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1.
The thick alluvial conglomerate sequences around the Tibetan Plateau have been notoriously difficult to date. Here we use the cosmogenic nuclide burial dating method to date the Yumen and Jiuquan formations, a ∼900 m thick fanglomerate found in the Hexi Corridor, the foredeep of the Qilian Shan, and exposed in the Laojunmiao anticline. We date 16 sites with simple burial dating and 2 sites with isochron burial dating, and use these dates to reinterpret the magnetostratigraphy of the section. We suggest that the bottom of the Yumen Formation, defined by a progressive unconformity, is around 5 My. Taking this timing as the initiation of anticline growth, the long-term crustal shortening rate at the ramp zone in western Qilian Shan is about 0.72 mm/yr, consistent with those obtained from middle and eastern Qilian Shan. The boundary between the Yumen and Jiuquan Formations is near ∼1.2 My. Three other angular unconformities are dated to ∼2.6–3.1, ∼2.2–2.5, and ∼1.2–1.7 My, respectively. Burial dating offers a robust chronology for these deposits, and when combined with paleomagnetic stratigraphy offers much tighter precision.  相似文献   

2.
Existing methods of cosmogenic nuclide burial dating perform well provided that sediment sources undergo steady rates of erosion and the samples experience continuous exposure to cosmic rays. These premises exert important limitations on the applicability of the methods. And yet, high mountain sediment sources are rife with transient processes, such as non-steady erosion by glacial quarrying and/or landsliding, or temporary cosmic-ray shielding beneath glaciers and/or sediment. As well as breaching the premises of existing burial dating methods, such processes yield samples with low nuclide abundances and variable 26Al/10Be ratios that may foil both isochron and simple burial-age solutions. P–PINI (Particle-Pathway Inversion of Nuclide Inventories) is a new dating tool designed for dating the burial of sediments sourced from landscapes characterized by abrupt, non-steady erosion, discontinuous exposure, and catchments with elevation-dependent 26Al/10Be production ratios. P–PINI merges a Monte Carlo simulator with established cosmogenic nuclide production equations to simulate millions of samples (10Be–26Al inventories). The simulated samples are compared statistically with 10Be–26Al measured in field samples to define the most probable burial age. Here, we target three published 10Be–26Al datasets to demonstrate the versatility of the P–PINI model for dating fluvial and glacial sediments. (1) The first case serves as a robust validation of P–PINI. For the Pulu fluvial gravels (China), we obtain a burial age of 1.27 ± 0.10 Ma (1σ), which accords with the isochron burial age and two independent chronometers reported in Zhao et al. (2016) Quaternary Geochronology 34, 75–80. The second and third cases, however, reveal marked divergence between P–PINI and isochron-derived ages. (2) For the fluvial Nenana Gravel (USA), we obtain a minimum-limiting burial age of 4.5 ± 0.7 Ma (1σ), which is compatible with unroofing of the Alaska Range starting ∼ 6 Ma, while calling into question the Early Pleistocene isochron burial age presented in Sortor et al. (2021) Geology 49, 1473–1477. (3) For the Bünten Till (Switzerland), we obtain a limiting burial age of <204 ka (95th percentile range), which conforms with the classical notion of the most extensive glaciation in the northern Alpine Foreland assigned to the Riss glaciation (sensu marine isotope stage 6) contrary to the isochron burial age presented in Dieleman et al. (2022) Geosciences, 12, 39. Discrepancies between P–PINI and the isochron ages are rooted in the challenges posed by the diverse pre-burial 26Al/10Be ratios produced under conditions characteristic of high mountain landscapes; i.e., non-steady erosion, discontinuous cosmic-ray exposure, and elevation-dependent 26Al/10Be production ratios in the source region, which are incompatible with the isochron method, but easily accommodated by the stochastic design of P–PINI.  相似文献   

3.
We applied both single-sample and isochron methods of cosmogenic-nuclide burial dating to determine the age of the sedimentary fill in Unaweep Canyon, western Colorado, USA. This stratigraphic sequence is of interest because it documents capture and diversion of the ancestral Gunnison River by the Colorado River during late Cenozoic incision of the Colorado Plateau. Seven 26Al–10Be burial ages from sedimentary infill penetrated by a borehole in central Unaweep Canyon, as well as a 26Al–10Be burial isochron age formed by multiple clasts and grain-size separates in a sample from the stratigraphically lower Gateway gravels, indicate that canyon blockage, initiation of lacustrine sediment accumulation, and presumed river capture, took place 1.41 ± 0.19 Ma. Lacustrine sedimentation ceased 1.34 ± 0.13 Ma.  相似文献   

4.
Our understanding of the chronological sequence of Pleistocene land mammal faunas (including hominids) preserved in caves and fissures in the karstic terrain of subtropical southern China has been greatly impeded by the discontinuous nature of the strata yielding the fossils. A contributory issue is the lack of suitable elements for numerical dating, especially for the Early Pleistocene cave sediments. Here we present new magnetostratigraphic dating results for five cave sedimentary sequences in the Bubing Basin, Guangxi Zhuang Autonomous Region, which contain the typical Pleistocene Gigantopithecus fauna and Ailuropoda–Stegodon fauna of southern China. The five karstic caves are situated at elevations ranging from 252 m to 191 m above sea level. The integrated dating results reveal that the five cave sedimentary sequences span an age range from the late Gauss normal Chron to the early Brunhes normal Chron. The faunas are assigned to >2.6 Ma (Mohui–houshan Cave), 1.95–1.78 Ma (Mohui Cave), 0.99–0.78 Ma (Baolai Cave) and ∼0.7 Ma (Upper Wuyun and Ganxian Caves). Furthermore, by combining our new magnetostratigraphy work with previously published magnetostratigraphic, U–series and electron spin resonance dating results, a chronological sequence of the Plio–Pleistocene mammalian faunas from cave sites in southern China has been established. The chronological sequence contributes to an improved understanding of the evolutionary processes of Plio–Pleistocene land mammals, as well as of early humans in subtropical southern China.  相似文献   

5.
We review the relative motion of India and Asia for the last 100 million years and present a revised reconstruction for the India–Antarctica–Africa–North America–Eurasia plate circuit based on published motion histories. Deformation of these continental masses during this time introduces uncertainties, as does error in oceanic isochron age and location. Neglecting these factors, the data ipso facto allow the inference that the motion of India relative to Eurasia was distinctly episodic. Although motion is likely to have varied more smoothly than these results would allow, the geological record also suggests a sequence of distinct episodes, at about the same times. Hence we suggest that no single event should be regarded as the collision of India with Asia. The deceleration of the Indian plate commencing at ∼65 Ma is matched by an equally significant prior acceleration and this aspect must be taken into account in geodynamic scenarios proposed to explain the collision of India with Asia.  相似文献   

6.
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.  相似文献   

7.
In this study, we use isochron‐burial dating to date the Swiss Deckenschotter, the oldest Quaternary deposits of the northern Alpine Foreland. Concentrations of cosmogenic 10Be and 26Al in individual clasts from a single stratigraphic horizon can be used to calculate an isochron‐burial age based on an assumed initial ratio and the measured 26Al/10Be ratio. We suggest that, owing to deep and repeated glacial erosion, the initial isochron ratio of glacial landscapes at the time of burial varies between 6.75 and 8.4. Analysis of 22 clasts of different lithology, shape, and size from one 0.5 m thick gravel bed at Siglistorf (Canton Aargau) indicates low nuclide concentrations: <20 000 10Be atoms/g and <150 000 26Al atoms/g. Using an 26Al/10Be ratio of 7.6 (arithmetical mean of 6.75 and 8.4), we calculate a mean isochron‐burial age of 1.5 ± 0.2 Ma. This age points to an average bedrock incision rate between 0.13 and 0.17 mm/a. Age data from the Irchel, Stadlerberg, and Siglistorf sites show that the Higher Swiss Deckenschotter was deposited between 2.5 and 1.3 Ma. Our results indicate that isochron‐burial dating can be successfully applied to glaciofluvial sediments despite very low cosmogenic nuclide concentrations. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

8.
Optically stimulated luminescence (OSL) dating has been applied to determine the burial age of sediment horizons within a sequence containing Lower Palaeolithic artefacts on an upland site associated with a solution feature (doline) at West Cliffe, located on the North Downs in Kent, UK. The study makes use of a novel extension of the single aliquot OSL measurement procedure to investigate the nature of significant overdispersion in equivalent dose values with very small aliquots of quartz inclusions, enabling single grain resolution to be approached with ∼90 μm diameter quartz extracted from the relatively fine-grained brickearth. A detailed examination of the uniformity of the distribution of radionuclide sources in the sampled volumes was also performed and this included the application of a spatially-resolved technique for beta dose rate measurement. The OSL ages, obtained for the burial of brickearth positioned stratigraphically below and above a clay and flint clast layer containing the artefacts and debitage, place the deposition of the artefacts to between ca 140 and 80 ka ago. This is significantly later than indicated by the artefact typology (>300 ka) and contrary to the expectation of in situ burial indicated by earlier research in this region. If displacement occurred on other upland sites this finding has important implications for establishing the timing of hominin use of the upland areas which, beyond broad attribution to Lower or Middle Palaeolithic origin, is uncertain and similar doubts apply to the interpretation of the environments that prevailed.  相似文献   

9.
The most extensive terrestrial outcrops of glacial and glaciomarine deposits in the Eastern Canadian Arctic are exposed in sea cliffs along the Clyde Foreland and Qivitu Peninsula of Baffin Island. Collectively known as the Clyde Foreland Formation (CFF), these stacked deposits record at least seven glacial advances. Despite having been the focus of numerous investigations spanning nearly 50 years, no numerical chronological framework for the age of the deposits has been established. Previous studies relied on biostratigraphy and amino acid racemization (AAR) geochronology and postulated that the oldest units were Late Pliocene to Mid-Pleistocene in age. In this paper, we use a cosmogenic radionuclide isochron approach to determine a minimum age for the burial of a paleosol preserved within the CFF. Abundant palynomorphs in the paleosol are dominated by cool-climate taxa. Combining the paleosol burial age with a compilation of published and new CFF AAR data for marine bivalves Hiatella arctica and Mya truncata, we statistically define seven CFF aminozones and develop a piecewise isoleucine AAR calibration model for Baffin Island. From this, we estimate the minimum age of each aminozone, although the propagation of errors through all calculations produces large uncertainties for each age estimate. The youngest three CFF units, known as the Kogalu, Kuvinilk, and Cape Christian members, were most likely deposited during glaciations in the Mid- to Late-Pleistocene. The paleosol formed prior to 1.15 ± 0.20 Ma, and the underlying aminozones represent sedimentation during Early Pleistocene or latest Pliocene glaciations and record early advances of Laurentide ice across Baffin Island.  相似文献   

10.
This study reconstructs the late stages in the exhumation history of a nappe derived from the Meliatic accretionary wedge in the Western Carpathians by means of zircon and apatite (U-Th)/He dating. The Meliatic accretionary wedge formed due to the closure of the Neotethyan Triassic–Jurassic Meliata–Hallstatt Ocean in the Late Jurassic. The studied fragments of the blueschist-bearing Meliatic Bôrka Nappe were metamorphosed at low-temperature and high- to medium-pressure conditions at ca. 160–150 Ma and included into the accretionary wedge. The time of the accretionary wedge formation constrains the beginning of the Bôrka Nappe northward thrusting over the Gemeric Unit of the evolving Central Western Carpathians (CWC) orogenic wedge. The zircon (U-Th)/He data on four samples recorded three evolutionary stages: (i) cooling through the ∼180 °C isotherm at 130–120 Ma related to starting collapse of the accretionary wedge, following exhumation of the high-pressure slices in the Meliatic accretionary wedge; (ii) postponed exhumation and cooling of some fragments through the ∼180 °C isotherm from 115 to 95 Ma due to ongoing collapse of this wedge; and (iii) cooling from 80 to 65 Ma, postdating the thrusting (∼100–80 Ma) of the Bôrka Nappe slices during the Late Cretaceous compression related to formation of the CWC orogenic wedge. The third stage already documents cooling of the Meliatic Bôrka Nappe slices in the CWC orogenic wedge. The apatite (U-Th)/He data may indicate cooling of a Bôrka Nappe slice to near-surface temperatures at ∼65 Ma. The younger AHe age clusters indicate that at least one, or possibly two, reheating events could have occurred in the longer interval from ∼40 to ∼10 Ma during the Oligocene–Miocene. These were related to sedimentary burial and/or the magmatism as documented in other parts of the CWC.  相似文献   

11.
Zircon U–Pb dating using LA-ICP-MS was applied to six Quaternary tephras in Boso Peninsula, central Japan: J1, Ks4, Ks5, Ks10, Ks11, and Ch2 in descending order. Accurate age determination of these tephras is of critical importance because they are widespread tephras in Japan and also relevant to a candidate site for the global boundary stratotype section and point of the early–middle Pleistocene boundary. Twenty grains were dated for each tephra and the following results were obtained. The J1 tephra had only 5 grains that yielded <2 Ma. The obtained age was ∼0.2 m.y. older than the stratigraphic age. No Quaternary ages were obtained from the Ks4 tephra. The Ks5 and Ks10 tephras had 10–12 grains that were ∼0.1–0.3 m.y. older than the stratigraphic age. The Ks11 tephra had 14 grains that yielded a weighted mean age of 0.52 ± 0.04 Ma (error reported as 95% confidence level), which was in agreement with the stratigraphic age. The Ch2 tephra had 16 grains that yielded a weighted mean age of 0.61 ± 0.02 Ma, which was also in agreement with the stratigraphic age. The good agreement between zircon U–Pb ages and the stratigraphy for Ks11 and Ch2 tephras validates the reliability of the established stratigraphy and our dating approach. The other tephras that yielded ∼0.1–0.3 m.y. older ages than the stratigraphy may indicate that the analyzed zircons were antecrysts that crystallized before eruption or they were detrital zircons incorporated during deposition.  相似文献   

12.
A much improved high-resolution magnetostratigraphy is presented for the interval between 8.5 and 12.5 Ma in the deep marine astronomically tuned Monte dei Corvi Beach section in northern Italy. This section contains the formally designated boundary between the Middle and Upper Miocene defined by the Tortonian GSSP. The natural remanent magnetization is carried by an unusual low-temperature component of primary origin. According to interpretations of isothermal remanent magnetization (IRM) acquisition curves, the palaeomagnetic signal is most probably carried by iron sulphides, such as greigite or pyrrhotite. The resultant magnetostratigraphy is reliable and can be straightforwardly calibrated to the Astronomically Tuned Neogene Time Scale (ATNTS2004) showing that the section ranges from Chron C5An.2n up to C4.2r-1. The inferred correlation of the Tortonian GSSP to the older part of Chron C5r.2n is confirmed, thereby guaranteeing global correlation potential. In addition, the Monte dei Corvi Beach section has been re-tuned to the new numerical solution La2004 for the interval between 8.5 and 12.9 Ma. This exercise shows that the characteristic cycle patterns are not consistent with a single solution with fixed values for dynamical ellipticity and tidal dissipation. For the younger part of the section, sedimentary cycles show an excellent fit with intricate details of precession/obliquity interference in the nominal La2004(1,1) solution. However, this solution produces serious mismatches between the sedimentary cycles and the precession/obliquity interference in the interval older than ∼ 10.2 Ma. For this interval changes in tidal dissipation value (while keeping dynamical ellipticity constant at its present-day value) are necessary to maintain a perfect fit. A detailed visual comparison reveals a very good to excellent fit with La2004(1,1.2) and, to lesser extent, with La2004(1,0.5). However, La2004(1,1.2) is the preferred tuning target for the moment in the interval older than 10.2 Ma. The astronomical (re)tuning on precessional scale to La2004(1,1) yielded slightly different astronomical ages for each basic cycle and, hence, also for the calcareous plankton events and magnetic reversal boundaries with uncertainties ranging within a few thousand years. The age of the Tortonian GSSP turns into 11.625 Ma instead of the previously published 11.608 Ma. Our ages should replace existing ages in the Astronomically Tuned Neogene Time Scale (ATNTS2004).  相似文献   

13.
We present a Bayesian isochron approach to interpret measurements of multiple cosmogenic nuclides from glacially modified bedrock surfaces with complex exposure histories. An isochron approach explicitly incorporating glacial erosion is ideally suited for this problem; such erosion must be accounted for but has traditionally been ignored. Previous methods required treating each sample individually (to account for glacial erosion) and subsequently averaging results for the entire dataset. Geological considerations, however, suggest a more robust approach is to treat samples in the dataset here (and samples from other conceivable datasets) simultaneously. The Bayesian isochron method is applied to a previously published set of in situ 14C and 10Be measurements from a set of samples spanning the forefield of the Rhone Glacier, Switzerland. Results indicate 6.4 ± 0.5 kyr of integrated exposure and 4.7 ± 0.5 kyr of cumulative burial, similar to previous estimates, but with much smaller uncertainties. The reduced uncertainties result from fitting the exposure and burial duration to the entire dataset, while explicitly accounting for glacial erosion. The method presented here should be applicable with minor modifications in a number of geologic settings, and further demonstrates the utility of paired in situ 10Be and 14C measurements for unraveling complex exposure histories over during the Holocene and late Pleistocene.  相似文献   

14.
In the Arctic Ocean, direct dating methods are needed as an alternative to the radiocarbon (14C) method and to various indirect approaches for a longer stratigraphy. In past attempts to develop a luminescence sediment dating, the use of fine-silt (4–11 μm) mixture of quartz and feldspar grains from core tops has often produced large age overestimates by several ka. A recent application of micro-focused laser (‘micro-hole’) photon-stimulated luminescence (PSL) to medium-silt to fine-sand quartz grains (11–105 μm) from the core tops at the Alaska margin has been usefully accurate. To extend this approach to the central Arctic Ocean and to a larger grain size range, we applied micro-hole PSL dating to >11 μm quartz grains from core tops (0.5–2 cm horizon) from two sites on the central Lomonosov Ridge. We obtain a burial age estimate of ca. 2 ka for 11–62 μm grains at a multicore site 18 MC within a perched intra-ridge basin, in accord with 14C ages obtained on foraminifers. At nearby site 19 MC on the erosive ridge top, the micro-hole PSL dating of >90 μm quartz grains produces a burial age estimate of ∼ca. 25 ka, in accord with a foraminiferal 14C age of ca. 26 ka. However, the 11–90 μm grains from the same sample produce a much younger burial age estimate of ca. 9 ka. Thus, these two size fractions of quartz grains record different burial times and different deposition agents (icebergs vs. sea ice), providing insight into past sedimentary processes. Overall, our results confirm an earlier conclusion from micro-hole PSL dating study at the Alaska margin that medium to coarse silt fractions of quartz grains (11–90 μm or at least 62 μm) is the preferred material for direct dating of the last daylight exposure of detrital sediment in the Arctic Ocean.  相似文献   

15.
In the central Western Alps, a combined structural, petrological and 40Ar–39Ar geochronological study of the Modane-Aussois and Southern Vanoise units yields important constraints on the timing of deformation and exhumation of the Briançonnais zone. These data help to decipher the respective roles of oceanic subduction, continental subduction and collision in the burial and exhumation of the main units through time. In the Modane-Aussois unit top to the NW thrusting (D1) was followed by top to the east shearing (D2) interpreted by some as normal faulting and by others as backthrusting. Pseudosection calculations imply that D1 deformation occurred at 1.0 ± 0.1 GPa and 350 ± 30 °C. Analysis of chlorite–phengite pairs yield P–T estimates between 0.15 and 0.65 GPa and between 220 and 350 °C for the D2 event. Phengites along the D1 schistosity (sample M80) yields an 40Ar–39Ar age of 37.12 ± 0.39 Ma, while D2 phengites yield ages of 35.42 ± 0.38 (sample M173) and 31.60 ± 0.33 Ma (sample M196). It was not possible to test whether these ages are altered by excess argon or not. Our interpretation is that the D1/D2 transition occurred at ∼37 Ma at the beginning of decompression, and that D2 lasted until at least ∼32 Ma. Pseudosection calculation suggests that the Southern Vanoise unit was buried at 1.6 ± 0.2 GPa and 500–540 °C. D1 deformation occurred during exhumation until 0.7–10.5 GPa and 370 ± 30 °C. Published ages suggest that D1 deformation possibly started at ∼50 Ma and lasted until ∼37 Ma. D2 deformations started at P–T conditions close to that recorded in Modane-Aussois unit and lasted until 0.2 ± 0.1 GPa and 280 ± 30 °C at ∼28 Ma. The gap of 0.6 ± 0.3 GPa and 150 ± 130 °C between peak metamorphic conditions in the two units was concealed by thrusting of the South Vanoise unit on top of the Modane-Aussois unit during D1 Deformation. Top to the east deformation (D2) affects both units and is interpreted as backthrusting.Based on these data, we propose a geodynamic reconstruction where the oceanic subduction of the Piedmont unit until ∼50 Ma, is followed by its exhumation at the time of continental subduction of the continental Southern Vanoise unit until ∼45 Ma. The Southern Vanoise is in turn underthrusted by the Modane-Aussois unit until ∼37 Ma (D1). Between 37 and 31 Ma the Modane-Aussois and Southern Vanoise units exhume together during backthrusting to the east (D2). This corresponds to the collision stage and to the activation of the Penninic Thrust. In the ∼50 Ma to ∼31 Ma time period the main thrusts propagated westward as the tectonic context switched from oceanic to continental subduction and finally to collision. During each stage, external units are buried while internal ones are exhumed.  相似文献   

16.
The Adelaide rift complex in South Australia contains the type sections for Sturtian and Marinoan glacial deposits. The litho- and chemo-stratigraphy of these deposits play a central role in evaluating global Neoproterozoic ice age hypotheses and Rodinia supercontinent reconstructions, but reliable depositional age constraints have been extremely limited. We report results of in situ Th–U–total Pb (electron microprobe) dating of detrital and authigenic monazite in two samples from the Umberatana Group (Sturtian Holowilena Ironstone and pre-Marinoan Enorama Shale) in the Central Flinders Ranges. Several texturally and chemically distinct detrital and authigenic populations are recognized. Detrital dates range from 1600 Ma to 760 Ma and most relate to well-known orogenic or igneous events in surrounding cratonic regions. Authigenic monazite grew in three or more pulses ranging from 680 Ma to 500 Ma. The date of 680 ± 23 Ma (2σ) for the earliest generation of authigenic monazite in sandstone from the Enorama Shale (1) provides an estimate for the age of the base of the Trezona carbon isotopic anomaly just beneath the Marinoan glacial deposits, (2) provides an absolute minimum age constraint on the underlying Sturtian glacial deposits, and (3) supports proposed correlations between type Marinoan deposits and precisely dated glacial deposits in Namibia and China, which bracket the presumed Marinoan equivalents between 655 and 635 Ma. This age is inconsistent with a Re–Os isochron age of 643 ± 2.4 Ma (2σ) on shales near the bottom of the Sturtian–Marinoan interglacial succession, stratigraphically > 3000 m below the Enorama Shale sample, and militate against the hypothesis that the type Marinoan is correlative with the 580 Ma Gaskiers glaciation. Monazite growth near 600 Ma and again at about 500 Ma probably represent hydrothermal fluid-flow events, the latter of which also corresponds to the well-known Delamerian Orogeny during which the Adelaide sediments were folded into their present structural pattern.  相似文献   

17.
To improve the French Plio-Pleistocene biostratigraphy scheme based on mammal biozone boundaries or “Mammal Neogene/Quaternary Zones” (MNQ) we collected volcanic material that could be dated using the 40Ar/39Ar method in five exceptional mammalian paleofauna sites located in the Massif Central (France). We present 40Ar/39Ar ages that we obtained for Perrier-Les Etouaires, Roca-Neyra, Chilhac, Senèze and le Creux de Peyrolles. We show that the overall stratigraphic position of these sites based on faunal assemblages is valid from the relative point of view. However, we greatly improve both the accuracy and precision of the age of these mammalian paleofaunas. We obtained 40Ar/39Ar ages varying between 2.78 ± 0.01 Ma (1σ external) for Les Etouaires (Lower MNQ 16b) and 1.47 ± 0.01 Ma for the Creux de Peyrolles site (MNQ 19). Based on these new dates we estimate the duration of several biozones including MNQ 16b, 17a and 17b. We suggest that the first Late Villafranchian biozone (MNQ 18) starts as early as the Reunion subchron or just after. The first occurrence of Equus stenonis in Roca-Neyra (i.e. 2.60 ± 0.02 Ma) is close to or synchronous with the Gauss–Matuyama transition (i.e. 2.59 Ma) and the Pliocene/Pleistocene boundary. The chronological framework we build shows the very rapid increase of the large grazers community in French faunal assemblages at the beginning of MNQ 17 (i.e. 2.6 to 2.4 Ma). This rapid faunal turnover is probably associated with a general decrease of woodland habitat in the Massif Central contemporaneous with the onset of the Northern Hemisphere glaciations. The faunal assemblages in France, Spain, and Italy covering the period between 2.1 and 2.0 Ma suggest that favorable conditions for early hominin settlement (mainly savannah prairies, grassland with open forest patches) existed in southwestern Europe at least 200 ka before the first traces of Homo in Eurasia. This period also shows the arrival of taxa originating in Asia and Africa, suggesting dispersal events within southwestern Europe well before the Olduvai subchron and with no indication (as yet) of Homo as a “fellow traveler”.  相似文献   

18.
Dating agricultural artefacts such as field walls and clearance cairns using radiocarbon can be challenging, especially since the association with datable material may be poor. Rock surface burial dating using luminescence offers an alternative. Here we report on the luminescence dating of a medieval circular stone-walled enclosure at Sønnebøe, northern Scania, Sweden, using both buried rocks and sediments. Luminescence burial profiles from IRSL signals measured at 50 °C (IR50) indicated significant prior light exposure in 7 of the 8 samples tested (5 granite, 2 felsic gneiss), in some cases multiple exposure burial cycles were indicated. These rock surfaces had apparently been exposed for sufficient time to allow accurate IRSL ages for the most recent burial event. In contrasts, no useful post-IR IRSL profiles were obtained indicating that this signal was not sufficiently reset to allow accurate determination of the burial dose on any of these rocks. IR50 fading corrections (typically ∼50%) were derived by comparing field saturation with that induced in the laboratory. Quartz extracted from sediments surrounding the rocks gave an average measured to given dose ratio of 1.03 ± 0.01 (n = 90), and these sediment samples were then dated using multigrain aliquots; the corresponding feldspar dose recovery ratio obtained using rock samples was 0.98 ± 0.05 (n = 28). A total of 15 ages were derived; 8 quartz OSL ages from the disturbed coarse grained sediments surrounding the structure, and 7 fading corrected IR50 ages from the surfaces of rocks (2–3 mm chips, ∼1 mm thick) used in the construction of the structure itself. The exposure events preserved by the ring enclosure stones unequivocally show wall building taking place at the site between 800 and 300 years ago.  相似文献   

19.
The Ceprano calvarium, found in 1994 in Italy and attributed to Homo cepranensis, is one of the most celebrated hominin remains of Europe. It was considered at least 700 ka-old until a recent investigation incorporating magnetostratigraphy and K-Ar ages from the literature assigned to the calvarium an age of ∼450 (+50, −100) ka. Here we pin down the age of the Ceprano calvarium to 353 ± 4 ka (±1σ external) by means of new 40Ar/39Ar dating on K-feldspars retrieved from the sediments that hosted the skull. In absence of evidence of reworking, this refined age sinks the conviction that H. cepranensis belonged to human evolution at the Brunhes–Matuyama boundary (c.a. 781 ka). Our refined age indicates that H. cepranensis lived in central Italy probably during the cold period of marine isotope stage (MIS) 10, and that despite his archaic morphology and lack of Neanderthal traits, he was contemporaneous with more advanced species such as H. heidelbergensis.  相似文献   

20.
The ～ 14 km diameter Jänisjärvi impact structure is located in Svecofennian Proterozoic terrain in the southeastern part of the Baltic shield, Karelia, Russia. Previous radioisotopic dating attempts gave K/Ar and 40Ar/39Ar ages of 700 ± 5 Ma and 698 ± 22 Ma, respectively, with both results being difficult to interpret. Recent paleomagnetic results have challenged these ages and proposed instead ages of either 500 Ma or 850–900 Ma. In order to better constrain the age of the Jänisjärvi impact structure, we present new 40Ar/39Ar data for the Jänisjärvi impact melt rock. We obtained five concordant isochron ages that yield a combined isochron age of 682 ± 4 Ma (2σ) with a MSWD of 1.2, P = 0.14, and 40Ar/36Ar intercept of 475 ± 3. We suggest that this date indicates the age of the impact and therefore can be used in conjunction with existing paleomagnetic results to define the position of the Baltica paleocontinent at that time. Argon isotopic results imply that melt homogenization was achieved at the hundred-micrometer scale certainly, because of the low-silica content of the molten target rock that allows fast 40Ar? diffusion in the melt. However, the large range of F(40Ar?inherited) (4.1% to 11.0%) observed for seven grains shows that complete isotopic homogenization was not reached at the centimeter and perhaps the millimeter scale. The F(40Ar?inherited) results are also in good agreement with previous Rb and Sr isotopic data.  相似文献   

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