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

Abstract

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 Section and several correlative sections in South China. Forty clastic, granular phosphorites from Meishucun yield tightly grouped δ 34 S values averaging 33‰ (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 ambient seawater, confirming further the existence of uniquely high δ 34 S values in the earliest Cambrian oceans. This novel use of trace-sulphate in phosphate to constrain seawater δ 34 S represents the first time that sulphate δ 34 S data for this period have been given precise biostratigraphic assignments. Superimposed on the overall trend are short-term, stratigraphic variations, which might reflect local variations in the sedimentary and early diagenetic environment. Our data, together with other published data, indicate that seawater sulphate δ 34 S rose from low values (15–20‰) during the pre-750 Ma Proterozoic to possibly all-time high values (>32‰) by the earliest Cambrian. We argue that this rise may, in part, relate to increases in the amount of sulphur isotopic discrimination during microbially mediated sulphate reduction as a result of increased sulphide reoxi-dation. On the other hand, the Neoproterozoic trend to high δ 34 S values appears to mirror a trend to decreasing seawater δ 13 C towards the Proterozoic–Phanerozoic transition, implying progressive increases in the efficiency of organic carbon recycling, which would normally be coupled with real increases in sulphate reduction on the global scale. We consider that both these changes in biogeochemical cycling derive ultimately from the introduction of macrofauna around this time and, in particular, from the influence of bioturbation on early diagenesis. Precise constraints on S-isotopic evolution during the Neoproterozoic require additional trace sulphate studies.