Evolution of the seawater sulfate sulfur composition through the Cambrian Period: Implications from carbonate-associated sulfate

Cambrian carbonate successions of Australia, W-Gondwana, Kazakhstan, Laurentia, and Siberia were investigated for their sulfur isotopic composition of carbonate-associated sulfate (CAS). For a secure CAS extraction a repeated leaching procedure with NaCl solution was applied as a standard protocol with supplementary analyses of pre-leach sulfate concentrations and δ³⁴S_NaCl, and chromium-reducible sulfur (CRS) concentrations and δ³⁴S_CRS as routine checks on possible contamination. Additionally, δ¹³C_carb, δ¹⁸O_carb, and elemental concentrations (Ca, Fe, Mg, Mn, Sr) of the carbonate host rock were analyzed in order to constrain diagenetic alteration of the measured δ³⁴S_CAS. About 200 δ³⁴S_CAS values were generated using this analytical procedure which allows the most precise state of the art methodology for CAS and CRS extraction. The most primary δ³⁴S_CAS values vary between 24‰ and 33‰ for successions of the transition from Cambrian Series 2 to Cambrian Series 3 (traditional Lower-Middle Cambrian boundary) and between 25‰ and 48‰ for the Cambrian Series 3-Furongian (Upper Cambrian) interval, respectively. These new δ³⁴S_CAS data are valuable proxies to verify paleoecological and paleoenvironmental information. They further close an obvious gap of the Cambrian δ³⁴S_CAS record, composed of δ³⁴S_CAS values, ranging from about 20‰ up to 70‰ (SPICE-event). However, the individual data sets of this Cambrian δ³⁴S_CAS pool were generated using various methods of CAS extraction, thus offering the potential of significant differences in the final δ³⁴S_CAS values which are consequently not per se comparable with each other. The new δ³⁴S_CAS data of worldwide Cambrian sections will be discussed in the context of this problematic.