Paleoceanographic Indicators for Early Cambrian Black Shales from the Yangtze Platform,South China: Evidence from Biomarkers and Carbon Isotopes |
| |
Authors: | CHEN Lan ZHONG Hong HU Ruizhong XIAO Jiafei TSAI Louis Loung-Yie LIN Andrew Tien-Shun and ZOU Yanrong College of Petroleum Engineering Chongqing University of Science and Technology Chongqing China |
| |
Institution: | key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou 550002, China 5 State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, Guangdong 510640, China |
| |
Abstract: | The lower Cambrian Niutitang Formation, a widespread black shale deposition, is of geological interest because of its polymetallic beds, Cambrian explosion, depositional ages, dramatic environmental changes and so on. Previous study focused mainly on inorganic geochemistry and few studies have investigated the organic fractions of upper Neoproterozoic‐lower Cambrian strata in South China. Here we report a study of biomarkers plus organic carbon isotopes for black shales from Ganziping, Hunan Province (China). All the saturated hydrocarbon fractions have a unimodal distribution of n‐alkanes, a high content of short‐chain alkanes and maximize at C19 or C20 (C23 for sample Gzh00–1). The C27/C29 sterane ratio ranges from 0.77 to 1.20 and 4‐methylsteranes are in low abundance. These parameters indicate that algae and bacteria are the important primary producers. Furthermore, biomarker maturity proxies show the samples to be higher maturity. The low Pr/Ph values (<0.7) suggest that the samples were deposited under anoxic conditions and, likely, under stratified water columns. In addition, 25‐norhopanes and gammacerane are present as diagnostic indicators of normal marine salinity and dysoxic to anoxic conditions. During the Early Tommotian, known to coincide with a transgression event, small shelly fossils increased in abundance and diversity. Moreover, positive δ13Corg excursions close to 1.4‰ occur at the base of the Tommotian stage. In summary, the Early Cambrian black shales were deposited under dramatic paleoenvironmental changes, including oceanic anoxia, higher primary productivity and sea‐level rise. |
| |
Keywords: | black shales biomarkers carbon isotope paleoceanography Early Cambrian South China |
本文献已被 CNKI 等数据库收录! |
| 点击此处可从《Acta Geologica Sinica》浏览原始摘要信息 |
| 点击此处可从《Acta Geologica Sinica》下载免费的PDF全文 |
|