Biodiversity and palaeoecological significance of tertiary fossil floras from King George Island, West Antarctica

Palaeogene volcanics with plant-bearing sediment intercalations crop out extensively on King George Island in the South Shetland Islands, West Antarctica. The plant fossil assemblages are the most complete Palaeogene terrestrial foliar record in Antarctica. Compositional variations in the flora have previously been used to construct climate change models for the Tertiary. King George Island is part of the late Triassic to Recent, Andean - West Antarctic subducting margin. Eastwards subduction oceanic crust beneath the Antarctic Peninsula resulted in mountain building and crustal melting at depth that in turn led to large stratovolcanoes and active pyroclastic volcanism (Leat et al., 1995). Consequently the flora is preserved in a range of primary and reworked volcaniclastic sediments, that were deposited in lacustrine and ephemeral lacustrine basins developed on the volcanic surface. The King George Island flora consists of impressions and carbonised compressions of leaves of angiosperms, gymnosperms, and ferns, in addition to new records of an angiosperm infloresence, fruits and coniferous cones. The flora comprises vegetation elements with a disjunct modern Southern Hemisphere distribution, such as Cunoniaceae, Nothofagaceae, Proteaceae, Sterculiaceae, Lauraceae and Myrtaceae. Close affinities are suggested with the cool to warm temperate forests of southern South America. Morphotype analyses based on leaf venation architecture have been used to group the flora into 85 morphotypes based on 428 specimens. The morphotypes have been used in foliar physiognomic and nearest living relative palaeoclimate analyses, which suggest warm microthermal climates for the Middle Eocene in West Antarctica. The Dragon Glacier and Mt. Wawel floras from Point Hennequin are currently regarded as impoverished Upper Oligocene, post-glacial floras. However, 40Ar/39Ar dating of the encapsulating lavas indicates that they are Middle Eocene in age (44 - 49 Ma) and field collections have yielded a diverse range of plant fossils from the localities. In composition, tectonic setting and climatic regin1e, the closest modern analogue for the King George Island flora is the Valdivian rainforests of Chile. The composition of these forests is controlled by disturbance, largely of volcanic origin. As such, local variations in vegetation composition previously attributed to climatic change during the Palaeogene could be explained in terms of disturbance related succession.