PRECLUDING VOLCANISM AS PRIMARY TRIGGER FOR DEVONIAN-CARBONIFEROUS GLOBAL ANOXIA EVENT: IMPLICATIONS OF THE HANGENBERG EVENT IN THE LATE DEVONIAN PHO HAN FORMATION, CAT BA ISLAND, VIETNAM

The Devonian-Carboniferous (D-C) transition is a period of mass extinction and rapid global faunal changes that affected both marine and terrestrial ecosystems. The Hangenberg Event is a D-C transition ocean anoxia and regression event characterized by black shale deposits in many locations throughout Europe and North America, whose history is complicated by the Appalachian/Variscan and Antler orogenies. Little is known about the Hangenberg Event in Southeast Asia, however, which was tectonically quiescent at that time. Conodont biostratigraphy indicates that the D-C boundary is present within the Pho Han Formation on Cat Ba Island in northeastern Vietnam, which represents a basinal facies on the South China carbonate platform. In contrast to many Hangenberg Event lithologies within Europe and North America, anoxic conditions were not constrained to the D-C boundary in the Pho Han Formation sediments. Significant enrichments in Mo, Cu, Ni, Zn, Cd, and other elements commonly used as anoxia proxies coincide with the biostratigraphic location of the D-C transition, and likewise indicate the presence of the Hangenberg Event in this basin. Whole rock geochemical results also indicate increased clastic input at the D-C transition, potentially due to the regression observed in many Hangenberg Event localities around the world, or conversely to a complete shutdown of the carbonate factory. There is no Hg enrichment in the Pho Han sediments, precluding extensive volcanism as the trigger mechanism for the Hangenberg Event. The presence and size distribution of pyrite framboids can be used as a proxy for water column redox conditions, and preliminary analysis shows abundant framboids less than 5 microns in diameter throughout the section. Framboid distribution analysis in conjunction with whole rock geochemical analysis indicates long-term anoxic conditions in the basin, with more severe anoxia at the Hangenberg Event. The ocean anoxia of the Hangenberg Event was therefore a global phenomenon with anoxia not only in the epicontinental basins surrounded by active tectonism in Europe and North America, but also in the sediment-starved basins of the stable South China carbonate platform.