The high-resolution analyses of fossil diatom assemblages in four cores recovered from the area off Sanriku Coast revealed the most complicated hydrographic history in this area. The fossil diatom assemblages changed in responding to the expansion of the warm-water eddies detached from the Kuroshio Extension and of the southward Oyashio Intrusion. Diatom abundances are higher in the cores from near shore than in the area off shore. The diatom assemblages were affected more seriously by the environmental changes in the cores from near shore. The diatom assemblages were also changed by the fluctuations at the intervals of ～40-kyr corresponding to the obliquity (tilt) band variations among the Earth's orbital parameters and ～60-kyr intervals suggesting changes in the sea level due to glacial/interglacial phases, with secondary and smaller fluctuations at ～20-kyr intervals corresponding to the precessional changes. Diatom assemblages in the southern margin of the Perturbed Area have been controlled mainly by the Kuroshio-Kuroshio Extension, but in the northern margin they changed in responding to the mixing of warm-water eddies and Oyashio Intrusion.

We carried out a deep wide-angle seismic experiment using a large airgun array and total 95 ocean bottom seismographs (OBSs) in the middle Izu-Ogasawara arc area, which was conducted by R/V Kaiyo of Japan Agency for Marine-Earth Science and Technology (JAMSTEC) from August 3, 2006 to September 1, 2006 (UTC) named as KY06-09 cruise. The seismic line runs from the Izu-Ogasawara trench to the eastern foot of the Kinan Seamount Chain in the Shikoku basin through the northern tip of the Ogasawara trough, the volcanic front located off south of the Sofugan and the Miocene Nishi-shichito ridge. Most important objective of this cruise is to construct a velocity structure across this area and clarify commonalities and differences by comparison of velocity structures obtained with previous studies in the whole of Izu-Ogasawara arc regions. The key structures are the continuity of the crustal layers of the Ogasawara ridge and trough, and the current active arc obtained by previous KY05-02 and KY05-11 cruises. And the high velocity lower crust with P-wave velocity of over 7.0 km/s is also one of the key structures because it was discovered at the arc-backarc transition zones in the western Izu-Ogasawara arc and the Mariana arc. The seismic signals propagated from an airgun array with a total capacity of 12,000 cu.in. were recorded by not only OBSs but also a 12-channnel streamer towed from stern. In this paper, we introduce the seismic experiment, especially the schedule and examples of OBS and reflection data.

Artificial noise, which was generated simultaneously as a data storage's hard disc drive started up, has been recorded in STS-1 broadband seismometers' output at two seismic stations where were established over western Pacific Ocean. If this periodical noise is included in the data, it may disturb our analyses of long period ground motions. For this reason, we conducted the experiments under several conditions to find the origin of this noise and to remove it. As a result, the noise wasn't generated by separating a ground line for frame ground from that for power return in the laboratory and the station, which means that the noise was generated by adding some impedance to the ground line for power return. We also propose some ideas to improve the instrument so that this noise may be not generated even if we short the ground line for frame ground and that for power return.

Temperature and conductivity sensors manufactured by Sea Bird Electronics, Inc. (SBE) are affixed to APEX-type Argo floats manufactured by Webb Research Co. Before shipping to users, calibration of the sensors is carefully carried out at SBE. However, the sensors are recalibrated at Japan Marine-Earth Science and Technology Agency (JAMSTEC) to reconfirm sensor accuracy considering a long transportation from SBE and quite a long storage time at JAMSTCE before deployment. In this paper, we describe and analyze these recalibration results. We used the same calibration technique as used by SBE, comparing outputs of temperature and conductivity sensors with those of highly quality-controlled standard temperature and conductivity sensors. We set thresholds of good or bad temperature and conductivity (salinity) sensors to be ± 0.0050 ℃ and ± 0.010 psu, respectively. The thresholds are selected to meet the target of the Argo project. Between 2001 and 2005, 228 sensors which are about 60 % of all were recalibrated at JAMSTEC. All the temperature sensors fall within the threshold. Of the conductivity sensors, 175 (77%) passed at the first measurement of recalibration series, while 30 (13%) passed after two or more measurements. The latter sensors were carefully cleaned to remove contaminations by dust or chemical pollutants. However, nine sensors (4%) did not pass recalibration test and were returned to SBE for further checking and repairs.

Multi-channel seismic (MCS) reflection experiments were carried out in KR06-13 cruise to investigate crustal structures and deformations of the arc-backarc transition zone in the Izu-Ogasawara-Mariana island arc, by using the R/V KAIREI of Japan Agency for Marine-Earth Science and Technology (JAMSTEC). Swath bathymetry, geomagnetic and gravity data are simultaneously observed during this cruise. Four MCS survey lines (Lines A02, A06, A07, and IBr12) with 1043.55km of total line length were obtained in this survey. An airgun array with a total capacity of 12,000 cubic inches (eight airguns with 1,500 cubic inches each) was shot with shot spacing of 50 m, air pressure of 2000 psi and towing depth of 10 m. These airgun signals are recorded on a 204-channel hydrophone streamer cable with group interval of 25 m. The record length and sampling interval were 18 sec and 4 msec, respectively. The MCS data has sufficient quality to assess the crustal deformation of the arc-backarc transition zone. The preliminary interpretations on the onboard processing result indicated unique features of the basement and sediment in the transition zone.

We carried out a deep seismic survey using a large airgun array with total capacity of 12,000 cubic inches and a 204-channel multi-channel hydrophone streamer system (MCS) in the southern Izu-Ogasawara arc-backarc area. The survey was conducted on the R/V Kairei of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) during 5 July --27 July 2006 (KR0607 cruise). Objectives of this cruise were to obtain crustal deformation data of the southern Izu-Ogasawara arc area, which was formed in the Eocene, Miocene, and by the currently active arc. During this cruise, three MCS seismic lines were set along the Ogasawara Ridge and across the entire southern Izu-Ogasawara arc.

Sea bottom exposure of a sherbet hydrate was not unusual as famous as in the Hydrate Ridge along the Oregon coast of United States. During the NT06-19 cruise Leg2 of R/V Natsushima and Hyper-Dolphin at the sea area southwest off the Sadogashima Island in the central Japan Sea, a fantastic outcrop of exact crystalline methane hydrate was discovered and exhumation of the world's largest scale was observed by the submersible investigation. It means that tectonic activity and related methane plume of this area is so active that formed a mud volcano with a conduit topped by such an outcrop. It is expected that the formation mechanism of the solid methane hydrate layer will be distinctly clarified by synthesizing our analytical results of each field concerning the cruise.