Optimizing radiocarbon chronologies in peat profiles with examples from Xinjiang,China

Reliable chronological frameworks are crucial to paleoenvironmental studies, and high precision ¹⁴C dating is the foundation, but many factors, such as dating materials, surficial deposition (influenced by nuclear bomb), and the ¹⁴C age plateau, will affect the reliability of the ¹⁴C ages and chronology frameworks. In this paper, we present 87 ¹⁴C dates of different peat fractions from three peat sites in Xinjiang, China. Plant macrofossils, rootlets, the fine fraction of <90 μm, the mid-size fraction of 90–250 μm and the coarse fraction of >250 μm from selected peat samples were measured to investigate the alternative suitable fraction for dating except for plant macrofossils. We discovered that the 90–250 μm component of peat can provide alternative and reliable results in case of plant macrofossils are not available. Additionally, more dating samples from surficial peat deposition were collected, and accurate surface chronological control points were produced by comparing ¹⁴C results of plant macrofossils with atmospheric ¹⁴C bomb data. Furthermore, multiple data sets with wiggle matching were used along the radiocarbon age plateau to minimize calibrated errors when dates on the ¹⁴C age plateau were shown. Finally, radiocarbon chronology frameworks in peat profiles were optimized. In conclusion, we not only focus on the reliable dating materials, but also highlight that the importance of surficial deposition (after 1950AD) and the anomalous ¹⁴C dates when establishing the dating framework in peat profiles. Furthermore, we propose that the obtaining chronological control points of surficial peat is an important part of the establishing and improving of peat chronological framework in future research.     

Holocene moist period recorded by the chronostratigraphy of a lake sedimentary sequence from Lake Tangra Yumco on the south Tibetan Plateau

Palaeolimnological studies together with geomorphological investigations of exposed lacustrine sections on the Tibetan Plateau provided valuable palaeoclimate records. Radiocarbon dating is the most commonly used method for establishing chronologies of lake sediments. However, ¹⁴C dating of such sediments could be problematic due to the lack of organic matter or a reservoir effect, which commonly appears in radiocarbon ages of lacustrine sediments from the Tibetan Plateau. OSL dating is an alternative for dating the lake sediments and also provides the opportunity to independently test radiocarbon chronologies. The current study tries to compare OSL and ¹⁴C dating results in order to evaluate the reservoir effect of ¹⁴C dating, and then based on quartz OSL dating and stratigraphic analysis, to construct the chronostratigraphy of a lacustrine sedimentary sequence (TYC section), an offshore profile from Tangra Yumco lake on the southern Tibetan Plateau. Results suggest that: (1) it is possible to obtain robust OSL age estimates for these lake sediments and the OSL ages of the three samples range from ca. 7.6 ka to ca. 2.3 ka; (2) The discrepancy between the OSL and ¹⁴C ages is ca. 4–5 ka, which possibly results from the age overestimate of ¹⁴C dating due to a reservoir effect in the studied lake; (3) the chronostratigraphy of TYC section and sedimentological environmental analysis show a large lake with a lake level distinctively above the present during ca. 7.6–2.7 ka indicating a wet mid-Holocene in the study area.     

Chronology and paleoenvironmental interpretation of talus flatiron sequences in a sub‐humid mountainous area: Tremp Depression,Spanish Pyrenees

Talus flatirons are debris‐covered relict slopes, disconnected from the source area, which are relatively common in arid and semi‐arid areas. Talus flatiron sequences record the alternation of accumulation and incision phases. These chronosequences may be used for infer temporal changes in the morphogenetic processes acting on the slopes as well as information on the local paleoclimatic history. Talus flatiron sequences developed in the Tremp Depression, eastern Spanish Pyrenees, are analysed from the geomorphological, chronological and paleoenvironmental perspective. The two groups of relict slopes differentiated by means of detailed geomorphological mapping have been dated by optically stimulated luminescence (OSL) and radiocarbon dating at 25–20 kyr (S3) and 5·4–1·7 cal kyr (S2). The talus flatiron group S3 is correlated with a fluvial terrace of the Noguera Pallaresa River (c. 23 kyr bp ). The comparison of the ages obtained in the Tremp Depression with chronologies published for talus flatiron sequences in semi‐arid areas and other paleoclimatic proxies suggests that the aggradation phases in the slopes occurred during periods with higher humidity and vegetation cover. The chronological differences observed between semi‐arid Spain and the Tremp Depression may be partly related to the more humid climate of the latter mountain area. Copyright © 2013 John Wiley & Sons, Ltd.     

Timing of Holocene lake highstands around Dawa Co in inner Tibetan Plateau: Comparison of quartz and feldspar luminescence dating with radiocarbon age

Lakes over the inner Tibetan Plateau (TP) are very sensitive to the regional environmental transformations and climate changes. Well-preserved lake sediments around these lakes provide critical geomorphological and sedimentary evidence that can be used to infer the past hydroclimate changes. In this study, a lacustrine section from a sandy shoreline (∼74 m above the modern lake) situated to the northwest of modern Dawa Co in the inner TP was investigated using both luminescence and radiocarbon dating methods. Our results demonstrated: (1) the quartz optically simulated luminescence (OSL) dating yielded much younger ages (∼4 ka) than that of the post-infrared IRSL (pIRIR) dating of the K-feldspar fraction; (2) fading test showed g-values ranging between 1.34 and 4.46%/decade for quartz OSL signals, which is considered to be responsible for the underestimation of the corresponding ages; (3) the AMS ¹⁴C age of the charcoal sample from the section is in line with the K-feldspar pIRIR₂₂₅ ages, confirming the reliability of the pIRIR₂₂₅ dates and the underestimation of the quartz OSL ages. The anomalous fading of quartz OSL signals and the consequent age underestimation have been reported in several other lakes on the TP, we presented here for the first time firm evidence of the phenomenon with the help of a robust independent control of AMS ¹⁴C age of the charcoal. Based on the pIRIR₂₂₅ and AMS ¹⁴C ages, we conclude that Dawa Co underwent a prominent highstand during the early Holocene (∼9–7 ka), which was probably controlled by the large amounts of glacial meltwater input and the increased monsoonal precipitation.     

Large 14C age offsets between the fine fraction and coexisting planktonic foraminifera in shallow Caribbean sediments

Absolute chronologies in paleoceanographic records are often constructed using the ¹⁴C dating of coarse fraction foraminifera (>150 μm). However, due to processes such as changes in sediment sources or abundances, sedimentation rates, bioturbation, reworking, the adsorption of modern carbon, etc., several studies conducted in different environmental settings have shown time-lags between records obtained from various granulometric fractions. In this study, we examined temporal phasing between the coarse foraminifera and fine fractions by studying changes in the abundances of δ¹⁸O, the ¹⁴C ages of the planktonic foraminifera Globigerinoides ruber (G. ruber, 250–350 μm), and the sediment fine fraction (<63 μm) over the last 45 ka in a core obtained from the northern Caribbean Sea. All of the records were found to be in phase during part of the Holocene (at least for the last ≈6 ka). As determined from δ¹⁸O records and ¹⁴C ages, the fine fraction was younger than G. ruber during the Last Deglaciation (of 1.89 ka). The coupling between bioturbation and changes in the fine fraction, and G. ruber abundances, as tested using a numerical model of the bioturbation record within a mixed-layer depth of 8 cm, was sufficient to explain the results. ¹⁴C age discrepancies increased from 5.64 to 8.5 ka during Marine Isotopic Stages (MIS) 2 and 3, respectively. These chronological discrepancies could not be explained by only one process and seemed to result from the interplay between mechanisms: size-differentiated bioturbation (for 1.5 to 2.5 ka), the adsorption of modern atmospheric CO₂ (for 3.04 to 5.92 ka), and variations in sedimentological processes that influenced the fine carbonate fraction. However, even if variations in the mineralogical composition of the fine carbonate fraction were identified using scanning-electron microscopy observations, X-ray diffraction measurements, and geochemical analyses (the mol % MgCO₃ of magnesian calcite and the Sr/Ca ratio of the bulk fine fraction), they can not account for the observed age differences. The results presented for core MD03-2628 extend beyond this case study because they illustrate the need for a detailed characterization of the various size fractions prior to paleoclimate signal interpretations, especially for chronological studies.     

Can abundance of geomorphological features be predicted using presence–absence data?

Geomorphological models are useful tools for assessing the impacts of changing environmental conditions on earth surface processes and landforms. Here we evaluate the feasibility of predicting the abundance (spatial coverage) of geomorphological features using presence–absence data. To achieve the study goals, generalized linear models (GLMs), spatial predictors and geomorphological data from a thoroughly inventoried area of 600 km² in sub‐Arctic Finland at a resolution of 25 ha were utilized in the exercise. The key finding was that the GLMs derived from presence–absence data performed as consistently, as a relative index of abundance, as models derived directly from the abundance data. Thus, predictions based on presence–absence data may serve as a reasonable surrogate for abundance of geomorphological features. Copyright © 2007 John Wiley & Sons, Ltd.     

Robust chronological reconstruction for young speleothems using radiocarbon

We have studied two young speleothems, SC4 from Smiths Cave (Christmas Island, eastern Indian Ocean) and WM7 from Wollondilly Cave (Wombeyan caves, SE Australia). Attempts to date these speleothems by the Th/U method have proved unsuccessful with some age reversals for SC4 due to multiple sources of non-authigenic Th. This method has also resulted in imprecise ages for WM7 because of low U concentrations (<10 ppb) and consequently very low levels of authigenic ²³⁰Th relative even to the very low levels of detrital ²³⁰Th present. Here, we present an alternative method for reliable dating of these young speleothems using radiocarbon. Approximately 100 carbonate samples from SC4 and WM7 were analysed for ¹⁴C by accelerator mass spectrometry (AMS). The AMS results indicate that bomb ¹⁴C was evident in the youngest parts of both stalagmites. Two different approaches were used to estimate dead carbon fraction (DCF) values for these stalagmites for the pre-bomb period. For SC4, the DCF values were estimated based on the timing of ¹⁴C dates for that period determined by high-resolution δ¹⁸O recorded in the speleothem, and the timing of the onset of bomb ¹⁴C. For WM7, a “maximum” range of pre-bomb DCF was determined. Chronologies of these speleothems were built based on a dense sequence of DCF-corrected ages using three different age-depth models: Clam (Classical method), and Bacon and OxCal (Bayesian statistical approach). Good agreement between these age-depth models were observed indicating that the top 170 mm of SC4 and the top 50 mm of WM7 grew during the past 550–750 years and 1360–1740 years, respectively.     

Coupled U-series and OSL dating of a Late Pleistocene cave sediment sequence,Morocco, North Africa: Significance for constructing Palaeolithic chronologies

Cave sequences provide some of the most important archives of Palaeolithic archaeology that are currently available. The potential value of such sequences is, however, frequently limited by the problems associated with constructing robust chronologies for archaeologically significant levels. In this study, we apply ²³⁰Th/U and OSL SAR techniques to the dating of deposits from the Calcareous Group at Grotte des Pigeons, Taforalt, eastern Morocco. The advantage of combining these techniques is: 1) they allow the cross-checking of chronologies derived from independent sets of measurements and assumptions, and 2) they date different facies of cave sediments (i.e. clastic cave sediments as opposed to cave precipitates such as speleothems) allowing the timing of deposition by different cave processes to be constrained more robustly. The age estimates derived by these techniques from different sampling locations are in good agreement with one another, suggesting the potential of these techniques when used in tandem for producing more robust chronologies of Palaeolithic sequences. The deposition of the Calcareous Group is placed within the middle to late part of MIS 5 (MIS 5c to 5a). Furthermore, the Bayesian analysis of the dates derived in this study along with those that have been previously published, allows the duration of deposition of this unit to be estimated at 25.4 ± 3.7 ka (mean ± 1σ). The paper concludes by discussing the significance of these ages for the archaeology of the Taforalt sequence. We suggest that the collection of samples for multiple independent dating methods in direct association combined with the use of careful, stratigraphic constraints in Bayesian models results in the most robust dates available for such cave sequences.     

Dating alluvial fan surfaces in Owens Valley,California, using weathering fractures in boulders

A wide range of sedimentological and geomorphological field research depends on the availability of accurate and detailed depositional age models. Although exposure dating techniques such as cosmogenic nuclide and luminescence dating are now widely available, they remain expensive and time‐consuming, and this frequently limits the density of age constraints and the resolutions of age models for many study areas. We present a simple and effective, field‐based approach for extending and correlating existing age models to un‐dated surfaces. In Owens Valley, California, we make use of detailed beryllium‐10 (¹⁰Be) chronologies reported for four different alluvial fan systems, to precisely calibrate the rate at which weathering fractures are enlarged in granitic surface boulders. We show that these fractures have widened at a time‐integrated rate of 1.05 ± 0.03 mm ka^?1 for at least 140 ka at this location, and this relationship can be represented by a linear regression that makes them ideal chronometers for surface dating. Our analysis offers a new approach to refining the uncertainties of both surface erosion rate and cosmogenic age estimates at this location. Ultimately, we integrate our observations to devise a robust age calibration for clast fracture widths in Owens Valley, and we demonstrate its application by estimating the ages of 27 additional local fan surfaces. We present an updated and extended stratigraphy for eight Sierra Nevada fan systems in total, with exceptional age control. This novel approach to dating sedimentary surfaces is inexpensive and easily applied in the field, and has the potential to significantly increase the temporal and spatial density of age constraints available for a particular study area. Copyright © 2014 John Wiley & Sons, Ltd.     

Radiocarbon-calibrated amino acid racemization ages from Holocene sand dollars (Peronella peronii)

Amino acid racemization (AAR) is widely used as a cost-effective method to date molluscs in time-averaging and taphonomic studies, but it has not been attempted for echinoderms despite their paleobiological importance and distinct biomineralization. Here we demonstrate the applicability of AAR geochronology for dating Holocene Peronella peronii (Echinodermata: Echinoidea) collected from Sydney Harbour (Australia). Using standard HPLC methods we determined the extent of AAR in 74 Peronella tests and performed replicate analyses on 23 tests. Replicate analyses from the outer edge of 23 tests spanning the observed range of D/L values yielded median coefficients of variation <4% for Asp, Glu, and Phe D/L values, which is comparable to the analytical precision. Correlations between THAA D/L values across 178 independently prepared subsamples of 74 individuals are also very high (Spearman ρ ≥ 0.95) for these three amino acids. The ages of 20 individuals spanning the observed range of D/L values were determined using ¹⁴C analyses, and Bayesian model averaging was used to determine the best AAR age model. Only three models fit to Glu D/L contributed to the final averaged age model. Modeled ages ranged from 14 to 5496 years, and the median 95% confidence interval for the 54 AAR ages was ±29% of the modelled age. In comparison, the median 2σ confidence interval for nine graphite target ¹⁴C ages was ±8% of the median age estimate and the median 2σ confidence interval for 20 carbonate target ¹⁴C ages was ±26% of the median age estimate. Overall Peronella yield high-quality D/L values and appear to be a good target for AAR geochronology.     

Methods and codes for reservoir–atmosphere 14C age offset calculations

Reservoir ¹⁴C age offsets are invaluable tracers for past changes in carbon cycle and oceanic circulation. Reconstruction of reservoir age offsets with time is also required for calibration purposes (reconstruction of atmospheric calibration curve, calibration of non-atmospheric radiocarbon ages). Thus, properly propagating the various uncertainties linked to reservoir age offset is important for proper interpretation. However, approaches for reservoir age offset calculation – especially when considering pairs of reservoir-derived ¹⁴C and calendar ages – are usually not detailed and inadequate for proper propagation of uncertainties. Here, the various ways to properly calculate reservoir age offsets are described with a focus on a new approach when considering pairs of ¹⁴C and calendar ages. This approach maps the calendar age distribution onto the ¹⁴C time scale prior to reservoir age offset calculation – the “uncalibration-convolution process”. R codes computing reservoir age offsets based on available data are presented. Finally, a case study focusing on the reconstruction of the speleothem-atmosphere ¹⁴C age offsets of speleothem ¹⁴C data used in the latest release of the atmospheric calibration curve is discussed.     

Geochronology and excess Ar geochemistry of the Lhotse Nup leucogranite, Nepal Himalaya

⁴⁰Ar/³⁹Ar dating experiments on several coexisting minerals from two close-by leucogranite outcrops near Lhotse Nup glacier (Nepal Himalaya) reveal a complex behaviour. Four biotite and muscovite ages cluster around 15.5 Ma, a lower value than literature Rb/Sr ages on splits of the same four micas, suggesting a discrepancy with the ideal cooling age sequence observed in the Alps.A strongly discordant Ar-Ar spectrum on tourmaline does not allow a chronological interpretation.A potassium feldspar shows a slow-cooling staircase spectrum with a superimposed saddle diagnostic of excess Ar. HF leaching removed excess Ar but caused great perturbations to the minimum step ages, isochron plots, and the release of reactor-produced Ar isotopes. The present data require that the currently fashionable interpretations of feldspar systematics be radically changed.The main chronological conclusions rely on the eight mica ages. Their decrease from 18.2 to 15.3 Ma dates the cooling of the Lhotse Nup leucogranite.     

Coastal slope terracing and relative sea-level changes: Deductions based on computer simulations

Although a traditional geological survey of terraced coastal slopes is an essential part of studies aimed to reconstruct relative sea-level changes, the stratigraphic and chronological data so obtained frequently prove inadequate to completely unravel the sometimes very complex history of sea-level fluctuation, especially where erosion has prevailed over deposition and/or deposits are difficult to date. On the basis of our experience we think that much additional information can be gained through geomorphological interpretation of the profiles of those slopes. In order to facilitate such interpretation, a computer simulation model is developed that is able to predict the morphogenic response to a variety of possible relative-sea-level histories. The results can be used to envisage some new interpretation keys for the analysis of real situations and, hopefully, as bases for the conception of general models of coastal slope evolution under the action of a variable sea level. Though preliminary, the results so far obtained permit identification of the geomorphological consequences (in terms of resulting slope profile) of ordered successions of transgressions and regressions of different amplitude, rate, order and style. Moreover they provide interesting insights into the role that different styles of tectonic uplift may play in the phenomenon of multiple terracing of coastal slopes.     

Can in-situ cosmogenic 14C be used to assess the influence of clast recycling on exposure dating of ice retreat in Antarctica?

Cosmogenic nuclide exposure dating of glacial clasts is becoming a common and robust method for reconstructing the history of glaciers and ice sheets. In Antarctica, however, many samples exhibit cosmogenic nuclide ‘inheritance’ as a result of sediment recycling and exposure to cosmic radiation during previous ice free periods. In-situ cosmogenic ¹⁴C, in combination with longer lived nuclides such as ¹⁰Be, can be used to detect inheritance because the relatively short half-life of ¹⁴C means that in-situ ¹⁴C acquired in exposure during previous interglacials decays away while the sample locality is covered by ice during the subsequent glacial. Measurements of in-situ ¹⁴C in clasts from the last deglaciation of the Framnes Mountains in East Antarctica provide deglaciation ages that are concordant with existing ²⁶Al and ¹⁰Be ages, suggesting that in this area, the younger population of erratics contain limited inheritance.     

OSL chronology of a sedimentary sequence from the inner-shelf of the East China Sea and its implication on post-glacial deposition history

Sedimentary records from the inner-shelf of the East China Sea (ECS) are unique for the reconstruction of post-glacial palaeoclimate and sea-level changes. So far, the chronology of sediment succession from this region has mainly been based on radiocarbon dating, which might be problematic due to reworked deposition or old carbon contamination. In this study we tested the applicability of optically stimulated luminescence (OSL) dating to a drilling core (ECS-DZ1) taken from the northern ESC. A total of 20 OSL samples and two radiocarbon samples were collected from the upper 58 m of this core. The results indicate the likely sufficient reset of OSL signal of fine-grained (4–11 μm) quartz before burial, and thus reliable chronology for the studied core sediments. For one sample, however, the extracted coarse-grained (100–200 μm) quartz overestimated the deposition age significantly, presumably resulting from partial bleaching prior to deposition. The fine-grained quartz ages are generally consistent with the stratigraphical order, and the reliability of these OSL ages are further validated by two selective robust ¹⁴C dates. The chronological framework of core ECS-DZ1 reveals striking sedimentation-rate changes. By comparison with other chronostratigraphical records, we infer that post-glacial deposition history (since ∼15 ka) of the study site is likely related to regional sea-level rise and delta-estuary environment evolution, as well as strengthened human activities and/or coastal currents.     

Improved age estimates for Holocene Ko-g and Ma-f~j tephras in northern Japan using Bayesian statistical modelling

The Ko-g and Ma-f~j tephras are two key isochronous marker layers in northern Japan, which are from the largest Plinian eruptions of Komagatake volcano (VEI = 5) and Mashu caldera (VEI = 6), respectively. Despite extensive radiocarbon studies associated with the two tephras, individual calibrated results show considerable variations and thus accurate ages of these important eruptions remain controversial. Bayesian statistical approaches to calibrating radiocarbon determinations have proven successful in increasing accuracy and sometimes precision for dating tephras, which is achieved through the incorporation of additional stratigraphic information and the combination of evidence from multiple records. Here we use Bayesian approaches to analyse the proximal and distal information associated with the two tephra markers. Through establishing phase and deposition models, we have taken into account all of the currently available stratigraphic and chronological information. The cross-referencing of phase models with the deposition model allows the refinement of eruption ages and the deposition model itself. Using this we are able to provide the most robust current age estimates for the two tephra layers. The Ko-g and Ma-f~j tephras are hereby dated to 6657-6505 (95.4%; 6586±40, μ±σ) cal yr BP, and 7670-7395 (95.4%; 7532±72, μ±σ) cal yr BP, respectively. These updated age determinations underpin the reported East Asian Holocene tephrostratigraphic framework, and allow sites where the tephra layers are present to be dated more precisely and accurately. Our results encourage further applications of Bayesian modelling techniques in the volcanically active East Asian region.     

Two contrasting petrotectonic domains in the Kokchetav megamélange (north Kazakhstan): Difference in exhumation mechanisms of ultrahigh‐pressure crustal rocks,or a result of subsequent deformation?

Abstract In northern Kazakhstan the WNW striking Kokchetav megamélange includes different crustal sequences with high‐pressure/ultrahigh‐pressure (HP/UHP) remnants of their 540–520 Ma subduction metamorphism. Two domains separated by the north‐east trending Chaglinka fault are distinguished. The western domain exhibits NE–SW structures within a single Kumdy–Kol megaunit of diamond‐bearing UHP metasediments and high‐temperature (HT) eclogites. The eastern domain consists of the composite Kulet megaunit with the Kulet UHP unit (coesite‐bearing metasediments, whiteschists and eclogites), the Enbek–Berlyk medium‐pressure (MP) unit (kyanite‐bearing, high‐alumina rocks with interleaved coronitic metagabbro), and ortho‐ and paragneisses with eclogites and amphibolites included. All eclogites in the eastern domain are of the relatively low temperature (LT) type. Sillimanite is common and appears after kyanite in the sheared MP unit. A regional and moderately ESE plunging linear fabric coincides with the fold‐axis of the foliation poles from the eastern domain. Whether this also reflects a regional top to the WNW transport, as inferred from the dextral strike‐slip on steeply to SSW dipping foliation, needs further study. Top to the WNW shear is shown by weakly inclined low pressure (LP) cordierite rocks that flank the eastern domain in the south. Some new ³⁹Ar/⁴⁰Ar mica cooling ages (519, 521 Ma) from the Kulet UHP micaschists reflect the same early stage evolutionary event as was previously shown for the Kumdy–Kol UHP rocks (515, 517 Ma) in the west. Similar ³⁹Ar/⁴⁰Ar ages (500, 517 Ma) are recorded by micas and amphibole that outline a top to NNW shear fabric in the non‐subducted Proterozoic basement, north of the megamélange. A 447 Ma overprint of the MP sequences is considered to reflect the strike‐slip deformation with sillimanite and the reworking of an early kyanite‐bearing tectonite. Biotites from the LP cordierite rocks yielded approximately 400 Ma ³⁹Ar/⁴⁰Ar ages. In case they reflect the WNW shear deformation, the latter is considered to be associated with a regional granite magmatism (420–460 Ma) extending south of the eastern domain. In their present different structural domains the Kulet and Kumdy–Kol UHP units display a similar early stage event. Subsequent LP deformation, which is likely to be associated with regional granite magmatism (420–460 Ma), is assumed to have obliterated any common or uniform early exhumation structure for the whole megamélange. The north‐east structured Kumdy–Kol domain is assumed to have preserved the most information about the early stage exhumation. This domain is at an angle to the regional WNW strike of the megamélange.     

Bayesian age-depth modelling of Late Quaternary deposits from Wet and Blanche Caves,Naracoorte, South Australia: A framework for comparative faunal analyses

Bayesian age-depth models were constructed for two Late Quaternary aged fossil-bearing sedimentary sequences from caves in south eastern South Australia. The deposits in Wet and Blanche Caves contain dense assemblages of vertebrate fossils, largely the result of owl pellet accumulation. While individually calibrated radiocarbon determinations from the fossil sequences have provided a chronology for their accumulation, there was limited capacity available with such data to (a) temporally constrain assemblages associated with different depositional units and layers within the two sites, (b) interpret the chronological relationships among successive units and layers and (c) correlate sedimentary units and layers of similar age between the two deposits. Here, Bayesian age-depth models were constructed in OxCal for the Wet and Blanche Cave sequences, incorporating the available radiocarbon data and stratigraphic information collected during their excavation. Despite the low precision of the age-depth models for Wet and Blanche Caves which results in part from there being only single radiocarbon determinations available for a number of units and layers, the models enabled the relationships within and between the two sites to be established. Of particular utility for future faunal analyses is quantification of the temporal relationship between strata from the two sites, where groups of individual layers from Blanche Cave were found to be temporally equivalent with the longer-duration units in Wet Cave. We suggest that the use of Phase modelling, as performed here, is useful for cave deposits that have complex depositional histories and even in such instances where, as is common for palaeontological sites, few radiocarbon data are collected relative to the time-spans of tens of millennia that are often represented by them.     

OSL and radiocarbon dating of a core from the Bohai Sea in China and implication for Late Quaternary transgression pattern

The Bohai Sea is a semi-enclosed continental shelf sea in northern China. Three transgression layers have been identified from the Late Quaternary strata in the western Bohai Sea and the coastal regions, which provide critical information on Late Quaternary sea-level fluctuations and landscape development. The three transgression layers were previously assigned to Marine Isotope Stage (MIS) 1 (transgression 1, T1), MIS 3 (T2) and MIS 5 (T3), respectively, mainly based on ¹⁴C dating. However, this chronological framework aroused an enigma that the regional sea level in MIS 3 was even higher than that of MIS 5, conflicting with the context of global sea-level pattern. In order to clarify this issue, here quartz optically stimulated luminescence (OSL) dating (four samples) was used to constrain the T2 chronology of borehole TJC-1 from the western Bohai Sea. Radiocarbon samples (eight) of peaty sediments were also measured for reference and comparison. All the four OSL samples showed saturation ages of >80 ka, suggesting that the T2 layer should have formed at least in MIS 5, instead of in MIS 3. Radiocarbon ages in T2 should have been severely underestimated, with a saturation age range of 22–30 cal ka BP, similar to all the previous published radiocarbon ages. The renewed OSL chronological framework for Late Quaternary transgressions in the western Bohai Sea is in better compliance with the history of global sea-level change.     

Optically stimulated luminescence and radiocarbon dating of sediments from Lop Nur (Lop Nor), China

Lop Nur is a playa lake occupying the lowest part of the Tarim Basin, northwestern China, and is now a desolate and barren region. In the past decades, the ages of the lacustrine sediments from the lake were determined mainly by radiocarbon dating on bulk sediment. In this study, both optically stimulated luminescence (OSL) and radiocarbon methods were used to date the sediments from a pit in the central part of the lake. The OSL ages obtained for ten samples range from 0.5 to 9.4 ka, and are in stratigraphic order except for one sample. The ¹⁴C ages obtained for twenty-two bulk sediment samples range from 5.8 to 30.2 cal ka BP with erratic distribution. Based on the comparison of ¹⁴C with OSL ages and their age-depth models, we argue that the OSL ages are relatively reliable. The disequilibria in the U decay chain for some samples are deduced from the comparison of the NAA and TSAC results. We suggest that the OSL dating technique should preferably be applied to the playa sediments from Lop Nur, but the disequilibria in the U decay chain should be considered in evaluating dose rates. Additionally, radiocarbon reservoir effects in lakes in western China are reviewed.