In situ fossil forest from the upper Fremouw Formation (Triassic) of Antarctica: paleoenvironmental setting and paleoclimate analysis

摘要

A structural and ecological study of a standing Triassic forest containing 99 silicified trunks from the Fremouw Formation of Antarctica is detailed. Utilizing quantitative methods, forest density, total forest cover per hectare, mean separation of trees, and basal area per stump were obtained. This information, integrated with sedimentologic, taxonomic, fossil wood, and biomechanical data, has allowed the reconstruction of a plant community that grew at very high paleolatitudes (～70-75°S). The paleoforest grew along river banks (levees) and within proximal floodplain environments, where a taphocoenosis of permineralized stumps and compression-impression foliage of the Dicroidium type has been preserved. In particular, the intimate association of this leaf type with dense, conifer-like wood provides additional confirmation that the Dicroidium foliage morphotype was attached to several types of stems. This riparian forest stand was apparently in a mature stage prior to the beginning of preservation. Tree ring analysis, as well as additional indirect evidence, indicates that this Middle Triassic ecosystem in Antarctica experienced a season very suitable for plant growth resulting from an overall favorable climatic regime. This conclusion contrasts with earlier paleoclimate models based on physical parameters in which temperature ranges would make plant or animal growth at these latitudes almost impossible. Finally, Antarctic climatic change from latest Permian through Late Triassic is considered in the context of tectonic and paleogeographic reconstructions of the East Antarctic craton.