ABSTRACT Robust chronologies of Late Pleistocene and Holocene volcanic eruptions are vital for hazard analysis but accurate and precise dating of these events is often difficult. Here we apply various luminescence techniques to quartz and polymineral extracts from heated crustal xenoliths enclosed in scoria and volcanically heated bedrock in the Quaternary Eifel Volcanic Field (EVF), Germany. Consistent results from red thermoluminescence (RTL) and optically stimulated luminescence (OSL) of quartz and from post‐infrared infrared stimulated luminescence of polymineral material demonstrate complete luminescence signal resetting during lava emplacement and sufficient signal stability. RTL and OSL age underestimation of one sample with independently known age from the lava contact zone could be eliminated by annealing before laboratory regenerative irradiation. The average luminescence age of 33.6 ± 2.4 ka for the Wartgesberg eruption is in good agreement with independent age control, while the average age of 15.5 ± 1.1 ka for the Facher Höhe scoria cone is much younger than previously assumed. This result represents the third youngest known eruption in the EVF. Our new data are in line with the hypothesized climate‐controlled triggering of Eifel volcanism and confirm that active volcanism related to asthenosphere upwelling migrates from NW to SE in the EVF.
The stratigraphic principle of superposition assumes that sediments at surface are youngest. And yet, patches of older preserved landscapes continue to be identified in the regions formerly covered by the Laurentide Ice Sheet. Two gravel pits, at the edge of the western Hudson Bay Lowland near the geographic centre of the North American Ice Sheet Complex, reveal additional patches where older glacial and nonglacial sediment is preserved. These sites expose 1–4 m of a darker, highly overconsolidated, clayier till, and 0–2 m of a lighter, less consolidated, sandier till over glaciofluvial and post-glacial gravels and sands; the waning stages of deposition occurred at 214 ± 22 ka (1σ minimum age model quartz grain optical age estimates, n = 2). This was during the latter end of the cool Marine Isotope Stage (MIS) 7-d (within 1σ range) or near the end of MIS-8 (within 2σ range). Next followed a warmer period similar to, or warmer than, present day with higher precipitation; inferred from pollen and macrofossils deposited in nonglacial low-energy floodplain or pond sediments. Our study highlights the need to accurately date Quaternary sediments, given that the shallowest nonglacial sediments at both gravel pits were deposited during an old (MIS 7) interglacial; there is likely no record of the youngest interglacial (MIS 5). Preservation of older sediment also means that in Quaternary stratigraphy, disjoint regional nonglacial “organic marker beds” should not automatically be considered correlative. Identification of similar “old” patches, together with till stratigraphy and composition, is essential to accurately model glacial sediment transport over time.
Abstract An understanding of the growth and demise of ice sheets over North America is essential to inform future climate models. One poorly studied subject is the glacial dynamics during interstadial Marine Isotope Stage (MIS) 3 (57–29 ka). To better constrain the southern margin of the Laurentide Ice Sheet during this time period, we re-examined a stratigraphic sequence in southeast Manitoba, Canada, and provide robust evidence for advance and retreat of ice. Around 46.6 ± 5.1 ka (1σ error), fluvial sands were deposited under similar precipitation and significantly cooler summer temperatures than present-day. Ice then advanced south over the area, before retreating once again and a return to boreal forest and grassland conditions. The area was then covered by proglacial Lake Vita, dammed by ice to the north. Geochronology constraints indicate Lake Vita existed from ca. 44.3 ± 3.6 to 30.4 ± 2.3 ka (1σ error), although gaps in the optical and finite radiocarbon ages suggest either a lack of data or plausible temporary ice-margin advances during this time period. Ice covering most of Manitoba during MIS 3 is in line with global δ 18 O records, and glacially influenced sediment deposition in the Mississippi River basin.
Abstract. Mass movements play an important role in landscape evolution of high mountain areas such as the Himalayas. Yet, establishing numerical age control and reconstructing transport dynamics of past events is challenging. To fill this research gap, we bring Optically Stimulated Luminescence (OSL) dating to the test in an extremely challenging environment: the Pokhara Valley in Nepal. This is challenging for two reasons: i) the OSL sensitivity of quartz, typically the mineral of choice for dating sediments younger than 100 ka, is poor, and ii) highly rapid and turbid conditions during mass movement transport hamper sufficient OSL signal resetting prior to deposition, which eventually results in age overestimation. Here, we first assess the applicability of single-grain feldspar dating of medieval mass movement deposits catastrophically emplaced in the Pokhara Valley. Second, we exploit the poor bleaching mechanisms to get insight into the sediment dynamics of this paleo-mass movement through bleaching proxies. The Pokhara valley is a unique setting for our case-study, considering the availability of an extensive independent radiocarbon dataset as a geochronological benchmark. Single-grain infrared stimulated luminescence signals were measured at 50 °C (IRSL-50) and post-infrared infrared stimulated luminescence signals at 150 °C (pIRIR-150). Our results show that the IRSL-50 signal is better bleached than the pIRIR-150 signal. A bootstrapped Minimum Age Model (bMAM) is applied to retrieve the youngest subpopulation to estimate the paleodose. However, burial ages calculated with this paleodose overestimate the radiocarbon ages by an average factor of ~23 (IRSL-50) and ~72 (pIRIR-150), showing that dating of the Pokhara Formation with a single-grain approach was not successful for most samples. Some samples, however, only slightly overestimate the true emplacement age and thus could be used for a rough age estimation. Large inheritances in combination with the scatter in the single-grain dose distributions show that the sediments have been transported under extremely limited light exposure prior to deposition, which is consistent with the highly turbid nature of the sediment laden flood and debris flows depositing the Pokhara gravels. To investigate the sediment transport dynamics in more detail, we studied three bleaching proxies: the percentage of grains in saturation 2D0 criteria, the percentage of best-bleached grains (2σ range of bMAM-De) and the overdispersion (OD). None of the three bleaching proxies indicate a spatial relationship with run-out distance of the mass movement deposits. We interpret this as evidence for the lack of bleaching during transport, which reflects the catastrophic nature of the event. While not providing reliable burial ages of the Pokhara mass movement deposits, single-grain feldspar dating can potentially be used as an age range finder method. Our approach shows the potential of luminescence techniques to provide insights in sediment transport dynamics of extreme and rare mass movement events in mountainous region.