Palaeoenvironmental changes and vegetation history during the Triassic-Jurassic transition

Abstract

The Triassic-Jurassic (T-J) boundary, ~200 Ma, is known as one of the ‘big five’ mass extinctions in Earth’s history. The T-J transition is characterized by a major extinction in the marine realm but evidence for floral turnover is ambiguous. Palynological records across the T-J boundary are controversially discussed because of the paucity of sections with a sufficient time resolution and/or well established stratigraphic framework. The presence of both well-preserved palynomorphs and ammonites in the investigated key T-J boundary sections from the Northern Calcareous Alps in Austria, (Hochalplgraben and Kuhjoch) and southern UK (St. Audrie’s Bay) allow for an integration of terrestrial microfloral events in a marine biostratigraphic framework. The high-resolution palynological records presented in this thesis were used to 1) obtain a firm palynostratigraphic framework for the T-J boundary interval, 2) make a reconstruction of past changes in vegetation and climate, and 3) understand the magnitude and nature of the floral turnover. The present study shows that the first occurrence of Cerebropollenites thiergartii is useful as a palynological marker to identify the approximate position of the base of the Jurassic. One particular event in the Eiberg Basin is the deposition of organic rich sediments during the end-Triassic, co-occurring with a major negative carbon isotope excursion (CIE), a prasinophyte mass occurrence, and vegetation changes. Increased terrestrial organic matter influx is related to enhanced seasonality and erosion of the hinterland. Stratification of the water column may have caused anoxic bottom water conditions and black shale deposition. The observed warming during the CIE is probably the result of CO2 outgassing from massive flood basalt volcanism, and additional marine methane release. Hochalplgraben shows a change from gymnosperms to spore producing plants and an increased diversification of spore types during the latest Rhaetian corresponding with a trend to wetter conditions across the T-J boundary. By contrast, in St. Audrie’s Bay a mixed gymnosperm forest is replaced by a monotonous vegetation consisting of Cheirolepidiaceae, which corresponds to a diversity decrease and a change to a warmer and more arid climate. A compilation of T-J boundary sections across the world demonstrates the presence of Cheirolepidiaceae dominated forests in the Pangaean interior and an increase in spore producing plants near the Tethys Ocean in the earliest Jurassic. Stomatal frequency data from Germany indicates rising CO2 levels (up to 2750 ppmv) during the T-J transition. The increase in greenhouse gases caused a warmer climate and an enhanced thermal contrast between the continent and the seas. Consequently, the monsoon system got stronger and induced a drier continental interior and more intensive rainfall near the margins of the Tethys Ocean. Furthermore, St. Audrie’s Bay contains an end-Triassic palynofloral transition interval with four pronounced spore peaks, which can be related to precession-induced variations in monsoon strength. There is no compelling evidence of a global end-Triassic spore spike related to a catastrophic mass extinction event. Climate change is a more plausible mechanism for explaining the increased amount of spores and the differences between T-J palynological datasets.