Deep convection seesaw controlled by freshwater transport through the Denmark Strait

Observations of deep ocean temperature and salinity in the Labrador and Greenland Seas indicate that there is negative correlation between the activities of deep convection in these two sites. A previous study suggests that this negative correlation is controlled by the North Atlantic Oscillation (NAO). In this study, we discuss this deep convection seesaw by using a coupled atmosphere and ocean general circulation model. In this simulation, the deep convection is realistically simulated in both the Labrador and Greenland Seas and their negative correlation is also recognized. Regression of sea level pressure to wintertime mixed layer depth in the Labrador Sea reveals strong correlation between the convection and the NAO as previous studies suggest, but a significant portion of their variability is not correlated. On the other hand, the convection in the Greenland Sea is not directly related to the NAO, and its variability is in phase with changes in the freshwater budget in the GIN Seas. The deep convection seesaw found in the model is controlled by freshwater transport through the Denmark Strait. When this transport is larger, more freshwater flows to the Labrador Sea and less to the Greenland Sea. This leads to lower upper-ocean surface salinity in the Labrador Sea and higher salinity in the Greenland Sea, which produces negative correlation between these two deep convective activities. The deep convection seesaw observed in the recent decades could be interpreted as induced by the changes in the freshwater transport through the Denmark Strait, whose role has not been discussed so far.     

Prediction of slope water intrusion into the Kii channel in summer

Intrusions of the warm, oligotrophic surface slope water (SSW) and the cold, nutrient-rich bottom slope water (BSW) from the continental slope influence the annual variations in water temperature and nutrient concentrations in the Kii Channel in August. In order to evaluate the relationships between both these intrusions and the distance of the Kuroshio axis from Cape Shionomisaki (Kuroshio distance), a Distance-Intrusion-Diagram (DID) for temperature, which can reproduce the vertical temperature profile of the channel, was constructed by analyzing the temperature and Kuroshio distance records in August for 1967–2001. DIDs for nutrients (nitrate and phosphate) are also constructed by using the relationship between the nutrient concentration and water temperature. The only explanatory variable in the DIDs is the Kuroshio distance. The DID for temperature predicts that the SSW occupies almost the entire water column when the Kuroshio approaches Cape Shionomisaki (Kuroshio distance = 18.5 km). When the Kuroshio distance lies in the range 18.5–74 km, the BSW thickness increases proportionally to the Kuroshio distance increment while the SSW thickness decreases. The BSW occupies the largest portion of the channel when the Kuroshio distance is 74 km. Further, beyond 74 km, the BSW thickness reduces gradually. Yearly variations in the temperature and concentrations of nitrate and phosphate were hindcast with the DIDs. The results revealed that the Kuroshio distance contributes 70%, 35%, and 30% of the variances in temperature, nitrate concentration, and phosphate concentration, respectively.     

Wave forces acting on a vertical circular cylinder with a constant forward velocity

Wave forces acting on submerged circular cylinders moving forward with a constant velocity in regular waves are investigated experimentally. Hydrodynamic forces acting on the cylinder forced to surge in a steady are also measured and hydrodynamic coefficients were obtained. Wave force coefficients obtained from wave force measurements are compared with the hydrodynamic coefficients from surging tests, and the similarity and difference between them are discussed. Experiments show that these coefficients are quite different from those of the cylinder without a forward velocity.     

A structure of the Kuroshio and its related upwelling on the East China Sea shelf slope

The ADCP on an advanced towed fish with controllable main and tail wings, called DRAKE measured a detailed sectional structure of the Kuroshio flowing to the NE along the East China Sea shelf slope west of Okinawa. At the observation period, a countercurrent directed to the SW formed in near-bottom water on the shelf slope. The horizontal flow perpendicular to the stream axis of the Kuroshio constructed a convergence zone around the boundary between the Kuroshio and the countercurrent. An intensive upwelling with the maximum velocity of 2.8 cm s^–1 was found to distribute on the shelf slope around the convergence zone. A dynamic cause of this intensive upwelling is discussed carefully.     

The Current Structure of the Tsushima Warm Current along the Japanese Coast

The branching of the Tsushima Warm Current (TWC) along the Japanese coast is studied based upon intensive ADCP and CTD measurements conducted off the Wakasa Bay in every early summer of 1995–1998, the analysis of the temperature distribution at 100 m depth and the tracks of the surface drifters (Ishii and Michida, 1996; Lee et al., 1997). The first branch of TWC (FBTWC) exists throughout the year. It starts from the eastern channel of the Tsushima Straits, flows along the isobath shallower than 200 m along the Japanese coast and flows out through the Tsugaru Strait. The current flowing through the western channel of the Tsushima Straits feeds the second branch of TWC (SBTWC) which develops from spring to fall. The development of SBTWC propagates from the Tsushima Straits to Noto Peninsula at a speed of about 7 cm sec⁻¹ following the continental shelf break with a strong baroclinicity. However, SBTWC cannot be always found around the shelf break because its path is influenced by the development of eddies. It is concluded that SBTWC is a topographically steered current; a current steered by the continental shelf break. Salient features at intermediate depth are the southwestward subsurface counter current (SWSCC) between 150 m and 300 m depths over the shelf region in 1995–1998 with the velocity exceeding about 5 cm sec⁻¹, although discrepancies of the velocity and its location are observed between the ADCP data and the geostrophic currents. This revised version was published online in August 2006 with corrections to the Cover Date.     

High magnetic susceptibility produced by thermal decomposition of core samples from the Chelungpu fault in Taiwan

We carried out thermomagnetic susceptibility analyses of fault rocks from core samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP) to investigate the cause of high magnetic susceptibilities in the fault core. Test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 °C and by high-velocity frictional tests before magnetic analyses. Thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 °C in the heating cycle, and showed three step increases, at 600 to 550 °C and at 300 °C during the cooling cycle. These behaviors are consistent with the presence of pyrite, siderite and chlorite, suggesting that TCDP gouge originally included these minerals, which contributed to the generation the magnetic susceptibility by thermomechanical reactions. The change in magnetic susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite, so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core. Color measurement results indicate that thermal decomposition by frictional heating took place under low-oxygen conditions at depth, which prevented the minerals from oxidizing to reddish hematite. This finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility. A kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite. Our results show that decomposition of pyrite to pyrrhotite, siderite and, to some extent, chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the Chelungpu fault zone.     

Physical property anisotropy of foliated fault rocks: Study from the Nobeoka Thrust,Shimanto Belt,southwest Japan

To investigate the physical property anisotropies of foliated fault rocks in subduction zones, the hanging wall phyllites and footwall cataclasites exhumed along the Nobeoka Thrust, a fossilized out‐of‐sequence‐thrust in the Shimanto Belt, Japan, was focused. Discrete physical property (electric resistivity, P‐ and S‐wave velocities, and porosity) measurements were conducted employing geologic coordinates (depth‐parallel direction, strike direction, and maximum dip direction of foliation), using the core samples obtained from the Nobeoka Thrust Drilling Project and compared the data to borehole geophysical logs. A higher sample P‐wave velocity (Vp), lower S‐wave velocity (Vs), higher Vp/Vs, and lower sample porosity and resistivity compared to the logs, are inferred to have been caused by the larger sampling scale of the logs and lower fluid saturation of the borehole. The phyllites and cataclasites exhibited substantial vertical and horizontal anisotropy of Vp (0.4–17.3 % and 2.7–13.8 %, respectively), Vs (0.5–56 % and 7.7–43 %, respectively), and resistivity (0.9–119 % and 2.0–65.9 %, respectively). The physical property anisotropies are primarily affected by the dip angles of foliation. The fault rocks that have gentler dip angles exhibit a higher Vp in the strike and maximum dip direction and a lower Vp in the depth‐parallel direction. In contrast, the fault rocks that have steeply dipping structures show a higher Vp in the strike and depth‐parallel directions with a lower velocity in the maximum dip direction. Resistivity anisotropy show a trend opposite to that of the Vp in relation to the dip angles. Our results show lower Vp anisotropy than those obtained in previous studies, which measured wave speeds perpendicular or parallel to foliation under confining pressure. This study highlights the significance of dip angles on vertical properties in geophysical surveys across foliated fault rocks.     

Stratigraphic records of tsunamis along the Japan Sea,southwest Hokkaido,northern Japan

The stratigraphy of tsunami deposits along the Japan Sea, southwest Hokkaido, northern Japan, reveals tsunami recurrences in this particular area. Sandy tsunami deposits are preserved in small valley plains, whereas gravelly deposits of possible tsunami origin are identified in surficial soils covering a Holocene marine terrace and a slope talus. At least five horizons of tsunami events can be defined in the Okushiri Island, the youngest of which immediately overlies the Ko‐d tephra layer (1640 AD) and was likely formed by the historical Oshima‐Ohshima tsunami in 1741 AD. The four older tsunami deposits, dated using accelerator mass spectrometry ¹⁴C, were formed at around the 12th century, 1.5–1.6, 2.4–2.6, and 2.8–3.1 ka, respectively. Tsunami sand beds of the 1741 AD and circa 12th century events are recognized in the Hiyama District of Hokkaido Island, but the older tsunami deposits are missing. The deposits of these two tsunamis are found together at the same sites and distributed in regions where wave heights of the 1993 tsunami (Hokkaido Nansei‐oki earthquake, Mw = 7.7) were less than 3 m. Thus, the 12th century tsunami waves were possibly generated near the south of Okushiri Island, whereas the 1993 tsunami was generated towards the north of the island. The estimated recurrence intervals of paleotsunamis, 200–1100 years with an average of 500 years, likely represents the recurrence interval of large earthquakes which would have occurred along several active faults offshore of southwest Hokkaido.     

Seismic imaging of the Formosa Ridge cold seep site offshore of southwestern Taiwan

Multi-scale reflection seismic data, from deep-penetration to high-resolution, have been analyzed and integrated with near-surface geophysical and geochemical data to investigate the structures and gas hydrate system of the Formosa Ridge offshore of southwestern Taiwan. In 2007, dense and large chemosynthetic communities were discovered on top of the Formosa Ridge at water depth of 1125 m by the ROV Hyper-Dolphin. A continuous and strong BSR has been observed on seismic profiles from 300 to 500 ms two-way-travel-time below the seafloor of this ridge. Sedimentary strata of the Formosa Ridge are generally flat lying which suggests that this ridge was formed by submarine erosion processes of down-slope canyon development. In addition, some sediment waves and mass wasting features are present on the ridge. Beneath the cold seep site, a vertical blanking zone, or seismic chimney, is clearly observed on seismic profiles, and it is interpreted to be a fluid conduit. A thick low velocity zone beneath BSR suggests the presence of a gas reservoir there. This “gas reservoir” is shallower than the surrounding canyon floors along the ridge; therefore as warm methane-rich fluids inside the ridge migrate upward, sulfate carried by cold sea water can flow into the fluid system from both flanks of the ridge. This process may drive a fluid circulation system and the active cold seep site which emits both hydrogen sulfide and methane to feed the chemosynthetic communities.     

Enrichment of adsorbed methane in authigenic carbonate concretions of the Japan Trench

Substantial amounts of adsorbed methane were detected in authigenic carbonate concretions recovered from sedimentary layers from depths between 245 and 1,108 m below seafloor during Ocean Drilling Program Leg 186 to ODP sites 1150 and 1151 on the deep-sea terrace of the Japan Trench. Methane contents were almost two orders of magnitude higher in the concretions (291–4,528 nmol/g wet wt) than in the surrounding bulk sediments (5–93 nmol/g wet wt), whereas methane/ethane ratios and stable carbon isotopic compositions were very similar. Carbonate content of surrounding bulk sediments (0.02–3.2 wet wt%) and methane content of the surrounding bulk sediments correlated positively. Extrapolation of the carbonate contents of bulk sediments suggests that 100 wt% carbonate would correspond to 1,886±732 nmol methane per g bulk sediment, which is similar to the average value observed in the carbonate concretions (1,321±1,067 nmol/g wet wt, n = 13). These data support the hypothesis that, in sediments, adsorbed hydrocarbon gases are strongly associated with authigenic carbonates.