Cosmogenic nuclide budgeting of floodplain sediment transfer

Abstract

Cosmogenic nuclides produced in quartz may either decay or accumulate while sediment is moved through a river basin. A change in nuclide concentration resulting from storage in a floodplain is potentially important in large drainage basins in which sediment is prone to repeated burial and remobilization as a river migrates through its floodplain. We have modeled depth-and time-dependent cosmogenic nuclide concentration changes for 10 Be, The cosmogenic nuclide composition of old deposits in currently inactive floodplains that have been isolated for periods of millions of years from the river that once deposited them are predicted to either increase or decrease in C, nuclide concentrations modeled for the in situ-produced variety of this nuclide are, however, sensitive to floodplain storage on residence times of < 20 ky. 10 Be and 26 Al concentration, depending on the depositional depth. These conditions can be evaluated using the 26 Al/ 10 We illustrate these models with examples from the Amazon basin. As predicted, modern bedload collected from an Amazon tributary, the Bolivian Beni River, shows no systematic change in nuclide concentration as sediment is moved through 500 km of floodplain by river meandering. In contrast, in the central Amazon floodplain currently untouched by the modern river system, low Be ratio that readily discloses the depth and duration of storage.26 Al/ 10 The important result of this analysis is that in all likely cases of active floodplains, cosmogenic Be ratios account for minimum burial depths of 5 to 10 m for a duration of > 5 My. 10 Be and 26 Al concentrations remain virtually unchanged over the interval sediment usually spends in the basin. Thus, spatially-averaged denudation rates of the sediment-producing area can be inferred throughout the entire basin, provided that nuclide production rates are scaled for the altitudes of the sediment-producing area only, because floodplain storage does not modify nuclide concentrations introduced from the sediment source area.

From source to sink: Preserving the cosmogenic 10Be-derived denudation rate signal of the Bolivian Andes in sediment of the Beni and Mamoré foreland basins

“…We therefore recalculated the denudation rates for the Beni River floodplain according to Section 3.4 to include the sediment-producing areas only (ε FC , see Eq. 1) following Wittmann and von Blanckenburg (2009). Results from this recalculation give an average floodplain denudation rate ε FC of 0.45 ± 0.06 mm/ yr that is very similar to our denudation rate estimate at Rurrenabaque (0.37 ± 0.06 mm/ yr).…”

“…For 600 km of floodplain within the Beni River basin from Rurrenabaque to Cachuela Esperanza, mean nuclide concentrations are 3.7 ± 0.5×10 4 at/g (Qz) (BE 2 to 17, n = 12, excluding tributaries; also see Wittmann and von Blanckenburg, 2009). This concentration compares well to that obtained from BE 1 at Rurrenabaque (3.8 ± 0.6×10 4 at/g (Qz) ).…”

“…The conclusion of the model presented by Wittmann and von Blanckenburg (2009) is that in most floodplains, the denudation signal of the sediment source area is preserved by its cosmogenic nuclide concentration, regardless of the duration of temporary storage. This finding is valid if the storage is short compared to the half life of the nuclide.…”

“…Yet, the calculation of a cosmogenic nuclide-derived denudation rate (ε cosmo ) requires that the nuclide production rate is scaled to the altitude of the sediment producing area. Because not all catchment area located upstream of a sampling point within a floodplain is actually producing sediment, we calculate floodplain-corrected cosmogenic nuclide-based denudation rates (termed ε FC in the following) (Wittmann and von Blanckenburg, 2009). To this end, all basin-averaged denudation rates of which the catchment contains a floodplain area are corrected by defining a "floodplain-corrected" sediment source area A FC .…”

“…In such basins, the transport of sediment includes residence in floodplains due to deposition in the floodplain and reincorporation of sediment in the active fluvial system by bank erosion. Wittmann and von Blanckenburg (2009) recently presented a nuclide budget to model the effect of temporary sediment storage in floodplains. This budget was modeled for a variety of settings including the Beni floodplain.…”