Mid Cretaceous fossil forests of Alexander Island, Antarctica

Abstract

Spectacular fossil trees and shrubs are preserved within fossil soils in their original growth positions in mid Cretaceous rocks on Alexander Island, Antarctic Peninsula. These fossils indicate that diverse forest communities grew upon the floodplain areas of Alexander Island during the Albian at a palaeolatitude of 69-75 ° S. The fossil forests are preserved within the fluvial sediments of the Triton Point Formation of the Fossil Bluff Group, which represents the infill of a fore-arc basin. The fluvial environment matured from a braided river system, with frequent floods and unstable channel banks, to a more mature meandering river system with more stable floodplain areas. Low energy floods preserved plants that grew on the river channel banks in fine muds and silts. Catastrophic floods occurred rarely, possibly as a result of volcanic activity upon the magmatic arc 200 km away. These high-energy floods covered floodplain areas in coarse sands, entombing trees. Well-drained mollisols formed on emergent surfaces becoming rich and fertile and supporting diverse forest communities. The flora within the forests occurred as well structured communities that occupied different parts of the river floodplain. Open-woodland forests dominated by araucarian conifers, ferns and shrubs, occupied stable areas of the floodplain characterised by low-energy floods. Disturbance vegetation dominated by Taeniopteris, liverworts, angiosperms and ferns, occupied river channel banks where frequent floods provided fresh surfaces for these early colonising plants to flourish. Patch forest communities dominated by podocarp conifers, ginkgo trees, cycadophytes and ferns, grew on stable areas of the floodplain distal to the river channel. These mature climax forests were rarely disturbed by catastrophic floods. Evidence from the palaeosols, sediments and fossil wood of Alexander Island suggest that the climate was warm, temperate, and semi-arid with seasonal precipitation and intermittent wet phases. In structure and composition the Alexander Island forest are very similar to the warm temperate rainforests of New Zealand which experience mean summer temperatures of 16-22 °C and mean winter temperatures of 3-8 ° C. This study and other palaeobotanical data suggests that the Alexander Island forests are likely to have grown under similar temperatures