopyroxene, clinopyroxene, and titanomagnetite. Eruptions of Izu-Oshima volcano have occurred at the summit and along the flanks. Summit eruptions tap only plagioclase-controlled magmas, while flank eruptions supply both magma types. It is considered unlikely that both magma types would coexist in the same magma chamber based on the petrology. In the case of the 1986 eruption, the flank magma was isolated sometime in the past from the summit magma chamber or central conduit, and formed small magma pockets, where further differentiation occurred due to relatively rapid cooling. In a period of quiescence prior to the 1986 eruption, new magma was supplied to the summit magma chamber, and the summit eruption began. The dike intrusion or fracturing around the small magma pockets triggered the flank eruption of the differentiated magma. This model can be applied to the large-scale flank eruption in 1338(?) which erupted differentiated magmas. In 1421(?), the flank eruption tapped plagioclase-controlled magma. In this case, the isolated magmas from the summit magma chamber directly penetrated the flank without differentiation.  相似文献