Rivers, Vegetation and Extinction

The "greening" of the terrestrial surface as plants started to colonize the landscape nearly 500 million years ago was among the Earth's most profound evolutionary changes. For the next 200 million years, plants diversified from poorly rooted herbs to deeply rooted forests, increased their cover of the landscape, and attacked the bedrock more effectively to generate mud. In parallel with these changes, our earlier work has shown that meandering and multi-thread rivers appeared, virtually for the first time, as rooted vegetation stabilized river banks and floodplains. This resulted in a total reworking of the Earth's landscapes, highlighting the importance of vegetation as a forcing factor in landscape evolution. As the supercontinent Pangea assembled, much of the land surface became more arid, and vast volcanic eruptions in Siberia contributed to the most profound extinction in Earth history, affecting both animals and plants at what is known as the Permian-Triassic boundary. Forests were replaced by a low herbaceous cover but were re-established after about 5 million years. Did this major floral dieback and reorganization cause a detectable change in river styles and the terrestrial landscape? If so, did the land surface briefly revert to pattern of simple braided rivers much as before vegetation evolved, as suggested by some local studies? Answers to these questions require a global assessment of the geological record. Based on a compilation of literature for ancient river deposits over a period of 100 million years that includes the extinction event, the proposed research will document the relative abundance of fluvial styles and fossil materials through this important time. During field investigations at selected sites, we will work closely with fossil experts to assess the interaction of vegetation and river systems, focusing on the degree of preservation of the plants (a key problem in assessing changes in the original plant communities) and links with animal evolution and extinction. Because of arid conditions in Pangea, vegetation may have been under pressure before the extinction period, and detailed studies of ancient river systems and plants will be conducted at fossiliferous sites in Texas, New Mexico, and Germany, among other sites. Over the next grant cycle, we aim to document the landscape effects of a major extinction, which is relevant to the ongoing anthropogenic extinction and loss of biodiversity in the 21st Century. A complementary programme will learn more about how rivers before landscapes were vegetated. The simple "sheet-braided" rivers that existed on Earth for more than three billion years before vegetation may be virtually unknown on Earth today: how did they function? This issue will be explored through field studies in northern Canada and the English Channel Islands. Although the development of meandering rivers was clearly promoted by plant evolution, there are other means of stabilizing river banks, including cohesive mud, early hardening of sand, and ice. Meandering rivers have been inferred for some pre-vegetational systems on Earth and have been identified in images of Mars. Evaluating bank strength and the dimensions of channels in these enigmatic rivers will be a key aspect of the studies.