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

点击翻译按钮获取中文摘要

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.