Moored instrument observations in the Kuroshio south of Kyushu

In each of the (fiscal) years 1979–1982, multilayer current measurements by means of moored instruments were carried out in the Kuroshio off Tanegashima Island, south of Kyushu, with particular interest in the baroclinic layer (0–1,000 m) of the intense current. Most of the moored instruments were in position for more than 30 days, the maximum duration being 145 days. This paper describes some noticeable features of the mean and fluctuation fields in the surface Kuroshio as inferred from the the new data sets.Vertical profiles of horizontal velocities show that the eastward current of the Kuroshio in the region is confined to the upper 600 m on average and below this level the mean current is directed to the southwest along the local isobaths. This is believed to be the first observation of the level of no motion in this region. It is argued that the indicated level of no motion would lead to much smaller values for the Kuroshio transport than previous values referred to the 1,000 m level — for example, earlier estimates by Nitani (1972).The fluctuation field in the upper intense current appears to be dominated by an organized motion with a time scale of about 30 days. This can be seen clearly in the time variations of water temperature and local volume transport, but less clearly in the velocity records. The horizontal transfer components of momentum and heat are calculated and it is suggested that the transverse momentum transfer, UV tends to be negative (offshore) in the region of anticyclonic shear of the upper Kuroshio; no evidence for a negative eddy viscosity effect is found.     

Zonation and species diversity of smaller macrobenthos in the westernmost part of Wakasa Bay (the Sea of Tango)

Grab samples of the smaller macrobenthos (>1 mm, <1g wet weight) were obtained seasonally at fourteen stations along two transects in the shallow coastal waters of the westernmost part of Wakasa Bay. According to Morisita's similarity index (C _i , two faunal groups were recognized to be present throughout the year. The boundary between them was found to lie between depths of 10 and 20 m where the silt-clay fraction increased abruptly in the sediment, although the boundary was less obvious in summer. Statistics on community structure (species diversity, species richness, and evenness) also showed marked differences between the two assemblages. The change in faunal features was found to be roughly parallel to changes in sediment characteristics. On the nearshore sandy bottom, the influence of wave action is considered to be the major factor affecting the fauna as it results in a decrease in evenness and species richness. On the other hand, on the offshore muddy bottom, stagnant conditions cause the benthic fauna to decrease in density and to bear some resemblance to those of enclosed bays. The highest species diversity was found on the silty sand bottom (20 m deep). This can be understood as an edge effect of an ecotone.     

Effects of temperature on the vegetative cell liberation of seven species of red-tide algae from the bottom mud in Osaka Bay

Mud samples were collected from the eastern coast of Osaka Bay from April 1984 to August 1985 and were incubated atin situ mud temperatures to examine the effects of temperature on the liberation of seven species of red-tide algae. Skeletonema costatum appeared extensively over a wide temperature range of 9–25°C. The percentage of liberation of benthicHeterosigma akashiwo cells tended to decrease with decreasing temperature, but this species also showed high percentages of liberation (40–100 %) in April whenin situ mud temperatures were still low (9.5–10°C). Germination of benthicHeterocapsa triquetra cells was active at temperatures lower than 10°C but was inhibited to below 20% at temperatures higher than 21°C. This indicates that the supply of vegetativeHeterocapsa cells from bottom muds to seawaters becomes inactive from summer to early autumn. The optimum temperature range for the liberation of benthicChattonella sp. cells was 16–21°C. ForProrocentrum micans, Prorocentrum minimum, andProrocentrum triestinum, no vegetative cells were released from bottom mud at any of the temperatures examined (9–25°C).Field observations on the population dynamics of vegetative cells conducted from April to September 1984 revealed that a close link exists between liberation ofS. costatum andH. triquetra and their blooms in the waters of Osaka Bay. On the basis of these findings, the ecological role of benthic cells in initiating red tides in Osaka Bay is discussed.     

Temporal variations in productivity and planktonic ecological structure in the East Sea (Japan Sea) since the last glaciation

We investigated the biogenic components (biogenic opal, calcium carbonate, and organic carbon) of the marine sediments in core TY99PC18 recovered from the southeastern part of the Ulleung Basin, East Sea (Japan Sea). Our results indicate that primary productivity by diatoms increased after the last glaciation (15,000 ¹⁴C years b.p.) probably because of the onset of vertical mixing of seawater and nutrient supply from the deep water. Between 5,000 and 10,000 ¹⁴C years b.p. a shift in the dominant primary producer, i.e. from diatoms to coccolithophores, coincides with decreased productivity, which could be related to the influx of warm, low-nutrient waters at that time. During the late Holocene (after 5,000 ¹⁴C years b.p.) the productivity of diatoms increased once more probably due to renewed ventilation and vertical mixing.     

Barium Increasing Prior to Opal during the Last Termination of Glacial Ages in the Okhotsk Sea Sediments

Ba and Ti in a sediment core (10 m long) from the Okhotsk Sea, covering the last 120 kyrs, were measured. The authigenic Ba (Ba_ex) contents were calculated and compared with the opal. The correlation coefficient between Ba_ex and opal was quite small (r = 0.34), but it greatly grew larger (r = 0.90), if the Ba_ex contents were multiplied by a simple function increasing with depth, except for two intervals. This may be due to the gradual change in the sedimentation environment during the glacial ages. One of the exceptional interval is found at 60–170 cm in depth, corresponding to 10–17 calendar kyr ago, the last termination period of glacial ages, where the Ba_ex began to increase prior to opal. Since the calcium carbonate contents similarly increased prior to the opal increase, the Ba_ex may be also related to calcareous organisms besides siliceous ones. The other is the last interglacial period around 120 kyr ago when the opal contents were high, but those of the Ba_ex were not increased. This can be explained, if the Ba_ex was reduced to sulfide and dissolved away in a strongly anoxic environment during the biologically productive period. During the glacial ages, the mass accumulation rate (MAR) of lithogenic Ti was about twice the amount of that during the interglacial ages. For opal, however, the contrast between the glacial and interglacial ages was more remarkable in its MAR than in its concentration in sediments, due to the larger variation in the bulk sedimentation rates. This revised version was published online in July 2006 with corrections to the Cover Date.     

Grain growth kinetics and the effect of crystallographic anisotropy on normal grain growth of quartz

Annealing experiments on agate were performed to investigate grain growth kinetics and the effect of crystallographic anisotropy on normal grain growth of quartz. The experiments were conducted using a piston-cylinder apparatus at 700–800°C and 0.5 GPa for 0–66 h. The grain growth rate was expressed by D ⁿ −D ₀ ⁿ  = kt with k = k ₀ exp(−H*/RT) where D ₀ is the initial grain size at t = 0, with n = 4.4 ± 0.3, and H* = 191.3 ± 11.0 kJ/mol is the activation enthalpy and logk ₀  = 19.8 ± 1.4. While the grain aspect ratios are nearly constant at ~0.7 (short/long) during grain growth, the longest axis in individual grains tends to be oriented parallel to their c-axis, indicating that a primary crystal-preferred orientation of c-axis of the agate could result in the development of a weak shape-preferred orientation during grain growth.     

Numerical analysis of overburden soil subjected to strike-slip fault: Distinct element analysis of Nojima fault

A distinct element method analysis is carried out to examine the development of shear bands in overburden soil subjected to a strike-slip fault. About 2.3 million spherical particles are used in the analysis and the results are compared with the shears observed at the Nojima earthquake fault during the 1995 Hyogoken Nanbu earthquake. En echelon shears and secondary shears which strike at lower angles to the basement fault – typical in strike-slip faults – are observed in the numerical analysis. Simple shear in the horizontal plane and drag due to the dependence of velocity on depth are confirmed to control the helicoidal shape of Riedel shears. Rotation of the compressional direction toward the fault strike as a result of slip along Riedel shears is also verified. It is found that the compressional direction is more horizontal within the area enclosed by Riedel shears than in outside areas and that these compressional directions produce secondary lower-angle shears that are less helicoidal. It is shown that the formation of column-like structures of particles and their subsequent buckling play significant micromechanical roles in three-dimensionally wrenched shears. The results of the numerical analysis, such as shear intervals and striking angles, show a resemblance to observational results at sites where sediment contains coarse grains and is subjected to strike slip with a small dip component, although they are not exactly the same as those observed at locations with similar overburden thicknesses.     

Spectrophotometric scan of comet 1973 XII Kohoutek

Photoelectric spectrophotometric scans of Comet 1973 XII Kohoutek were made on November 25.85 and 28.85 UT, 1973. The vibrational temperature of C₂, the total numbers of CN and of C₂, and the production rates for CN and C₂ are derived from the observed fluxes. The temperature of C₂ was 4900 K on 25.85 November and 4300 K on 28.85 November. Mean abundance ratio of C₂ to CN was about 2.7. From the variation of production rates with heliocentric distance, it is shown that there was an outburst on 25.85 November in both CN and C₂ productions. For C₂, the production rates are derived by using two coma models, i.e. the Haser's parent-daughter model and the model relevant to a proposition of Yamamoto (1981b) that C₂ is formed via two-step photodissociation of its parent molecules. By comparing the production rate derived from the two models, it can be supported that C₂ is formed via two-step photodissociation rather than one-step photodissociation. In consequence, it is shown that the variation of the production rate with heliocentric distance is largely modified compared with that derived from the Haser's model.