PLIOCENE VEGETATION AND CLIMATES OF THE SIRIUS GROUP, ANTARCTICA: EVIDENCE FROM FOSSIL PLANTS, CLIMATE AND BIOME MODELS

Sirius Group strata in the Transantarctic Mountains contain fossil leaves and woody stems of Nothofagus beardmorenesis , a type of southern beech. Asymmetrical growth patterns in the stems suggest that these 'trees' were small dwarf shrubs with prostrate habit. A tundra environment was proposed for these Pliocene environments with mean annual temperatures of about -12 to -15°C, with a mean summer maximum temperature of +5°C and minimum winter temperature of -22°C (Francis and Hill, 1996; Hill et al., 1996).

However, this interpretation conflicts with the boundary conditions determined from the PRISM2 data set (Dowsett et al., 1999), which predicts deciduous forest vegetation in Antarctica characteristic of cool temperature climates, following earlier interpretations of the Sirius Group fossils. New simulations of global vegetation using the BIOME4 vegetation model, forced with climate estimates from an advanced numerical General Circulation Climate Model (HadAM3), now suggest that vegetation consisting only of cushion forb lichen moss tundra, prostrate shrub tundra, dwarf shrub tundra or shrub tundra could have survived in Antarctica during the middle Pliocene.

This consensus supports previous proposals (e.g. Harwood and Webb, 1990) that this Pliocene interval represents a period of warming between more frigid conditions during which the Antarctic ice sheets retreated, allowing the establishment of periglacial tundra-like conditions at the ice margins. More substantial warming, allowing the growth of deciduous forests in Antarctica, is not predicted by the models nor shown by the geological data, suggesting that PRISM boundary conditions should be changed and deciduous forests on Antarctica reclassified as tundra. The agreement between the new model results and geological data indicates that the Sirius Group sits comfortably in this Pliocene environmental setting, supporting the age suggested for these strata.

References: Dowsett et al., 1999. USGS Open File Report 99-535, http://pubs.usgs.gov/openfile/of99-535. Francis, J. E. and Hill, R. S. 1996. Palaios, 11, 389-396. Harwood, D. M. and Webb, P. N. 1990. Trans. Am. Geophys. Union, 71, 538-539. Hill, R. S., Harwood, D. M. and Webb, P. 1996. Rev. Palaeobot. Palynol., 94, 11-24.