Interstitial water chemistry in deep-sea sediments from the Japan Sea

Vertical profiles of pH, Eh, and major and minor chemical constituents in interstitial waters have been studied in four piston cores from deep-sea basins of the Japan Sea. Sulfate concentration decreases remarkably with increasing depth in three cores, and hydrogen sulfide is observed in two cores although the overlying bottom water is highly aerated. Three types of interstitial waters are observed in the four cores: (1) alkalinity and manganese concentration increase gradually with increasing depth, and sulfate reduction is not appreciable, (2) alkalinity increases and sulfate concentration decreases remarkably, and hydrogen sulfide is not detected, and (3) alkalinity increases and sulfate concentration decreases remarkably, and hydrogen sulfide is observed. Factors controlling the chemical composition of interstitial waters are discussed.     

Oceanographic analysis of the North Pacific,with special reference to the Kuroshio

I have been concerned with oceanographic observations made since 1948 by the oceanographic group of the Japan Meteorological Agency. My primary interest has been to describe physical aspects of the Kuroshio current system and its adjacent regions. Much efforts have been devoted to observing current and water characteristics of the Kuroshio south of Japan and the Kuroshio extension east of Japan and to describing various kinds of fluctuations ranging from diurnal to seasonal or year-to-year variations. I have also attempted to study the equatorial current system particularly in the western Pacific and those characteristic water-types in the central region of the North Pacific.     

Temporal variation in mass fluxes and the major components of sinking particles in Sagami Bay, off Japan

Sinking particles were collected using time-series sediment traps deployed at 350 and 20 mab at Site SB (34° 58.5’N, 139° 20.9’E, 1544 m depth) near the center of Sagami Bay, off Japan with high time resolutions of 5-8 days (March 1997 to August 1998) and 3-4.5 days (March 1998 to August 1998), respectively. The major components (CaCO₃, OM, opal, and clay) of these sinking particles and surface bottom sediments were determined using a stepwise leaching method combined with gravimetry. Average total mass fluxes were 1480, 5560 and 3068 mg/m²/year at 350 mab, at 20 mab, and in the surface sediments, respectively, indicating an enhanced collection of sinking particles at 20 mab. Clay was the dominant component and biogenic components (opal+OM+CaCO₃) were dominated mainly by opal and secondly by OM. On average, opal and CaCO₃ contents decreased gradually as clay content increased with increasing depth from 350 mab-20 mab and in the surface sediments, indicating dissolution of opal and CaCO₃ through sinking, rebound, resuspension or sedimentation processes. Thirteen total mass flux peaks at 17--40-day intervals were observed at 350 mab during the period from March 1997 to August 1998 except for winter, while eight peaks were observed at 20 mab for the period from March 1998 to August 1998. Two types of total mass peaks can be distinguished: one with a clear increase in biogenic flux (opal+OM+CaCO₃) and little or no increase in clay flux and termed a bloom type (B-type), and the other with a clear increase in clay flux, little increase in biogenic flux and termed a resuspension type (R-type). Some R-type peaks, but not all, coincided with total mass flux peaks observed at the mouth of Tokyo Bay and suggested the possibility of the effect of particulate materials transported from Tokyo Bay to site SB. The enormously large peak observed at 20 mab in late May 1998 and that at 350 mab in early June 1998 were considered to be due to some physical perturbations from an earthquake swarm near site SB during the period from April to June 1998. The 17--40-day periodicity was associated clearly with the change in biogenic flux dominated by opal flux and is thought to reflect the periodicity of biological productivity dominated by diatoms in the euphotic zone of Sagami Bay.     

A Late Holocene geomagnetic secular variation record from Erhai Lake, southwest China

A secular variation record of the geomagnetic field direction for the last 6.5  kyr has been obtained from the magnetization of sediment cores from Erhai Lake, southwest China. In order to make a comparison with this record, secular variation in east-central China was investigated by combining available magnetic field data from historical records and archaeomagnetic measurements since about 350 bc . The secular variation in Erhai Lake shows features consistent with the combined record, except for the oldest three observed declination swings in Sian from 720 to 900 ad . Many features of declination and inclination in China also occur in Japan. From 500 to 1000 ad , declination was westerly ranging from about −20° to −5° in Erhai Lake, east-central China, and Japan.     

Pb and Cs in sediments from Sagami Bay, Japan: sedimentation rates and inventories

Profiles of the radioisotopes ²¹⁰Pb and ¹³⁷Cs were determined in 15 sediment cores collected from Sagami Bay, Japan. The activities of ²¹⁰Pb_ex (unsupported) in core top sediments increased with water depth from 25 dpm g⁻¹ on the upper continental slope off the mouth of Tokyo Bay to an average of 283 dpm g⁻¹ at the deep-sea station SB. The high ²¹⁰Pb trapping efficiency of settling particles expected from the results of the sediment trap experiment near the SB site suggests that effective ²¹⁰Pb enrichment in surface sediments may occur during resuspension and lateral transportation of particles via the benthic nepheloid layer on the continental slope. In several cores, ¹³⁷Cs profiles showed an increase, a distinct peak, and then a decrease to an undetectable level downcore. These profiles can be compared with the temporal change of bomb-produced ¹³⁷Cs fallout.The mean sedimentation rates estimated by the ²¹⁰Pb_ex inventory method, rather than using ²¹⁰Pb_ex profiles, ranged from 0.06 g cm⁻² y⁻¹ to 0.14 g cm⁻² y⁻¹. The average value of the rates in SB cores was calculated to be 0.11 g cm⁻² y⁻¹, which was similar to that calculated under the assumption that the age of the ¹³⁷Cs peak corresponds to its maximum fallout year in 1963.Although ¹³⁷Cs inventories represented one tenth of the anthropogenic fallout of ¹³⁷Cs until 1997, they correlated with the increase in ²¹⁰Pb_ex inventory. This suggests that the scavenging of refractory ¹³⁷Cs as well as ²¹⁰Pb by settling particles in the water column can lead to the formation of a time marker layer even in deep-sea sediment core, such as at the SB site.