ANOXIA BETWEEN EDIACARAN FAUNA AND CAMBRIAN BIORADIATION: CARBON AND SULPHUR ISOTOPE STUDIES

Carbon, sulphur and iron abundances and carbon and sulphur isotopes were measured in shales, cherts and carbonates from a shelf and a basin Lower Cambrian (Nemakit-Daldynian to Atdabanian) sections on the Yangtze Platform, South China. The objective of our study was to constrain the marine redox conditions for these sedimentary environments immediately preceding the rapid diversification of metazoans. High DOP and low δ³⁴S values for sulphide in the upper Nemakit-Daldynian and lower Tommotian, indicate bacterial sulphate reduction not only in the sediment but also in a euxinic water column. δ¹³C of organic carbon is consistently low throughout this period, reflecting organic matter recycling through sulphate reducing bacteria, superimposed on the temporal evolution of the global carbon cycle. Covarying δ¹³C carbonate values could have been accomplished by an oceanographic situation in which the chemocline was situated within the photic zone. The apparent isotope fractionation between organic and carbonate carbon is > 32‰ in the upper Nemakit-Daldynian and lower Tommotian, suggesting an input of chemoautotrophic biomass which is in agreement with the record of previously published organic carbon isotope data (Hayes et al., Chem. Geol., 161, 103-125, 1999). In the upper Tommotian, low DOP values introduce water column oxygenation. δ³⁴S values for sulphide rises. This suggests that bacterial sulphate reduction occurred primarily in interstitial waters where sulphate limitation was soon established. Organic and carbonate carbon isotope ratios increase, reflecting open marine conditions. However chemical proxies measured from the deeper water section suggest that anoxic conditions were still present or re-established in the basinal region and became oxic to dysoxic only in the lower Atdabanian. Our geochemical data are consistent with a postulated stagnant anoxic basin during the upper Nemakit-Daldynian and lower Tommotian. Euxinic conditions may have terminated some organisms, but on the other hand given rise to new life after retrieved oxygenation. High δ³⁴S values for pyrite and negative carbon isotope excursions have also been documented from other parts of the world during this time. This suggests that such euxinic conditions were not solely restricted to the Yangtze Platform.