From Source to Sink: linking the Andean source signal to coastal eolian sand-dunes using cosmogenic 10Be/14C residence times in Central Chile

The formation of dune and paleodune sequences along the Pacific coast of Central Chile (30-35° S) remains poorly understood. The Andean glaciated landscape seems to exert a major influence on sediment supply, although the coupling between glacier fluctuations and dune build up in the Pacific coast remains speculative. Drainage basins, modulated by climatic and tectonic forcings, produce and transfer sediment from source areas to continental and marine sinks. However, transport pathways are complex as sediment may be temporarily stored and mixed along fluvial systems, decoupling source signals from downstream sinks by delaying or partially erasing them. Sediment residence time integrates both transport duration and temporary storage within a basin. Constraining it is essential to evaluate whether climatic signals such as glacier fluctuations can be faithfully transferred to the Pacific coast and recorded in dune deposits, allowing a better understanding on the formation of these systems. Here, we estimate sediment residence times in the Rapel Basin using paired in situ cosmogenic ¹⁴C and ¹⁰Be measured in modern fluvial and dune sands. As ¹⁴C has a much shorter half-life than ¹⁰Be, this pair is particularly sensitive to periods of sediment storage and can be used to quantify the time sediments spend in transient reservoirs along their transport pathways. Therefore, we adopt a source-to-sink sampling strategy, collecting sediments from distinct geomorphic domains along the basin, from the Andean headwaters to the coastal dune systems. Our results indicate that all samples experienced some degree of storage during transport, with minimum residence times of ~2.5 kyr in the Andean glacial domain and maximum values of ~14 kyr in dunes at the Pacific coast. At the source zone, residence times ranging from ~2.5 to ~7.5 kyr suggest that sediments record a millennial scale residence signal prior to entering the fluvial network, likely due to storage in glacial environments and on hillslopes. Once sediments enter the channel network, transport through the medium and lower basin appears to be largely efficient, with little additional storage until reaching the Pacific coast. From the river mouth to the dune systems, sediments record up to ~4 kyr of additional residence time relative to the Andean source signal. Such addition indicates a millennial scale lag time in source signal transmission towards the dunes. Our dataset suggests that sediment residence times within the Rapel Basin (Central Chile) are primarily controlled by sediment generation rather than by complex fluvial transport histories itself. Only at the river mouth, significant additional residence times are added to the signal as sediment is transferred towards the dunes, as part of the long-shore littoral drift. These results demonstrate that even with a millennial timescale lag at the coast, the dune systems remain sensitive archives that record the primary Andean signal.