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Marine redox evolution in the early Cambrian Yangtze shelf margin area: evidence from trace elements, nitrogen and sulphur isotopes

Part of: From Snowball Earth to the Cambrian Explosion: Evidence from China

Published online by Cambridge University Press:  22 March 2017

GUANG-YI WEI ,
HONG-FEI LING ,
DA LI ,
WEI WEI ,
DAN WANG ,
XI CHEN ,
XIANG-KUN ZHU ,
FEI-FEI ZHANG  and
BIN YAN
GUANG-YI WEI
Affiliation:
State Key Laboratory for Mineral Deposits Research, School of Earth Science and Engineering, Nanjing University, Nanjing 210023, China
HONG-FEI LING*
Affiliation:
State Key Laboratory for Mineral Deposits Research, School of Earth Science and Engineering, Nanjing University, Nanjing 210023, China
DA LI
Affiliation:
State Key Laboratory for Mineral Deposits Research, School of Earth Science and Engineering, Nanjing University, Nanjing 210023, China
WEI WEI
Affiliation:
State Key Laboratory for Mineral Deposits Research, School of Earth Science and Engineering, Nanjing University, Nanjing 210023, China
DAN WANG
Affiliation:
MLR Key Laboratory of Isotope Geology, State Key Laboratories of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
XI CHEN
Affiliation:
State Key Laboratory for Mineral Deposits Research, School of Earth Science and Engineering, Nanjing University, Nanjing 210023, China
XIANG-KUN ZHU
Affiliation:
MLR Key Laboratory of Isotope Geology, State Key Laboratories of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
FEI-FEI ZHANG
Affiliation:
School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6004, USA
BIN YAN
Affiliation:
MLR Key Laboratory of Isotope Geology, State Key Laboratories of Continental Tectonics and Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
*
†Author for correspondence: [email protected]
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Abstract

Nitrogen is an essential element for biological activity, and nitrogen isotopic compositions of geological samples record information about both marine biological processes and environmental evolution. However, only a few studies of N isotopes in the early Cambrian have been published. In this study, we analysed nitrogen isotopic compositions, as well as trace elements and sulphur isotopic compositions of cherts, black shales, carbonaceous shales and argillaceous carbonates from the Daotuo drill core in Songtao County, NE Guizhou Province, China, to reconstruct the marine redox environment of both deep and surface seawater in the study area of the Yangtze shelf margin in the early Cambrian. The Mo–U covariation pattern of the studied samples indicates that the Yangtze shelf margin area was weakly restricted and connected to the open ocean through shallow water flows. Mo and U concentrations, δ15Nbulk and δ34Spy values of the studied samples from the Yangtze shelf margin area suggest ferruginous but not sulphidic seawater and low marine sulphate concentration (relatively deep chemocline) in the Cambrian Fortunian and early Stage 2; sulphidic conditions (shallow chemocline and anoxic photic zone) in the upper Cambrian Stage 2 and lower Stage 3; and the depression of sulphidic seawater in the middle and upper Cambrian Stage 3. Furthermore, the decreasing δ15N values indicate shrinking of the marine nitrate reservoir during the middle and upper Stage 3, which reflects a falling oxygenation level in this period. The environmental evolution was probably controlled by the changing biological activity through its feedback on the local marine environment.

Keywords

Yangtze shelf marginearly Cambrianlocal redox conditionnitrogen cyclebiological feedback
Type
Original Articles
Information
Geological Magazine , Volume 154 , Issue 6: THEMATIC ISSUE: From Snowball Earth to the Cambrian Explosion: Evidence from China , November 2017 , pp. 1344 - 1359
Copyright
Copyright © Cambridge University Press 2017 

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