MARINE COMMUNITY DYNAMICS THROUGH THE EARLY TOARCIAN (EARLY JURASSIC) GLOBAL WARMING EVENT

The Pliensbachian-Toarcian interval was a period of severe environmental changes. Available geochemical data indicate a cooling event in the late Pliensbachian followed by prominent warming in the early Toarcian. Northern European epicontinental basins developed prolonged anoxic/euxinic conditions in the sediments and in the water column, which caused the deposition of organic rich facies in the early Toarcian. At the same time, a regional biotic crisis occurred in the marine realm, when approximately 5% of families and 26% of genera became extinct. Although the extinction is well documented in the literature, there is debate concerning the dynamics of the event. Key questions remain, e.g. how many extinction horizons were there, and which paleoenvironmental factors controlled species distribution? To address these questions we studied in detail the macro-invertebrate fauna from the coastal sections of the Cleveland Basin, Yorkshire (UK). Unlike previous studies based on the analysis of stratigraphic range data, we collected quantitative abundance data and analyzed the resulting dataset with multivariate statistical methods. Ordination of samples shows a high faunal turnover during the whole studied interval, during progressive environmental change, with no overlap between samples belonging to consecutive lithostratigraphic units. However, cluster analysis and analysis of similarity demonstrate the existence of just a single extinction horizon in the upper part of the D. semicelatum ammonite subzone, almost at the top of the Grey Shale Member (Whitby Mudstone Formation). This interval coincides with an abrupt decrease in species richness, and it is characterized by the short stratigraphic occurrence of a bivalve belonging to the Nucinellidae. Symbionts were recently discovered in one modern Nucinella species, and chemosymbiosis inferred for all the family. Its occurrence might thus mark the onset of more strongly reducing conditions in the sediments prior to anoxia. Ordination of samples coupled with sequence stratigraphy and paleotemperature proxy data indicates that the extinction horizon coincides with a rise in paleotemperature and sea level. The pattern of faunal degradation and recovery of the benthic community closely resembles the paths observed in modern habitats affected by anoxia.