Collaborative Research: Testing Critical Zone Controls on Mountain-Scale Relief in a Tropical Climate

临界带是地球上薄薄的一层，从土层的底部延伸到树顶，支撑着我们星球上大多数的陆地生命。本项目通过对波多黎各热带岛屿上两种不同基岩单元的对比实验，研究了临界带过程的差异如何影响地形。在岛上，古老的火山岩形成陡峭、高耸的山峰，土壤富含粘土，而花岗岩则位于相对较平缓、起伏的山丘上，上面覆盖着砂质土壤。这项研究将评估这些差异对滑坡发生的影响，河道中沉积物颗粒的大小，以及洪水的大小和频率。主要的假设是，山体滑坡在富含粘土的土壤中切割得更深，因此，向河流输送了更粗的沉积物。较粗的沉积物反过来更好地保护了河道免受侵蚀，因此需要更大的洪水来冲刷下面的基岩。此外，富含粘土的火山土保留了更多的雨水，导致洪水比砂质花岗岩土覆盖的地区更少。最终的结果是，火山岩保持了比花岗岩地形更高的海拔和更陡峭的地形。该项目将通过详细的滑坡测绘、粒度测量、流量测量数据的统计分析和长期景观侵蚀率测量来检验这些想法。该项目将成为研究生和本科生研究经验的基础，并吸引波多黎各大学、mayag<e:1> ez和科罗拉多州立大学的其他学生参与体验式学习、研究和文化交流项目，重点是比较和对比波多黎各和科罗拉多州的景观。地形起伏度、陡度和高度的变化通常与基岩地质的差异有关。大多数情况下，这些变化被解释为下伏基岩可蚀性的差异。在波多黎各的热带岛屿上，中基性火山碎屑单元位于陡峭的高山之下，而富含硅质的花岗质-花岗闪长岩则以平缓起伏的丘陵为特征。初步结果表明，两种岩石单元之间的未风化基岩强度和裂缝密度相似，表明原生岩石强度或可蚀性的差异不能解释地形的明显差异。以前的研究表明，在这些岩石单元之上发育的土壤是不同的，更基性的单元由富含粘土的土壤组成，而砂质土壤在更长的单元中发育。这些土壤特征的差异和相关的临界带结构促进了相对于花岗岩单元的火山碎屑单元的浮雕生产。具体而言，临界带的差异通过控制层质粒度分布来影响切口阈值的大小，并通过调节浅层地下水文来影响突破这些阈值的洪水频率。这些假设将通过绘制滑坡位置、测量沉积物中的粒度分布、分析现有河流测量站的洪水统计数据以及计算宇宙形成放射性核素的长期侵蚀率来进行检验。数据集将被综合到一个建模框架中，该框架预测河流景观起伏作为岩石强度、切口阈值大小和阈值决口洪水频率的函数。该项目由地球科学地貌学和土地利用动力学项目和促进竞争研究的既定项目（EPSCoR）共同资助。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。

The Critical Zone is the thin layer of Earth that extends from the base of the soil layer to treetops and supports most terrestrial life on our planet. This project examines how differences in Critical-Zone processes influence topography through a comparative experiment of two different bedrock units on the tropical island of Puerto Rico. On the island, ancient volcanic rocks form steep, high peaks and have clay-rich soils, whereas granitic rocks underlie relatively more subdued, rolling hills covered by sandier soils. This study will evaluate the impact of these differences on the occurrence of landslides, the size of sediment grains that populate river channels, and the size and frequency of floods. The principal hypothesis is that landslides cut more deeply in the clay-rich soils and, as a result, deliver coarser sediment to rivers. The coarser sediment, in turn, better shields the river channel from erosion such that larger floods are needed to carve the underlying bedrock. In addition, the clay-rich volcanic soils retain more rainwater, resulting in fewer floods than areas covered by sandy granitic soils. The net result is that the volcanic rocks maintain a higher elevation and steeper topography than the granitic terrain. This project will test these ideas through detailed landslide mapping, grain-size measurements, statistical analyses of streamflow gaging data, and measurements of long-term landscape erosion rates. This project will form the basis for graduate and undergraduate research experiences and also engage additional students at the University of Puerto Rico, Mayagüez and Colorado State University in an experiential learning, research, and cultural exchange program centered on comparing and contrasting the landscapes in Puerto Rico and Colorado.Variations in topographic relief, steepness, and height are often observed to correlate with differences in bedrock geology. Most often, these changes are interpreted to result from differences in the erodibility of the underlying bedrock. On the tropical island of Puerto Rico intermediate-to-mafic volcaniclastic units underlie high relief, steep mountains, whereas felsic-rich granitic-to-granodioritic rocks are characterized by gentler, rolling hills. Preliminary results indicate that unweathered bedrock strength and fracture density between these two rock units is similar, implying the differences in primary rock strength or erodibility cannot explain the stark contrast in topography. Previous studies show that the soils developed atop each of these rock units are distinct, with the more mafic units comprised of clay-rich soils and sandier soils developed in the more felsic units. These differences in soil characteristics and associated Critical Zone architecture promote relief production in the volcaniclastic units relative to the granitic units. Specifically, differences in the Critical Zone affect the magnitude of incision thresholds by controlling bedload grain size distributions and the frequency of floods that breach these thresholds by modulating shallow, subsurface hydrology. These hypotheses will be tested by mapping landslide locations, measuring grain size distributions in sediment deposits, analyzing flood statistics from existing stream gaging stations, and calculating long-term erosion rates from cosmogenic radionuclides. Datasets will be synthesized into a modeling framework that predicts fluvial landscape relief as a function of rock strength, the magnitude of incision thresholds, and the frequency of threshold breaching floods. This project is jointly funded by the Geomorphology &amp; Land-use Dynamics program in Earth Sciences and the Established Program to Stimulate Competitive Research (EPSCoR).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.