Shoaling of anoxic water as a driver of the Cambrian ROECE event: new sulfur isotope evidence from the Tarim Basin

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 sulfur isotope records across this boundary are crucial for resolving this debate. In this study, we present sulfur isotope data of carbonate-associated sulfate (CAS) across the Cambrian Series 2–3 boundary from 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 Cambrian Series 2 but showed an abrupt increase across the Series 2–3 boundary, marking a shift from coupled to decoupled carbon‑sulfur isotope behaviour. According to the prevailing ROECE hypothesis, this decoupling of carbon and sulfur isotopes may be caused by isotopically light carbon inputs to surface water from Kalkarindji volcanism or from anoxic bottom water. These two hypotheses are tested quantitively using a biogeochemical box model. The modelling results show that light carbon emission from volcanism is insufficient to explain 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 upwelled and oxidised by oxygen during a large-scale transgression, leading to a negative seawater δ13C excursion. Along with oxygen consumption and 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 sulfur isotope behaviour at this boundary and highlights the significant role of reduced oxygen levels in the Redlichiid–Olenellid extinction.