Quantifying denudation and sediment–accumulation systems (open and closed lakes): basic concepts and first results
Abstract The quantitative aspects of denudation in the drainage areas and sediment accumulation in the corresponding closed or open lakes are approached using two methods: (1) from known mean rates of mechanical and chemical denudation and the ratio of the drainage/lake area, average terrigenous and potential chemical (biogenic and evaporites) sedimentation rates in the lake are determined; or (2) from known sedimentation rates the average denudation rate is found. In closed systems the total mass denuded during a certain unit of time is deposited in the lake as terrigenous, biogenic and evaporitic sediments, whereas open lake systems lose a major part of the dissolved river input derived from chemical denudation. Applying elementary equations describing these relationships and using ratios between mechanical and chemical denudation (weathering ratio) established in other regions, e.g. chemical weathering in the lake drainage can be estimated. Likewise, the knowledge of the `bio-index' (ratio of biogenic sediment/total dissolved mass) allows the calculation of the loss of dissolved matter through the outflow from the lake, etc. In a first step, existing data from modern lake systems are evaluated. Later, idealized lake systems in various climatic and morphotectonic zones can be simulated and the results utilized to better understand ancient lake systems. Modern Alpine lakes reflect increased mechanical denudation rates (100 to ≥300 mm/ka) in highly elevated drainage areas of the Central Alps (mainly crystalline rocks) and high chemical denudation (up to ca. 100 mm/ka) in carbonate rocks of the Northern Calcareous Alps. Mechanical denudation is significantly enhanced by glaciation, but transport of the detritus into lakes may be delayed until the time immediately after glacial retreat. Average rates of terrigenous lake sedimentation amount to 5 to ≥15 mm/a (crystalline rocks, high relief), or are ≤5 mm/a (carbonate rocks). Delta outbuilding was a main factor in the filling of many Alpine lakes. Lakes draining low-altitude, semi-arid parts along the eastern margin of the Alps (Lake Balaton in Hungary) have low sedimentation rates and reflect the influence of increased temperature and vegetation. In East Africa, the transition from the Late Pleistocene (arid) to the early Holocene (humid) and again back to somewhat drier conditions caused substantial rises in lake levels and changes from closed to open lake systems. In such cases, the calculation of denudation from accumulation rates is biassed as a result of intermittent sediment storage at the lake margins (low sedimentation rates in the lake centre during lake highstands) or redeposition of sediment during lowstands (lowstand shedding). The modern denudation rates of East African rift lake systems vary by a factor of ≥15 (Lake Tanganyika, crystalline rocks and more dense vegetation cover, total denudation rate ca. 4 mm/ka; Lake Turkana, young volcanic rocks and tephra, sparse vegetation, mechanical denudation rate ca. 60 mm/ka).