Chemostratigraphy of the Ediacaran basinal setting on the Yangtze platform, South China: Oceanographic and diagenetic aspects of the carbon isotopic depth gradient. (23rd June 2017)

• Record Type: Journal Article
• Authors: Furuyama, Seishiro; Kano, Akihiro; Kunimitsu, Yoko; Ishikawa, Tsuyoshi; Wang, Wei; Liu, Xin‐chun
• Is Part Of: Island arc. Volume 26:Number 5(2017)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2017-06-23
• Subjects: carbon isotope -- chemostratigraphic correlation -- diagenesis -- Ediacaran -- South China -- strontium isotope; Plate tectonics -- Periodicals; Island arcs -- Periodicals; Geodynamics -- Periodicals; 551.136
• DOI: 10.1111/iar.12196
• Languages: English
• ISSNs: 1038-4871

Abstract

Abstract: The Ediacaran Yangtze platform in South China, which represents depositional settings ranging from coastal to basinal, provides valuable information for understanding climate changes and animal evolution during the Ediacaran Period. Although the shallower settings have been investigated, research on the basinal sections has been limited. This has hampered efforts to establish stratigraphic correlations and understand the oceanographic setting of the Yangtze platform. In this paper, the chemostratigraphy of a basinal section at Fengtan, Hunan Province, is reported based on analyses of stable carbon isotope profiles in carbonates (δ 13 Ccarb ), organic matter (δ 13 Corg ), total organic carbon, 87 Sr/ 86 Sr ratios, and Mn, Rb, and Sr concentrations. The basinal section of the Doushantuo Formation, which is represented at Fengtan, provides data supporting regional correlations and oceanography. Three intervals in the Doushantuo Formation are correlated with the Three Gorges: (i) a negative δ 13 Ccarb anomaly with stable δ 13 Corg values and altered 87 Sr/ 86 Sr ratios in the lower section can be correlated to the boundary between Doushantuo Members 2 and 3 (Interval A); (ii) a relatively high δ 13 Ccarb anomaly with unaltered 87 Sr/ 86 Sr ratios (up to 0.7086) in the middle section corresponding to the lower part of Doushantuo Member 3 (Interval B); and (iii) a negative δ 13 Ccarb anomaly with lowered δ values in the upper section can be correlated to the long interval of negative δ 13 Ccarb (Interval C). The Gaskiers glaciation is likely represented in Interval A, and Interval C corresponds to the Shuram excursion reported for other Ediacaran localities. Our correlations confirm the depth gradient of δ 13 Ccarb in the Yangtze platform and imply that reductive conditions prevailed in the basinal section from the Early to Middle Ediacaran. Under such conditions, anaerobic degradation of organic carbon or methane perturbed the inorganic carbon isotopic compositions and was at least partly responsible for the depth gradient of δ 13 Ccarb . Abstract : The chemostratigraphy of a basinal section of Ediacaran Yangtze platform is reported based on analyses of some isotopes. Our correlations confirm the depth gradient of inorganic carbon isotope in the Yangtze platform and imply that reductive conditions prevailed in the basinal section from the Early to the Middle Ediacaran. Abstract : 南中国の揚子地塊上には, 浅海域〜海盆域で堆積したエディアカラ系が広く分布しており, 多細胞動物進化と古環境変動の関係を解明する上で格好のフィールドとなっている. しかし海盆域からのデータは限定的で, 浅海域との層序対比が確立されていない. そこで本研究では浅海域−海盆域間の層序対比を目的として, 海盆域で堆積した中国湖南省鳳灘地域の下部エディアカラ系陡山沱層を対象に, 有機・無機炭素及びストロンチウム同位体比による同位体層序を明らかにした. これらを浅海域(湖北省三峡地域)で報告されているものと比較した結果, 負の無機炭素同位体比異常が認められる鳳灘地域の陡山沱層下部をガスキエス氷期(582 Ma)とされる層準に, 同層上部をシュラムエクスカーションとされる層準に, それぞれ対比できた. 両地域間で無機炭素同位体比に深度勾配が認められることから, 当時の海盆域は還元的環境に支配されており, 有機物やメタンの嫌気的分解で放出された炭素が両地域の無機炭素同位体比に影響したと考えられる.