Optical Dating of Eolian Dune Sand from the Canadian Prairies*

Abstract

Optical dating is a well-established method that is used to determine the time elapsed since quartz or feldspar grains were last exposed to sunlight, which usually dates the time elapsed since their burial in a sedimentary deposit. Only a few seconds of direct sunlight are needed to reset the “mineral clock”, making the method ideal for dating the formation of eolian landforms. In this paper we describe how we use violet luminescence that is emitted in response to near-infrared excitation of sand-sized K-feldspar grains to date the time of formation of eolian dunes on the Canadian prairies. Using the multiple-aliquot with thermal transfer correction method, together with a test for sufficient sunlight exposure, and a correction for anomalous fading, we are able to produce accurate optical ages ranging from 150 years to more than 11 ka with precision usually better than 10 % at 2σ. Ages are given for dunes in the Dundurn and Elbow sand hills of south-central Saskatchewan, the climatic significance of which is provided in a paper also in the present volume. Knowledge of when the dunes were stable, and when they were not, is used to help understand the nature and timing of Holocene climate change in the area.La datation optique est une méthode reconnue employée pour déterminer le temps écoulé depuis que des grains de quartz ou de feldspath potassique ont été pour la dernière fois exposés à la lumière du soleil, ce qui correspond généralement au temps écoulé depuis leur enfouissement dans un dépôt sédimentaire. Quelques secondes seulement de lumière du soleil directe suffisent pour remettre à zéro « l’horloge minérale », ce qui en fait une méthode parfaitement appropriée pour dater la formation de dépôts éoliens. Nous décrivons ici comment utiliser la luminescence violette émise en réponse à une excitation à l’infrarouge des grains de feldspath de la taille du sable pour dater la formation des dunes dans les Prairies. Grâce à la méthode des portions aliquotes multiples comprenant une correction pour le transfert thermique, ainsi qu’un test d’exposition suffisante au soleil et une correction pour la décroissance anormale, nous pouvons obtenir des âges optiques précis à partir de 150 ans jusqu’à plus 11 ka avec une précision généralement supérieure à 10 % à 2σ. Les âges proviennent des dunes des collines Dundurn et Elbow, dans le centre sud de la Saskatchewan ; leur signification au plan climatique est donnée dans un autre article du présent numéro. La connaissance des périodes de stabilité des dunes sert à comprendre la nature et le synchronisme des changements climatiques survenus dans la région, au cours de l’HolocèneLa datación óptica es un método conocido que se emplea para determinar el tiempo transcurrido desde que los granos de cuarzo y feldespato fueron expuestos a la luz del sol, este proceso generalmente refleja el tiempo transcurrido durante el cual los sedimentos terminan por ser sepultados. Unos pocos segundos de exposición a la luz del sol bastan para poner en marcha el “reloj mineral”, esta característi...

Interpretations and common challenges of aeolian records from North American dune fields

“…A common limitation with dating of dune sands in the northern Great Plains is the low luminescence signal of quartz grains. This occurs, both in regards to low luminescence emitted for young samples, such as those younger than 1000 years (Lian et al, 2002), and for sands buried on the order of 10,000 years ago, with less than 1% of grains producing a measurable signal (Munyikwa et al, 2011). For these reasons, multiple aliquot or single aliquot methods on feldspars or quartz are most common, and single grain methods on quartz have not been utilized.…”

“…Notably, anomalous fading corrections have been made on some individual samples with delays after laboratory irradiation ranging in time from 1.7 to 641 days (Lian et al, 2002). A common limitation with dating of dune sands in the northern Great Plains is the low luminescence signal of quartz grains.…”

“…In the northern Great Plains, most luminescence ages have been acquired from IRSL using feldspar, and this method has been tested extensively for zero-age determinations, anomalous fading corrections, comparison to radiocarbon and OSL age determinations, and comparison to historical rates of dune movement and stabilization (Lian et al, 2002;Wolfe et al, 2002a;b;. Notably, anomalous fading corrections have been made on some individual samples with delays after laboratory irradiation ranging in time from 1.7 to 641 days (Lian et al, 2002).…”

Relict Late Wisconsinan Dune Fields of the Northern Great Plains, Canada*

“…Wintle (1997), Aitken (1998), Huntley and Lian (1999), and Lian and Huntley (2001) describe techniques, and Lian et al (2002) provide methodological details applied to this study. Sand sized K-feldspar grains (180-250 µm diameter) were dated by excitation with near-infrared (1.4 eV, ~880 nm) photons and measuring the violet (3.1 eV, 400 nm) photons emitted in response.…”

Assessing the impact of porosity variations on the reflectance and transmittance of natural sands

“…It represents an essential input for basin modeling and geophysical studies involving erosion of regions more vulnerable to environmental changes, such as coastal areas and river deltas. 7 For example, the effective optical dating 10 of sand deposits found in these regions 11 relies on the careful appraisal of their light penetration (transmission) profiles. These are affected by the packing conditions of these deposits, 12 which are directly associated with their porosity.…”