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 delta (super 34) S (sub py) range from 13.5 ppm to 35.8 ppm, and the values of Delta (super 33) S (sub py) range from -0.044 ppm to 0.063 ppm. The first-order observation of highly positive delta (super 34) S (sub py) is consistent with sulphur isotope records from other sedimentary successions (with various degrees of enrichment in (super 34) S), 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 Delta (super 33) S (sub py) 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 Delta (super 33) S) relative to sulphate, which may reflect enhanced exposure of pyrites in continental margins due to reorganization of continents at this time. Both delta (super 34) S (sub py) and Delta (super 33) S (sub py) 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 delta (super 34) S and negative Delta (super 33) S) 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.