Quaternary Geochronology最新文献

The Atacama Desert is generally considered the driest non-polar desert on Earth and is therefore an ideal study area for exploring the water and biota free endmember of Earth's Critical Zone (ECZ). Single grain (SG) luminescence dating has successfully identified processes in the ECZ. However, SG luminescence dating of Atacama Desert feldspars is challenging and time consuming since only a small fraction of grains emits sufficient luminescence and their potassium (K) contents, needed for internal dose rate calculations, are highly variable. Here we present an adaption of the standardised growth curve (SGC) method adjusted to the conditions of Atacama Desert sediments and a correlation of single-grain geochemistry and luminescence properties.

To evaluate if SGCs are suitable for our study site and to determine the influence of the K-content on our luminescence age calculations, we used a set of five samples from the Atacama Desert and five chemically and structurally different feldspar sediment extracts from various geological origins worldwide. We performed a dose recovery test (DRT) using a post-infrared infrared stimulated luminescence (pIRIR) protocol and measured nine major element concentrations, including K, on a single grain level using a scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDX). The DRT dataset was then used to test the application of SGCs. The accuracy of Atacama feldspar pIRIR measurements fitted onto SGCs frequently suffers from odd values in single measurement cycles, since the SGC approach developed for SG feldspar luminescence (Li et al., 2015b) uses one L_x/T_x measurement to project the L_n/T_n values onto a SGC. We investigate the influence of calculating a synthetic regenerative signal (sR) for SGC fitting, to reduce the effect of those odd values on individual grain measurements. Furthermore, we reduced the regenerative cycles used for our sR approach, to test if shorter protocols would result in equivalent dose (D_e) estimates in agreement with longer protocols. We then calculated Spearman rank correlations between the results obtained with our modified SGC and the SAR protocol, luminescence signal intensities, and the geochemical dataset.

Finally, we present a new method of fitting data onto a SGC which significantly decreases measurement time, without risking the inclusion of outliers. We furthermore show that the luminescence signal intensities, the D_e values and their dose recovery ratios obtained with our SGC method and a SAR protocol, are independent of the sample geochemistry.

Evaluating remnant doses is crucial for luminescence dating of late Holocene sedimentary deposits, especially when slow-bleaching feldspar pIRIR signals are employed. Previous research demonstrated that the bleaching capacity varies between individual grains, but only a few attempts have been made to quantify this. In this research, we evaluate variance in bleachability at a single-grain level for a low-temperature pIRIR signal from K-rich feldspar extract. The research is conducted through experimental exposure to natural sunlight of two natural samples. The degree of zeroing is monitored over exposure times up to 100 min and related to the recuperation values obtained on the same grains from the standard pIRIR sequence. We find a positive correlation between recuperation doses of grains and their residual doses, which suggests that recuperation provides a proxy for grain bleachability. In addition, we combine our dataset of bleachability with information on the difference between IRSL₅₀ and pIRIR₁₇₅ equivalent doses determined on the same grain (D_e ratio), which allows us to identify grains for which light exposure was too limited to reset both signals. The results demonstrate that prior inheritance, light exposure, and grain bleachability all may impact the residual dose of grains, resulting in overdispersion in equivalent dose distributions. Combining the assessment of bleachability with the information on the D_e ratio can support the identification of best-bleached grains in a natural sample. Finally, the different bleachability of individual grains demonstrates that it may not be advisable to subtract the mean residual dose from paleodoses obtained with a minimum age model and for Holocene-aged deposits.

While flint and similar materials are some of the most often preserved finds from the pre-historic archaeological records, trapped charge techniques can currently only be used on heated flint pieces. Here, we investigate if rock surface luminescence dating using the light-sensitive OSL signal is applicable to unheated flint nodules and cobbles by measuring OSL signal-depth profiles in flint samples collected from eastern Zealand and Møn, Denmark. We demonstrate that a detectable, natural OSL signal is present in the centre in eight out of the nine investigated flint samples. Additionally, the OSL signal had been bleached to various depths at the flint surfaces, which were exposed to daylight at the time of sampling. The field-saturated OSL is 50% or lower compared to the laboratory OSL after a 6 kGy dose was administered to field-saturated flint slices; this discrepancy could be related to signal loss occurring over geological time scales, but the exact nature of the mechanism of signal loss remains unclear. The presence of a natural OSL signal that bleaches into the flint when exposed to daylight suggests that rock surface dating of unheated flints is feasible, and the method should be further tested on known-age samples.

In this study we apply optically stimulated luminescence and infra-red stimulated luminescence to the sediments from the interfluve dividing three river basins: the Pechora, the Kama and the Severnaya Dvina, hoping to gather new age data on glaciofluvial sediment. We also studied the valley of the Berezovka, a medium-scale river cutting through this interfluve. We obtained a chronology from 17 quartz and 15 feldspar ages. Sedimentological analyses and luminescence dating show that most of the sediments examined here are water-transported (gully alluvium, slopewash deposits), and their ages mark a period of accumulation during MIS 2. Some of these sediments were reworked by aeolian processes; these took place over a time span consistent with such processes in other parts of the East European Plain (19-12 ka). The Berezovka river experienced incision before ∼22 ka and another erosion event with extremely high floods at 15–16 ka. Its terrace formed in MIS 5, constraining the minimum age of the valley. In the bottom part of the sections, much older, possible MIS 8, glaciofluvial sediments were found; the apparent absence of MIS 6 sediments may argue for moving the MIS 6 ice sheet limit further to the north of the study area.

As a vital component of the “Asian water tower”, lakes on the Tibetan Plateau (TP) significantly influence the regional ecosystems and economies and they are also an effective sentinel of climate change. However, the temporal and spatial patterns of lakes and the related hydroclimatic evolution on the northwestern TP (NWTP) remain unclear. We reconstructed the lake level variations of a non-glacier-fed lake, Longjue Co, on the NWTP, using optical dating of paleo-shorelines. The optically stimulated luminescence signals of quartz grains were unsuitable for dating due to high contributions of the medium component, and thus the post-infrared infrared stimulated luminescence signals (pIR₅₀IR₁₇₀, pIR₅₀IR₂₂₅) of K-feldspar single grains were used. Internal checks including dose recovery tests, residual dose tests, and anomalous fading tests showed that the pIR₅₀IR₁₇₀ signal was suitable for paleo-shoreline dating in Longjue Co. However, some of the samples were affected by the incomplete bleaching of pIRIR signals before deposition, and in this case the Minimum Age Model was used to constrain the ages. We also examined the dependence of the K-feldspar equivalent dose (D_e) on grain brightness and explored the possible mechanisms, and the brightest grains were then used for D_e calculations. The results show that Longjue Co reached its maximum Holocene level (+34 m) during the early Holocene (10.06 ± 1.39 ka), and then after ∼5 ka it commenced a shrinking trend, punctuated by two rapid lake level decreases. Reference to independent paleoclimate records suggests that the Holocene lake level variations of Longjue Co and the regional hydroclimate were mainly controlled by the Indian summer monsoon.

The optically stimulated luminescence (OSL) sensitivity of the fast component in quartz has been increasingly used in provenance analysis of Quaternary sediments. Quartz OSL natural sensitization is thought to be mainly controlled by the formation conditions of the source bedrock and surface processes occurring mainly in sediment source areas. Thus, quartz OSL sensitivity can be linked to distinct sediment source regions, based on their tectonic setting and erosion conditions. In this way, changes in quartz OSL sensitivity within siliciclastic successions would track variations in sediment provenance. So far, few works evaluated how the OSL sensitivity of quartz sand grains varies in sedimentary successions that experienced long-term cycles of deep burial, exhumation, and erosion. Therefore, this work aims to characterize the sensitivity of the fast OSL component of quartz sand grains retrieved from the Cenozoic sedimentary rocks of the Northern Andes basins and assess its spatiotemporal changes. We found that quartz grains with the lowest OSL sensitivity are sourced by crystalline and volcanic rocks related to the Andean Continental Arc emplaced in the Colombian Central Cordillera, reflecting the onset of denudation in orogenic sources during the Paleocene. Subsequently, increasing trends in OSL sensitivity are related to the sedimentary recycling during the Andean orogeny, reaching maximum values as a result of the progressive unroofing of Cretaceous rocks in the Colombian Eastern Cordillera, originally sourced from the low-relief Amazon Craton. Changes in quartz OSL sensitivity measured in the Cenozoic sedimentary basin-fill sequences of the Northern Andes vary according to the shifts in sediment provenance related to the orogenic construction and sediment recycling of the Andean range.

Differing from the classic fine silt-dominated loess on the southern and central Chinese Loess Plateau (CLP), sandy loess is extensively distributed along the southern bank of the middle Yellow River and reaches thickness of up to ∼ 200 m. However, its deposition time and formation processes remain unclear. In this study, we dated three representative sandy loess sections along the middle Yellow River using single-aliquot regenerative-dose optically stimulated luminescence (OSL) and multiple-aliquot regenerative-dose recuperated OSL (ReOSL) dating protocols on fine quartz grains (4–11 μm). The reliability of these methods was robustly verified through traditional luminescence dating checks. Our dating results showed that the sandy loess was predominantly deposited during the last glacial period and exhibited an exceptionally high dust accumulation rate surpassing that of the classic loess on the southern and central CLP. By considering atmospheric circulation, geomorphology, and proxy data from both the sandy loess and the Yellow River fluvial sediments, we propose that the sandy loess primarily originated from the proximal riverbeds and fluvial plains of the middle Yellow River, transported by northerly East Asian winter monsoon (EAWM) winds. During the last glacial period, weakened East Asian summer monsoon (EASM) in central and northern China resulted in reduced runoff and vegetation cover in the middle Yellow River region. Consequently, the riverbeds and fluvial plains served as main sources of dust material. Subsequently, the strengthened EAWM winds carried and deposited the dust along the southern bank of the middle Yellow River, culminating in the formation of the sandy loess. Our study underscores the importance of aeolian and fluvial processes interactions in contributing to the formation of the sandy loess along the middle Yellow River and suggests that the investigated sandy loess holds potential for high-resolution paleoclimate reconstruction.

The Galería karstic complex, situated within the Atapuerca Multilevel Karst system (Burgos, Spain), records an impressive sedimentary sequence from the Early to Middle Pleistocene period. One of the notable features of this complex is the presence of thick fluvial deposits (waterlain facies), providing a detailed record of the transitional moment between the formation of the middle and lower Atapuerca karst levels. Examining these sediments, barely dated thus far, is crucial for gaining a comprehensive understanding of fluvio-karstic dynamics at the Atapuerca site. In our study, we conducted dating analyses on sediment samples from different areas of the Galería complex using Electron Spin Resonance (ESR) on quartz grains (4 samples from Covacha de los Zarpazos and 1 from Tres Simas Norte outcrops) and paleomagnetism (12 samples). The aim was to refine the chronological framework of the Galería complex, providing ages for the formation of these passages. Considering the geological context, our results revealed underestimated Aluminium (Al) ESR ages ranging from 372 ± 78 to 569 ± 80 ka in Covacha de los Zarpazos, along with consistent dates of 1184 ± 120 and 1001 ± 302 ka in Covacha de los Zarpazos and Tres Simas Central, respectively. Our results agree with previous estimates obtained through luminescence dating methods, indicating Middle Pleistocene ages for the samples. In contrast, reverse polarity estimates disagree with those from radiometric dating methods, except for samples ZAR1501 and TSC1501. Caution should be taken when interpreting these ages, as various geological factors including leaching processes from upper zones, post-sedimentary structures, and lithological variations may indicate alterations in the primitive geochemical composition of the sediment. Consequently, these factors can potentially affect the total Dose rate parameter and introduce uncertainties into the age calculations.