Osmium-isotope evidence for volcanism across the Wuchiapingian–Changhsingian boundary interval

Two negative carbon isotope excursions (3.5–6.5‰) across the Wuchiapingian-Changhsingian boundary (WCB) are observed globally (sections in China, Canada, and Iran); however, the causative mechanism of these excursions is debated. Here, high-resolution osmium isotope (¹⁸⁷Os/¹⁸⁸Os or Os_i) chemostratigraphy of four globally correlated WCB sections (3 in China - Meishan, Shangsi and Lianyuan and 1 in Canada - Buchanan Lake) show two separate Os_i excursions to less radiogenic compositions that are coincident with the carbon isotope shifts for two of the South China sections (Lianyuan, Meishan). In contrast, only a single Os_i excursion to less radiogenic compositions, coinciding with the earliest Changhsingian carbon isotope shift, is observed for the Shangsi and Buchanan Lake sections. The Os_i shift is interpreted to reflect increased unradiogenic Os input from basaltic magmatism in South China, possibly related to the Emeishan large igneous province (LIP). However, ¹⁸⁷Os/¹⁸⁸Os data suggest that only the earliest Changhsingian volcanism had global impact on both the ocean and atmosphere. The lack of any evidence for a biotic event associated with the WCB therefore may have been due to the more regional rather than global impact of volcanism during the latest Wuchiapingian. In contrast, during the earliest Changhsingian, volcanism was sufficient to cause a more global signal in the ocean osmium record, but was inadequate, or too prolonged, to drive any significant environmental change. Volcanism, however, may have provided the isotopically light carbon that drove the negative carbon isotope excursions across the WCB.