It takes times to transport sediments from the source area down to the sinks which can be continental or marine. On their way to the marine realm, sediments can be stored in temporary sinks and remobilized afterwards due to river avulsion for example. In consequence, signals produced in source areas (such as a climatic or a tectonic perturbation) are not transmitted immediately to the sediment archives. 

This has important consequences when we try to reconstruct past conditions from sedimentary records. In fact, because sediment grains have a stochastic transit from mountain sources to sedimentary basins, a fraction of grains are stored for a long time while others are recycled from old deposits. Consequently, the temporal representativity of a population of grains in a sedimentary stratum is uncertain. The potential recycling of old material is a major concern in the reconstruction of paleo-environments and this recycling is usually difficult to evaluate. 

We use a landscape evolution model (Cidre) that traces grains to analyse the distribution of residence times in an alluvial apron at the foot of a mountain relief. We study an end-member scenario that is the least favourable for the storage of grains: when the mountain is eroding at the same rate as rock is uplifting. In this case, the alluvial apron behaves as a by-pass zone, when averaging sediment flux over Ma, and the storage of grains of any size should be minimal. 

We observe that some grains are stored for hundreds of thousands of years before exiting the alluvial apron. Consequently, the mean residence time of sediment grains is much higher than the observed residence time of 95% of the grains exported by the alluvial apron rivers. This process may explain very long residence times found in fluvial systems by geochemical methods based on bulk measurements of sediment. In addition, it suggests that grains stored for a very long time, although a minority, can bias time-dependent proxies.

Related publications:

The distribution of sediment residence times at the foot of mountains and its implication for proxies recorded in sedimentary basins - S. Carretier, L. Guerit, R. Harries, V. Regard, P. Maffre, S. Bonnet - Earth Surface Processes and Landforms, 546, 2020 (here).

Times associated with source-to-sink propagation of environmental signals during landscape transience -  S. Tofelde, A. Bernhardt, L. Guerit, B. Romans - Frontiers in Earth Science  (here).