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

美国中西部地区的地貌至少部分被黄土覆盖，这些黄土是晚更新世时期沉积的风沙，然后由于山坡侵蚀而广泛重新分布。许多观察结果表明，薄的黄土地幔的存在可以增加土壤形成过程的速度，并影响生态上重要的特性，如养分或水分保持。该研究项目将探讨黄土地幔在土壤链带演变中的相对重要性，土壤链带是可预测的土壤沿山坡变化模式。在可能的解释范围的一个极端，黄土厚度的局部格局可能在晚更新世确定，因为灰尘沉积并迅速重新分布到下坡。从那时起，黄土地幔对土壤的形成起着至关重要的作用，但主要是静态的控制。另一方面，生物过程，如穴居动物对土壤物质的混合和分选，可能增强甚至取代黄土地幔在确定当前土壤性质空间格局中的作用。这一解释暗示了一个更动态的链带发展历史，因为生物力学过程逐渐将土壤下坡并产生有利于土壤快速发展的细颗粒表面地幔。在这个项目中，研究人员将对粒度和矿物学进行分析，以估计明尼苏达州和威斯康星州研究地区土壤链中存在的黄土的实际数量。在有黄土存在的地方，下坡再分布的时间将由斜坡脚下天然“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，研究了黄土和生物力学过程对生态重要土壤性质的相对影响。这个项目的结果将对中西部和世界上许多其他地区的土壤景观的长期和短期变化产生广泛的影响，在这些地区，黄土地幔可能会影响链带的演变。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，那么黄土的加速侵蚀可能会不可逆转地改变土壤发育的速度或方向，这反过来会改变植物的生产力和对许多人类土地利用的适用性。气候变化加速的史前侵蚀或最近土地利用变化引发的快速侵蚀都是如此，在估计土壤侵蚀的可容忍率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中起主要作用，那么从长远来看，即使在严重侵蚀之后，土壤景观最终也可以得到更新。这项研究的这些影响将通过在对可持续土地利用有强烈和日益增长的兴趣的研究地区的公众宣传向科学界提出。研究生和本科生都将参与该项目，学习许多最重要的土壤成因和地貌学研究技术。

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.