OVERTURNING IDEAS ABOUT OCEANIC OVERTURNING IN THE LATE DEVONIAN: INSIGHTS FROM THE CENTRAL ASIAN OROGENIC BELT

Oceanic anoxia events in the Late Devonian are hypothesized to be the causal mechanisms of two of the top six major mass extinctions in Earth’s history: the Kellwasser Anoxia Event at the Frasnian-Famennian (F-F) boundary that decimated coral-stromatoporoid reef ecosystems and the Hangenberg Anoxia Event at the Devonian-Carboniferous (D-C) boundary that primarily affected pelagic marine communities including fish and cephalopods. Although these events have been studied in locations around the globe, >77% of published studies have been conducted on the continental margins and epeiric seas of Europe and North America. This concentration of paleogeographically similar study sites has limited the accepted mechanism of ocean anoxia (oceanic upwelling/overturning) to those that were probable on a tropical/equatorial paleocontinent undergoing a major orogenesis. These unusual conditions, however, may not be representative of global oceanic conditions at that time.

In contrast, the Central Asian Orogenic Belt (CAOB) consisted of a series of island arcs located far from continental-driven sediments, and represents an ideal location to study Late Devonian anoxia events a) in the open ocean and b) outside of the influence of the rising Appalachian/Variscan mountain chain. Late Devonian sediments from the CAOB have been collected from both China (Boulongour Reservoir section of the Junggar Basin in Xinjiang Province) and Mongolia (southwest end of the Gobi fold megazone in the Baruunhuurai Terrane, Khovd Province, Mongolia). Multiproxy geochemical evidence indicates that both the Kellwasser and Hangenberg Events are present in the sediment sequences in China, although they do not contain the characteristic black shales commonly associated with these intervals. New field mapping, geochronology, geochemistry, and paleontology data tentatively suggest the presence of the Kellwasser Event in analogous Mongolian sediments. The presence of anoxia events on two separate isolated island arcs in the CAOB means that the oceanic upwelling/overturning model must itself be overturned, as it is highly unlikely that the entire ocean overturned simultaneously. Eutrophication triggered by climate change from orbital forcing or large igneous provinces is a more likely model for Late Devonian ocean anoxia.