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 (similar to 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 delta S-34(SO4) and delta O-18(SO4) 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 delta S-34 and delta O-18 of sulphate was measured in phosphorites and carbonates from different paleoenvironmcntal 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 delta S-34 with sulphate delta O-18, pyrite delta S-34 and Fe speciation. Some samples are characterized by comparably low delta O-18 and delta S-34(SO4), 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 delta O-18(SO4) of between 13 and 15 parts per thousand for the sulphate oxygen isotopic composition of Ediacaran to early Atdabanian seawater. For the post-glacial Ediacaran stage, seawater delta S-34(SO4) values of around 34 parts per thousand are indicated, with a possible decrease to similar to 25 parts per thousand in the late Ediacaran Dengying Formation, due to influx of S-34 depleted sulphate driven from oxidised sulphide, and a possible decrease in pyrite burial. A rise in delta S-34(SO4) to ca. 40 parts per thousand in the early Cambrian may be connected to more pronounced basinal euxinia and an enhanced draw-down of S-32 via intense bacterial sulphate reduction in the water column. This rise in S-34 is not accompanied by an increase in delta O-18(SO4), which could reflect a change in either the oxidation mechanism Of SO4 or an oceanic fluctuation in delta O-18. (c) 2005 Elsevier B.V. All rights reserved.