首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 547 毫秒
1.
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   

2.
通过对柴达木盆地察尔汗古湖贝壳堤剖面沉积物、生物壳体碳酸盐同位素及沉积物碳酸盐含量等多项指标的分析,探讨了贝壳堤剖面记录39.6~17.1kaB.P.(未校正14 C测年,下同)期间碳酸盐和贝壳化石碳氧稳定同位素的特征和相关关系,指出壳体化石及沉积物碳酸盐的碳氧同位素是湖泊水体温度和盐度的反映,其中δ18 O对温度的指示意义更敏感,是湖泊和古气候演化很好的代用指标。根据这些指标重建的古气候与环境变化显示,在39.6~35.5kaB.P.期间,柴达木盆地处于温暖湿润期,周围山地降水增加,盆地内湖泊发育;自35.5kaB.P.开始,气候较前期更加湿润,是湖泊发育的最佳期;22.1kaB.P.以后,气候逐步转入较温暖的干燥期,湖泊开始萎缩、退化;末期17.1kaB.P.气候环境急剧恶化,形成石盐结晶,湖泊高湖面演化史结束。  相似文献   

3.
利用常规14C、加速器(AMS)方法对柴达木盆地察尔汗湖一典型古湖相沉积剖面系统测年,并对测年结果进行不同组分和不同实验室对比,确定该剖面发育于察尔汗古湖高湖面时期(39.7 ka B.P.～17.5 ka B.P.).通过对削面沉积物正构烷烃的系统分析,结果表明正构烷烃携带了重要的湖泊演化的环境和生物信息,其中nC2...  相似文献   

4.
柴达木盆地石炭系是中国北方新区新层系油气勘探调查研究的热点,盆地东部地区石炭系出露较全,但多数学者认为柴东地区不存在二叠系。针对扎布萨尕秀组归属于石炭纪还是二叠纪的争议,采集扎布萨尕秀组的砂岩碎屑锆石开展了U-Pb LA-ICP-MS同位素测年,获得40个和谐年龄分布在2488±34 Ma至288±3 Ma之间,最年轻的一组碎屑锆石的加权平均年龄288.0±2.0 Ma（MSWD=0.112,n=21）。这意味着该扎布萨尕秀组中的砂岩沉积于288 Ma之后,晚于国际石炭—二叠纪界限（299 Ma）。依据野外调查和已有研究成果,扎布萨尕秀组中赋存丰富的虫筳科化石,其中膨胀型虫筳属Pseudoschwagerina或Sphaeroschwagerina的出现是进入二叠纪的重要标志。结合碎屑锆石年代学和虫筳属化石特征,扎布萨尕秀组应归属早二叠世,说明柴达木东部地区存在二叠纪沉积记录。该认识为深化柴达木盆地晚古生代—中生代沉积建造和构造演化提供了重要的地层学证据。   相似文献   

5.
Reliable dating of glaciomarine sediments deposited on the Antarctic shelf since the Last Glacial Maximum (LGM) is challenging because of the rarity of calcareous (micro‐) fossils and the recycling of fossil organic matter. Consequently, radiocarbon (14C) ages of the acid‐insoluble organic fraction (AIO) of the sediments bear uncertainties that are difficult to quantify. Here we present the results of three different methods to date a sedimentary unit consisting of diatomaceous ooze and diatomaceous mud that was deposited following the last deglaciation at five core sites on the inner shelf in the western Amundsen Sea (West Antarctica). In three cores conventional 14C dating of the AIO in bulk samples yielded age reversals down‐core, but at all sites the AIO 14C ages obtained from diatomaceous ooze within the diatom‐rich unit yielded similar uncorrected 14C ages between 13 517 ± 56 and 11 543 ± 47 years before present (a BP). Correction of these ages by subtracting the core‐top ages, which probably reflect present‐day deposition (as indicated by 210Pb dating of the sediment surface at one core site), yielded ages between ca. 10 500 and 8400 cal. a BP. Correction of the AIO ages of the diatomaceous ooze by only subtracting the marine reservoir effect (MRE) of 1300 a indicated deposition of the diatom‐rich sediments between 14 100 and 11 900 cal. a BP. Most of these ages are consistent with age constraints between 13.0 and 8.0 ka for the diatom‐rich unit, which we obtained by correlating the relative palaeomagnetic intensity (RPI) records of three of the sediment cores with global and regional reference curves. As a third dating technique we applied conventional radiocarbon dating of the AIO included in acid‐cleaned diatom hard parts extracted from the diatomaceous ooze. This method yielded uncorrected 14C ages of only 5111 ± 38 and 5106 ± 38 a BP, respectively. We reject these young ages, because they are likely to be overprinted by the adsorption of modern atmospheric carbon dioxide onto the surfaces of the diatom hard parts prior to sample graphitisation and combustion for 14C dating. The deposition of the diatom‐rich unit in the western Amundsen Sea suggests deglaciation of the inner shelf before ca. 13 ka BP. The deposition of diatomaceous oozes elsewhere on the Antarctic shelf around the same time, however, seems to be coincidental rather than directly related. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

6.
Primnoa resedaeformis is a deep-sea gorgonian coral with a two-part skeleton of calcite and gorgonin (a fibrillar protein), potentially containing long-term records of valuable paleo-environmental information. For various reasons, both radiocarbon and U/Th dating of these corals is problematic over the last few centuries. This paper explores aspartic acid racemization dating of the gorgonin fraction in modern and fossil specimens collected from the NW Atlantic Ocean. Radiocarbon dating of the fossil specimen indicates a lifespan of 700 ± 100 years, the longest yet documented for any octocoral. Gorgonin amino acid compositions were identical in the fossil and modern specimens, indicating resistance to organic diagenesis. Similar to bone collagen, the fibrillar protein of gorgonin may impose conformational constraints on the racemization of Asp at low temperatures. The rate of racemization of aspartic acid (d/l-Asp) was similar to previously published results from an 1800 year old anemone (Gerardia). The age equation was: age (years BP 2000 AD) = [(d/l − 0.020 (±.002))/.0011 (±.0001)]2 (r2 = 0.97, p < .001). The error in an age estimate calculated by d/l-Asp was marginally better than that for 14C dating over the most recent 50-200 years, although the dating error may be improved by inclusion of more samples over a broader time range. These results suggest that d/l-Asp dating may be useful in augmenting 14C dating in cases where 14C calibrations yield two or more intercept ages, or in screening samples for further 14C or U/Th dating.  相似文献   

7.
The stable isotopic composition of the bivalve shell has been widely used to reconstruct the pa-laeo-climate and palaeo-environment. The climatic and environmental significance of carbon isotopic composition of the bivalve shell is still in dispute, and incorporation of metabolic carbon can obscure carbon isotope records of dis-solved inorganic carbon. This study deals with freshwater bivalve, Corbicula fluminea aragonite shell. The results indicated that the δ13C values of bivalve shells deposited out of equilibrium with the host water and showed an onto-genic decrease, indicating that there are metabolic effects and more metabolic carbon is incorporated into larger shells. The proportion of metabolic carbon of shells varies between 19.8% and 26.8%. However, δ13CS can still be used as qualitative indicators of δ13CDIC and environmental processes that occurred during shell growth.  相似文献   

8.
max vs the present depth of the Kupferschiefer, soluble organic matter (SOM) yields, and relative proportions of saturated and aromatic hydrocarbons of the SOM provide evidence for an oxidative alteration of organic matter in highly mineralized Kupferschiefer samples near the Rote F?ule zones. This is confirmed by differences in the composition of the saturated and aromatic hydrocarbon fractions of the soluble organic matter: Saturated hydrocarbons from Rote F?ule samples are dominated by short-chain n-alkanes and higher abundances of pristane and phytane relative to heptadecane (n-C17) and octadecane (n-C18), respectively, compared with samples more distant to the Rote F?ule zone. Compositional changes of the aromatic hydrocarbon fractions with decreasing distance to that zone are characterized by the occurrence of polycyclic aromatic hydrocarbons and elevated ratios of phenanthrene to methylphenanthrenes that are attributed to demethylation reactions and resulted in a decrease of the methylphenanthrene index (MPI 1). Kupferschiefer samples from the barren zone of the Polish Basin do not show these alteration patterns. The observed variations in organic matter composition with burial depth are consistent with changes due to increasing thermal maturation. Maturity assessment is achieved from MPI 1 and the methyldibenzothiophene ratio (MDR). From the relationship between the maturity of organic matter in terms of vitrinite reflectance values and depth of the Kupferschiefer strata, a continuous increase in reflectance of vitrinite is obtained within the Polish Basin. The alteration pattern of organic matter related to base metal mineralization of the Kupferschiefer corresponds to changes in the isotopic composition of organic carbon and calcite. Kerogen within, or close to, Rote F?ule zone is enriched in 13C caused by the preferential release of isotopically light organic compounds through progressive degradation of organic matter. The opposite tendency towards lower δ 13C and δ 18O values of calcite provides evidence for isotopic exchange between carbonate and the oxidizing, ore-bearing solutions and for organic matter remineralization. In contrast, organic matter and calcite from the Kupferschiefer do not show regular trends in δ 13C with increasing thermal maturation. Received: 25 June 1999 / Accepted: 1 December 1999  相似文献   

9.
Lipids extracted from lacustrine deposits in the paleolake Qarhan of the Qaidam basin in the northeastern Tibetan Plateau were determined by conventional gas chromatography-mass spectrometry. Several series of biomarkers were identified, mainly including n-alkanes, n-alkan-2-ones, n-alkanoic acids, branched alkanes, triterpenoids and steroids, indicative of various biogenic contributions. On the basis of cluster analysis, the n-C15, n-C17, n-C19 alkanes were proposed to be derived from algae and/or photosynthetic bacteria, the n-C21, n-C23, n-C25 homologues from aquatic plants, and the n-C29, n-C31 homologues from vascular plants. In contrast, the n-C27 alkane is not categorized in the n-C29 and n-C31 group of alkanes, probably due to more complex origins including both aquatic and vascular plants, and/or differential biodegradation. Stratigraphically, layers-2, 4 and 5 were found to show a close relationship in n-alkane distribution, associated with a positive shift in carbon isotope composition of bulk organic matter (δ13Corg), inferring a cold/dry period. Layers-1 and 6 were clustered together in association with a negative δ13Corg, excursion, probably indicating a relatively warm/humid climate. The potential coupling between the n-alkane distributions and δ13Corg, suggests a consequence of vegetation change in response to climate change, with the late MIS3 being shown to be unstable, thought to be the climatic optimum in the Tibetan Plateau. Our results suggest that the cluster analysis used in this study probably provides an effective and authentic method to investigate the n-alkane distribution in paleolake sediments.  相似文献   

10.
Radiocarbon Dating of Soil Organic Matter   总被引：2，自引：0，他引：2  
Radiocarbon ages of soil organic matter are evaluated with a model which incorporates the dynamics of the14C content of soil organic matter. Measured14C ages of soil organic matter or its fractions are always younger than the true ages of soils due to continuous input of organic matter into soils. Differences in soil C dynamics due to climate or soil depth will result in significantly different14C signatures of soil organic matter for soils of the same age. As a result, the deviation of the measured14C age from the true age of soil formation could differ significantly among different soils or soil horizons. Our model calculations also suggest that14C ages of soil organic matter will eventually reach a steady state provided that no climatic or ecological perturbations occur. Once a soil or a soil horizon has reached a steady state,14C dating of soil organic matter will provide no useful information regarding the age of the soil. However, for soils in which steady state has not been reached, it is possible to estimate the age of soil formation by modeling the measured14C contents of soil organic matter. Radiocarbon dating of buried soils could, in general, overestimate the true age of the burial by as much as the steady-state age of the soil or soil horizon.  相似文献   

11.
柴达木盆地察尔汗贝壳堤剖面记录了约39.7～17.514CkaB.P.期间古湖泊高湖面扩张、收缩的演化历史。剖面中丰富的介形类,皆为我国东部山间盆地～平原区系与西部高原区系常见种。依据青海地区现代介形类分布调查,可知研究剖面高湖面发育期为淡水湖,其中高分异度介形类组合代表近静水的浅湖环境,单种或近单种土星介大量出现层位指示缓流水环境,大量原生双壳类共生可能指示静水、较深水环境。据地层中介形类分布,自下而上划分为7个组合-沉积环境带,分别为:①少量湖沼种出现（积水洼地形成带）——湖泊前期,②单种缓流水种连续出现（湖泊初始形成带）——高湖面初始形成期,③高分异度组合、介形类富集（浅水大湖带）——高湖面水深加大期,④中分异度组合、介形类少量出现（深湖带）——高湖面最盛期,⑤中分异度组合、介形类少量出现、双壳类连续富集（湖泊变浅带）——高湖面略下降期,⑥缓流水介形类繁盛、双壳类多少不等（湖泊水位下降带）——古湖泊退缩期和⑦介形类稀少带——古湖泊快速消亡期。贝壳堤剖面所见双壳类在现代介形类调查区域没有发现,它们在地层中的出现,进一步指示大体相当MIS3阶段时期柴达木盆地比现代更温暖的气候背景,降水条件好于现代。  相似文献   

12.
Biogenic amino acids, taken as representative of organic matter, were analyzed to determine the apparent degradation rate constant in boreal terrestrial sediment. Age determination using 14C dating gave two rate constants: the initial degradation rate constant for glycine (kGLY 1), the simplest amino acid, was 1.5 × 10−3 yr−1 (r = 0.97) until about 2200 yr BP. After the inflection point, the rate constant kGLY 2 was 9.1 × 10−5 yr−1 (r = 0.73). The degradation of amino acids in the labile organic matter in the sediment was markedly affected by rapid processes. After the inflection point, the rate constant profiles for sub-surface amino acids were shown to have discontinuous relationships with sediment age. One pattern which emerged in the vertical distribution is that the biogenic amino acid degradation rate constant k was far greater in the labile organic matter phase than that in the refractory organic matter over the past 10,000 years.  相似文献   

13.
The accuracy of Arctic lake chronologies has been assessed by measuring the14C activities of modern carbon sources and applying these isotopic mass balances to dating fossil lake materials. Small (<1 km2) shallow (<25 m) Arctic lakes with watersheds <12 km2have soil and peat stratigraphic sections with14C activities ranging from 98 to 51% Modern. The14C activity of particulate organic carbon, dissolved organic carbon, and dissolved inorganic carbon from lake and stream waters ranges from 121 to 95% Modern. The sediment–water interface of the studied lakes shows consistent14C ages of ∼100014C yr, although the14C activity of living aquatic vegetation is 115% Modern. Radiocarbon measurements of components of the lacustrine carbon pool imply that the ∼100014C yr age of the sediment–water interface results from deposition of14C-depleted organic matter derived from the watershed.  相似文献   

14.
The present study focused on detecting variations in trophic relationships among blue crab (Callinectes sapidus) consumers according to water quality along two estuaries in North Carolina. Stable isotope (δ15N and δ13C) analyses of particulate organic matter and bivalve(Rangia cuneata andCorbicula fluminea) food sources were examined in combination with an Isosource mixing model. Results suggest that blue crab δ13C values increased significantly with increasing salinity from upper to lower sites along the Neuse River estuary (NRE; R2 = 0.87, p < 0.01) and Alligator River estuary (R2 = 0.92, p < 0.01). There was a positive relationship between blue crab δ15N values and nitrate concentrations for the NRE (R2 = 0.48, p = 0.12). This study found that blue crab δ13C values increased with salinity from upper to lower regions along both estuaries. Results suggest that blue crab production may have used alternative food sources that were isotopically (δ13C) depleted, especially in the upper NRE, and enriched sources in the mid to lower regions of both estuaries. Consumers sampled from the upper NRE may be influenced by higher nitrogen input from urban land use and municipal wastewater.  相似文献   

15.
Both aquatic and land snails are common in the geologic record, but their utility in dating is greatly restricted by their well-documented tendency to yield14C dates inconsistent with true14C ages. In this study, we examine the use of14C ages from (1) small, previously unstudied, terrestrial snails to date hosting spring deposits and from (2) cooccuring aquatic snails to constrain groundwater travel times during the last glacial period. Our study area in the southern Great Basin encompasses Yucca Mountain, site of the proposed high-level nuclear waste repository, where information on the age and extent of past high water tables and on groundwater flow times is crucial to several licensing issues. Our results show that shells of small terrestrial snails belonging toValloniasp. yield14C dates consistent with14C ages of associated carbonized wood. These results imply that these taxa can provide reliable14C age control on the broadly distributed deposits in which they have been described. In contrast, cooccurring aquatic snails from fossil spring deposits yield14C ages generally greater than the control age. This is because the aquatic shells often formed in spring waters that had an initial14C deficiency. However, the magnitude of the deficiency is much less than that observed in nearby modern springs, arguing for much higher average14C contents in late Pleistocene groundwaters in these basins. If representative, this implies shorter groundwater travel times through aquifers in southern Nevada during late-glacial time.  相似文献   

16.
The Bohemian Cretaceous Basin represents a complex hydrogeological system composed of several aquifers with very favourable hydrogeological properties. These aquifers have been exploited for many years. The sustainability of such resources might be guaranteed by well organised water management, which requires a detailed knowledge about the functioning of the hydrogeological system. Although many efforts have previously been made to evaluate groundwater residence time, the many intricate geochemical processes complicate groundwater dating. The current study clarifies the functioning of this complex hydrogeological basin using hydrogeochemical and isotopic investigations. Chemical data and a combination of 13C and 14C isotopes within the Cenomanian and the Turonian layers indicate groundwater interactions with deep-seated CO2, rock matrix, surface waters and fossil organic matter. Very depleted δ13C values (average δ13C ∼ −13.4‰) suggest interactions with fossil organic matter, whereas enriched values account for the interaction with deep CO2 gas ascending from the upper mantle via the numerous faults and fractures, and also, to a lesser extent, from calcite dissolution. Geochemical processes that take place in the system cause a clear depletion in 14C that greatly complicates groundwater residence time evaluation. Different dilution correction models have been applied considering the different C origins. The stable isotope content, mainly 18O values, indicates both the contribution of modern precipitation and the partial infiltration of palaeowaters during colder climatic conditions from the end of the Pleistocene. The apparent 14C groundwater ages range from modern to 11.1 ka BP, which suggests some post glacial infiltration from melting ice sheets. Finally, all the acquired information was used to propose a conceptual model of C origin within the basin.  相似文献   

17.
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。  相似文献   

18.
A series of confirmed and suspected dammed palaeo‐lake sedimentary successions is scattered within the middle Yarlung Tsangpo valley in Tibet. However, the chronology, the genesis of the dam and its location, the water level of the dammed lake, the process of dam failure and the spatiotemporal relationships between the sedimentary successions remain controversial. Here, we focus on one sedimentary succession of the suspected dammed palaeo‐lake at Xigazê. We measured the grain‐size distribution, magnetic susceptibility, organic and inorganic carbon content, and δ13Corg and δ15Ntotal ratios of the sediments. In addition, we measured the δ18Oshell and δ13Cshell values of modern and fossil Radix sp. shells, and the δ18Owater and δ13CDIC values of the ambient water with different hydrological regimes. The results indicate that the δ18Oshell values of modern Radix sp. and the δ18Owater of the ambient water body significantly depend on its hydrological status. In addition, a strong positive relationship was observed between δ18Oshell values of modern Radix sp. shells and the δ18Owater of the ambient water on the Tibetan Plateau. According to this correlation, the δ18Owater values of the palaeo‐water body are reconstructed using the δ18Oshell values of Radix sp. fossil shells in the Xigazê section. Further, based on the δ18Oshell values of fossil Radix sp., the reconstructed δ18Owater of the palaeo‐water body and the specific habitats of Radix sp., we infer that the sedimentary succession in the Xigazê broad valley was mainly formed within the backwater terminal zone of a dammed palaeo‐lake and that the elevation of the water level of the lake was approximately 3811 m a.s.l. AMS 14C dating indicates that the deposits of the dammed palaeo‐lake were formed at about 33–22 cal. ka BP. Finally, the presence of Radix sp. fossil shells within the Xigazê section suggests that Radix sp. survived the late Last Glacial Period on the Tibetan Plateau.  相似文献   

19.
Lake Xingcuo is a small closed,hard-water lake ,situated on eastern Tibet Plateau.Stable isotope data(δ^18O and δ^13C) from the freshwater snail Gyraulus sibirica(Dunker)in a34 cm long,radioactive isotope-dated sediment core represent the last 200 years of Lake Xingcuo environmental history.Carbon and oxygen isotope ratios in the shells of the freshwater snail bear information on the isotopic composition of the water in which the shells were formed ,which in turn characterizes the climatic conditions prevailing during the snail‘s life span.Whole-shell and incremental growth data were collected from modern and fossil shells from Lake Xingcuo.The δ18^O values of modern shells from Lake Xingcuo are in equilibrium with high δ^13CTDIC.By calibrating δ^18O and δ^13C in the shell Gyraulus sibirica(Dunker)with in-strument-measured data for the period 1954-1992,we found that the δ^18O of the snail shells is an efficient indicator to reveal air temperature in the warmer half year instead of that around the whole year,and that there is a certain positive correlation between index δ^18O and the run-ning average temperature in the warmer half-yiar period.Climatic variability on eastern Tibet Plateau,for the last two centuries,has been successfully inferred from the δ^18O record in freshwater snails in the sediments of Lake Xingcuo.As such,the last 200 years of palaeocli-matic record for this region can be separated into three periods representing oscillations between warming and cooling,which are confirmed by the Guliya ice record on the Tibet Plateau.  相似文献   

20.
长江铀矿田位于诸广山复式岩体中南部,是典型的花岗岩型铀矿田.前人采用锆石U-Pb定年方法对赋矿花岗岩进行了年代学研究,但由于全岩和锆石铀含量较高,锆石往往发生了蜕晶化,可能导致锆石U-Pb定年数据散乱,影响锆石U-Pb年龄的可靠性.独居石是花岗岩中广泛存在的含铀副矿物,铀和钍含量均较高,可达10000×10-6,普通铅...  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号