MULTIPLE S-ISOTOPIC RECORD ACROSS THE SILURIAN-DEVONIAN BOUNDARY IN SOUTH CHINA: IMPLICATIONS FOR THE END-SILURIAN BIOTIC CRISIS

Fossil records indicate that there was a biotic crisis across the Silurian-Devonian (S-D) boundary. Several studies have shown a major positive excursion in δ¹³C recorded within the S-D boundary and discussed their implications for seawater chemistry changes and their possible link to the biotic crisis. However, the causes of the Silurian-Devonian biotic crisis have remained unsolved. In this study, we report multiple S-isotopic data (δ³⁴S and ∆³³S) of pyrite from Silurian-Devonian boundary sedimentary rocks from Qujing and Dahe sections in southwestern China. Prior to the S-D boundary, except the two negative δ³⁴S values at the basal Yulungssu Formation at Qujing, the δ³⁴S show positive δ³⁴S values from +10.92‰ to +27.423‰, coupled with the positive ∆³³S values from 0‰ to +0.038‰. We observe little δ³⁴S change across the S-D boundary though minor ∆³³S values are shown. After the S-D boundary, both the δ³⁴S and ∆³³S display positive compositions.

Like the Qujing section, the positive δ³⁴S values from +13.06‰ to +23.34‰ combined with the positive ∆³³S values from -0.002‰ to +0.020‰ are observed. However, unlike the Qujing section, the δ³⁴S and ∆³³S show abrupt changes across the S-D boundary. After the S-D boundary, the δ³⁴S and ∆³³S are dominated by the positive values. On the basis of the multiple S-isotopic data, we will discuss the environmental and redox implications for the biotic crisis across the S-D boundary. Our results provide evidence for change of seawater redox conditions possibly caused by sea-level fall that resulted in the mass extinction.