High-resolution210Pb dating of Lake Constance sediments: Stable lead in Lake Constance

Two sediment cores (BO90/13b and BO90/17b) from Lake Constance were investigated by-spectrometry for²¹⁰Pb,¹³⁴Cs,¹³⁷Cs,²⁴¹Am,²³⁴Th, and other members of the²³⁸U decay chain. The sediments were dated using the constant-flux model for²¹⁰Pb, and accumulation rates were determined. These range from 0.04 to 0.65g/cm ²/yr (BO90/13b) and 0.04 to 0.8g/cm ²/yr (BO90/17b), respectively. The mean accumulation rate amounts to 0.16g/cm ²/yr for both cores. The cores had already been dated by lamination counting and reconstruction of high-water events at the Limnological Institute at Constance, so that a very precise time scale was available. Both ages derived are in agreement within statistical error up to 1900, which means dating with the constant-flux model for²¹⁰Pb was confirmed up to that age. The position of the maxima of bomb cesium and americium confirm the stratigraphic and²¹⁰Pb datings. With²⁴¹Am a further radioactive isotope is available, which can, due to the half-life of²⁴¹Pu (t _1/2=14.4yr) be detected now by-spectrometry and can serve as an additional time indicator, the maximum being dated at 1963. By applying the various time scales, the depth profiles of stable lead and zinc of core BO90/13b were dated. Both heavy metals show a very significant maximum located beneath the layer of the maxima of bomb cesium and americium, showing that these maxima are older than those of the bomb isotopes. It is remarkable in this context that the maximum of zinc concentration occurs a little later than that of stable lead. Similar concentration profiles are observable in core BO90/17b and other, older sediment cores (CS6-CS10) on a transect across the lake. In contrast to a former assumption, the depth profile of stable lead in Lake Constance sediments does not reflect the anthropogenic gasoline lead emissions into the atmosphere for Germany, their maximum being dated at 1971.     

Assessment of 210Pb data from Canadian lakes using the CIC and CRS models

 Much discussion has centered around which ²¹⁰Pb dating method should be used, the constant initial concentration (CIC) model or the constant rate of supply (CRS) model. In this study, the activity data from 22 lacustrine sediment cores from the Canadian prairies were used to compare the determination of sediment accumulation using the two models. Other relative and absolute dating techniques have been used to calibrate the methodology. For half of the core sites examined, the mass sedimentation rate was constant, and thus both the CIC and CRS models were found to be valid. For the other half, variability was observed in the CRS mass accumulation rate trend. The validity of the CIC model for these cores was dependent on the degree of variability of the mass sedimentation rate. Where the variability is moderate to high, the CRS model may be more satisfactory. Caution should be exercised when using chronological data determined with the CRS model, however, as the accuracy of chronology in the lower reaches of a profile is questionable. Received: 11 May 1995 · Accepted: 16 August 1995     

黄海和渤海沉积物 210Pb活度的分布特征

利用α多道能谱仪测定了黄海和渤海海域 58个表层沉积物样品的 210Pb活度,探讨了黄海和渤海表层沉积物中 210Pb活度的空间分布特征.渤海中部、北黄海中部泥质沉积区和南黄海中部泥质沉积区是 210Pb活度的高值区.渤海湾内、莱州湾内及北黄海西南部沿岸海域则是 210Pb活度的低值区.黄海和渤海海区 210Pb的富集、分布特征受到研究海区水动力条件和沉积物粒级的制约.根据位于黄海和渤海泥质沉积区的 3个岩芯 210Pb活度的垂直分布资料,并结合 2个站位所取岩芯的 210Pb活度的垂直分布资料,探讨了 210Pb活度的垂直分布与沉积环境之间的关系.黄海和渤海 210Pb活度呈现高值的泥质沉积区,其 210Pb活度随岩芯深度衰减的垂直分布很有规律且沉积速率较低,由此可以反映泥质沉积区稳定的沉积环境.     

Determination of 210Pb, 226Ra and 137Cs in sediments

A method is described for bringing a sediment sample into solution and subsequently carrying out analysis for ²¹⁰Pb, ²²⁶Ra and ¹³⁷Cs. Silica is removed from the sample by cyclic HNO₃HF treatments. ¹³⁷Cs is separated from ²¹⁰Ra in a carbonate fusion, extracted by absorption on ammonium molybdophosphate, precipitated directly with BiI₃ in presence of citric acid, and β-counted. ²¹⁰Pb and ²²⁶Ra are separated out by 70–75% HNO³ precipitation. Further purification and mutual separation of the two radionuclides is achieved by solvent extraction and anion-exchange techniques. ²¹⁰Pb and ²²⁶Ra are determined by β- and α- counting of their chromate precipitates, respectively, after allowing suitable ingrowth periods for their daughters. The procedural steps effectively eliminate possible interference from other natural or fall-out radioactivities.     

A comparison of the210Pb dating technique with three other independent dating methods in an oxic estuarine salt-marsh sequence

The²¹⁰Pb dates from a salt-marsh sediment sequence at Tites Point, Severn Estuary, southwest England, were compared with the results of three other independent dating techniques. These methods are inspection of dated topographic maps for the presence of absence of the salt marsh, dating of artifacts in a well-defined horizon within the sequence, and counting annual bands. All four sets of dates showed good agreement to about 30 cm depth from the present marsh surface; thereafter, dates from the²¹⁰Pb method are earlier than those from the other methods, which are themselves in agreement. It is concluded that a wide range of methods can be successfully used for dating salt-marsh sediments (e.g., radiometric techniques, topographic maps, archaeological evidence, annual banding). Where possible, it is important to confirm dates using at least two independent techniques.     

Chronology of a lacustrine core from Lake Linggo Co using a combination of OSL, 14C and 210Pb dating: implications for the dating of lacustrine sediments from the Tibetan Plateau

Vast palaeolakes once occupied the Qinghai‐Tibetan Plateau (TP). Analyses of the sedimentary records of these lakes could potentially provide an extensive dating archive. Many previously constructed age‐depth models simulating lacustrine cores have been principally based on radiocarbon (¹⁴C) dating. However, such dating could have been hampered by the so‐called ‘lake reservoir effect’ (LRE) and the reworking of lakebed sediments, resulting in inaccurate ¹⁴C age‐depth models and limiting interpretations of existing lacustrine palaeoclimatic records. Lake Linggo Co is located on the central TP, in one of the coldest and most arid regions of Tibet. We dated a 9.87‐m‐long lacustrine core extracted from the lakebed at a water depth of ~60 m using a combination of ²¹⁰Pb, ¹⁴C and optically stimulated luminescence (OSL) techniques. Some ¹⁴C ages showed significant age reversals; all the ¹⁴C ages were much older than the ²¹⁰Pb and OSL ages for the same depths. This could possibly be attributed to the presence of old, inherited carbon, causing the inherited ¹⁴C age to appear unstable during the deposition period. The ²¹⁰Pb and OSL ages were roughly concordant, and were also consistent with the stratigraphical succession. We therefore suggest that ¹⁴C dating may, on its own, be inadequate for accurate dating of lacustrine sediment sites on the TP, and that the OSL method should also be applied in order to evaluate the reliability of any ¹⁴C ages. With this approach, we constructed an age‐depth model, revealing sedimentation rates of 1.7, 0.8, 6.8 and 0.6 mm a⁻¹ between 0–1.9, 1.9–4.2, 4.2–4.4 and 4.4–9.4 ka, respectively.     

Determination of diffusion coefficients associated with the transport of210Pb radionuclides in lake bed sediments

An iterative least-squares optimization technique is utilized in conjunction with a one-dimensional representation of the mass transport equation to generate theoretical²¹⁰Pb concentration/depth profiles beneath the water-sediment interface that are best-fit approximations to directly measured²¹⁰Pb concentration/depth profiles at various locations within the Great Lakes system. The outputs of such an optimization analysis are the diffusion coefficientsD _M (molecular) andD _B (bioturbation) associated with the transport of²¹⁰Pb radionuclides in lake bed sediments. For all stations studied, the estimated values ofD _B are consistently larger than the estimated values ofD _M , emphasizing the importance of accounting for the effects of bioturbation in the modelling of contaminant transport through lake bed sediments.     

Thermoluminescence dating of sediments

The potential for using the thermoluminescence behaviour of sediments for dating them was first recognized by Soviet scientists G.V. Morozov and V.N. Shelkoplyas, and for over a decade their TL dates, obtained from a variety of sediments, have appeared in the Soviet literature. Since 1977 TL sediment dates have been published by six additional groups using a variety of methods.In this review we describe the principles of TL dating, the various methods used, and contrast TL dating of sediments with the now well-accepted TL dating of pottery. We conclude that while TL dating has the potential to solve many sedimentary problems, more fundamental research needs to be carried out before such dates should be accepted. A set of criteria for acceptable dates is proposed.     

Reconstructing seawater column 90Sr based upon 210Pb/226Ra disequilibrium dating of mollusk shells

The shells of marine and fresh water mollusks can serve as effective archives in retrieving information on natural and anthropogenic environmental changes. The advantage of using bivalves is that they integrate water chemistry changes into their shells during their life span. Retrospective study of environmental changes and pollutants using bivalve shells requires precise determination of the time of incorporation into the abiotic environmental matrix (here after age) of the specimen. For the first time, a set of archived bivalve samples (for which date of the death/collection is known) has been analyzed to establish the ages of mollusk shells using the ²¹⁰Pb–²²⁶Ra disequilibrium method. In addition, Sr and ⁹⁰Sr were analysed. The ages obtained using the ²¹⁰Pb/²²⁶Ra disequilibrium dating method agrees well with the calendar years calculated from the date of death/collection. The ages obtained can be utilized to reconstruct the ⁹⁰Sr levels in the water column at sites where the mollusk shells were collected.     

~(210)Pb和~(137)Cs测年在中国海岸带古环境演变研究中的应用

海岸带地处陆地和海洋交界地区,是物质和能量交换的中心地带,沉积环境复杂。而对其沉积记录古环境信息的正确理解和解释在很大程度上依赖于年代框架的建立。210Pb和137Cs年代学的问世,解决了高速沉积环境中时间量程为百年或更短的沉积物的定年,近年来得到广泛的应用。在实际应用中,我们应该根据研究目的、范围等因素,选择合适的测年手段测年。此外,由于每种方法还存在不足,且可能同时会有几种方法适用,所以为了增加测年数据的准确性和可靠性,我们应尽量选择多种适当方法进行对比测年。     

Estimation of mixing rates in western Lake Ontario sediments by finite element analysis of210Pb profiles

A finite element model is used to generate theoretical excess²¹⁰Pb concentration/depth profiles that are best-fit approximations to directly measured profiles at five locations in western Lake Ontario. The best-fit approximations are derived using a minimum error analysis technique. The results indicate that, while mixing is prevalent at all the study locations, its influence is severe at locations closer to the mouth of the Niagara River. The nonoccurrence of the implicit effects of sediment mixing in measured pollutant profiles at these locations is pointed out and it is postulated that the general assumptions of constant flux of excess²¹⁰Pb and constant sedimentation rate in such models lead to unrealistic estimates of mixing.     

TL dating of partially bleached sediments by the regeneration method

Thermoluminescence dating of sediments in the Warsaw laboratory is based on blue-to-green emission. The ‘partial bleach + regeneration’ method is proposed for dating sediments which have not been completely bleached at the time of burial. Correct TL ages are reported for sediments covering organic deposits of the Eemian Interglacial and also for glacial sediments of Würm and Riss age. We obtained good results for dating till using the partial bleach + regeneration method. The TL zeroing in till is explained by the ‘rubbing’ or compression of grains beneath the glacier and by the freeze-thaw activity occurring during glacial transport and deglaciation.     

The relationship of soil 210Po and 210Pb geochemical dispersion patterns to uranium mineralization

²¹⁰Po and ²¹⁰Pb measurements of soils delineated uranium anomalies at three out of four test sites in Ontario, Canada. Measurements were made of ²¹⁰Po in solutions produced by both complete digestion and partial leaching of soil samples. Direct plating of ²¹⁰Po onto metal plates was followed by measuring the alpha activity. Subsequent plating of ²¹⁰Po in-grown from ²¹⁰Pb in solution several months later confirmed the anomalies.The ²¹⁰Po and ²¹⁰Pb anomalies at three of the test sites coincided with ²²⁶Ra and ²²²Rn anomalies. Samples from the fourth uranium occurrence associated with a known ²²²Rn anomaly failed to show either a ²²⁶Ra anomaly on the one hand, or ²¹⁰Po or ²¹⁰Pb anomalies on the other. This suggests that the ²¹⁰Po and ²¹⁰Pb anomalies were probably produced by the decay of ²²⁶Ra contained within secondary dispersion haloes.Although anomalies due to the ²¹⁰Po and ²¹⁰Pb products of ²²²Rn have now been documented, prospecting methods based on their use as direct tracers of the migration paths of ²²²Rn require much further development.     

An evaluation of the zircon method of isotopic dating in the Southern Arabian Craton

A zircon study has been made on eleven samples of igneous rocks from the Saudi Arabian Craton. Ages of sized and magnetic fractions of zircon concentrates show variable degrees of discordance which seem to result from a very young disturbance that produces linear arrays in the Concordia plot. Model age calculations based on a statistically and geologically reasonable lower intercept produce very consistent internal relationships. The Pan African Orogeny, considered to be responsible for loss of radiogenic argon and strontium from minerals of many rocks, does not appear to have affected the zircon data, even though uplift had exposed the rocks of the Arabian Shield at that time.Tonalite, granodiorite, and crosscutting leucoadamellite bodies in the southern part of the An Nimas Bathylith yield ages in the time range 820–760 Ma. A narrow time range of 660 to 665 million years was indicated for ages of widely separated and compositionally different intrusive bodies all to the east of the An Nimas Bathylith. This work suggests that the younger end of the age spectrum established from regional K-Ar and Rb-Sr measurements may be underestimated, and that magmatic activity could be more episodic than previously assumed.     

U-Th dating of carbonate platform and slope sediments

Absolute chronology of marine sediment beyond the ¹⁴C age range provides a test for models of climate change and has many other applications. U-Th techniques have been used for such chronology by dating corals, but extending these techniques to marine sediment is complicated by the presence of significant initial ²³⁰Th—both in detrital material and scavenged from seawater. In this study, we investigate four methods of solving the initial ²³⁰Th problem for a particular type of marine sediment—the aragonite-rich sediments of carbonate platforms and slopes. Bulk sediment U-Th analyses can be corrected for initial Th to yield ages with ≈2 to 3 kyr precision for highstand periods when sediment aragonite contents are particularly high. Uncertainty on the corrections causes inadequate precision for sediment from other periods, however. Removal of scavenged Th before analysis would enable a dramatic increase in this precision but has not proved successful despite a range of chemical leach approaches. Using heavy liquids to separate the various carbonate minerals found in Bahamas sediment enables an isochron approach to correct for initial Th, but the presence of initial Th from two sources requires correction or removal of one source of initial Th before the other is deconvolved by the isochron. Quantitative removal of detrital material before isochron analysis proves a successful approach. Such isochron data demonstrate that, although sediment remains closed to U-Th on a centimetre scale, nuclides are moved from grain to grain by α-recoil. Such intergrain exchange is expected to be observed in all sediments containing mineral grains with different U concentrations. Measured ²³⁴U/²³⁸U allows the recoil movement to be corrected and results in isochron ages with precision sometimes as low as 3 kyr. The accuracy of this approach has been proved by dating samples within the ¹⁴C age range. Sediments spanning the penultimate deglaciation have also been dated. After a small correction for bioturbation, the age for this event is found to be 135.2 ± 3.5 ka. This date is ≈8 kyr before the peak in northern hemisphere insolation and suggests that deglaciation is initiated by a mechanism in the southern hemisphere or tropics. This isochron approach shows considerable promise for dating of sediments older than this event, which will provide further information about the timing and mechanisms of global climate change.     

山地灾害沉积物的测年综述

山地灾害事件发生的年代是理解其发育机制并作出预测的基础.在长时间尺度上,常用的测牟方法有光释光、14C、宇宙成因核素和火山灰测年等,其中的关键是能否在剖面中寻找到合适的测年物质.短时间尺度的测年方法以树轮为主,辅以地衣测年.在山地灾害中,不仅灾害沉积本身,其相关沉积物对事件年代也具有指示意义.在实际应用中,根据测年材料...     

海洋沉积物测年方法综述

海洋沉积物是研究古气候变化、海平面升降、地球化学循环等重大科学问题的载体之一,精确的测年结果是上述研究的重要前提.随着海洋沉积物测年理论、技术设备不断突破,测年方法如210pb法、137Cs法、AMS14C测年、光释光法、古地磁定年、天文轨道调谐法、氧同位素曲线对比、电子自旋共振法、铀系法等层出不穷.文章总结了海洋地质...     

Luminescence dating of Late Quaternary sediments from East Greenland

Luminescence dating based on K-feldspars and using both TL and OSL methods have been performed on 94 sediment samples from East Greenland. The ages go back more than 380 ka, but are mainly from the last interglaciation and the subsequent period and include both shallow-marine/coastal-fluvial and ice proximal meltwater sediments. Independent age control indicates that the dates in the first group generally fall within ± 15% of the expected age, although a few samples show larger deviations. The ice proximal sediments, on the other hand, show a much larger spread and exaggerated ages, probably owing to incomplete bleaching. OSL may give better results than TL in these sediments.     

Luminescence dating of loessic sediments from the Loess plateau, China

Loess/palaeosol sequences from the Loess plateau in China were investigated by combined infrared optically stimulated luminescence (IRSL) and thermoluminescence (TL) dating techniques in order to study the luminescence properties of the loessic sediments and to provide a direct chronological link for correlation and position of the last interglacial soil in Central Asia and the Loess plateau in China. Sensitivity changes were found for all samples through artificial bleaching of the samples. The greatest sensitivity changes, of up to 50%, were found for very old loess samples designated to be older than the Matu-yama/Brunhes magnetic boundary and hence older than 788,000±1,800 years. The upper dating limit, as investigated by the very old loess samples, ranges from 250,000 to 300,000 years, if the TL additive dose method is applied. The chronological position of the last interglacial soil S1 at the section near Lanzhou indicates luminescence age estimates ranging from 82,000 to 75,000 years for the marine-isotope stage 5 to 4 transition. However, the loess from below S1 yielded luminescence age estimates between 153,200±14,200 and 110,100±20,100 years for TL and IRSL additive dose methods, respectively. Thus, a direct correlation between the S1 and the first intercalated pedocomplex PC1 in Central Asia is most likely. Received: 31 March 1998 / Accepted: 25 October 1998