Gas hydrate volume% filled in pore space of sediments by in situ bacterial methane production is calculated as a function of total organic carbon (TOC) contents in sediments, assuming that all the excess amount of methane beyond the solubility in pore water can form the hydrate under ordinary conditions of outer continental margins. The results suggest that at least 0.5% TOC is required for the hydrate formation. Average volume of gas hydrates filled in pore space of hydrate-bearing sediments is estimated to be about 5∼6% at a site on the Blake Ridge (Matsumoto et al., 1996). TOC required to fill the 5% pore volume as gas hydrate is calculated to be about 2% in the case of water depth of 3000 m and utilizable organic carbon for methanogenesis of 10%. This TOC value is comparable to the measured TOC values in the sediments at the site (0.8 to 2.3%, average 1.4%; Shipboard Scientific Party, 1996b). Therefore, hydrate formation by in situ bacterial methanogenesis can roughly explain the average amount of gas hydrates in the sediments. However, for the formation of locally concentrated massive gas hydrates, some accumulation processes are required. Accumulation of gas hydrates near the base of gas hydrate stability zone (BGHS) is possible by the recycling of methane and migration of methane from depths below the BGHS.

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The partition coefficients of rare earth elements (REE) and Sr between olivine and silicate melt and those of Ga, In and TI between metallic phase and silicate melt were experimentally determined under atmospheric pressure using L6 chondrite (ALH-76009) as a starting material. Temperature and oxygen fugacity were finely controlled; temperature = 1320, 1370, 1420 and 1440°C and oxygen fugacity = 0.5, 1.0, 1.5 and 2.0 log units lower than the Fe-FeO boundary on the iron-oxygen phase diagram. Olivine formed at 1320°C shows REE partition patterns with horizontal or heavy-REE-enriched features, having partition coefficients from 0.02 to 0.08. Inclination of light-REE increases with increase of temperature of olivine formation. Finally, at the temperature of 1440°C near the liquidus of the chondrite, olivine/liquid partitioning of REE displays a V-shaped feature with partition coefficients of 0.15-0.8 for La, 0.08-0.3 for Lu and 0.05-0.09 for Dy and Er. The V-shaped REE patterns are known to be characteristic of olivine in the main group pallasites, for which our REE results imply the formation at higher temperatures near the liquidus of olivine. Our results of major element partitioning also support the idea that such pallasites were formed at a temperature near the liquidus. Experiments of this study show that compositions of olivine formed at 1440°C are Fa₈-Fa₁₃ comparable to those of the main group pallasites, while those formed at 1320°C are Fa₁₅-Fa₁₈. The REE concentrations in the ferrous olivine of two pallasites, Springwater (Fa_18.0) and Eagle Station (Fa_20.0), were determined here. It was found that their chondrite-normalized REE patterns also show V-shaped features, as observed for the olivine of the main group pallasites, the Brenham (Fa_12.5) and Thiel Mts. (Fa_13.0) pallasites. These results suggest that the olivines of Springwater and Eagle Station were also formed at temperatures near liquidus around 1450°C. The difference in major element compositions between the main group pallasites (Brenham and Thiel Mts.) and the other pallasites (Springwater and Eagle Station) may be due to differences in major element compositions of their sources, oxygen fugacity at their formation and/or the cooling history of the pallasites. In the last case, it is reasoned that at lower temperatures (e.g., ca. 1300°C), Fe and Ni in olivine of non-main group pallasites were reequilibrated with those in the metal phases, but V-shaped REE patterns were retained because the diffusibility of REE is lower than those of Fe and Ni. The main group pallasites are considered to be free from the lower temperature reequilibrium processes.

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The excess ²¹⁰Pb dating method was applied to two young straw stalactites collected from a cave in southern Okinawa Island, Japan. The ²¹⁰Pb profiles only in the matrix of the straws exhibited an exponential decrease with distance from the tip. The ²¹⁰Pb profiles of the two straw samples gave the values of 2.2 and 5.9 mm/y for longitudinal growth rates. The excess ²¹⁰Pb is distributed uniformly over the outer and inner surfaces of the straws. Such an excess of ²¹⁰Pb is presumably produced from the airborne ²²²Rn which exists with high concentrations in the cave air and the ground water running through the central channel of the straws. During growing process of the straws, a part of the ²¹⁰Pb will be adsorbed on the tip of straw, and they are supposed to be incorporated into matrix part during reprecipitation of CaCO₃.

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This study investigates the distribution and enrichment of trace metals in suspended and sinking particulate matter from southern East China Sea (ECS) north of Taiwan during the period of April 1992 to April 1993. According to these results, concentrations of suspended particulate matter (SPM) in the inner shelf of southern ECS, the upwelling-influenced shelf break, and Kuroshio water are 1.2 (surface)-4.2 (bottom) mg/l, ca. 0.3-0.5 mg/l, and 0.1-0.3 mg/l, respectively, reflecting various influences of terrestrial inputs. A benthic nepheloid layer (BNL), apparently owing to resuspension of local and/or remote bottom sediments, formed over the shelf region. Temporal variation in trace metal contents and enrichment in suspended matter from the shelf region reflect the variability of metal inputs from Chinese rivers, particularly from Changjiang runoff. The abundance of a particulate metal is largely determined by the SPM content. Hence, the abundant profiles of most particulate metals closely correspond to the distribution pattern of SPM. However, concentrations of most particulate metals (except for Al and Fe), based on the dry weight of SPM, increase from the China coastal water to the Kuroshio water. The enriched metals are more likely to have derived from terrestrial inputs, rather than solely from biological accumulation. In addition, a decrease in metal contents and an increase in salinity confirm the transport of suspended particulate metals from the ECS shelf to the open ocean. Such an occurrence is verified by the feature of metal plume in the intermediate layer (550-800 m) of Kuroshio water. Moreover, the sinking particles collected from sediment traps deployed on the upper slope are relatively enriched in lithogenic matter and trace metals. Terrestrial inputs apparently control the distribution of most trace metals in suspended and sinking particulates in the southern ECS.

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We measured ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios and carried out analytical test of Sr and Nd isotopic standards for the seven GSJ rock reference samples (hereafter GSJ-RRSs); JA-1, JB-1a, JB-2, JB-3, JG-1a, JGb-1 and JR-1. Our mean values of five separate analyses for the GSJ-RRSs are summarized in Table 1, in agreement with the ranges of previously published data. Our mean values for ⁸⁷Sr/⁸⁶Sr ratios of JB-2 and JGb-1 and ¹⁴³Nd/¹⁴⁴Nd ratio of JR-1, however, show a slightly large variation as compared with the other JGS-RRSs. Further data accumulation with specification of split/position number is required to evaluate homogeneity in Sr and Nd isotopic compositions.

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