Steady state reach‐scale theory for radioactive tracer concentration in a simple channel/floodplain system

Abstract

A steady state analytical model is presented for reach‐scale variation in the concentration of a decaying radioactive tracer associated with sediment particles that regularly pass through an off‐channel floodplain. The floodplain is represented as a series of well‐mixed sediment reservoirs that continually exchange sediment with the channel. The model allows for tributary input and valley‐wide aggradation or degradation. Tracer concentration depends on the upstream boundary concentration, the tracer and sediment load, floodplain geometry, and the rates of in‐floodplain tracer production and/or decay. The theory predicts relatively modest down‐channel change in the concentration of long‐lived isotopes but implies that significant change may occur for (1) tracers with a short‐enough half‐life (such as 14C) or (2) floodplains with sediment residence times that are large enough for cosmogenic production or meteoric fallout to increase tracer concentration in the down‐valley direction. The profiles are shown to be strongly dependent on the grain size distributions of both the sediment load and the floodplain. The results imply that down‐channel 14C profiles have the potential to constrain Holocene bed material loads in systems with sufficient storage. The theory concisely describes the general importance of a floodplain for modifying in situ produced cosmogenic tracer concentration and can also characterize floodplain importance for fallout radioisotopes (i.e., 10Be, 210Pb, or 7Be) or organic 14C.

Model predictions of long-lived storage of organic carbon in river deposits

“…Most previous models of sediment storage in meandering river systems have assumed that storage durations are exponentially distributed (Malmon et al, 2003;Lauer and Parker, 2008a;Lauer and Willenbring, 2010). This assumption requires that deposits of all ages are equally likely to be eroded at any given time, which is inconsistent with some field data (Nakamura and Kikuchi, 1996;Lancaster and Casebeer, 2007) as well as the common observation that the position of the river channel is persistent in time (Bradley and Tucker, 2013).…”

“…After being eroded from an upland source, fluvial sediments are routed through a transport network that includes multiple temporary storage reservoirs (e.g., channel and floodplain deposits; Malmon et al, 2003;Lauer and Parker, 2008a;Lauer and Willenbring, 2010;Pizzuto et al, 2017). These sediment reservoirs can also store POC, potentially leading to a decrease in its radiocarbon content due to radioactive decay.…”

Quantifying sediment discharge from the Bolivian Andes into the Beni foreland basin from cosmogenic 10Be-derived denudation rates

“…Overall, the Andean erosion signal is preserved throughout transfer of sediment through the fl oodplain. These results are supported by a more sophisticated numerical reach-scale theory model by Lauer & Willenbring (2010), who also conclude that only relatively modest down-channel changes in the concentration of long-lived nuclides occur.…”

210 Pb geochronology of flood events in large tropical river systems

“…The concepts developed [23] for 210 Pb (downstream decline in sediment activity, accumulation event plateaus and meteoric caps) may also be applied to other clay-associated meteoric radionuclides, such as 137 Cs and 7 Be, as has recently been suggested [38]. In environments with more complex delivery of sediment than in the Bolivian study area, our procedures could be combined with a numerical model [38,48] to simulate an alternating process scenario of constant sedimentation interrupted by episodic, time-dependent sediment accumulation.…”

“…The significance of this exchange process has implications for the down-channel budget of clay-associated 210 Pb because the cutbanks contain material that is radioactively 'older' (less XS activity) than the river sediment. Therefore, the process of rapid channel migration during floods implies a continuous downstream dilution of the 210 Pb activity of riverborne clay [23]-a process related to that proposed to explain an observed decline in 210 Pb activity in the sub-aqueous environment of the Wilmington Submarine Canyon [37] and recently developed in more theoretical detail in the study of Lauer & Willenbring [38]. Because we have estimated (i) these migration fluxes from field and image surveys [23], (ii) average clay abundances for all types of river deposits, and (iii) the average radionuclide content of the cutbanks across the Beni Foreland (electronic supplementary material, figure S4), we can calculate an approximate budget of clay and the associated downstream decline in 210 Pb activity for the Beni River.…”