Eutrophication-induced changes in Lake Nakaumi,southwest Japan

Lake Nakaumi, southwest Japan, is an enclosed lagoon characterized by polyhaline and halocline conditions. Since the last century, its ecological state has been altered by eutrophication. We used a paleolimnological approach and studied multiple proxies, including chemical compounds, diatoms, foraminifera and molluscs, to infer the eutrophication history of the ecosystem. Eutrophication in Lake Nakaumi was associated with several factors, including increased nutrient loading, input of herbicides, and dike building since the 1920s. The ecological condition of this lake was divided into several stages that reflect the eutrophication process after the 1940s. A catastrophic “regime shift” from a clear state with aquatic vegetation to a turbid one with phytoplankton occurred in the early 1950s. Environmental degradation in the Honjo area, a part of Lake Nakaumi, was attributed primarily to physical changes caused by the construction of an enclosing dike. Eutrophication occurred almost simultaneously with the physical changes to the Honjo area in the 1970s. Until recently, no regime shift was observed in this area, though the core-top sediments show possible symptoms of incipient change.     

Operational Data Assimilation System for the Kuroshio South of Japan: Reanalysis and Validation

We describe an operational ocean data assimilation system for the Kuroshio and its validation using a nine-year reanalysis (historical run from 1993 to 2001) dataset of upper-ocean state estimation in the North Pacific. The horizontal structure of volume transport of the Ryukyu Current System (RCS) is shown from the reanalysis: The RCS is connected to the flow of the subtropical gyre, and its volume transport gradually increases from south-east of Okinawa (5–10 Sv) to the east of Amami-Ohshima Island (20 Sv). Comparing the reanalysis with independent observations on the southeast slope of the Amami-Ohshima Island indicates that the root mean square differences (RMSDs) are 0.076 (0.037) m/s in the period of December 1998 to November 1999 (November 1999 to November 2000) respectively. The reanalysis field has a bias (3.1 Sv) of the volume transport of the RCS and the RMSD (3.5 Sv) which is larger than the observed variability (2.81 Sv). Surface velocity and the Kuroshio axis south of Japan are also examined. Comparison of the reanalysis and ADCP data gave maximum RMSD of 0.749 (0.271) m/s in the strong (weak) current regions, respectively. The annual mean value of the axis error is 19 km in 1998. The RMSD of the error is at most 50 km, in 294 cases in the observation period, which is smaller than the observed root mean square variability of the axis (64 km). This revised version was published online in July 2006 with corrections to the Cover Date.     

Nanoindentation approach characterizing strain rate sensitivity of compressive response of asphalt concrete

This paper presents the results of a study on the use of nanoindentation test to characterize the strain rate-dependent compressive response of asphalt concrete. Nanoindentation is now widely used for characterization and testing of composite as well as single-phase materials. Using a small piece of sample, nanoindentation tests can evaluate material behavior and structure in terms of the elasticity, time-dependent response, yield strength, damage, crack advance, debonding, and fatigues. In this study, a mixture of asphalt and calcium carbonate filler powder filling the intergranular void space of the asphalt concrete was characterized in terms of strain rate sensitivity at room temperature. The indentation hardness is observed to continuously decrease during constant indentation strain rates, but the hardness response clearly indicates positive strain rate dependency when compared at the same indentation depths. Following the constant strain rate tests, indentation creep response of the asphalt–filler mixture was tested at constant load conditions. The strain rate sensitivity values characterized from double logarithmic relationships between indentation hardness and strain rate during constant strain rate and constant load tests are comparable with that determined from uniaxial compression test of cylindrical asphalt concrete samples. The observed indentation size effect on hardness value was analyzed based on an existing size effect model. The size effect in the asphalt–filler mixture, which is stronger than that defined by the model, could be attributed to a plastically graded surface of asphalt–filler sample.     

Scientific role of ACA for low-mass star-formation study

We discuss the scientific role of the Atacama Compact Array (ACA), the Japanese contribution to the ALMA project, for low-mass star-formation study. Our recent observations of several low-mass protostellar envelopes in the submillimeter CS (J=7–6) and HCN (J=4–3) lines with the SMA and ASTE have revealed that these submillimeter emissions are more extended than ∼2000 AU and show different velocity structures from those traced by millimeter lines. These results suggest the importance of taking short-spacing informations the ACA can offer. Our comprehensive imaging simulations of these protostellar envelopes, as well as prestellar cores and debris disks, unprecedentedly demonstrate the scientific importance of ACA.     

Surface current structure of the Tsushima Warm Current region in the Japan Sea derived by satellite-tracked surface drifters

We examined the surface current structure in the Tsushima Warm Current (TWC) region of the Japan Sea based on a dataset from satellite-tracked surface drifters, including our new deployments during 2004–2008. The gridded mean current velocity and mean kinetic energy (KE) fields calculated from all available drifter data revealed the structure and seasonality of the three branches of the TWC. By comparing the eddy kinetic energy (EKE) field to the KE field, we found four regions where the ratio of EKE to KE was high. In particular, this ratio became extremely high east of the Oki Islands located between the first branch of the TWC (FBTWC) and the second branch of the TWC (SBTWC). Our analysis suggests that the generation of alternate warm and cold eddies east of the Oki Island induced this high ratio. The occurrence of a warm or cold eddy was related to the route—either nearshore or offshore—of the SBTWC. When the offshore route of the SBTWC became dominant, a warm eddy was frequently generated east of the Oki Islands. In contrast, when the nearshore route of the SBTWC became dominant, a cold eddy was more likely to be generated.     

High-resolution observation and detailed photometry of a great Hα two-ribbon flare

We propose that when all sources on the solar disc are taken into account, the S component at 10.7 cm wavelength is dominated by thermal free-free (bremsstrahlung) emission. It is not produced only in the vicinity of sunspots; more than 60% of the total flux may be due to a widely-distributed emission associated with the hot complexes of activity. Using a model for the solar atmosphere based upon an assumption of weak (or vertical) magnetic fields, the spectrum of the S-component is calculated and its sensitivity to changes in the model parameters investigated. Variation of the thicknesses of the chromosphere, transition region and mixed zone cause only small changes in the S-component spectrum; there is a much stronger dependence upon the plasma density, particularly at the base of the corona. The behaviour of the S-component at 10.7 cm wavelength is examined in more detail. We find that the largest contribution to the 10.7 cm flux originates in the low corona, that structural changes affect it only slightly, but that it is strongly density-related. This dependence upon few quantities, together with its relative localization in the low corona, contributes to the usefulness of the 10.7 cm flux as an index of solar activity.Summer Student Worker, 1988.     

Tectonic setting and formation of the Setouchi volcanic rocks in the western Seto Inland Sea,Japan

The Setouchi volcanic rocks include high-Mg andesites (HMAs) and garnet-bearing dacite–rhyolite, and are sporadically distributed along the Median Tectonic Line, Japan. New U–Pb zircon ages and geological and geochemical data are presented for those rocks in the Western Setouchi region (W-Setouchi). Previous studies referred to the altered andesite in the W-Setouchi as “pre-Setouchi volcanic rocks.” However, on the basis of the new U–Pb age (14.4 Ma ± 0.3 Ma) and geochemical characteristics, we redefine it as the Jikamuro Formation, part of the Setouchi volcanic rocks. Incompatible elements are more enriched in the Jikamuro Formation rocks than in the Setouchi HMAs. The characteristic element compositions may be explained by mixing of compositionally different magmas, including subducted sediment melts, plus a contribution from crustal contamination. A stress-inversion technique with Bingham distribution method was applied to the orientations of felsic and mafic dikes within the Setouchi volcanic rocks, and indicates paleo-stress conditions during the period of Setouchi volcanism in the W-Setouchi. The analysis reveals NNW-extensional stresses and a strike-slip stress. We infer that the former represents extensional conditions during the main period of volcanism and the latter represents a stress transition during the most recent period of volcanism (after 12 Ma).     

Characteristics of submarine pumice-rich density current deposits sourced from turbulent mixing of subaerial pyroclastic flows at the shoreline: field and experimental assessment

This study investigates the types of subaqueous deposits that occur when hot pyroclastic flows turbulently mix with water at the shoreline through field studies of the Znp marine tephra in Japan and flume experiments where hot tephra sample interacted with water. The Znp is a very thick, pumice-rich density current deposit that was sourced from subaerial pyroclastic flows entering the Japan Sea in the Pliocene. Notable characteristics are well-developed grain size and density grading (lithic-rich base, pumice-rich middle, and ash-rich top), preponderance of sedimentary lithic clasts picked up from the seafloor during transport, fine ash depletion in coarse facies, and presence of curviplanar pumice clasts. Flume experiments provide a framework for interpreting the origin and proximity to source of the Znp tephra. On contact of hot tephra sample with water, steam explosions produced a gas-supported pyroclastic density current that advanced over the water while a water-supported density current was produced on the tank floor from the base of a turbulent mixing zone. Experimental deposits comprise proximal lithic breccia, medial pumice breccia, and distal fine ash. Experiments undertaken with cold, water-saturated slurries of tephra sample and water did not produce proximal lithic breccias but a medial basal lithic breccia beneath an upper pumice breccia. Results suggest the characteristics and variations in Znp facies were strongly controlled by turbulent mixing and quenching, proximity to the shoreline, and depositional setting within the basin. Presence of abundant curviplanar pumice clasts in submarine breccias reflects brittle fracture and dismembering that can occur during fragmentation at the vent or during quenching. Subsequent transport in water-supported pumiceous density currents preserves the fragmental textures. Careful study is needed to distinguish the products of subaerial versus subaqueous eruptions.     

Basin-scale distribution of NH<Subscript>4</Subscript><Superscript>+</Superscript> and NO<Subscript>2</Subscript><Superscript>−</Superscript> in the Pacific Ocean

We used more than 25,000 nutrient samples to elucidate for the first time basin-scale distributions and seasonal changes of surface ammonium (NH₄ ⁺) and nitrite (NO₂ ^?) concentrations in the Pacific Ocean. The highest NH₄ ⁺, NO₂ ^?, and nitrate (NO₃ ^?) concentrations were observed north of 40°N, in the coastal upwelling region off the coast of Mexico, and in the Tasman Sea. NH₄ ⁺ concentrations were elevated during May–October in the western subarctic North Pacific, May–December in the eastern subarctic North Pacific, and June–September in the subtropical South Pacific. NO₂ ^? concentrations were highest in winter in both hemispheres. The seasonal cycle of NH₄ ⁺ was synchronous with NO₂ ^?, NO₃ ^?, and satellite chlorophyll a concentrations in the western subtropical South Pacific, whereas it was synchronous with chlorophyll-a but out of phase with NO₂ ^? and NO₃ ^? in the subarctic regions.