Isotopic tracers for water masses in the coastal region of eastern Hokkaido

In this study we used two stable isotopes, δ¹³C and δ¹⁸O, for water mass classification in the coastal region off eastern Hokkaido. δ¹³C* values, which were corrected for the biological effect, and δ ¹⁸O values up to 300 m depth suggested that the isotopic character of the onshore and offshore water in the southern Okhotsk Sea, the Nemuro Strait and the western North Pacific could be explained by the mixing of three source waters: the Oyashio water (OYW), Soya Warm Current water (SWCW) and East Sakhalin Current water (ESCW). In summer, δ ¹³C*-δ ¹⁸O plots indicated mixing between SWCW from the southern Okhotsk Sea and OYW in the Pacific coast of southeastern Hokkaido, while temperature-salinity plots of the onshore water showed minimal difference from the offshore OYW. In winter, on the other hand, the mixed water of ESCW and OYW (or SWCW) appeared in the Pacific coastal region, distributed as cold, low salinity onshore water. Finally, we estimated mixing ratios of OYW, SWCW and ESCW in the coastal region of western North Pacific using their mean values of δ ¹³C* and δ ¹⁸O as endmembers. These results suggest seasonal and yearly changes of water mass combination en route from the southern Okhotsk Sea to the western North Pacific.     

Mineralogy of interstitial rim materials of the Y74123 and Y790981 ureilites and their origin

Abstract— Y74123 is an olivine-rich, relatively unshocked ureilite and contains more interstitial pigeonitic materials than do ureilites which have been reported previously. Thus, Y74123 is especially suited for detailed study of the interstitial materials. We have studied these materials by optical microscope, electron microprobe, scanning electron microscope, high resolution transmission electron microscope (TEM) and analytical TEM to gain a better understanding of their nature and origin. Y790981, with shock partial melts, has also been examined by the same techniques. Bulk chemical compositions of the interstitial materials in Y74123 are pyroxene-like and have higher CaO and Al₂O₃ contents than the large pigeonite and olivine core. Interstitial materials at olivine-pigeonite grain boundaries are richer in CaO and Al₂O₃ than those at olivine-olivine grain boundaries. TEM observations of the interstitial material of Y74123 show that it consists of alternating pigeonite-augite lamellae more than 3.5 μm thick on (001). This texture suggests that the rim material had already crystallized before the parent body breakup. The shock-produced glassy veins in Y790981 cut through the rim materials. These observations are consistent with the idea that the interstitial materials in this ureilite are a mixture of residual liquids of high Ca melts and shock-produced partial melts of olivine and pigeonite. This mixture accumulated along the grain boundaries and some of it is trapped within grains.     

Age determination of marine sediments in the western North Pacific by as partic acid chronology

The ages of fossil planktonic foraminifera,Pulleniatina obliquiloculata, in sediments (core 3bPC) from the western North Pacific were determined by aspartic acid chronology, which uses the racemization reaction rate constant of aspartic acid (k_Asp). Aspartic acid racemization-based ages (Asp ages) ranged from 7,600 yrBP at the surface, to 307,000 yrBP at a depth of 352.9 cm in the sediments. This sediment core was also dated by the glacial-interglacial fluctuation of δ¹⁸O chronology, and the ages determined by both chronologies were compared. The ages derived from aspartic acid chronology and δ¹⁸O stratigraphy were more or less consistent, but there appeared to be some differences in age estimates between these two dating methods at some depths within the core. In the core top sediments, the likely cause for the age discrepancy could be the loss of the surface sediment during sampling of the core. At depths of 66.3 and 139 cm within the core, Asp ages indicated reduced sedimentation rates duringca. 60,000-80,000 yrBP andca. 140,000–190,000 yrBP. The maximum age differences in both chronologies are 33,000 yr and 46,600 yr during each of these periods. These anomalous reductions in sedimentation rates occurring during these periods could possibly be related to some geological events, such as an increased dissolution effect of the calcium carbonate in the western North Pacific. Another possible reason for these age differences could be the unreliability in δ¹⁸O ages of core 3bPC as they were estimated by δ¹⁸O ages of another core, 3aPC.     

Abundance and biomass of scyphomedusae, Aurelia aurita and Chrysaora melanaster, and Ctenophora, Bolinopsis mikado, with estimates of their feeding impact on zooplankton in Tokyo Bay, Japan

The abundance of a scyphomedusae, Aurelia aurita and Chrysaora melanaster, and a ctenophore, Bolinopsis mikado, in Tokyo Bay was investigated from 1995 to 1997. Aurelia aurita appeared throughout the year with a peak in abundance occurring from spring to summer. The average abundance and biomass during this period for the three successive years was 4.8, 43.8 and 3.2 ind. m⁻², and 1.02, 10.0 and 0.42 gC m⁻², respectively. The values in 1995 and 1997 were comparable with those previously reported for A. aurita abundance from 1990 to 1992. Values were very high in 1996, but the size composition of the bell diameter did not differ from other years, which suggested the absence of food limitation for A. aurita in 1996. C. melanaster was scarce over the survey period (<1.0 ind. m⁻²) while Bolinopsis mikado was more abundant during September to December, with maximum values of 172 ind. m⁻² and 0.33 gC m⁻² observed in December 1997. The weight-specific clearance rate for A. aurita on zooplankton (mainly copepods and their nauplii) was 0.16 ± 0.05 lgWW⁻¹ h⁻¹ (n = 13). Population clearance rate peaked from spring to summer, with average levels of 14.2%, 162% and 5.0% day⁻¹ obtained from spring to summer for respective years. Population clearance rates for B. mikado, calculated based on minimum carbon requirements, was 7.1% day⁻¹ in December 1997. Consequently, the trophic role of gelatinous zooplankton as predators in Tokyo Bay is important all the year round, considering the high impact of A. aurita from spring to summer and B. mikado from autumn to winter.     

Early diagenesis of fatty acids in lacustrine sediments—I. Identification and distribution of fatty acids in recent sediment from a freshwater lake

Fatty acids have been isolated and quantitatively determined from a 1.5 m sediment core of Lake Suwa, a eutrophic lake in the central districts of Japan.The fatty acids identified by combined gas chromatography-mass spectrometry were straight-chain saturated (C₁₂ to C₃₄), monounsaturated with even carbon number (C₁₆ to C₂₄) and branched-chain (iso, anteiso, 10-methyloctadecanoic) acids. The concentrations of the higher molecular weight (? C₂₀) saturated fatty acids remained nearly constant throughout the core, suggesting a high degree of preservation of those acids, whereas the monounsaturated and the lower molecular weight saturated fatty acids indicated a great decrease in concentration with depth to an approximately 20cm level. It is suggested that the microbial activity in sediments causes a significant reconstruction of the fatty acid distribution during early diagenesis.     

Determination of the role of entrapped air in water flow in a closed soil pipe using a laboratory experiment

Soil pipes, continuous macropores parallel to the soil surface, are an important factor in hillslope hydrological processes. However, the water flow dynamics in soil pipes, especially closed soil pipes, are not well understood. In this study, the water and air dynamics within closed soil pipes have been investigated in a bench‐scale laboratory experiment by using a soil box with an artificial acrylic soil pipe. In order to grasp the state of water and air within the soil pipe, we directly measured the existing soil pipe flow and air pressure in the soil pipe. The laboratory experiment showed that air in the soil pipe had an important role in the water flow in the closed soil pipe. When air entrapment occurred in the soil pipe before the soil matrix around the soil pipe was saturated with water, water intrusion in the soil pipe was prevented by air entrapped in the pipe, which inhibited the soil pipe flow. This air entrapment in the soil pipe was controlled by the soil water and air flow. Moreover, after the soil pipe flow started, the soil pipe was not filled completely with water even when the soil pipe was completely submerged under the groundwater table. The entrapped air in the soil pipe prevented further water intrusion in the soil pipe.     

Vertical distributions of large ontogenetically migrating copepods in the Oyashio region during their growing season

Neocalanus flemingeri, Neocalanus plumchrus, Neocalanus cristatus and Eucalanus bungii are large and dominant mesozooplankton occurring throughout the subarctic Pacific. They are an important trophic link and transporter of organic matter to the mesopelagic zone. Vertical distributions of these copepods were investigated from March to October 2000 in the Oyashio region of the western subarctic Pacific. Neocalanus plumchrus and N. flemingeri were distributed in the surface layer (0–50 m) and N. cristatus and E. bungii in the subsurface layer (50–100 m). However, when examined in detail, clear seasonal and vertical differences were observed. Neocalanus plumchrus was concentrated in the top 20 m from late April to the end of July, and N. flemingeri showed a little deeper distribution from May to July. Neocalanus cristatus showed a deeper distribution than that of grazing individuals of E. bungii from April to early July, but grazing individuals of E. bungii (C3–C6) showed a deeper distribution than that of N. cristatus from the end of July to October. Early copepodites of E. bungii were distributed much shallower than late copepodite stages and overlapped with copepodites of N. plumchrus and N. flemingeri. These results suggest that the four species of large copepods have established habitat segregation by season, vertical distribution and food resource partitioning in the Oyashio region as well as other regions of the North Pacific.     

Flux of low salinity water from Aniva Bay (Sakhalin Island) to the southern Okhotsk Sea

In this study, we examined the relationship between the low salinity water in the shelf region of the southern Okhotsk Sea which was seasonally sampled (0–200 m), and fluxes of low salinity water from Aniva Bay. To express the source of freshwater mixing in the surface layer, we applied normalized total alkalinity (NTA) and stable isotopes of seawater as chemical tracers. NTA-S diagrams indicate that NTA of low salinity water in the upper 30 m layer just off the Soya Warm Current is clearly higher than in the far offshore region in summer and autumn. Using NTA-S regression lines, we could deduce that the low salinity and high NTA water in the upper layer originates from Aniva Bay. For convenience, we defined this water as the Aniva Surface Water (ASW) with values S < 32, NTA > 2450 μmol kg⁻¹. Formation and transport processes of ASW are discussed using historical data. The interaction between the maximum core of high NTA water on the bottom slope of eastern Aniva Bay and an anticyclonic eddy at the mouth of Aniva Bay are concluded to control ASW formation. Upwelling of the Cold Water Belt water at the tip of Cape Krillion is considered to cause ASW outflow from Aniva Bay.     

Installation of ocean bottom bases for observation of seafloor crustal movement

Abstract

Ocean bottom bases (OBBs) have been installed on both sides of the axis of the Sagami Trough east of the Izu Peninsula, central Japan, as the first step toward long‐term geodetic and geophysical observations at the plate boundary (subduction zone). The OBB is a platform for seafloor measurements; otherwise it is difficult to find an appropriate place for precise seafloor measurements in the subduction zones. It is made of a nonmagnetic concrete block of size 1100 × 1100 × 500 mm. It was lowered from a ship using a winch wire and installed on a predetermined place with its position being monitored by an acoustic transponder system and a 30‐kHz bottom pinger with an accuracy of about 2 m.

It was confirmed later during the divings on board the submersible Shinkai 2000 that the OBB was installed on a flat mud bottom in normal condition. No change has been recognized in the installation condition in 3 years; the OBB is stable enough to be used for acoustic range measurements on the seafloor as well as for several geophysical measurements.

The resolution of seafloor range measurement can be improved by two orders by using phase measurement techniques with the aid of pulse compression. Precise acoustic range measurement of the order of 10^?5 is feasible under the following conditions: two‐way measurements between the two OBBs installed on the slope facing each other with angles larger than 1.5°. Correction is necessary for the effect of long‐term temperature variation.