Reconstructing virtual velocity and fluvial dynamics using MET-IRSL from single grains of sand

Over the past decade, several research groups have developed and applied methods for using Infra-Red Stimulated Luminescence (IRSL) from sand-sized grains of alkali feldspar collected from the active channels of different rivers. These methods used either conventional multiple grain IRSL measurements, or single grain IRSL determinations, but all depend on comparisons of results from different sampling locations to reconstruct virtual velocity. In its simplest form, this approach relies on the Ergodic principle as the basis for time-space equivalence of different samples. While this can often represent a successful approach, recent anthropogenic disturbances to fluvial systems may in some cases render this method problematic. For example, where channel engineering or dam construction cuts off or modifies the natural sediment supply, samples collected downstream of these locations may provide signals that are inconsistent with those from upstream.

For this reason, our research team has been developing IRSL approaches to attempt to reconstruct sediment storage times and virtual velocity by inverting measured Multiple Elevated Temperature (MET) IRSL signals from single grains of alkali feldspar. Some grains preserve a record that is shaped by multiple episodes of storage during burial and light exposure during transport; storage causes trapped charge populations responsible for IRSL signals to grow in a predictable manner, while light exposure causes a reduction in each population. Multiple IRSL signals measured at a range of temperatures in the laboratory display different sensitivity to light, resulting in different degrees of “bleaching” (reduction in trapped charge). When a grain is subject to multiple episodes of burial and bleaching, the different IRSL signals move away from being in an equilibrium ratio with each other, allowing us to constrain their past burial and bleaching histories, within some limits. In this presentation, we shall compare results from this novel single grain MET-IRSL inversion approach with conventional IRSL sediment transport approaches, and assess performance from grains subject to laboratory simulations of different burial-bleach cycles. The new technique has great potential to help understand contemporary and past fluvial dynamics and sediment storage, as well as determination of sediment sources and channel erosional processes, and can contribute significantly to applications such as catchment carbon dynamics, or assessing impacts of engineering structures.