Luminescence dating of the last glaciation maximum in the southern Black Forest, Germany: Preliminary results

<p>During the last glaciation maximum, an ice cap and its more than 20 kilometres-long outlet glaciers covered the highest summit of the Black Forest, the Feldberg (1493 m above sea-level), and the surrounding region (Hofmann et al., 2020). This event is tentatively correlated with the global last glacial maximum (at ca. 22-19 ka). However, this hypothesis has never been tested by applying up-to-date geochronological techniques to glacial deposits or landforms. Due to the lack of significant topographic control, the climate probably mainly controlled the mass balance of the ice cap. Dating its last maximum extent may thus have important implications for the reconstruction of atmospheric circulations patterns during the Pleistocene. A last glaciation maximum out of phase with the Alps would strengthen the hypothesis that a meridional atmospheric circulation prevailed over Europe during the last glaciation maximum in the Alps (at ca. 25 ka).</p><p>We aim at filling this gap by re-investigating a well-preserved multi-ridged terminal moraine complex several kilometres north-west of the Feldberg. Since the beginning of the twentieth century, this landform is undisputedly assigned to the last glaciation maximum. As units of sorted sediments occur in two sections on one ridge of the terminal moraine complex, luminescence dating was deemed a suitable technique to infer the age of the landform.</p><p>Luminescence dating was applied to both feldspar and quartz for suitable comparison, as these dosimeters have different drawbacks. Since quartz from crystalline source areas often shows no or only a relatively weak luminescence signal, we anticipated that this also applies to the sampled sediments from the crystalline part of the Black Forest. Optically-stimulated luminescence (OSL) measurements revealed a bright signal in quartz as well as equivalent doses in the order of 200 Gy considerably below the saturation level. We did not observe any obvious signs for partial bleaching, but this possibility has to be confirmed via further OSL measurements, since the sampled sediments were not transported over large distances. Component analysis will also be performed.</p><p>Infrared-stimulated luminescence (IRSL) measurements on feldspar revealed higher preliminary luminescence ages. It is expected that anomalous fading and partial bleaching will be two major future challenges. To overcome the first problem, post-infrared IRSL dating will be applied. As resetting of this signal is more difficult when compared to IRSL and OSL signals, problems associated with incomplete bleaching may arise in future work.</p><p>Our preliminary quartz OSL ages tentatively suggest that the last glaciation maximum in the southern Black Forest preceded the last glaciation maxima in other formerly glaciated uplands in Central Europe and the global last glacial maximum. This result needs to be evaluated by luminescence dating of other ice-marginal landforms correlative with the last glaciation maximum. Other geochronological techniques, such as <sup>10</sup>Be cosmic-ray exposure dating, will also be applied to reveal their age.</p><p> </p><p><strong>Reference</strong></p><p>Hofmann, F. M., Rauscher, F., McCreary, W., Bischoff, J.-P., and Preusser, F.: Revisiting Late Pleistocene glacier dynamics north-west of the Feldberg, southern Black Forest, Germany, E&G Quaternary Science Journal, 69, 61–87, https://doi.org/10.5194/egqsj-69-61-2020, 2020.</p><p> </p><p> </p><p> </p>

Luminescence properties and dating of glacial to periglacial sediments from northern Switzerland

Luminescence dating has become a pillar of the understanding of Pleistocene glacial advances in the northern foreland of the Swiss Alps. However, both quartz and feldspar from the region are equally challenging as dosimeters with anomalous fading and partial bleaching being some of the obstacles to overcome for the establishment of decisive chronologies. In this study, luminescence properties of coarse- and fine-grained quartz, feldspar, and polymineral fractions of eight samples from a palaeovalley, Rinikerfeld in northern Switzerland, are systematically assessed. Standard performance tests are conducted on all four fractions. Deconvolution of luminescence signals of the quartz fractions is implemented and shows the dominance of stable fast components. Reader-specific low preheat temperatures are investigated on the infrared stimulated luminescence (IRSL) signal of feldspar. Thermal stability of this signal is found for low preheats, and thermal quenching could be excluded for higher preheats. However, anomalous fading is observed in the feldspar and polymineral IRSL signals and two correction approaches are applied. For one approach, fading corrected coarse-grained feldspar ages are consistent with those derived from quartz. In general, coarse-grained quartz and feldspar, as well as the fine-grained polymineral fraction of one sample, are in chrono-stratigraphic agreement and present negligible evidence for partial bleaching. However, ages derived from fine-grained quartz are found to underestimate those of the coarse-grained quartz fractions. Hence, the impact of alpha efficiency and water content on the dose rate and thus the ages are assessed. A finite explanation for the observed discrepancies remains lacking, but this systematic investigation of different luminescence signals allows for the establishment of a chronology for the palaeovalley fill dating back to at least Marine Isotope Stage 6 (MIS 6).

Luminescence properties and dating of proglacial sediments from northern Switzerland

Luminescence dating has become a pillar of the understanding of Pleistocene glacial advances in the northern foreland of the Swiss Alps. However, both quartz and feldspar from the region are equally challenging as dosimeters with anomalous fading, partial bleaching and unstable components being some of the obstacles to overcome. In this study, luminescence properties of coarse- and fine-grained quartz, feldspar and polymineral fractions of eight samples from a palaeovalley, Rinikerfeld, in northern Switzerland are assessed and found appropriate for dating. While anomalous fading of the IRSL signal of the feldspar and polymineral fraction is observed, fading corrected coarse-grained feldspar ages are consistent with those derived from quartz. In general, coarse-grained quartz and feldspar as well as the fine-grained polymineral fractions are in agreement and present negligible evidence for partial bleaching. However, ages derived from fine-grained quartz are found to underestimate those of the coarse-grained quartz fractions. Impact of total dose rate on finite ages was assessed but age underestimation is likely due to grain size dependent luminescence properties. The top six samples indicate sedimentation of at least 16.6 m during Marine Isotope Stage 6 with a rapid transition from a lacustrine environment to a landscape dominated by colluvial deposits. For the two lowest samples, no finite ages are derived.

OSL Dating of Earthen Mortars from a Medieval Building in Northwestern Spain: Crypt of Basílica da AscensiÓn (Allariz, Ourense)

ABSTRACTThere are few papers that focus on optically stimulated luminescence (OSL) dating of earthen mortars. These mortars are abundant in historical buildings in northwestern Spain. The Basílica da AscensiónyForno da Santa building is an unfinished church built on a previous structure that was transformed into a crypt (Allariz, Ourense, NW Spain). Previous archaeological studies established a sequence of phases of construction, the first dating back to the Iron Age, with significant changes occurring in the Early and Late Medieval ages. The only datable material in the crypt is earthen mortar. Thus, eight mortar samples (seven joint mortars and one wall infill) were taken, seven of them dated by OSL. The dose rate was assessed, and the expected equivalent doses estimated based on the established archaeological age. Several grain sizes (from fine to coarse) were used in small multigrain aliquots to assess the equivalent doses and ages. No evidence of partial bleaching was observed in most samples and grain sizes. The resulting ages are younger than expected for most samples. This is explained by the fact that joints were repaired with new mortar from the 16th century onwards.

New chronological constraints on the timing of Late Pleistocene glacier advances in northern Switzerland

Deposits of the Reuss Glacier in the central northern Alpine foreland of Switzerland are dated using luminescence methodology. Methodological considerations on partial bleaching and fading correction of different signals imply the robustness of the results. An age of ca. 25 ka for sediment directly overlying basal lodgement till corresponds well with existing age constraints for the last maximal position of glaciers of the northern Swiss Alpine Foreland. Luminescence ages imply an earlier advance of Reuss Glacier into the lowlands during Marine Isotope Stage 4. The presented data are compared to findings from other parts of the Alps regarding glacier dynamics and palaeoclimatological implications, such as the source of precipitation during the Late Pleistocene.

Identifying the degree of luminescence signal bleaching in fluvial sediments from the Inner Mongolian reaches of the Yellow River

The partial bleaching of the luminescence signal prior to deposition results in age overestimation, and can be a problem in delineating fluvial evolution within an OSL chronological framework. The Inner Mongolian reaches of the Yellow River are characterised by a high sediment load and complex sources of sediments. To test the incomplete bleaching occurring in this type of environment, the residual doses and the luminescence signal characteristics of different particle size fractions from 14 modern fluvial sediment samples were investigated. Furthermore, 26 OSL ages derived from drilling cores were compared with 11 radiocarbon ages. Our results show that the residual equivalent doses principally range between 0.16 and 0.49 Gy for silt grains, and between 0.35 and 3.72 Gy for sand grains of modern samples. This suggests that medium-grained quartz has been well bleached prior to deposition, and is preferable to coarse-grained quartz when dating fluvial sediments in this region. The results also show that the De values of coarse-grained fractions display a stronger correlation with distance downstream. In addition, a comparison of OSL and radiocarbon ages from drilling cores establishes further confidence that any initial bleaching of these sediments was sufficient. As a result, we believe that the studied fluvial samples were well bleached prior to deposition.

LUMINESCENCE DATING OF AN ANCIENT WALLED SETTLEMENT IN ORKHON VALLEY, MONGOLIA

We investigated the potential of the optically stimulated luminescence (OSL) method to date young (<1000 years) samples collected in the Orkhon Valley Cultural Landscape, Mongolia. Quartz showed an infrared signal; therefore the post-IR OSL method was applied to small aliquots which are considered proxies for single grain measurements. Statistical analysis of the dose distribution produced CAM De of 5.14±0.10 Gy and over dispersion of 47.5%, and MAM De of 3.7±0.6 Gy. Since no partial bleaching was suspected, the analysis of signal composition was done and the fast quartz post-IR OSL lead to De of 4.9±0.2 Gy. Based on the quartz fast component and CAM De we propose the new chronology of ancient construction at 785±80 AD, rather than 906-1125 AD as suggested by archaeological evidence. However, the MAM age is in good agreement with independent age control for construction of the ramparts suggesting the date of reconstruction, collapse or reuse for the square walled enclosure MOR3 during 1090±80AD.

Testing the suitability of dim sedimentary quartz from northern Switzerland for OSL burial dose estimation

We investigate the suitability of sedimentary quartz associated with former glacial advances in northern Switzerland to provide reliable burial dose estimates using Optically Stimulated Luminescence (OSL). Previous studies on northern alpine quartz show that its signal characteristics can be poor and potentially problematic. We analyse quartz signals of small aliquots, which reveal the presence of a prominent medium or slow component in the initial part of some signals. Nonetheless, rejection of aliquots with unfavourable signal composition does not alter the burial dose estimates, but significantly reduces the data set for De determination. Signal lifetimes from isothermal decay measurements cover a wide range of values, yet the lowest lifetimes are high enough to guarantee a reliable burial dose estimate for samples of < 400 ka. Comparison of small aliquot and single grain burial dose distributions reveals that signal averaging masks partial bleaching in some of the samples. We therefore strongly recommend single grain measurements for samples from this setting and area, in order to exclude age overestimation due to partial bleaching.