Reconstructing marine redox conditions for the Early Cambrian Yangtze Platform: Evidence from biogenic sulphur and organic carbon isotopes

Abstract

In order to constrain marine redox conditions for the sedimentary environments immediately preceding the rapid diversification of metazoans carbon and sulphur isotopes, as well as carbon, sulphur and iron abundances, were measured in shales, cherts, and carbonates from two Lower Cambrian (Nemakit-Daldynian to Atdabanian) sections belonging to a shallow shelf and deeper water basinal environment on the Yangtze Platform, South China. A largely anoxic and most probably sulphidic deeper water body for most of the Nemakit-Daldynian and lower Tommotian is supported by positive δ 34S values for sulphide, as well as high degree of pyritization (DOP) and total organic carbon (TOC) values. Additionally, an isotope fractionation between organic and carbonate carbon of > 32‰ suggests that sulphate reducing and sulphide oxidizing bacteria flourished and added to the biomass. Furthermore a positive correlation between δ 34S of sulphide and δ 13C of organic and possibly inorganic carbon is observed. In the lower Tommotian, low δ 13C values for organic and inorganic carbon together with lower δ 34S values for pyrite, if compared to underlying and overlying samples, have been measured. This is indicative of bacterial sulphate reduction taking place not only in the interstitial waters but also in an anoxic, possibly euxinic water column, and thus leading together with chemoautotrophic bacteria to recycling of organic matter, superimposed on the temporal evolution of the global carbon cycle. In the upper Tommotian and Atdabanian, decreasing DOP and TOC and high δ 34S values for pyrite and organic sulphur suggest that the water column became oxic and that bacterial sulphate reduction occurred primarily in the sediment, where sulphate limitation was soon established. Organic carbon and carbonate carbon isotope ratios increase, reflecting open marine conditions and oxic photosynthesis dominating carbon cycling in the shelf environment. In the basinal deeper water section, anoxic conditions were still present or re-established during the upper Tommotian. In the lower Atdabanian, δ 34S is variable, but mostly high in the deeper section, with DOP indicating an oscillation between oxic and dysoxic conditions.