The Geochemical Carbon Cycle: A Vadose-Hydrologic Study of the Effect of Plants on Chemical Weathering

二氧化碳（CO2）是一种温室气体。了解大气CO2浓度与气候关系的一个途径是研究地质时期的这种关系。这就要求我们在长时间尺度上了解影响大气CO2的过程。CO2从地球大气层长期转移到地壳的主要途径是（a）土壤、底土和地下水系统中常见矿物的溶解或“化学风化”，然后是（B）溶解的物质通过大陆水系输送到海洋，在那里（c）发生沉淀和沉积到海底。这一顺序影响了CO2的转移，因为矿物溶解消耗了含CO2的酸，并将产物碳酸氢盐夹带在排水沃茨中，碳酸氢盐随后作为碳酸盐矿物沉积在海洋中。在地球化学碳循环中，这一过程被火山作用和变质作用从俯冲和埋藏的碳酸盐矿物返回大气的CO2所抵消。这些过程的相对速率决定了任何时候大气和岩石圈中二氧化碳的质量，并在此过程中影响地球的气候。因此，了解控制周期中各个步骤速率的因素非常重要。其中一个因素是植物群落及其在地质时期的演变对化学风化速率的影响。相对于无植被的条件，植物可以提高地下酸的浓度，促进新鲜矿物表面的暴露，并将矿物颗粒结合到土壤的保水结构中，从而增强化学风化。然而，由于难以将其他变量从比较性实地研究中剔除，因此对这些机制的风化增强的实际程度了解甚少。这项研究利用了正在进行的植物养分循环的长期，田间规模的实验。这些实验田是为了监测排水而设计的，现在已经有了一个十年的数据库。美国国家科学基金会的一笔探索性研究小额赠款已被授予资助（刚刚完成）安装一个广泛的渗流孔隙水采样器网络，以观察溶液化学（化学风化“负荷”）如何随深度和时间在土壤和底土下的各种地块演变。本提案是申请资金继续开展工作，即，进行地球化学和水文研究，这些设施现在可以支持。这些结果将有助于检验关于风化增强机制的相对重要性的假设。??

9628296 Keller Carbon dioxide (CO2) is a greenhouse gas. One avenue to understanding the relationship of atmospheric CO2 concentration to climate, is to study that relationship over geologic time. This requires that we understand the processes affecting atmospheric CO2 on long time scales. The principal pathway for long-term transfer of CO2 from Earth's atmosphere to its crust is (a) dissolution or "chemical weathering" of common minerals in soils, subsoils, and groundwater systems, followed by (b) transport of the dissolved material to the oceans in continental drainage, where (c) precipitation and deposition onto the ocean floor occur. This sequence effects transfer of CO2 because mineral dissolution consumes CO2 -containing acids and entrains the product bicarbonate in drainage waters, which is later deposited in the oceans as carbonate minerals. In the geochemical carbon cycle, this process is countervailed by the return of CO2 from subducted and buried carbonate minerals to the atmosphere by volcanism and metamorphism. The relative rates of these processes determine the masses of CO2 in the atmosphere and lithosphere at any one time and, in the process, affect Earth's climate. It is therefore important to understand the factors controlling the rates of various steps in the cycle. One such factor is the effect of plant communities, and their evolution over geologic time, on the rate of chemical weathering. Plants could enhance chemical weathering--relative to unvegetated conditions--by raising subsurface concentrations of acids, facilitating the exposure of fresh mineral surfaces, and binding mineral grains into water-retaining structures characteristic of soils. The actual extent of weathering enhancement by these mechanisms is, however, poorly understood because of the difficulty of factoring other variables out of comparative field studies. This study takes advantage of ongoing long-term, field-scale experiments in plant nutrient cycling. The experimental plots were designed for monitoring of drainage water, and a ten-year database now exists. An NSF Small Grant for Exploratory Research has been awarded to fund the (just-completed) installation of an extensive network of vadose pore-water samplers to observe how solution chemistry (the chemical weathering "load") evolves with depth and time in soils and subsoils beneath the various plots. This proposal is application for funds to continue the work, i.e., to do the geochemical and hydrologic research which these facilities can now support. The results will facilitate testing of hypotheses regarding the relative importance of weathering-enhancement mechanisms. ??