Applying a Bayesian approach for refining the chronostratigraphy of the Yumidong site in the Three Gorges region,central China

Yumidong (Corn Cave) is a newly discovered Paleolithic site in the Three Gorges region of central China. Numerous Paleolithic artifacts have been excavated from the sedimentary deposits of the cave in association with faunal remains attributed to the Middle-Late Pleistocene Ailuropoda-Stegodon fauna of southern China. To establish the chronology of the sedimentary sequence (>5 m thick), ¹⁴C dating was applied to bone and charcoal samples (n = 6); the U-series method was used to date in situ precipitated speleothems (n = 12), transported speleothem samples (n = 6) and 18 subsamples of a fossil tooth; and the coupled ESR/U-series method was used to date fossil teeth (n = 6). The derived dates were combined using a hierarchical Bayesian approach to generate a unified chronostratigraphy for the Yumidong sequence. In our Bayesian analyses, the ¹⁴C and coupled ESR/U-series dates were considered to provide direct age estimates for the target layers, while the U-series dates of the in situ precipitated speleothems and fossil tooth were used as minimum age constraints and those of the transported speleothem fragments as maximum age constraints. The Bayesian analyses provided robust time intervals for the archeological layers: L2-Upper (14–23 ka), L2-Lower (27–63 ka), L3 (106–171 ka), L4 (140–192 ka), L10 (157–229 ka), L11 (181–256 ka), and L12 (214–274 ka) with a probability of 95%, allowing the establishment of a ∼300 ka long geological and archeological history for the Yumidong site and placing it as a reference site for Paleolithic cultural evolution in the Three Gorges region from the late Middle Pleistocene to Late Pleistocene.     

Uplift rates defined by U-series and 14C ages of serpulid-encrusted speleothems from submerged caves near Siracusa,Sicily (Italy)

We have established a plausible rate of uplift near Siracusa in southeastern Sicily (Italy) over the last glacial–interglacial cycle using U-series ages of submerged speleothem calcite and ¹⁴C ages of calcite serpulid layers that encrust the speleothems during cave submergence. The precisely determined ages of these sea level benchmarks were compared with expected relative sea level position based on glacio-hydro-isostatic modeling to assess the rate of uplift in this region. When combined with the age of various late Holocene archaeological sites that have been recently described and characterized in terms of their functional position relative to sea level these data collectively define a rate of uplift ≤0.4 mm a^?1 along this portion of the Sicilian coastline. These results are consistent with an age assignment of marine isotope stage (MIS) 5.3 or 5.5 for the Akradina terrace, which in turn places temporal constraints on paleoshorelines above and below this level.     

Chronology building using objective identification of annual signals in trace element profiles of stalagmites

Determination of annual lamination provides important additional constraints to radiometric dates on speleothems, both for dating the duration of specific growth intervals and optimizing growth models. In the absence of visible laminae, however, speleothem age models are reliant upon curve fitting through discretely dated points and are therefore inherently more uncertain than annual chronologies from laminae. Given that the impact of seasonality on speleothems is expected to be strong enough to generate an annual pulse in trace element chemistry regardless of whether or not visible or fluorescent growth laminae are visible, we demonstrate the potential for deriving high-resolution stalagmite chronologies from non-laminated samples using annual chemical variations in stalagmites from two Alpine caves (Obir, Austria and Ernesto, NE Italy). Trace element data were obtained by ion microprobe analyses for H, P, Mg, Na, Sr and Ba and the annual signal was sought using spectral and wavelet analysis. An automated chemical peak-counting software tool was developed in MATLAB^©. It counts significant annual peaks using criteria of minimum amplitude in relation to the local standard deviation of signal variation and minimum separation between peaks determined by the thickness of the preceding layers. Verification of the tool using visibly laminated samples suggests the software is a reliable and accurate method of chronology building, with hit ratios greater than 0.93 and less than 0.75% false alarm occurrences. Used in conjunction with other dating methods such as radiocarbon, U–Th and sulphur peak dating, the automated chemical laminae chronology-building approach provides a more meaningful alternative to simple age-depth curve fitting for non-laminated samples.     

High-resolution U–Pb dating of an Early Pleistocene stalagmite from Corchia Cave (central Italy)

Recent developments in the uranium–lead (U–Pb) dating of speleothems have opened up new opportunities in palaeoclimate research. An important goal in this new frontier is to produce palaeoclimate records underpinned by precise and accurate age models, which together will increase the range of palaeoclimate questions that can be addressed by the speleothem research community. In this paper, we investigate the level of age-model precision that is achievable by applying high-resolution U–Pb dating of a stalagmite (CC8) from Corchia Cave (Italy) whose period of growth spans part of the Middle Pleistocene Transition (∼970–810 ka). Focussing largely on the periods encompassing three glacial terminations, we carried out age sampling of CC8 at a density similar to that performed in many studies of younger (i.e. U–Th-dated) speleothems. Using a combination of Tera–Wasserburg isochron and model age approaches, coupled with age-depth modelling, our results show that age-model uncertainties of ∼4 kyr are possible, equivalent to a precision of about 0.4%. At this level of precision palaeoclimate time series derived from speleothems of this age can be used to test hypotheses of orbital forcing.     

StalAge – An algorithm designed for construction of speleothem age models

Here we present a new algorithm (StalAge), which is designed to construct speleothem age models. The algorithm uses U-series ages and their corresponding age uncertainty for modelling and also includes stratigraphic information in order to further constrain and improve the age model. StalAge is applicable to problematic datasets that include outliers, age inversions, hiatuses and large changes in growth rate. Manual selection of potentially inaccurate ages prior to application is not required. StalAge can be applied by the general, non-expert user and has no adjustable free parameters. This offers the highest degree of reproducibility and comparability of speleothem records from different studies. StalAge consists of three major steps. Firstly, major outliers are identified. Secondly, age data are screened for minor outliers and age inversions, and the uncertainty of potential outliers is increased using an iterative procedure. Finally, the age model and corresponding 95%-confidence limits are calculated by a Monte-Carlo simulation fitting ensembles of straight lines to sub-sets of the age data.We apply StalAge to a synthetic stalagmite ’sample’ including several problematic features in order to test its performance and robustness. The true age is mostly within the 95%-confidence age limits of StalAge showing that the calculated age models are accurate even for very difficult samples. We also apply StalAge to three published speleothem datasets. One of those is annually laminated, and the lamina counting chronology agrees with the age model calculated by StalAge. For the other two speleothems the resulting age models are similar to the published age models, which are both based on smoothing splines. Calculated uncertainties are in the range of those calculated by combined application of Bayesian chronological ordering and a spline, showing that StalAge is efficient in using stratigraphic information in order to reduce age model uncertainty.The algorithm is written in the open source statistical software R and available from the authors or as an electronic supplement of this paper.     

Estimation of an upper limit on prehistoric peak ground acceleration using the parameters of intact speleothems in Hungarian caves

The examination of speleothems in the Hajnóczy and Baradla caves (northeastern Hungary) allows estimating an upper limit for horizontal peak ground acceleration generated by paleoearthquakes. The density, the Young’s modulus and the tensile failure stress of the samples originating from a broken speleothem have been measured in a laboratory, whereas the natural frequency of intact speleothems was determined by in situ observations. The value of horizontal ground acceleration resulting in failure, the natural frequency and the inner friction coefficient of speleothems were assessed by theoretical calculations. The ages of the samples taken from a stalagmite 5.1 m in height (Baradla cave) have been determined by inductively coupled plasma mass spectrometry analysis and alpha spectrometry. The measured ages fall between 140,000 and 70,000 years; therefore, we assume the speleothem has not been changed since the end of this time interval. According to our modeling results, this speleothem has not been excited by a horizontal acceleration higher than 0.05 g during the last 70,000 years.     

Early diagenetic imprints and U–Th isotope systematics of fossil land snail shells from the Chinese Loess Plateau

The loess-paleosol sequence on the Chinese Loess Plateau provides a unique archive that records climate change in East Asia in the Quaternary, yet absolute dating of the loess deposits is challenging due to the lack of directly datable materials. Fossil land snail shells, which are made from aragonite, are widely preserved in the loess deposits and have long been used to reconstruct past environmental changes. U-series dating of fossil land snail shells has the potential to provide a route for absolute dating of the loess deposits but remains largely unexplored. In this study, we present the first systematic investigation on the U-series isotope geochemistry as well as the early diagenetic imprints of fossil land snail shells (Cathaica sp.) from the Mangshan loess-paleosol sequence in Henan province, central China. Several geochemical techniques, including Raman microscopy, SEM, LA-ICPMS, LA-MC-ICPMS, solution-MC-ICPMS, and AMS ¹⁴C dating, were employed to investigate the mineralogy, chemical and isotopic compositions of both modern and fossil snail shells to micrometer level. Our results show that the fossil Cathaica sp. shells are overall characterized by a higher degree of porosity and elevated contents of organic matter compared to live-collected shells of the same species. The layers with higher porosity and organic matter content in the fossil Cathaica sp. shell are also found to be enriched in Na, Mg, Mn, Ba, and U, indicating diffusion and adsorption of these elements by specific surface binding sites of either aragonitic lattice or organic compounds of the fossil shell. Combining in-situ measurements using LA-MC-ICPMS with solution U-series determination, we further demonstrate that fossil Cathaica sp. shell is relatively homogeneous regrading both [²³⁴U/²³⁸U] and [²³⁰Th/²³⁸U] values although the distribution of U in the fossil shell is sample-specific. The comparison of different dating results suggests that the calculated apparent closed system U–Th ages are all systematically younger (∼6000 to 13,000 years) than the corresponding shell ¹⁴C ages and quartz SAR (single-aliquot regenerative-dose) ages from the Mangshan section. We suggest that the underestimation of U–Th ages of fossil Cathaica sp. shells is very likely caused by diagenetic uptake of U that started immediately after the burial of the shell and effectively ceased when the fossil shell was isolated from the pore waters due to persistent deposition of eolian dust at the Mangshan section. Our work on both modern and fossil Cathaica sp. shells thus provides detailed morphological and geochemical characterization for the diagenetic alteration of fossil snail shells and suggests that U-series dating of fossil land snail shells may provide age constraints for dust deposits in the semi-arid region although the timing of early diagenetic U-uptake by the fossil shells need to be better quantified for reliable age determination.     

Direct U–Th dating of vertebrate fossils with minimum sampling destruction and application to museum specimens

Although vertebrate fossils are commonly abundant in museum palaeontological collections, they are only rarely accompanied by contextual data (e.g., stratigraphic and taphonomic information) that allow them to be placed independently into reliable temporal frameworks critical for testing significant evolutionary and extinction hypotheses. Moreover, where critical samples do exist in such collections, sampling for direct geochronological analyses becomes a significant concern, especially where such sampling is destructive in nature. Here we apply a direct fossil dating, micro-drilling sampling approach that minimises damage to and destruction of precious museum specimens. We carried out a systematic U–Th dating study (n = 28 ages) of an isolated museum specimen of the extinct Palorchestes azael (megafaunal ‘marsupial tapir’) originally collected in 1977 from Tea Tree Cave, Chillagoe, northeastern Australia. We obtained 21 U–Th ages and constructed ²³⁰Th-age profiles across three teeth exposed in cross-section, using micro-drilling and thermal ionisation mass spectrometry. Individual sample masses were as little as 0.18 mg (U concentration 33–82 ppm), meaning that the sampling resulted in only minimal destruction of the specimen. The results show no evidence of U leaching, suggesting that the dates represent reliable minimum ages. For independent age control, we also dated calcite that had encrusted the sample (thus, providing a minimum age; n = 6) and an older calcite clast that had been reworked into the surrounding breccia at the time of burial (thus, providing a maximum age; n = 1). U–Th ages of the teeth are older than the calcite overgrowths and younger than the reworked calcite, consistent with their demonstrable relative age relationships. Collectively, the results unequivocally bracket the age of the fossil between 199.1 ± 8.9 ka and 137.4 ± 1.1 ka (2σ), adding another rare datum to inform the timing and geographic distribution of last occurrences of the species. The benefits of our dating approach of museum fossil specimens are threefold: 1) it is minimally destructive even compared with laser-ablation method; 2) the use of U vs. apparent age approach allows direct testing for potential U leaching as occasionally seen in fossil dating; and 3) the combination of fossil and associated speleothem dating provides the most robust means of securely bracketing the age of fossils that lack firm stratigraphic control.     

Luminescence dating without sand lenses: An application of OSL to coarse-grained alluvial fan deposits of the Lost River Range,Idaho, USA

Optically stimulated luminescence (OSL) dating is increasingly used to estimate the age of fluvial deposits. A significant limitation, however, has been that conventional techniques of sampling and dose rate estimation are suitable only for thick (>60 cm) layers consisting of sand size or finer grains. Application of OSL dating to deposits lacking such layers remains a significant challenge. Alluvial fans along the western front of the Lost River Range in east-central Idaho, USA are one example. Deposits are typically pebble to cobble sheetflood gravels with a sandy matrix but thin to absent sand lenses. As a result, the majority of samples for this project were collected by excavating matrix material from gravelly deposits under light-safe tarps or at night. To examine the contributions of different grain-size fractions to calculated dose-rates, multiple grain-size fractions were analyzed using ICP–MS, high resolution gamma spectrometry and XRF. Dose rates from bulk sediment samples were 0.4–40% (mean of 18%) lower than dose-rate estimates from the sand-size fractions alone, illustrating the importance of representative sampling for dose rate determination. We attribute the difference to the low dose-rate contribution from radio-nuclide poor carbonate pebbles and cobbles that occur disproportionately in clast sizes larger than sand. Where possible, dose rates were based on bulk sediment samples since they integrate the dose-rate contribution from all grain sizes. Equivalent dose distributions showed little evidence for partial bleaching. However, many samples had significant kurtosis and/or overdispersion, possibly due to grain-size related microdosimetry effects, accumulation of pedogenic carbonate or post-depositional sediment mixing. Our OSL age estimates range from 4 to 120 ka, preserve stratigraphic and geomorphic order, and show good agreement with independent ages from tephra correlation and U-series dating of pedogenic carbonate. Furthermore, multiple samples from the same deposit produced ages in good agreement. This study demonstrates that with modified sampling methods and careful consideration of the dose rate, OSL dating can be successfully applied to coarse-grained deposits of climatic and tectonic significance that may be difficult to date by other methods.     

Speleothems in a north Cuban cave register sea‐level changes and Pleistocene uplift rates

A flight of marine terraces along the Cuban coast records Quaternary sea‐level highstands and a general slowly uplifting trend during the Pleistocene. U/Th dating of these limestone terraces is difficult because fossil reef corals have been affected by open system conditions. Terrace ages are thus often based on geological and geomorphological observations. In contrast, the minimum age of the terraces can be constrained by dating speleothems from coastal mixing (flank margin) caves formed during past sea‐level highstands and carving the marine limestones. Speleothems in Santa Catalina Cave have ages >360 ka and show various cycles of subaerial–subaqueous corrosion and speleothem growth. This suggests that the cave was carved during the MIS 11 sea‐level highstand or earlier. Some stalagmites grew during MIS 11 through MIS 8 and were submerged twice, once at the end of MIS 11 and then during MIS 9. Phreatic overgrowths (POS) covering the speleothems suggest anchialine conditions in the cave during MIS 5e. Their altitude at 16 m above present sea level indicates a late Pleistocene uplift rate of <0.1 mm/ka, but modelling also shows uplift to have been insignificant over a long timespan during the middle Pleistocene since the cave was carved. Our study shows that some flank margin caves in the region of Matanzas are older than commonly believed (i.e. MIS 11 rather than MIS 5). These caves not only can be preserved but are good markers of interglacial sea‐level highstands, more reliable than marine abrasion surfaces. Copyright © 2018 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.     

Radiometric dating of Meipu hominin site in China by coupled ESR/U-series and cosmogenic 26Al/10Be burial dating methods

The Three Gorges and Western Hubei area in the geographic central part of China was a potential migration corridor for early hominin and mammals linking South and North China during the Pleistocene period. Some key early hominin sites are known in this region where limestone cave and fissure sites are numerous but difficult to date as beyond the dating range of OSL and mass spectrometry U-series method. Here, we report radiometric dating study for such a hominin site, Meipu (Hubei Province), by coupled ESR and U-series dating of nine fossil teeth and cosmogenic ²⁶Al/¹⁰Be burial dating of one quartz sediment. The burial age calculated by simple burial model (573 ± 266 ka) gives a minimum age constraint of the sediment. The fossil dating provided two main age groups at 541 ± 48 ka and 849 ± 39 ka, the older age group is in agreement with the U-series age (>630 ka) of the flowstone overlying the fossil layer and the paleomagnetic data which placed the Brunhes-Matuyama boundary in the fossil layer. The reason of this age difference is probably caused by the U-content discrepancy in the enamel of the dated fossil samples. This study exhibits the limitation of ESR/U-series fossil dating and the importance of using multiple dating approach when it is possible in order to identify the problematic ages.     

Accurate chronological construction for two young stalagmites from the tropical South Pacific

Modern to Holocene tropical Pacific stalagmites are commonly difficult to date with the U-series, the most commonly used dating method for speleothems. When U-series does not provide robust age models, due to multiple sources of ²³⁰Th or little U, radiocarbon is, potentially, the best alternative. The ¹⁴C content of two stalagmites (Pu17 and Nu16) collected from Pouatea and Nurau caves in the Cook Island Archipelago of the South Pacific were measured to obtain accurate chronology for their most modern parts. The bomb-pulse soil continuum modelling indicates that bomb radiocarbon in Pu17 onsets in 1956 and reaches its maximum in 1966 CE, suggesting a fast transfer of atmospheric carbon to the stalagmite of <1 year. The modelling for Pu17 suggests a 20% contribution from C₁ - an instantaneous carbon source, which renders possible an immediate transfer of atmospheric signal into the cave. Nu16 shows a slower transfer of atmospheric carbon to the stalagmite than Pu17, with bomb radiocarbon onsetting in 1957 CE and peaking in 1972 CE. The less negative δ¹³C values in Nu16 than Pu17, and also the modelling corroborated this, which points out no contribution from the instantaneous carbon source. The radiocarbon age models and laminae counting age models were then spliced to achieve a single master chronology for the top part of each stalagmite. This study is an example of ¹⁴C age modelling combined with visible physical and chemical laminae counting and how it can improve the accuracy and precision of dating for otherwise hard-to-date tropical Pacific speleothems. Such accurate and precise age models pave the way to obtain sub-annually resolved paleoclimate records by further improving the calibration of climate proxy data with the current and instrumental weather parameters.     

Transferring radiometric dating of the last interglacial sea level high stand to marine and ice core records

In order to derive a radiometric age marker for the end of the penultimate glacial–interglacial transition, we compiled published U-series isotope measurements on corals from the period extending from stage 6 to the middle of the last interglacial, and computed the corresponding open-system ages using Thompson et al. model (Thompson, W.G., Spiegelman, M.W., Goldstein, S.L., Speed, R.C., An open-system model for U-series age determinations of fossil corals. Earth Planet. Sci. Lett. 210 (2003) 365–381). We obtain a global mean age of 126 calendar kyr BP (ka) ± 1.7kyr (2σ) for the beginning of the last interglacial sea level high stand. After showing that the phase relationships observed between changes in sea level, North Atlantic benthic and planktonic foraminifera oxygen isotopic records, and atmospheric methane over the last deglaciation were likely also valid over the penultimate deglaciation, we derive an age of 131.2ka ± 2kyr (2σ) for the abrupt increase in atmospheric CH₄ and North Atlantic surface temperature marking the end of the penultimate glacial–interglacial transition. This age is consistent with U–Th dates of the penultimate glacial–interglacial transition recorded in speleothems from sites where speleothems isotopic records are synchronous with North Atlantic temperature records over the last deglaciation. Finally, we show that the phase obtained between the climatic response and northern hemisphere summer insolation is not constant from Termination II to Termination I, implying that northern hemisphere summer insolation alone cannot explain the timing of terminations.     

High-precision U–U–Th disequilibrium dating of the recent past: a review

Although it was demonstrated 20 years ago that mass spectrometric U–Th methods are capable of high-precision dating of young corals, the use of this approach to decipher recent environmental, climatic and archaeological records is still restricted and its potential has not yet been widely recognised. U–Th methods are typically used to determine the ages of carbonate materials such as speleothem and coral. Dating young carbonates of this sort is challenging. Their extremely low ²³⁰Th content necessitates stricter instrumental and laboratory conditions than those required for dating older samples. Moreover, analyses must be corrected for the presence of non-radiogenic ²³⁰Th, which is proportionally far more significant than in older samples. Nevertheless, 2σ precisions of around ±1–10 years are readily achievable for pristine coral samples dating from the last 500 years. Although the range of analytical precisions for speleothems may vary more widely depending on U concentrations and levels of non-radiogenic ²³⁰Th correction, published studies reveal 2σ precisions of around ±10–80 years for typical speleothems. This paper demonstrates how the U–Th method may be applied to establish the causes of recent coral mortality, to determine the recurrence interval of extreme wave events, to investigate earthquake frequency and neotectonic uplift, to reconstruct recent climatic history, and to understand settlement patterns and sociopolitical changes in Polynesia prior to European contact.     

U–Pb dating of speleothems from Spannagel Cave,Austrian Alps: A high resolution comparison with U–series ages

U–Pb dating is increasingly used to date speleothems that are too old for precise U–Th disequilibrium dating; however there is little data that can independently validate its application to such material. This study presents U–Pb ages for speleothems from the Spannagel Cave in the Austrian Alps including a detailed comparison with U–Th ages from an unusually U–rich sample that yields precise ages by both methods. Sample SPA4 is a flowstone with three growth phases separated by distinct hiatuses. For the youngest growth phase the U–Pb and U–Th ages are 267 ± 1 ka and 267 ± 5 ka respectively; the middle growth phase is 291 ± 1 versus 295 ± 11 ka while for the oldest growth phase a single sub-sample, assuming the same initial Pb composition as for the younger phases, yields an age of 340 ± 2 ka compared to 353 ± 9 ka by U–Th. Correlation of these ages with the marine isotope stages confirms that these speleothems grew during glacial stages as suggested by previous work on the same sample. Sample SPA 15 has U–Th isotopic compositions indistinguishable from secular equilibrium; the U–Pb data on the main growth phase of this sample give an age of 551 ± 10 ka, whereas a single analysis from the oldest phase suggests it may be on the order of 40 ka older. This detailed comparison of U–Pb and U–Th ages provides important support for the potential validity of the U–Pb method in older samples beyond the range of U–Th.     

FACTORS IMPACTING THE ACCURACY OF APATITE(U-TH)/HE DATING

Apatite (U-Th)/He dating has gained popularity since its rejuvenation as geochronometry and thermochronometry applied in the deduction of the geological processes of the upper-three-km crust.However,this irreplaceable method,which has the lowest known closure temperature (~70℃),sometimes is suffering from large dispersion and deviation because of its dating theory,its analytical method,and its diffusion process.In this paper,we summarized ten factors impacting the accuracy of (U-Th)/He dating.They are grain size,fluid and mineral inclusions,α-particle ejection,α-particle implantation,U-Th zonation,radiation damage,chemical composition,samarium concentration,multiple thermal events,and U-series disequilibrium.We discussed how these ten factors would affect the (U-Th)/He ages and how to reduce and/or avoid the deviation caused by them.The factors of grain size (different size,different diffusion domain) and inclusions (parentless ⁴He) can be suppressed in the procedure of grain selection under binocular.It is the precise measurement of a homogenous crystal that endows the (U-Th)/He dating method credible due to the correction of α-particle ejection based on the dimension of crystals.The possible implantation of α-particles can be evaluated by the negative correlation of age and eU (eU=[U]+0.235[Th]).U-Th zonation,a heterogeneous distribution of parent nuclides,makes the correction of α-particle ejection inaccurate;besides,this factor also amplifies the side effects of radiation damage and grain size.LA-ICP-MS can detect this phenomenon.Radiation damage outstands when the samples experience reheating or long residence time in partial retention zone (40~70℃),indicated by the positive correlation between age and eU/[⁴He].Apatite (U-Th)/He age can be entangled by higher Cl content,with which the crystal accumulates more radiation damage,leading to a larger age,meanwhile this factor needs more investigation.Having larger half-life than U-series nuclides,the ¹⁴⁷Sm is not a big problem in the dating of a large timescale,while more precise event needs the data of ¹⁴⁷Sm in the age calculation.The multiple thermal events should be evaluated to exclude thermal perturbation,when applying the (U-Th)/He geo/thermochronology to the deduction of a particular thermal event.U-series disequilibrium has an impact on the ages smaller than 1Ma,making the age results larger than the true one.U-series disequilibrium method and mineral couples can solve this problem.Inclusion,α-particle ejection and implantation,and U-Th zonation are at the mercy of analytical methods.The factors stemming from helium diffusion in the crystals are grain size,radiation damage,Cl content,multiple thermal events.The dating theory of (U-Th)/He method renders U-series disequilibrium and ¹⁴⁷Sm outstand as side effects in some specific conditions.It would be our pleasure if this paper could provide some useful information for the works relevant to this dating method.     

Effect of deposit alterations on the dating of herbivorous teeth from Arago cave by the ESR–U-series method

Geochemical alteration processes in archaeological sites may affect the environmental dose rate during the burial history and cause inaccuracy in age determination by palaeodosimetric dating methods, such as luminescence or ESR. In Arago Cave (Tautavel, France), the original composition of the sandy aeolian sediments of palaeoanthropological level G, where the Homo heidelbergensis Arago XXI skull was found, has been modified by both carbonation and phosphatization processes. Eight fossil teeth were collected from different geochemically affected parts of level G and analyzed by the ESR–U-series method. All the teeth are presumably contemporaneous. The results show that the samples from the phosphated area agree within error with those from the carbonated area. Surprisingly, the samples from the non-altered area show ages 100–150 ka younger. This difference is mainly due to the remarkable changes in the gamma dose rate over time. The measured in situ dose rate accounts for more than 50% of the total dose rate for all the samples. We observed that the samples' equivalent dose (D_E) were generally 20% lower in the non-altered area than in the carbonated and phosphated ones. These results show the crucial effect of the geochemical processes affecting the age calculation by ESR–U-series method in comparison with independent chronological data.     

High-precision U-series measurements of more than 500,000 year old fossil corals

Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ～ 600 ka samples with an uncertainty of better than ± 15 ka (2σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [Villemant B. and Feuillet N. (2003) Dating open systems by the ²³⁸U–²³⁴U–²³⁰Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman M.W., Goldstein S.L., and Speed R.C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of ²³⁰Th and ²³⁴U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model.