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

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

An exceptionally long paleoseismic record of a slow-moving fault: the Alhama de Murcia fault (Eastern Betic Shear Zone, Spain)

Most catastrophic earthquakes occur along fast-moving faults, although some of them are triggered by slow-moving ones. Long paleoseismic histories are infrequent in the latter faults. Here, an exceptionally long paleoseismic record (more than 300 k.y.) of a slow-moving structure is presented for the southern tip of the Alhama de Murcia fault (Eastern Betic shear zone), which is characterized by morphological expression of current tectonic activity and by a lack of historical seismicity. At its tip, the fault divides into a splay with two main faults bounding the Góñar fault system. At this area, the condensed sedimentation and the distribution of the deformation in several structures provided us with more opportunities to obtain a complete paleoseismic record than at other segments of the fault. The tectonic deformation of the system was studied by an integrated structural, geomorphological, and paleoseismological approach. Stratigraphic and tectonic features at six paleoseismic trenches indicate that old alluvial units have been repeatedly folded and thrusted over younger ones along the different traces of the structure. The correlation of the event timing inferred for each of these trenches and the application of an improved protocol for the infrared stimulated luminescence (IRSL) dating of K-feldspar allowed us to constrain a paleoseismic record as old as 325 ka. We identifi ed a minimum of six possible paleoearthquakes of Mw = 6-7 and a maximum mean recurrence interval of 29 k.y. This provides compelling evidence for the underestimation of the seismic hazard in the region

Pooled sputum to optimise the efficiency and utility of rapid, point-of-care molecular SARS-CoV-2 testing

Background As SARS-CoV-2 testing expands, particularly to widespread asymptomatic testing, high sensitivity point-of-care PCR platforms may optimise potential benefits from pooling multiple patients’ samples. Method We tested patients and asymptomatic citizens for SARS-CoV-2, exploring the efficiency and utility of CovidNudge (i) for detection in individuals’ sputum (compared to nasopharyngeal swabs), (ii) for detection in pooled sputum samples, and (iii) by modelling roll out scenarios for pooled sputum testing. Results Across 295 paired samples, we find no difference (p = 0.1236) in signal strength for sputum (mean amplified replicates (MAR) 25.2, standard deviation (SD) 14.2, range 0–60) compared to nasopharyngeal swabs (MAR 27.8, SD 12.4, range 6–56). At 10-sample pool size we find some drop in absolute strength of signal (individual sputum MAR 42.1, SD 11.8, range 13–60 vs. pooled sputum MAR 25.3, SD 14.6, range 1–54; p < 0.0001), but only marginal drop in sensitivity (51/53,96%). We determine a limit of detection of 250 copies/ml for an individual test, rising only four-fold to 1000copies/ml for a 10-sample pool. We find optimal pooled testing efficiency to be a 12–3-1-sample model, yet as prevalence increases, pool size should decrease; at 5% prevalence to maintain a 75% probability of negative first test, 5-sample pools are optimal. Conclusion We describe for the first time the use of sequentially dipped sputum samples for rapid pooled point of care SARS-CoV-2 PCR testing. The potential to screen asymptomatic cohorts rapidly, at the point-of-care, with PCR, offers the potential to quickly identify and isolate positive individuals within a population “bubble”

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

Combining dating techniques in paleo-earthquake chronologies: the example of El Saltador (Alhama de Murcia fault)

Presentamos la cronología de paleoterremotos establecida para la rama sur de la Falla de Alhama de Murcia (Béticas Orientales) a partir del estudio paelosismológico de la localidad de el Saltador (Lorca). Mostramos la aplicación combinada de cuatro métodos de datación; luminiscencia en cuarzo y feldespato (OSL en cuarzo y pIRIRSL), datación de series de Uranio en carbonatos pedogénicos, y radiocarbono. Para acotar la edad de los paleoterremotos con la mejor definición posible, se han integrado las edades de las capas que definen temporalmente los paleosismos en un modelo deposicional. Para ello se usó el programa OxCal (versión 4.2), que realiza un tratamiento estadístico bayesiano de las funciones de probabilidad de las edades de materiales afectados según un orden estratigráfico conocido y calcula la recurrencia media y la recurrencia particular entre eventos sucesivos. La secuencia de materiales aluviales muestra el registro de ocho eventos paleosísmicos ocurridos desde hace 54,3 ± 9,1 ka. El tiempo de recurrencia medio varía entre 3,5- y 5,3 ka , localizándose el último evento entre el s. XIII y el s. XVIII.We present a chronology of paleo-earthquakes established at El Saltador (Lorca), a paleoseismical site in the southern branch of the Alhama de Murcia Fault (Eastern Betics). Because of the difficulties in dating the affected sediments using a single dating approach, we combined four types of methods; Two of them based on the luminescence of quartz and feldspar (Quartz-OSL and pIR-IRSL), U-series on pedogenic carbonates and radiocarbon dating. To obtain the best possible accuracy for the age of each paleoearthquake, we integrated the age of the layers that constrain the seismic events into a depositional model. To this end, we relied on the OxCal program (version 4.2), which performs a Bayesian statistical treatment of the probability functions of the ages assigned to a sequence of layers of known stratigraphical order, at the time that it provides the average recurrence and the single recurrence between successive events. The sequence of alluvial deposits analyzed shows the record of eigth paleoseismic events occurred since 54,3 ± 9.1 ka. The average recurrence intervals ranges between 3,5- and 5,3 ka with the last event taking place between the XIII and XVII centuries.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEMinisterio de Ciencia e Innovación (MICINN)Generalitat de Catalunyapu

Contemporary and paleo-rockfalls as proxies for strong ground motions

A major hazard accompanying earthquake shaking in areas of steep topography is the detachment of rocks from bedrock outcrops that subsequently slide, roll, or bounce downslope (i.e. rockfalls). The 2010-2011 Canterbury earthquake sequence caused recurrent and severe rockfall in parts of southern Christchurch. Coseismic rockfall caused five fatalities and significant infrastructural damage during the 2011 Mw 6.2 Christchurch earthquake. Here we examine a rockfall site in southern Christchurch in detail using geomorphic mapping, lidar analysis, geochronology (cosmogenic 3He dating, radiocarbon dating, optically stimulated luminescence (OSL) from quartz, infrared stimulated luminescence from K-feldspar), numerical modeling of rockfall boulder trajectories, and ground motion prediction equations (GMPEs). Rocks fell from the source cliff only in earthquakes with interpolated peak ground velocities exceeding ~10 cm/s; hundreds of smaller earthquakes did not produce rockfall. On the basis of empirical observations, GMPEs and age chronologies we attribute paleo-rockfalls to strong shaking in prehistoric earthquakes. We conclude that earthquake shaking of comparable intensity to the strongest contemporary earthquakes in Christchurch last occurred at this site approximately 5000 to 7000 years ago, and that in some settings, rockfall deposits provide useful proxies for past strong ground motions