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A revised chronological framework for the deglaciation of the Lake Michigan lobe of the south‐central Laurentide Ice Sheet is presented based on radiocarbon ages of plant macrofossils archived in the sediments of low‐relief ice‐walled lakes. We analyze the precision and accuracy of 15 AMS 14C ages of plant macrofossils obtained from a single ice‐walled lake deposit. The semi‐circular basin is about 0.72 km wide and formed of a 4‐ to 16‐m‐thick succession of loess and lacustrine sediment inset into till. The assayed material was leaves, buds and stems of Salix herbacea (snowbed willow). The pooled mean of three ages from the basal lag facies was 18 270 ± 50 14C a BP (21 810 cal. a BP), an age that approximates the switch from active ice to stagnating conditions. The pooled mean of four ages for the youngest fossil‐bearing horizon was 17 770 ± 40 14C a BP (21 180 cal. a BP). Material yielding the oldest and youngest ages may be obtained from sediment cores located at any place within the landform. Based on the estimated settling times of overlying barren, rhythmically bedded sand and silt, the lacustrine environment persisted for about 50 more years. At a 67% confidence level, the dated part of the ice‐walled lake succession persisted for between 210 and 860 cal. a (modal value: 610 cal. a). The deglacial age of five moraines or morainal complexes formed by the fluctuating margin of the Lake Michigan lobe have been assessed using this method. There is no overlap of time intervals documenting when ice‐walled lakes persisted on these landforms. The rapid readvances of the lobe during deglaciation after the last glacial maximum probably occurred at some point between the periods of ice‐walled lake sedimentation. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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博斯腾湖湖泊沉积物光释光年代测量* 总被引：2，自引：0，他引：2
使用光释光年代学的单片再生法测量了博斯腾湖沉积剖面中碳酸盐泥及粉砂质泥底部的浅湖相灰色粉细砂和风成沙的年龄,对剖面上部碳酸盐层中陆生植物残体进行了AMS 14 C测年。通过不同测片的等效剂量(De)值的分布状况评价了样品的晒褪程度,选择不随灵敏度校正后的自然释光信号变化的相对集中的等效剂量(De)值计算了样品的埋藏年龄。通过这些年龄结果的对比,发现石英矿物的OSL年龄和AMS 14 C年龄在地层上是一致的,表明尽管在浅湖相细砂中存在不完全晒褪,但根据相对较小而集中的De值计算得到的年龄结果是可靠的。这些年龄结果和地层资料揭示末次冰消期以来至早全新世,博斯腾湖处于无水干盆地向深水湖泊转化的浅水湖泊状态,现代深水博斯腾湖大约形成于距今8ka前后。 相似文献
4.
Eolian and subaqueous landforms composed of gypsum sand provide geomorphic evidence for a wet episode at the termination of glacial climate in southwestern North America. Drying of pluvial Lake Estancia, central New Mexico, occurred after ca. 12,000 14C yr B.P. Thereafter, eolian landforms on the old lake floor, constructed of gypsum sand, were overridden by rising lake water, modified by subaqueous processes, and organized into beach ridges along the lake's eastern shore. Preservation of preexisting eolian landforms in the shallow lake suggests abupt changes in lake level and climate. Available radiocarbon ages suggest that the final highstand recorded by beach ridges may have developed during the Younger Dryas (YD) stade. The beach ridges provide information about lake surface area, which was 45% of the lake area reached during the maximum highstands of the late Pleistocene. A similar proportional response has been reported for YD climate changes outside the North Atlantic region. 相似文献
5.
Jessica L. Blois John W. Williams Eric C. Grimm Stephen T. Jackson Russell W. Graham 《Quaternary Science Reviews》2011,30(15-16):1926-1939
Mapping past vegetation dynamics from heterogeneous databases of fossil-pollen records must face the challenge of temporal uncertainty. The growing collection of densely sampled fossil-pollen records with accurate and precise chronologies allows us to develop new methods to assess and reduce this uncertainty. Here, we test our methods in the context of vegetation changes in eastern North America during the abrupt climate changes of the last deglaciation. We use the network of fossil-pollen records in the Neotoma Paleoecology Database (www.neotomadb.org) and data contributed by individual investigators. Because many of these records were collected decades before the current generation of 14C and age-model technologies, we first developed a framework to assess the overall reliability of 14C chronologies by systematically evaluating individual 14C ages and associated chronologies. We developed a qualitative ranking scheme for individual 14C ages that combines information about their accuracy and precision. ‘Benchmark’ pollen records were defined to have at least one 14C age with an accuracy within 250 years and a precision less than 500 years that is within 1000 years of the time interval of interest, and at least five pollen samples per 1000 years across this time period. Only 22 of >350 late-Pleistocene pollen cores in eastern North America met the benchmark criteria.We then used Bayesian change-point analysis to identify widespread ecological events (Picea decline, Quercus rise, and Alnus decline), and interpolated the ages of these events from the benchmark sites to non-benchmark sites. Leave-one-out cross-validation analyses with the benchmark sites indicated that the spatial error associated with interpolation was less for inverse distance-weighting (IDW) than thin-plate splines (TPS) and was about 500 years for the three biotic events. By comparison, the difference between the original ages of events at poorly constrained sites and the biostratigraphic ages interpolated from the benchmark sites was close to 1000 years, suggesting that the use of biostratigraphic ages can significantly improve the age models for poorly constrained sites. Overall, these analyses suggest that the temporal resolution of multi-site syntheses of late-Pleistocene fossil-pollen data in eastern North America is about 500 years, a resolution that allows analysis of ecological responses to millennial-scale climate change during the last deglaciation. 相似文献
6.
Groundwater systems in the San Luis Valley, Colorado, USA have been re-evaluated by an analysis of solute and isotopic data. Existing stream, spring, and groundwater samples have been augmented with 154 solute and isotopic samples. Based on geochemical stratification, three groundwater regimes have been identified within 1,200 m of the surface: unconfined, upper active confined, and lower active confined with maximum TDS concentrations of 35,000, 3,500 and 600 mg/L, respectively. The elevated TDS of northern valley unconfined and upper active confined systems result from mineral dissolution, ion exchange and methanogenesis of organic and evaporate lake sediments deposited in an ancient lake, herein designated as Lake Sipapu. Chemical evolutions along flow paths were modeled with NETPATH. Groundwater ages, and δ13C, δ2H and δ18O compositions and distributions, suggest that mountain front recharge is the principle recharge mechanism for the upper and lower confined aquifers with travel times in the northern valley of more than 20,000 and 30,000 14C years, respectively. Southern valley confined aquifer travel times are 5,000 14C years or less. The unconfined aquifer contains appreciable modern recharge water and the contribution of confined aquifer water to the unconfined aquifer does not exceed 20%. 相似文献
7.
Glenn W. Berger Martin Melles Debabrata Banerjee Andrew S. Murray Alexandra Raab 《第四纪科学杂志》2004,19(5):513-523
A 10.5 m core from Changeable Lake in the Severnaya Zemlya Archipelago just north of the Taymyr Peninsula intersects ca. 30 cm of diamicton at its base, interpreted as a basal till. Because the upper 10.13 m of this core consists of non‐glacial sediments, a maximum numeric age for these non‐glacial sediments would provide a clear lower limit to the timing of the last glaciation in the area of Changeable Lake. Radiocarbon (14C) dating of several materials from this core yielded widely scattered results. Consequently we applied photonic dating to sediments above the diamicton. The experimental single‐aliquot‐regenerative (SAR) dose fine‐grain method was applied to two samples, using the ‘double SAR’ approach. With one exception, these fine‐grain SAR results and the results of application of the SAR method to sand‐sized quartz grains from two samples, at ca. 9.95 m and ca. 10.05 m depth, are discrepant with age estimates from the multi‐aliquot infrared‐photon‐stimulated luminescence (IR‐PSL) method applied to fine grains. Multi‐aliquot IR‐PSL dating of 10 samples produces ages increasing monotonically from ca. 4 ka at 2 m to 53 ± 4 ka at 9.97 m. These self‐consistent multi‐aliquot IR‐PSL ages, along with limiting 14C ages of >47 ka at ca. 10 m, provide direct evidence that glacial ice did not advance over this lake basin during the Last Glacial Maximum, and thus delimit the northeastern margin of the Barents–Kara Sea ice‐sheet to somewhere west of this archipelago. The last regional glaciation probably occurred during marine isotope stage (MIS) 4 or earlier. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
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Radiocarbon age inversions and progression: source and causes in Late Holocene sediments from Lake Chapala,western Mexico 总被引：1，自引：0，他引：1
Pedro F. Zárate-del-Valle Hermes U. Ramírez-Sánchez François Fernex Bernd R. T. Simoneit Isabel Israde-Alcántara 《Environmental Earth Sciences》2011,63(5):1011-1019
The results of eight radiocarbon datings of Lake Chapala sediments (site T46) are presented, the age inversions (AI) observed
and their age progression discussed. As deduced from some AIs and the 210Pb activity (site CHP4), the bioturbation zone in the lake varies over a depth of 5–25 cm. The linear sedimentation rates
(LSRs) calculated from 14C ages do not match the LSR calculated from unsupported 210Pb activity for the upper sediments. This demonstrates the usefulness of dating sediments with complementary radiometric techniques
such as short-lived isotope counting (SLIC), i.e., 210Pb and 137Cs. This approach leads to the following conclusions: (1) The incorporation of detrital particles with ancient carbon into
the sedimentary column of the lake occurred by a combination of: (a) the presence of outcrops of hydrothermal petroleum with
ages >40 ka (ka = thousands of years) in the lake, and (b) mass transport due to the presence of two elongated gyre circulation
patterns integrated by cyclonic circulation (counterclockwise) in the north portion of the lake and anticyclonic circulation
in the southern part. (2) Consequently, the 14C ages of shallow lake sediments have geologic ages one order of magnitude greater compared to their ages determined by the
210Pb method. (3) A bioturbation mechanism is not necessary to explain the 14C AI in the top 70 cm and from 110 to 150 cm depth of the sediments. (4) According to the biological proxies data for the
last 600 years B.P., the paleoclimate at Lake Chapala has changed from sub-humid to dry environmental conditions, and eutrophication
has increased over the past 100 years due to local input from ongoing agricultural activities. 相似文献
10.
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
11.
Uranium-Series Dating of Carbonate (Tufa) Deposits Associated with Quaternary Fluctuations of Pyramid Lake, Nevada 总被引：1，自引：0，他引：1
Uranium-series dating of dense tufa deposited in a small cave, at former lake margins, and in large tufa mounds clarifies the timing of lake-level variation during the past 400,000 yr in the Pyramid Lake basin. A moderate-sized lake occasionally overflowed the Emerson Pass sill at elevation of 1207 m between ca. 400,000 and 170,000 and from ca. 60,000 to 20,000 yr B.P., as shown by230Th/234U ages of the cave samples,230Th-excess ages of tubular tufas, and average isochron-plot ages of shoreline-deposited tufas. (By comparison, modern Pyramid Lake is 50 m below this sill). There is a lack of tufa record during the intervening period from ca. 170,000 to 60,000 yr B.P. After ca. 20,000 yr, Pyramid Lake underwent abrupt changes in level and, based on previous14C ages, reached its highest elevation (ca 1335 m) at ca. 14,000 yr B.P. The youngest uranium-series ages are comparable with previously reported14C ages. 相似文献
12.
This study evaluates the accuracy of optically stimulated luminescence to date well-preserved strandline sequences at Manistique/Thompson bay (Lake Michigan), and Tahquamenon and Grand Traverse Bays (Lake Superior) that span the past ∼4500 yr. The single aliquot regeneration (SAR) method is applied to produce absolute ages for littoral and eolian sediments. SAR ages are compared against AMS and conventional 14C ages on swale organics. Modern littoral and eolian sediments yield SAR ages <100 yr indicating near, if not complete, solar resetting of luminescence prior to deposition. Beach ridges that yield SAR ages <2000 yr show general agreement with corresponding 14C ages on swale organics. Significant variability in 14C ages >2000 cal yr B.P. complicates comparison to SAR ages at all sites. However, a SAR age of 4280 ± 390 yr (UIC913) on ridge77 at Tahquamenon Bay is consistent with regional regression from the high lake level of the Nipissing II phase ca. 4500 cal yr B.P. SAR ages indicate a decrease in ridge formation rate after ∼1500 yr ago, likely reflecting separation of Lake Superior from lakes Huron and Michigan. This study shows that SAR is a credible alternative to 14C methods for dating littoral and eolian landforms in Great Lakes and other coastal strandplains where 14C methods prove problematic. 相似文献
13.
Extensive coastal dunes occur in the Great Lakes region of North America, including northwestern Michigan where some are perched on high (~ 100 m) bluffs. This study focuses on such a system at Arcadia Dunes and is the first to systematically generate optical ages from stratigraphic sections containing buried soils. Dune growth began ca. 4.5 ka during the Nipissing high lake stand and continued episodically thereafter, with periods of increased sand supply at ca. 3.5 ka and ca. 1.7 ka. The most volumetrically dominant phase of dune growth began ca. 1.0 ka and continued intermittently for about 500 years. It may have begun due to the combined effects of a high lake phase, potential changes in lake hydrodynamics with final isostatic separation of Lake Superior from Lakes Michigan and Huron, and increased drought and hydrologic variability associated with the Medieval Warm Period. Thus, this latest eolian phase likely reflects multiple processes associated with Great Lakes water level and climate variability that may also explain older eolian depositional events. Comparison of Arcadia ages and calendar corrected 14C ages from previous studies indicate broad chronological agreement between events at all sites, although it appears that dune growth began later at Arcadia. 相似文献
14.
The Dead Sea is surrounded by chemical and detrital sediments that were deposited in its larger precursor lakes, Lake Samra and Lake Lisan. The sedimentary history of these lakes was recon-structed by means of 230Th/234U ages of 30 samples, mostly of argonite laminae, from 8 columnar sections up to 110 km apart. The general validity of the ages was demonstrated by subjecting them to tests of internal isotopic consistency, agreement with stratigraphic order, and concordance with 14C ages. In the south, only the part of the Samra Formation older than 170,000 yr is exposed, while the aragonite-detritus rhythmites found in the central and northern region are generally younger than 120,000 yr. The Lisan Formation started accumulating about 63,000 yr B.P., with the clay and aragonite beds in the south-central area reflecting a rise in water level to at least −280 m. The upper part of the Lisan Formation, the aragonite-rich White Cliff Member, started accumulating about 36,000 yr B.P. The lake probably reached its highest level sometime after this, based on the ages of Lisan sediments preserved in the southernmost reaches of the basin. 相似文献
15.
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
16.
Rewi M. Newnham Marcus J. Vandergoes Mark H. Garnett David J. Lowe Christine Prior Peter C. Almond 《第四纪科学杂志》2007,22(1):37-51
Previous attempts to radiocarbon date sediments >10 kyr from the high rainfall region of Westland, New Zealand, a critical location for investigation of interhemispheric patterns of climate change, have been problematic. This study, building on recent work by Vandergoes and Prior ( 2003 ), shows that AMS 14C dating of pollen concentrates has potential to provide more reliable ages than other sediment constituents, including plant macrofossils. The method was applied to sediments from three sites containing the 22.6k 14C yr Kawakawa Tephra, which provided an independent test of the 14C ages. Although some minor laboratory contamination was detected in tests on background standards, the modelled relationship between sample mass and measured 14C content permitted an appropriate correction to be determined. Improved pollen concentrations derived by density separation between 1.4 and 1.2 specific gravity and sieving in the range 10–50 μm provided either older ages than other fractions of the same sample or, where in situ contamination was not evident, equivalent ages. Differences in degree of in situ contamination between depositional environments indicated that, in Westland, lake sites may be less susceptible to contamination by younger carbon than peat sites, where this process may be facilitated by root penetration into underlying sediments. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
17.
《Quaternary Science Reviews》2003,22(10-13):1131-1138
In this investigation we evaluate several proposed optically stimulated luminescence single-aliquot regeneration (OSL SAR) procedures to determine which technique has the greatest potential to yield accurate ages for samples collected from tuff-derived alluvial sediments within the narrow, sharply incised canyon systems of the Pajarito Plateau of northern New Mexico. The SAR data collection methods evaluated are: infrared-stimulated luminescence (IRSL), post-IR blue-OSL, IRSL with TL annealing cycles on polymineral fine-grains, and blue-OSL on quartz fine sand. A single-grain laser luminescence (SGLL) procedure for quartz sand is also evaluated. Age estimates obtained from these methods are compared with radiocarbon, soil PDI (profile development index), and IRSL multi-aliquot additive dose (MAAD) age constraints. Our results indicate that the modal De of quartz sand SGLL dose distributions yield ages that are consistent with radiocarbon and PDI age constraints for the tuff derived sediments in this investigation and appears to be the most promising method for studies in this area. Additionally, two fine-grained polymineral methods, IRSL SAR and traditional IRSL MAAD, produced ages that were generally in agreement with the SGLL ages and with available 14C and PDI age constraints. At the present stage of research, we advocate using quartz sand SGLL in conjunction with IRSL SAR or even IRSL MAAD for polymineral fine-grains to provide the most robust and reliable luminescence age data sets for tuff-derived sediments. 相似文献
18.
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
19.
华南早期新石器的14C断代和问题 总被引：2，自引：0，他引：2
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
20.
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