Early Jurassic Toarcian Timescale Calibration

The Jurassic timescale is relatively poorly constrained because of a lack of precise radio-isotopic ages from (bio)stratigraphically well-constrained sedimentary archives. For example, the Early Jurassic (Hettangian, Sinemurian, Pliensbachian and Toarcian stages) is characterized by only two relatively robust temporal anchor points, at the Triassic–Jurassic boundary (~201.4 ma) and at the Pliensbachian–Toarcian boundary (~183.7 Ma). Recent advances in Early Jurassic timescale development largely stems from cyclostratigraphic study of key geological (core and outcrop) archives spanning entire Stages.Here, we present and discuss an example of Jurassic timescale calibration, and the opportunity that new data and approaches provide. We discuss the Toarcian Oceanic Anoxic Event (T-OAE), one the largest and best studied global change events of the past 200 million year. The T-OAE is characterized by global climatic/environmental disturbance related to massive (volcanogenic) carbon release. Sedimentary archives of the T-OAE are consequently marked by a 4–7‰ negative carbon isotope excursion (nCIE) with a cyclostratigraphically estimated duration of ~1 million year, and with astronomical forcing of its stepwise onset. A recent study presenting U-Pb radio-isotopic dating of ash-horizons in a marine sedimentary archive from Japan capturing this event however challenged this view, suggesting a mere ~200 kyr duration for the T-OAE nCIE. We here present cyclostratigraphic analyses of data spanning the entire Toarcian Stage in the Mochras borehole (Wales, UK) as well as new U-Pb radio-isotopic analyses of ash-beds in the Toarcian successions of the Neuquen Basin (Argentina). We utilize this integrated dataset to present a new numerical timescale for the entire Toarcian Stage. We combine this data with new insight from International Continental Drilling Programme (ICDP) coring projects to discuss the implications of these findings for, and the future opportunities in Jurassic timescale calibration.