Structural characteristics controlling the seismicity crustal structure of southern Japan Trench fore-arc region, revealed by ocean bottom seismographic data

The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the Japanese islands. Many earthquakes occur associated with plate convergence, and the hypocenter distribution is variable along the Japan Trench. In order to investigate the detailed structure in the southern Japan Trench and to understand the variation of seismicity around the Japan Trench, a wide-angle seismic survey was conducted in the southern Japan Trench fore-arc region in 1998. Ocean bottom seismometers (15) were deployed on two seismic lines: one parallel to the trench axis and one perpendicular. Velocity structures along two seismic lines were determined by velocity modeling of travel time ray-tracing method. Results from the experiment show that the island arc Moho is 18–20 km in depth and consists of four layers: Tertiary and Cretaceous sedimentary rocks, island arc upper and lower crust. The uppermost mantle of the island arc (mantle wedge) extends to 110 km landward of the trench axis. The P-wave velocity of the mantle wedge is laterally heterogeneous: 7.4 km/s at the tip of the mantle wedge and 7.9 km/s below the coastline. An interplate layer is constrained in the subducting oceanic crust. The thickness of the interplate layer is about 1 km for a velocity of 4 km/s. Interplate layer at the plate boundary may cause weak interplate coupling and low seismicity near the trench axis. Low P-wave velocity mantle wedge is also consistent with weak interplate coupling. Thick interplate layer and heterogeneous P-wave velocity of mantle wedge may be associated with the variation of seismic activity.     

Absorption of Atmospheric CO2 and Its Transport to the Intermediate Layer in the Okhotsk Sea

In the southwestern Okhotsk Sea off Hokkaido we observed chemical components related to the carbonate system for 1 year from August 1997 to June 1998. Using the conservative components salinity and water temperature, we confirmed the existence of two water masses flowing into the intermediate layer of the Okhotsk Sea, the East Sakhalin Current Water (ESCW) which becomes denser by mixing of brine water, and the Forerunner of Soya Warm Current Water (FSWW) which becomes denser due to cooling of the saline Kuroshio water. The ΔNTC_x values were calculated by comparing the ESCW and the FSWW with the Pacific Deep Water (PDW). The ΔNTC_x values obtained are 100–110 μmol/kg and 70–100 μmol/kg for the ESCW and the FSWW off Hokkaido, respectively, which are considerably larger than that of the Kuroshio water. These large ΔNTC_x values may be due to both low DIC concentration in the surface water and intense gas exchange under the cold and stormy winter conditions for the ESCW and the cooling of the FSWW as it flows northward. Since the flow rates of dense waters concerned with the ESCW and the FSWW have previously been estimated as 0.9 Sv and 0.2 Sv, respectively, the amount of atmospheric CO₂ absorbed and transported to the intermediate layer turns out to be 3.9−4.1 × 10¹³ gC/yr. This flux is small on a global scale, but the flux divided by the surface layer of the Okhotsk Sea is 30 gC/m²/yr, which is 5 times greater than the mean absorption flux of anthropogenic CO₂ in the world's oceans. It is thus considered that atmospheric CO₂ is efficiently absorbed in the Okhotsk Sea. This revised version was published online in July 2006 with corrections to the Cover Date.     

Complexation of dimethylsulfide with mercuric ion in aqueous solutions

The tendency of dimethylsulfide (DMS) to form complexes with heavy metal ions in aqueous solutions and the factors that influence it have been investigated. Among five heavy metal ions examined (Cu²⁺, Cd²⁺, Zn²⁺, Pb²⁺ and Hg²⁺), only Hg²⁺ bound significantly with DMS in aqueous solutions in which Hg²⁺ concentration was increased to much higher levels than that of natural seawater. The complexation capacity of Hg²⁺ for DMS was influenced by pH and media. The affinity of Hg²⁺ for DMS was generally lower at high than at low pH, presumably due to the competition of hydroxide ion to form hydroxomercury species. In different solutions, the affinity of Hg²⁺ for DMS followed the following sequence: ultra-purified water > 35‰ NaCl solution > seawater. It seems apparent that chloride had a negative impact on the complexation of DMS by Hg²⁺, owing to the competition of chloride with DMS for complexing Hg²⁺. In addition, the affinity of Hg²⁺ for DMS in the bulk seawater appeared to be higher than that in the surface microlayer seawater. The tendency of Hg²⁺ to form complexes with DMS in aqueous solution can be reduced by the presence of 2 mM amino-acid such as glycine, alanine, serine and cysteine, as these ligands give stable mercury complexes. However, the presence of 2 mM acetate in experimental solutions had no significant effect on the complexation of Hg²⁺ with DMS, even though this ligand has a relatively strong complexing capacity for Hg²⁺. Although mercury ions appeared to have a strong affinity for DMS, the concentration of mercury in seawater is too low to produce a great effect on the distribution of DMS in oceans.     

Dimethylsulfide in coastal zone of the East China Sea

Dimethylsulfide (DMS) in seawater were observed four times from February 1993 to August 1994 along a fixed section (PN line) in the East China Sea. The DMS concentrations showed remarkable temporal and spatial variations. The DMS concentrations were generally higher in the upper euphotic layer of the continental shelf zone in summer. The spatial variation, however, was more pronounced even in well mixed winter water, where the concentration of DMS varied widely from 3 to 106 ng-S/l in the continental shelf zone while the salinity was vertically almost uniform. This means that DMS in seawater is rapidly produced and decomposed with a time scale less than one month in the water column. The largest value of 376 ng-S/l was obtained at 5 m depth near the mouth of Changjiang River in August 1994. The mean concentrations in the surface 30 m layer in the continental shelf zone were 21, 54, 126 and 57 ng-S/l in February, October, June and August, respectively, which were about twice as large as those in the Kuroshio region. The mean fluxes of DMS from the East China Sea to the atmosphere are estimated to be 49 g-S/m²/day in winter and 194 g-S/m²/day in summer in the continental shelf zone, and to be 32 and 107 g-S/m²/day in the Kuroshio region.     

Relation between the concentrations of DMS in surface seawater and air in the temperate North Pacific region

The concentrations of DMS were simultaneously measured in both water and air at the sea surface on board a vessel during a trans-Pacific cruise around 40° N in August 1988. Those in the surface seawater varied widely with a mean of 162 ng S/1 and a standard deviation of 134 ng S/1 (n=37), but the variation was not a mere fluctuation and the high concentration (376 ng S/1) was found in the area between 145° W and 170° W. The atmospheric DMS concentration varied more widely with a mean value of 177 ng S/m³ and a standard deviation of 203 ng S/m³ (n=23). The diurnal variation of DMS was not significant in the air near the sea surface. However, the concentrations in the surface water was fairly well correlated with those in the surface air. The correlation coefficient (r ²=0.86) was larger than that between the atmospheric concentration and outflux of DMS (r ²=0.64). These findings mean that the turnover time of DMS in the atmosphere is not extremely short. Based on the linear relation between the atmospheric and seawater DMS, the turnover time of the atmospheric DMS has been calculated to be 0.9 days with an uncertainty of around 50%. The oxidation rate agrees fairly well with that expected from the OH radical concentration in the marine atmosphere.     

East-west distribution of POC fluxes estimated from <Superscript>234</Superscript>Th in the northern North Pacific in autumn

Observations of primary productivity, ²³⁴Th, and particulate organic carbon (POC) were made from west to east across the northern North Pacific Ocean (from station K2 to Ocean Station Papa) during September–October 2005. Primary productivities in this region varied longitudinally from approximately 236 to 444 mgC m⁻²d⁻¹ and clearly indicate the West High East Low (WHEL) trend. We estimated east-west variations in the POC flux from the surface layer (0–100 m) by using ²³⁴Th as a tracer. POC fluxes in the western region (44–53 mgC m⁻²d⁻¹) were higher than those in the eastern region (21–34 mgC m⁻²d⁻¹). However, the export ratios (e-ratios) ranged from approximately 8% to 16% and did not show the WHEL trend. Contrary to our expectation, no relation between POC flux (or e-ratio) and diatom biomass (or dominance) was apparent in autumn in the northern North Pacific.