THE END-TRIASSIC EXTINCTION EVENT: THE CASE FOR AN EXTRATERRESTRIAL RATHER THAN TERRESTRIAL CAUSE

Most theories about the end-Triassic extinction (ETE) event involve a relationship with the early Jurassic central Atlantic magmatic province (CAMP) LIP. We report new evidence from the Čanj section in Montenegro that indicates an impact event at the Tr-J boundary.

Most studied sections containing the Tr-J boundary are limited to shallow marine or terrestrial sections, which are prone to discontinuities and hiatuses. However, the Čanj section in Montenegro provides a continuous deep-marine succession containing the Tr-J boundary. The section exposes Triassic limestones of the Halobia limestone fm, which are overlain by carbonate poor Jurassic red beds of the Passée Jaspeuse fm. The boundary between the formations is sharp and is marked by a few-cm thick grey clay layer, which coincides with a negative δ¹³C_org excursion that is interpreted as the start of the ETE (Črne et al., 2011). An abrupt loss of CaCO₃, enrichment of Ni, Cr and Co and a modest enrichment of Ir (±600 ppt; <50 ppt average background) is observed at this clay layer. Fourier transform of the cyclicity in the section yields sedimentation rates of 3.7 – 5.0 mm/ky for the Triassic and ± 3.4 mm/ky for the Jurassic. Extrapolation of the sedimentation rates to the boundary layer suggests it is deposited within 15 ky. We note no progressive changes in geochemistry or lithology of the Triassic limestones towards the boundary implying a sudden termination of calcareous sedimentation following the deposition of an Ir-enriched clay layer. Such an abrupt change is comparable to that of pelagic environments across the K-Pg boundary (e.g. Alvarez et al., 1990).

Stratigraphical correlation of Tr-J sections around the world indicates the global negative δ¹³C_org isotope excursion to be contemporaneous with the onset of CAMP. Observations at Čanj fail to explain how the onset of CAMP had a major, abrupt effect on the global δ¹³C_org during the first 15 ky, whereas hardly any effect is observed during the continued outpouring of CAMP over the next 600 ky. Considering the enrichment of Ir, Ni, Cr and Co, the sudden lithological change but absence of shocked quartz and spherules at the boundary layer in Čanj we argue the ETE to be caused by a catastrophic large impact event in the oceanic realm.

Alvarez et al. (1990) Science 250(4988) 1700-1702; Črne et al. (2011) Geol Soc Am Bull 123(1-2), 40-50.