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 265 毫秒
1.
A chronology based on optically stimulated luminescence (OSL) dating is presented for the late- and post-glacial evolution of the southern Baltic Sea (15 ka to present). During this period, large water level and salinity changes occurred in the Baltic Basin due to opening and closing of connections to the North Atlantic. Previous attempts to establish a chronology for these palaeoenvironmental changes have mainly been conducted in coastal settings where organic material for 14C dating is abundant. Many of these records are, however, discontinuous due to the large water level fluctuations. In contrast, in the relatively deep water of the Arkona Basin, the sediment record is expected to be more or less continuous. The single aliquot regenerative dose (SAR) procedure was used to date 32 samples of fine quartz sand from a 10.86 m long sediment core from the centre of this basin (45 m water depth). Tests of luminescence characteristics confirmed the suitability of the material for OSL dating and the ages agree well with the available AMS 14C ages on shells. The Baltic Ice Lake drainage to the North Atlantic appears to occur 11.6 ka, agreeing with other published evidence. However, we suggest that the main marine Littorina transgression appears in the Arkona Basin at about 6.5 ka, rather than at 8.5 ka, as previously thought.  相似文献   

2.
We report single-grain quartz luminescence ages for the Puritjarra rock shelter, with the aim of resolving an apparent discrepancy between ages obtained by 14C and a variety of luminescence methods, previously reported. Ages now found at all depths to 75 cm (ages to 30 ka) can be interpreted as largely resolving the differences. This implied caveat arises because single-grain methods are statistically inefficient. As a consequence, a degree of interpretation is inevitable in analysing the data. The emphasis in the present paper is an analysis making use of weighted histograms. The measurements by single grain OSL and 14C, including ABOX–SC, taken together, can be regarded as compatible. They indicate human occupation of the Puritjarra rock shelter at least as early as 30 ka BP.  相似文献   

3.
Changes in oceanic radiocarbon (14C) reservoir ages through the deglaciation and Holocene can provide important information on ocean circulation as Earth's climate warmed. Here, we present reservoir ages for the western tropical Pacific that span the mid-Holocene transition from less to more frequent El Niño events. Reservoir ages were calculated using paired U–Th and conventional 14C dating of eight individual fossil coral samples from Koil and Muschu Islands, northern coastal Papua New Guinea (PNG). AMS 14C and MC-ICPMS U–Th dating of additional samples from six of the fossil corals were used to confirm the TIMS U–Th and conventional 14C ages. The combined results show average reservoir ages of 185±30 14C yr (n=4) for 7220–5850 yr BP compared to 420 14C yr for a modern coral from Muschu Island. From 5850 to 5420 yr BP reservoir ages increase to modern values. The relatively young reservoir ages from 7220 to 5850 yr BP are best explained by greater influx of well-equilibrated sub-tropical water from the southern branch of the South Equatorial Current (SEC). This is consistent with strengthening trade winds (facilitating air–sea exchange) and a more northerly position of the Intertropical Convergence Zone thought to have occurred at this time. The transition to more modern-like reservoir ages from 5850 to 5420 yr BP suggests modern oceanic circulation patterns were established during this interval. The onset of modern El Niño activity around this time would have served to enhance the intrusion of 14C-depleted equatorial waters via the south equatorial branch of the SEC. Overall, the changes in reservoir age presented here for the western tropical Pacific suggest that Holocene changes in the El Niño–Southern Oscillation state of the tropical Pacific resulted in reorganisation of oceanic circulation in this region.  相似文献   

4.
In order to establish a reliable chronology for lacustrine sediments of the Frickenhauser See (central Germany) different dating methods have been applied. A total of 17 AMS 14C dates, all within the last 2000 years, were supplemented with 137Cs/210Pb dating and varve counting of the uppermost sediments (131 years). The age–depth model for the Frickenhauser See has to cope with highly variable sedimentation rates and overlapping probability distributions of calibrated 14C dates. The uncertainty of calibrated 14C dates could be considerably reduced by including the stratigraphic relationship of the dated samples, the age information derived from short-lived isotopes and varve counting as well as an upper and lower limit of realistic sedimentation rates as ‘a priori’ information in the calibration procedure. Sets of possible age combinations obtained by repeated sampling from the modified probability distributions were used to calculate continuous age–depth relationships based on monotonic smoothing splines. The obtained age–depth model for the sediment record of the Frickenhauser See represents the average of over 16,000 such model runs and suggests a drastic increase in sedimentation rates from around 1–2 mm a−1 (200–1000 AD) to over 25 mm a−1 for the period between 1100 and 1300 AD. From then on, sedimentation rates exhibit relatively stable values around 3–9 mm a−1. ‘Conventional’ age–depth models such as general polynomial regression or cubic splines either do not include the obtained age-information in a satisfying manner (the model being too “stiff”) or exhibit “swings” causing age-reversals in the model. Although the age–depth relationships obtained for monotonic smoothing splines and mixed-effect regression are generally very similar, they differ in their respective sedimentation rates as well as in their uncertainties. Mixed-effect regression resulted in much higher sedimentation rates of more than 37 mm a−1. These results suggest that monotonic smoothing splines give better control of the age–depth model characteristics and are well suited in situations, where the integrity of 14C dates is high, i.e. the dated material represents the age of the respective layer.  相似文献   

5.
In this study, we explored the spatial and temporal relations between boulders and their original in-situ locations on sandstone bedrock cliffs. This was accomplished by combining field observations with dating methods using cosmogenic isotopes (10Be and 14C) and optically stimulated luminescence (OSL). Our conclusions bear both on the landscape evolution and cliff retreat process in the hyperarid region of Timna and on the methodology of estimating exposure ages using cosmogenic isotopes.

We recognize three discrete rock fall events, at 31 ka, 15 ka, and 4 ka. In this hyperarid region, the most plausible triggering mechanism for rock fall events is strong ground acceleration caused by earthquakes generated by the nearby Dead Sea fault (DSF). Our record, however, under represents the regional earthquake record implying that ongoing development of detachment cracks prior to the triggering event might be slower than the earthquake cycle.

Cliff retreat rates calculated using the timing of rock fall events and estimated thickness of rock removed in each event range between 0.14 m ky− 1 and 2 m ky− 1. When only full cycles are considered, we derive a more realistic range of 0.4 m ky− 1 to 0.7 m ky− 1. These rates are an order of magnitude faster than the calculated rate of surface lowering in the area. We conclude that sandstone cliffs at Timna retreat through episodic rock fall events that preserve the sharp, imposing, landscape characteristic to this region and that ongoing weathering of the cliff faces is minor.

A 10%–20% difference in the 10Be concentrations in samples from matching boulder and cliff faces that have identical exposure histories and are located only a few meters apart indicates that cosmogenic nuclide production rates are sensitive to shielding and vary spatially over short distances. However, uncertainties associated with age calculations yielded boulder and matching cliff face ages that are similar within 1 σ . The use of external constraints in the form of field relations and OSL dating helped to establish each pair's age. The agreement between calculated 14C and 10Be ages indicates that the accumulation of 10Be at depth by the capture of slow deep-penetrating muons was properly accounted for in the study.  相似文献   


6.
Radiocarbon calibration beyond the extent of tree-ring records depends on U-series dating of fossil corals or speleothem, both of which can provide independent calendar ages. Less direct methods rely on layer counting and comparison with other well-dated records. In spite of considerable effort to provide a reliable radiocarbon calibration curve beyond 25,000 years, the majority of the data show large atmospheric radiocarbon peaks which are inconsistent both in magnitude and timing between different determinations. The results of the most recent work [Chiu, T.-C., Fairbanks, R.G., Mortlock, R.A., Bloom, A.L., 2005. Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals. Quaternary Science Reviews 24 (16–17), 1797–1808], from Araki Island fossil corals, indicate a monotonic variation from about 33 to 49 ka, with no radiocarbon peaks, but with some gaps in the data. The difference between this and previous results, from fossil corals, has been attributed to selection of better-quality samples and rigorous analytical methods. However, previous results from Huon Peninsula [Yokoyama, Y., Esat, T.M., Lambeck, K., Fifield, L.K., 2000. Last ice age millennial scale climate changes recorded in Huon Peninsula corals. Radiocarbon 42 (3), 383–401; Cutler, K.B., Gray, S.C., Burr, G.S., Edwards, R.L., Taylor, F.W., Cabioch, G., Beck, J.W., Cheng, H., Moore, J., 2004. Radiocarbon calibration and comparison to 50 kyr BP with paired 14C and 230Th dating of corals from Vanuatu and Papua New Guinea. Radiocarbon 46 (3), 1127–1160] show radiocarbon peaks exclusively located within the gaps in the Araki data. The timing of the gaps are not random, but appear to be related to severe climate and sea-level variations associated with Heinrich events initiated in the North Atlantic. We propose that the Huon and Araki data sets are complementary rather than exclusive and that the absence of coral growth at Araki Island during Heinrich events presumably reflect local adverse conditions for coral growth.  相似文献   

7.
Over the last two decades, in-situ cosmogenic 14C dating has become an import tool in Quaternary geology and is recognized to geoscientists because of its potential to provide information on exposure age and process rate estimates for geomorphic studies. The in-situ cosmogenic radiocarbon has a relatively short half-life(5730 yr)and is substantially more sensitive than all the other cosmogenic nuclides used so far. It is therefore particularly useful to determine surface-exposure ages of Holocene landforms and quantify erosion rates in rapidly denuding landscapes during the past few tens of thousands of years. Moreover, in situ 14C is produced in quartz which is both highly resistant to weathering and common in nature, so it can be used in combination with other in-situ cosmogenic nuclides such as 3He, 10Be,21Ne,26Al, and 36Cl to constrain complex exposure histories involving burial and/or erosion occurring over the past 25ka. The age and slip rate of Holocene normal fault have been undoubtedly a challenge for seismologists to be faced with as result from lack of appropriate late Quaternary sediment. Recently, the cosmogenic nuclides such as 36Cl of preserved, seismically exhumed normal fault scarps were used to identify the last few major earthquakes and recover their ages and displacements through the modeling of the content of 36Cl in the scarp rocks. This paper mainly summarizes the development of in-situ 14C dating, including its research history, production rate estimate, production mechanism, chemical behavior and experimental method. The potential application of in-situ 14C dating to recovering past earthquakes, their timing, and the regularity of their recurrence for preserved, seismically exhumed normal fault scarps is also introduced.  相似文献   

8.
The Horqin sand-field in northeastern Inner Mongolia, China, had been the fertile grassland in North China, but desertification and sand-dust storm have increasingly occurred in the past decades [Zhu and Wang, 1992. Theory and practice of sandy desertification in China (in Chinese with English abstract). Quaternary Sciences 2, 97]. To understand the Holocene sand dune activities in this region, five sand dune profiles were investigated, and 32 coarse grain quartz samples were dated by OSL using the single-aliquot regenerative-dose (SAR) protocol [Murray and Wintle, 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiation Measurements 32, 57–73]. For cross-checking, six organic-rich samples from the paleosols and sandy peat/mud were dated by both 14C and quartz OSL. With one exception, 14C and quartz OSL dating results show good agreements. Based on the consistent results of five sand dune profiles, a chronology of Holocene dune activity in Horqin sand-field is established as follows: (i) active sand dunes built up widely before 10 ka; (ii) dunes semi-stabilized between 10 and 7.5 ka ago; (iii) the dunes solidify and chernozem soils developed between 7.5 and 2.0 ka ago; and (iv) partially re-mobilization of dunes occurred since about 2.0 ka ago.  相似文献   

9.
Oxygen and carbon data from eight stalagmites from northwest South Island are combined to produce composite records of δ18O and δ13C from 23.4 ka to the present. The chronology is anchored by 43 thermal ionization mass spectrometry (TIMS) uranium series ages. Delta 18O values are interpreted as having a first order positive relationship to temperature, but also to be influenced by precipitation in a complex manner. Delta 13C is interpreted as responding negatively to increases in atmospheric CO2 concentration, biological activity and precipitation amount.

Six climatic phases are recognized. After adjustment of 1.2‰ for the ice volume effect, the δ18O record between 23 and 18 ka varies around −3.72‰ compared to the Holocene average of −3.17‰. Late-glacial warming commenced between 18.2 and 17.8 ka and accelerated after 16.7 ka, culminating in a positive excursion between 14.70 and 13.53 ka. This was followed by a significant negative excursion between 13.53 and 11.14 ka of up to 0.55‰ depth that overlapped the Antarctic Cold Reversal (ACR) and spanned the Younger Dryas (YD). Positive δ18O excursions at 11.14 ka and 6.91–6.47 ka represent the warmest parts of the Holocene. The mid-Holocene from 6 to 2 ka was marked by negative excursions that coincide with increased glacial activity in the South Island. A short positive excursion from 0.71 to 0.57 ka was slightly later than the Medieval Warm Period of Europe.

Delta 13C values were high until 17.79 ka after which there was an abrupt decrease to 17.19 ka followed by a steady decline to a minimum at 10.97 ka. Then followed a general increase, suggesting a drying trend, to 3.23 ka followed by a further general decline. The abrupt decrease in δ-values after 17.79 ka probably corresponds to an increase in atmospheric CO2 concentration, biological activity and wetness at the end of the Last Glaciation, but the reversal identified in the δ18O record from 13.53 to 11.14 ka was not reflected in δ13C changes. The lowest δ13C values coincided with the early Holocene climatic suboptimum when conditions were relatively wet as well as mild.

Major trends in the δ18Oc record are similar to the Northern Hemisphere, but second order detail is often distinctly different. Consequently, at the millennial scale, a more convincing case can be made for asymmetric climatic response between the two hemispheres rather than synchronicity.  相似文献   


10.
For about the last 30 years it has been recognized that the high frequency component of the tree rings 14C/12C record is dominated by the modulation of the cosmic ray flux by the solar wind. In particular, it has been demonstrated that the three most recent periods of low sunspot occurrence were characterized by high values of atmospheric 14C/12C. During the last millennium other periods of high 14C/12C values were observed but their solar origin is still debatable. In the present work we compare these fluctuations with an independent record of cosmogenic 10Be measured in ice from the South Pole to check the solar origin of the observed 14C/12C variations. In order to compare quantitatively the results obtained on 10Be and 14C, it is necessary to take into account the different behaviour of these two cosmogenic isotopes, and especially the damping effect of the carbon cycle in the case of 14C. As an input to a 12-box numerical model we used the relative fluctuations of the 10Be concentrations record measured in South Pole ice and converted it into a synthetic 14C record. We took into account the fact that 10Be modulation is enhanced in polar regions due to the orientation of the geomagnetic field. As expected, the fluctuations of the modelled 14C record are much smaller (a factor of 20) than those observed for the raw 10Be record. In addition, the variations are smoother and shifted in time by a few decades. The 10Be-based 14C variations closely resemble the 14C measurements obtained on tree rings (R = 0.81). In particular, it is easy to identify periods of maximal 14C/12C which correspond to solar activity minima centred at about 1060, 1320 (Wolf), 1500 (Spörer), 1690 (Maunder) and 1820 (Dalton) yr A.D. Cross-correlation calculations suggest that there is no significant lag between the 10Be-based 14C and the tree-ring 14C records. Our study strongly suggests the dominance of the solar modulation on the cosmonuclide production variations during the last millennium.  相似文献   

11.
Primitive basaltic single eruptions in the Big Pine Volcanic Field (BPVF) of Owens Valley, California show systematic temporal–compositional variation that cannot be described by simple models of fractional crystallization, partial melting of a single source, or crustal contamination. We targeted five monogenetic eruption sequences in the BPVF for detailed chemical and isotopic measurements and 40Ar/39Ar dating, focusing primarily on the Papoose Canyon sequence. The vent of the primitive (Mg# = 69) Papoose Canyon sequence (760.8 ± 22.8 ka) produced magmas with systematically decreasing (up to a factor of two) incompatible element concentrations, at roughly constant MgO (9.8 ± 0.3 (1σ) wt.%) and Na2O. SiO2 and compatible elements (Cr and Ni) show systematic increases, while 87Sr/86Sr systematically decreases (0.7063–0.7055) and εNd increases (− 3.4 to − 1.1). 187Os/188Os is highly radiogenic (0.20–0.31), but variations among four samples do not correlate with other chemical or isotopic indices, are not systematic with respect to eruption order, and thus the Os system appears to be decoupled from the dominant trends. The single eruption trends likely result from coupled melting and mixing of two isotopically distinct sources, either through melt-rock interaction or melting of a lithologically heterogeneous source. The other four sequences, Jalopy Cone (469.4 ± 9.2 ka), Quarry Cone (90.5 ±17.6 ka), Volcanic Bomb Cone (61.6 ± 23.4 ka), and Goodale Bee Cone (31.8 ± 12.1 ka) show similar systematic temporal decreases in incompatible elements. Monogenetic volcanic fields are often used to decipher tectonic changes on the order of 105–106 yr through long-term changes in lava chemistry. However, the systematic variation found in Papoose Canyon (100–102 yr) nearly spans that of the entire volcanic field, and straddles cutoffs for models of changing tectonic regime over much longer time-scales. Moreover, ten new 40Ar/39Ar ages combined with chemistry from all BPVF single eruption sequences show the long-term trend of BPVF evolution comprises the overlapping, temporal–compositional trends of the monogenetic vents. This suggests that the single eruption sequences contain the bulk of the systematic chemical variation, whereas their aggregate compositions define the long-term trend of volcanic field evolution.  相似文献   

12.
Analytical procedures in the determination of iodine-129 (half-life: 1.6×107 y) have been studied using accelerator mass spectrometry (AMS), with special references to the separation procedures of iodine from soil samples for the AMS measurement. Iodine was successfully volatilized from soil samples by pyrohydrolysis at 1000 °C and collected in a trap solution. Iodine was purified from the matrix by solvent extraction. Finally, it was precipitated as silver iodide to make a target for AMS. In order to obtain information on the 129I/127I ratio in a chemical blank (or iodine carrier), we have determined the ratios in several iodine reagents and found that the ratios fell in a narrow range around 1.7×10−13. The detection limit for soil sample (1 g material) by the present method was about 0.01 mBq/kg or 4×10−11 as the ratio of stable iodine (129I/127I ratio), i.e. these values were much better than that by neutron activation analysis (NAA) used in our previous studies. We have applied this method in the analysis of soil samples collected from different places in Japan. We could successfully determine 129I in soil samples with low 129I concentrations, which could not be detected by NAA. Sample size necessary for the soil analysis by AMS was only about 0.5 g or less, whereas about 100 g of the sample were required for NAA [Muramatsu, Y., Ohmomo, Y., 1986. Iodine-129 and iodine-127 in environmental samples collected from Tokaimura/ Ibaraki, Japan. Sci. Total Environ. 48, 33-43]. Using this method, new data were obtained for the 129I levels in 20 soil samples collected from background areas far from nuclear facilities, and the ranges were 1.4×10−5−4.5×10−3 Bq/kg as 129I concentrations and 3.9×10−11−2.2×10−8 as 129I/127I ratios. These values are useful in understanding the 129I levels in Japanese environments. Higher 129I concentrations were observed in forest soils than those in field and rice paddy soils should be related to the interception effect of atmospheric 129I due to tree canopies. Relatively high 129I/127I ratios found in rice paddy soils could be explained by their low stable iodine concentrations which were caused by the desorption of stable iodine from the rice paddies during the cultivation.  相似文献   

13.
Gas is extracted from large (6–31 kg) Antarctic ice samples to obtain sufficient CO2 for14C measurements with small low-level proportional counters. The14C ages of Byrd core ice are in accord with glaciological estimates ranging from (2.2−1.1+1.4)×103 yr at 271 m depth to more than 8 × 103 yr at 1071 m depth. The CO2 abundances in gas extracted from Byrd core ice range from 0.0216 to 0.051%, with below present-day atmosphere CO2 abundances for ice from 1068 and 1469 m depths. The CO2 abundance in gas from Allan Hills surface ice samples ranges between four and six times the atmospheric value and the CO2 had a specific activity three times that of contemporary carbon. A possible explanation for the anomalously high specific activity is surface melting with the incorporation into CO2 of14C produced by cosmic ray spallation of oxygen in ice. The CO2 abundance in gas extracted from subsurface Allan Hills ice ranged from 0.030 to 0.065%, and the specific activities are below contemporary carbon, indicating ages greater than 5×103 yr. The18O/16O ratio of oxygen in the trapped gas is the same as that of atmospheric oxygen and differs markedly from the18O/16O ratio in the ice. The O2, N2, and Ar abundances and isotopic compositions are similar to those in contemporary air, except for positive15N/14N ratios in a few samples.  相似文献   

14.
The cosmogenic nuclide 10Be was analyzed by using accelerator mass spectrometry on an ice core drilled at the Dome Fuji station, inland Antarctica, for 700–1900 yr CE. The measured concentration of 10Be in the Dome Fuji ice core and the derived 10Be flux show similar fluctuations, with both increasing at known solar-activity minima over the last millennium in agreement with earlier observations of 10Be and 14C. Based on the similar nature of the 10Be flux to the reconstructed 14C production rate patterns, a 10Be–14C correlation age model for the Dome Fuji ice core was successfully constructed. This age model agrees well with the initial version of the tephrochronology of the core. The 10Be-flux record contains information on variability in the amount of cosmic radiation incident on the atmosphere, which is mainly attributable to high-frequency change in solar activity and low-frequency background intensity adjustment of the geomagnetic field. High-resolution 10Be analyses of the Dome Fuji ice cores promise to provide potentially important information on the history of cosmic radiation intensity over the past several hundred thousand years.  相似文献   

15.
Dust depositions are critical archives for understanding interior aridification and westerly climatic changes in Central Asia. Accurate and reliable dating of loess is very important for interpreting and correlating environmental records. There remains a disparity between luminescence ages and radiocarbon dating of late Quaternary loess from the Ili Basin in Central Asia. In this study, we establish a closely spaced quartz optically stimulated luminescence (OSL) chronology for the 20.5-m-thick Nilka loess section in the Ili Basin. Based on OSL ages, two intervals of higher mass accumulation rate occurred at 49–43 ka and 24–14 ka. We further compare these OSL ages with 23 accelerator mass spectrometry (AMS) 14C ages of bulk organic matter. The results indicate that the OSL and radiocarbon ages agree well for ages younger than ca. 25 14C cal ka BP. However, beyond 30 cal ka BP, there is no consistent increase in AMS 14C age with depth, while the OSL ages continue to increase. These differences confirm the observation that the AMS 14C ages obtained using conventional acid–base–acid (ABA) pretreatment are severely underestimated in other terrestrial deposits in Central Asia, which could be due to 2–4% modern carbon contamination. However, OSL dating is applicable for constructing an accurate chronology beyond 30 cal ka BP. We suggest caution when interpreting paleoenvironmental changes based on radiocarbon ages older than 25 cal ka BP.  相似文献   

16.
Keiichi  Sasaki  Akio  Omura  Tetsuo  Miwa  Yoshihiro  Tsuji  Hiroki  Matsuda  Toru  Nakamori  Yasufumi  Iryu  Tsutomu  Yamada  Yuri  Sato  Hiroshi  Nakagawa 《Island Arc》2006,15(4):455-467
Abstract   High-resolution seismic reflection profiles delineated the distribution of mound-shaped reflections, which were interpreted as reefs, beneath the insular shelf western off Irabu Island, Ryukyus, southwestern Japan. A sediment core through one of the mounded structures was recovered from the sea floor at a depth of −118.2 m by offshore drilling and was dated by radiometric methods. The lithology and coral fauna of the core indicate that the mounded structure was composed of coral–algal boundstone suggesting a small-scaled coral reef. High-precision α-spectrometric 230Th/234U dating coupled with calibrated accelerator mass spectrometric 14C ages of corals obtained reliable ages of this reef ranging from 22.18 ± 0.63 to 30.47 ± 0.98 ka. This proves that such a submerged reef was formed during the lowstand stage of marine oxygen isotope stages 3–2. The existence of low-Mg calcite in the aragonitic coral skeleton of 22.18 ± 0.63 ka provides evidence that the reef had once been exposed by lowering of the relative sealevel to at least −126 m during the last glacial maximum in the study area. There is no room for doubt that a coral reef grew during the last glacial period on the shelf off Irabu Island of Ryukyus in the subtropical region of western Pacific.  相似文献   

17.
OSL, radiocarbon dating of pedogenic carbonate and tephrochronology have been used in an attempt to provide a detailed and reliable chronology for Birdlings Flat loess, a thick, proximal loess found on the lower flanks of Banks Peninsula, Canterbury, New Zealand. In a ca 15 m thick section at Ahuriri Quarry the Kawakawa tephra isochron of 26,500 cal. yr BP is identified at a depth of ca. 1.35 m on the basis of glass counting and electron microprobe fingerprinting. Radiocarbon ages of filamentous or root pseudomorph carbonate increase down section from ca. 10,000 cal. yr BP at 2.80 m to ca. 30,000 14C yr BP at 10.90 m. Ages from carbonate lining cracks are often out of sequence, indicating deep percolation by bypass flow down preferential flow paths. OSL ages show reversals with respect to each other, carbonate radiocarbon ages, and with the position and accepted age of Kawakawa tephra. Coincident radiocarbon and OSL ages at 3.85 m depth indicate that OSL ages are underestimates by at least 20%. Before OSL can be deemed a high accuracy dating method of the quatzo-feldspathic loess of South Island, New Zealand, more research into the causes of age underestimation and age reversals must be carried out. The last major episode of loess accumulation on the flanks of Banks Peninsula in Canterbury began before ca. 30,000 14C yr BP (ca. 35,000 cal. yr BP) and possibly before 43,000 yr based on OSL.  相似文献   

18.
Groundwater recharge and palaeoclimate in the Sirte and Kufra basins, Libya   总被引：1，自引：0，他引：1  
Stable and radio-isotope results (C, H, O) for groundwaters from the Sirte and northern Kufra basins are used to determine the recharge history during the Holocene and late Pleistocene. Radiocarbon ages have been corrected on the basis of their stable carbon isotope ratios and on environmental samples from the areas, and two groups may be recognised: (1) low 14C activity groundwaters (13000–34000 yr. BP) with δ 13C-5.6 to −11.7‰; and (2) higher 14C activity groundwaters (5000–7800 yr. BP) enriched in 13C up to δ 13C = −3.2‰. There is a general correlation of age with depth.

A well defined freshwater (< 50 mg/l Cl−) channel can be traced within the aquifer for some 130 km through the region, which is considered to represent recharge from a former wadi. This water with an age of ± 7800 yr. BP is chemically and isotopically distinct from the regional groundwaters and provides direct evidence of a significant recharge event during the Holocene.

The stable isotope (O and H) composition of groundwater from the Kufra and Sirte basins are all related by an evaporative line with slope δ D = 4.5δ 13O − 35 with an intercept on the meteoric line of -11‰. This suggests a recharge source continuing into the Holocene from air masses, analogous to current heavy monsoon rain derived from south of the Sahara. The spatial and temporal distribution of groundwaters in relation to the evaporative line suggests a progressive change in character of the recharge which is controlled by a shift towards strongly convective rainfall during the Holocene.

The direct hydrogeological and geochemical evidence supports climatic models proposed by several workers in which discrete humid episodes during the Holocene are inferred.  相似文献   


19.
We have determined the production yields for radionuclides in Al2O3, SiO2, S, Ar, K2SO4, CaCO3, Fe, Ni and Cu targets, which were irradiated with slow negative muons at the Paul Scherrer Institute in Villigen (Switzerland). The fluences of the stopped negative muons were determined by measuring the muonic X-rays. The concentrations of the long-lived and short-lived radionuclides were measured with accelerator mass spectrometry (AMS) and γ-spectroscopy, respectively. Special emphasis was put on the radionuclides 10Be, 14C and 26Al produced in quartz targets, 26Al in Al2O3 and S targets, 36Cl in K2SO4 and CaCO3 targets, and 53Mn in Fe2O3 targets. These targets were selected because they are also the naturally occurring target minerals for cosmic ray interactions in typical rocks. We also present results of calculations for depth-dependent production rates of radionuclides produced after cosmic ray μ− capture, as well as cosmic ray-induced production rates of geologically relevant radionuclides produced by the nucleonic component, by μ− capture, by fast muons and by neutron capture.  相似文献   

20.
Radiocarbon dating is the most widely applied and reliable dating technique for providing chronological control during Marine Isotope Stage 3 (MIS3; ∼60–27 cal kyr BP). Past variations in the atmospheric concentration of radiocarbon mean a calibration curve is required. IntCal20 and SHCal20 calibration curves covering MIS3 are presently largely based on non-atmospheric records which, in combination with larger radiocarbon (14C) dating uncertainties, results in significant smoothing and reduced resolution in calibration curve structure. Floating tree ring radiocarbon chronologies that are wiggle-matched to other palaeo records (particularly to Hulu Cave speleothems) have the potential to provide detailed structure to the MIS3 portion of extant calibration curves. New Zealand subfossil kauri (Agathis australis) trees are long-lived and are useful for constructing temporally-floating MIS3 atmospheric radiocarbon datasets. This paper presents extant and emerging data from several important Northland subfossil kauri locations (Omaha, Babylon Coast, Bream Bay, Kai Iwi Lakes, Mangawhai). We show the span of seven floating MIS3 kauri sequences (individual trees and chronologies) from which sequential radiocarbon series covering a total of 7556 years is now in development (representing 23% of the period 60–27 cal kyr BP). We also report radiocarbon dates for an additional 34 ancient kauri from MIS3 that can provide additional coverage. After these floating subfossil wood sequences have been produced, close to 40% of MIS3 (12,420 years) will be covered by contiguous subfossil kauri radiocarbon measurements. Based on our findings, we discuss the prospects and limitations for obtaining a highly resolved and precise atmospheric radiocarbon calibration curve comprehensively covering MIS3 using subfossil kauri.  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