Collaborative Research: Timescales and drivers of floodplain disequilibrium at climatic transitions

该奖项全部或部分由《2021年美国救援计划法案》（公法117-2）资助。河流和洪泛平原对它们所经过的景观和环境产生了重大影响。这些系统表现出一系列的形式和特征（形态），由松散岩石（沉积物）的可用性和河流输送这些沉积物的能力之间的平衡控制。由于气候对这些变量起着重要的控制作用，预计随着地球气候变暖，洪泛平原的形态将发生重大变化。在气候变暖的情况下，沉积物供应往往相对于运输能力而减少，而洪泛平原的特点是垂直切割旧沉积物，形成阶梯状的景观特征，即河流梯田。这项研究的目的是加深我们对洪泛平原如何对过去不同规模和时间尺度的气候变化作出反应的理解，以提高我们预测未来变化程度的能力。这一目标将通过确定河流阶地形成的时间，并将其与地球最近地质历史中已知的气候变化进行比较来实现。该项目还为来自历史上代表性不足群体的中学女生举办了一个关于景观演变的夏季讲习班，开发了一个关于梯田形成的无障碍本科虚拟实地项目，并促进了一个社区实地讲习班，以分享项目成果。具体的研究问题包括：(1)气候变化的持续时间和强度需要多大才能驱动洪泛平原形态的显著变化？(2)在显著的气候驱动变化之后，洪泛平原需要多长时间才能形成新的、稳定的形态？(3)非气候机制在多大程度上抵消或加强了气候对洪泛过程的控制？这些问题将通过解决苏格兰东部河流阶地中保存的约14000年洪泛区恶化和退化的时间，并将其与主要和次要气候转变和冰川作用的时间进行比较来解决。苏格兰东部是进行这项工作的理想地点，因为它是构造不活跃的，有一个很好的气候年代学，经历了大陆冰盖和高山冰川作用，并包含河流阶地序列。阶地年代学将采用放射性碳、光激发发光和宇宙成因核素暴露定年法进行研究。将对具有相同气候和冰川历史但集水区性质不同的实地地点之间河流形态变化的时间和持续时间进行比较，以确定非气候变量如何影响恶化和退化周期。研究还将探讨先前的冰川侵蚀对洪泛平原形态和现代沉积物路径的影响。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117-2).Rivers and floodplains significantly influence the landscapes and environments through which they pass. These systems exhibit a range of forms and characteristics (morphologies) controlled by a balance between the availability of unconsolidated rock (sediment) and the river’s capacity to transport that sediment. As climate exerts a major control on these variables, floodplain morphologies are expected to undergo major changes as Earth’s climate warms. Under a warming climate, sediment supply often declines relative to transport capacity and narrowing floodplains are characterized by vertical cutting through older deposits creating stair-stepped landscape features called river terraces. The goal of this study is to deepen our understanding of how floodplains have responded to different magnitudes and timescales of climatic changes in the past to enhance our ability to predict the extent to which they will change in the future. This goal will be achieved by determining the timing of river terrace formation and comparing it to known climate shifts throughout Earth’s recent geological past. The project also supports a summer workshop on landscape evolution for middle school girls from historically underrepresented groups, the development of an accessible undergraduate virtual field project on terrace formation, and the facilitation of a community field workshop to share project results.Specific research questions include: (1) What duration and intensity of climatic change is required to drive significant changes to floodplain morphology? (2) How long does it take for floodplains to establish new, stable morphologies after significant climate-driven changes? (3) To what extent do non-climatic mechanisms counter or enhance climatic controls on floodplain processes? These questions will be addressed by resolving the timing of ~14,000 years of floodplain aggradation and degradation preserved in river terraces in eastern Scotland and comparing them against the timing of major and minor climatic transitions and glaciations. Eastern Scotland is an ideal location to undertake this work because it is tectonically inactive, has an excellent climate chronology, has undergone both continental ice sheet and alpine glaciation, and contains river terrace successions. The terrace chronology will be developed using radiocarbon, optically stimulated luminescence, and cosmogenic nuclide exposure dating. A comparison of the timing and duration of river morphology changes between field sites with the same climatic and glacial history, but with different catchment properties, will be undertaken to determine how non-climatic variables influence aggradation and degradation cycles. The research will also address the role of prior glacial erosion on influencing floodplain morphology and modern sediment routing.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.