The cosmogenic record of mountain erosion transmitted across a foreland basin: Source-to-sink analysis of in situ 10Be, 26Al and 21Ne in sediment of the Po river catchment

We analyze the source-to-sink variations of in situ Be-10, Al-26 and Ne-21 concentrations in modern sediment of the Po river catchment, from Alpine, Apennine, floodplain, and delta samples, in order to investigate how the cosmogenic record of orogenic erosion is transmitted across a fast-subsiding foreland basin. The in situ Be-10 concentrations in the analyzed samples range from similar to 0.8 x 10(4) at/g(QTZ) to similar to 6.5 x 10(4) at/g(QTZ) . The Be-10-derived denudation rates range from 0.1 to 1.5 mm/yr in the Alpine source areas and from 0.3 to 0.5 mm/yr in the Apenninic source areas. The highest Be-10-derived denudation rates are found in the western Central Alps (1.5 mm/yr). From these data, we constrain a sediment flux leaving the Alpine and the Apenninic source areas (>27 Mt/yr and ca. 5 Mt/yr, respectively) that is notably higher than the estimates of sediment export provided by gauging (similar to 10 Mt/yr at the Po delta). We observe a high variability in Be-10 concentrations and Be-10-derived denudation rates in the source areas. In the Po Plain, little variability is observed, and at the same time, the area-weighed Be-10 concentration of (2.29 +/- 1.57) x 10(4) at/g(QTZ) (+/- 1 SD of the dataset) from both the Alps and the Apennines is poorly modified (by tributary input) in sediment of the Po Plain ((2.68 +/- 0.78, +/- 1 SD) x 10(4) at/g(QTZ)). The buffering effect of the Po floodplain largely removes scatter in Be-10 signals. We test for several potential perturbations of the cosmogenic nuclide record during source to sink transfer in the Po basin. We find that sediment trapping in deep glacial lakes or behind dams does not significantly change the Be-10-mountain record. For example, similar Be-10 concentrations are measured upstream and downstream of the postglacial Lake Maggiore, suggesting that denudation rates prior to lake formation were similar to today's. On the scale of the entire basin, the Be-10 concentration of basins with major dams is similar to those without major dams. A potential modification of the Be-10-mountain record during sediment storage in the subsiding Po Plain can be excluded as measured Al-26/Be-10 ratios do not show the addition of deeply buried material. A barely resolvable excess Ne-21 signal in the Po plain indicates recycling of previously eroded sediment rather than accumulation of cosmogenic nuclides during surficial floodplain transport. Our results demonstrate that the cosmogenic record of mountain erosion is effectively transmitted from the source areas to the sediment sink, even across a strongly subsiding foreland basin. We show that this record is poorly influenced by a range of potential geological and anthropogenic sources of bias, and is largely independent from upstream sediment interception and sediment storage in the floodplain. (C) 2016 Elsevier B.V. All rights reserved.