Palaeoenvironmental changes in the Moesian Basin (Bulgaria) during the Toarcian

There is a wide range of data behind the concept that the late Pliensbachian–early Toarcian span (184.5–181.5 Ma) was a time of Earth-scale palaeoenvironmental changes. Rapid global warming, sea-level rise, perturbations in the carbon cycle and the development of widespread anoxia, known as the early Toarcian Oceanic Anoxic Event (T-OAE), are deemed amongst the major biogeochemical disturbances in the Mesozoic. A biotic crisis in the marine realm, with ~30% taxonomic loss, known as the early Toarcian mass extinction (T-ME), was broadly recognized. Many studies linked the T-OAE and the T-ME with the eruption of the Karoo–Ferrar igneous province in the southern hemisphere. The volcanism–warming–anoxia–extinction link has been well established and continues to collect evidence in numerous sites from around the world. The T-OAE has been indicated in several Bulgarian stratigraphic sections to date. The latter represent inner-shelf deposits of the Moesian Basin, which was developed proximal to the southern Eurasian passive continental margin. The overall depositional trend fits into the regressive phase of the second-order Ligurian (R6) T-R cycle, which is widely recognizable in the Jurassic basins of NW Europe. Locally, the deposition took place within a rapid late Pliensbachian–early Toarcian transgression, followed by a gradual regression to the mid-Bajocian times. It is related with widespread condensations and a collapse of the carbonate platform productivity. The manifestations of anoxia have been weak and more enhanced at post-T-OAE levels. Oxygen deficiency was evidenced in narrow and diachronous redox intervals with elevated TOC contents, V/Cr, V/(Ni+V) and Th/U ratios, and smallest mean framboid diameters. The δ¹³C_bel and δ¹⁸O_bel profiles showed similar trends to coeval European sections, in having a decline in oxygen-isotope signatures and rising in carbon-isotope values, both of >3‰, around the Pliensbachian/Toarcian (P/T) boundary. A decrease in ⁸⁷Sr/⁸⁶Sr ratios through the upper Pliensbachian, reaching a minimum around the P/T boundary, and followed by gradual increase throughout the Toarcian was also recorded. The isotope data revealed an increased freshwater influx and rapid seawater warming. The warming was attributed to the eruption of the Karoo-Ferrar, reflected by enrichments in mercury (Hg) recorded as a shift in sedimentary Hg/TOC ratios and concomitant with the δ¹³C_bel and δ¹⁸O_bel excursions. A biotic crisis was recorded amongst the benthic (bivalve and brachiopod) faunas and considered to be part of the T-ME. Both bivalves and brachiopods displayed clear NW European affinity, but characteristic ‘black shale’ taxa are virtually absent. A loss of ~40% of extinct bivalve species was evidenced around the P/T boundary. In brachiopods, the loss is even greater, with a prominent P/T boundary-hiatus and straggling occurrences above it, consisting mainly of autochthonous taxa. In the absence of ‘true’ anoxic setting, the T-OAE–T-ME link remains unclear. Therefore, rapid warming rather than oxygen deficiency has been driving the T-ME in the Moesian Basin.

This account takes part of the National Science Program “Environmental Protection and Reduction of Risks of Adverse Events and Natural Disasters”, approved by the Resolution of the Bulgarian Council of Ministers (No. 577/17.08.2018).