In October to November, 2009, a multi-beam bathymetry survey had been successfully completed in the outer-arc high off northwest Sumatra during the KY09-09 cruise using R/V KAIYO of JAMSTEC (Japan Agency for Marine-Earth Science and Technology). Then KY09-09 bathymetry data were integrated with a previous NT05-02 bathymetry data to make a new detailed bathymetry map, gridded at approximately ∼37 m cell size, in the middle part of the outer-arc high. The most predominant morphological feature is the NNW trending sigmoidal structures consisting of a series of ridges and troughs parallel to the local trench, which is more evident trenchward than landward. Secondary predominant structure is the N-S to NE-SW trending ridges and adjoining V shaped valleys that offset the most predominant structures from north to south in the middle part of the integrated map area. Third predominant structure is the NNE to NE narrow valleys that cut the NNW trending sigmoidal structures elsewhere in the whole integrated map area.

View full abstract

An Os isotope stratigraphy is one of the most powerful tools to determine the sedimentation age of Fe-Mn crusts. Since a typical growth rate of Fe-Mn crust is very slow such as several mm/Myr, the Re-Os isotope analysis with a high-resolution sampling is required for precise age determination. Fine-scale sampling with a metallic drill is a common technique, but such sampling decreases the amount of sample powders, which in turn arises a necessity for a low-level Re and Os blank analysis to obtain an accurate isotope composition. Thus, in the present study, our main goals are (1) evaluation of Re and Os contamination amounts through drill sampling using three types of metallic drills, (2) measuring Os amounts in acid reagents that we use for sample digestion, and (3) investigating Os blank during a Carius tube digestion method under various heating temperature and time conditions. The Re contamination amounts through drill sampling reached ca. 50 pg despite of the drill material. On the other hand, the Os contamination amount through drill sampling can be negligible only when a tungsten carbide (WC) drill was used for sample preparation. The Os amount and ¹⁸⁷Os/¹⁸⁸Os ratio of TAMAPURE AA-10 HNO₃ solution were much higher than those of electric industrial grade (EL) and special grade HNO₃ solutions. It has been also turned out that the Os blank did not depend on the heating temperature and time conditions in the Carius tube. Consequently, drill sampling using a WC drill combined with low Os blank acid digestion by EL inverse aqua regia in a Carius tube allows us an accurate Os isotope analysis of Fe-Mn crusts.

View full abstract

For the measurement of ¹⁴³Nd/¹⁴⁴Nd ratios, Nd must be separated from the major matrix and isobaric interfering elements before mass spectrometry measurements are performed. We report an improved technique for the separation of Nd from the other rare earth elements (REE), in particular Ce and Sm, in geological samples. This method uses Ln resin chromatography, combining oxidative and conventional “Ln-resin” procedures in order to remove Ce and Sm, respectively, from the Nd fraction. Repeated analyses of ¹⁴³Nd/¹⁴⁴Nd in standard JNdi-1 prepared using the newly developed technique are in good agreement with the values measured by direct analysis. The procedural Nd blanks are extremely small. The recovery yield of Nd after the entire separation process was always better than 90%, which is better than the recovery realized in previous studies owing to the collection of wider Nd elution points by the effective removal of neighbour elution elements (Ce and Sm). The ¹⁴³Nd/¹⁴⁴Nd ratios of four igneous rock reference materials (JB-1a, JB-2, JA-1, JR-1) were also analysed, and the mean values were consistent with those given in previous reports, indicating the reliability of our new Nd separation chromatography technique.

View full abstract

There are two problems with the shortwave penetration scheme used in Modular Ocean Model version 3 (MOM3): (i) the spatiotemporal variability of the thickness of the first layer resulting from the free surface is not considered, and (ii) shortwave irradiance penetrates the ocean bottom. Because both of these problems can cause artificial heat sources or sinks, their effects are evaluated in the present study using a MOM3-based ocean general circulation model. The first problem creates an artificial heat sink (source) in the regions of positive (negative) sea surface height (SSH) with a maximum amplitude greater than 10 W m^-2 and decreases (increases) sea surface temperature (SST) by up to 0.3°C on the basis of annual mean. This change in SST leads to a reduction in global mean evaporation and, as a result, an increase in SSH, which enhances the artificial heat sink. After several years of integration, this positive feedback amplifies the effects of the first problem in cases of stand-alone ocean simulations forced by freshwater flux. The estimated artificial heat sink induced by the second problem reaches 50 W m^-2, and the decrease in SST exceeds 1.0°C. However, the effects of this problem are restricted within shallow coastal areas and do not involve positive feedback.

View full abstract

Marine sediment cores are usually stored in archives after collection until they are utilized. Few studies, however, have investigated changes in the physical and chemical properties of sediment cores during storage. Therefore, it has not been known how long archived sediment cores are useful for determining certain physical, geochemical or chemical components. To clarify changes in physical and chemical properties of archived sediments, we monitored moisture ratio, magnetic susceptibility, lightness, color reflectance, total carbon, total nitrogen, and organic carbon contents in archived sediments stored at 20-25°C, 4°C, or -20°C, using sediment cores collected from the North Pacific seafloor near Japan. We also monitored magnetic susceptibility in foraminiferal ooze and diatomaceous pelagic clay sediments from the North Pacific. The moisture ratio changed toward a constant value at all depths with increasing time because pore water in the sediment could easily move throughout the sediment core. There was no significant difference in magnetic susceptibility in hemipelagic and diatom-bearing clay sediments archived at 4°C and at 20-25°C. In foraminiferal ooze, diatom-bearing foraminiferal ooze, and diatom-bearing pelagic silty clay, magnetic susceptibility showed a reducing trend throughout the monitoring period, and the magnitude of reduction was larger at 20-25°C than at 4°C. Changes in lightness and color reflectance were significant and rapid, occurring within weeks of the collection date. Slight differences in the preservation of carbon and nitrogen were observed at different storage temperatures, with a smaller degradation rate at -20°C than at 4°C or 20-25°C. The presence of an inert gas, argon, was not effective at preserving organic materials. The possible effectiveness of other inert gases for this purpose should be investigated. Our results will be useful for estimating the alteration rate of physical and chemical properties of archived sediment samples under various storage conditions.

View full abstract

DONET is a network of permanent ocean-bottom seismic stations aimed at improving the detection capability and earlier detection of earthquakes and tsunamis off the Kii Peninsula, where the Tonankai mega-thrust earthquake is anticipated to occur in the near future. DONET consists of 20 stations each of which seismometers and pressure gauges are installed. The orientation of the horizontal components of the seismometer at each station has been measured by using video of a remotely operated vehicle (ROV), which is difficult to measure again for the confirmations. We estimated the seismometer orientations by using the seismic observation data based on three methods; correlation of long-period seismic waveforms with observations on land, direction of P-wave first motion of distant earthquakes, and particle motion of airgun signal. The obtained results are well consistent with each other. The differences between the estimations are at maximum 5 degrees. The difference from the measurement of the ROV video is up to about 10 degrees for most stations, but in some stations the difference is about 50 degrees.

View full abstract