COSMOGENIC NUCLIDES: NEW USES AS SEDIMENT TRACERS AND IN DEVELOPING SEDIMENT BUDGETS

Terrestrial cosmogenic nuclide (TCN) concentrations provide a measure of near-surface residence time, and have been used to document surface exposure ages and erosion rates. Using TCNs as a sediment tracer is a promising new application. Here we provide an overview of tracing sediment with TCNs, including determining sediment provenance, tracing sediment through linked geomorphic systems, and balancing long-term sediment budgets, with examples from two landscapes. We also develop the theoretical framework for the general case of balancing the TCN budget in sedimentary systems, including erosion and deposition events in poorly-mixed systems.

The high W. Alps landscape includes broad U-shaped valleys, steep hillslopes, and valley glaciers. Sediment traps indicate a long-term erosion rate of about 0.12 m / k.y. during the Holocene. If landscape denudation were uniform, the spallation TCN component in the subaerially eroding hillslopes should approach steady state in about 5-6 k.y., with a concentration of about 200,000 atoms ¹⁰Be / g qtz. The measured concentration in subglacial outwash is a tenth of this, less than 20,000 atoms ¹⁰Be / g qtz. River concentrations are similar to the subglacial sediment concentrations, suggesting that the sediment provenance is largely glacial. Given the likely storage of TCNs on slowly eroding hillslopes, we would expect a pulse of high concentration sediment at the onset of large-scale ice sheet glacial advance.

The active margin at Santa Cruz, California is characterized by broad flat marine terraces, steep fluvial basins, and narrow discontinuous beaches. TCNs are used to examine rates of sediment production in basins, to determine the duration of sediment storage on marine terraces, and to constrain fluvial and cliff-derived sediment inputs to the littoral cell. The TCN-derived fluvial erosion rates in the five largest basins are typically around 0.2 mm / yr, with concentrations of ~20,000 atoms ¹⁰Be / g qtz. The duration of sediment storage on the lowest terrace is about 65 ka, with an average concentrations of about 200,000 atoms ¹⁰Be / g qtz. The measured pattern of beach TCN concentrations can be reproduced by a spatially uniform, long-term cliff-backwearing rate of ~10 cm / yr, punctuated by fluvial inputs from the various basin sizes.