Heterogeneity in the Ediacaran–Cambrian coastal oceans: a sulphur isotope perspective

Multiple sulphur isotope compositions of sedimentary pyrites across the Ediacaran–Cambrian (Ed–C) transition and into the early Cambrian from the Xiaotan section, Yunnan, South China, are presented to explore the evolution of the sulphur cycle. The values of δ34Spy range from 13.5 ‰ to 35.8 ‰, and the values of Δ33Spy range from −0.044 ‰ to 0.063 ‰. The first-order observation of highly positive δ34Spy is consistent with sulphur isotope records from other sedimentary successions (with various degrees of enrichment in 34S), reflecting a common feature in cycling of sulphur among ocean basins. The positive values suggest that pyrite was formed in a depositional setting with limiting availability of sulphate that suppressed the expression of microbial fractionations. The first-order observation of a 10-million-year period of negative Δ33Spy beginning around the Ed–C boundary likely reflects changes in isotopic compositions of sulphur influx to the oceans. Such changes are suggested to be linked to a pulse of preferred weathering of sulphides (with negative Δ33S) relative to sulphate, which may reflect enhanced exposure of pyrites in continental margins due to reorganization of continents at this time. Both δ34Spy and Δ33Spy data imply low seawater sulphate levels, and possibly heterogeneity in sulphate concentrations in the world’s coastal oceans. The predictions about sulphur isotope signatures of evolved seawater (with highly positive δ34S and negative Δ33S) at the Xiaotan section are testable with future measurements of carbonate-associated sulphate (CAS), a proxy of ancient oceanic sulphate that carries information about the operation of sulphur cycling on a global scale.