ANOXIA AND THE UPPER KELLWASSER EVENT: A HIGH RESOLUTION STUDY IN NEW YORK STATE

The Late Devonian Upper Kellwasser (uK) event is interpreted to represent a biocrisis of mass extinction magnitude, preserved globally as black shale at the Frasnian-Famennian boundary. Although these black shales are typically interpreted to represent anoxia, and oxygen stress is widely considered a contributing mechanism for this bioevent, they have yet to be analyzed on the cm scale; in this study, we characterize bottom water oxygen conditions at this high resolution to specifically test for persistent anoxia through the black shale interval.

Four biostratigraphically well-defined localities in western New York were examined, using trace fossil and trace metal signals at a mm-cm scale to constrain relative oxygen levels in the bottom waters at the time of the uK black shale deposition. At each locality, the ichnofabric index (ii), a metric to quantify the amount of bioturbation correlative with relative bottom water oxygen, varies at the cm scale between commonly ii1 and ii2 and rarely up to fully bioturbated ii5 through the ~ 30 cm black shale interval; typically laminations are broken as result of infaunal activity. The trace metals also vary through the interval, but have an overall pattern of decreasing enrichment up section; Mo concentrations range from crustal values (2-3 ppm) to moderate enrichment (~40 ppm). The absence of thick packages of laminated intervals, mm-scale variation in the amount of bioturbation, and low enrichments of Mo and other trace metals provide no evidence for euxinia and indicate that anoxia was not persistent through the uK interval in the Appalachian Basin. We furthered studied lipid biomarkers, and the organic geochemical data confirm these units as appropriate for geochemical study and corroborate the interpretations of the ichnofabric indices and inorganic proxies toward fluctuating bottom water oxygen levels during uK black shale deposition.

These results contrast the view of persistent anoxia contributing to black shale deposition and Late Devonian extinction and demonstrate that any model of anoxia-driven biotic crisis must include parameters of fluctuating bottom water oxygen and non-persistent anoxia.