Sulphur isotopic evolution of neoproterozole-Cambrian seawater:: new francolite-bound sulphate δ34S data and a critical appraisal of the existing record

Francolite-bound sulphate sulphur isotope compositions from the 570 +/- 20 Ma Doushantuo Formation are used to constrain seawater delta(34)S during the late Neoproterozoic. Phosphorites of the lower ore have delta(34)S compositions between 32.3parts per thousand and 37.8parts per thousand, averaging 36.1parts per thousand, delta(13)C values between -4parts per thousand and -1parts per thousand, and do not exhibit anomalous cerium depletion. By contrast, phosphorites of the upper ore, which contains some of the earliest animal fossils known, have delta(34)S between 31.6parts per thousand and 34.5parts per thousand, averaging 33.3parts per thousand, delta(13)C values between -1parts per thousand and +2parts per thousand, and exhibit a distinct negative cerium anomaly. The retention of a cerium anomaly in these upper Doushantuo phosphorites implies that oxidising conditions prevailed during early diagenetic REE scavenging, indicating that these samples are more likely to retain a seawater sulphur isotope composition. On this basis we argue that seawater delta(34)S was 32parts per thousand (+/-2parts per thousand) during the deposition of the upper Doushantuo unit. The slightly higher values of the lower Doushantuo unit may reflect either slightly higher ambient seawater delta(34)S or isotopic fractionation due to bacterial sulphate reduction. Almost identical constraints on seawater delta(34)S have been reported from lowermost Cambrian phosphorites across South China, indicating that seawater delta(34)S was similar at similar to570 and similar to540 Ma. A critical examination of all published data provides no firm evidence for major fluctuations in seawater sulphate 6,4 S during the Neoproterozoic-Phanerozoic transition interval. This indicates that the onset of pervasive bioturbation had only a negligible effect oil global pyritisation rates and questions recent reports of massive fluctuations in seawater sulphate delta(34)S in response to the Snowball Earth glaciations. The existing delta(34)S record is consistent with a sustained increase in seawater delta(34)S between 750 and 570 Ma. Shorter-term fluctuations cannot be ruled out entirely, but will require more detailed study, incorporating rigorous methodologies to constrain the effects of non-marine influences. (C) 2004 Elsevier B.V. All rights reserved.