Paleoenvironmental interpretations for the End Devonian faunal crises using carbon and nitrogen isotope distributions from the Western Canada Basin

The Devonian-Carboniferous boundary marks a critical interval in the Earth's history when the marine community experienced a pronounced global loss of biodiversity. The Hangenberg event, which eliminated almost 21% of marine genera and 16% at the family level (Simakov, 1993; Sepkoski, 1996), represents the final phase of the Late Devonian mass extinction. The widespread biological overturn apparently due to climatic perturbations is associated with the deposition of organic rich black shales in epicontinental and epeiric basins that indicate eutrophication and marine anoxia as a potential trigger to the faunal crisis. In the present study, stable isotopes of carbon and nitrogen were measured for black shales (Exshaw Formation) deposited during the Devonian-Carboniferous interval in seven locations representing a range of water depths in the Western Canada basin. Positive excursions in δ¹³C profiles and high total organic carbon values reflect increase in the fraction of organic carbon burial. Similar to other major mass extinction events, the Hangenberg event was followed by a shift in the phytoplankton community from algae to cyanobacteria, which was discernible from the isotopic records. Bulk δ¹⁵N values between 0-3‰ can be ascribed to increased seawater denitrification of nitrate which results in enhanced nitrogen fixation by cyanobacteria and a decrease in δ¹⁵N of the ecosystem. The residence time of fixed nitrogen in modern deep ocean settings is relatively short (~1.5 to 3kyrs) whereas in the surface waters it is significantly shorter, on the order of days, due to use by primary productivity. For this reason the seawater fixed nitrogen has the potential to show local dissimilarities (Liu et al., 2015). Spatial and temporal variations in δ¹⁵N are observed due to regional differences in primary productivity, nutrient utilization and denitrification rates. Paleogeographic and paleolatitudinal reconstructions suggest that these black shales were deposited in the low latitudes on the eastern margin of Panthalassic Ocean. The climate was significantly warmer facilitating continental weathering and increased nutrient fluxes to the ocean. Tropical trade winds and divergent surface ocean currents caused localized upwelling of nutrients with heavier δ¹⁵N from anoxic/suboxic bottom layers. Such conditions are reflected as perturbations in the nitrogen isotope signal. Transgressive events led to oscillating redox conditions in the coastal regions from oxic to partial and total euxinia. Small perturbations in the carbon and nitrogen signal could be combined with sequence stratigraphy to understand the contribution of eustatic changes in photic zone euxinia.