Rivers, Vegetation and Extinction

近5亿年前，随着植物开始在陆地上定居，陆地表面变绿，这是地球上最深刻的进化变化之一。在接下来的2亿年里，植物多样化，从生根不良的草本植物到根深蒂固的森林，增加了它们对风景的覆盖，并更有效地攻击基岩以产生泥浆。在这些变化的同时，我们早期的工作表明，由于扎根的植被稳定了河岸和泛滥平原，实际上第一次出现了曲流河和多线河。这导致了地球景观的彻底重塑，突显了植被作为景观演变中的一个强迫因素的重要性。随着超级大陆盘古大陆的聚集，陆地表面的大部分变得更加干旱，西伯利亚的大规模火山喷发导致了地球历史上最深刻的物种灭绝，影响了被称为二叠纪-三叠纪分界线的动植物。森林被低草本植物覆盖所取代，但在大约500万年后重新建立起来。这种主要的植物死亡和重组是否导致河流风格和陆地景观发生了明显的变化？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，短暂地恢复到植被进化之前的简单辫状河流的模式？ 这些问题的答案需要对地质记录进行全球评估。基于对1亿年来包括灭绝事件在内的古代河流沉积的文献汇编，拟议的研究将记录这一重要时期河流沉积风格和化石材料的相对丰富。在选定地点的实地考察期间，我们将与化石专家密切合作，评估植被和河流系统的相互作用，重点是植物的保存程度(评估原始植物群落变化的关键问题)以及与动物进化和灭绝的联系。由于盘古的干旱条件，植被在灭绝之前可能已经受到了压力，对古代水系和植物的详细研究将在德克萨斯州、新墨西哥州和德国等地的化石遗址进行。在下一个赠款周期，我们的目标是记录一次重大物种灭绝的景观影响，这与21世纪持续的人为灭绝和生物多样性丧失有关。 一个补充方案将学习更多关于景观之前的河流是如何被植被覆盖的。在植被出现之前，在地球上存在了30多亿年的简单“编织”河流在今天的地球上可能几乎是未知的：它们是如何发挥作用的？将通过在加拿大北部和英吉利海峡群岛的实地研究来探讨这一问题。尽管植物进化明显促进了曲流河的发展，但还有其他稳定河岸的方法，包括粘性泥浆、沙子早期硬化和冰。蜿蜒的河流被推断为地球上的一些前植被系统，并在火星的图像中被识别出来。评估河岸强度和这些神秘河流中的河道尺寸将是研究的一个关键方面。

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.