Climatic and Erosional Effects on Chemical Weathering Rates, Measured Using Steep Climatic Gradients in Mountainous Terrain

化学风化和物理侵蚀是相互依存的过程，它们塑造了景观，调节了土壤的发展，并将溶质和沉积物输送到溪流中。它们在地球的长期气候演变中也起着关键作用，因为硅酸盐风化是大气中二氧化碳的最终长期汇。对化学风化的气候控制的实地研究通常在分布广泛的地点测量风化速率（以便跨越广泛的气候范围），因此难以控制基岩矿物学的变化和物理侵蚀造成的矿物供应速率。由于气候、岩性和侵蚀的影响很难相互分离，它们在调节化学风化速率方面的相对重要性仍然不确定。知识价值：最近开发的方法现在允许在侵蚀景观中同时测量物理侵蚀和化学风化的长期速率。宇宙形成核素对长期剥蚀速率的测量可以与土壤及其母基岩的总体化学和矿物学相结合，从而产生对单个造岩元素和单个矿物的长期化学风化速率的测量。这种方法明确地解释了不同地点在侵蚀速率和基岩组成上的差异可能造成的混淆效应。该提案采用这些新方法来测量萨蒙河南叉峡谷两个气候样带的物理侵蚀和化学风化率。从相邻山脉的山顶上看，南福克峡谷和大峡谷一样深，平均坡度是大峡谷的两倍。这种戏剧性的地形起伏产生了陡峭的气候梯度，形成了研究气候对生物地球化学过程影响的天然实验室：在距离小于6公里的范围内，起伏变化1300-1500米，平均积雪含水量变化4- 5倍，年平均气温变化7-9摄氏度（大于中纬度地区伴随冰期到间冰期转变的温度变化）。在沿着这些气候样带的19个研究地块中，我们对土壤和基岩进行了广泛的采样，并安装了数据记录热电偶和土壤湿度探测器，这些探测器现在全天候全天候地监测地下风化环境。因此，风化速率的变化可以直接与当地的土壤小气候以及这些气候样带的地上条件联系起来。拟议的项目利用地球化学和矿物学分析以及宇宙形成核素测量的投资，量化这些气候梯度的长期化学风化率。该项目将量化气候对野外长期化学风化速率的影响，方法是将气候强迫的变化最大化，并考虑岩性和侵蚀速率的任何混杂变化。这些结果将有助于建立长期气候演变模型，评估山区地形的沉积物和溶质输送，以及定量了解气候变化将如何影响山地生态系统中的土壤形成和生物地球化学过程。更广泛的影响：本研究将为一名博士候选人提供研究培训。本科生将获得实地考察、样品制备和实验室分析的实践研究经验。这项工作还将进一步实现关键区域勘探网络（CZEN）倡议的目标，通过建立一个“自然实验室”，严格量化气候对长期风化率的影响，从而为我们自己和其他人研究风化过程的生物地球化学、微生物学、水文学和地貌学创造机会。

Chemical weathering and physical erosion are interdependent processes that sculpt landscapes, regulate the development of soils, and deliver solutes and sediment to streams. They also play critical roles in Earth's long-term climatic evolution, since silicate weathering is the ultimate long-term sink for atmospheric CO2. Field studies of climatic controls on chemical weathering have typically measured weathering rates at widely distributed sites (in order to span a wide range of climates), making it difficult to control for variations in bedrock mineralogy and in mineral supply rates from physical erosion. Because the effects of climate, lithology, and erosion have been difficult to isolate from one another, their relative importance in regulating chemical weathering rates remains uncertain.Intellectual Merit: Recently developed methods now allow long-term rates of physical erosion and chemical weathering to be measured simultaneously in eroding landscapes. Cosmogenic nuclide measurements of long-term denudation rates can be combined with the bulk chemistry and mineralogy of soils and their parent bedrock, yielding measurements of long-term chemical weathering rates, both for individual rock-forming elements and individual minerals. This approach explicitly accounts for the potentially confounding effects of site-to-site differences in erosion rates and bedrock composition.This proposal employs these new methods to measure rates of physical erosion and chemical weathering across two climate transects in the canyon of the South Fork of the Salmon River. From the summits of adjacent mountains, the canyon of the South Fork is as deep as the Grand Canyon, and on average twice as steep. This dramatic topographic relief generates steep climatic gradients, forming a natural laboratory for studying climatic effects on biogeochemical processes: within distances of less than 6 km, relief varies by 1300-1500 m, mean snow-pack water content varies by 4- to 5-fold, and mean annual temperature varies by 7-9 degrees C (greater than the temperature changes accompanying glacial-to-interglacial transitions in the mid-latitudes).In 19 study plots along these climate transects we have extensively sampled soils and bedrock, and have installed datalogging thermocouples and soil moisture probes, which are now monitoring the subsurface weathering environment around the clock and throughout the year. Thus variations in weathering rates can be directly linked to the local soil microclimates, as well as above-ground conditions, across these climate transects. The proposed project leverages this investment with geochemical and mineralogical analyses and cosmogenic nuclide measurements, to quantify long-term chemical weathering rates across these climate gradients.This project will quantify effects of climate on long-term chemical weathering rates in the field, by maximizing the variation in climatic forcing, and accounting for any confounding variations in lithology and erosion rates. These results will contribute to models of long-term climatic evolution, to assessments of sediment and solute delivery from mountainous terrain, and to quantitative understanding of how climate change will affect soil formation and biogeochemical processes in montane ecosystems.Broader Impacts: This study will provide research training for one Ph.D. candidate. Undergraduates will gain hands-on research experience in fieldwork, sample preparation and laboratory analyses. This work will also further the goals of the Critical Zone Exploration Network (CZEN) initiative, by establishing a 'natural laboratory' in which climatic effects on long-term weathering rates have been rigorously quantified, thus creating opportunities for ourselves and others to study the biogeochemistry, microbiology, hydrology, and geomorphology of weathering processes.