LONG-TERM OCEANIC STABILITY AND ORBITAL CONTROL ON CARBON CYCLE PRIOR THE LATE TRIASSIC MASS-EXTINCTION

We established a new high-resolution carbonate isotope record from the lower Norian to the late Rhaetian in the Northern Calcareous Alps in Austria. The new curve has an excellent biostratigraphic control based on ammonoids and conodonts. Among the four sections sampled was the proposed GSSP section (Steinbergkogel) for the Norian-Rhaetian Boundary. The middle Norian to lower Rhaetian is composed of a sequence of different Hallstatt-type limestone. These consist of fine-grained bioclastic wackestone deposited from periplatform ooze. The Rhaetian terrigenous event of the Zlambach Formation ended the Hallstatt facies deposition. Its background sedimentation of alternating marls and subordinate micritic limestone is episodically overlain by allodapic carbonate sedimentation. The Zamblach sequence was deposited in a toe-of-slope to basin environment. The carbon isotope curve display a gentle decrease from the late early Norian (3.5‰) to the base of the Rhaetian (1.8‰) with two accelerated steps, one in the middle Norian and the other one just after the Norian-Rhaetian Boundary. This last 1‰ decrease correspond however to a change in lithology between the Hallstatt facies and the alternation of marls and limestone. The values show then a small increase during the early Rhaetian, with a maximum in the middle Rhaetian (at 2.4‰). The general stability of the curve even through the Norian-Rhaetian boundary crisis event describes a stable oceanic structure prior the mass extinction.

Superposed to this long-term trend, the δ¹³C isotopic curve in the Zamblach Formation records distinctive cycles. First results of the spectral analyses reveal prominent 400 kyr. cyclicity in the curve, which correlates with Milankovitch long eccentricity changes. Cycles occurring in our record resemble those observed in several Cenozoic and Cretaceous records, suggesting that a link between orbital forcing and carbonate cycling existed also in the Late Triassic time. These 400kyr cycles in the Late Triassic could have been linked to sea-level changes influencing the carbonate export from the platform or, as during the Cretaceous, be related to a fluctuating monsoonal regime.