EXPLORING THE TEMPORAL LINK BETWEEN THE KAROO LARGE IGNEOUS PROVINCE AND BIOTIC AND ENVIRONMENTAL CHANGE IN THE EARLY JURASSIC

The causal link between the volcanic activity of Large Igneous Provinces (LIPs) and periods of biotic and environmental disturbance in the geological past can be investigated through their contemporaneity. High-precision U/Pb dating of igneous zircon yields ²⁰⁶Pb/²³⁸U age information of 0.05 % uncertainty, which allows for resolution of 30-50 ka magmatic pulses in Mesozoic LIPs and their correlation with carbon isotope excursions (CIE), biodiversity crises, or geological boundaries. The geochronology community thus targets causality of events at the 10⁴ year level via contemporaneity. With this resolution, we investigate the relationship between pulses of magmatic activity in the Karoo Large Igneous Province (K-LIP) and repeated biotic and environmental crises during the late Pliensbachian and early Toarcian, between ~186 and 178 Ma.

The initial and short-lived intrusive pulse of the Karoo Large Igneous Province (K-LIP) formed the sill/dyke complex of the Karoo basin, South Africa. New U/Pb dates confirm previously reported magmatic activity at around 183.4-183.0 Ma, as well as its synchronicity with the lower Toarcian carbon cycle disturbance and oceanic anoxic event (T-OAE) at around an age of 183.22 ± 0.25 Ma (Sell et al., 2014). Carbon isotope excursions in the late Pliensbachian (~185.5 Ma) and at the Pliensbachian-Toarcian boundary (>183.5 Ma) are therefore at least partly older than any known magmatic activity of the K-LIP (Lena et al., 2019). This requires non-volcanic drivers to explain these instabilities of the carbon cycle. The younger igneous events of the Ferrar LIP 182.8-182.4 Ma (Burgess et al., 2015), and other magmatic pulses associated with the K-LIP at around 180.5 Ma and 176.8 Ma in the Lebombo and Mwenezi monoclines, respectively indicate that younger pulses of magmatic activity may have been at the origin of subsequent, repeated biotic-environmental disturbances. These examples underline that high-precision U-Pb geochronology is required for correlation of planetary-scale events in geo-, atmo- and biospheres at the 10⁴ years resolution.

References: Burgess et al. (2015) Earth Planet. Sci. Lett. 415, 90–99; Greber et al., subm. to Results Geochem., Lena et al. (2019) Sci. Rep. 9, 18430, Sell et al. (2014) Earth Planet. Sci. Lett. 408, 48–56