| Popis: |
The generally dry Late Triassic climate was interrupted by a wet phase during the Carnian termed Carnian Pluvial Epi sode (CPE) accompanied by facies changes, biotic turnovers and global carbon cycle perturbation that lasted ca. 1 mil lion years. In SW England, the terrestrial succession of the Mercia Mudstone Group exposes a Carnian sedimentary se ries that is equivalent of the Keuper Marls elsewhere in NW Europe and records the CPE. The mainly lacustrine MMG provides evidence for a climate change from a different terrestrial realm compared to the classical areas of the Carnian Pluvial Episode e.g., the fluvial-alluvial facies of the Schilfsandstein (Stuttgart Formation) in the Germanic Basins. Quantitative and qualitative palynological data from the Sidmouth, Dunscombe and Branscombe Mudstone Formations within the Mercia Mudstone Group are presented in order to reveal vegetation changes and paleoclimate trends. Palynostratigraphy and the integrated bulk organic carbon isotope data enable the correlation to other CPE successions. A total of 104 samples were processed for palynological analysis from three outcrops and the Wiscombe Park-1 borehole. From the total of 104 processed samples only 36 provided palynomorph assemblages. A total of 81 spore and pollen taxa and five aquatic palynomorphs are distinguished. Our new palynostratigraphy contributes to the biostratigraphic subdivision of the MMG. The previous palynological approaches assigned the Dunscombe Mudstone Formation to the Carnian without further dating to a substage level. The palynological assemblages indicate that the Dunscombe Mudstone Formation is Julian in age and the lower studied part of the Branscombe Mudstone Formation is still Tuvalian. The Aulisporites astigmosus acme, which is present in the Tethyan realm and the Schilfsandstein, is entirely missing in the studied successions. The quantitative palynological record suggests the predominance of the xerophyte floral ele ments through the whole CPE with only few horizons with the increase in the hygrophytes. The lack of the clear humid signal might be related to the strong seasonality in the precipitation in the generally dry inner part of Pangea and lack of suitable habitat for the permanent growth of hygrophyte vegetation (e.g. Aulisporites parent plants. However, the most likely explanation for the lack of clear humid signal is the overrepresentation of regional pollen rain in the lacustrine setting of the MMG with the predominance of xerophyte upland floral elements. The bias towards regional pollen rain is further enhanced by the potential increase in continental runoff related to more humid conditions |
