HOW ROBUST IS YOUR FAVORITE ANOXIA PROXY? LESSONS WE'RE LEARNING FROM THE LATE DEVONIAN (Invited Presentation)

Oceanic anoxic events that result in the sequestration of organic carbon are common in deep time are generally associated with major bioevents. These anoxia events have a variety of driving mechanisms, so their cause-and-effect relationships with bioevents can be difficult to prove. The Late Devonian Frasnian-Famennian (F-F) boundary and Devonian-Carboniferous (D-C) boundary bioevents are associated with widespread oceanic anoxia (the Kellwasser Events and the Hangenberg Event, respectively) and elevated (mass) extinctions as well as organic carbon sequestration, but there is still no consensus around the mechanisms for and relationship between oceanic anoxia and extinction during this time period. Furthermore, the extent, severity, and expression of anoxia differs across paleoenvironment and water depth, and can fluctuate with time.

The utility and accuracy of the most commonly used marine anoxia proxies (total organic carbon, organic walled microfossils, stable carbon (δ¹³C) and sulfur (δ₃₄S) isotopic trends, trace element geochemistry, pyrite framboid distributions, and ichnofabrics) are influenced by localized paleoenvironmental and diagenetic factors, and may provide conflicting signatures. Calibration of the most commonly used redox proxies across depositional conditions has yet to be rigorously performed. Proxies that are only accurate in specific paleoenvironments at specific water depths prevent us from fully comprehending the global extent and severity of oceanic anoxia. Therefore, a comprehensive set of best practices for recognizing and evaluating oxygen loss in ancient marine environments is urgently needed. Proxies must be compared not only across different paleoenvironments and sedimentary regimes, but also across baseline pre-anoxic conditions, anoxic conditions, and post-anoxic rebound conditions. We are currently in the process of comparing and validating proxies across a wide variety of paleoenvironments and water depths from Late Devonian sites around the globe that include epeiric seas, carbonate reefs, deep carbonate epicontinental basins, clastic tectonic basins, and volcanic island arcs at a variety of water depths.