Sediment Transit Time and Floodplain Storage Dynamics in Alluvial Rivers Revealed by Meteoric 10Be

Quantifying the time scales of sediment transport and storage through river systems is fundamental for understanding weathering processes, biogeochemical cycling, and improving watershed management, but measuring sediment transit time is challenging. Here we provide the first systematic test of measuring cosmogenic meteoric Beryllium-10 (Be-10(m)) in the sediment load of a large alluvial river to quantify sediment transit times. We take advantage of a natural experiment in the Rio Bermejo, a lowland alluvial river traversing the east Andean foreland basin in northern Argentina. This river has no tributaries along its trunk channel for nearly 1,300 km downstream from the mountain front. We sampled suspended sediment depth profiles along the channel and measured the concentrations of(10)Be(m)in the chemically extracted grain coatings. We calculated depth-integrated(10)Be(m)concentrations using sediment flux data and found that(10)Be(m)concentrations increase 230% from upstream to downstream, indicating a mean total sediment transit time of 8.4 +/- 2.2 kyr. Bulk sediment budget-based estimates of channel belt and fan storage times suggest that the(10)Be(m)tracer records mixing of old and young sediment reservoirs. On a reach scale,(10)Be(m)transit times are shorter where the channel is braided and superelevated above the floodplain, and longer where the channel is incised and meandering, suggesting that transit time is controlled by channel morphodynamics. This is the first systematic application of(10)Be(m)as a sediment transit time tracer and highlights the method's potential for inferring sediment routing and storage dynamics in large river systems.