Sediment production in large river basins throughout the Quarternary
Final Report Abstract
Using modern river sediments from large global rivers in comparison to their sedimentary archives spanning the past 2.5 Myr, we measured terrestrial cosmogenic nuclides (10Be, 26Al) to infer changes in erosion rates throughout this time. The overall aim was to test whether an increase in sedimentation rates, and thus also erosion rates, due to enhanced mountain building or changes in climate, may have resulted in increased silicate weathering that consumes CO2 and hence may have caused global late Cenozoic cooling. When measured in modern river sediments, cosmogenic 10Be-derived erosion rates integrate over millennial time scales and record the flux of sediment produced in mountainous source areas. A prerequisite however is that cosmogenic nuclide concentrations that are characteristic of the erosion rate of the sediment-producing area are transferred from source to sink without modifications by floodplain processes. By using paired nuclide analysis (26Al/10Be ratios) in more than 50 large global modern rivers, in total covering 30.9% of the Earth’s ice-free terrestrial surface, we tested under which conditions the source area-derived cosmogenic signal is modified, finding that mainly for slowly-eroding rivers with low water runoff, the cosmogenic signal is modified. For rivers where the cosmogenic signal is not modified, cosmogenic nuclide-derived sediment fluxes compare well to the sediment flux obtained from decadalscale sediment gauging. This agreement over different erosional time scales indicates that large rivers are capable of buffering short-term, high amplitude fluctuations in sediment flux caused by climate variability or anthropogenic soil erosion. From the analysis of cosmogenic nuclides in sedimentary archives paleo-erosion rates can be obtained, provided that similar to modern settings, the cosmogenic nuclide-derived erosional signal is unmodified from source to sink. We found this to be the case for three out of eight archives of large global rivers, namely for the Amazon, Indus, and Nile rivers. These three archives all show a vague trend towards increasing erosion over the past 2.5 Myr towards modern times. This increase was obliterated in the other archives (Yangtze, Rhine, Rhone, Danube, Murray-Darling) due to significant sediment burial that modified nuclide concentrations from source to sink and hence hamper the interpretation of these archives. Previous paleo-denudation records have also noted an increase in denudation rates over time, but our records comprise the first analysis in sediment of large global rivers whose sediment fluxes are relevant on a global scale. We conclude that changes in erosion could be responsible for atmospheric cooling over the late Cenozoic.
Publications
- (2020) A global rate of denudation from cosmogenic nuclides in the Earth's largest rivers. Earth-Science Reviews 204 103147
Wittmann, H.; Oelze, M.; Gaillardet, J.; Garzanti, E.; Blanckenburg, F. von
(See online at https://doi.org/10.1016/j.earscirev.2020.103147) - 2016, "The geological significance of cosmogenic nuclides in large lowland river basins”, 159, 118-141
Wittmann, H. and von Blanckenburg, F.
(See online at https://doi.org/10.1016/j.earscirev.2016.06.001)
