Palaeoenvironmental and biotic change through the Lower Jurassic in Bulgaria

The early Toarcian was a time of widespread environmental and biotic changes linked to the eruption of the Karoo-Ferrar large igneous province (LIP). Despite the purported global nature of these changes, the majority of the data come from western European sections, with a particular focus on settings in which the early Toarcian Oceanic Anoxic Event (T-OAE) was clearly expressed. This thesis examines a series of Lower Jurassic successions in northwest Bulgaria that were deposited on an open-ocean facing carbonate shelf in the northwest Tethys Ocean. The Bulgarian δ13C and δ18O profiles show similar trends through the Lower Jurassic to coeval European sections, suggesting that seawater in the Moesian Basin was recording global palaeoceanographic conditions. The carbon isotope record reveals a broad positive carbon isotope excursion (CIE) of 3 ‰ in δ13Cbel through the early Toarcian, interrupted by a negative CIE of 3.5 ‰ in δ13Corg. Rapid warming of seawater temperatures and increased influx of freshwater into the basin through the early Toarcian is indicated in δ18Obel. Such changes are attributed to the eruption of the Karoo-Ferrar LIP, reflected in the Moesian Basin by an enrichment in Mercury (Hg) recorded as a shift in sedimentary Hg/TOC values, synchronous with the negative CIE. A biotic crisis is recorded amongst bivalves in Bulgaria for the first time. Although a significant loss amongst bivalves during the early Toarcian often coincides with the spread of anoxia elsewhere, this link is not clearly seen in Bulgaria. Framboid analysis and a lack of laminated sediments during the early Toarcian indicate oxygen-deficiency as an unlikely driver of the biotic crisis. Rapid warming of seawater temperatures are tentatively suggested as a key driving mechanism in this central Tethyan region.