Long-term riverbed response of lower Tedori River,Japan, to sediment extraction and dam construction

Effects of sediment extraction and dam construction on changes of riverbed characteristics over yearly to decadal scales in the lower Tedori River of Japan are clarified. Over 1950–1991, the riverbed degraded in excess of 0.5–3.5 m. Concurrently, riverbed sediment volume of the 0–16 km reach decreased by 12.7 × 10⁶ m³. Intensive sediment extraction was the dominant cause of riverbed degradation during the period. During 1991–2007, an increase in riverbed sediment volume of 0.6 × 10⁶ m³ resulted in accretion of the riverbed by average depth 0.04 m. The cessation of sand and gravel mining (SGM), coupled with Tedorigawa Dam operation since 1980, was responsible for that accretion. Temporal change in riverbed elevation during 1950–2007 indicates that there were five phases of vertical adjustment. Combination of nonlinear regression models described four of these phases well. During 1950–1979, the first four modes of empirical orthogonal function analysis successfully captured temporal and spatial responses of the riverbed to natural and anthropogenic impacts. That is, the first mode explained the mean riverbed profile and temporal variation in riverbed sediment volume. The second through fourth spatial eigenfunctions reflected spatial variation in vertical adjustment rate for phases II, III and I, respectively. The corresponding temporal eigenfunctions explained the respective effects on the riverbed of SGM, of imbalance between sediment transport capacity and sediment supply, and of dredging activity.     

Geochemistry of Late Permian to Early Triassic pelagic cherts from southwest Japan: implications for an oceanic redox change

Major, trace, and rare earth element abundances were determined for the southwestern Japanese pelagic chert sequence from the early Late Permian to early Early Triassic to investigate a redox change in deep-sea pelagic environments before and at the Permo-Triassic boundary (PTB) (251 Ma). The sequence was primarily deposited in the deep-sea of the superocean Panthalassa, and then was accreted to Japan in the Middle Jurassic. A remarkable lithostratigraphic change from red chert to siliceous∼carbonaceous claystone through gray chert is observed in this sequence. Constituent elements for these sedimentary rocks are essentially derived from two sources: (1) an ancient seawater via biogenic (mainly radiolarian), hydrothermal, and authigenic materials and (2) an average shale-like terrigenous material. The present measurement demonstrates significant stratigraphic changes of the Ce/Ce_non-ter* value (estimated Ce anomaly value of non-terrigenous component, recalculated by subtraction of terrigenous REEs from bulk REEs) and the (Mn/TE)_sample/(Mn/TE)_PAAS value (excess Mn component other than terrigenous one; TE=terrigenous elements including Ti, Al, Nb, Hf, Th; PAAS=Post-Archean Average Australian Shale) in the Permian chert. The Ce/Ce_non-ter* values increase from <0.2 to 1 and the (Mn/TE)_sample/(Mn/TE)_PAAS values decrease up-section, suggesting that the redox condition of deep-sea open-ocean changed from oxic to suboxic in an interval of approximately 10 Myr. The (∑Fe/TE)_sample/(∑Fe/TE)_PAAS and (Mn/TE)_sample/(Mn/TE)_PAAS values of carbonaceous claystone near or at the PTB are less than unity, suggesting that reductive dissolution of iron and manganese occurred under an anoxic condition. This supports the idea of the PTB oceanic anoxia in the superocean Panthalassa. The present data suggest that the anoxic condition prevailed in the deep-sea pelagic regions for an extremely long period, much more than 10 Myr, from the middle Late Permian to early Early Triassic. This long-term development of widespread oceanic anoxia may have been linked to the greatest mass extinction of the Phanerozoic.     

Extracting fair-weather data from atmospheric electric-field observations at Syowa Station, Antarctica

At Syowa Station (69.0°S, 39.6°E), located on East Ongul Island near the continent of Antarctica, atmospheric electric-field observations started in 1968 and had been carried out intermittently. An improved electric-field mill at Syowa Station had and obtained better-quality atmospheric electric-field data from February 2005 to January 2006. After a 1-year interruption, the observations resumed in January 2007.The atmospheric electric-field data from Syowa Station are often contaminated due to local disturbances caused by near-ground meteorological phenomena. We examined correlations between the atmospheric electric field and near-ground weather from February 2005 to January 2006 and from February 2007 to January 2008, and proposed a criterion to extract “fair-weather” electric-field data based on wind speed and cloud coverage data. The diurnal variation of fair-weather data in January followed the shape of the so-called Carnegie curve. Fair-weather data obtained during a substorm showed some correspondence between the atmospheric electric field and variations in the geomagnetic field. This newly developed extraction method may enable the use of atmospheric electric-field data for studying the solar terrestrial environment.     

Relationship between sinking organic matter and minerals in the shallow zone of the western Subarctic Pacific

We used time-series sediment trap data for four major components, organic matter and ballast minerals (CaCO₃, opal, and lithogenic matter) from 150, 540, and 1000 m in the western subarctic Pacific (WSAP), where opal is the predominant mineral in sinking particles, to develop four simple models for settling particles, including the “ballast model”. The ballast model is based on the concept that most of the organic matter “rain” in the deep sea is carried by the minerals. These four models are designed to simultaneously reproduce the flux of each major component of settling particles at 540 and 1000 m by using the data for each component at 150 m as initial values. Among the four models, the ballast model, which considers the sinking velocity increase with depth, was identified as the best using the Akaike information criterion as a measure of the model fit to data. This model successfully reproduced the flux of organic matter at 540 and 1000 m, indicating that the ballast model concept works well in the shallow zone of the WSAP on a seasonal timescale. This also suggests that ballast minerals not only physically protect the organic matter from degradation during the settling process but also enhance the sinking velocity and reduce the degree of decomposition.     

Earthquake core inferred from near field observations

The source processes of large shallow earthquakes are investigated based on the various field phenomena and on the seismograms recorded at short focal distances. The results from coseismic and postseismic field surveys in some source regions strongly show that there must be a particular region characterized by a large dislocation, large acceleration and extremely low aftershock activity. This specific region seems to have a relatively small dimension compared with the length of the main fault.The predominant short-period waves on the strong-motion seismograms are concentrated within the short intervals at the initial parts of P and S waves. This fact also suggests that the rupture elements generating the predominant short-period waves are not distributed over the entire surface of a single main fault but are concentrated in a small region.We call this confined small region in the source area “earthquake core”. The earthquake core is formed a little later than the start of smoothing dislocation and it may be located at some distance from the starting point of rupture.     

Two recent flowslides in Yamashina area, Kanazawa City, Japan

Due to a continual rainfall, a flowslide occurred in Yamashina area, Kanazawa City, Japan on November 8, 2002 in the Tertiary mudstone area. The sliding mass was fully fluidized during the motion and moved downward the slope for a long distance. On December 31, 2003, slope failure was triggered by intensive rainfall and snowmelt water at the same site again, and resulted in the second occurrence of flowslide. The total displacement of the slope was recorded with an extensometer. Through field investigation, the difference of the sliding mechanism between the two flowslides was examined.     

An Ecosystem Model Including Nitrogen Isotopes: Perspectives on a Study of the Marine Nitrogen Cycle

We have developed an ecosystem model including two nitrogen isotopes (¹⁴N and ¹⁵N), and validated this model using an actual data set. A study of nitrogen isotopic ratios (δ¹⁵N) using a marine ecosystem model is thought to be most helpful in quantitatively understanding the marine nitrogen cycle. Moreover, the model study may indicate a new potential of δ¹⁵N as a tracer. This model has six compartments: phytoplankton, zooplankton, particulate organic nitrogen, dissolved organic nitrogen, nitrate and ammonium in a two-box model, and has biological processes with/without isotopic fractionation. We have applied this model to the Sea of Okhotsk and successfully reproduced the δ¹⁵N of nitrate measured in seawater and the seasonal variations in δ¹⁵N of sinking particles obtained from sediment trap experiments. Simulated δ¹⁵N of phytoplankton are determined by δ ¹⁵N of nitrate and ammonium, and the nitrogen f-ratio, defined as the ratio of nitrate assimilation by phytoplankton to total nitrogenous nutrient assimilation. Detailed considerations of biological processes in the spring and autumn blooms have demonstrated that there is a significant difference between simulated δ¹⁵N values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of δ ¹⁵N values of phytoplankton, nitrate and ammonium in the spring and autumn blooms may indicate the ratios of nutrient selectivity by phytoplankton. In winter, most of the simulated biogeochemical fluxes decrease rapidly, but nitrification flux decreases much more slowly than the other biogeochemical fluxes. Therefore, simulated δ¹⁵N values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized with observations of δ¹⁵N of ammonium in winter and a sensitive study varying the parameter of nitrification rate.