Unraveling natural and human-accelerated erosional and weathering processes in the past: The new "comminution dating" approach based on uranium isotope analyses applied on a lake sediment record

侵蚀和风化主要受构造抬升和气候条件的控制，因此受自然变化的影响。在不久的将来，人为导致的快速气候变暖可能会在这些过程中引发明显的变化。此外，人为的林木砍伐和农用土地利用可能会加速陆地领域的侵蚀。尽管对侵蚀和风化如何形成地貌的深刻理解对未来的预测至关重要，但将侵蚀和风化的历史与地质史中的古环境和气候替代数据联系起来的适当分析方法很少。缩小这一认识差距的一种新的有希望的方法是对碎屑沉积物材料进行铀同位素分析，这使得能够确定所谓的粉碎年龄，即自风化形成碎屑颗粒以来所经过的时间。通过比较沉积盆地中的粉碎年龄和沉积年龄，可以量化侵蚀过程和集水动力学。碎屑物质在风化层中的停留时间长(风化层停留时间长)，表明表层物质被片层冲刷去除缓慢，而风化层停留时间短，则表明冲沟侵蚀加剧和加深了剥蚀。这一相对较新的方法将应用于奥赫里德湖(阿尔巴尼亚马其顿)的湖泊沉积物序列。利用沉积学、(生物)地球化学、(O，C)同位素和孢粉资料对奥里德湖的古气候环境史进行了较好的研究。这些信息将与铀同位素数据结合起来，以调查气候、环境和侵蚀之间的联系。因此，将特别关注人类在全新世晚期通过砍伐森林和农业土地使用对景观演变的影响。由于铀同位素分析作为过去地貌演化的替代方法是一种相对较新的方法，直到今天才应用于湖泊沉积物序列，因此拟议的研究项目还包括对该方法的校准和评估。将测试化学前处理方法在湖泊沉积物上的适用性，并将在达尔文港(澳大利亚北领地)最近的沉积物上研究席流与沟谷侵蚀的概念模型。达尔文港集水区的侵蚀过程在地理上可分为两个末端，分别以席流和沟谷侵蚀为特征。这项研究能够通过铀同位素分析在达尔文港沉积物中检测到这两种侵蚀类型的信号。

Erosion and weathering are predominately controlled by tectonic uplift and climate conditions and are therefore affected by natural variations. Rapid anthropogenic induced climate warming can trigger distinct modifications in these processes in the near future. Furthermore, anthropogenic wood clearance and agricultural land use can accelerate the erosion in the terrestrial realm. Although a profound understanding of how erosion and weathering form landscapes are fundamentally important for future predictions, appropriate analytical methods for linking the history of erosion and weathering to paleoenviromntal and -climatic proxy data in the geological history are sparse. A new promising method to close this gap of knowledge is the application of uranium isotope analyses on clastic sediment material, which enables the determination of the so-called comminution age, i.e. the time that has elapsed since a detrital grain was formed by weathering. By comparing the comminution age with the depositional age in a sedimentary basin, erosional processes and catchment dynamics can be quantified. A long residence time of clastic material in the weathering horizon (long regolith residence time) suggests slow removal of topsoil material by sheet wash, whereas a short regolith residence time points to an intensification and deepening of the denudation by gully erosion. This relatively new method shall be applied on a lacustrine sediment succession from Lake Ohrid (Macedonia, Albania). The paleoclimatic and -environmental history of Lake Ohrid is already well studied by using sedimentologic, (bio-)geochemical, isotope (O, C), and pollen data. This information will be combined with the uranium isotope data in order to investigate the link between climate, environment and erosion. Thereby, particular focus will be drawn on the human impact on the landscape evolution by wood clearance and agricultural land-use during the Late Holocene period. Since uranium isotope analysis as a proxy for past landscape evolution is a relatively new approach and was not applied to lake sediment successions until today, the proposed research project also encompasses a calibration and evaluation of the method. Chemical pre-treatment methods will be tested for their applicability on lake sediments, and the conceptual model of sheet wash versus gully erosion will be studied on (sub-)recent sediments from Darwin Harbor (Northern Territory, Australia). Erosional processes in the catchment area of Darwin Harbor can geographically be separated into two endmembers, characterized by sheet wash and gully erosion, respectively. This study enables a detailed examination if the signal of these two erosion types can be detected in the Darwin Harbor sediments by means of uranium isotope analyses.