The effect of test dose and first IR stimulation temperature on post-IR IRSL measurements of rock slices

Abstract Optically stimulated luminescence (OSL) is increasingly applied to the dating of rock surfaces. There is at present no practical way of separating pure minerals (quartz and feldspar) from hard rocks for OSL measurement without losing the grain-size dependent dosimetric information and there is little information about the performance of the single-aliquot regeneration-dose (SAR) measurement protocol on the post-infrared infrared stimulated luminescence (pIRIR) signals from rock slices. The latter is investigated here. Our data indicate that there is a systematic increase in dose response curve saturation (or D o) with test dose size when the regeneration doses are first given in increasing order, and then decreasing order. This trend disappears if these orders are reversed. The reproducibility of dose response curves is dependent on the size of the test dose (poorer for small test dose). For rock slices given a saturation dose in the laboratory, it is observed that the sensitivity corrected pIRIR290 signal lies close to saturation level of the dose response curve, for first IR stimulation at temperatures between 50 and 250°C. However, the pIRIR290 signal from naturally saturated slices lies close to the laboratory saturation levels only for higher first IR stimulation temperatures e.g. 200°C or 250°C. Our data confirm earlier suggestions based on sand-grain measurements that, for older samples, accurate measurements close to saturation require that a higher first IR temperature is used.</jats:p

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- (&lt;102 a) and long-term (&gt;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 &lt;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

Reply to Simon and Reed: Independent and Converging Results Rule Out Historic Disturbance and Confirm Age Constraints for Barrier Canyon Rock Art

We welcome this further discussion of our results on the age of the Great Gallery rock art in the Canyonlands of Utah. The comment by Simon and Reed (1) focuses on just one of the three components of our study (2), which is presented in greater technical detail in ref. 3 and is surely our best-constrained and least-surprising result: the dating of a rock-fall that removed some of the art and thus provides a minimum age. Simon and Reed (1) point out that the Great Gallery panel is not pristine and relate the sordid human history of visitation and possible disturbance to the site. Indeed, being aware of this during our research, one of our initial hypotheses was that the rock fall may be historic. Despite the possibility of recent disturbance to some of the talus boulders, our results document that the rock fall occurred ∼900 y ago, and for the boulder we sampled a scenario of historic disturbance and exposure such as postulated by Simon and Reed (1) can be ruled out

Towards dating prehistoric rock art from central Iberia using luminescence from rock surfaces

Rock art is notoriously difficult to date numerically using established geochronological techniques. Recently, optically stimulated luminescence (OSL) rock surface dating has been used successfully to constrain the age of a Native American rock art in Utah, USA (Chapot et al., 2012). In this study, we follow a similar approach to constrain the age of two important rock art sites in the Iberian Peninsula: i) a Levantine Style rock art (Peña del Escrito, Villar del Humo, ~300 km SE of Madrid) characterised by naturalistic depictions and scenes of people and animals interacting, and ii) a Schematic rock art (Peña Escrita, Fuencaliente, ~400 km S of Madrid) of a more abstract style with human- and animal-like figures and more abstract elements as the main features. The time frame of the Levantine Style is controversial with dates proposed from before and after the Neolithic. In contrast, the Schematic Style is traditionally considered to be of Chalcolithic or Bronze Age. At both sites, rockfall has removed a part of the rock art and buried it on the toe of the cliff. For luminescence measurements, we collected three samples from the pigmented buried rock face at the sandstone Levantine Style site and a similar one from the quartzite Schematic Style site, together with corresponding dose rate samples from the underlying sediment/rock. Whole rock samples were cut in the usual manner and the luminescence signals measured as a function of depth. The sandstone samples have a weak fast-component OSL signal but are dominated by IRSL. In contrast, the quartzite sample is dominated by a strong fast component OSL signal with only weak IRSL. As expected, the observed OSL and IRSL profiles all show evidence of light exposure prior to burial. We present these profiles and relevant luminescence characteristics for the two styles of rock art

Age of Barrier Canyon-style rock art constrained by cross-cutting relations and luminescence dating techniques

Rock art compels interest from both researchers and a broader public, inspiring many hypotheses about its cultural origin and meaning, but it is notoriously difficult to date numerically. Barrier Canyon-style (BCS) pictographs of the Colorado Plateau are among the most debated examples; hypotheses about its age span the entire Holocene epoch and previous attempts at direct radiocarbon dating have failed. We provide multiple age constraints through the use of cross-cutting relations and new and broadly applicable approaches in optically stimulated luminescence dating at the Great Gallery panel, the type section of BCS art in Canyonlands National Park, southeastern Utah. Alluvial chronostratigraphy constrains the burial and exhumation of the alcove containing the panel, and limits are also set by our related research dating both a rockfall that removed some figures and the rock’s exposure duration before that time. Results provide a maximum possible age, a minimum age, and an exposure time window for the creation of the Great Gallery panel, respectively. The only prior hypothesis not disproven is a late Archaic origin for BCS rock art, although our age result of A.D. ∼1–1100 coincides better with the transition to and rise of the subsequent Fremont culture. This chronology is for the type locality only, and variability in the age of other sites is likely. Nevertheless, results suggest that BCS rock art represents an artistic tradition that spanned cultures and the transition from foraging to farming in the region. Archaeology is focused upon material records, contextualized in time. Rock art is a record with the potential to provide unique insight into the dynamics and evolution of culture, but it generally lacks stratigraphic or chronologic context. Interpretation of the origin and meaning of rock art is indirect at best, or simply speculative. In the case of some pictographs, pigments may include or have enough accessory carbon for accelerator mass spectrometry (AMS) radiocarbon dating (1⇓⇓–4). In other special situations, such as caves, minimum age constraints have been obtained by various techniques of dating material that overlies or entombs rock art (5⇓–7). However, most rock art remains undatable and researchers rely upon stylistic comparison and indirect associations with artifacts at nearby sites (8, 9). The case in point for this study is arguably the most compelling and debated rock art in the United States—the Barrier Canyon style (BCS) of the Colorado Plateau. Previous attempts to derive an absolute chronology have failed and its age remains unknown, with widely ranging hypotheses that have remained untested until now. The continued development of dating techniques offers new possibilities for hypothesis testing. The optically stimulated luminescence (OSL) signals from mineral grains make it possible to date the deposition of most sediment that is exposed to a few seconds of full sunlight before burial, and its use in the earth and cultural sciences has greatly increased (10, 11). Among the latest applications of OSL are techniques dating the outer surfaces of rock clasts that have become shielded from light, including those with archaeological context (12⇓⇓–15). Recent work has furthermore used the “bleaching” profile of decreasing luminescence signal toward the surface of rock to estimate exposure time to sunlight (16, 17). Using these dating tools, we can constrain the age of rock art and gain new insight into past cultures and landscapes. Here, we synthesize results from three approaches to dating the type section of BCS art, the Great Gallery in Canyonlands National Park of southeastern Utah. Through dating the full alluvial stratigraphy and a rockfall event that both have incontrovertible cross-cutting relations with the rock art, and then by determining the exposure duration of a painted rock surface, we greatly narrow the window of time when the rock art was created. These approaches do not require direct sampling of rock art and have strong potential for application to other archaeological and surface processes research. Although our results are only for the type section of BCS art, and chronological variability should be expected for the style across the region, they suggest that BCS art coincides with the transition to agriculture in the northern Colorado Plateau and may not have been limited to a specific archaeological culture