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1.
Andesitic and basaltic andesitic tephra layers are abundant in Holocene deposits from the Antarctic Peninsula. Visually discernible tephra horizons occur in three lakes on Livingston Island. Tephra in two other lakes and in a moss bank on Elephant Island, with very low ash concentrations, were detected magnetically. Deception Island is the most likely volcanic source for the tephra. With direct 14C dating, age/depth curves, and cross-correlations at least 14 tephra horizons dating to between ca. 4700 and 250 yr B.P. were identified and now form the basis for a preliminary regional tephrochronology that will be a valuable dating tool for investigating the Holocene climatic history of Antarctica.  相似文献   

2.
This paper describes the petrological features and the ages of rock fractures filled mainly with carbonates at coastal outcrops of Yakushima Island, Japan. Microscopic observation and geochemical analysis were used to investigate the petrological features and the compositions of the fracture fillings. In addition, AMS 14C dating was also performed to estimate the ages of them. Microscopic study indicated that the fracture fillings contain not only cementing materials but also lithic fragments from host rock and bioclasts. SEM observation showed that the cements exhibit the characteristic textures of beachrocks. The cements were identified as Mg-calcite from XRD analysis. It was also observed from geochemical analysis that there were at least two stages of precipitation within the fractures. In addition, AMS 14C dating for seventeen samples of fracture fillings showed that the ages range from 2,460 to 5,130 years BP which is found to be younger than that of the uplifted coral at Yakushima Island (approximately 5,300–5,600 years BP). The results suggest that the fractures were solidified after the coral was uplifted.  相似文献   

3.
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4.
Thorium-230 dating on saline of the Lower Salt unit in pluvial Searles Lake, California, shows that this unit was formed between 24,500 and 32,000 years ago. The initial apparent 14C age of the lake is estimated to be about 900 years. After correcting for nonradiogenic 230Th and for the initial 14C age, excellent agreement between 230Th and 14C ages is obtained. The reliability of 230Th dating on salt deposit opens a new way for continuation of absolute chronology below the Lower Salt in Searles Lake.  相似文献   

5.
Thorium-230 dating on saline of the Lower Salt unit in pluvial Searles Lake, California, shows that this unit was formed between 24,500 and 32,000 years ago. The initial apparent 14C age of the lake is estimated to be about 900 years. After correcting for nonradiogenic 230Th and for the initial 14C age, excellent agreement between 230Th and 14C ages is obtained. The reliability of 230Th dating on salt deposit opens a new way for continuation of absolute chronology below the Lower Salt in Searles Lake.  相似文献   

6.
《Applied Geochemistry》2000,15(8):1191-1201
The impact of climatic and vegetation conditions over the past 15 Ka on the chemical composition and 14C dating of groundwater from the Franconian Albvorland aquifer system is discussed. Seven groundwaters over a flow distance of 25.5 km are investigated. Groundwater dating is made by 14C of dissolved inorganic carbon (DIC) and aqueous fulvic acid as well as 18O. 14C dating via fulvic acid gives groundwater ages consistent with climatic and vegetation records and variations in the groundwater composition. No correction for geochemical processes is required, since under these geochemical conditions fulvic acid remains stable over this time period and flow-distance. On the other hand, 14C dating via DIC requires correction of the 14C value due to perturbation by different geochemical processes. Up to a groundwater flow distance of approximately 17 km and an age of about 10 Ka, the 14C dating by DIC shows considerable dependence on the 14C-correction model applied. Beyond this groundwater age, 14C-DIC dating results in an overestimation by two to three 14C half-lives (T1/2=5730 a). This deviation may result from different groundwater recharge conditions at the end of the past glaciation and geochemical processes acting on DIC that cannot be adequately characterized. The present study has implications for humic substance mediated transport of pollutants in natural aquatic systems over long time periods.  相似文献   

7.
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.  相似文献   

8.
通过分析近30000年以来地球磁场强度和大气中14C 含量变化,讨论了14C年龄偏离实际值的原因,提出以地球磁场强度随时间的变化率校正14C 年龄Tc 偏离实际年龄Tt 的公式,即Tt=Tc-b(Tc-T?|F（1）ΔF/ΔT。用这一公式校正了14C 测年技术给出的新仙女木事件发生的时限,并获得了与树轮年龄和U-Th 年龄一致的结果。本文还对12000年以来地球磁场强度变化与气候变迁的相关性做了探讨。  相似文献   

9.
Stable isotopes (δ18O, δ2H), tritium (3H), and helium isotopes (3He, 4He) were used for evaluating groundwater recharge sources, flow paths, and residence times of three watersheds in the Cape Verde Islands (West Africa). Stable isotopes indicate the predominance of high-elevation precipitation that undergoes little evaporation prior to groundwater recharge. In contrast to other active oceanic hotspots, environmental tracers show that deep geothermal circulation does not strongly affect groundwater. Low tritium concentrations at seven groundwater sites indicate groundwater residence times of more than 50 years. Higher tritium values at other sites suggest some recent recharge. High 4He and 3He/4He ratios precluded 3H/3He dating at six sites. These high 3He/4He ratios (R/Ra values of up to 8.3) are consistent with reported mantle derived helium of oceanic island basalts in Cape Verde and provided end-member constraints for improved dating at seven other locations. Tritium and 3H/3He dating shows that São Nicolau Island’s Ribeira Fajã Basin has groundwater residence times of more than 50 years, whereas Fogo Island’s Mosteiros Basin and Santo Antão Island’s Ribeira Paul Basin contain a mixture of young and old groundwater. Young ages at selected sites within these two basins indicate local recharge and potential groundwater susceptibility to surface contamination and/or salt-water intrusion.  相似文献   

10.
Extensive field investigations were carried out for the first time in the meizoseismal area of the great 1950 Assam Earthquake aimed at exploring the paleoseismic history of the NE Indian region through documentation of liquefaction features and radiocarbon (14C) dating. Trenching at more than a dozen locations along the Burhi Dihing River valley and within the alluvial fans adjoining the Brahmaputra and Dibang Rivers resulted in the identification of more than a dozen very prominent liquefaction features (sand dykes, sills, sand blows etc.) as evidences of large to great earthquakes. 14C dating of the organic material associated with some of the features indicates a paleoseismic record of about 500 yrs archived by the sediments in this region. Compelling geological evidence(s) of the great 1950 earthquake are well constrained by 14C dating. Out of the two historically reported seismic events (1548 AD and 1697 AD) from this region, 14C dating could constrain the 1548 AD event though not distinctly. Further studies using combined 14C and OSL dating may better constrain the seismo-chronology of the study region.  相似文献   

11.
本文介绍了国外利用AMS14C法研究极冰、海水中溶解的碳、大气中含碳,深海沉积含碳物等的最新进展和成果。这些成果表明,由于AMS14C法测定需要的样品量仅是常规测定用量的千分之一,因而为在地学领域中的应用开辟了广阔的前景。我国的AMS14C方法研究已经起步,新方法的建成和使用必将推动有关学科的进一步发展。  相似文献   

12.
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   

13.
《Applied Geochemistry》1993,8(5):483-493
Information regarding the origin, composition and transport of natural dissolved organic carbon (DOC) in groundwater is necessary to understand the transport of metals and organic pollutants, as well as for the use of14C in DOC as an isotopic groundwater dating method. Previous research in several groundwater systems has suggested soil organic C is the predominant source of high molecular weight DOC to the subsurface. Through the use of stable isotopes,14C and geochemical analyses, this study shows that significant concentrations of DOC and CH4 in a regional confined aquifer can be generated in situ from subsurface sedimentary organic sources. The DOC and CH4 produced is a combined result of degradation of buried peats and bacterial action, resulting in high DOC concentrations and strongly methanogenic conditions in the aquifer. The DOC and CH4 comprise, on average, nearly 50% of the total dissolved C pool in the central part of the aquifer. Methanogenic conditions complicate isotopic groundwater dating by the conventional dissolved inorganic carbon (DIC) method. Estimates of isotopic groundwater residence time using DOC14C data are proposed by the application of14C isotope and mass balance corrections.  相似文献   

14.
Archaeological sites on gravel beach ridge plains offer a treacherously facile method of reconstructing cultural chronology based on the assumption that settlements were preferentially situated nearest the sea. the initial phase of beach ridge methodology in Alaska dates from its 1930s use by Henry Collins at St. Lawrence Island and its 1950s use by Louis Giddings in Kotzebue Sound. Numerous questions of cultural and depositional chronology remain unresolved. At Gambell, on St. Lawrence Island, three sets of ridges span the period since about 2000 B.P., with a prominent disconformity after Punuk culture times at ca. 1100 B.P. Reviewing the 14C dates (n = 83) we find that the Gambell sequence (n = 50) broadly parallels that of Kotzebue Sound, especially in the similar erosional disconformity at 1200-1000 B.P., related to increased storminess in the North Pacific. the Cape Krusenstern sequence is only loosely constrained by 14C dates (n = 33) disproportionately concentrated on 7 of the 114 ridge fragments. the dating of early Choris culture is especially problematic, which seems to occur both before and after the Old Whaling culture, well-dated at 2900-2800 B.P. on the 53rd ridge. However, reanalyzing the depositional sequence, we find that some of the more easterly Choris ridges probably represent erosional events after the Old Whaling occupation.  相似文献   

15.
A Bison antiquus cranium and partial skeleton from Ayer Pond wetland on Orcas Island, San Juan Islands, Washington, date to 11,760 ± 70 14C yr BP. They lay in lacustrine sediments below peat, unconformably above emergent Everson Glaciomarine Drift (> 12,000 14C yr BP). Several bison finds in similar contexts on Orcas and Vancouver Islands dating between 11,750 and 10,800 14C yr BP indicate an early postglacial land mammal dispersal corridor with reduced water barriers between mainland and islands. New bison dates and published shell dates allow estimation of early postglacial relative sea-level trends for the San Juans, with a drop below modern datum ∼ 12,000 14C yr BP, and assist in evaluation of marine reservoir corrections. Emergence by ∼ 60 m is suggested by data from nearby areas. A tundra-like or meadow community and succeeding open pine parkland before 11,000 14C yr BP supported bison but horn-core reduction suggests suboptimal forage or restricted habitat. Expanding mixed-conifer forests after 11,000 14C yr BP contributed to bison extirpation. Dispersing ungulates such as bison must have influenced island vegetation establishment and early succession. Possible evidence for butchering by early coastal people adds significance to the Ayer Pond discovery, given its pre-Clovis age.  相似文献   

16.
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   

17.
Pollen analysis of a sediment core from Zagoskin Lake on St. Michael Island, northeast Bering Sea, provides a history of vegetation and climate for the central Bering land bridge and adjacent western Alaska for the past ≥30,000 14C yr B.P. During the late middle Wisconsin interstadial (≥30,000-26,000 14C yr B.P.) vegetation was dominated by graminoid-herb tundra with willows (Salix) and minor dwarf birch (Betula nana) and Ericales. During the late Wisconsin glacial interval (26,000-15,000 14C yr B.P.) vegetation was graminoid-herb tundra with willows, but with fewer dwarf birch and Ericales, and more herb types associated with dry habitats and disturbed soils. Grasses (Poaceae) dominated during the peak of this glacial interval. Graminoid-herb tundra suggests that central Beringia had a cold, arid climate from ≥30,000 to 15,000 14C yr B.P. Between 15,000 and 13,000 14C yr B.P., birch shrub-Ericales-sedge-moss tundra began to spread rapidly across the land bridge and Alaska. This major vegetation change suggests moister, warmer summer climates and deeper winter snows. A brief invasion of Populus (poplar, aspen) occurred ca.11,000-9500 14C yr B.P., overlapping with the Younger Dryas interval of dry, cooler(?) climate. During the latest Wisconsin to middle Holocene the Bering land bridge was flooded by rising seas. Alder shrubs (Alnus crispa) colonized the St. Michael Island area ca. 8000 14C yr B.P. Boreal forests dominated by spruce (Picea) spread from interior Alaska into the eastern Norton Sound area in middle Holocene time, but have not spread as far west as St. Michael Island.  相似文献   

18.
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   

19.
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   

20.
华南早期新石器的14C断代和问题   总被引：2，自引：0，他引：2  
安志敏 《第四纪研究》1989,9(2):123-133
14C断代在中国史前考古学中取得了显著的成果,但华南早期新石器的五十多个年代数据却呈现偏老的趋势,甚至还有明显的误差。由于华南的石灰岩地带属于14C断代的异常区,某些样品的层位和年代反映出一定的矛盾,所测定的陶器年代过于古老,特别是时代和文化的发展过程,也与其它地区不相平衡。因此,华南早期新石器的年代数据尚待深入验证,至少目前还不可能作为可以信赖的绝对年代。  相似文献   

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