Collaborative Research: Spatial Variability in Eroded Sediment Size and Geomorphic Processes Inferred From Detrital Thermochronometry and Cosmogenic Nuclides

了解山区的侵蚀和沉积模式对地球科学家和土地利用管理者都很重要。在管理时间范围内，被侵蚀的沉积物影响水质，影响河流水生栖息地的数量和质量，并通过设定天然和人造水库的沉积速度来决定它们的寿命。在更长的地质时间范围内，被侵蚀的沉积物还通过为河流提供切割底层基岩所需的工具来影响景观侵蚀，从而适应气候和构造作用力的变化。因此，泥沙侵蚀既是景观变化的产物，也是其背后的关键驱动力。由这笔赠款资助的工作将开发新的方法来询问沉积物来自哪里，如何产生，以及它在景观中移动的速度有多快。这将有助于在了解侵蚀过程及其如何影响地形对气候和构造强迫的反应方面取得进展。地质学家早就认识到，沉积物包含着丰富的信息，从山坡上的完整岩石到覆盖现代河床并填充古代沉积沉积物的颗粒混杂。随着最近的技术和方法的进步，这种信息的提取变得越来越复杂。因此，对表面过程的了解变得越来越定量。例如，斜坡的侵蚀速度现在通常可以通过宇宙源核素来测量，这些核素在靠近地球表面的沉积物颗粒中积聚。与此同时，最近的研究表明，这种矿物磷灰石中放射性成因氦的钟表状堆积可以作为集水区斜坡上沉积物来源的指纹。在这里，这种磷灰石-氦示踪技术将以一种全新的方式与宇宙成因核素一起使用，同时揭示基岩风化在山坡上产生的侵蚀速度和沉积物尺寸的空间变化。这标志着沉积物追踪方面的一项重要进展；到目前为止，还没有办法量化侵蚀沉积物的大小如何随集水规模的变化而变化。初步结果表明，斜坡的高程与它们风化和侵蚀产生的沉积物的大小之间存在联系。海拔越高的斜坡越冷，植被越少，产生的沉积物就越粗糙。这表明气候是沉积物大小的关键调节因素。这项拟议的研究将在陡峭的集水区测试新的沉积物追踪方法，并探索关于气候和侵蚀沉积物大小之间联系的假设。这项研究的预期成果包括对常见但尚未完全了解的下游景观趋势的新见解，例如：山脉河床的粒度细化；从辫状河道形态向曲折河道形态的转变；以及水生栖息地和栖息在其中的生物的变化。

Understanding patterns of erosion and sedimentation in mountains is important for Earth scientists and land-use managers alike. Over management timescales, eroded sediment affects water quality, influences the amount and quality of aquatic habitat in rivers, and determines the lifespan of both natural and manmade reservoirs by setting the pace of sedimentation within them. Over longer, geologic timescales, eroded sediment also influences landscape erosion by providing rivers with the tools they need to cut into underlying bedrock and thereby adjust to changes in climate and tectonic forcing. Thus, the erosion of sediment is both the product of and a key driving force behind landscape change. Work funded by this grant will develop new methods for interrogating sediment about where it comes from, how it is generated, and how fast it moves across landscapes. This should permit progress on understanding erosional processes and how they influence landscape response to climatic and tectonic forcing. Geologists have long recognized that sediment contains a wealth of information about its journey from intact rock on hillslopes to the jumbles of particles that cover modern riverbeds and fill ancient sedimentary deposits. Extraction of this information has become increasingly sophisticated with recent technological and methodological advances. As a result, understanding of surface processes has become increasingly quantitative. For example, rates of erosion from slopes can now often be measured from cosmogenic nuclides, which build up in sediment grains when they are near Earth's surface. Meanwhile, recent studies have shown that the clockwork-like buildup of radiogenic helium in the mineral apatite can be used as a fingerprint of where sediment comes from on catchment slopes. Here, this apatite-helium tracing technique will be used together with cosmogenic nuclides in a completely new way, to simultaneously unveil spatial variations in both the erosion rates and sizes of sediment produced on hillslopes by bedrock weathering. This marks an important advance in sediment tracing; until now, there was no way to quantify how the sizes of eroded sediment vary over catchment scales. Preliminary results show a connection between the elevation of slopes and the sizes of sediment that they produce by weathering and erosion. Higher-elevation slopes, which are colder and less vegetated, produce coarser sediment. This points to climate as a key regulator of sediment size. The proposed research will test the new sediment tracing approach in a steep catchment and explore hypotheses about linkages between climate and the sizes of eroded sediment. Expected outgrowths of this research include fresh insight on common, but as-yet incompletely understood downstream trends in landscapes, such as: fining in grain size in mountain streambeds; shifts from braided to meandering channel forms; and changes in aquatic habitats and the organisms that populate them.