"Life finds a way”: Palaeoceanographic reconstructions of the Early Jurassic using stable isotope analyses and their relationship to extinctions and bivalve shell size changes.

Abstract

The Early Jurassic is a turbulent time for ecology and climate. The end – Triassic extinction (ETE) at the beginning of the Jurassic was caused by the large igneous province: Central Atlantic Magmatic Province (CAMP), and ecology fluctuated between radiation during the recovery period of this extinction and another biotic crisis at the Pliensbachian – Toarcian boundary and the Toarcian Oceanic Anoxic Event. Between these two extinctions, great change within both ecology and climate occurred. In producing long term, high resolution stable isotope records for the Cleveland Basin core: Felixkirk, (13Corg, 15N and 34S) climactic changes were suggested across the Early Jurassic. These records indicated: the Triassic – Jurassic boundary, CAMP and ETE, are coeval to a large excursion and increased anoxia within deeper waters which spread into the upper water column. The Sinemurian – Pliensbachian experienced anaerobic bottom waters and enhanced pyrite burial relating to increased continental weathering and a global low stand. Bivalves were measured at the Triassic – Jurassic and Sinemurian – Pliensbachian boundaries within the Felixkirk core and analysed statistically against evolutionary models. At each boundary, the size of three genera were compared to evolutionary models. Some bivalves indicated they exploit the ecological niche and grow in size after the TJB (Pteromya). However, there is a 54% chance that this change in size was representative of an unbiased random walk model. The other two genera measured (Protocardia and Modiolus) were determined to suggest stasis to a 93% significance. Across the Sinemurian – Pliensbachian, the number of available fossils to measure was incredibly low, further indicative of anoxia. These fossils however appeared to shrink then immediately grow after the excursion. When comparing this data to evolutionary models, both Pseudoliema and Camptonectus indicated stasis, yet Protocardia was indictive of the Ornstien- Uhlenbeck model indicating a distinct evolutionary change because of a distinct niche.