RECOGNITION OF THE KELLWASSER AND HANGENBERG ANOXIA EVENTS IN THE CENTRAL ASIAN OROGENIC BELT

The Devonian was a time of dynamic long-term climate change and substantial changes in biodiversity. The rapid rise of land plants during the Middle Devonian was coupled with rapidly decreasing atmospheric CO₂ values. Changing environmental conditions resulted in widespread anoxia and extinction that characterize many Events throughout the Devonian. Two of the most important of these events are the Kellwasser Event and Hangenberg Event which occur at the beginning and end of the Famennian Stage of the Late Devonian. Both anoxic events are associated with major mass extinction events. The Famennian in northwestern Xinjiang, China, records the Upper Kellwasser Anoxic Event at the Frasnian / Famennian boundary and the rebound from the F/F extinction event in a highly fossiliferous shallow marine setting associated with a Devonian oceanic island arc complex. Preliminary analysis of multiple geochemical proxies and mineralogical data (pyrite framboids) suggest that the Hangenberg Anoxic Event can be recognized in the same sequence although our biostratigraphic control is less precise.

The Devonian of northwest Xinjiang is part of the Central Asian Orogenic Belt (CAOB), bordered on the north by the Siberian Craton and on the south by the North China-Tarim Craton. The CAOB has a complex tectonic history and is thought to have formed as part of a complex amalgamation of intra-oceanic island arcs and continental fragments prior to the end of the latest Carboniferous. These results allow us to confirm the presence of the Kellwasser and Hangenberg Events in the open oceanic part of Paleotethys in contrast to the sections previously studied, which were located near Laurussia, Gondwana, Siberia, and South China. This indicates that the Kellwasser and Hangenberg Events were global in scope. The presence of an abundant diverse Famennian fauna between these anoxia (and extinction) events suggest that the shallow marine ecosystems in the CAOB were somewhat protected due to their tectonic location and relative isolation within an open ocean system. Due to the lack of studies from locations outside subequatorial North America and Europe the global extent of the extinction event is not well-constrained and studies such as this one aid in understanding the nature of ocean anoxia during this time period.