Early Triassic palaeoenvironmental and floral changes on North Gondwana

Abstract

The end-Permian mass extinction is preceding the Early Triassic representing the time period of biotic recovery. The recovery patterns of the different marine faunal groups show great differences. Ammonoids and conodonts are the two marine clades that recovered quickly after the end-Permian extinction event. Their recovery was interrupted by Early Triassic marine extinction events such as the late Smithian event. The recovery of benthic organism was rather slow (reefs) or repeatedly reset by multiple minor extinction events during the Early Triassic (gastropods and bivalves). Consequences of the end-Permian event proposed for the continental vegetation range from a rapid and total collapse of terrestrial ecosystems to a rather long-term gradual change. The recovery patterns in the marine realm and terrestrial ecosystem changes are thought to be closely linked to environmental perturbation triggered by late pulses of the Siberian Trap volcanism and reflected in the Early Triassic C-isotope records, which are characterised by recurrent positive and negative anomalies. In order to investigate changes in marine palaeoenvironments, and their link to carbon cycle perturbations, late Permian to Middle Triassic deposits have been studied with a combined approach of sedimentology, palynology, palynofacies, carbon isotopes and organic geochemistry. The sedimentary sequences from the Trondelag and Finnmark platforms (Norway) represent the most expanded Permian- Triassic boundary sections containing well preserved organic matter. In order to assess the timing of ecosystem changes across the Permian-Triassic transition in Norway a high-resolution δ13Corg dataset has been generated. This dataset allowed the establishment of a chemostratigraphic scheme that could be correlated to other globally distributed Permian-Triassic boundary sections. The global correlation reveals that the extinction of marine organisms occurred during the stepwise negative decline of Cisotopes across the Permian-Triassic boundary. A reference area for the Early Triassic of the North Indian Margin is located in Pakistan. The sedimentary archives from the Nammal, Chhidru, Chitta-Landu and Narmia gorges in the Salt Range and Surghar Range have been studied to describe the marine environmental conditions in the aftermath of the end-Permian extinction. The sections are characterised by excellently preserved organic matter. Sedimentological and palynofacies data reveals several seal-level changes during the Early Triassic that could be correlated to a global Early Triassic sea-level curve. For the Dienerian poor oxygenation levels are indicated by high amounts of amorphous organic matter in the particulate organic matter assemblages. In contrast to previous studies our results do not support an anoxic event in the Griesbachian. Biomarker studies of a few samples suggest proliferating bacteria during the Early Triassic. In order to investigate short-termed climatic and vegetation patterns from the late Permian to the Middle Triassic, terrestrial palynomorph assemblages from the sedimentary archives of the North Indian Margin have been studied. Changing humidity conditions have been inferred from the spore-pollen ratios of the palynological records from Pakistan and South Tibet. The results demonstrate increased humidity in the Griesbachian persisting with short-termed fluctuations up to the middle Smithian. In the late Smithian, climate changed abruptly, humidity decreased and dryer climates prevailed in the Spathian and Anisian. Comparison to previous climate interpretations on Early Triassic palynological datasets from Norway reveals that the late Smithian climate change is not a local phenomenon of the North Indian Margin but has global significance. This climate change coincides with a major extinction event of ammonoids and conodonts. Probable causes for the observed climatic patterns are elevated atmospheric CO2 levels combined with orbital forced insolation changes that influenced the monsoon intensity on the North Indian Margin. The vegetation patterns on North Gondwana inferred from the palynological records from Pakistan document high lycopod abundances from the Griesbachian to the middle Smithian where the climax of lycopods is reflected in a spore spike. This spore spike is closely associated with a C-isotope negative excursion and followed by a marine faunal extinction event (late Smithian event). The striking similarities with the end-Permian spore spike documented in previous studies support the conclusion that both events (end-Permian and late Smithian) are driven by comparable environmental perturbations. The resurgence of gymnosperms in the Spathian 2.1 Ma after lycopod dominated ecosystems suggests the fading of volcanically induced environmental stress. Based on the palynological dataset the Early Triassic palynostratigraphy of Pakistan could be refined. Eight successive assemblage zones are distinguished and calibrated against ammonoid biostratigraphy and C-isotope chemostratigraphy. A comparison with other floral provinces reveals distinct local differences in the floral assemblages in the Permian and in the Spathian to Anisian interval. During the Griesbachian to Smithian time interval, floral assemblages show a more cosmopolitan character.