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 (δ¹⁵N_bulk) and organic carbon (δ¹³C_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 ¹⁵N-depleted (−1.0–+2.0‰) and have significantly lower δ¹⁵N_bulk than interbedded grey shales (+0.5–+4.0‰), a trend consistent with many instances of black shale deposition in the Phanerozoic. Organic carbon isotopes exhibit the broad, positive excursions (~+3.5‰ from background) that are typical of the KWEs globally. Superimposed over these positive excursions in δ¹³C_org are sharp decreases of up to ~3.0‰ within the black shale beds, to as low as −30.5‰. The pattern of δ¹⁵N_bulk and δ¹³C_org values suggests that the depth of the chemocline and the degree of water-column stratification exert a primary control on both δ¹⁵N_bulk and δ¹³C_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.