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1.
The short residence times of Th and Pa in seawater make them very responsive to changes in the ocean environment. We use a new multi-ion-counting technique to make Th and Pa isotope measurements in seawaters from a near-shore environment in which oceanic chemical tracers are not overwhelmed by terrestrial inputs (the Bahamas). An unusual feature of the Bahamas setting is the shallow depth of water residing on the bank tops. These waters have significantly lower 232Th/230Th (∼10,000) than those immediately adjacent to the banks (24,000-31,000) and a (231Pa/230Th) near the production ratio (∼0.1). The change in 232Th/230Th and (231Pa/230Th) on the bank tops is explained by almost quantitative removal of Th and Pa by scavenging, and their replacement with a mixture of 230Th and 231Pa alpha-recoiled from the underlying carbonates, together with Th from dust dissolution. Analysis of a water profile in the Tongue of the Ocean, which separates the Great and Little Bahama Banks, allows us to trace the movement of bank-top water to depth. A distinct minimum in both 232Th/230Th (∼13,000) and (231Pa/230Th) (∼0.5) is observed at ∼430 m and is interpreted to reflect density cascading of bank-top water with entrained carbonate sediment. These results suggest that Th and Pa can be used as water-mass tracers in near-shore environments. Uranium concentration measurements on the same waters demonstrate that U is conservative across a range in salinity of 2 psu, with a concentration of 3.33 ppb (at a salinity of 35).The incorporation of U and Th isotopes into marine carbonates has also been assessed by analyzing carbonate samples from the same location as these Bahamas waters. Such incorporation is critical for U-Th geochronology. U isotope analyses demonstrate that seawater δ234U averages 146.6 and does not vary by more than 2.5%o, and that carbonates capture this value. Additional high precision measurements (≈±1%o) on modern carbonates confirm that all oceans have identical δ234U. Modern marine carbonates are shown to have 232Th/230Th ratios that reflect the local seawater in which they formed.  相似文献   

2.
Deep-sea coral has proved useful for paleoceanographic reconstructions and for documenting 14C-ages of water masses using 230Th-ages. However, for precise and accurate U-series dating, further information on coral-age structure, growth rate and diagenetic evolution is still needed. To document such processes, we used U-Th-226Ra systematics in a 40 cm diameter, Lophelia pertusa specimen collected in 1912 from the Eastern Atlantic (Sea of the Hebrides). External parts of the specimen are thought to have been alive when collected whereas more internal parts were likely dead. The “live-collected” and “dead” parts of the skeleton were sampled and analyzed for their 230Th, 232Th, 234U, 238U, 226Ra and Ba contents by thermal ionization and multicollector inductively coupled plasma mass spectrometry. 230Th/234U ratios in the most recent parts yielded ages of 86 ± 6 a and 92 ± 9 a, in agreement with the date of recovery. The older parts yielded 230Th ages ranging from 169 ± 15 to 211 ± 10 a (n = 5), but had a 31% higher U content than more recent parts of the skeleton. This raises concerns about the possibility of secondary diagenetic U uptake, although an environmentally controlled U/Ca shift between coral growth stages cannot be ruled out. 226Ra/Ba measurements were made, and model- 226Ra/Ba ages averaging 250 ± 12 yr were calculated for the older part, assuming a constant initial 226Ra/Ba ratio in bottom waters. These ages are slightly older than 230Th-ages, suggesting either that 226Ra/Ba ratios of ambient-seawater changed over time or that a diagenetic phenomenon have affected the U-series system, or both. Scanning electron microscope observations revealed bioerosion and secondary biomineralization in the older part of the coral skeleton, supporting the hypothesis that diagenetic processes may have influenced the ages obtained by the U-series toolbox. Modeled U-series ages for such an open system are discussed. However, a comparison between 14C- and 230Th-ages performed on both pristine and bioeroded parts of the coral gives coherent values (ca 450 a) for the preindustrial 14C-reservoir age of North Atlantic waters. It remains to be determined, however, whether diagenesis occurs rapidly over a short period of time, or whether it continues for longer periods. In the latter case, diagenetic processes would hamper paleoceanographic interpretations as well as the precise calculation of 14C ages of deep-water masses, based on comparative U/Th- and 14C-chronologies.  相似文献   

3.
The organic deposits derived from the mangrove swamps form reliable stratigraphic markers within the Late Quaternary sequence of Kerala–Konkan Basin. Three generations of such deposits have been identified. The older one is dated to around 43,000–40,000 14C yr B.P., with a few dates beyond the range of radiocarbon. The younger ones date from the Middle Holocene to latest Pleistocene (10,760–4540 14C yr B.P.) and the Late Holocene (<4000 14C yr B.P.). Pollen analyses confirm that the deposits are mostly derived from the mangrove vegetation. Peat accumulation during the period 40,000–28,000 14C yr B.P. can be correlated with the excess rainfall, 40–100% greater than modern values, of the Asian summer monsoon. The low occurrence of mangrove between 22,000 and 18,000 14C yr B.P. can be attributed to the prevailing aridity and/or reduced precipitation associated worldwide with Last Glacial Maximum, because exposure surfaces and ferruginous layers are commonly found in intervals representing this period. The high rainfall of 11,000–4000 14C yr B.P. is found to be the most significant as the mangrove reached an optimum growth around 11,000 14C yr B.P. but with periods of punctuated weaker monsoons. From the present and previous studies, it has been observed that after about 5000 or 4000 14C yr B.P., the monsoons became gradually reduced leading to drying up of many of the marginal marine mangrove ecosystems. A case study of Hadi profile provided an insight to the relevance of magnetic susceptibility (χ) to record the ecological shift in Late Holocene.  相似文献   

4.
We present the first application of cross-dating (Th/U measured by thermo-ionization mass spectrometry (TIMS) and 14C measured by accelerator mass spectrometry (AMS)) of calcite covering prehistoric paintings. Th/U age estimates of cave drapery range from 9800 to 27,300 yr B.P. while conventional 14C age is estimated between 9900 and 7610 yr B.P. depending on the dead carbon correction. The age discrepancy is attributed to a disturbance of Th/U and/or 14C geochemical systems, showing the limits of the geochronological approach applied to this kind of material. For the Th/U system, the poor consistency of U data (U content, 234U/238U activity ratios) and apparent ages argue for open system conditions. For 14C system, variation of the dead carbon fraction (dcf) and a possible mixing of successive generations of calcite could account for age discrepancy. Nevertheless, one sample shows concordant ages for the two methods. Compatible ages through corrections for open system conditions are assumed for other samples. Then, the cross-dating suggests 9900 yr as the minimum age of the piece of drapery; the underlying painting must be older. This study of rock art demonstrates the presence of a Pleistocene population before 9900 yr in the southeast of Borneo, whereas previously the only population in evidence in this area was of Austronesian type from ∼5000 to 6000 yrs ago.  相似文献   

5.
6.
Palynologic and stratigraphic data from Laguna Tahui (42°50′S, 73°30′W) indicate cool-temperate and humid conditions there between 14,000 and 10,000 14C yr B.P., followed by warmer and drier-than-present conditions between 10,000 and 7000 14C yr B.P., and subsequent cooling and rise in precipitation over the last 5800 14C yr. The thermophilous Valdivian trees Eucryphia cordifolia and Caldcluvia paniculata reached their maximum abundance during the early Holocene warm-dry phase (10,000-7000 14C yr B.P.), followed by a rise in lake levels and reexpansion of North Patagonian conifers starting at 7000 and 5800 14C yr B.P., respectively. Variations in the stratigraphic and geographic distribution of temperate rainforests in southern Chile suggest multimillennial trends in temperature and westerly activity, which are spatially and temporally coherent with paleoclimate records from neighboring regions. Climate variability at millennial and submillennial time scales may account for the establishment and persistence of fine-scale mosaics of Valdivian and North Patagonian rainforest species in low- to mid-elevation communities since ∼5800 14C yr B.P.  相似文献   

7.
《Applied Geochemistry》2003,18(11):1691-1703
Calcareous deposits, mainly consisting of mollusc shell accumulations, which have been dated by the U/Th disequilibrium method, mark the shorelines of paleolake highstands in the Great Chotts Area of Southern Tunisia. The 5 sites studied consist of discontinuous accumulations of fossils of marine-like organisms e.g.: Cerastoderma glaucum, Melania tuberculata, Melanopsis praemorsa, Cerithium rupestre. U/Th isochron plots and age frequency histograms for 39 shell samples are reported here. Limited variations for U content and 234U/238U activity ratios (AR) of shells support the hypothesis of closure of the geochemical system with respect to this element. It is remarkable that 234U/238U AR of shells collected in Chott Fejej or Chott Jerid are clustered around different values, reflecting probably different groundwater recharge from the Continental Intercalaire (CI) or Complexe Terminal (CT) aquifers. Furthermore waters collected near Wadi el Akarit show 234U/238U AR values comparable to those observed for shells. 14C determinations made on aliquots of some of these samples suggested an age distribution between 18 and 34 ka BP. The U/Th data of these 39 shell samples imply that 4 distinct flood episodes of these lakes occurred at about 30, 95–100, 130–150 and 180–200 ka. For the episode centred around 30 ka, the frequency histogram of ages shows a multimodal age group that could represent the existence of several humid pulses rather than a unique event. Moreover, the comparison of δ13C and δ18O with those of older humid Pleistocene phases, when very large palaeolakes have been recorded, suggests that these young carbonate shells are not related to a true highstand lake. It is suggested that they represent a period of less humid climatic conditions with carbonate accumulation in minor water ponds in which intensive biological activity could have taken place. It should be noted that this period was less arid than the present.  相似文献   

8.
Reliable and precise ages of Quaternary pedogenic carbonate can be obtained with 230Th/U dating by thermal ionization mass spectrometry applied to carefully selected milligram-size samples. Datable carbonate can form within a few thousand years of surface stabilization allowing ages of Quaternary deposits and surfaces to be closely estimated. Pedogenic carbonate clast-rinds from gravels of glacio-fluvial terraces in the Wind River Basin have median concentrations of 14 ppm U and 0.07 ppm 232Th, with median (230Th/232Th) = 270, making them well suited for 230Th/U dating. Horizons as thin as 0.5 mm were sampled from polished slabs to reduce averaging of long (≥105 yr), and sometimes visibly discontinuous, depositional histories. Dense, translucent samples with finite 230Th/U ages preserve within-rind stratigraphic order in all cases. Ages for terraces WR4 (167,000 ± 6,400 yr) and WR2 (55,000 ± 8600 yr) indicate a mean incision rate of 0.26 ± 0.05 m per thousand years for the Wind River over the past glacial cycle, slower than inferred from cosmogenic-nuclide dating. Terrace WR3, which formed penecontemporaneously with the final maximum glacial advance of the penultimate Rocky Mountain (Bull Lake) glaciation, has an age of 150,000 ± 8300 yr indicating that it is broadly synchronous with the penultimate global ice volume maximum.  相似文献   

9.
南海珊瑚礁铀系年龄及其地质意义   总被引:2,自引:0,他引:2  
珊瑚礁是铀系方法测定年龄相当理想的对象,在国外已获得十分有价值的成果。主要用于讨论更新世以来海平面变化和构造运动上升速率,研究反映环境的珊瑚生长速度。本文珊瑚礁样品选取于海南岛鹿回头、雷州半岛、西沙和中沙群岛等地。成功地测试了铀系年龄,揭示出与14C 数据的不一致,初步讨论了构造上升速率和南海海面的变化。  相似文献   

10.
High-resolution paleomonsoon proxy records from peat and eolian sand–paleosol sequences at the desert–loess transition zone in China denote a rapid oscillation from cold–dry conditions (11,200–10,60014C yr B.P.) to cool–humid conditions (10,600–10,20014C yr B.P.), followed by a return to cold–dry climate (10,200–10,00014C yr B.P.). Variations in precipitation proxies suggest that significant climatic variability occurred in monsoonal eastern Asia during the Younger Dryas interval. Late-glacial climate in the Chinese desert–loess belt that lies downwind from Europe was strongly influenced by cold air from high latitudes and from the North Atlantic via the westerlies. The inferred precipitation variations were likely caused by variations in the strength of the Siberian high, which influenced the pressure gradient between land and ocean and therefore influenced the position of the East Asian monsoon front.  相似文献   

11.
Uranium-series dating of dense tufa deposited in a small cave, at former lake margins, and in large tufa mounds clarifies the timing of lake-level variation during the past 400,000 yr in the Pyramid Lake basin. A moderate-sized lake occasionally overflowed the Emerson Pass sill at elevation of 1207 m between ca. 400,000 and 170,000 and from ca. 60,000 to 20,000 yr B.P., as shown by230Th/234U ages of the cave samples,230Th-excess ages of tubular tufas, and average isochron-plot ages of shoreline-deposited tufas. (By comparison, modern Pyramid Lake is 50 m below this sill). There is a lack of tufa record during the intervening period from ca. 170,000 to 60,000 yr B.P. After ca. 20,000 yr, Pyramid Lake underwent abrupt changes in level and, based on previous14C ages, reached its highest elevation (ca 1335 m) at ca. 14,000 yr B.P. The youngest uranium-series ages are comparable with previously reported14C ages.  相似文献   

12.
A comparison of uranium-series geochronology and radiocarbon dating, the first systematic test of its kind applied to authigenic marine apatite, has been made for nine seafloor phosphate nodules sampled from the Peruvian and Chilean shelves. Radiocarbon ages agree well with the 230Th ages for all six samples with 230Th ages younger than the practical dating limit of the 14C technique for marine samples. Two of three “old” samples contain no significant 14C activities, as expected. However, one sample with a high concentration of phosphate pellets contains more than 30% of the modern 14C activity. The discrepancy between the 14C and U-series age for this sample is thought to be due to post-depositional adsorption of the particle-reactive elements thorium and protactinium onto pellet surfaces.  相似文献   

13.
A new packrat midden chronology from Playas Valley, southwestern New Mexico, is the first installment of an ongoing effort to reconstruct paleovegetation and paleoclimate in the U.S.A.–Mexico Borderlands. Playas Valley and neighboring basins supported pluvial lakes during full and/or late glacial times. Plant macrofossil and pollen assemblages from nine middens in the Playas Valley allow comparisons of two time intervals: 16,000–10,000 and 4000–0 14C yr B.P. Vegetation along pluvial lake margins consisted of open pinyon–juniper communities dominated by Pinus edulis, Juniperus scopulorum, Juniperus cf. coahuilensis, and a rich understory of C4 annuals and grasses. This summer-flowering understory is also characteristic of modern desert grassland in the Borderlands and indicates at least moderate summer precipitation. P. edulis and J. scopulorum disappeared or were rare in the midden record by 10,670 14C yr B.P. The late Holocene is marked by the arrival of Chihuahuan desert scrub elements and few departures as the vegetation gradually became modern in character. Larrea tridentata appears as late as 2190 14C yr B.P. based on macrofossils, but may have been present as early as 4095 14C yr B.P. based on pollen. Fouquieria splendens, one of the dominant desert species present at the site today, makes its first appearance only in the last millennium. The midden pollen assemblages are difficult to interpret; they lack modern analogs in surface pollen assemblages from stock tanks at different elevations in the Borderlands.  相似文献   

14.
The Dead Sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes, Lake Samra and Lake Lisan. The sedimentary history of these lakes was recon-structed by means of 230Th/234U ages of 30 samples, mostly of argonite laminae, from 8 columnar sections up to 110 km apart. The general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency, agreement with stratigraphic order, and concordance with 14C ages. In the south, only the part of the Samra Formation older than 170,000 yr is exposed, while the aragonite-detritus rhythmites found in the central and northern region are generally younger than 120,000 yr. The Lisan Formation started accumulating about 63,000 yr B.P., with the clay and aragonite beds in the south-central area reflecting a rise in water level to at least −280 m. The upper part of the Lisan Formation, the aragonite-rich White Cliff Member, started accumulating about 36,000 yr B.P. The lake probably reached its highest level sometime after this, based on the ages of Lisan sediments preserved in the southernmost reaches of the basin.  相似文献   

15.
柴达木盆地察尔汗贝壳堤剖面年代学研究*   总被引:9,自引:6,他引:9  
利用常规14 C、加速器(AMS)和230 Th等测年方法,对位于青藏高原东北部柴达木盆地察尔汗古湖贝壳堤剖面化石贝壳、湖泊沉积(包括现代沉积)样品有机质(包括碱性残留和胡敏酸组分)、碳酸盐(CaCO3)和剖面顶部石盐(NaCl)晶体在不同实验室进行了系统的实验研究,探讨了不同样品测年结果的可靠性和精确性,并通过与已有研究结果的对比,对柴达木盆地贝壳堤剖面记录的察尔汗古湖高湖面演化历史进行了探讨,得出察尔汗古湖高湖面形成于约39.7~17.5 14 C kaB.P. ; 研究证明柴达木盆地沉积物中有机质含量很低,且主要来源于菌藻类等低等藻类和微生物,受老碳和溶解物质影响强烈,因此必须经过多种方法综合测年才能比较可靠地确定所测样品的形成年代; 测年结果的不确定性不仅来源于样品自身元素(同位素)的特性和沉积过程中再沉积作用的影响,也可能源自于放射性14 C产生过程的不稳定性; 对比发现贝壳化石老于同层位有机质14 C年龄15~18ka,这可能与柴达木盆地所处的特殊地理位置和巨大的古湖泊面积及水体有关; 同时,不平衡铀系测年在确定开放体系矿物晶体年代应用中的可靠性尚须进一步深入分析,其机理也还需要探讨和实验研究。  相似文献   

16.
This article presents a combined pollen and phytolith record of a 1.70-m sediment core from the wetlands of India Muerta (33° 42′ S, 53° 57′ W) in the lowland Pampa (grasslands) of southeastern Uruguay. Six 14C dates and the pollen and phytolith content of the samples permitted the recognition of four distinct climatic periods between 14,850 14C yr B.P. and the present. The Late Pleistocene period (between ca. 14,810 and ca. 10,000 14C yr B.P.) was characterized by drier and cooler conditions indicated by the presence of a C3-dominated grassland. These conditions prevailed until the onset of the warmer and more humid climate of the Holocene around 9450 14C yr B.P. The early Holocene (between around 10,000 and 6620 14C yr B.P.) was characterized by the establishment of wetlands in the region as evidenced by the formation of black peat, the increase in wetland taxa, and the replacement of C3 Pooideae by C4 Panicoideae grasses. During the mid-Holocene, around 6620 14C yr B.P., began a period of environmental change characterized by drier climatic conditions, which resulted in the expansion of halophytic communities in the flat, low-lying areas of the wetlands of India Muerta. About 4020 14C yr B.P. a massive spike of Amaranthaceae/Chenopodiaceae coupled with a radical drop in wetland species indicates another major and more severe period of dryness. After ca. 4000 14C yr B.P., a decrease of halophytic species indicates the onset of more humid and stable climatic conditions, which characterized the late Holocene.The findings reported in this article substantially improve our knowledge of the late Glacial and Holocene climate and vegetation in the region. The data provide a detailed record of the timing and severity of mid-Holocene environmental changes in southeastern South America. Significantly, the mid-Holocene drying trend coincided with major organizational changes in settlement, subsistence, and technology of the pre-Hispanic populations in the region, which gave rise to early Formative societies. This study also represents the first combined pollen and phytolith record for southeastern South America reinforcing the utility of phytoliths as significant indicators of long-term grassland dynamics.  相似文献   

17.
In order to determine the geochemical evolution of a freshwater limestone cave system located in central Switzerland (Hell Grottoes at Baar/Zug,) young postglacial tufaceous limestone and travertine precipitates were investigated using the 230Th/234U ingrowth system. Additional analyses of further radionuclides within the 238U decay chain, i.e. 226Ra and 210Pb, showed that the Th/U chronometer started with insignificant inherited 230Th over the entire formation period of the travertine setting (i.e. 230Th(0)=0). A contribution from detrital impurities with 230Th/234U in secular equilibrium could be precisely subtracted by applying isochron dating of cogenetic phases and recently formed travertine. The resulting precise 230Th/234U formation ages were found to be consistent with the geological stratigraphy and were furthermore used to demonstrate the applicability of the next geologically important chronometer in the 238U-decay series, based on decay of excess 226Ra normalized to the initial, i.e.226Raex/226Ra(0). This system is suitable for dating phases younger than 7000 yr when the correction of a detritus component increasingly limits the precision of the 230Th/234U chronometer. Analytical solutions of the coupled 234U/230Th/226Ra radionuclide system predicted that the 226Raex/226Ra(0) chronometer is independent of the actual 230Th activity build up from decay of 234U, if the systems starts with zero inherited 230Th(0). The data set confirmed this hypothesis and showed furthermore that the initially incorporated 226Ra excess must have remained almost uniform in all limestone over a period of at least 7000 yr, i.e. 4–5 half-lives of 226Ra. This is concluded because (i) the 226Raex/226Ra(0) ages agreed well with those derived from 230Th/234U, (ii) all data plot within uncertainty on the 226Raex/226Ra(0) decay curve and (iii) the atomic Ba/Ca ratio was found to be constant in the travertine material independent of the sample ages. Provided that such boundary conditions hold, 226Raex/226Ra(0) should be applicable to materials which are suitable for 230Th/234U dating in sedimentology and oceanography, i.e. travertine, corals, phosphorites, etc., and should strongly support 230Th/234U for samples that have been formed a few thousand years ago.  相似文献   

18.
U-series disequilibria measured in waters and rocks from a chalk aquifer in France have been used as an analog for long-term radionuclide migration. Drill core samples from a range of depths in the vadose zone and in the saturated zone, as well as groundwater samples were analyzed for 238U, 234U, 232Th and 230Th to determine transport mechanisms at the water/rock interface and to quantify parameters controlling the migration of radionuclides. Isotope measurements in rocks were done by TIMS, whereas (234U/238U) and (230Th/232Th) activity ratios in water samples were measured by multi-collector-ICP-MS. Both depletion and enrichment in 234U relative to 238U were observed in carbonate rock samples resulting from chemical weathering in the unsaturated zone and calcite precipitation in the zone of water-table oscillation, respectively. The correlation between (230Th/232Th) activity ratios and 87Sr/86Sr ratios found in the chalk samples indicates that thorium is mainly contained in a minor silicate phase whose abundance is variable in chalk samples. Water samples are all characterized by (234U/238U) > 1 resulting from α-recoil effect of 234Th. Groundwaters are characterized by a more radiogenic signature in 87Sr/86Sr than the rocks. Moreover, (230Th/232Th) activity ratios in the waters are lower than in the rocks, and increase with distance from the water divide, which suggests that Th transport is controlled by colloids formed during water infiltration in the soil. A 1-D transport model has been developed in order to constrain the U-series nuclide transport considering a transient behavior of radionuclides in the aquifer and a time-dependent composition for the solid phase. This model permits a prediction of the time scale of equilibration of the system, and an estimation of parameters such as weathering rate, distribution coefficients and α-recoil fractions. Retardation factors of 10-35 and from 1 × 104 to 2 × 105 were predicted for U and Th, respectively, and can be used to predict the migration of radionuclides released as contaminants in the environment. At the scale of our watershed (∼32 km2), a characteristic migration time from recharge to riverine discharge of 200-600 yr for U and 0.2-3.7 Myr for Th was obtained.  相似文献   

19.
The 230Th/234U/238U age dating of corals via alpha counting or mass spectrometry has significantly contributed to our understanding of sea level, radiocarbon calibration, rates of ocean and climate change, and timing of El Nino, among many applications. Age dating of corals by mass spectrometry is remarkably precise, but many samples exposed to freshwater yield inaccurate ages. The first indication of open-system 230Th/234U/238U ages is elevated 234U/238Uinitial values, very common in samples older than 100,000 yr. For samples younger than 100,000 yr that have 234U/238Uinitial values close to seawater, there is a need for age validation. Redundant 230Th/234U/238U and 231Pa/235U ages in a single fossil coral fragment are possible by Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry (MC-MS-ICPMS) and standard anion exchange column chemistry, modified to permit the separation of uranium, thorium, and protactinium isotopes from a single solution. A high-efficiency nebulizer employed for sample introduction permits the determination of both 230Th/234U/238U and 231Pa/235U ages in fragments as small as 500 mg. We have obtained excellent agreement between 230Th/234U/238U and 231Pa/235U ages in Barbados corals (30 ka) and suggest that the methods described in this paper can be used to test the 230Th/234U/238U age accuracy.Separate fractions of U, Th, and Pa are measured by employing a multi-dynamic procedure, whereby 238U is measured on a Faraday cup simultaneously with all minor isotopes measured with a Daly ion counting detector. The multi-dynamic procedure also permits correcting for both the Daly to Faraday gain and for mass discrimination during sample analyses. The analytical precision of 230Th/234U/238U and 231Pa/235U dates is generally better than ±0.3% and ±1.5%, respectively (2 Relative Standard deviation [RSD]). Additional errors resulting from uncertainties in the decay constant for 231Pa and from undetermined sources currently limit the 231Pa/235U age uncertainty to about ±2.5%. U isotope data and 230Th/234U/238U ages agree with National Institute of Standards and Technology (NIST) reference materials and with measurements made by Thermal Ionization Mass Spectrometry (TIMS) in our laboratory.  相似文献   

20.
The end of the Pleistocene in North America was marked by a wave of extinctions of large mammals, with the last known appearances of many species falling between ca. 11,000–10,000 14C yr BP. Temporally, this period overlaps with the Clovis Paleoindian cultural complex (11,190–10,530 14C yr BP) and with sudden climatic changes that define the beginning of the Younger Dryas chronozone (ca. 11,000–10,000 14C yr BP), both of which have been considered as potential proximal causes of this extinction event. Radiocarbon dating of enamel and filtered bone collagen from an extinct American Mastodon (Mammut americanum) from northern Indiana, USA, by accelerator mass spectrometer yielded direct dates of 10,055 ± 40 14C yr BP and 10,032 ± 40 14C yr BP, indicating that the animal survived beyond the Clovis time period and into the late Younger Dryas. Although the late survival of this species in mid-continental North America does not remove either humans or climatic change as contributing causes for the late Pleistocene extinctions, neither Clovis hunters nor the climatic perturbations initiating the Younger Dryas chronozone were immediately responsible for driving mastodons to extinction.  相似文献   

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