Sulphur and oxygen isotope signatures of late Neoproterozoic to early Cambrian sulphate, Yangtze Platform, China: Diagenetic constraints and seawater evolution

A wide range of sulphate sulphur isotope data has previously been reported for the late Neoproterozoic and early Cambrian (∼600 to 542 Ma), whereas hardly any data exists with respect to the isotopic composition of sulphate oxygen for this time period. Thus, a refinement of the seawater sulphate records for δSSO434 and δOSO418 at this time, following an evaluation of the diagenetic history of the studied sediments, can contribute to a better understanding of ocean chemistry. For this purpose δ34S and δ18O of sulphate was measured in phosphorites and carbonates from different paleoenvironmental settings within the Yangtze Platform in China. Fe speciation was also applied in order to constrain water column redox conditions. A number of secondary processes in pore waters are identified by combining sulphate δ34S with sulphate δ18O, pyrite δ34S and Fe speciation. Some samples are characterized by comparably low δ18O and δSSO434, attributed to preferential incorporation of H2O oxygen during sulphide oxidation. Other samples have elevated oxygen and sulphur isotope values, implying not only bacterial sulphate reduction under closed conditions with respect to sulphate availability, but also bacterial disproportionation of intermediate sulphur species. Samples exhibiting no apparent diagenetic overprint give a range in δOSO418 of between 13 and 15‰ for the sulphate oxygen isotopic composition of Ediacaran to early Atdabanian seawater. For the post-glacial Ediacaran stage, seawater δSSO434 values of around 34‰ are indicated, with a possible decrease to ∼25‰ in the late Ediacaran Dengying Formation, due to influx of 34S depleted sulphate driven from oxidised sulphide, and a possible decrease in pyrite burial. A rise in δSSO434 to ca. 40‰ in the early Cambrian may be connected to more pronounced basinal euxinia and an enhanced draw-down of 32S via intense bacterial sulphate reduction in the water column. This rise in 34S is not accompanied by an increase in δOSO418, which could reflect a change in either the oxidation mechanism of SO4 or an oceanic fluctuation in δ18O.