Dating dry-stone walls with rock surface luminescence: A case study from the Italian Alps

Dating the construction of dry-stone walls is challenging since such structures are typically built without any mortar that can be used for dating. Rock surface luminescence dating is a developing dating method that could advance chronological insights from structures constructed using dry-stone techniques. This study explores rock surface luminescence dating by targeting dry-stone walls from two enclosure complexes and a hut located in the pastoral upland landscape in Val di Sole, Italy. Gneissic rocks were collected from the dry-stone walls, including surfaces that were either exposed or covered (buried) during the time of sampling. Their respective exposure and burial histories were investigated by measuring the luminescence intensity in feldspar minerals from polymineral rock slices. From covered rock surfaces from one enclosure complex, we calculated recent burial ages (~200 a) from one rock, and burial of ~500 a (bottom surface) and 3750 & PLUSMN; 660 a (top surface) from a second rock. The top surfaces of two additional rocks date the construction of an adjacent hut to the Early Middle Ages. The luminescence-depth profile from one such rock has a complex exposure and burial history, including events that predate the hut's construction. Fitted exposure ages from a second enclosure complex suggest with significant errors either a recent age (< 10 a) or construction during the 19th century AD. Burial dating using rock surface luminescence dating appears feasible for dry-stone walls provided that the rock surface was sufficiently exposed before being incorporated into the structure; here, the gneissic surfaces were bleached to depths of ~0-2 mm before the last burial. Contrariwise, exposure dating generally underestimates the expected age. The variation in ages observed from our rock surfaces indicates that the degree of preservation of the wall, the position of the rock, erosion, and knowledge regarding the general archaeological setting are essential to interpreting the estimated ages. In this study, rock surface luminescence dating provides new, previously inaccessible chronological data with implications for interpreting human activities in the alpine areas of Val di Sole

Investigating the resetting of IRSL signals in beach cobbles and their potential for rock surface dating of marine terraces in Northern Chile

Inactive shorelines represent valuable records for sea level change, shoreline variations and tectonics if we can constrain the timing of their formation. Where the associated beaches are cobble dominated, luminescence rock surface dating is a promising alternative to established dating approaches, since unlike other techniques it offers the potential to identify clasts unaffected by inherited ages. While luminescence rock surface dating has successfully been used on Holocene and Late Pleistocene beach ridges previously, in this study the potential of IRSL rock surface dating is evaluated for the magmatic cobbles of uplifted Pleistocene terraces along the tectonically active coast of northern Chile. Cobbles from an active beach were used to investigate the influence of cobble lithology on IRSL signal properties and the effectiveness of IRSL signal resetting in the rock. While alkaline and andesitic cobbles yield low IRSL intensities and limited signal resetting due to strong light attenuation, more favourable characteristics for dating were observed for some diorite and granite cobbles. Their IRSL signals were well reset in the uppermost few mm without any systematic difference between upper and lower surface. Some of them revealed bleaching plateaus with inherited ages close to zero after correction for laboratory residuals. For dating, cobbles from three Pleistocene marine terraces, for which new uranium-thorium and ESR control ages on molluscs provide age control, were targeted. None of the associated IRSL rock surface burial ages agrees with the MIS 5 control ages of the terraces. Most of the selected cobbles are either too dark to allow for effective signal resetting or yield IRSL properties unsuitable for dating. Only one of the targeted cobbles shows both signs of signal resetting at its surface and sensitive IRSL signals, but its signal was already in field saturation due to dose rates >6 Gy/ka. In conclusion, our data indicate that beach cobbles with granitic to dioritic lithology combine appropriate IRSL properties and sufficient IRSL signal resetting for dating Holocene landforms. Last interglacial terraces may already be beyond the limit of IRSL dating for most cobbles of this lithology since they show large dose rates compared to IRSL sediment dating

Rock surface IRSL dating of buried cobbles from an alpine dry-stone structure in Val di Sole, Italy

Here, we investigate the application of rock surface IRSL dating to chronology restrain archaeological structures related to upland pastoralism. We applied the method to cobbles collected from archaeological units in an excavation of a dry-stone structure in Val di Sole in the Italian Alps. At this site, archaeological finds and previous radiocarbon analyses have dated an initial human occupation of the site to the Early Bronze Age (ca. 2200-1600 BC), and a possible second occupation to the Middle Bronze Age (ca. 1600-1350 BC). These archaeological units have later been buried by colluvial sediments. Theoretically, the luminescence-depth profiles from rock surfaces from inside such structures could record the exposure and burial of these archaeological units. We collected buried gneiss cobbles from these archaeological units and measured rock slices and chips from 1 to 4 cm long cores with a low-temperature pIR-IRSL protocol to investigate the signal resetting in these cobbles. Only the IRSL50 signal was deemed appropriate for dating. Measured luminescence-depth profiles demonstrate varying levels of signal resetting before burial. Dating of two paragneiss cobbles from the lower unit yielded corrected burial ages of similar to 1450-700 BC and similar to 19 ka. The older date is clearly not associated with human occupation; the younger date slightly underestimates the Early Bronze Age occupation, which was confirmed by new radiocarbon dating of charcoal (1731-1452 and 2124-1773 cal. BC). The burial of the upper archaeological unit was dated to similar to AD 1000, based on ages derived from the bottom surface of an orthogneiss cobble and the top surface of a paragneiss cobble. This is slightly younger than two new radiocarbon ages (426-596 and 537-654 cal. AD) from charcoal fragments sampled from the same unit. This new chronological data show longer exposure of the upper archaeological unit than was previously known. Furthermore, the paragneiss cobble from the upper unit has been exposed to sufficient heat to reset the IRSL50 and pIR-IRSL290 signals throughout the cobble; an event which can be dated to similar to AD 100-1500 BC. Comparisons between fading-corrected IRSL50 ages and pIR-IRSL290 ages from the heated cobble are in agreement, which suggests that the conventional g-value approach accurately corrects for signal loss during burial. Overall, our research suggests that rock surface IRSL dating can provide complementary chronological data for archaeological settings

Occupazione pastorale delle alte quote alpine nell’età del Bronzo: primi dati dal sito MZ051S (Camp da Ortisé, Val di Sole, Trento)

Pastoral occupation of the uplands in the Bronze Age: first data from the MZ051S site (Camp da Ortisé, Val di Sole, Trento, Italy) - Available data on human occupation of mountain areas in late Prehistory are still rather scarce. Despite the growing archaeological interest in the uplands, few upland archaeological sites have been investigated in the Alps. In this paper, a new Bronze Age site from Val di Sole (Trentino, Italy) is presented. The site, a large dry-stone enclosure named MZ051S, has been investigated as part of the ALPES project (Alpine Landscapes: Pastoralism and Environment of Val di Sole). MZ051S is located in Val Poré, on a glacial plateau at 2240 metres of elevation. The excavations carried out between 2015 and 2019 uncovered a modest and poorly characterised archaeological assemblage, which includes lithic artefacts and potsherds. Scientific analyses provided significant multidisciplinary data on the site stratigraphy, chronology and formation, on its geographical and environmental context and on the characteristics of sediments and soils in the area. These new data contributed to inferring the evolution of this Alpine landscape, the dynamics of human occupation of the uplands of Val di Sole in the last 4000 years and the human impact on the vulnerable local environment, which preserves glacial and periglacial morphologies. Preliminary reconstructions suggest a first human occupation, related to the pastoral use of the valley, during the Early Bronze Age, followed by a more ephemeral presence in the Middle Bronze Age and the Iron Age. Evidence of human activities during the Medieval and Modern period, identified in other sites of the valley, has not been recorded at MZ051S. Geoarchaeological and palaeobotanical data seem to suggest the establishment of practices that led to the acceleration of soil-erosion processes during the Bronze Age and the subsequent degradation of soils in the area. The data derived from the investigation of MZ051S, albeit preliminary, are crucial to understand the processes of exploitation of the uplands in late Prehistory in the central-eastern Alps and to infer the complex interactions between anthropic and natural factors in mountain environments