STABLE CARBON ISOTOPE (Δ13CCARB AND Δ13CORG) EVIDENCE OF THE TOARCIAN OCEAN ANOXIC EVENT FROM CENTRAL ITALY: EVIDENCE FOR A GLOBALLY MIXED TETHYS SEA.

The Toarcian Ocean Anoxic Event (T-OAE) occurred during the latest Early Jurassic and is one of several globally recognized Mesozoic OAEs. Geochemical evidence of this event is measured from biostratigraphically-constrained carbonate rocks and organic-rich (black) shales worldwide. Previous studies record a positive stable carbon (δ¹³C) increase of ~5‰, but the sulfur cycle and perturbation is less well constrained. Previous work indicates that a positive sulfur isotopic (δ³⁴S) excursion of ~6‰ is coincident with the δ¹³C excursion from several sections in Europe. The similar magnitude of these excursions to other OAEs suggests they had similar causes and that the carbon and sulfur reservoirs did not vary in size through time, despite the fact that several gypsum and evaporitic deposits formed during the Mesozoic. Here we explore the global expression of T-OAE by studying another section in central Italy and test what environmental constraints possibly characterize the carbon and sulfur cycles during this time.

We sampled 28 lime mudstone hand samples from the Marne Del Monte Serrone Formation of the Colle d'Orlando section in central Italy (Umbria region) to identify whether the T-OAE is preserved in these rocks. This 65-m-thick measured section comprises marly limestone and shale facies, with a thin (<30-cm-thick) black shale preserved in the lower part of the measured interval. Bulk carbonate (δ¹³C_carb) and bulk organic matter (δ¹³C_org) were sampled for chemostratigraphic correlation as the palaeontologic record here is not well constrained. Hand samples were drilled on fresh surfaces to obtain sufficient powder for isotopic analysis. We observe a clearly defined δ¹³C_carb increase from 0‰ to ~3.5‰. However, the δ¹³C_org record is less clearly defined, but δ¹³C_org values increase from ~ -30‰ to -26‰ at the peak of the excursion. The magnitudes of these results are similar to published studies, suggesting that these sections deposited in well-connected basins that were able to freely exchange with the global reservoir. Future work will analyze the same samples to generate a sulfur isotopic record, from carbonate-associated sulfate and pyrite, to better characterize the global to regional expression of this geochemical perturbation.