5 research outputs found
Luminescence Dating in Fluvial Settings: Overcoming the Challenge of Partial Bleaching
Optically stimulated luminescence (OSL) dating is a versatile technique that utilises the two most ubiquitous minerals on Earth (quartz or K-feldspar) for constraining the timing of sediment deposition. It has provided accurate ages in agreement with independent age control in many fluvial settings, but is often characterised by partial bleaching of individual grains. Partial bleaching can occur where sunlight exposure is limited and so only a portion of the grains in the sample was exposed to sunlight prior to burial, especially in sediment-laden, turbulent or deep water columns. OSL analysis on multiple grains can provide accurate ages for partially bleached sediments where the OSL signal intensity is dominated by a single brighter grain, but will overestimate the age where the OSL signal intensity is equally as bright (often typical of K-feldspar) or as dim (sometimes typical of quartz). In such settings, it is important to identify partial bleaching and the minimum dose population, preferably by analysing single grains, and applying the appropriate statistical age model to the dose population obtained for each sample. To determine accurate OSL ages using these age models, it is important to quantify the amount of scatter (or overdispersion) in the well-bleached part of the partially bleached dose distribution, which can vary between sediment samples depending upon the bedrock sources and transport histories of grains. Here, we discuss how the effects of partial bleaching can be easily identified and overcome to determine accurate ages. This discussion will therefore focus entirely on the burial dose determination for OSL dating, rather than the dose-rate, as only the burial doses are impacted by the effects of partial bleaching
Long-term temporal trajectories to enhance restoration efficiency and sustainability on large rivers: an interdisciplinary study
While the history of a fluvial hydrosystem can provide essential
knowledge on present functioning, historical context remains rarely
considered in river restoration. Here we show the relevance of an
interdisciplinary study for improving restoration within the framework of a
European LIFE+ project on the French side of the Upper Rhine (Rohrschollen
Island). Investigating the planimetric evolution combined with historical
high-flow data enabled us to reconstruct pre-disturbance hydromorphological
functioning and major changes that occurred on the reach. A deposition
frequency assessment combining vertical evolution of the Rhine thalweg,
chronology of deposits in the floodplain, and a hydrological model revealed
that the period of incision in the main channel corresponded to high rates of
narrowing and lateral channel filling. Analysis of filling processes using
Passega diagrams and IRSL dating highlights that periods of engineering works
were closely related to fine sediment deposition, which also presents
concomitant heavy metal accumulation. In fact, current fluvial forms,
processes and sediment chemistry around Rohrschollen Island directly reflect
the disturbances that occurred during past correction works, and up to today.
Our results underscore the advantage of combining functional restoration with
detailed knowledge of the past trajectory to (i) understand the functioning
of the hydrosystem prior to anthropogenic disturbances, (ii) characterize the
human-driven morphodynamic adjustments during the last 2 centuries,
(iii) characterize physico-chemical sediment properties to trace
anthropogenic activities and evaluate the potential impact of the restoration
on pollutant remobilization, (iv) deduce the post-restoration evolution
tendency and (v) evaluate the efficiency and sustainability of the
restoration effects. We anticipate our approach will expand the toolbox of
decision-makers and help orientate functional restoration actions in the
future