Palynology and Paleobotany of Permo-Triassic Beacon Supergroup at Allan Hills, South Victoria Land, Antarctica: stratigraphical and paleoenvironmental change implications

Nowadays the Antarctic continent is almost entirely covered by ice (around 98% of the total land surface) and the conditions are inhospitable for vegetation, apart from very few species such as mosses and lichens. During the geological time however, conditions were very different and the Phanerozoic fossil record documents several occurrences of vegetation remains also indicating the presence of wide high latitude forests. The life of plants in the continent was obviously strictly influenced by the evolving paleogeography and paleoenvironmental conditions and their mutual interactions during each time age of vegetation record. The thesis project has been finalized to define, constrain and discuss with new field and laboratory data the most likely Late Permian and Triassic paleoenvironmental reconstructions for the Victoria Land region in Antarctica, on the basis of a new set of paleobotany and palynological investigations of the unique fossiliferous strata recently found in the Beacon Supergroup of Allan Hills (South Victoria Land). The study was developed following a broad multidisciplinary and multi-analytical methodology in which paleobotany (including innovative approaches), palynology and palynostratigraphy methods and techniques play a key role in the reconstruction of the paleoenvironmental conditions and their changes through the time. In the palynostratigraphic sequence of Allan Hills were recovered the EPE, a strata horizon with long-shaped inertinite probably referred to a paleo-fire, situated in the last level of coal of the Permian sequence; going up in the sequence the paleoflora is affected by deeply change, as an adaptation to the new environmental condition up to the PTB, were the major samples were completely inert, due to a poor presence of flora and a changing of the sedimentary condition. After the PTB the first palynomorphs recovered are associated to an intensive fungal and algae activity, and just at the end of the Early Triassic, the flora came back to be flourishing, even if with a quite completely new association of pollen and spores, where is dominant the genus of Alisporites in the whole Middle and Late Triassic. The genus of Alisporites is probably linked to the Dicroidium macroflora, playing a central role during the Triassic time, replacing the Glossopteris flora which, on the other hand, dominated the Permian landscape. The Dicroidium, together with other genera, were recovered in great abundance in the Middle Triassic deposits of the Lashly Formation in Allan Hills, particularly within a horizon containing a so-called “Allan Hills Fossil Forest”, even if the term “forest “ is not strictly pertinent, as a way the trunks were deposited and transported by a massive flow, and no more in the growth-position. With an innovative technique was possible to reconstruct the main differences between the trunks there deposited and some other Permian fossil trunks outcropping in other areas of South Victoria Land. From this study, after a completely reconstruction of 237 years, that is the longest life-time never done for so old samples, were possible to note that the amplitude of the rings is probably linked to the amount of hours of sunlight during the years, so they are linked to the latitude to which the land was at time life-time of the trees. The last topic analysed about the “Allan Hills fossil forest” is the peculiar kind of fossilization, with a multianalytic approach were in fact possible to reconstruct modalities of coalification/charcoalification and multi-phase fossilization. The PAHs analysis highlights the presence of a high percentage of PAHs >4 ringed polynuclear aromatic hydrocarbons, that are typical attribute to pyrogenetic materials as fluoranthene, 9,10-dimethylanthracene and Pyrene, moreover also the δ13C measured on the growth tree rings shows a shifting of the curve in the more external rings, as an enrichment of 13C due to a partial combustion. After a first phase where the trunks were partially burned due to paleofires, they were transported through massive water flows. Subsequently, they were buried, deformed and petrified by a massive silica gel, probably occurred mainly during the Jurassic sill intrusion. The last step of the fossilization is the precipitation of calcite in a rounded shape occurred for some samples. Taken together the results of the study provide new important constraints and implications for reconstructing the history of sedimentary processes and coupled changing of the paleo-flora during a transition of deeply transformation of the biodiversity linked to the onset of greenhouse conditions, which occurred after the Permo-Triassic boundary, particularly during the Early Triassic.