Questioning a widespread euxinia for the Furongian (Late Cambrian) SPICE event: indications from delta C-13, delta O-18, delta S-34 and biostratigraphic constraints

摘要

Results from a high-resolution study of delta C-13(carb), delta O-18(carb), delta S-34(CAS), delta S-34(CRS) and elemental concentrations (Ca, Fe, Mg, Mn and Sr) in the Furongian Kyrshabakty section, southern Kazakhstan, are reported here. The investigated interval covers the Drumian to Jiangshanian stages of the Cambrian Period, respectively the regional Ptychagnostus atavus to Ivshinagnostus ivshini-Irvingella major trilobite zones. delta C-13(carb) data include the Steptoean positive carbon isotope excursion (SPICE) with a local peak value of + 5 parts per thousand. The onset of SPICE corresponds to the Kormagnostus simplex Glyptagnostus stolidotus zones and pre-dates the base of the Paibian Stage/Furongian Series. delta S-34(CAS) data already increase during the Lejopyge armata biozone culminating in three positive excursions prior and after the SPICE maximum. Differences in onset, peak values and shape of the delta C-13(carb) and/or delta S-34(CAS) excursions at Kyrshabakty, but also in almost all sections characterized by the SPICE, are pointing towards Furongian seawater that was low in sulphate concentration and heterogeneous in its carbonate carbon and sulphate sulphur isotopic composition. The occurrence of benthic faunal elements in almost all SPICE-related sections strongly supports oxygenated conditions at the seafloor, therefore excluding widespread anoxia or euxinia. Regional anoxic conditions are most probable. A positive delta O-18(carb) excursion parallel to the SPICE could probably be explained by a decline in seawater pH associated with a sea-level rise. Again, no euxinic conditions would be mandatory for explaining the SPICE event.