International Geologiical Congress - Oslo 2008

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HPS-01 General contributions to stratigraphy

 

Rare earth and other trace element in biogenic fossils from the Cambrian-Ordovician Boundary section

 

Peng Shiming Peng , School of Life Science (China)
Wang Yuanyuan, Chengdong College (China)
Wang Yinxi* , Center of Modern Analysis (China)
Li Huiming, Center of Modern Analysis (China)
 

 

Introdution
Currently a high attention has been paid to the biogenic apatite because it retains greatly enriched but unfractionated geochemical signatures of the sea water under which the fossil apatite was deposited. Aside from on sample belonging to an undetermined fossil group, all the 14 sample under the present investigation are inarticulate brachiopods of the family Acroteretidae which were found in the acetic acid insoluble residues from micritic limestone beds. Analytical method All the REE and trace elements are measured with ICAP-AES instrument except Sr and Rb which are investigated by TIMS isotopic dilution analysis.
Discussion of Result
In a summary, a notable tendency of enrichment in REE and other trace elements as indicated in the text-fig and Table occur in the critical interval at or slightly below the proposed Cambrian-Ordovician Boundary level and its overlying 3.3m interval. The REE patterns of 14 inarticulate brachiopod samples all bear a strong resemblance to each other, indicating a dominating stable pale environment in the duration of the studied interval, and also assuming an enclosed geochemical environment which might have been involved in the host rocks of the biogenic apatite fossil samples after their sedimentation. The greatest enrichment of REE and other trace elements appear in a 4m vertical interval across the proposed Cambrian-Ordovician Boundary, which seems related to a significant chemical event. Ce abnormal values of all the investigated samples are higher than -0.10, suggesting the dominant anoxic conditions of water mass in times of the investigated interval from the Proconodontus Zone to the lower part of the Cordytodus lindstromi Zone.
Conclusions
Ce abnormal values fluctuate conspicuously near to but slightly above the proposed Cambrian-Ordovician boundary. A possible chemical event represented by the greatest enrichments of rare earth and other trace elements together with the rising of Ce abnormal values occurred in coincidence with remarkable biotic events in a short period probably less than on million years during the Cambrian-Ordovician interval, and both the chemical and biological events seem intimately related to each other as cause and effect.
Reference
Wang Yinxi, Yang Jiedong ,Tao Xiangchong and Li Huiming. 1988. A study of the Sm-Nd method for fossil mineral and rock and its applications. Journal of Nanjing University (Natural Sciences Edition), 24(2): 297-308 (In Chinese with English abstract)

 

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