Seasonal controls on meteoric 7Be in coarse-grained river channels

Abstract

Cosmogenic 7Be is a natural tracer of short-term hydrological processes, but its distribution in upland fluvial environments over different temporal and spatial scales has not been well described. We measured 7Be in 450 sediment samples collected from perennial channels draining the middle of the Connecticut River Basin, an environment that is predominantly well-sorted sand. By sampling tributaries that have natural and managed fluctuations in discharge, we find that the 7Be activity in thalweg sediments is not necessarily limited by the supply of new or fine-grained sediment, but is controlled seasonally by atmospheric flux variations and the magnitude and frequency of bed mobilizing events. In late winter, 7Be concentrations in transitional bedload are lowest, typically 1 to 3Bqkg-1 as 7Be is lost from watersheds via radioactive decay in the snowpack. In mid-summer, however, 7Be concentrations are at least twice as high because of increased convective storm activity which delivers high 7Be fluxes directly to the fluvial system. A mixed layer of sediment at least 8cm thick is maintained for months in channels during persistent low rainfall and flow conditions, indicating that stationary sediments can be recharged with 7Be. However, bed mobilizing rain on snowmelt events in late Spring can 'reset' 7Be amounts and concentrations in the channel as previously buried 'old' sediment with low 7Be is mixed into the thalweg. We conclude that given proper temporal and spatial sampling, 7Be is a valuable tracer of seasonal-timescale mass transport and exchange in coarse-grained fluvial systems. © 2013 John Wiley & Sons, Ltd.