Asteroids with large secular orbital variations

As the classical linear theory of secular perturbations for asteroids is known not to be adequate for computing the perturbations of asteroids with high eccentricities and/or inclinations, a seminumerical method to calculate the secular perturbations by including higher-degree terms in the disturbing function has been developed. It is here applied to asteroids with small values of $\text{(1 ? e}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{cos}\text{i}$, since the secular variations as well as their deviations from the results derived by the classical linear theory are generally large for such asteroids. It is found that the arguments of perihelion for five of the numbered asteroids are librating around 90 or 270°. For asteroids with $\text{(1 ? e}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext><mtext>cos</mtext><mtext>\; i</mtext>}}$ less than 0.85 the results of the secular variations are tabulated. Also the stability of such orbits is discussed by comparing the orbital properties of short-periodic comets with them. Generally speaking, orbits of the asteroids are more stable than those of the short-periodic comets, and asteroids with librating arguments of perihelion are more stable than those with circular coplanar orbits although their orbital elements are changed more by secular perturbations.     

Organic components in size-separated aerosols from the western North Pacific

Marine aerosols in the western North Pacific were collected using a cascade impactor. Size-separated aerosols were analyzed for organic carbon, alkanes and polycyclic aromatic hydrocarbons (PAH). The results showed that the rate of decrease of the atmospheric concentrations of these organic components with increase in distance from Japan as well as from the coast of the Eurasian Continent was in the order PAHalkanes>organic carbon. The bulk of all these organic components occurred in the smallest size fraction of particles (<1m). Analysis of the alkanes and PAH indicated that the hydrocarbons in aerosols in Japanese coastal marine areas are primarily derived from terrestrial anthropogenic sources which also contribute to a lesser extent to aerosols in marine areas about 1,000 km off the coast of Japan. In remote marine areas the hydrocarbons on small particles (<1m) have principally a natural terrestrial origin while those on larger particles are marine in origin.     

Temporal Variations in the Dissolved Nutrient Stocks in the Surface Water of the Western North Atlantic Ocean

Changes in patterns of undetectability and molar ratios of dissolved nutrients in the euphotic zone of the oligotrophic western North Atlantic Ocean were investigated utilizing the Bermuda Atlantic Time-series Study (BATS) data set of the US Joint Global Ocean Flux Study (JGOFS). Our aim was to examine the temporal dynamics of nutrient stocks over a decade (1989∼1998) and to gain insight into the interactions between the different biotic and abiotic factors underlying BATS. Patterns of nutrient undetectability clearly revealed the depleted nature of the nutrients in surface water at the BATS location, particularly phosphorous. The N:P ratio was consistently far above the nominal Redfield ratio (mean, 38.5) but was significantly lower during the 1993∼1994 period (22.1). Over the same period the proportion of samples depleted in N only increased while the proportion of samples depleted in P only decreased. This indicates an overall reduction of N relative to P in the surface water at BATS during the 1993∼1994 period, the reasons for this anomaly, though, are not clear. The correlation analysis between the biotic and abiotic variables at BATS has indicated some interesting relationships that can help understand some of the parameters affecting nutrient stocks in the euphotic zone and their consequent impacts on marine biota. Although nutrient stocks in the oligotrophic environment are limited, they might be subject to interannual variation that may become anomalous in some cases. These variations might underlay significant feedback mechanisms by affecting marine productivity, the prime factor controlling the sequestration of atmospheric CO₂ by the oceans. This revised version was published online in July 2006 with corrections to the Cover Date.     

A precise and continuous monitoring system of the distance between the near-bottom instruments and the sea floor

A relatively simple and inexpensive distance monitoring system (DMS) was devised to monitor continuously and accurately the position of an underwater camera with an attached pinger relative to the sea floor. Although the precision graphic recorder (PGR) or precision depth recorder (PDR) have hitherto been used to record the pinger to bottom distance, these complicated and expensive instruments are not always expected on all vessels.This newly developed system, which measures with a counting timer the interval between the direct sound pulses generated by a pinger and the bottom echoes, yielded results equal to or better than that of the PGR. The distance information thus obtained was accurate within ±2.5% under good conditions (calm weather and flat and level topography) just above the sea bed. This system simplified the photographing operations and made it possible to obtain a precise and quantitative assessment of the epibenthic fauna with the use of a single camera.The DMS is also applicable for determining the precise location of a bottom water sampler, grab sampler, or other underwater oceanographic equipment as well as that of an underwater camera.A part of this research was supported by science research fund from the Ministry of Education.     

Hydrothermal plumes in the eastern Manus Basin, Bismarck Sea: CH4, Mn, Al and pH anomalies

We present evidence for strong hydrothermal activity in the eastern Manus Basin (depth: 1700–2100 m), the existence of large scale triple-layered buoyant plumes at depths of 1100 m (“shallow plume”), 1700 m (“deep plume”), and 1400 m (“middle plume” with less extent than the other two plumes) that were revealed from water column anomalies of CH₄, Mn, Al and pH observed in November to December 1990. Judging from the horizontal distribution of these parameters, the deep plume seems to originate from two distinct hydrothermal sites (eastern and western sites) in the research area, the eastern site being visually ascertained with deep-tow observations at the same time. The CH₄/Mn ratio (mol mol⁻¹) of the deep plume (0.02–0.05) is the lowest yet observed in hydrothermal plumes. The order of magnitude difference of CH₄/Mn ratios between the shallow plume and the deep plume suggests that different kinds of fluid-rock interaction occurred to make the hydrothermal end members for the deep and shallow plumes. The shallow plume, which had an areal extent of more than 50 km, may be an episodic “megaplume”, because it was not recognized in the previous CH₄ profiles in 1986, and because it has a similar CH₄/Mn ratio as the megaplume observed in the North Fiji Basin. We found that the eastern deep plume is characterized by enormously high aluminium concentrations (0.6– 1.5 μmol kg⁻¹), pH anomalies (0.1) and high Al/Mn ratios (10–17). The endmember fluid for the eastern deep plume may have an unusually low pH value to dissolve this much aluminum during fluid-rock interaction, or this plume may originate from an eruption-influenced fluid.     

Observation of behavior of a sergestid shrimpSergestes lucens Hansen by underwater photography

Some behavior ofSergestes lucens by day was observed by underwater camera with an electronic flash. A number of species was snapped at about 2–60 m above the steep continental slope where the bottom ranged between 341 and 437 m depths. They were often abundant from 3–4 to 10 m above the bottom. It suggests that the shrimps scatter widely along the continental slope by day. The maximum concentration of the shrimp was 1.2 individuals/m³. None of the shrimps was found on the bottom, and the majority were swimming in horizontal position.     

Seasonal variations of the deep-sea macrobenthos communities in the coastal and bathyal zones off sanriku,northeastern Japan

Seasonal variations of macrobenthos communities were analyzed over six years at two fixed stations at the depths of 500 m and 1,000 m in the bathyal zone off Sanriku, northeastern Japan, and were compared with those of the 80 m station established at the mouth of Otsuchi Bay, northeastern Japan. Significant seasonal variations of macrobenthos density were detected at the 80 m and 500 m stations. While the density increased in late spring and decreased until July at the 80 m station, it increased between May and August, and decreased in September at the 500 m station. At the 1,000 m station, no seasonal variation of macrobenthos abundance could be detected. The seasonal variations of the density observed at the 80 m and 500 m stations are probably related to that of the supply of organic materials derived from surface phytoplankton. Differences in the patterns of seasonal variations among the three stations may reflect the different periods of food supply to the sea floor. Significant seasonal variation in the feeding structure of the polychaete communities could be detected at the 80 m and 1,000 m stations.     

Response and Stability of Underground Structures in Rock Mass during Earthquakes

Underground structures are well known to be earthquake resistant. However, the recent earthquakes showed that underground structures are also vulnerable to seismic damage. There may be several reasons such as high ground motions and permanent ground movements. This study attempts to describe various forms of damage to underground structures such as tunnels, caverns, natural caves and abandoned mines during major earthquakes. Results of various model tests on shaking table are also presented to show the effect of ground shaking on the response and collapse of underground structures in continuum and discontinuum. Furthermore, some empirical equations are proposed to assess the damage to underground structures, which may be useful for quick assessments of possible damage.     

Rapid and frequent turbidite accumulation in the bottom of Izu-Ogasawara Trench: Chemical and radiochemical evidence

Two sediment cores (pilot gravity and piston) were obtained from the bottom of the Izu-Ogasawara Trench at 9750 m and analyzed for various elements and radioisotopes. The results showed a history of complex and frequent turbidite deposition: In the gravity core, eight layers rich in manganese were observed, of which five are enriched in Cu and Co as well. The other three are also enriched in Mo but no other heavy metals, suggesting the presence of at least two mechanisms of formation. Trapping of iron manganese micronodules can account for the enrichment of Mn, Cu and Co. The other three layers rich in Mn and Mo appear to be formed by a post-depositional diagenetic process of Mn mobilization and redeposition in the sediment column. A strong correlation between ²²⁶Ra and Cu in the gravity core suggests that the ²²⁶Ra was also carried into the bottom of the trench in turbidites in association with Mn micronodules. Little excess of ²¹⁰Pb over ²²⁶Ra was found at the top but the excess was significant at mid-depths from 30 to 70 cm, indicating that those sediments were deposited within the last 200 y.

In the piston core there is a sharp discontinuity of chemical and radiochemical composition around a depth of 250 cm. Below that depth the sediments appear to be dominated by materials derived from terrestrial sources, as compared with those in the upper layer which are of contemporary marine origin. ²²⁶Ra is deficient relative to ²³⁰Th throughout the sediment column down to 6 m. This finding is consistent with the finding that the average rate of sediment accumulation is 1–2 orders of magnitude faster than that in the western North Pacific abyssal plain, suggesting the convergence of materials into the bottom of the trench.     

Speciation of Chromium in Artificially Contaminated Soil Reference Material GSJ JSO-2 Using XANES and Chemical Extraction Methods

The oxidation states of chromium in GSJ JSO-2 (artificially contaminated soil) and three other geochemical reference materials (GSJ JSO-1, JLS-1 and JMS-1) were observed using X-ray near edge structure (XANES). For comparison, other artificially contaminated soil materials (mimic-JSO-2) were prepared by adding Cr(VI) into JSO-1. Their oxidation states of chromium were determined using XANES. The chromium contents were 1118 μg g^-1 for JSO-2, 1352 μg g^-1 for mimic-JSO-2 and 69-113 μg g^-1 for the other reference materials. Most chromium was present as hexavalent in mimic-JSO-2. No hexavalent species were detected in other samples. These results for chromium oxidation state in JSO-2 and mimic-JSO-2 obtained with XANES resembled those obtained from a chemical extraction method. The present JSO-2 has no trace of Cr(VI), although Cr(VI) was added as a major species during preparation. On the other hand, the content of Cr(VI) obtained in mimic-JSO-2 agreed with the original Cr(VI) content. A time-elapse study showed that Cr(VI) contents in mimic-JSO-2 decreased gradually to 70% of the original abundance during 240-day preservation in dry conditions. Moreover, the abundance of Cr(VI) decreased markedly to 15% after 240 days in the wet mimic-JSO-2 containing 20% m/m of water. These experiments suggested that soil humidity enhanced the reduction of Cr(VI) and that Cr(VI) was reduced even in dry conditions. Consequently, it is reasonable to infer that Cr(VI) doped into JSO-2 was completely reduced to Cr(III) during the preservation period of 5 years. The certification of the long-term stability of the chemical form in reference materials will be much more important in future.