Sediment Transit Time and Floodplain Storage Dynamics in Alluvial Rivers Revealed by Meteoric¹⁰Be

Abstract

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 (10 Be 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. Plain Language Summary Understanding how long sediment takes to travel downstream in rivers, also known as sediment "transit time," is crucial for responsible watershed management and constraining global biogeochemical cycles. We aim to measure transit times for large rivers and determine the processes regulating this time scale. We present a new transit time proxy based on beryllium-10 (10 Be), a rare isotope produced in the atmosphere and delivered to Earth by rain. If river sediment collected downstream has more 10 Be than sediment upstream, this indicates that sediment was trapped in the floodplain for many years before continuing to travel downstream. We collected river sediment at multiple locations from upstream to downstream along a large, undammed river, the Rio Bermejo in Argentina. We found that 10 Be increased 230% from upstream to downstream, translating to a transit time of~8,500 years. This long time scale implies that sediment and carbon delivered to rivers have enough time to be weathered or oxidized to CO 2 before they are buried in the ocean. We also show that transit time is controlled by the river's shape and lateral mobility, suggesting that human alteration of channel shape or sediment supply will reduce a river's ability to use its floodplain for natural flood and erosion control.

River Sediment Geochemistry as a Conservative Mixture of Source Regions: Observations and Predictions From the Cairngorms, UK

“…These temporal and anthropic fluctuations will contribute to the geochemical variability not explained by our model. We note that cosmogenic nuclide studies suggest that the average residence time of sediment in drainage basins is likely to be of the order of hundreds to thousands of years (Repasch et al., 2020). These observations suggest that the composition of sediment in drainage basins might integrate processes operating on timescales of hundreds to thousands of years.…”

Times Associated With Source-to-Sink Propagation of Environmental Signals During Landscape Transience

“…The duration of sediment storage outside the active river channel can also be measured from the ratio of cosmogenic meteoritic 10 Be over 9 Be within sediments (Wittmann et al, 2015). Repasch et al (2020) tested this method along the Rio Bermejo (Andean foreland basin, northern Argentina) and observed that the suspended load travels ∼1200 km from sources to sink in 8.4 ± 2.2 kyr.…”

Times associated with source-to-sink propagation of environmental signals during landscape transience

“…The duration of sediment storage outside the active channel can also be measured from the ratio of cosmogenic meteoritic 10 Be over 9 Be within sediments (Wittmann et al, 2015). Repasch et al (2020) tested this method along the Rio Bermejo (Andean foreland basin, northern Argentina) and observed that the suspended load travels ~1200 km from sources to sink in 8.4±2.2 kyr. Within the Indus SRS, Clift et al (2008) suggests based on Nd composition on a limited set of samples, that the clay-sized fraction travels as suspended load rapidly through the system after an increase in monsoon strength with no resolvable lag time, while the bedload transport is decoupled from the suspended load and heavy mineral (zircon) grains travel about of 7-14 kyr from source to sink (Clift and Giosan, 2014).…”