Quaternary Geochronology最新文献

We introduce a novel age-depth modeling approach called CosmoChron that integrates both cosmogenic nuclide concentrations and other age constraints, such as radiocarbon and OSL ages, from different depths in a sedimentary sequence. Based on probabilistic inverse modeling, CosmoChron constrains the age-depth relationship of a sedimentary sequence along with associated uncertainties. Knowledge about the sample origins and the accumulation process is incorporated in the prior model. The ²⁶Al/¹⁰Be ratio is computed at different depths in the forward model by accounting for different pre-burial scenarios, radioactive decay and post-burial production of ²⁶Al-¹⁰Be, which is directly tied to the age-depth relation itself. Synthetic test cases demonstrate the method's ability to construct accurate age-depth relationships given by the posterior distribution, even for complex scenarios that include slow and varying accumulation rates, complex pre-burial histories, hiatuses, and unconformities. Based on observed unconformities, users have the option to manually input hiatuses into the model at specific depths, which allows estimation of their durations. Application of CosmoChron to real ²⁶Al/¹⁰Be data from the Laujunmiao section in China yields ages that are similar to those obtained with conventional burial dating methods for specific stratigraphic layers. However, the associated uncertainties are significantly reduced with CosmoChron (by ∼47 % on average) because it exploits the vertical coupling of data combined with knowledge of the relative age of the samples, which must become younger towards the top of the profile. Additionally, the age-depth model reflets the duration of three hiatuses inferred from unconformities observed in the field. When CosmoChron is applied to OSL-derived ages from Jingbian section A on the Chinese Loess Plateau, covering the last ∼140 ka, the method produces results that are almost identical to those obtained with the well-established Bacon age-depth modeling approach. CosmoChron consequently offers a new, versatile and reliable tool to construct age-depth models for Quaternary sediment sequences.

Inferring the pattern and rate of past sea-level changes from uncertainty-prone proxy records requires formal statistical analyses, preferably in a hierarchical framework. The commonly used error-in-variables method treats the relative sea level as a collection of random variables drawn from the multivariate Gaussian distribution. However, this method does not make any use of prior information about the sea-level index points as constraints in the inferential process, thereby leading to anomalously large uncertainties for the time periods when observational data are absent. Here, a hierarchical Bayesian model of past sea-level changes is presented. Specifically, the stochastically varying relative sea level is modeled as a piecewise linear process with an additive independent Brownian increment arriving in a Gaussian fashion. The treatment of temporal uncertainties associated with the sea-level index points in the partially observed proxy records also differs from the existing methods. Instead of calibrating the radiocarbon ages individually, the corresponding calendar ages are treated as random variables and inferred recursively according to their temporal order. Illustrative studies using synthetic and real-world data demonstrate the promise of this model.

It is very important to distinguish the strata of different periods in the Quaternary period, especially the Middle Pleistocene and Late Pleistocene strata, for stratigraphic division, depositional environmental analysis and climate evolution analysis. In this study, the Shinaimiao (SNM) borehole sequence in Hebi City, Henan Province (China) was established by both electron spin resonance (ESR) and magnetostratigraphy methods. Both Al (eighteen samples) and Ti-Li (eight out of the eighteen samples) centers were measured using the Multiple Aliquots Additive Dose (MAAD) method. In addition, hundreds of magnetostratigraphic samples were measured to determine the precise position of the Matuyama/Brunhes (M/B) boundary. The results show that: (1) The equivalent dose (D_E) values of the Al and Ti-Li centers are consistent within the error range, demonstrating that the Multiple Center (MC) approach has worked. (2) The ESR dating results are consistent with the order of sedimentation (the ages get older with depths) and with magnetostratigraphic results of the B/M boundary within the error range, indicating that the ESR ages obtained from the Al and Ti-Li centers are reliable in this study. (3) By comprehensive analysis of the dating results and lithology characteristics, the stratigraphic ages of the SNM borehole with a depth of 126.93 m spans from Neogene to Holocene, but part of the early Pleistocene strata are missing.

Sediment dynamics in five sites within the Mobile River Basin, Alabama, impacted by (i) dam-and-lock use, (ii) urbanization, (iii) industrial/mining practices, (iv) flooding, and (v) storm surge events were evaluated to understand better anthropogenic impacts on regional sediment budgets. Three widely used sediment dating models based on excess ²¹⁰Pb (ex²¹⁰Pb) (i.e., Constant Rate of Supply, CRS, Constant Initial Concentration, CIC, and Advective Dispersion Equation, ADE) and two recently developed ones (i.e., Porosity Variation, PV and Porosity Variation Without Diffusion, PVWD) were tested to determine their applicability to these anthropogenically altered lacustrine and coastal areas. To verify results from ex²¹⁰Pb models, we used conventional time markers, including regional hydrograph records and historical aerial images. We found that the traditionally used time-marker ¹³⁷Cs is no longer useful due to its current low inventories in the sediments. Drainage-to-lake area ratios were used to determine relative runoff and atmospheric radionuclide contributions. Statistical analysis of physical properties such as porosity, bulk and dry density, water content, and sediment geochemical compositions were utilized to support sediment transport and site development hypotheses. When constructing the sediment history at each site using these proxies, we found that the CIC and ADE models produced unreliable ages because of violation of requirements for exponentially decreasing porosity and ex²¹⁰Pb activity with depth. We found that two new models, PV and PVWD, that account for heterogeneous porosity, produced more reliable sediment depositional ages. These two models produced ages with lower uncertainties than the CRS model, outperforming the other conventional models tested. We conclude that the PV and PVWD models are more applicable for environments experiencing erosional and abrupt depositional events, which in our study resulted from dam construction and storm-surge events. Model sensitivity analysis showed that decreasing average particle density produces younger sediment ages by the PV and PVWD models. Higher ex²¹⁰Pb activity analytical uncertainty resulted in lower sedimentation rates and higher estimated ages by all five models.

It is crucial to establish a robust chronology for understanding late Quaternary sedimentation processes and environment changes in response to sea-level fluctuation on shelves. The shelf of the East China Sea (ECS) is featured by huge terrigenous sediment input and striking land -sea interaction during the late Quaternary. However, there remains controversy in chronostratigraphic rebuilding of transgression and regression deposits during eustatic-glacial cycles, mainly due to a lack of reliable dating data. In this study, an extensive comparison of 17 optically stimulated luminescence (OSL) and 15 AMS ¹⁴C ages from core TBF-1 (upper 40 m) provides a constraint in chronostratigraphic reconstruction since Marine Isotope Stage (MIS) 5. OSL ages range from 9.5 ± 0.7 to 67.3 ± 4.8 ka at depths from 2.00 to 28.00 m, while ¹⁴C ages range from 6645 to 41435 cal a BP at depths from 0.02 to 17.52 m. Regression analysis demonstrates a high level of agreement between OSL and ¹⁴C ages, with r² values of 0.91 and 0.90, respectively. For Holocene sediments (U1), both OSL and ¹⁴C methods are applicable in tidal sand ridges, however, it should be noted that ¹⁴C ages may exhibit a young bias of up to 3 ka. For pre-Holocene sediments (U6-U2), OSL ages are robust within saturation limitation of OSL signal (D_e up to ca. 166 Gy), cross-checked by feldspar post-IR IRSL (pIRIR) dating protocol. Nevertheless, the shell ¹⁴C ages (11620 and 10390 cal a BP at depths of 13.95–14.56 m) in Last Glacial Maximum (LGM) fluvial sediments of U2 are significantly underestimated, because of their susceptibility to recrystallization and overgrowth, resulting in carbon exchange with environment. While the peaty layer's¹⁴C age in deltaic deposition (17.50 m, U5) was determined to be 36420 cal a BP, coincided with OSL age of 39.6 ± 3.9 ka. The chronostratigraphy since MIS 5 for core TBF-1 exhibits a significant correlation with previously published cores SFK-1, DZQ4, and DH02, on the ECS outer shelf. Through the evaluation of those underestimated ¹⁴C ages and saturated OSL ages, we re-elucidated the primary sedimentary facies and their respective formation periods. This sedimentary stratigraphic illustration exhibits significant consistency with eustatic-glacial cycles. The renewed chronological framework for ECS shelf allows better correlation between the late Quaternary sedimentary evolution and the land-sea interaction on open shelves dominated by siliciclastic sedimentation.

The emerging use of quartz luminescence properties to characterize Earth-surface processes shows promise, with optically stimulated luminescence (OSL) sensitivity proposed as a valuable tool for provenance or sediment history tracing. However, the geologic processes that lead to quartz sensitization remain unclear. Here we study the impact of source rock and surface processes on the luminescence properties of quartz sand from bedrock and modern and Late Pleistocene alluvium generated from a mountainous catchment in northern Utah, USA. Continuous wave and linear modulated OSL are used to characterize the luminescence sensitivity and intensity of the fast-decay component. We compare the OSL sensitivity with sand-grain provenance and with proxies for surface processes such as topographic metrics, cosmogenic ¹⁰Be-derived erosion rates, chemical weathering indices, and magnetic susceptibility. Late Pleistocene sediment has low OSL sensitivity and a weak fast-decay component, similar to bedrock samples from the source area. In contrast, modern alluvium is dominated by the fast-decay component and has higher and more variable OSL sensitivity, with no clear relationship to upstream bedrock source. There is, however, an inverse relationship between OSL sensitivity and catchment-averaged erosion rates and a positive relationship with chemical weathering indices and magnetic susceptibility. These metrics suggest that the modern alluvium has experienced increased residence time in the shallow critical zone compared to the Late Pleistocene sediments. We suggest that changes in hillslope processes between the effectively wetter, cooler Pleistocene and the dryer, warmer conditions of the Holocene enhanced the luminescence properties. The results suggest that climatic controls on rates and processes of chemical and mechanical weathering and sediment transport and residence within the critical zone are encoded in the luminescence properties of quartz sand.

The Pamir Mountains are one of the highest mountain systems in the world; they act as sources of fresh water for the main rivers of Central Asia: the Amudaria and Syrdaria. Throughout the Quaternary, the Pamirs played a major role in controlling atmospheric circulation and land-surface processes, and provided great volumes of terrigenous sediments for transport by large rivers to the depressions in the Aral and Caspian regions. These ultimately provided broad aeolian cover in the sandy deserts, and finer dust for the widely distributed loess-palaeosol sequences. The glaciation history of this highly dynamic region provides an important basis for understanding climate change, sediment source and landscape evolution in Central Asia during the Quaternary. The question of the number, distribution, extent and timing of Pleistocene glaciations in the Pamir is debated. One of the main obstacles to research, together with difficulties of access and severity of current climate, is the varying degree of preservation of traces of previous glaciations in the western and eastern Pamir. As a result of a geological survey, we for the first time identified a thick lacustrine deposit at high altitudes in a tributary of the Panj – the valley of the Sary-Shitharv River – this records the damming of the Panj River valley by a large glacier. Luminescence measurements were undertaken to obtain the age of the Sary-Shitharv glacially-dammed lake. As often in mountain catchments the quartz OSL signal was unsuitable for dose estimation, and so the chronology of the Sary-Shitharv section is based entirely on post-IR IRSL signals from K-rich feldspar. We used pIRIR_50,290 and pIRIR_200,290 protocols and obtained indistinguishable ages from both protocols. Given the high sedimentation rates deduced from the structure of lacustrine deposits, the entire sequence must have been accumulated rather quickly, over a period of no more than a few thousand years. The average age over the whole series of dates is 165 ± 11 ka. This places the existence of the glacially-dammed lake at Sary-Shitharv in late MIS 6, a result that fits well with the general course of the glacial history of the Pamirs.

The spatiotemporal patterns of late Quaternary lake evolution, along with their responses to climatic changes and glacial meltwater in the westerlies-dominated Central Asia, remain unclear primarily due to the lack of well-dated records spanning across glacial and interglacial cycles. In this study, we investigated five well-preserved paleolake shoreline sequences, 15–27 m above the modern lake basin of the now-dry Manas Lake, a representative terminal lake in the Junggar Basin of arid Central Asia. Both single aliquot and single-grain K-feldspar post-infrared infrared stimulated luminescence (pIRIR) dating protocols were applied to 26 shoreline samples to reconstruct a lake level variation for Manas Lake over the past 90 kyr. The reliability of the K-feldspar pIRIR dating was tested through assessment of luminescence characteristics and comparison of single-grain and single-aliquot K-feldspar pIRIR D_es. The results indicate that the highest water levels (∼25 m deep) occurred during late Marine Isotope Stage (MIS) 5 (∼80 ka) and MIS 3 (31–27 ka). A lake level 20 m above modern lake basin (a.m.l.b.) occurred from the last deglaciation to the early Holocene (14–10 ka) and again in the late Holocene (3.4–0.3 ka). The lake level changes of Manas Lake are decoupled from observed Westerlies precipitation changes over the past 90 kyr. This decoupling suggests enhanced glacial meltwater sourced from the high Tianshan Mountains, triggered by higher mean summer temperatures, worked together with Westerlies precipitation drove periods of lake level highstand in Manas Lake during late MIS 5, late MIS 3, and early Holocene periods.