Decomposition characteristics of particulate organic matter in hiroshima bay

The decomposition characteristics of particulate organic matter (POM) sampled with plankton nets in Hiroshima Bay were investigated under aerobic conditions in a laboratory experiment.The POM derived from plankton consisted of both a labile fraction (70–80 % of the whole) and a refractory fraction (20–30%). The labile fraction was completely decomposed within 40 days at 20°C. Although the concentrations of particulate organic carbon (POC) decreased gradually with time, an apparent lag phase was recognized in the decomposition of particulate phosphorus (PP) at an early stage, which might result from a specific uptake of dissolved inorganic phosphorus (DIP) by bacteria. A comparison of the metabolic activity between dissolved organic matter (DOM) and POM by measuring ATP contents showed that the former was one order of magnitude larger than the latter.On the other hand, there was no significant difference among the decomposition rates of POM collected at various depths. The change of the first-order rate constant (k) for the POM decomposition by temperature was expressed ask=0.0329 exp(0.0644T), and the Q₁₀ value was 1.94. There were fairly large variances ink values obtained from the various plankton species. Thek values averaged 0.144 day^–1 and ranged from 0.078 to 0.20 day^–1 at 20°C.     

Water exchange between Tokyo Bay and the Pacific Ocean during winter

Variability in water-exchange time between Tokyo Bay and the Pacific Ocean during winter is investigated based on the results of intensive field observation from November 2000 to March 2001. Water-exchange time between Tokyo Bay and the Pacific Ocean during winter mainly depends on the strength of northerly monsoon, being about 16 days under the weak monsoon and about 12 days under the strong monsoon. Moreover, it becomes longer by about 1 day in spring tide and shorter in neap tide due to the coupling effect of estuarine circulation and vertical mixing. Water-exchange time also varies depending on the open-ocean condition. When the warm water mass approaches from the Pacific Ocean to the mouth of Tokyo Bay through the eastern channel of Sagami Bay, which connects Tokyo Bay and the Pacific Ocean, water-exchange time becomes longer by about 2 days because the warm water mass is blocked in the surface layer at the bay mouth. On the other hand, when the warm water mass approaches to the mouth of Tokyo Bay through the western channel of Sagami Bay, water-exchange time becomes shorter by about 1 day because the warm water mass intrudes into the middle or lower layers of Tokyo Bay. Such different behavior of warm water mass at the mouth of Tokyo Bay is due to the difference in density of approaching warm water masses, that is, the density of the warm water mass through the eastern channel is smaller than that of the warm water mass through the western channel of Sagami Bay.Responsible Editors: Yens Kappenberg     

Active spreading and hydrothermalism in North Fiji Basin (SW Pacific). Results of Japanese French Cruise Kaiyo 87

The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.     

Life habit of vesicomyid clam,Calyptogena soyoae,and hydrogen sulfide concentration in interstitial waters in Sagami Bay,Japan

In and around the beds of vesicomyid clam (Calytogena soyoae) located off Hatsushima Island in Sagami Bay, central Japan, hydrogen sulfide concentration in bottom water and interstitial water was measured every 10 cm from just above seafloor to 40 cm deep usingin situ separative dialysis bags. While hydrogen sulfide over 0.01 mmol/kg was not measured from the seawater just above the dense clam beds, the concentration of hydrogen sulfide increased rapidly below 10 cm deep. The results indicate that the habit of the clam is correlated with high concentration of hydrogen sulfide contained in pore waters of sediments between depths of 10 and 20 cm from the bottom surface. Concentrations of hydrogen sulfide ranging from approximately 0.05 mmol/kg to 0.6 mmol/kg might be suitable requirement for the habitat ofC. soyoae.     

A model with rigid rotations and slip deficits for the GPS-derived velocity field in Southwest Japan

We interpret the GPS-derived velocity field in southwest Japan by a superposition of the elastic deformation caused by fault interactions (slips or slip deficits) on the rigid motion of tectonic blocks (or plates). Based on the strain rate field and crustal seismicity, we apply a model with three blocks (Inner Arc, Outer Arc, and the northern Ryukyu block) and slip deficits along the block boundaries.Several characteristics of the synthesized contributions are found:

(1) Westward motion of the outer arc relative to the Amurian plate and the inner arc,

(2) southeastward motion of the northern Ryukyu block relative to the Amurian plate,

(3) 2−4 mm/yr deficits of left lateral slip rates along the boundary at 32°N in southern Kyushu,

(4) 0−8 mm/yr deficits of right lateral slip rates along the Median Tectonic Line and the Beppu-Shimabara Graben,

(5) slip deficit rates on the plate interface smaller than those in the case without any consideration for rigid block motions,

(6) clockwise deflection of slip deficit rate vector on the plate interface from that estimated when not taking rigid block motions into consideration.

Stratigraphy and optically stimulated luminescence (OSL) dating of a Quaternary sequence along the Dong Nai River, southern Vietnam

The pre-Holocene Cenozoic sequence outcrops in the terrestrial part of the eastern margin of the Mekong Basin. However, the stratigraphy of the sequence is still unclear. Its detailed stratigraphy and chronology were therefore studied along the Dong Nai River, southern Vietnam, and the lithofacies and the relations among the formations were investigated from the outcrops. The ages of the deposits were determined by using optically stimulated luminescence (OSL) dating.The Ba Mieu Formation was deposited about 176±52 ka during marine isotope stage (MIS) 7–6. The Thu Duc Formation was deposited about 97±27 ka during MIS 5. Both the Ba Mieu and Thu Duc formations are composed of fluvial and tidally influenced coastal deposits. The newly proposed Nhon Trach Formation was originally an eolian (blanket) deposit, but it has been partly reworked by fluvial processes. The Nhon Trach Formation was deposited about 10.9±4.7 ka, in the last part of the Pleistocene to the beginning of the Holocene. The OSL ages for the Ba Mieu, Thu Duc, and Nhon Trach formations are younger than the ages previously assigned to these formations.     

Characteristics and implication of clay minerals in the northern and southern parts of the Chelung-pu fault, Taiwan

In this study, we report the characteristics of clay minerals present in the Chelung-pu fault in Taiwan. In the 1999 Chi-Chi earthquake, different slip motions were recorded along the Chelung-pu fault in the northern and southern parts of the fault. The characteristics of clay minerals present in the fault zones can be attributed to the differences in motion. We analyzed the shallow drill core samples obtained from the northern and southern sites penetrating the fault. The clay minerals identified in most of the samples from both the sites are smectite, illite, and chlorite. There are illite–smectite mixed layers with a high illite content and no chlorite–smectite mixed layer. In some samples, no smectite is detected. We also examined the iron content and symmetry of iron and magnesium in the silicate and hydroxide layers in chlorite. At the northern site, the total iron content in chlorite of gouge is larger than that of the host rocks. On the other hand, at the southern site, the total iron content varies widely. It is hypothesized that the smectite consumption and differences in the characteristics of chlorite may be controlled by the differences in the lithology, fluid chemistry, fluid temperature, or fault activities (heating or breakage) between the northern and southern sites.     

Bathymetry and Canyons of the western Solomon Sea

The floor of the western Solomon Sea (for new bathymetric map see inside back cover of this issue) is dominated by the arched and ridged basement of the Solomon Sea Basin, the partly-sediment-filled New Britain Trench, and a more completely filled trench, the Trobriand Trough. There is a deep basin where the trenches join (149° Embayment), and a silled basin west of the New Britain Trench (Finsch Deep). Submarine canyons descend from the west and south to the 149° Embayment. Abyssal fans and plains are structurally defined and locally disturbed by young faults. Probable submerged pinnacle reefs stand in water depths as great as 1,200 m.     

Gonadal histology and serum vitellogenin levels of bigeye tuna Thunnus obesus from the Northern Pacific Ocean--absence of endocrine disruption bio-indicators

Endocrine disrupting chemicals such as organochlorines have been detected in a large number of marine fish. Histological observation of the gonads, measurement of serum vitellogenin (VTG) level and of liver polychlorinated biphenyl (PCB) content were performed to evaluate the reproductive health and the contamination with endocrine disruptors in bigeye tuna Thunnus obesus, collected in the northern Pacific Ocean in 1999 and 2000. Abnormalities commonly found in species affected by endocrine disruptors such as the presence of oocytes in the testis or elevated serum VTG levels were not found in any of males examined. Both males and females had only small amounts of liver PCB content. The results suggest that currently there is little if any risk of organochlorine contamination or endocrine disruption of gonadal function in bigeye tuna from the northern Pacific Ocean. However, further studies are necessary to evaluate the health status of the open sea fishery resources.     

Influence of cluster environments on X-ray cluster morphology

We investigated the influence of environment on cluster morphology by examining the alignment of the cluster position angle with respect to the nearest neighbour cluster. The cluster position angle and ellipticity were measured using high spatial resolution X-ray data taken from the Chandra archive, while the nearest neighbour sample was extracted from the Abell cluster catalogue. We found high confidence for an alignment when neighbour distance (Dn) was less than  70  h ⁻¹  Mpc, while we found no significant variations of the confidence level at various values of Dn less than  70  h ⁻¹  Mpc.