Sulphur isotope compositions of sedimentary phosphorites from the basal Cambrian of China: implications for Neoproterozoic-Cambrian biogeochemical cycling

The Meishucun Section (Yunnan Province, South China) is considered to be an important Precambrian-Cambrian boundary section, primarily because of its rich small shelly fossil record. In this article, we report the results of a sulphur isotope study of phosphate-bound sulphate from the Meishucun Sect ion and several correlative sections in South China. Forty elastic, granula r phosphorites from Meishucun yield tightly grouped delta(34)S values avera ging 33 parts per thousand (CDT), which agree well with published evaporite data for the lower Cambrian of Siberia and elsewhere. We argue that these strongly positive values reflect the sulphur isotopic composition of ambien t seawater, confirming further the existence of uniquely high delta(34)S va lues in the earliest Cambrian oceans. This novel use of trace-sulphate in p hosphate to constrain seawater delta(34)S represents the first time that su lphate delta(34)S data for this period have been given precise biostratigra phic assignments. Superimposed on the overall trend are short-term, stratig raphic variations, which might reflect local variations in the sedimentary and early diagenetic environment. Our data, together with other published d ata, indicate that seawater sulphate delta(34)S rose from low values (15-20 parts per thousand) during the pre-750 Ma Proterozoic to possibly all-time high values (>32 parts per thousand) by the earliest Cambrian. We argue th at this rise may, in part, relate to increases in the amount of sulphur iso topic discrimination during microbially mediated sulphate reduction as a re sult of increased sulphide reoxidation. On the other hand, the Neoproterozo ic trend to high delta(34)S values appears to mirror a trend to decreasing seawater delta(13)C towards the Proterozoic-Phanerozoic transition, implyin g progressive increases in the efficiency of organic carbon recycling, whic h would normally be coupled with real increases in sulphate reduction on th e global scale. We consider that both these changes in biogeochemical cycli ng derive ultimately from the introduction of macrofauna around this time a nd, in particular, from the influence of bioturbation on early diagenesis. Precise constraints on S-isotopic evolution during the Neoproterozoic requi re additional trace sulphate studies.