Centennial- to millennial-scale hard rock erosion rates deduced from luminescence-depth profiles

The measurement of erosion and weathering rates in different geomorphic settings and over diverse temporal and spatial scales is fundamental to the quantification of rates and patterns of earth surface processes. A knowledge of the rates of these surface processes helps one to decipher their relative contribution to landscape evolution – information that is crucial to understanding the interaction between climate, tectonics and landscape. Consequently, a wide range of techniques has been developed to determine short- (<102 a) and long-term (>104 a) erosion rates. However, no method is available to quantify hard rock erosion rates at centennial to millennial timescales. Here we propose a novel technique, based on the solar bleaching of luminescence signals with depth into rock surfaces, to bridge this analytical gap. We apply our technique to glacial and landslide boulders in the Eastern Pamirs, China. The calculated erosion rates from the smooth varnished surfaces of 7 out of the 8 boulders sampled in this study vary between <0.038±0.002 and 1.72±0.04 mmka-1 (the eighth boulder gave an anomalously high erosion rate, possibly due to a recent chipping/cracking loss of surface). Given this preferential sampling of smooth surfaces, assumed to arise from grain-by-grain surface loss, we consider these rates as minimum estimates of rock surface denudation rates in the Eastern Pamirs, China

Exploring IRSL 50 fading variability in bedrock feldspars and implications for OSL thermochronometry

International audienceOptically Stimulated Luminescence (OSL) is a well-established Quaternary dating method, which has recently been adapted to application in low-temperature thermochronometry. The Infra-Red Stimulated Luminescence (IRSL) of feldspar, which so far is the most promising target signal in thermochronometry, is unfortunately prone to anomalous fading. The fading of feldspar IRSL is at times not only challenging to measure, but also laborious to incorporate within luminescence growth models. Quantification of IRSL fading is therefore a crucial step in OSL thermochronometry, raising questions regarding (i) reproducibility and reliability of laboratory measurements of fading, as well as (ii) the applicability of existing fading models to quantitatively predict the level of IRSL field saturation in nature. Here we investigate the natural luminescence signal and anomalous fading of IRSL measured at 50 °C (IRSL50) in 32 bedrock samples collected from a variety of lithologies and exhumation settings (Alaska and Norway). We report a large span of IRSL50 fading rates between samples (g2days ranging from ∼0.5 to ∼45%/decade), which further demonstrates (i) a good reproducibility between two common fading measurement protocols, and (ii) the ability of tunnelling models to predict the level of feldspar IRSL50 field saturation in nature. We observe higher IRSL50 fading in feldspar with increasing Ca content, although other factors cannot be dismissed at present. Finally, our dataset confirms that the applicability of feldspar IRSL50 in OSL thermochronometry is limited to rapidly-exhuming settings or warm subsurface environments

Radiation-induced growth and isothermal decay of infrared-stimulated luminescence from feldspar

Optically stimulated luminescence (OSL) ages can determine a wide range of geological events or processes, such as the timing of sediment deposition, the exposure duration of a rock surface, or the cooling rate of bedrock. The accuracy of OSL dating critically depends on our capability to describe the growth and decay of laboratory-regenerated luminescence signals. Here we review a selection of common models describing the response of infrared stimulated luminescence (IRSL) of feldspar to constant radiation and temperature as administered in the laboratory. We use this opportunity to introduce a general-order kinetic model that successfully captures the behaviour of different materials and experimental conditions with a minimum of model parameters, and thus appears suitable for future application and validation in natural environments. Finally, we evaluate all the presented models by their ability to accurately describe a recently published feldspar multi-elevated temperature post-IR IRSL (MET-pIRIR) dataset, and highlight each model's strengths and shortfalls

The bleaching limits of IRSL signals at various stimulation temperatures and their potential inference of the pre-burial light exposure duration

Infrared Stimulated Luminescence (IRSL) techniques are being increasingly used for dating sedimentary feldspars in the middle to late Quaternary. By employing several subsequent stimulations at increasing temperatures, a series of post-IR IRSL (pIRIR) signals with different characteristics (stability and bleachability) can be obtained for an individual sample. It has been experimentally demonstrated that higher-temperature pIRIR signals are more stable, but they tend to exhibit larger residual doses up to few tens of Gy, potentially causing severe age overestimation in young samples. In this study we conducted comprehensive bleaching experiments of IRSL and pIRIR signals using a loess sample from China, and demonstrated that non-bleachable components in the IR (and possibly pIRIR) signals do exist. The level of such non-bleachable signal shows clearly positive correlation with preheat/stimulation temperature, which further supports the notion that lower temperature pIRIR are advantageous to date young samples and sediments especially from difficult-to-bleach environments. These results display a potential in constrain the pre-burial light exposure history of sediment utilizing multiple feldspar post-IR IRSL (pIRIR) signals. For the studied loess sample, we infer that prior to its last burial, the sample has received an equivalent of >264 h exposure to the SOL2 simulator (more than 2,000 h of natural daylight)

Violet stimulated luminescence: geo- or thermochronometer?

The method of quartz optically stimulated luminescence (OSL) dating is widely used, but generally limited to the past ~0.1 million years (Ma) due to early saturation of the desired signal. Violet stimulated luminescence (VSL) of quartz has previously been shown as a promising alternative, with a dose saturation level ~20 times higher compared to that of OSL, excellent thermal stability on the 1011 year time scale, and agreement between VSL and OSL ages up to ~0.3 Ma. Here we explore the usability of the VSL signal to date older quartz samples from palaeosols, whose ages are bracketed by KeAr ages and palaeomagnetic data of the interbedded basalts, emplaced between 1.6 and 0.7 Ma. VSL ages from three palaeosols largely underestimate the independent ages of their overlying basalts. This can be explained either by a low-temperature thermal anomaly resetting the VSL signal in nature, and/or by an insufficient measurement protocol, unable to correctly translate the natural signal into the equivalent laboratory dose

OSL-thermochronometry of feldspar from the KTB borehole, Germany

The reconstruction of thermal histories of rocks (thermochronometry) is a fundamental tool both in Earth science and in geological exploration. However, few methods are currently capable of resolving the low-temperature thermal evolution of the upper ∼2 km of the Earth's crust. Here we introduce a new thermochronometer based on the infrared stimulated luminescence (IRSL) from feldspar, and validate the extrapolation of its response to artificial radiation and heat in the laboratory to natural environmental conditions. Specifically, we present a new detailed Na-feldspar IRSL thermochronology from a well-documented thermally-stable crustal environment at the German Continental Deep Drilling Program (KTB). There, the natural luminescence of Na-feldspar extracted from twelve borehole samples (0.1–2.3 km depth, corresponding to 10–70 °C) can be either (i) predicted within uncertainties from the current geothermal gradient, or (ii) inverted into a geothermal palaeogradient of 29±2 °C km−1, integrating natural thermal conditions over the last ∼65 ka. The demonstrated ability to invert a depth–luminescence dataset into a meaningful geothermal palaeogradient opens new venues for reconstructing recent ambient temperatures of the shallow crust (200 °C Ma−1 range). Although Na-feldspar IRSL is prone to field saturation in colder or slower environments, the method's primary relevance appears to be for borehole and tunnel studies, where it may offer remarkably recent (<0.3 Ma) information on the thermal structure and history of hydrothermal fields, nuclear waste repositories and hydrocarbon reservoirs

Fundamentals of luminescence photo- and thermochronometry

With its classic applications rooted in archaeology and sedimentology, the field of luminescence dating has, in the past decade, experienced a remarkable bloom of innovation and novel applications in Earth science. In the field of thermochronometry, luminescence has begun to successfully complement mainstream noble-gas and fission-track techniques, by constraining thermal histories of bedrock at low temperatures (100 degree C) over previously inaccessible timescales (105 years). In the field of surface exposure dating (hereafter: photochronometry), luminescence has been put on a solid theoretical footing similar to that of cosmogenic nuclide techniques, and can now be used to determine the duration (105 years) and degree of rock surface preservation on unprecedented spatial (millimeter) scales. In this chapter, we present a uniform mathematical description of luminescence photo- and thermochronometers, highlighting the close theoretical similarity between the two. We first introduce and discuss key theoretical concepts (partial retention, apparent age, and system closure), and demonstrate them using familiar luminescence signals obeying simple first-order reaction kinetics. We then proceed to show how these concepts may be deployed for reconstructing past environmental conditions (static or variable), involving temperature or light. We conclude the chapter by discussing some of the current methodological conundrums, including the description of non-first-order reaction kinetics, the incorporation of quantum mechanical tunneling effects, and the utilization of multi-signal luminescence systems.</p

Trapped-charge thermochronometry and thermometry: A status review

Trapped-charge dating methods including luminescence and electron spin resonance dating have high potential as low temperature ( 200 °C Ma− 1), or elevated-temperature underground settings (> 30 °C). Despite this limitation, trapped-charge thermochronometry comprises a diverse suite of versatile methods, and we explore potential future applications and research directions.</p

Investigating the thermal stability of TT-OSL main source trap

Thermally-Transferred Optically Stimulated Luminescence (TT-OSL) from quartz is an extended-range luminescence dating technique, with an assumed potential to date sediments as old as early Pleistocene (0.8–2.6 Ma). However, one of the main drawbacks of the TT-OSL signal is its relatively low thermal stability. The few and scattered estimates in the literature of the relatively low thermal stability highlight the possibility that some reported TT-OSL ages are thermal artefacts (i.e. minimum ages only). In this study, we rigorously investigate the thermal stability of the main TT-OSL source trap, using a combination of laboratory and analytical techniques (varying heating rates, isothermal decay, alongside several models) on multiple aliquots of a modern sand sample from the eastern Mediterranean coastal plain. The varying heating rates method constrains the Arrhenius parameters of the TT-OSL main trap to E = 1.50 ± 0.06 eV and s = 1012.8±0.6 s−1; these values translate into a trap lifetime of 3.2−1.9 +4.8 Ma at 10 °C. Isothermal decay data further exhibits significant departures from first-order kinetic behavior, which can be well captured by either the general order kinetics model, or a Gaussian distribution of first-order systems. However, extrapolations of these models to geological timescales are at odds with a large volume of observations, thus suggesting that the deviation from first-order kinetics may be a laboratory artefact. Overall, our study reinforces the concern, that thermal loss progressively affects the TT-OSL signal in the Ma-scale age range, and that some previously reported results may be “minimum ages” only.</p