Baroclinic circulation and its high frequency variability in Otsuchi Bay on the Sanriku ria coast,Japan

Hydrographic observations were made in Otsuchi Bay on the Sanriku ria coast, Japan, to provide clear images of the baroclinic circulation extending over the bay together with the associated intrusion of lower-layer water (bottom water) from outside the bay. In summer, a prominent baroclinic circulation with flow speeds \({>} 0.1\ \text{ m }\ \text{ s }^{-1} \) extends over the greater part of the bay. A main pycnocline (thermocline), which separates the upper and lower layers, is located at a depth of 15–40 m in and around the bay. The direction of the lower-layer flow (inflow into and outflow from the bay) is opposite to that of the upper-layer flow, which are baroclinically coupled to each other. Moreover, with regard to the lower-layer flow, the inflow tends to occur mainly through the northwestern part of the bay mouth, whereas the outflow tends to occur mainly through the southeastern part. The inflow and outflow alternate on time scales of several to a few tens of hours, and the flow directions are sometimes related to the tidal ones, although the relationship is not applied persistently. In winter, the baroclinic circulation is considerably weaker than in summer, because the stratification breaks down.     

Tectonic reconstruction of batholith formation based on the spatiotemporal distribution of Cretaceous–Paleogene granitic rocks in southwestern Japan

The spatiotemporal distribution of Cretaceous–Paleogene granitic rocks in southwestern Japan is investigated to understand the origin of the granitic batholith belt and to reconstruct the tectonic setting of emplacement. New U–Pb zircon ages for 92 samples collected from a region measuring 50 km (E–W) by 200 km (N–S) reveals a stepwise northward younging of granitic rocks aged between 95 and 30 Ma with an age‐data gap between 60 and 48 Ma. Based on the spatiotemporal distribution of granite ages, we examine two plausible models to explain the pattern of magmatic activity: (i) subduction of a segmented spreading ridge and subsequent slab melting (ridge‐subduction model), and (ii) subduction with a temporally variable subduction angle and corresponding spatial distribution of normal arc magmatism (subduction angle model). We optimize the model parameters to fit the observed magmatism in time and space, and compare the best‐fit models. As to ridge subduction model, the best‐fit solution indicates that the spreading ridge started to subduct at approximately 100 Ma, and involved a 45‐km‐wide section of the ridge segment, a subduction obliquity of 30°, and a slow migration velocity (～1.6 cm/y) of the ridge. These values are within the ranges of velocities observed for present‐day ridge subduction at the Chile trench. On the other hand, the best‐fit solution of subduction angle model indicates that the subduction angle decreases stepwise from 37° at 95 Ma, 32° at 87 Ma, 22° at 72 Ma, to 20° at 65 Ma, shifting magmatic region towards the continental side. These results and comparison, together with constraints on the geometry of the tectonic setting provided by previous studies, suggest that the ridge subduction model better explains the limited duration of magmatism, although both models broadly fit the data and cannot be ruled out.     

Contribution of slab-fluid in arc magmas beneath the Japan arcs

Identifying the amount and composition of slab-derived fluid and its spatial variation is key to quantifying fluid processes in subduction zones. Based on the isotopic systematics of arc lavas, we found regional variations over the Japan arcs in terms of the amount and composition of slab-derived fluid added to the melting source region. The average amounts of slab-derived fluid differ among the arcs: 2.6 wt.% for Central Japan, 0.69 wt.% for Ryukyu, 0.17 wt.% for NE Japan, and 0.12 wt.% for both Kurile and Izu–Bonin. These differences may be attributed to the arc setting (oceanic or continental) and the geometry of the slabs. Contribution of sediment involved in the slab-derived fluid is dominant in NE Japan compared to the Izu–Bonin and Central Japan arcs. This could be attributed to mechanical features such as fractures near the subducting plate surface, in addition to the arc setting and the slab geometry. Therefore, the amount and composition of slab-derived fluid are thought to be controlled not only by the thermal conditions, but also by the tectonic and mechanical features around the subduction zone. On top of the variability of slab-derived fluid, the mantle wedge shows the regional variation in terms of proportion of the Pacific-type and Indian-type MORB-source mantle components, which also contributes to the compositional variations of arc magmas.     

Monitoring of DSP toxins in small-sized plankton fraction of seawater collected in Mutsu Bay,Japan, by ELISA method: relation with toxin contamination of scallop

Monitorings were conducted on DSP toxins in mid-gut gland of scallop (mouse assay), cell numbers of toxic dinoflagellate species of Dinophysis, and diarrhetic shellfish poisoning (DSP) toxins in small-sized (0.7-5 microm) plankton fraction of seawater collected from surface (0 m) and 20 m depth at a station in Mutsu Bay, Aomori Prefecture, Japan, in 2000. A specific enzyme-linked immunosorbent assay (ELISA) was employed for the analysis of DSP toxins in small-sized plankton fraction using a mouse monoclonal anti-okadaic acid antibody which recognizes okadaic acid, dinophysistoxin-1, and dinophysistoxin-3. DSP toxins were detected twice in the mid-gut gland of scallops at 1.1-2.3 MU (mouse units) g(-1) on 26 June and at 0.6-1.2 MU g(-1) on 3 July, respectively. Relatively high cell densities of D. fortii were observed on 26 June and 11 September, and may only contribute to the bivalve toxicity during late June to early July. D. acuminata did not appear to be responsible for the toxicity of scallops in Mutsu Bay in 2000. ELISA monitoring of small-sized plankton fraction in seawater could detect DSP toxins two weeks before the detection of the toxin in scallops, and could do so two weeks after the loss of the bivalve toxicity by mouse assay. On 17 July, toxic D. fortii was detected at only small number, <10 cells l(-1), but DSP toxins were detected by the ELISA assay, suggesting a presence of other toxic small-sized plankton in seawater. For the purpose of reducing negative impacts of DSP occurrences, monitorings have been carried out hitherto on DSP toxins of bivalve tissues by mouse assay and on cell densities of "toxic" species of Dinophysis. Here we propose a usefulness of ELISA monitoring of plankton toxicity, especially in small-sized fraction, which are possible foods of mixotrophic Dinophysis, as a practical tool for detecting and predicting DSPs in coastal areas of fisheries grounds of bivalve aquaculture.     

Seasonal Trend in the Occurrence of Nocturnal Drainage Flow on a Forested Slope Under a Tropical Monsoon Climate

Using data for one year, we examined the vertical wind speed profileson a mountain slope covered with forest in northern Thailand undera tropical monsoon climate. We defined two profile patterns: higherwind speeds at greater heights (Pattern 1) and lower wind speeds atgreater heights (Pattern 2). We classified 9.4% of the data as Pattern 2;this pattern tended to occur during the night, at low wind speeds, and with high outgoing longwave radiation. In addition, stable stratification anddecoupling between the canopy surface air and the overlying layers wereobserved when Pattern 2 occurred frequently. These facts suggested thatPattern 2 was caused by a nocturnal drainage flow. The occurrence ofPattern 2 showed a clear seasonal trend, indicating that there is a seasonaltrend in the occurrence of nocturnal drainage flows. Pattern 2 was observedmore frequently between August and February and less frequently betweenMarch and July. This corresponded to the seasonal trend in wind speed, butdid not correspond to the seasonal trend in the outgoing longwave radiation.     

Paleoshoreline geomorphology of Böön Tsagaan Nuur, Tsagaan Nuur and Orog Nuur: the Valley of Lakes, Mongolia

We conducted a preliminary study of paleoshoreline features associated with Böön Tsagaan Nuur, Tsagaan Nuur, and Orog Nuur, lakes located in the Gobi–Altai transition zone of the Valley of Lakes (Dolina Ozor) which stretches from central to western Mongolia. The paleoshoreline features were first identified on RADARSAT satellite SAR imagery. We investigated the features during the 1998 field season of the Joint Mongolian–Russian–American Archaeological Expedition to the Gobi–Altai region. We identified paleoshorelines of multiple elevations in the field, which are considered to be relict beach ridges and wave-cut terraces. Other paleolake landforms include spits and Gilbert-type deltas. These landforms are complex, large and well established, implying that the paleolakes were stable for extended periods. The reconstructed paleolakes cover extensive areas of the valley floor, implying that hydrological and climatic conditions were very different in the past. Paleolake expansions may have occurred under a variety of circumstances. One hypothesis is that the high lake stands occurred during the wetter period corresponding to the Oxygen Isotope Stage 3 prior to the Last Glacial Maximum (LGM), during the warmest early Holocene and the late Holocene, or during all these periods. If low evaporation rates due to lower temperatures, glacier meltwater and possibly increased precipitation are important factors, then the expansions may have occurred during the terminal Late Glacial period after the Last Glacial Maximum. The greatly expanded lakes in the Gobi–Altai could have significantly affected the Quaternary human demography and migration in the region.     

The Tsenkher structure in the Gobi-Altai, Mongolia: Geomorphological hints of an impact origin

The Tsenkher structure, in the Gobi-Altai region of Mongolia, was studied using a wide array of remote sensing data and field observations. The structure has a shallow, 3.6 km wide, central depression bordered by a near-circular ridge (putative crater rim) with breaches to the northwest. The central depression is obliterated partially by fluvial infill through these breaches. Outside the ridge, the western side is a rugged terrain, but the eastern half is characterized by a concentric outer ridge that occurs at about one radius distance from the inner ridge. Although intrusion, salt tectonics and maar crater hypotheses cannot be completely ruled out, its morphology strongly implies an impact origin for the Tsenkher structure. If of impact origin, it has a well-preserved morphology and its position in the basin fills indicates that the formation may have occurred relatively recently, sometime during the late Tertiary or Quaternary. The outer ridge morphology is similar to rampart craters on Mars, whose formation has been attributed to fluidization of a water-rich target layer and ejecta materials, or to atmospheric entrainment and deposition of ejected materials. However, other hypotheses including erosional remnant of ejecta blanket, erosional scarp of structural rim uplift, multi-ring or deeply eroded crater rim of a peak-ring crater are also possible at this stage. A complex fluvial and probable lacustrine history is envisaged within the Tsenkher structure. The structure is also associated with archaeology, including Paleolithic and Bronze Age remains.     

Kumamotoa, an early Late Cretaceous non-marine bivalve, from Fujian, south China

The non-marine bivalve species Trigonioides (Kumamotoa) quadratus Gu and Ma is described from the Fourth Member of the Hekou Formation in western Fujian, southeastern China. This species previously was regarded to belong to the subgenus Trigonioides (s.s.), which is restricted to the Early Cretaceous. Herein, it is included into the subgenus Kumamotoa. It is compared to Cenomanian (early Late Cretaceous) species from Japan and Korea such as Trigonioides (Kumamotoa) mifunensis Tamura, Trigonioides (Kumamotoa) matsumotoi Kobayashi and Suzuki and Trigonioides (Kumamotoa) paucisulcatus Suzuki. The age of the Fourth Member of the Hekou Formation is discussed.     

Neutron diffraction study of aluminous hydroxide δ-AlOOD

We have determined the position of deuterium atoms in δ-AlOOD by neutron powder diffraction at ambient pressure. As previously reported by theoretical and experimental studies, the deuterium atoms are located in the tunnel formed by the chains of AlO₆ octahedra. The data are best fit with the P2₁ nm structure, producing bond lengths of D–O1 of 1.552(2) Å, O2–D of 1.020(2) Å and O1–O2 of 2.571(2). This study confirms that the hydrogen bond is asymmetric at ambient conditions in agreement with recent single-crystal synchrotron study for δ-AlOOH.