Ca isotopic compositions of dolomite, phosphorite and the oldest animal embryo fossils from the Neoproterozoic in Weng'an, South China |
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| Authors: | Tsuyoshi Komiya Akinobu Suga Tsuyoshi Ohno Jian Han Junfeng Guo Shinji Yamamoto Takafumi Hirata Yong Li |
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| Institution: | aDepartment of Earth and Planetary Sciences, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo 152-8551, Japan;bDepartment of Geology and Key Laboratory for Continental Dynamics, Northwest University, Xi'an 710069, China;cSchool of Earth Sciences and Resources Management, Chang'an University, Xi'an 710054, China |
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| Abstract: | Deciphering the drastic changes of surface environment and the emergence of animals after the Neoproterozoic Snowball Earth event are important for understanding the changes of surface environment and its influence on the evolution of life through geologic time. Especially, the emergence of two types of Metazoan animals such as animal embryo fossils, cnidarians or sponges, and Ediacaran fauna in the late Neoproterozoic was one of the critical turning points in the biological evolution.Calcium is one of the essential elements for the growth of most animals. In this study, in order to evaluate the Ca cycles in the Neoproterozoic, we have measured Ca isotopic ratios (44Ca/42Ca and 43Ca/42Ca) for phosphorite, dolostone and phosphatic animal embryo fossils with a multiple collector, inductively coupled plasma-mass spectrometer (MC-ICP-MS). The resulting 44Ca/42Ca ratio defined by the relative deviation from the ratio of NIST SRM915a (δ44/42CaNIST915) for phosphorite and dolostone ranges from 0.83 to 0.95‰, demonstrating that the fractionation between phosphorite/dolostone and seawater was very small. This evidence indicates that at the emergence of the Weng'an biota seawater was deficient in Ca probably due to mass deposition of phosphorite/dolostone and to the beginning of Ca-biomineralization.Three phosphatic animal embryo fossils have lower δ44/42Ca values than the phosphorite and dolomite, implying that the precursor of the phosphatic embryo fossils was able to fractionate Ca isotopes through Ca-biomineralization, consistent with marine gastropods. |
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| Keywords: | Ca stable isotopes Neoproterozoic Weng'an Metazoan Snowball Earth Phosphorite |
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