系列^14C样品贝叶斯法日历年龄校正研究进展

系列^14C样品贝叶斯法日历年龄校正研究已成为当前^14C年代学中一个热点,系统综述了时序已知、时序间隔已知和时序间隔近似已知系列^14C样品日历年龄校正研究的历史、原理、计算方法及其在树轮、泥炭、地层等领域上应用进展,对影响系列样品贝叶斯分析获得^14C日历年龄区间的主要因素以及未来发展方向作了归纳。大量事例研究结果表明系列样品贝叶斯分析的实质在于贝叶斯方法从统计的角度灵活地把地层信息以概率方式加入到^14C日历年龄校正过程中,获得比单个^14C年代校正精度更高的日历年龄。     

14C测年技术新进展

¹⁴C测年技术的进展主要表现为3个方面：1)¹⁴C常规测定技术向高精度发展比较成熟，目前已可以普及；2)加速器质谱技术的建立和普及，使测定要求的样品碳量减少到mg级，甚至μg级，虽然目前的精度尚未超过常规方法，但由于所需样品量极少，测定时间短而工效高，大大拓宽了应用范围；3)高精度¹⁴C－树轮年龄校正曲线的建立，不但可将样品的¹⁴C年龄转换到日历年龄，而且对有时序的系列样品的¹⁴C年龄数据通过曲线拟合方法转换到日历年龄时年龄误差大为缩小。     

黄土剖面古土壤和生物化石14C测年对比

精确、可靠测定古土壤和生物化石年代是重建环境变化过程、探讨人地关系的关键.研究古土壤中不同组分年代的异同对于分析土壤中碳附存状态和碳储存库十分重要.对甘肃省榆中县境内兴隆山典型黄土剖面采集的动物化石、土壤无机质、土壤有机质3个样品进行常规14C和AMS测年,发现同一地层相同点样品不同组分的测年结果相差悬殊,样品无机质比其有机质年龄(3 682±70)a偏老2 624 a.对其14C测年可靠性对比分析发现,常规14C和AMS对骨骼和牙齿化石测年相差仅为13 a,校正为日历年后几乎相等,认为实验室产生的误差很小,而碳的来源、组成及其"死碳"混入的比例是影响测年结果的主要因素.化石年龄与土壤有机质年龄之间的差别,揭示该剖面可能存在持续时间达千年以上的沉积间断.由于同一层位骨化石、土壤有机质、土壤无机质14C测年结果差异,在进行化石年代确定、考古及古文明研究、生态植被恢复、土壤无机碳存储库研究中,年龄的界定应选择相应的测年数据.     

环渤海海岸带14C数据集（Ⅱ）

作为晚更新世与全新世地层年代测定的一种重要手段,¹⁴C测年被广泛应用于地质样品的年代测定。但对于¹⁴C数据的系统校正,迄今未得到应有的重视。文章在环渤海海岸带¹⁴C数据集(Ⅰ)的基础上,对所收集的421个由其他研究者获得的环渤海海岸带¹⁴C数据进行了系统校正。并讨论了百年、千年与万年尺度的¹⁴C数据校正效果。结果表明,直接测定值与系统校正值之间的差值常有数百年至两千年的差别。经统一校正的¹⁴C年龄,最大限度地接近样品太阳历纪年的“真实”年龄,从而有助于更加准确地重建该地区的晚更新世晚期以来的地质年代史,并可与考古纪年及其他测年方法获得的绝对年龄直接对比。     

关中地区2万年以来黄土序列中蜗牛化石及其有机质的14C AMS测年初探

前人在黄土—古土壤地层的¹⁴C年代学研究中,探讨了不同测年物质的可靠性,但不同物质组分获得的年代差异较大,这是由于选用的样品不符合定年条件,所以选择可靠的样品组分和有效的前处理方法显得尤为重要。对关中地区2万年以来黄土序列中有机质、花粉浓缩物和蜗牛化石进行了AMS¹⁴C年代学研究。运用现生蜗牛壳体的AMS¹⁴C测定来校正化石蜗牛的年龄,结合剖面中孢粉浓缩物、土壤有机质的AMS¹⁴C年代学对比研究,发现蜗牛化石壳体的文石可以提供较可靠的年代数据,为¹⁴C测年的可靠性研究提供了一种可能的途径。     

华北平原深层地下水14C年龄的TDIC校正与对比

用14C确定地下水年龄需要进行多种校正。在系统测定华北平原深层地下水14C含量的基础上,应用TDIC(Total Dissolved Inorganic Carbon,即总溶解无机碳)校正地下水14C年龄。将补给区地下水样的14C和TDIC含量作为"初始14C活度"和"初始TDIC"含量,对其他样品进行"初始TDIC"的14C活度校正,并用CAL-IB5.0分别计算了样品校正年龄(14C校)和TDIC校正后的校正年龄(14CTDIC校)。华北平原深层地下水14C校正年龄变化比例达-27.5%~44.4%,TDIC校正对14C年龄影响明显,地下水样品进行14C定年时需要先进行TDIC校正。根据华北平原地下水14C校正年龄和Cl-含量分布,华北平原深层地下水受到明显的海水入侵的影响,衡水—河间—任丘一线西侧地下水14C校正年龄代表了其真实年龄,而衡水—献县—天津一线东侧地下水14C校正年龄不能代表其真实年龄,只是海水和地下水混合后的表观年龄。     

AMS ~(14)C测年揭示的渤海湾沿海低地第Ⅱ海相层年龄

在渤海湾沿海低地地区获得4个机械钻孔,结合AMS14C测年方法,对Ⅱ海的年龄进行了研究。结果显示,在Ⅱ海所在层位获得的14C样品年龄均大于43.5ka BP,说明渤海湾沿海低地地区Ⅱ海形成于MIS 3早期或MIS 5,而不是前人认为的23~39ka。建议在前人地层结构研究成果的基础上,系统开展Ⅱ海年龄研究。     

AMS ^14C测年揭示的渤海湾沿海低地第Ⅱ海相层年龄

在渤海湾沿海低地地区获得4个机械钻孔,结合AMS 14C测年方法,对Ⅱ海的年龄进行了研究。结果显示,在Ⅱ海所在层位获得的14C样品年龄均大于43.5ka BP,说明渤海湾沿海低地地区Ⅱ海形成于MIS 3早期或MIS 5,而不是前人认为的23-39ka。建议在前人地层结构研究成果的基础上,系统开展Ⅱ海年龄研究。     

“死碳”对14C年代测定影响的初步研究

在回顾14 C测年技术发展的基础上,指出了影响14 C测年精度的各种因素,并初步讨论了“死碳”对14 C年代的影响。通过对岩溶区“死碳”的成因分析以及对碳酸盐样品的14 C年代学研究,认为酸的来源不同,是造成岩溶区样品14 C年代偏老的原因;局部环境中“死碳”释放是造成水生样品14 C表观年龄偏老的主要原因。由于影响岩溶区样品14 C年龄的因素是多方面的,目前尚无有效的校正办法。为此,作者在讨论了洞穴碳酸盐样品年代的可靠性及影响因素的基础上,提出了用交叉对比定年的方法解决“死碳”对14 C测年的影响问题和在岩溶地区谨慎使用14 C年龄的建议。      

环渤海海岸带14C数据集（Ⅰ）

环渤海泥质海岸带近30年来获得的600余个放射性碳测年数据,确定了该地区晚更新世晚期以来的基本年代地层序列。但是,对¹⁴C数据的系统校正,迄未得到应有的重视,致使在应用¹⁴C数据解释地层和地质现象的年代时,存在着一定程度的混乱。文章对作者自己所采集和由国内外有关实验室测定的126个¹⁴C数据进行了系统校正,主要步骤包括分馏效应、地区性海洋贮存库效应及大气¹⁴C含量变化校正等。建议以-2.68‰PDB作为环渤海晚更新世晚期以来半咸水和开放浅海区(潮间带与潮下带上部)贝壳的地区性^δ13C平均值。另外,讨论了在地区性海洋贮存库效应值获得之前的1990年代,以MARINE93与INTCAL93程序的平均校正年龄近似地视作该地区贝壳的校正值,以抵消这一类地区小型水体与大气迅速进行¹⁴CO₂交换而对年龄值的影响。CALIB4.4校正结果表明,前述近似校正是可行的,其结果仍可沿用。文章经统一校正的年龄,最大限度地接近其太阳历纪年的"真实"年龄,从而有助于更加准确地重建该地区的晚更新世晚期以来的地质年代史,并可与考古纪年及其他测年方法获得的绝对年龄直接对比。     

Cumulative probability functions and their role in evaluating the chronology of geomorphological events during the Holocene

Cumulative probability functions (CPFs) for large numbers of radiocarbon age determinations are increasingly being used by scientists as a methodology to discern environmental histories. While the recent compilation of regional databases of the radiocarbon dating control for fluvial sediment sequences has been beneficial for identifying gaps in knowledge and stimulating new research, there are a number of problems that critically undermine the use of these CPFs as sensitive hydroclimatic proxies. (i) The CPF method is underpinned by the assumption that each radiocarbon measurement is a true age estimate for a point in time, whereas each measured age in fact forms a scatter around the true age of the sample; (ii) calibration of radiocarbon ages is responsible for much of the structure in CPFs and compounds the problem of scatter and smears the chronological control; (iii) the databases incorporate multiple types of environmental changes differing chronological relationships between the ¹⁴C measurements and the dated events, with pre‐dating, dating or post‐dating chronological control each displaying variable length temporal lags all mixed together in the same analysis; and (iv) the radiocarbon ages from individual case studies need to be more robustly tested before being incorporated into regional databases. All these factors negate the value of CPFs as sensitive proxies of environmental change, because peaks in probability for individual radiocarbon measurements are likely to be an incorrect estimate for the age of a geomorphological event and this problem is compounded by combining probabilities for multiple unrelated events. In this paper we present a critical analysis of CPFs and their interpretation before suggesting alternative approaches to analysing radiocarbon geochronologies of geomorphic events, which include: (i) Bayesian age modelling of river terrace development; (ii) developing regional databases that test specific geomorphic hypotheses; (iii) Bayesian age modelling of palaeoflood records; and (iv) analysis of sedimentation rates. Copyright © 2011 John Wiley & Sons, Ltd.     

High-resolution chronology for the Mesoamerican urban center of Teotihuacan derived from Bayesian statistics of radiocarbon and archaeological data

A high-resolution ¹⁴C chronology for the Teopancazco archaeological site in the Teotihuacan urban center of Mesoamerica was generated by Bayesian analysis of 33 radiocarbon dates and detailed archaeological information related to occupation stratigraphy, pottery and archaeomagnetic dates. The calibrated intervals obtained using the Bayesian model are up to ca. 70% shorter than those obtained with individual calibrations. For some samples, this is a consequence of plateaus in the part of the calibration curve covered by the sample dates (2500 to 1450 ¹⁴C yr BP). Effects of outliers are explored by comparing the results from a Bayesian model that incorporates radiocarbon data for two outlier samples with the same model excluding them. The effect of outliers was more significant than expected. Inclusion of radiocarbon dates from two altered contexts, 500 ¹⁴C yr earlier than those for the first occupational phase, results in ages calculated by the model earlier than the archaeological records. The Bayesian chronology excluding these outliers separates the first two Teopancazco occupational phases and suggests that ending of the Xolalpan phase was around cal AD 550, 100 yr earlier than previously estimated and in accordance with previously reported archaeomagnetic dates from lime plasters for the same site.     

The evolution of Paleoindian geochronology and typology on the Great Plains

The Great Plains contain many of the best‐known Paleoindian sites in North America, and a number of these localities were key to determining the chronology of Paleoindian occupations in the years before, during, and since the development of radiocarbon and other chronometric dating methods. Initial attempts at dating were based on correlation with extinct fauna, the “geologic‐climatic” dating method, and stratigraphic relationships of artifacts within sites. By the time radiocarbon dating was developed (1950), the basic Paleoindian sequence (oldest to youngest) was: Clovis‐Folsom‐unfluted lanceolates (such as Plainview, Eden, and Scottsbluff). Initial applications of radiocarbon dating in the 1950s did little to further resolve age relationships. In the 1960s, however, largely through the efforts of C. V. Haynes, a numerical geochronology of Paleoindian occupations on the Great Plains began to emerge On the Southern Great Plains the radiocarbon‐dated artifact chronology is: Clovis (11,600–11,000 yr B.P.); Folsom and Midland (10,900–10,100 yr B.P.); Plainview, Milnesand, and Lubbock (10,200–9800 yr B.P.); Firstview (9400–8200 yr B.P.); St. Mary's Hall, Golondrina, and Texas Angostura (9200–8000 yr B.P.). The chronology for the Northern Great Plains is: Clovis (11,200–10,900 yr B.P.); Goshen (ca. 11,000 yr B.P.); Folsom (10,900–10,200 yr B.P.); Agate Basin (10,500–10,000 yr B.P.); Hell Gap (10,500–9500 yr B.P.); Alberta, Alberta‐Cody (10,200–9400 yr B.P.); Cody (Eden‐Scottsbluff) (9400–8800 yr B.P.); Angostura, Jimmy Allen, Frederick, and other parallel‐oblique types (9400–7800 yr B.P.). Fifty years after the development of radiocarbon dating, the basic typological sequence has not changed significantly except for the realization that there probably was significant temporal overlap of some point types, and that the old unilinear sequence does not account for all the known typological variation. The chronology has been continually refined with the determination of hundreds of radiocarbon ages in recent decades. © 2000 John Wiley & Sons, Inc.     

第四纪骨化石样品的多方法对比测年

本文报道作者用¹⁴C、铀系和ESR等多种测年技术对比测定第四纪骨化石样品年龄结果,对比分析骨化石中各含碳组分¹⁴C年龄的异同。在此基础上讨论诸测年方法的可靠性和测年精度,分析哪种含碳组分最能代表骨质样品的真实年龄。对晚于40 000aB.P.的骨质样品,作者倾向于样品中氨基酸的¹⁴C测年,而对更老的样品,铀系法应优先被选用。本文还对北京周口店山顶洞遗址骨化石样品中不同含碳组分的¹⁴C测年结果做了讨论。     

The use of ‘bomb spike’ calibration and high-precision AMS C analyses to date salt-marsh sediments deposited during the past three centuries

A combination of ‘bomb spike’ calibration and conventional calibration of AMS ¹⁴C dating has been used to determine a detailed age-depth model for a 1-m sediment section collected from a salt marsh in Poole Harbour, southern England. These data were compared with the chronology obtained from ²¹⁰Pb analysis and ¹³⁷Cs age markers. We report post bomb values of over 1.46 F¹⁴C (> 146% modern ¹⁴C), and both the rising and falling limbs of the atmospheric ‘bomb spike’ are identified. Five pre-bomb samples were analysed using multi-target high-precision 2‰ AMS analysis, and after the replicates were combined the one-sigma uncertainty was as low as ± 9 ¹⁴C yr on some ages. These data, and an additional three normal-precision pre-bomb ¹⁴C samples, were calibrated using CALIB 5.0 and the chronology constrained using the ‘prior knowledge’ of independent age markers obtained from the analysis of pollen and spheroidal carbonaceous particle (SCPs). No agreement was found between the ¹⁴C ‘bomb spike’ dates and the CRS ²¹⁰Pb chronology modelled for this sequence. In addition, poor agreement was found between the signal of the 1960s weapons test fallout indicated by the ¹⁴C ‘bomb spike’ dates and the timing suggested by the ¹³⁷Cs data. This disagreement is attributed to the influence of the local discharge of ¹³⁷Cs from the former UKAEA site at Winfrith. We use our new chronology to confirm the existence of an acceleration in sedimentation rates in Poole Harbour during the last 100 yr previously reported for this site by Long et al. (Long, A.J., Scaife, R.G., Edwards, R.J. 1999. Pine Pollen in intertidal sediments from Poole Harbour, UK; implications for late-Holocene sediment accretion rates and sea-level rise. Quaternary International, 55, 3–16.), and conclude that ‘bomb spike’ ¹⁴C calibration dating may offer a more robust alternative to the use of ²¹⁰Pb chronologies for dating sediment deposition in salt-marsh environments. In addition, we demonstrate how the use of high-precision AMS analysis has the potential for reducing some of the uncertainties involved in the high-resolution dating of recent salt-marsh sediments.     

A revised chronology of the lowest occupation layer of Pedra Furada Rock Shelter,Piauı́, Brazil: the Pleistocene peopling of the Americas

The present work revisits the chronology of the archaeologically controversial Pedra Furada Rock Shelter of Southeast Piauı́, Brazil, using an improved radiocarbon laboratory pre-treatment and measurements on charcoal samples. The procedure, known as ABOX-SC (acid–base–wet oxidation followed by stepped combustion), has previously been used to secure radiocarbon dates of >40 ka for the antiquity of human occupation of Australia and South Africa, and now has been applied to charcoal from the previously dated oldest occupation layer of the Pedra Furada site. Previous radiocarbon dating had obtained only lower limits of 40–45 ka BP for the Pedra Furada basal layer. Nine charcoal samples from well-structured hearths were subjected to the ABOX-SC procedure and their radiocarbon content determined by accelerator mass spectrometry. Measurements on five of the samples returned ages of greater than 56 ka BP, from graphites produced from ABOX pre-treated charcoal combusted at 910°C. Two other samples were greater than 50 ka BP. The remaining two samples were essentially completely combusted at 650°C, with no material surviving to make a 910°C CO₂ fraction. Their ages were 41.3 and 47.2 ka BP. Ages obtained from graphites generated from the 650°C combusted fraction are considered minimum ages.     

Marine-derived C calibration and activity record for the past 50,000 years updated from the Cariaco Basin

An expanded Cariaco Basin ¹⁴C chronology is tied to ²³⁰Th-dated Hulu Cave speleothem records in order to provide detailed marine-based ¹⁴C calibration for the past 50,000 years. The revised, high-resolution Cariaco ¹⁴C calibration record agrees well with data from ²³⁰Th-dated fossil corals back to 33 ka, with continued agreement despite increased scatter back to 50 ka, suggesting that the record provides accurate calibration back to the limits of radiocarbon dating. The calibration data document highly elevated Δ¹⁴C during the Glacial period. Carbon cycle box model simulations show that the majority of observed Δ¹⁴C change can be explained by increased ¹⁴C production. However, from 45 to 15 ka, Δ¹⁴C remains anomalously high, indicating that the distribution of radiocarbon between surface and deep ocean reservoirs was different than it is today. Additional observations of the magnitude, spatial extent and timing of deep ocean Δ¹⁴C shifts are critical for a complete understanding of observed Glacial Δ¹⁴C variability.