Hypersaline and anoxia in the Devonian Frasnian–Famennian transition: Molecular fossil and mineralogical evidence from Guangxi, South China

Abstract

Abundant normal alkanes, isoprenoid hydrocarbons, terpanes and steranes were detected in 23 samples taken from the carbonate platform to basin facies in the Devonian Frasnian–Famennian (F–F) transition of Guangxi, South China. They were mainly derived from the phytoplankton or bacteria and algae. Molecular stratigraphic parameters, abundant micron gypsums and pyrite framboids show that hypersaline and anoxia prevailed in the F–F transition, the highly stressed environments occurred near the F–F boundary. Orbital cyclostratigraphic studies indicate that the hypersaline and anoxia prevailed in the F–F transition spanned an interval at least from the Palmatolepis linguiformis conodont Zone to the Middle Palmatolepis triangularis conodont Zone and lasted about 1.2 Ma. The Devonian marine ecosystem underwent Early Devonian gradual optimization, Middle Devonian to the F–F transitional stepwise deterioration and late Famennian recovery. A positive feedback among higher surface ocean temperatures, hypersaline, water column anoxia, enhanced nutrient regeneration, phytoplankton productivity and eutrophication was probably substantial in the F–F transition in Guangxi, South China, even in other regions of the world, such as Holy Cross Mountains of Poland, Montagne Noire in southern France, Alberta in Canada. The point that should be emphasized is that the decisive and direct causes of the F–F transitional mass extinction would be the stepwise deterioration of the marine ecosystem since the Middle Devonian other than a bolide impact on the earth. The hypersaline and anoxia prevailed in the F–F transition favor the formation and preservation of hydrocarbon source rocks in South China.