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Paleoceanographic Indicators for Early Cambrian Black Shales from the Yangtze Platform, South China: Evidence from Biomarkers and Carbon Isotopes
Author(s) -
Lan CHEN,
Hong ZHONG,
Ruizhong HU,
Jiafei XIAO,
Louis LoungYie TSAI,
Andrew TienShun LIN,
Yanrong ZOU
Publication year - 2012
Publication title -
acta geologica sinica ‐ english edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.444
H-Index - 61
eISSN - 1755-6724
pISSN - 1000-9515
DOI - 10.1111/j.1755-6724.2012.00737.x
Subject(s) - sedimentary depositional environment , geology , anoxic waters , sterane , oil shale , total organic carbon , maturity (psychological) , isotopes of carbon , geochemistry , marine transgression , paleontology , hopanoids , δ13c , abundance (ecology) , source rock , stable isotope ratio , environmental chemistry , oceanography , ecology , chemistry , structural basin , psychology , developmental psychology , physics , quantum mechanics , biology
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 C 19 or C 20 (C 23 for sample Gzh00–1). The C 27 /C 29 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 δ 13 C org 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.