MID-LATE GIVETIAN CARBON-CYCLE PERTURBATIONS IN BLACK SHALE AND ASSOCIATED DEPOSITS OF THE NORTHERN APPALACHIAN BASIN

The Mid-Late Givetian (Middle Devonian) was a volatile time. Marine faunal migration and extinction, eustatic fluctuation, carbon-cycle perturbation, warming/cooling, and anoxia occurred over ~2 Myr through the complex Taghanic Crisis and into the onset of the Frasnes Crisis. In the Northern Appalachian Basin, an expanded stable isotope profile of inorganic carbon δ¹³C_carb in the Lansing Core spans an incredible range of ~8‰ (~+3 to near -5) through a series of carbon-cycle perturbations in black shale and associated deposits. This overall positive-to-negative shift in values is seen globally as the lightest Devonian values, but this magnitude of negative shift is more extreme than previously reported.

­­In the Northern Appalachian Basin, carbon-cycle perturbations correspond to a series of paleoclimate, sea level, and faunal events that compose these crises. Recognized events include: 1) the Taghanic Lower Tully Event, during which a negative δ¹³C_carb excursion is immediately followed by warming, transgression, and Appalachian Basin invasion by warm water taxa; 2) the Taghanic Upper Tully Event, seen as a positive δ¹³C_carb excursion, cooling, sea level fluctuations including a major transgression (Taghanic Onlap) and the return of endemic cooler water taxa; 3) the latest Taghanic Geneseo Event, with the onset of a long-term negative δ¹³C_carb shift, black shale deposition, warming, maximum flooding and the loss of a majority of Appalachian Basin endemic taxa; 4) the post Taghanic disparilis Event, a short-term ‘stabilization’ of the δ¹³C_carb record, cooling and substantial sea level fall followed by transgression; 5) the Hubbard Quarry Event, the resumption of the long-term negative δ¹³C_carb shift, black shale deposition, resumed warming and maximum flooding; and, 6) the Lodi Event at the onset of the Frasnes Crisis, with the lightest δ¹³C_carb values observed (~-5‰) during warming, sea level fluctuations and warm water taxon incursion.

The light δ¹³C_carb values and negative isotopic shifts in the Lansing Core coincident with transgression and warming suggest pulsed release of methane to the global ocean-atmosphere system. Future work will document carbon isotope patterns in Mid-Late Givetian age rocks along transects in the Appalachian Basin and globally to differentiate regional and global patterns.