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，塑造景观，调节土壤的发展，并提供溶质和沉积物的溪流。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CO2的最终长期汇。化学风化的气候控制的实地研究通常在广泛分布的地点测量风化率（为了跨越广泛的气候范围），这使得很难控制基岩矿物学和物理侵蚀的矿物供应率的变化。由于气候，岩性和侵蚀的影响已经很难相互隔离，它们在调节化学风化率的相对重要性仍然uncertainty Merit：最近开发的方法，现在允许长期的物理侵蚀和化学风化率被同时测量侵蚀景观。长期剥蚀率的宇宙成因核素测量可以与土壤及其母岩的整体化学和矿物学相结合，产生对单个造岩元素和单个矿物的长期化学风化率的测量。这种方法明确地解释了潜在的混淆效应的站点到站点的侵蚀速率和基岩composition.This建议采用这些新的方法来测量跨两个气候断面的鲑鱼河的南叉峡谷的物理侵蚀和化学风化率的差异。从邻近山脉的山顶看，南叉的峡谷与大峡谷一样深，平均是大峡谷的两倍。这种巨大的地形起伏产生了陡峭的气候梯度，形成了研究气候对地球化学过程影响的天然实验室：在不到6公里的距离内，地形起伏在1300-1500米之间，平均积雪含水量变化4- 5倍，年平均气温变化7-9摄氏度（大于中纬度地区冰川到间冰期的温度变化）。在沿着这些气候横断面的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，这些探测器现在全年不间断地监测地下风化环境。因此，风化率的变化可以直接联系到当地的土壤小气候，以及地面上的条件，在这些气候断面。该项目将通过地球化学和矿物学分析以及宇宙成因核素测量来利用这一投资，以量化这些气候梯度上的长期化学风化速率。该项目将通过最大化气候强迫的变化，并考虑岩性和侵蚀速率的任何混杂变化，来量化气候对野外长期化学风化速率的影响。这些结果将有助于长期气候演变的模型，从山区地形的沉积物和溶质输送的评估，以及气候变化将如何影响山地生态系统中的土壤形成和土壤地球化学过程的定量理解。候选人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临界区勘探网络（CZEN）倡议的目标，建立一个“自然实验室”，在这个实验室中，气候对长期风化速率的影响被严格量化，从而为我们和其他人创造机会，研究风化过程的地球化学、微生物学、水文学和地貌学。

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.