Collaborative Research: Tropical cyclone imprint on late Quaternary alluvial fans of Baja California: Key for understanding arid regions landscape evolution

沙漠环境中的冲积扇形成仍然是一个有争议的主题，将特定地区的冲积扇形成的概念模型应用于不同气候环境的地区通常缺乏强有力的定量支持。这个国际合作项目旨在通过测试墨西哥下加利福尼亚半岛南部晚第四纪冲积扇形成的替代假设，将数据输入到这些概念模型中。中心假设是，热带气旋在这一时期完成了大部分地貌工作，并且季节性高强度降水与基岩的快速风化相结合，产生了响应千年尺度气候变率的循环冲积扇沉积。对晚更新世-全新世沉积单元的沉积学和土壤发育进行详细分析，为宇宙成因核素深度剖面年龄测定和光激发光深度剖面定年提供支持。这些技术的目的是同时确定山坡上的沉积物产量和冲积单元的输送和积累速率。这些结果将使我们清楚地了解冲积单元是何时建成的，冲积面是何时稳定下来的，沉积物沉积的速度有多快，以及沉积物在运输和沉积之前在山坡上储存了多长时间。该项目（由美国国家科学基金会的地貌和土地利用动力学项目、P2C2、EPSCoR和国际科学与工程办公室支持）将产生更多科学受众和更广泛公众感兴趣的数据集和知识。首先，我们将在北美西南部的沙漠中测试一个广泛使用的冲积扇沉积模型。其次，我们将测试热带气旋在该地区千年时间尺度上的相关性是如何演变的，并帮助分析热带气旋对世界其他干旱和半干旱地区的影响(特别是，我们将结合评估过去几个世纪东太平洋热带气旋活动及其对美国西南部和墨西哥西北部快速发展地区的生态系统和人口影响的研究，以改进这些风暴的危害特征。第三，我们对通常用作古地震标志的冲积扇表面的更精确的相关性将改善地震复发间隔和局部和区域断层滑动率的确定，增加我们对断层运动学的理解，不仅在下加利福尼亚州的邻近地区，而且在所有干旱的北美西南部和其他地区，冲积扇表面被用来推断地震复发率。

Alluvial fan formation in desert environments is still a debated subject, and application of conceptual models developed for fan formation in specific regions to areas with different climate settings generally lacks robust quantitative support. This international collaborative project aims to input data into these conceptual models by testing alternate hypotheses on the formation of Late Quaternary alluvial fans in the southern Baja California peninsula in Mexico. The central hypothesis is that tropical cyclones have performed most of the geomorphic work during this period and that seasonal high intensity precipitation has been coupled to rapid weathering of bedrock to generate cyclic alluvial fan aggradation in response to millennial-scale climate variability. Combined cosmogenic nuclide depth profile age determination and optically stimulated luminescence depth profile dating will be supported with detailed analyses of sedimentology and soil development of the sedimentary units in the Late Pleistocene-Holocene. These techniques are aimed to determine simultaneously sediment production on the hillslopes and delivery and accumulation rates in the alluvial units. The results will yield a clear picture of when the alluvial units were built, when the alluvial surfaces stabilized, how fast sediment was deposited, and how long the sediment was stored in the hillslopes before transport and deposition. This project (supported by NSF's programs in Geomorphology and Land Use Dynamics, P2C2, and EPSCoR, and Office of International Science and Engineering) will produce datasets and knowledge that are of interest to a larger scientific audience and to the broader public. First, we will test a widely used model of alluvial fan aggradation for the deserts of southwestern North America. Second, we will test how the relevance of tropical cyclones has evolved over millennial timescales in this area and help to analyze tropical cyclone effects in other arid and semi-arid regions of the world (in particular, we will link with studies that assess activity of Eastern Pacific tropical cyclones over the last centuries and their effects on ecosystems and human population in the rapidly growing areas of southwestern U.S. and northwestern Mexico to improve hazard characterization of these storms). Third, our more precise correlation of alluvial fans surfaces normally used as paleoseismological markers will improve determination of earthquake recurrence intervals and of local and regional fault slip rates, increasing our understanding of fault kinematics not only in neighboring areas in Baja California but also in all arid southwestern North America and other regions where alluvial surfaces are used to infer earthquake recurrence rates.