An Integrated Geochemical and Paleontological Investigation of Environmental and Biotic Change Associated with Late Devonian Mass Extinctions in the Appalachian Basin, USA

The Upper Kellwasser (uK) black shale, a global unit at the Frasnian-Famennian boundary, closely associated with the Late Devonian extinction event, is commonly linked to oxygen limitation in the water column. In spite of the significance of this time interval, the nature of the ocean redox geochemistry is poorly understood. Using a multi-proxy approach, this study tests the appropriateness of three distinct oceanographic models for ocean redox chemistry at this time: 1) an oxic setting with sub-oxic bottom waters but with sulfide production confined to sedimentary porewaters; 2) an expanded oxygen minimum zone within a highly stratified marine redox column with only intermittent photic zone (shallow water) euxinia; and 3) a persistently euxinic water column extending up into the photic zone. Bottom water oxygen conditions are described at a high resolution for 4 uK black shale localities in western New York State, using inorganic and organic geochemical proxies and trace fossils to constrain relative oxygen levels and identify signals of anoxia and euxinia in the Devonian Appalachian Basin. Mo concentrations typically range from crustal (2-3 ppm) to moderately enriched values suggestive of suboxic conditions (typically less than 30 ppm), with some higher values between 30 and 40 ppm perhaps suggesting intermittent euxinia, indicating that the uK black shale preserves reduced oxygen bottom water conditions. The levels of enrichment are muted, though, such that these are inconsistent with persistent anoxia or euxinia for the interval, especially as compared to other Phanerozoic euxinic black shale intervals. Other trace metals suggest similarly suboxic to intermittently anoxic bottom water conditions. Lipid biomarker patterns are typical for Paleozoic marine rocks, indicating that the biomarker molecules in the extracted bitumens are syndepositional and not significantly affected by contamination. Independent thermal maturity screening data indicating peak oil window maturity levels from Rock-Eval pyrolysis confirm that these units are appropriate for organic geochemical analyses. No appreciable amount of aromatic carotenoid biomarkers diagnostic for green sulfur bacteria (isorenieratane and aryl isoprenoids) has been detected as yet at any of the Appalachian localities, which line of evidence favors a local paleoredox model without a persistently shallow sulfidic zone. Thus, photic zone euxinia is not always associated with extinction horizons in the Late Devonian, though it is possible that a complex redox stratified water column with an oscillating chemocline existing for extended periods below photic zone water depths (>100 m depth) was the depositional setting in the marine realm around ocean margins. The trace fossil record through the uK black shale interval complements the geochemical proxies; the presence of horizons with abundant bioturbation between well-laminated intervals provides biological support for intermittently oxygenated bottom waters.