Biogeochemical controls on black shale deposition during the Frasnian-Famennian biotic crisis in the Illinois and Appalachian Basins, USA, inferred from stable isotopes of nitrogen and carbon

The Frasnian-Famennian biotic crisis is marked by two distinct intervals known as the Lower and Upper Kellwasser Events (KWEs) that in many locations are associated with deposition of organic-rich shales. Sedimentary nitrogen and carbon isotopes offer insight into the biogeochemical processing of nutrients, production of organic matter, and palaeoceanographic conditions during the KWEs. Here we present new bulk nitrogen (delta N-15(bulk)) and organic carbon (delta C-13(org)) isotope data from the Late Devonian Appalachian and Illinois Basins (AB and IB), with a focus on intervals encompassing the KWEs. Black shales from the IB and AB, including the KWEs, are N-15-depleted (-1.0-+2.0 parts per thousand) and have significantly lower delta N-15(bulk) than interbedded grey shales (+0.5-+4.0 parts per thousand), a trend consistent with many instances of black shale deposition in the Phanerozoic. Organic carbon isotopes exhibit the broad, positive excursions (similar to+3.5 parts per thousand from background) that are typical of the KWEs globally. Superimposed over these positive excursions in delta C-13(org) are sharp decreases of up to similar to 3.0 parts per thousand within the black shale beds, to as low as -30.5 parts per thousand. The pattern of delta N-15(bulk) and delta C-13(org) values suggests that the depth of the chemocline and the degree of water-column stratification exert a primary control on both delta N-15(bulk) and delta C-13(org) during black shale deposition. In the context of the Frasnian-Famennian biotic crisis, the oscillating redox state and changing temperatures would have likely placed extreme stress on organisms within the marine environment of the AB and IB and may potentially have been a contributing factor to diversity loss over this time period.