2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 26-6
Presentation Time: 9:15 AM

THE RADIOGENIC SR ISOTOPE EXCURSION DURING THE EDIACARAN AND EARLY CAMBRIAN


SAWAKI, Yusuke, Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo, 152-8551, Japan, MARUYAMA, Shigenori, Earth-Life Science Institute, Tokyo Institute of Techology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan and SATO, Tomohiko, Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo, 152-8551, Japan

The Ediacaran to Cambrian period records one of the most dramatic biological episodes in Earth’s history. However, the scarcity of well-preserved outcrops of Ediacaran rocks still leaves ambiguity in decoding ambient surface environmental changes and biological evolution.

The Ediacaran strata in South China are almost continuously exposed, comprise mainly carbonate rocks with subordinate black shales and sandstones, and they contain many fossils, suitable for study of environmental and biological changes in the Ediacaran. We conducted drilling from the Liantuo, via the Nantuo, Doushantuo, Dengying and Yanjiahe to the Shuijingtuo formation at four sites in the Three Gorges area to obtain continuous, fresh samples without surface alteration and oxidation. We analyzed 87Sr/86Sr of the fresh carbonate rocks with a MC-ICP-MS.

The chemostratigraphy of the 87Sr/86Sr ratios of the drilled samples displays a smooth curve and three large positive shifts during Ediacaran time. The first large positive excursion of 87Sr/86Sr occurred together with negative δ13C and positive δ18O excursions. Global regression due to global cooling enhanced the oxidative decay of exposed marine organic sediments and continental weathering.

The second large positive shift of 87Sr/86Sr firstly accompanied a positive δ13C excursion, and continued through the Shuram δ13C negative excursion. The higher 87Sr/86Sr values are the first compelling evidence for enhanced continental weathering, which was responsible for the large δ13C anomaly through the remineralization of the DOC by more active sulfate reduction due to a higher sulfate influx.

The 87Sr/86Sr ratios display a large positive anomaly just below the PC/C boundary at the lower part of Yanjiahe Formation. The data indicates that the 87Sr/86Sr excursions preceded the δ13C negative excursion, and suggests that global regression or formation of the Gondwana supercontinent, possibly together with a high atmospheric pCO2, caused biological depression.

Session No. 26

T236. The Ediacaran-Cambrian Ecosphere (R)evolution: Emerging Records from Central and East Asia
Sunday, 19 October 2014: 8:00 AM-12:00 PM
207 (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.86
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