Collaborative Research: The Significance of the Loess Mantle in Midwestern Soil Catena Evolution

美国中西部的地貌至少部分被黄土覆盖，风尘在晚更新世沉积，然后被山坡侵蚀广泛重新分布。许多观察表明，薄黄土地幔的存在可以增加土壤形成过程的速度，并影响重要的生态特性，如养分或水分保持。这项研究项目将调查黄土地幔在土壤连带演变中的相对重要性，土壤连带是土壤沿山坡变化的可预测模式。在可能的解释范围中的一个极端，黄土厚度的局部模式可能在晚更新世时被确定，因为粉尘沉积并迅速重新分布在山坡上。从那时起，黄土地幔对土壤的形成起到了至关重要但基本上是静态的控制作用。另一方面，生物过程，如穴居动物对土壤物质的混合和分类，可能会增强甚至取代黄土地幔在决定目前土壤性质的空间格局中所起的作用。这一解释意味着连锁区发育的历史更加动态，因为生物力学过程逐渐将土壤向下移动，并产生有利于土壤快速发育的细粒表层地幔。在这个项目中，研究人员将进行粒度和矿物学分析，以估计明尼苏达州和威斯康星州研究地区土壤链条中存在的实际黄土量。在有黄土存在的地方，斜坡下部重新分布的时间将根据斜坡脚下天然“圈闭”中的沉积物来解释。几种方法，包括光激发发光测年，将被用来表征土壤混合的程度。最后，将研究黄土和生物力学过程对这两个地区连锁带生态重要土壤性质的相对影响。该项目的结果将对中西部和世界上许多其他黄土地幔可能影响连锁带演化的地区土壤景观的长期和短期变化具有广泛的意义。如果当地的黄土厚度在很大程度上是对土壤形成过程的静态控制，那么去除黄土的加速侵蚀可能会不可逆转地改变土壤发展的速度或方向，这反过来又会改变植物生产力和对许多人类土地用途的适宜性。气候变化加速的史前侵蚀或近期土地利用变化引发的快速侵蚀也是如此，在估计可容忍的土壤侵蚀速度时显然需要考虑到这一点。另一方面，如果生物力学过程在促进土壤形成方面发挥了重要作用，那么土壤景观最终可以在长期内得到更新，即使在严重侵蚀之后也是如此。这项研究的这些影响将介绍给科学界，并通过在研究地区的公众宣传，这些地区对可持续土地利用的兴趣日益浓厚。研究生和本科生都将参加这个项目，学习土壤发生学和地貌学研究的许多最重要的技术。

Landscapes across the Midwestern United States are at least partially mantled by loess, wind-blown dust deposited during the Late Pleistocene and then extensively redistributed by hillslope erosion. Many observations suggest that the presence of a thin loess mantle can increase the rates of soil-forming processes and influence ecologically important properties such as nutrient or water retention. This research project will investigate the relative importance of a loess mantle in the evolution of soil catenas, which are predictable patterns of soil variation down hillslopes. At one extreme in the range of possible interpretations, the local pattern of loess thickness may have been determined in the Late Pleistocene, as dust was deposited and rapidly redistributed downslope. Since that time, the loess mantle has acted as a critically important but largely static control on soil formation. On the other hand, biological processes, such as mixing and sorting of soil materials by burrowing animals, may augment or even replace the role played by a loess mantle in determining the present spatial pattern of soil properties. This interpretation implies a much more dynamic history of catena development, as biomechanical processes progressively move soil downslope and generate a fine-grained surface mantle that is conducive to rapid soil development. In this project, the investigators will conduct analyses of particle size and mineralogy to estimate the actual amount of loess present in soil catenas of study areas in Minnesota and Wisconsin. Where loess is present, the timing of downslope redistribution will be interpreted from the sediments in natural "traps" at the foot of slopes. Several methods, including optically stimulated luminescence dating, will be used to characterize the extent of soil mixing. Finally, the relative influence of loess and biomechanical processes on ecologically important soil properties will be investigated in catenas of both study areas.The results of this project will have broad implications for long- and short-term change in the soil landscapes of the Midwest and many other regions of the world where a loess mantle may influence catena evolution. If local loess thickness is a largely static control over soil-forming processes, then accelerated erosion that removes loess may irreversibly change the rates or directions of soil development, which in turn can alter plant productivity and suitability for many human land uses. This would be true of prehistoric erosion accelerated by climatic change or rapid erosion triggered by recent land use change and would clearly need to be taken into account in estimating tolerable rates of soil erosion. On the other hand, if biomechanical processes play a major role in enhancing soil formation, then the soil landscape can ultimately be renewed over the long run, even after severe erosion. These implications of this research will be presented to the scientific community and through public outreach in the study areas, where there is a keen and growing interest in sustainable land use. Both graduate and undergraduate students will participate in the project, learning many of the most important techniques of research on soil genesis and geomorphology.