GEOCHEMICAL JOURNAL
Online ISSN : 1880-5973
Print ISSN : 0016-7002
ISSN-L : 0016-7002
34 巻 , 4 号
選択された号の論文の6件中1~6を表示しています
  • Hodaka Kawahata, Toshio Ishizuka
    2000 年 34 巻 4 号 p. 247-261
    発行日: 2000/08/20
    公開日: 2008/04/08
    ジャーナル フリー
    Biogenic calcareous and siliceous sediments were drilled at ODP Sites 689 and 690 on the Maud Rise, Antarctic Ocean. We analyzed dissolved combined amino acids (DCAA) and dissolved free amino acids (DFAA) in interstitial waters in order to characterize the amino acids in dissolved organic matter. The DFAA was predominant over the DCAA in interstitial waters at Sites 689 and 690, which contradicted the previous results from interstitial water and seawater studies. The DCAA in the interstitial waters probably originated from calcareous biogenic debris with less amounts of siliceous debris. Although glutamic acid constituted 41% of the total concentration of DCAA, it accounted for only 1% of the total concentration of DFAA due to the adsorption and/or reaction with biogenic carbonate. Ornithine, a nonprotein amino acid, is a decomposed product of arginine and made up 17 mol% of the total DFAA and. The total hydrolyzable amino acids (=DCAA + DFAA) accounted for 5 to 28% of the dissolved organic carbon (DOC) concentration, which implied that high molecular weight organic matter was a major contributor for the DOM (dissolved organic matter) in interstitial waters. Fairly positive correlation between the dissolved manganese and the total DCAA values suggested that the redox condition plays a significant role in controlling the total DCAA content. A small decrease in the sulfate concentration in the interstitial waters from both sites suggested fairly low microbial activity by sulfate-reducing bacteria.
  • Yuelong Chen, Zhongfang Yang
    2000 年 34 巻 4 号 p. 263-270
    発行日: 2000/08/20
    公開日: 2008/04/08
    ジャーナル フリー
    Sm-Nd isotopic compositions of Late Neoproterozoic (Sinian), Early Cambrian, Early Ordovician, Late Triassic, and Jurassic sedimentary rocks from the northwestern Yangtze Craton, Guangyuan, Sichuan province, China show that the averaged Nd model age is 1.9 Ga. Based on Nd model ages (1.0 Ga to 3.3 Ga) combining geochronological data, three end-members are identified, which are represented by ca. 3.3 Ga, and 1.0 Ga, and 2.0 Ga respectively. The 2.0 Ga is postulated from geographical distribution of Nd model ages and partly geochronological data. Because the highest εNd(t) values only occurred in Cambrian and Ordovician sedimentary rocks, Early Paleozoic was a very important stage of orogen, which led to newly formed crustal end-member elevated and denudated widely. The 1.0 Ga growth episode occurred in the circum-Yangtze Craton principally.
  • Naoyoshi Iwata, Ichiro Kaneoka
    2000 年 34 巻 4 号 p. 271-281
    発行日: 2000/08/20
    公開日: 2008/04/08
    ジャーナル フリー
    Based on the 40Ar-39Ar results for Deccan Traps samples, we have investigated the relationships between the 40Ar-39Ar age release pattern and conditions of basaltic samples. 40Ar-39Ar age release patterns are divided into five types, which might be related to the conditions of samples such as their petrological textures and alteration state. A peak of non-radiogenic component of Ar is observed at the temperature around 800°C in the Ar release patterns of Deccan Traps samples. Based on the above observations, criteria for sample selection and data treatment for the 40Ar-39Ar dating are summarized as follows. (1) It is recommended to use fresh samples as much as possible and minimize the use of samples which contain mesostasis. (2) Fine-grained holocrystalline rocks are preferable to get reasonable 40Ar-39Ar ages. (3) If a sample is affected by secondary alteration and shows a high 36Ar release in the intermediate temperature fractions, in age calculation it is better to exclude the isotopic data for the temperature fractions lower than about 800°C where the high 36Ar release is observed.
  • Y. Ternois, K. Kawamura, N. Ohkouchi, L. Keigwin
    2000 年 34 巻 4 号 p. 283-293
    発行日: 2000/08/20
    公開日: 2008/04/08
    ジャーナル フリー
    A sediment core collected from the Sea of Okhotsk, a marginal sea of the western North Pacific, has been analyzed for alkenones to reconstruct changes in the sea surface temperatures (SST) over the last 15 kyrs. A comparison of the core-top U37K' value with the modern surface temperatures indicates that alkenones are produced at present in summer-fall season, a production seasonality probably identical to the one prevailing in the past, as suggested by evidences of enhanced sea ice cover and subsequent limitation of the phytoplankton growth period to summer-fall at the glacial/deglacial transition. The alkenone downcore profile indicated that summer SST at the early deglaciation period (15 kyrs BP) were around 5°C lower than today (ca. 11°C). A rapid SST increase (more than 3°C) was found to occur from 13.7 to 11.6 kyrs BP, following the melting water pulse event (MWP-1A). This warming was assumed to be the consequence of a radical change in the atmospheric/oceanic circulations which induced an enhanced heat transport from the sub-tropical to the northern North Pacific. A significant surface water cooling was evidenced from 11 to 8 kyrs BP, possibly caused by the supply of cold Siberian permafrost melt-water to the Okhotsk Sea through the Amur River.
  • R. Ramesh, Al. Ramanathan, S. Ramesh, R. Purvaja, V. Subramanian
    2000 年 34 巻 4 号 p. 295-319
    発行日: 2000/08/20
    公開日: 2008/04/08
    ジャーナル フリー
    Rare earth elements (REEs) are considered as useful tracers of various surface geological processes in the river system. REEs and heavy metals (V, Pb, Cr, Co, Ag, Zn, Cd, Ni) were analyzed from the suspended sediments of Ganges and surficial sediments of Yamuna, Brahmaputra, Jamuna, Padma and Meghna, using VG Thermal Ionization Mass Spectrometer. Physical weathering process seems to be a major controlling factor for the distribution of REE and trace metals in the sediments of the Himalayan rivers. Weathering of the surface crustal area in the drainage basin shows significant variations due to changes in lithology and influence of tributaries. The distribution of REE shows an almost uniform pattern due to factors such as river transportation processes and high level of terrigenous mixing in the bed sediments. The results show that finer grain size and high content of clay mineral (illite) in these sediments are possible traps for the accumulation of metals. Anthropogenic activities seem to have very little influence in controlling the elemental distribution in the Himalayan rivers.
  • A. Suzuki, H. Kawahata, Y. Tanimoto, H. Tsukamoto, L. P. Gupta, I. Yuk ...
    2000 年 34 巻 4 号 p. 321-329
    発行日: 2000/08/20
    公開日: 2008/04/08
    ジャーナル フリー
    Based on skeletal oxygen and carbon isotope analyses, we found a remarkable decrease in skeletal linear extension rate of a Porites colony during the bleaching event of 1998 around the Ryukyu Islands. Water temperature in the summer of 1998 was recorded as the highest for at least last 50 years in the region. This colony, growing in a channel of fringing reef in Ishigaki Island, has experienced complete bleaching, as evidenced from direct observations of discoloration of the colony. Growth increment between the summer of 1997 and winter of 1997–98 is about 7 mm long while the corresponding increment between the summer of 1998 and winter of 1998–99 is only 2 mm long. Since the surface colonization by algae and excavating sponges cannot be recognized by visual inspection of X-radiographs, the slow growth might have been caused by the temporal perturbations in coral-algal symbiotic system, which inhibits calcification. The shift of skeletal carbon isotope ratios toward negative during bleaching also suggests the decrease of photosynthetic activity of symbiotic algae. Presumably, high temperature during the summer of 1998 could have led to massive decrease in carbonate production by coral reefs around the Ryukyu Islands through a large-scale coral bleaching event.
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