River incision, ¹⁰Be production and transport in a source-to-sink sediment system (Var catchment, SW Alps)

Abstract

Abstract. Detrital 10Be from continental river sands or submarine sediments has been extensively used to determine the average long-term denudation rates of aerial catchments, based on the assumption that the rate of cosmonuclide production by interaction of source rocks with cosmic radiations balances out the loss of these elements by surface denudation. However, the 10Be signal of in-situ produced sediments may be altered by the response time of mountainous catchments to high-frequency forcings; besides, transient sediment storage in piedmonts, alluvial plains, lakes or near the coast may also induce a difference between the erosive signal and its record in the sedimentary sink. Consequently, a significant part of the signal recorded in shallow-water sediments can be lost, as deep marine sediments may record simultaneously a signal coming from newly eroded source rocks along with one coming from the destabilization of previously deposited sediments. In this paper, we use the Surface Process Model Badlands to simulate erosion, deposition and detrital 10Be transfer from a source-to-sink sedimentary system (the Var River catchment, Southern French Alps) over the last 100 kyr. We first compare real denudation rates with the ones that would be extracted from in-situ produced sediments and from off-shore deposited sediments over time in order to examine how the 10Be record in sediments provides an accurate estimate of continental denudation rates. Then, we examine which conditions (precipitation rate, flexure, ice cover) permit to satisfy published measured river incision rates and 10Be concentration in submarine sediments. Our results, based on the Var catchment cosmic ray exposure dating and modelling indicate that, while river sands do accurately estimate the average denudation rate of continental catchments, it is much less the case for submarine deep sea sediments. We found that deep sea sediments have a different, and often much noisier 10Be signature than continental ones, and record a significant time lag with respect to actual precipitation rate changes, representing the geomorphological response of the submarine margin. The model which best fits both measured 10Be concentration in marine sediments and river incision rates on-land involves an increase in precipitation rates from 0.3 to 0.7 m.yr-1 after 20 ka, hence suggesting more intense precipitations starting at the end of the Last Glacial Maximum.