A global biogeochemical perturbation during the Middle Frasnian punctata Event: Evidence from muted carbon isotope signature in the Appalachian Basin, New York State (USA)

摘要

A modest positive stable carbon isotope excursion coincident with the Upper Devonian Middlesex Formation of western New York State and beneath the Palmatolepis transitans - Palmatolepis punctata zonal boundary is interpreted to define the beginning of the Middle Frasnian punctata Event. The broad positive excursion that defines this global isotopic event elsewhere, however, appears to have been muted in the northern Appalachian Basin by a tectonically induced regression and associated deposition of the organic-deficient Cashaqua Formation. delta C-13(org) increases upward through the lower two-thirds of the Cashaqua, peaking approximately 4.5 m below its contact with the overlying Rhinestreet Formation in the mid punctata Zone. Above this stratigraphic level, delta C-13(org) diminishes 2.2 parts per thousand over a vertical distance of 41 cm that encompasses a newly recognized interval of microtektite-like spherules. Thus, that part of the studied section between the middle of the Middlesex Formation and the peak delta C-13(org) value in the upper Cashaqua Formation embraces the stratigraphic range of the punctata positive excursion dampened by the regional base level reduction and associated increased flux of C-13-depleted terrestrial weathering runoff, including dissolved inorganic carbon (DIC). delta C-13(org) values recover to pre-negative excursion levels through the top of the Cashaqua Formation into the lower Rhinestreet Formation. The sharp negative excursion terminating the punctata Event of western New York likely records the sudden introduction of C-12 into the ocean - atmosphere system, perhaps related to destabilization of sea-floor methane hydrates. However, hydrate dissociation concurrent with rising global sea level and in the absence of a closely preceding thermal event sufficient to raise bottom-water temperature is problematic. The coincidence of the stratigraphic interval containing the microtektite-like spherules and the onset of the negative carbon isotope excursion suggests that impact-induced mechanical disruption of the ocean floor may have triggered the release of methane.