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1.
In three different areas in western Norway, large errors are obtained in the radiocarbon dates from lacustrine sediments close to marine/lacustrine sediment boundaries. Differences occur between radiocar-bon and pollenanalytic dates and between dates at isolation/ingression contacts for lakes of the same altitude above the present sea level. Younger radiocarbon dates are also obtained below older ones in undisturbed sediments. When divergent dates occur, the radiocarbon dates always seem to be the youngest. Large differences are also found between NaOH soluble and insoluble fractions of the same sediment samples. Insoluble fractions generally yield younger dates than the soluble. Differences are not found, however, for dates younger than about 8,000 B.P. The dating errors are connected to periods with more oligotrophic conditions with isoetides. Their roots penetrate older sediments. Due to contamination of the organic part of the sediment from partly decomposed roots, some radiocarbon dates will be too young. The isoetide vegetation and the dating errors disappear when the lakes become dystrophic.  相似文献   

2.
沉积物14 C的年龄测定一直是第四纪年代学研究的热点。文章对中国干旱、半干旱地区的内蒙古库伦泥炭剖面的泥炭全样、植物残体和孢粉浓缩物进行了AMS 14 C测年比较研究。从泥炭沉积物中提取用于AMS 14 C测年的孢粉浓缩物的关键步骤包括:过筛(125μm,63μm和10μm),重液浮选(比重1.9)及在180倍的体视显微镜下进行手工挑选。测年结果表明:孢粉浓缩物的测年值比同一层位的泥炭沉积物全样或植物残体的测年值老225~340年; 植物残体与孢粉浓缩物的测年结果较为接近。但是,距地表24~27cm处孢粉浓缩物给出了610~780A.D.的年龄值,远老于其下样点的年龄,而泥炭全样为现代样品,二者相差1255年,这可能与该样品孢粉纯度较低、掺杂有一些老碳物质燃烧后形成的微粒杂质有关。因此,提高孢粉纯度对孢粉浓缩物14 C测年至关重要。  相似文献   

3.
New radiocarbon dates, carbon and oxygen isotope analyses of mollusks and marl, and palynologic analysis have clarified age relationships at the Corry kettle bog in northwestern Pennsylvania. A 4000-yr difference in radiocarbon dates between basal peat and marl top, previously interpreted as caused by the presence of an unconformity, is reinterpreted as due to “hard-water effects” from carbonate bedrock and glacial deposits. A radiocarbon date of 14,000 yr from basal marl, previously used as a minimum age for glacial deposition, is also likely too old; instead a similar age is estimated for the base of the underlying lake clay. Radiocarbon dates on marl should be used with caution and supported by isotopic and pollen analyses.  相似文献   

4.
流域生物、化学风化对小冰期气候变化的响应   总被引：2，自引：3，他引：2  
岱海近500年沉积物的孢粉，介形类和地球化学记录表现了流域生物，化学风化对全球性小冰期气候波动明显而快速的响应，孢粉量的下降，化学风化强度的减弱（高Rb/Sr比值）以及意外湖花介（Limnocythere inopinata)的繁盛反映了小冰期最盛期岱海地区以干旱为特征的气候环境及其造成的高矿化度湖水，其中，介形类丰度及种类对环境变化的反应最迅速，植被生长的调节则要缓慢一些，内陆湖泊流域植被的衰弱与干旱环境下化学风化强度的减弱以及风尘作用的加强是相辅相成的。  相似文献   

5.
In order to quantify the reservoir age in Baltic Sea sediment, one sedimentary sequence from an isostatically isolated basin was subject to high-resolution AMS radiocarbon dates. Diatom analysis confirmed deposition during the Litorina Sea stage and later, in a freshwater lake. Macrofossils from well preserved seeds and other remnants from terrestrial plants were used for AMS datings. It is assumed that these fragile plant remains are not redeposited or affected by internal ages. The ages obtained from the macrofossils range from 6460±125 to 5580±75 14C yr BP. By comparing these radiocarbon ages with those obtained by bulk sediment dates, it was obvious that the bulk samples were affected by reservoir ages, resulting in too old ages. The reservoir ages varied within the sediment column; during the most saline phase, the reservoir age was approximately 750 yr, shortly after the isolation ca 400 yr and in the freshwater lake, the age differences between the two series were neglectable.  相似文献   

6.
湖泊沉积物的14 C和光释光测年* ——以固城湖为例   总被引：3，自引：0，他引：3  
富含有机质的湖泊沉积物被认为是14 C测年建立古环境记录年代标尺的理想材料,光释光测年方法近年开始应用于水成沉积物的定年。应用14 C和光释光两种方法对江苏固城湖湖心钻孔岩芯进行了年龄测定,结果表明全样有机质的14 C年龄与石英的光释光年龄存在系统差异,后者较前者年轻约2000年。系统光释光测年研究排除了光释光年龄低估的可能性,所以,二者的差异可能是湖泊沉积物碳库效应的反映。  相似文献   

7.
Palynology and sediment slumping in a high arctic Greenland lake   总被引：1，自引：1，他引：1  
A 103-cm core from a high arctic lake (80°49'N, 15°00'W) on Amdrup Land, NE Greenland has been analysed for pollen and other microfossils, as well as bulk samples from 10 cores for macrofossils. AMS 14C-dating of aquatic mosses ( Drepanocladus exannulatus and Scorpidiurn scorpioides ) revealed that the lowermost 90 cm of the core was deposited during a 1.5-2 ka interval in the early Holocene, before the appearance of Salix arctica which immigrated to N Greenland c . 7 ka BP. Armeria scabra , now extinct in the area, larval cases and apotomes of Apatcmiu zonella , today not known north of c. 77°N, and other indices point to a climate warmer than that of today. A thin, sandy layer 13 cm below the lake bottom, accompanied by marked changes in the content of pollen and macrofossils, such as the sudden occurrence of large amounts of Salix pollen and leaves, indicates a large hiatus. This is confirmed by the dating to c . 7 ka BP of mosses just below the sand, and of Salix leaves just above it to c . 1 ka BP. The hiatus must be caused by sediment slumping which transported all the sediment from the intervening period to the deeper part of the lake. A similar, sandy layer in the uppermost 2–6 cm may be caused by younger, (sub-)recent slumping, or both may be the result of the same disturbance. The possibility of a reservoir effect causing too old 14C-ages for the moss is discussed.  相似文献   

8.
In this paper we briefly compare previously published data on four lateglacial sediment profiles from lakes Holzmaar and Meerfelder Maar in the Eifel region. Two of these profiles (one from each lake) are varvedated, whereas the other two pollen profiles are on depth scales. In general, the palynological signal in all profiles compares well, proving the regional validity of the individual data sets. There are some discrepancies between Holzmaar and Meerfelder Maar, however, mainly concerning varve dating of major lateglacial features, and these are explained after detailed correlation using a combination of varve dating and palynological signals. The result is a consistent varve-dated biostratigraphy for the Lateglacial in the Eifel region. Moreover, it is demonstrated that a combination of varve chronology and high resolution palynostratigraphy in several lake profiles enables even gaps of a few decades up to a few centuries to be detected, thus providing a valuable tool for precise regional palaeoenvironmental study. This multi-core study on two lakes from the same region demonstrates the likelihood of undetected errors in single-core studies on non-varved sediments.  相似文献   

9.
Palaeomagnetic and palynological studies have been made of the sediments of two Finnish lakes. Geomagnetic field changes in Finland can be traced since lake sediments record the ambient geomagnetic field at, or close to, the time they are deposited. Pollen analyses provided a useful method of dating these changes since 14C dates proved to be misleading.  相似文献   

10.
Radiocarbon dating of bulk organic matter is the most commonly used method for establishing chronologies of lake sediments for palaeoclimate reconstructions on the Tibetan Plateau. However, this method is likely to be problematic because the dated material often suffers from old carbon contamination. Recently, advances in luminescence‐based chronological techniques have provided new options for dating lacustrine sediments. In the current study, we tested for the first time the applicability of a new post‐IR IRSL (pIRIR) measurement protocol for dating fine‐grained polymineral material from a deep‐lake sediment core from the central part of Tangra Yumco, on the southern Tibetan Plateau. Our results show that: (i) radioactive disequilibria in the uranium decay chain were observed in the studied lake sediments, and thus taken into account for dose rate calculation by using a dynamic modelling approach; (ii) the suitability and robustness of the pIRIR protocol measured at 150°C (pIRIR150) for our samples are confirmed by a set of luminescence characteristic tests as well as the agreement with an independent age control; (iii) turbidite deposition partly caused an insufficient resetting of luminescence signals and thus apparent overestimation in luminescence dating; (iv) compared with the luminescence‐based age‐depth model, the 14C ages of bulk organic matter from the studied core generally yielded an age difference of ～2 ka, which is attributed to hardwater reservoir effects in Tangra Yumco. This study highlights the need for multi‐dating approaches of lake sedimentary archives on the Tibetan Plateau.  相似文献   

11.
Varved lake sediments can be used to set multiple environmental proxies within a calendar year time scale. We undertook a systematic survey of lakes in the Province of Värmland, west central Sweden, with the aim of finding continuous varved lake sediment sequences covering the majority of the Holocene. In Fennoscandia, such sediments have previously only been recorded in northern Sweden and in southern and central Finland. By following a selective process and fieldwork we discovered three new varved sites (i.e. Furskogstjärnet, Mötterudstjärnet and Kälksjön). We found that lakes with varved sediments have several common lake morphometry properties and lake catchment characteristics such as maximum water depth, maximum water depth/lake surface area ratio, catchment soil types, altitude and number of inflows. Varve chronologies, supported by AMS-14C dating and tephrochronology were established for two of the sediment profiles. These varve chronologies are the longest geological records with an annual resolution known to exist in Sweden. In Furskogstjärnet, the AMS- 14C dates based on terrestrial plant macrofossils at several levels deviate significantly from the varve based time-depth curve. In Motterudstjarnet, a fully reasonable time-depth model based on the 14C dates gives older ages in the lower part of the sequence compared to the varve chronology. These results highlight that seemingly acceptable AMS radiocarbon dates may be erroneous. They also point to the fact that varved lake sediments are reliable geological archives with respect to chronological control and accuracy. Thus, these archives should be of prime interest for studies of climate and environmental change undertaken with the aim of providing sub-decadal resolution proxy data sets.  相似文献   

12.
The near-surface layer of the dominant microhabitat types on a peatland in the Rainy River area, northwestern Ontario, Canada was dated using a number of previously published methods. No single method was found to be appropriate in all microhabitat types, and discrepancies of up to 15 yr over a time-scale of 80 yr were encountered. The results suggested that the post-depositional mobility of 210Pb, and possibly its atmospheric supply, are affected by microhabitat type. 210Pb dates consistently underestimated independently derived dates in the uppermost sections of the cores. Ages derived by measuring the length of annual increments on moss stems were in general agreement with the 210Pb dates, but were subject to bias where the moss remains were poorly preserved. 14C analyses on material from the base to the near-surface horizon confirmed the modern (post-AD 1950) origin suggested by the 210Pb and moss increment dates. Increased ash content at the base of the near-surface layer was correlated with the dustbowl of the 1930s, but assignment of dates to other stratigraphic markers (charcoal, Ambrosia pollen, and Larix needles) was less certain. The ash peak (55 yr before AD 1990) was used as datum for relative dating methods based on assumptions of a constant rate of peat accumulation (constant bulk density method) and constant rate of pollen accumulation (pollen density method). The constant bulk density method provided quick, inexpensive estimates of maximum age, but could not be extrapolated beyond datum, nor applied to cores showing marked stratigraphic changes. The pollen density method was a valuable but time-consuming alternative for dating peatland depressions, which lack annual moss increments and are subject to 210Pb leaching.  相似文献   

13.
Reconstruction of temporal and spatial climate development on a seasonal basis during the last few centuries may help us better understand modern-day interplay between natural and anthropogenic climate variability. The objective of this paper is to reconstruct hydrology and landscape changes of East Siberia during the termination of the Little Ice Age and the subsequent Recent Warming. We analysed sediment samples from the saltwater Sulfatnoe Lake, Bolshoye Alginskoe and freshwater Shuchie Lake using high-resolution X-ray fluorescence spectroscopy at 1-mm scan resolution, Fourier-transform infrared techniques and pollen analyses. The depth–age models of the cores were constructed by 210Pb activity using the constant rate of supply model. The lake sediment cover of these lakes began to form from ca. 1870. Three significant periods (1870–1895, 1895–1925 and from 1925 to the present) were defined in hydrology and chemical regime of these lakes for the past 140 years. Lake levels were extremely low and high saturated with salts during the final period of the Little Ice Age. Lake levels began to slowly rise from 1870 to 1895 and vegetation was poor at that period. Intensive desalination of the lakes occurred in 1895–1925, and environment conditions were temperate and favourable for the majority of the taxa of the regional vegetation. Regional precipitation significantly increased and water saturation of the catchments was high from 1925 to the present. The chemical precipitation of carbonate stopped completely in Lake Shichie and reduced considerably in Lake Sulfatnoe and B. Alginskoe. Strong increasing trend of weathering of the lake catchments began in 1970 and still continues.  相似文献   

14.
Changes in vegetation were tracked from a well-dated sediment core from a boreal lake, Lake 239, at ～200-year resolution over the Holocene. This presently oligotrophic lake is located ～100-km east from the present-day parkland-forest ecotone in northwestern Ontario. Near-shore sediment core transects from Lake 239 have previously shown this lake was at least 8-m lower than present in the mid-Holocene, or ～58% less lake volume in comparison to today. Large shifts were expected in the terrestrial vegetation if the low lake levels were related to climate. The core from Lake 239 shows increases in the relative abundance and concentration of pollen such as Cupressaceae and Ambrosia, indicating a more open boreal forest between ～4500–8000 cal yr BP. Pollen-based inferences of average, summer and winter temperatures suggest that temperatures were on average up to 1–2 °C warmer than today, with winter temperatures up to 4 °C warmer. The pollen inferences also suggest enhanced precipitation, likely in the summer, but with an overall increase in evaporation and evapotranspiration resulting in reduced effective moisture. To assess regional climate changes, pollen-based reconstructions of temperature and precipitation were developed and synthesized from sediment cores from eight previously published lakes, from which pollen sites were available to both the west and east of Lake 239, spanning present-day prairie lakes to forested lakes up to 300 km east of the prairie-boreal ecotone. All sites show shifts in pollen assemblages that indicate a warm mid-Holocene period; prairie sites west of the Experimental Lakes Area (ELA) show mid-Holocene decreases in precipitation relative to today, whereas sites near or east of ELA show consistent increases in precipitation, but with increased temperatures and enhanced evaporation during the mid-Holocene.  相似文献   

15.
Altogether 62 radiocarbon dates from 17 pollen diagrams (16 sites) of Late Weichselian lake sediments in S Sweden have been determined to establish the chronostratigraphic sequence as defined by the pollen stratigraphy. This method of controlling the dates and the validity of the assumed synchronous Nordic climatostratigraphy are discussed. Most of the dates sufer from age erors, mainly caused by contamination by redeposited organic material. The effect of this source of error increases with decreasing percent of organic carbon in the deated samples. The significance of these problems for the Late Weichselian chronostratigraphic subdivision as well as ways of solving them are discussed.  相似文献   

16.
中国北方第四纪孢粉提取方法研究   总被引：3，自引：0，他引：3  
孢粉提取方法是第四纪孢粉学研究的基础和前提，中国北方地区具有独特的自然地理条件，第四纪以来形成了特有的沉积物类型，不同沉积物的孢粉提取研究一直以来是中国孢粉学研究的难点和热点。本文选取了中国北方不同沉积相的沉积物样品（黄土与古土壤样品17个、湖泊样品沉积物20个、风成砂样品11个、泥炭沼泽样品10、考古点文化层6个、表土样品10个）共74个，通过对同一种样品不同处理方法的对比研究，就中国北方不同沉积物的孢粉提取方法进行了探讨。对比研究发现，黄土及古土壤中的孢粉提取是以氢氟酸筛选法效果佳，值得注意的是盐酸与氢氟酸对样品的处理过程，进行多次彻底处理才能达到很好的效果；湖泊沉积物孢粉提取可采用传统的氢氟酸法，对干旱区含砂量较高的湖泊沉积物及风成砂样品需要配合重液浮选法进行；泥炭、沼泽中孢粉提取应采用重液浮选法，配合筛选法进行；考古点文化层沉积物孢粉提取应采用重液浮选法，并将重液浮选出来的样品配合以氢氟酸处理能达到很好效果；表土样品可以采用无酸碱重液浮选法。同时还探讨了孢粉提取中的分散剂和硝酸的使用及酸碱的使用顺序。  相似文献   

17.
《Quaternary Science Reviews》1999,18(4-5):611-630
The Late Quaternary environmental history of the Konya plain, in south central Turkey, is used to examine sediment facies changes in a shallow non-outlet basin which has experienced major climatically driven changes in lake extent. Two principal types of sedimentary archive are used to reconstruct a palaeoenvironmental record, namely alluvial sequences on the Çarşamba alluvial fan and sediments from residual lakes. The latter have been used to investigate broader climatic and vegetational histories via palaeolimnological techniques including pollen, diatom and stable isotope analysis. These changes are dated here by radiometric techniques including radiocarbon (AMS and conventional), OSL, and U–Th. Chronological agreement is generally good between the different dating techniques, although typically there is greatly reduced precision beyond ca. 25 ka. Lake sediment cores investigated have basal ages beyond the range of 14C dating, but contain hiatuses as a result of subsequent alternation between phases of lacustrine sedimentation and aeolian deflation. In contrast to most deepwater non-outlet lake systems, the Konya basin may have been occupied by a single extensive lake for as little as 10% of Late Quaternary time, mainly around the time of the LGM. This lake highstand was followed by an important arid interval. In the absence of unbroken chronostratigraphic sequences, palaeohydrological investigation of shallow non-outlet lakes may require analysis of basin-wide changes in sedimentation rather than reliance on single core records. Stratigraphic continuity in such sedimentary environments cannot be assumed, and requires independent chronological control through radiometric dating.  相似文献   

18.
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.  相似文献   

19.
汪苗  鹿化煜 《第四纪研究》2019,39(5):1071-1082
我国南部雷州半岛是亚洲季风影响的核心区,区域内田洋、青桐洋和九斗洋这3个干玛珥湖沉积序列可为揭示低纬地区亚洲季风的轨道尺度变化提供关键证据。查明这些玛珥湖的形成年代、基底玄武岩的地球化学组成及其风化产物特征,为进一步获得沉积序列的年代控制、揭示沉积物替代指标的古气候意义提供了基础。本研究对3个干玛珥湖基底的玄武岩进行了K-Ar测年、地球化学成分和玛珥湖周围风化剖面粘土矿物的测试分析,得出了以下结论:1)田洋玛珥湖湖盆的形成年代为0.73~0.87Ma,首次报道的青桐洋和九斗洋湖盆的形成时代分别为0.82 Ma和0.81~1.08 Ma,它们均属于早更新世晚期到中更新世早期的石峁岭期火山射气喷发形成的。2)田洋、青桐洋和九斗洋玛珥湖基底玄武岩均属于石英拉斑玄武岩。同一时期喷发的石英拉斑玄武岩广泛分布在玛珥湖的周围,这些玄武岩风化的产物以高岭石、三水铝石以及赤铁矿为主,在降水较为充沛的时期,物理或化学风化的产物随地表径流汇入湖盆,是玛珥湖沉积的主要来源。本研究为正在进行的低纬地区更新世轨道尺度季风降水重建提供了基础。  相似文献   

20.
长江中下游地区浅水湖泊密布,全新世该区湖泊沉积的模式还不清晰。本研究在长江中下游的南漪湖、升金湖和菜子湖这3个湖泊开展了多钻孔AMS^14C测年工作,测年结果显示这些湖泊沉积地层中广泛出现长时间尺度的沉积物缺失。南漪湖湖泊钻孔的沉积物14C年龄介于5668~7828cal.aB.P.,菜子湖湖泊钻孔的沉积物^14C年龄介于6221~7929cal.aB.P.,升金湖围垦区钻孔14C年龄介于6302~7049cal.aB.P.。结合该地区以往湖泊钻孔研究资料,发现全新世长江中下游两岸洼地湖泊存在较广泛的6~3ka的沉积间断。结合长江水位重建资料,笔者提出关于全新世湖泊沉积存有长期间断的新认识:即6~3ka,长江水位相对平稳,湖泊沉积物虽有堆积,但易于被侵蚀搬运造成沉积间断;与此对应的是,在约8~7ka,海面上升造成长江水位较快上升,由于顶托作用,湖泊沉积物持续堆积;在约3ka以来,由于人类活动的影响,以及长江水位的进一步上升,湖泊沉积物也易于堆积,但在一些湖区沉积物也会被侵蚀。在6~3ka之间湖泊沉积物易于被侵蚀的一个可能原因是该时段长江上游来沙来水减少,自然堤易被破坏,对两岸湖泊洼地的封堵作用减少,使得湖泊泥沙易被侵蚀入江。  相似文献   

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