When does large woody debris influence ancient rivers? Dendrochronology applications in the Permian and Triassic, Antarctica

Abstract

Accumulations of woody debris and in situ forests in and near ancient river systems are studied in Permian through Triassic strata of Antarctica. These Permian and Triassic fossil ecosystems represent paleo-polar and paleo-high-latitude environments, respectively, and therefore represent unique ecosystem end-members for assessing the role of woody debris in influencing fluvial deposition and paleoecologic processes. A new application of dendrochronology, the cross-dating of ring widths from individual trees, is applied to these fossil woods to develop a chronology of wood growth in the transported woody debris relative to in situ stumps. Cross-matching of woody debris relative to in situ stumps for the Permian indicates that woody debris deposition in river channels was contemporaneous with the development of vegetated macroforms in in-channel fluvial deposits. In contrast, Triassic woody debris occurs without evidence of vegetated in-channel macroforms, yet the volume and areal distribution of woody debris is greater than that measured in the Permian. Floral diversity increased dramatically across the Permian–Triassic transition in Antarctica, in addition to an increase in tree density in Triassic forests relative to Permian forests. However, these observations do not clearly explain the difference between the woody debris influence on Permian river systems relative to Triassic river systems. Paleoclimate simulations, however, indicate that the Permian polar environment was in surplus of water during the austral summer relative to the Triassic, consistent with paleosol morphology, suggesting that paleoclimate variation exerted a prominent control on vegetation-river dynamics.