PERMIAN-TRIASSIC PALEOCLIMATE AT HIGH PALEOLATITUDES IN GONDWANA: TREE RINGS AND FOSSIL FLORAS AS PROXY INDICATORS

The Beacon Supergroup in the Central Transantarctic Mountains (CTM) has yielded a number of well preserved plant fossils which provide proxy information on paleoclimate. The best sources of anatomical information, including fossil tree rings, are two sites with permineralized peat, one in the upper Buckley Formation (Late Permian) and the other from the upper Fremouw Formation (Middle Triassic). The peat preserves cellular detail of plants, and thus provides information on tree ring structure in woody plants, as well as details of floral diversity and plant growth.

During both periods, the Earth had a greenhouse climate, with temperatures in polar regions sufficient and even favorable for plant growth. Paleogeographic reconstructions suggest that the CTM floras were growing above 70°S in the Triassic and above 75-80°S in the Permian. Triassic tree rings average 1.71 mm in width, and Permian ones are only slightly below that size. Although ring width is similar from both time periods, floral diversity is not. Late Permian floras throughout Gondwana were of very low diversity; in Antarctica some sites show up to 90% coverage for glossopterid seed fern leaf types (i.e., Glossopteris and Gangamopteris). In contrast, the Middle Triassic flora is of medium diversity, and includes a number of small ferns, as well as plants that were probably frost-sensitive, such as cycads.

Permian and Triassic woods both exhibit a large amount of earlywood and very little latewood, suggesting a long, favorable growing season and a short transition to dormancy. At these paleolatitudes, the plants must have experienced 24 hours of light during the summer. Minimal change in radial cell diameter across individual rings and the small amount of latewood may be due to a rapid transition to low light levels at the end of the growing season. Mean sensitivity, which measures the difference in ring widths from one year to the next, suggests that there was some climatic variability from year to year, but that the plants were only slightly stressed. However, mean sensitivities do not vary much between Permian and Triassic woods, signifying that there was not a dramatic change in climate variability between these time periods. Tree ring analysis data will be presented and compared with floral data and with previously published paleoclimatic reconstructions.