Rivers, Vegetation and Extinction

近5亿年前，随着植物开始在地球上定居，地球表面的“绿化”是地球上最深刻的进化变化之一。在接下来的2亿年里，植物从根系较差的草本植物到根系较深的森林变得多样化，增加了它们对景观的覆盖，并更有效地攻击基岩以产生泥浆。与这些变化并行的是，我们早期的工作表明，由于扎根的植被稳定了河岸和洪泛平原，蜿蜒和多线程的河流实际上是第一次出现。这导致了地球景观的全面改造，突出了植被作为景观演变的强迫因素的重要性。随着超级大陆盘古大陆的形成，大部分陆地表面变得更加干旱，西伯利亚的大规模火山爆发导致了地球历史上最严重的灭绝，影响了二叠纪-三叠纪边界的动植物。森林被低矮的草本覆盖所取代，但在大约500万年后重新建立起来。这种主要的植物枯死和重组是否引起了河流风格和陆地景观的明显变化？如果是这样的话，陆地表面是否像一些当地研究表明的那样，在植被进化之前短暂地恢复到简单的辫状河流的模式？

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.