DEADLY WATERS: PALEOENVIRONMENTAL ANALYSIS OF THE LATE DEVONIAN HANGENBERG BIOEVENT IN THE APPALACHIAN BASIN

The Cleveland Shale member of the Ohio Shale, cropping out in eastern Ohio, is an organic-rich shale associated with the final pulse of Late Devonian extinctions: the Fammenian Hangenberg Bioevent. This unit records a period of deep water deposition within the Appalachian Basin and is exposed as a nearly homogeneous black shale interval ranging from one meter to over 30 meters in thickness. This study utilizes multiple geochemical and paleontological proxies to decipher the paleo-redox and biotic history throughout the Cleveland Shale as it records the lead-up, crisis, and recovery periods of the Hangenberg Bioevent. Rock-Eval pyrolysis indicates the Cleveland Shale has experienced a mild thermal history and is in prime condition for lipid biomarker analysis.

Data from two Cleveland Shale localities (Big Creek, near Cleveland, Ohio; Rocky Branch, near Gahanna, Ohio) exhibit complex signals. Trace fossil data, recorded as ichnofabric index (a semi-quantitative metric of relative bioturbation), show variation from laminated to sparsely burrowed strata interpreted to indicate depositional conditions ranging from dysoxic to anoxic in the bottom waters. Trace metal data (including Mo, V, Ni, and U) demonstrate variation, but also reveal extended episodes of euxinia (sulfidic waters) with possible evidence of transition metal drawdown. Lipid biomarkers reveal evidence for abundant isorenieratane, a tracer for photic zone euxinia, wide-spread throughout the Cleveland Shale strata. Hopane/sterane ratios are fairly low, typically below 0.5, which is less than the Phanarozoic average (0.5-2) for organic-rich source rocks; values increase into the overlaying shallower Carboniferous rocks. Consistently low hopane/sterane values indicate a stable microbial community structure and a relative abundance of eukaryotic phytoplankton versus bacteria throughout the extinction event. Sterane biomarker distributions encompass a great variety of structures but in terms of abundance are heavily dominated by C₂₉ steranes derived from green algal clades—a general characteristic of Paleozoic marine settings. Together, these proxies reveal a history of prolonged anoxia/euxinia punctuated with brief oxygenation episodes across the duration of the Hangenberg Bioevent in the Appalachian Basin.