Comparing Erosion Rates From 10Be With Other Methods in the Northern Apennines, Italy: Evidence for Dynamic Equilibrium

We have measured cosmogenic 10Be in quartz from modern stream sediment to infer erosion rates across a wide swath of the northern Apennines, Italy. This area provides an opportunity to compare cosmogenic nuclide-based results with erosion rates estimated from a variety of other methods, including (1) modern sediment yields estimated over annual timescales, (2) reservoir sedimentation rates estimated over decadal timescales, (3) basin sedimentation rates estimated over the Quaternary, and (4) exhumation rates from thermochronometry over >5 million years. The northern Apennines are a tectonically active mountain belt, having been uplifted mainly in the Plio- Pleistocene. Hillslope erosion in our study area takes place largely by shallow landsliding. Land use here has included intense forestry beginning in the Bronze Age, and agriculture in valley bottoms since at least Roman times. Land use over such long timescales may influence not only modern sediment yields, but cosmogenic nuclide rates as well. We sampled at more than 20 locations in 7 different watersheds over a span of 200 km on the Adriatic side of the Apennines. Erosion rates inferred from sand-sized sediments fall within a narrow range of 0.2-0.6 mm/yr, regardless of lithology, position in the watershed, or location within the mountain range. Erosion rates inferred from gravel are somewhat higher, at 0.5-0.8 mm/yr. This is consistent with gravels being derived from landsliding, as observed in some previous cosmogenic nuclide applications. We also inferred paleo-erosion rates from quartz-bearing fluvial and littoral sedimentary deposits in the same region, which have ages ranging from Late to Middle Pleistocene. Paleo-erosion rates are indistinguishable from modern rates. This suggests that the cosmogenic nuclides accurately reflect long-term erosion rates, and are not unduly influenced by recent land use. Our cosmogenic nuclide erosion rates are also consistent with estimates from reservoir and basin sedimentation over decades to millions of years, and with river incision rates since the Middle Pleistocene. Together these data suggest that the northern Apennines may be in a state of dynamic equilibrium that has persisted for the past million years. Erosion rates inferred from modern sediment yields are much higher than our cosmogenic nuclide results. Interestingly, long-term exhumation rates inferred from thermochronometry are also somewhat higher than our erosion rates. This may be because thermochronometry represents either (1) exhumation from beneath a more erodible overburden, or (2) tectonic exhumation that occurred during wedge growth.