Hikari Kamioka

JNdi-1 : a neodymium isotopic reference in consistency with LaJolla neodymium

Abstract A neodymium oxide with relative 143Nd/144Nd ratio 1.000503±1(1 σm) to LaJolla Nd was prepared as a new isotopic reference. The neodymium reagent was selected from two points of view as follows. The first is low abundance of neighboring elements Ce and Sm, which affects isobaric interference. The second is high 143Nd/144Nd ratio, which is closer to those of chondritic and mantle-derived materials. The 143Nd/144Nd ratio of the reagent was measured alternately with LaJolla Nd to get a coherency with LaJolla Nd using 12 mass spectrometers in 11 laboratories in Japan. Aliquots of this neodymium oxide reagent named JNdi-1 are available upon request from the Geological Survey of Japan and may be useful for precise interlaboratory calibration of Nd isotopes.

Asian continental nature of87Sr/86Sr ratios in north central Pacific sediments

Strontium isotopic compositions were determined for silicate detritus in 152 deep-sea and continental shelf surface sediment samples from the Pacific Ocean to trace their provenance. The major characteristics of the spatial variation of the Sr isotopes and the interpretation of this variation may be summarized as follows: (1) Sediments in the Pacific have a lower average87Sr/86Sr ratio (0.7112) with a narrower range (0.703–0.724) than in the Atlantic (0.7166; 0.704–0.743). Many western Pacific volcanoes together with young continental crust supply material with low87Sr/86Sr ratios to the Pacific, in contrast to the large old cratonic crust that supplies material with high Sr ratio to the Atlantic. (2) The87Sr/86Sr ratio is high in the north central Pacific (0.711–0.719). The Izu-Ogasawara and Mariana arcs supply low87Sr/86Sr material to the western Pacific, but farther to east the abundant supply of loess carried by middle-latitude westerlies from the Asian continent (87Sr/86Sr=ca.0.720) dominates the north central Pacific.

Provenance of the north Pacific sediments and process of source material transport as derived from Rb–Sr isotopic systematics

Abstract Rb–Sr isotopic systematics of 111 samples of sediments in nine cores from the north Pacific of Quaternary and Pliocene ages have been investigated. They provide information on the provenance, the process of particle transport and temporal variation in the flux of source material. The Rb–Sr isotopic systematics of the core sediments show well-correlated pseudo isochrons. The pseudo isochrons reflect the mixing of two types of material, i.e., the Asian continental material with high 87 Rb / 86 Sr ratios (4.5–6.5) and high 87 Sr / 86 Sr ratios (0.723–0.727) and the volcanic material with low 87 Rb / 86 Sr ratios and low 87 Sr / 86 Sr ratios, from island–arc volcanics such as the Izu–Ogasawara–Mariana and the Japanese Islands and oceanic islands such as the Hawaiian Islands (0.0–2.0; 0.703–0.708). The clearness of the pseudo isochrons implies that the Rb–Sr isotopic composition of the weathering products derived from the Asian continental crust is extremely homogenized. The fine fraction (a few μm) with a high 87 Rb / 86 Sr ratio (5.0–6.5) is widely transported into the north Pacific by the middle-latitude westerlies and contributes largely to pelagic sediments. The coarser fraction with a lower 87 Rb / 86 Sr ratio (

Characterization and geochemical behaviour of uranium series nuclides in sandy sediments at Kanamaru area, Yamagata and Niigata Prefectures, Japan

Uranium series nuclides in the Tertiary arkosic sands from a high rainfall area in Japan were determined and their characteristics and behaviour were studied. Uranium was concentrated in the upper (c. 241 ppm) and lower (c. 581 ppm) layers although no specific U mineral was observed. The filtration experiment showed that U in the groundwater is in the dissolved form rather than colloidal or particulate forms. Most U in the concentrated layers was in easily soluble fractions (such as AcONa/AcOH soluble) while the U content in the water around the site was low. The fact that the 230Th/234U activity ratios of a sample in the upper layer were much greater than unity suggests that U migration occurred within 0.3 Ma. However, the 234U/238U and 230Th/234U activity ratios of samples in the lower layer were close to unity, suggesting that the material has not been strongly leached. Using chemical leaching techniques, a small disequilibrium among the fractions is observed, even though the bulk sample was near equilibrium. It is inferred that sorption of U is more effective than precipitation in this area.

Natural attenuation of dissolved uranium within a small stream of central Japan

In this study, we investigated the natural attenuation of the uranium (U) load in the surface water within a humid forest in Japan. Surface water and sediments that had accumulated behind dams in the area were investigated in terms of their mineralogy and chemistry. The chemistry of the surface water was analyzed by field measurements of pH, dissolved oxygen (DO) concentration, and electrical conductivity (EC), as well as laboratory analyses via inductively coupled plasma mass spectrometry and ion chromatography. We determined U levels in the surface water; the distribution of U in phases within the sediment was estimated using a sequential extraction procedure. The results of this investigation indicate that U, which within the study area is derived from pegmatites at a mine, is attenuated by uptake onto the surface of organic material and uptake by amorphous material that forms over time within the dammed sediments. The U concentration within the sediment was as great as 8 mg kg−1, whereas the downstream U concentrations and loads in surface water decreased from 1.69 μg l−1 and 11.0 mg min−1 to 0.115 μg l−1 and 2.31 mg min−1, respectively.