Heterogeneity in the Ediacaran–Cambrian coastal oceans: a sulphur isotope perspective. (8th July 2020)

• Record Type: Journal Article
• Authors: Lin, Ying; Wu, Nanping; Li, Da; Ling, Hong-Fei
• Is Part Of: Geological magazine. Volume 157:Number 7(2020)
• Page Start: 1112
• Page End: 1120
• Publication Date: 2020-07-08
• Subjects: sulphur isotope, -- sedimentary pyrite, -- sedimentary sulphur cycle, -- Ediacaran–Cambrian transition; Geology -- Periodicals; 551.05
• DOI: 10.1017/S0016756819001213
• Languages: English
• ISSNs: 0016-7568

Abstract

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