Comparison of carbon cycle between the western Pacific subarctic and subtropical time-series stations: highlights of the K2S1 project     

Makio C. Honda  Masahide Wakita  Kazuhiko Matsumoto  Tetsuichi Fujiki  Eko Siswanto  Kosei Sasaoka  Hajime Kawakami  Yoshihisa Mino  Chiho Sukigara  Minoru Kitamura  Yoshikazu Sasai  Sherwood L. Smith  Taketo Hashioka  Chisato Yoshikawa  Katsunori Kimoto  Shuichi Watanabe  Toru Kobari  Toshi Nagata  Koji Hamasaki  Ryo Kaneko  Mario Uchimiya  Hideki Fukuda  Osamu Abe  Toshiro Saino 《Journal of Oceanography》2017,73(5):647-667

A comparative study of ecosystems and biogeochemistry at time-series stations in the subarctic gyre (K2) and subtropical region (S1) of the western North Pacific Ocean (K2S1 project) was conducted between 2010 and 2013 to collect essential data about the ecosystem and biological pump in each area and to provide a baseline of information for predicting changes in biologically mediated material cycles in the future. From seasonal chemical and biological observations, general oceanographic settings were verified and annual carbon budgets at both stations were determined. Annual mean of phytoplankton biomass and primary productivity at the oligotrophic station S1 were comparable to that at the eutrophic station K2. Based on chemical/physical observations and numerical simulations, the likely “missing nutrient source” was suggested to include regeneration, meso-scale eddy driven upwelling, meteorological events, and eolian inputs in addition to winter vertical mixing. Time-series observation of carbonate chemistry revealed that ocean acidification (OA) was ongoing at both stations, and that the rate of OA was faster at S1 than at K2 although OA at K2 is more critical for calcifying organisms.  相似文献   

Representative rainfall thresholds for landslides in the Nepal Himalaya   总被引：14，自引：0，他引：14  

Ranjan Kumar Dahal  Shuichi Hasegawa 《Geomorphology》2008,100(3-4):429-443

Measuring some 2400 km in length, the Himalaya accommodate millions of people in northern India and Pakistan, Nepal, Bhutan, and parts of other Asian nations. Every year, especially during monsoon rains, landslides and related natural events in these mountains cause tremendous damage to lives, property, infrastructure, and environment. In the context of the Himalaya, however, the rainfall thresholds for landslide initiation are not well understood. This paper describes regional aspects of rainfall thresholds for landslides in the Himalaya. Some 677 landslides occurring from 1951 to 2006 were studied to analyze rainfall thresholds. Out of the 677 landslides, however, only 193 associated with rainfall data were analyzed to yield a threshold relationship between rainfall intensity, rainfall duration, and landslide initiation. The threshold relationship fitted to the lower boundary of the field defined by landslide-triggering rainfall events is I = 73.90D^− 0.79 (I = rainfall intensity in mm h^− 1 and D = duration in hours), revealing that when the daily precipitation exceeds 144 mm, the risk of landslides on Himalayan mountain slopes is high. Normalized rainfall intensity–duration relationships and landslide initiation thresholds were established from the data after normalizing rainfall-intensity data with respect to mean annual precipitation (MAP) as an index in which N_I = 1.10D^− 0.59 (N_I = normalized intensity in h^− 1). Finally, the role of antecedent rainfall in causing landslides was also investigated by considering daily rainfall during failure and the cumulative rainfall to discover at what point antecedent rainfall plays an important role in Himalayan landslide processes. Rainfall thresholds presented in this paper are generalized so they can be used in landslide warning systems in the Nepal Himalaya.  相似文献   

Predictive modelling of rainfall-induced landslide hazard in the Lesser Himalaya of Nepal based on weights-of-evidence   总被引：12，自引：0，他引：12  

Ranjan Kumar Dahal  Shuichi Hasegawa  Atsuko Nonomura  Minoru Yamanaka  Santosh Dhakal  Pradeep Paudyal 《Geomorphology》2008,102(3-4):496-510

Landslide hazard mapping is a fundamental tool for disaster management activities in mountainous terrains. The main purpose of this study is to evaluate the predictive power of weights-of-evidence modelling in landslide hazard assessment in the Lesser Himalaya of Nepal. The modelling was performed within a geographical information system (GIS), to derive a landslide hazard map of the south-western marginal hills of the Kathmandu Valley. Thematic maps representing various factors (e.g., slope, aspect, relief, flow accumulation, distance to drainage, soil depth, engineering soil type, landuse, geology, distance to road and extreme one-day rainfall) that are related to landslide activity were generated, using field data and GIS techniques, at a scale of 1:10,000. Landslide events of the 1970s, 1980s, and 1990s were used to assess the Bayesian probability of landslides in each cell unit with respect to the causative factors. To assess the accuracy of the resulting landslide hazard map, it was correlated with a map of landslides triggered by the 2002 extreme rainfall events. The accuracy of the map was evaluated by various techniques, including the area under the curve, success rate and prediction rate. The resulting landslide hazard value calculated from the old landslide data showed a prediction accuracy of > 80%. The analysis suggests that geomorphological and human-related factors play significant roles in determining the probability value, while geological factors play only minor roles. Finally, after the rectification of the landslide hazard values of the new landslides using those of the old landslides, a landslide hazard map with > 88% prediction accuracy was prepared. The methodology appears to have extensive applicability to the Lesser Himalaya of Nepal, with the limitation that the model's performance is contingent on the availability of data from past landslides.  相似文献   

The 21st Himalayan-Karakoram-Tibet workshop     

Masaru Yoshida  Shuichi Hasegawa  Ranjan Kumar Dahal   《Gondwana Research》2006,10(3-4):396-397

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Lignin and fatty acid records in Lake Baikal sediments over the last 130 kyr: A comparison with pollen records   总被引：1，自引：0，他引：1  

Ryoshi Ishiwatari  Shuichi Yamamoto  Soichi Shinoyama 《Organic Geochemistry》2006,37(12):1787

A 10 m sediment core from Academician Ridge in Lake Baikal was analyzed for its molecular composition using on-line TMAH (tetramethylammomium hydroxide) thermochemolysis. Major products are lignin phenols, n-C₁₄ to C₃₀ fatty acids (alkanoic acids), cutin acids, hydroxy acids and aliphatic dicarboxylic acids. Lignin phenols are abundant in warmer periods (the interglacial: marine isotope stage (MIS) 5e and MIS 1), but extremely low in the other (colder) periods. This result coincides well with pollen records reported for a core near the present site, where an expansion of coniferous forests in sub-stage 5e and MIS 1 was implied. Normal C₂₄–C₃₀ alkanoic acids, important components of plant wax esters, are abundant in 5e and MIS 1 and are present in significant amounts in the other (colder) periods, unlike the lignin phenols. A high abundance of n-C₂₄ to C₃₀ alkanoic acids relative to lignin phenols in the Bølling–Allerød warm period suggests an enhanced development of herbs.It is implied from comparison of the sedimentary lignin phenol record with fossil pollen records and lignin phenol analysis of modern pollen that the ratio of cinnamyl phenols to vanillyl phenols may serve as an indicator of pollen contribution to sedimentary organic matter.  相似文献   

Sedimentary organic matter contents and porewater chemistry at upper bathyal depths influenced by the 2011 off the Pacific coast of Tohoku Earthquake and tsunami     

Hidetaka Nomaki  Kazuno Arai  Hisami Suga  Takashi Toyofuku  Masahide Wakita  Takuro Nunoura  Kazumasa Oguri  Takafumi Kasaya  Shuichi Watanabe 《Journal of Oceanography》2016,72(1):99-111

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Distribution and photo-physiological condition of phytoplankton in the tropical and subtropical North Pacific     

Tetsuichi Fujiki  Kazuhiko Matsumoto  Toshiro Saino  Masahide Wakita  Shuichi Watanabe 《Journal of Oceanography》2013,69(1):35-43

To better understand the vertical distribution of phytoplankton in the tropical and subtropical North Pacific, we used fast repetition rate fluorometry to investigate the photo-physiological condition of the phytoplankton assemblage in this region between February and March 2007. Along 155°E, between the equator and 24°N, the peak of fluorescence (F _m), an indication of the deep chlorophyll maximum (DCM), was deeper than the top of the nitracline and occurred at the 2.4 ± 1.3 % (mean ± SD) light depth (relative to 0 m). The photochemical efficiency (F _v/F _m) and effective absorption cross-section of photosystem II (σ_PSII) were low at the surface but increased rapidly at depths between the top of the nitracline (40–138 m) and the DCM (70–158 m), an indication that the photo-physiological condition of the phytoplankton improved below the top of the nitracline. The depth of the maximal F _v/F _m [Z(F _v/F _{m max})] was 18–32 m deeper than the DCM and corresponded to the 0.8 ± 0.2 % light depth. The values of F _v/F _m at the Z(F _v/F _{m max}) were 20 % higher than those at the DCM and averaged 0.48 ± 0.01. These results suggest that the phytoplankton assemblage beneath the DCM had a high potential photosynthetic performance capacity and was growing by using the very low ambient light in this region.  相似文献   

琵琶湖中浊度的季节变化     

远藤修一  Kunihiko SAGI  Naonori FUKUYAMA  Munetsugu KAWASHIMA  奥村康召 《湖泊科学》1998,10(S1):267-273

Seasonal variation of the turbidity (suspended substance) has been investigated in Lake Biwa. During the last five years, vertical and horizontal distributions of water temperature, turbidity, electric conductivity and chlorophyll-a have been obtained both in the south basin and the southern part of the north basin of Lake Biwa. The benthic nepheloid layer (BNL) developed in the seasons of thermal stratification, and is not detectable in the non-stratification period (winter). The BNL is mainly maintained by the organic matter such as phytoplankton under decomposition. However, the turbidity in the nepheloid layer was much affected by the turbid water from rivers after heavy rainfall. In this case, the major component of the suspended substance (SS) in the nepheloid layer was inorganic soil. The particulate P concentration, which is originated from phytoplankton, also increased after a rain fall. This suggests that phytoplankton in the surface layer sinks with clay and silt coming through rivers. From summer to the end of the stratification period, another kind of turbidity appeared in the bottom layer. This is caused by the chemical reaction of manganese under the anoxic condition. The resuspension of bottom sediment by strong currents also occurred, but it is not a major process for maintaining the BNL.  相似文献   

North Pacific Intermediate Water studied chiefly with radiocarbon     

Shizuo Tsunogai  Shuichi Watanabe  Makio Honda  Takafumi Aramaki 《Journal of Oceanography》1995,51(5):519-536

The importance of the North Pacific Intermediate Water as a sink for the anthropogenic carbon dioxide has been examined by mapping chemical and radiochemical properties at two isopycnal surfaces of of 26.6 ad 27.2 obtained in 1970's. Its radiocarbon contents in 1980's were determined for comparison. The isopleths of depth and salinity at the two isopycnal surfaces obviously show that the intermediate layer of the entire mid-latitudes of the North Pacific is occupied by a similar water mass. The distributions of dissolved oxygen contents and Si/N ratios in the intermediate water indicate its source in the northwestern North Pacific and its sink in the eastern Pacific. The ¹⁴C values clearly designate the intrusion of the artificial radiocarbon of mostly 1960's origin into the upper intermediate water of the western North Pacific having its maximum in the subarctic zone of 40–45°N and 160–180°E in 1973. The maximum region for tritium is much broader extending to the north. These suggest that the subboreal region is active in the gas exchange and/or the warm water residing for a long time at the surface and flowing into the region across the subarctic front sinks quickly in winter. At the lower isopycnal surface, the increase ¹⁴C value for 14±4 years was around 27, which is smaller than that expected from the total carbonate increase, indicating an active isopycnal mixing.  相似文献