Shift in Sedimentary Dynamics in the Belluno Basin (Southern Alps, Italy) after the Peak of the Early Toarcian Oceanic Anoxic Event

The early Toarcian Oceanic Anoxic Event (T-OAE, ∼183 Ma) provides an analog to the consequences of extreme climate change and major perturbation of the global carbon cycle. This disruption of the Earth’s system in the Early Jurassic caused the extension of anoxic and euxinic waters, notably in the Alpine-Mediterranean Tethys and across the northern European epicontinental shelf and north African margin, as indicated in the geological record by the widespread occurrence of organic-rich black shales. This event has been extensively studied in marine and continental records. However, the environmental dynamics during the recovery phase of the T-OAE are still poorly understood.

The Southern Alps of northern Italy are a critical region for reconstructing the evolution and dynamics of the T-OAE. The best outcrops of the associated black shales are found in the Belluno Basin, a narrow pelagic trough that formed between two carbonate platforms in the earliest Jurassic. Here, they are found in successions featuring large-scale rhythmic alternations between limestone and marlstone, which are interpreted as having a mostly primary origin.

In this study, we integrate petrographic, geochemical, and mineralogical data from the Vajont Gorge section near Longarone to investigate short-term signals (on a couplet scale) and long-term paleoenvironmental trends (on an outcrop scale) after the peak of the T-OAE.

Our results reveal that the recovery phase from the negative carbon-isotope excursion of the T-OAE is accompanied by a gradual increase in carbonate deposition or preservation. We argue that a significant portion of this mud-grade carbonate originated from calcareous nannoplankton and aragonitic muds shed off platforms. However, the short-term signal of the initial variations in the aragonite input into the deeper basin was lost due to early diagenetic carbonate redistribution processes.

Furthermore, our findings suggest that the lithological rhythms are linked to cyclic variations in the strength of bottom-water currents. We hypothesize that these variations in bottom current activity were caused by episodic reactivation of the thermohaline circulation in the Belluno Basin, which could have facilitated the amelioration of bottom-water oxygen conditions after prolonged phases of water mass stagnation during the peak of the T-OAE.