Sr isotope excursion across the Precambrian-Cambrian boundary in the Three Gorges area, South China

摘要

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life. However, the scarcity of well-preserved outcrops across the boundary leaves an obstacle in decoding surface environmental changes and patterns of biological evolution. In south China, strata through the PC/C boundary are almost continuously exposed and contain many fossils, suitable for study of environmental and biological change across the PC/C boundary. We undertook deep drilling at four sites in the Three Gorges area to obtain continuous and fresh samples without surface alteration and oxidation. Sr-87/Sr-86 ratios of the fresh carbonate rocks, selected based on microscopic observation and geochemical signatures of Mn/Sr and Rb/Sr ratios, aluminum and silica contents, and delta C-13 and delta O-18 values, were measured with multiple collector-inductively coupled plasma-mass spectrometric techniques. The chemostratigraphy of Sr-87/Sr-86 ratios of the drilled samples displays a smooth curve and a large positive anomaly just below the PC/C boundary between the upper part of Baimatuo Member of the Dengying Formation and the lower part of the Yanjiahe Formation. The combination of chemostratigraphies of delta C-13 and Sr-87/Sr-86 indicates that the Sr-87/Sr-86 excursions preceded the delta C-13 negative excursion, and suggests that global regression or formation of the Gondwana supercontinent, possibly together with a high atmospheric pCO(2), caused biological depression.