On the onset of CAMP volcanism, environmental change, and the magnitude of Carbon-cycle change at the Triassic?Jurassic mass extinction (Neuquén Basin, Argentina)

RUHL, M.; HESSELBO, S.; MANCEÑIDO, M.O.; AL-SUWAIDI, A.; JENKYNS, H.; DAMBORENEA, S.E.; STORM, M; RICCARDI, A.C

Milano

Congreso; 3rd International Congress on Stratigraphy; 2019

International Union of Geological Sciences

The Triassic?Jurassic transition is marked by the end-Triassic mass extinctionapproximately synchronous with the onset of emplacement of the Central Atlantic Magmatic Province (CAMP), and associated with a major negative carbon-isotope excursion (CIE) in the ocean?atmosphere system. New ammonite collections allow us to present a bio- and chemostratigraphically constrained and expanded Triassic?Jurassic boundary succession from the Arroyo Alumbre section in the Rio Attuel Region of the Neuquén Basin, Argentina. We show that the end-Triassic mass extinction is marked here by a limited (2?3?) negative CIE in bulk organic matter, similar in magnitude to that observed in other eastern Panthalassic marine basins, and to δ13CCARB records from the Tethyan realm. However, this value of 2?3? in the Argentinian section is significantly smaller than that recorded in contemporaneous Tethyan 5?6? negative excursions in δ13CTOC. We present a model that suggests that extreme aridity, extending between latitudes of 60º north and south across the western Pangaean landmass, may have resulted in very limited terrestrial organic-matter flux to the sedimentary realm in eastern Panthalassic marine basins, and that mixing and changing marine/terrestrial organic-matter sources explain the observed larger amplitude negative CIE in δ13CTOC records from the Tethyan realm.We further show that increased accumulation rates of sedimentary Hg (and Hg/TOC) inthe marine Neuquén Basin began significantly before the end-Triassic mass extinction andassociated negative CIE, and before the commencement, in North America and Africa, ofCAMP-related basalt emplacement. The onset of sedimentary Hg enrichment in the Neuquén Basin directly coincided with the early emplacement of CAMP-associated dykes and sills, suggesting thermal alteration of intruded country rocks as a potential major source of elevated Hg fluxes to the atmosphere at this time. We also show that the Neuquén Basin was marked locally by the development of oxygendepleted marine conditions across the Triassic?Jurassic transition as a result of increased primary productivity and/or watermass stratification, enabling increased preservation of organic matter. Combined with similar observations across the Panthlassic margin and the north-west Tethyan seaway, burial rates of organic matter must have been relatively elevated in a global context. Using simple mass-balance calculations, we show that enhanced carbon burial rates, either during or directly succeeding the end-Triassic mass extinction, and in line with the major phase of CAMP basalt emplacement, can explain the observed evolution of the global exogenic carbon cycle across the Triassic?Jurassic transition.