EUTROFICATION OF BASINAL WATERS AROUND THE FRASNIAN-FAMENNIAN BOUNDARY AT THE DEVILS GATE SECTION (NEVADA) : A RELATION WITH MASS EXTINCTION?

The 330 m thick Devils Gate section (North of Nevada), from early Frasnian to early Famennian, shows sedimentary environments evolving from semi restricted emerging inner shelf to outer shelf and finally passing to a deep pelagic basin with slumps and turbidites. The latter deep facies are reached shortly before de Frasnian-Famennian boundary (FFB). Whole rock geochemical (traces, minor and major elements) and stable isotopes (¹⁸O/¹⁶O, ¹³C/¹²C and ⁸⁷Sr/⁸⁶Sr) analyses have been performed on the entire section with a special focus on the FFB. The 50 samples used for ⁸⁷Sr/⁸⁶Sr analyses reveal a strong positive excursion right below the boundary with a highest value of 0,709434±0,000006, which is higher than the highest of the Sr isotope curve of the Phanerozoic. The Sr excursion comes along with a short magnetic susceptibility positive peak and a δ¹³C positive excursion from 0‰ to +4‰ value. The δ¹³C excursion spans from the ⁸⁷Sr/⁸⁶Sr peak and then keeps growing beyond the FFB in corollary with a high organic matter content and before getting back down. This short interval (around the FFB) characterized by peaks of ⁸⁷Sr/⁸⁶Sr, ¹³C/¹²C and organic matter, is probably related to an eutrophication of the bassinal water as suggested by the evolution of the Ba/Al and Ba/Th ratios. All these results are corroborating a FFB major eustatic rise associated with a continental washing and a nutrient influx that induced a strong organic productivity increase. Oxygen pumping by strong organic activity is followed by intense organic carbon burial.