Low oxygen levels and fluctuated redox states of continental shelf seawater after the Cambrian explosion

Abstract

The Cambrian Miaolingian and Furongian (509–485 Ma) represent a critical transitional interval in evolutionary history of animals, bridging the Cambrian explosion to Great Ordovician Biodiversification Event (GOBE). Despite its significance, scant attention has been directed toward understanding the shifts in marine redox conditions and biogeochemical cycles during this epoch, hampering the broader understanding of linkages between marine environment and early animal radiation. Here we present new paired sulfur isotope records from carbonate-associated sulfate (δ³⁴S_CAS) and pyrite (δ³⁴S_py), along with nitrogen isotopes (δ¹⁵N_decarb) from a continuous carbonate succession in South China, aiming to better constrain marine redox states and biogeochemical cycles during the middle-late Cambrian (ca. 509–495 Ma). Overall low sulfur isotope offsets Δ³⁴S_CAS-py (Δ³⁴S_CAS-py = δ³⁴S_CAS-δ³⁴S_py, 1.1–46.6‰，median = 21.0‰) indicate continuously low seawater sulfate concentrations during middle-late Cambrian. The consistently low δ¹⁵N_decarb values (−1.4–2.3‰，median = 0.4‰) suggest that this period was featured by active nitrogen fixation and a small nitrate reservoir in the shallow ocean. Taken together, the sulfur and nitrogen isotopic data can be best explained by persistently low global oxygenation levels of the middle-late Cambrian oceans. Meanwhile, high variabilities of Δ³⁴S_CAS-py and δ¹⁵N_decarb recorded by the studied section also suggest fluctuated marine redox conditions on continental margins during this period. In combination with paleontological records, we propose that persistently low marine oxygenation levels and frequent expansion of anoxic seawater on continental margins may have induced a trough period in animal biodiversification after the Cambrian explosion.