Carbonate facies changes and Carbon isotope excursions across the Triassic-Jurassic transition (Lombardy Basin, Italy): Local effects or global changes?

Global climatic, atmospheric and oceanographic changes may affect the Carbon cycle and be recorded in the C stable isotope signature of marine carbonate and terrestrial and marine organic matter deposits. Besides global events, δ13C excursions recorded in carbonate successions and organic matter-rich deposits may result from diverse local factors, masking or mimicking global C-cycle perturbations. Distinguishing the contribution of global C-cycle perturbations or local and regional sedimentary and environmental factors to the observed changes in the δ13C vertical trends is fundamental for the understanding of coupled sedimentological and geochemical studies. To delve into this issue, a previously well-studied, continuous carbonate succession across the Triassic-Jurassic transition in the Lombardy Basin (Southern Alps, N Italy) has been revisited investigating changes in facies association and variations of δ18O, δ13Ccarb, δ13Corg and C/N (atomic) values. The Triassic-Jurassic transition witnessed a cascade of global environmental perturbations responsible for the End Triassic Extinction, triggered by the injection of volcanic CO2 in the atmosphere from the Central Atlantic Magmatic Province. Besides the demise of Rhaetian biota, the studied carbonate succession is marked by abrupt facies changes: from marlstone-limestone cyclothems with corals, bivalves and foraminifers (Zu Limestone) to laminated calci-mudstone and marlstone (Malanotte Formation) overlain by progradational ooidal grainstone/packstone shoals (Albenza Formation). The δ13Ccarb and δ18O values are within those expected from Rhaetian-Hettangian marine carbonates, but relationships between the δ13Corg and C/N curves suggest a possible influence by changes in the ratio of terrestrial vs. marine organic matter. The integrations of vertical facies superposition with the δ18O, δ13Ccarb, δ13Corg values and C/N ratios are in tune in the three studied successions, suggesting possible alternative interpretations to global C-cycle perturbations. Changes in the δ13Corg values (in the range of ±1 ‰) may be explained by the variability of siliciclastic supply associated with different (terrestrial vs. marine) types of organic matter.