Precipitation as a key control on erosion rates in the tectonically inactive northeastern Sonoran Desert, central Arizona, USA

Langbein and Schumm (1958) connected precipitation to erosion in a right-skewed curve used in earth science textbooks for over six decades, where denudation increases with precipitation on the arid/semiarid limb and decreases in humid regions. Development of the catchment-averaged 10Be denudation method a quarter-century ago led geomorphologists to evaluate this hypothesis using data not influenced by the Anthropocene, with mixed findings. The Sonoran Desert in Arizona, USA, is optimal for investigating the longstanding hypothesis of increased erosion from arid to semiarid climates due to: (i) the modern orographic effect aligning elevated precipitation with altitude, mirroring Neotoma packrat midden paleoecology research for the Holocene and late Pleistocene; (ii) the region has been tectonically quiet for the residence times of analyzed 10Be ranging from ca. 8,000-110,000 years. Our significant finding echoes Langbein and Schumm's work, revealing heightened erosion along an elevation-precipitation gradient from arid to semiarid conditions. Notably, the significance of precipitation-elevation contrasts with the absence of significant correlation between 10Be denudation and attributes like slope, drainage area, relief, or landform type (e.g., alluvial fan, pediment, mountain watershed). Modern faunalturbation, increasing along this gradient, exposes more ground to rainsplash and overland flow at higher elevations, adding complexity to these results. Further insights unveil that (i) catchments in areas with substantial Quaternary base level reduction imitate tectonic effects, tripling 10Be denudation rates; (ii) basaltic boulders and cobbles yield an armoring influence; (iii) historical erosion acceleration due to urbanization and wildfires insignificantly affects 10Be denudation rates in the Sonoran Desert; and (iv) minute desert catchments yield anomalous erosion rates.朗贝恩与舒姆（Langbein and Schumm, 1958）用一条右偏曲线关联降水与侵蚀的关系，该曲线已被地球科学教科书沿用逾六十年。在该曲线中，干旱/半干旱分支的剥蚀速率随降水增加而升高，而湿润分支的剥蚀速率则随降水增加而降低。25年前，流域平均10Be剥蚀速率测定法（catchment-averaged 10Be denudation method）得以开发，这使得地貌学家能够利用未受人类世（Anthropocene）影响的数据验证该假说，但相关研究结果存在分歧。美国亚利桑那州的索诺兰沙漠（Sonoran Desert）是验证"干旱至半干旱气候下侵蚀加剧"这一长期假说的理想研究区域，原因如下：其一，现代地形效应（orographic effect）使得降水随海拔升高而增加，这与针对全新世（Holocene）与晚更新世（late Pleistocene）开展的北美林鼠（Neotoma）贮食堆古生态研究结果相吻合；其二，在本次分析的10Be样品对应的约8000至110000年的滞留时间尺度内，该区域始终处于构造稳定状态。本研究的重要发现与朗贝恩与舒姆的研究成果相呼应，揭示了沿海拔-降水梯度从干旱到半干旱条件下，侵蚀速率显著升高的现象。值得注意的是，降水-海拔梯度的显著影响与另一现象形成鲜明对比：10Be剥蚀速率与坡度、流域面积、地形起伏或地貌类型（如冲积扇（alluvial fan）、山麓剥蚀面（pediment）、山地流域）等参数均无显著相关性。沿该梯度递增的现代生物扰动（faunalturbation）作用，使得高海拔区域更多地表暴露于雨滴溅蚀（rainsplash）与地表径流（overland flow）的侵蚀之下，为本次研究结果增添了复杂性。进一步研究揭示：其一，第四纪（Quaternary）基准面大幅下降区域的流域，其10Be剥蚀速率可达常规水平的三倍，这一现象与构造活动影响下的结果类似；其二，玄武岩巨砾与砾石会产生抗侵蚀装甲效应；其三，城市化与野火引发的历史侵蚀加速现象，对索诺兰沙漠的10Be剥蚀速率并无显著影响；其四，小型荒漠流域会出现异常的侵蚀速率。