Dating on earth sub
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The most important incision is observed in the peri-urban sub-basins where urban inflow (UI) and/or combined sewer overflow (CSO) increase peak flow and consequently the energy level during floods. However, incision also occurs in the upland rural area were CSO is absent and UI is much less common.
Elles indiquent que tous les sédiments étudiés sont plus jeunes que 1500 ans ap. A partir de ces données une histoire complexe de phases de dépôt et d’incision est reconstituée sur les 500 dernières années.Establishing a robust chronology in this region is required to answer the question regarding the forcing factors behind sediment deposition and incision on hillslopes.In contrast to most previous studies, the sediments under consideration are very young (Palaeoenvironmental research in the Yzeron Basin was initiated by problems with managing the morphological dynamics of headwater streams in this region.A review of different dating methods has been provided by Lang (1999) showing that radiocarbon dating and dendrochronology are problematic for such sediments, as these methods will rely on the dating of reworked material (e.g., wood).As a consequence, such dating approaches may overestimate the true time of sediment accumulation (e.g., Lang & Hönscheidt, 1999).It is concluded that sediment deposition was mainly forced by sediment supply from ploughing areas.
Two short-lived phases of incision during the first half of the XIX century were probably caused by a decrease of sediment supply due to a decline in the frequency of heavy precipitation events.
Our hypothesis is that the current incision in the rural area could be the result of a decline in sediment supply from hillslopes and the watershed, due to reduced agricultural activity (i.e. Conversely, the hillslope deposits in which incision occurs could have been deposited during times of higher agricultural activity.
This hypothesis could also explain why incision is so important in the peri-urban area, as it is enhanced by UI and CSO.
The channel incised during this period was later partly filled with sediment.
The decline of agricultural land use from about 1915 onwards decreased sediment supply, while the increase of urbanisation from about 1950 amplified the flow energy of flooding by pluvial waters and overflows from storm basins, causing the presently ongoing incision in the area that began about AD 1975.
Luminescence methods are now frequently used for the dating of colluvial sediments, as originally suggested by Wintle & Catt (1985) and recently reviewed by Fuchs & Lang (2009).