The stepwise increase of continental weathering in the Ediacaran and early Cambrian: evidenced from radiogenic Sr isotope excursion in the strata at Three Gorges, South China.

Abstract:

The Ediacaran to Cambrian period records one of the most dramatic biological episodes in Earth’s history. The weathering influx from continents is thought to be a major influence on the change in composition of ancient seawater and on biological evolution. Its flux can be constrained from the ⁸⁷Sr/⁸⁶Sr ratio of ancient carbonate rocks. 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 contain many fossils. Therefore, they are suitable for study of a link between environmental and biological changes in the Ediacaran. We conducted on-land drilling from Liantuo, via Nantuo, Doushantuo, Dengying and Yanjiahe to Shuijingtuo formations at four sites in the Three Gorges area to obtain continuous, fresh samples. We analyzed radiogenic Sr isotope of the fresh carbonate rocks with a MC-ICP-MS at Kyoto University.

The ⁸⁷Sr/⁸⁶Sr excursion preserved in the drilled samples displays a smooth curve and three large positive shifts in the Ediacaran. The first large positive excursion occurred together with negative δ¹³C and positive δ¹⁸O excursions. We interpret that 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 negative δ¹³C excursion. The higher ⁸⁷Sr/⁸⁶Sr values are the first compelling evidence for enhanced continental weathering in the Ediacaran, which was responsible for the large δ¹³C anomaly through the re-mineralization of the dissolved organic carbon (DOC) reservoir 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 locally high continental weathering influx caused the re-mineralization of the dissolved organic carbon (DOC) reservoir at the beginning of the Cambrian.