The first trilobite mass extinction, known as the redlichiid–olenellid extinction, occurred at the Cambrian Series 2–3 boundary and is associated with the Redlichiid–Olenellid Extinction negative Carbon isotope Excursion (ROECE). The causes of the ROECE event remain debated, with some researchers attributing it to the Kalkarindji volcanism and others linking it to a coeval large-scale transgression event. Continuous seawater sulphur isotope records across this boundary are crucial for resolving this debate. In this study, we present sulphur isotope data of carbonate-associated sulphate (CAS) across the Cambrian Series 2–3 boundary in the Xiaoerbrak section, Tarim Basin and reconstruct a seawater δ³⁴S variation curve for this critical period. Our findings show that seawater δ³⁴S values were likely positively coupled with δ¹³C values during the Cambrian Series 2 but showed an abrupt increase across the Cambrian Series 2–3 boundary, marking a shift from coupled to decoupled carbon-sulphur isotope behaviours. According to the prevailing ROECE hypothesis, this decoupling of carbon and sulphur isotopes may be caused by significant light carbon inputs from the Kalkarindji volcanisms or from the anoxic bottom water. These two hypotheses are tested quantitively using a biogeochemical box model. The modelling results show that light carbon emission from the volcanism does not align with the recorded seawater isotope variations. Instead, the best fit comes from a dissolved organic carbon (DOC) oxidation hypothesis, an updated version of the anoxic bottom water shoaling/upwelling hypothesis. This hypothesis posits that DOC-enriched anoxic bottom water was oxidised during a large-scale transgression, leading to negative seawater δ¹³C excursions. Along with expanded anoxia, increased pyrite burial resulted in a sharp rise in seawater δ³⁴S at the Cambrian Series 2–3 boundary. The proposed DOC oxidation hypothesis effectively explains the decoupled carbon and sulphur isotope behaviours at this boundary and highlights the significant role of reduced oxygen levels in the redlichiid–olenellid extinction.
 

sulphur isotopes, Cambrian Series 2-3, Redlichiid–olenellid extinction, biogeochemical modelling