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1.
Concentrations of polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), and polybrominated diphenyl ethers (PBDEs) were measured in 32 species inhabiting the Yellow Sea to assess their bioaccumulation potentials. The concentrations in these samples were lower than those reported for other countries or locations. Relatively high levels of BDE 209 in biota suggest an ongoing source of deca-BDE technical mixing within the Yellow Sea. The accumulation profiles of PCBs were uniform between species, but the concentrations of OCPs and PBDEs varied widely. Pelagic and benthic food-chain components were separated by their δ13C values. Significant positive correlations between δ15N and PCB 153, PCB 138, p,p′-DDE, oxy-chlordane, and trans-nonachlordane were found only for pelagic consumers, indicating that the pelagic food chain is an important bioaccumulation pathway for selected PCB and OCP compounds. The other compounds did not show any biomagnification through benthic and pelagic food chains, suggesting the lower bioaccumulation potentials of these contaminants.  相似文献   

2.
In the Chinese Loess Plateau, land snail shells are often the only material available for dating in paleoenvironmental and archaeological research. However, the geochronological suitability of land snail shells is limited because of poor knowledge about their deposition dynamics, particularly with regards to the incorporation of inorganic carbonate and the resulting age anomalies. To evaluate the factors controlling these age anomalies, radiocarbon and stable carbon analyses were carried out on surface soils, as well as the shells and organic bodies of different modern snail species from different ecological habitats. The results showed that all specimens were depleted in 14C, indicating the influence of inorganic, radiocarbon-free carbonate on the 14C-activity of the snail shells. The apparent 14C-deficiencies and the resultant age anomalies of both the Cathaica and the Bradybaena snail shells were within close ranges across the Chinese Loess Plateau, indicating that the shells of these species could, after corrections for radiocarbon anomalies, provide reliable age estimates. The apparent 14C-deficiencies were closely associated with the ecological habitats of the snails. The shells of the ground-dwelling Bradybaena had the smallest age offsets (533 ± 150 a), followed grass-dwelling Cathaica (1107 ± 138 a) and Cathaica living on trees (1550 ± 345 a). These results suggest that the availability of calcium in the respective ecological habitats is an important factor in explaining the apparent 14C-deficiencies. The influence of carbonate on the stable carbon isotope composition of shells is overwhelmed by the organic diets of snails, making δ13C unsuitable for identifying and correcting shell age anomalies. The radiocarbon activities of surface soils (Acalc) increase with weathering intensity. Thus, a significant uncertainty could be caused by assuming that Acalc is zero when estimating the proportions of different carbon sources in shells, as has been the case in most previous studies.  相似文献   

3.
Fossil shells of small terrestrial gastropods are commonly preserved in wetland, alluvial, loess, and glacial deposits, as well as in sediments at many archeological sites. These shells are composed largely of aragonite (CaCO3) and potentially could be used for radiocarbon dating, but they must meet two criteria before their 14C ages can be considered to be reliable: (1) when gastropods are alive, the 14C activity of their shells must be in equilibrium with the 14C activity of the atmosphere, and (2) after burial, their shells must behave as closed systems with respect to carbon. To evaluate the first criterion, we conducted a comprehensive examination of the 14C content of the most common small terrestrial gastropods in North America, including 247 AMS measurements of modern shell material (3749 individual shells) from 46 different species. The modern gastropods that we analyzed were all collected from habitats on carbonate terrain and, therefore, the data presented here represent worst-case scenarios. In sum, ～78% of the shell aliquots that we analyzed did not contain dead carbon from limestone or other carbonate rocks even though it was readily available at all sites, 12% of the aliquots contained between 5 and 10% dead carbon, and a few (3% of the total) contained more than 10%. These results are significantly lower than the 20–30% dead carbon that has been reported previously for larger taxa living in carbonate terrain. For the second criterion, we report a case study from the American Midwest in which we analyzed fossil shells of small terrestrial gastropods (7 taxa; 18 AMS measurements; 173 individual shells) recovered from late-Pleistocene sediments. The fossil shells yielded 14C ages that were statistically indistinguishable from 14C ages of well-preserved plant macrofossils from the same stratum. Although just one site, these results suggest that small terrestrial gastropod shells may behave as closed systems with respect to carbon over geologic timescales. More work on this subject is needed, but if our case study site is representative of other sites, then fossil shells of some small terrestrial gastropods, including at least five common genera, Catinella, Columella, Discus, Gastrocopta, and Succinea, should yield reliable 14C ages, regardless of the local geologic substrate.  相似文献   

4.
Data on the content of the 14C cosmogenic isotope in tree rings, which were obtained as a result of laboratory measurements, are often used when solar activity (SA) is reconstructed for previous epochs, in which direct observations are absent. However, these data contain information not only about SA variations but also about changes in the Earth climatic parameters, such as the global temperature and the CO2 content in the Earth’s atmosphere. The effect of these variations on the 14C isotope content in different natural reservoirs after the last glacial termination to the middle of the Holocene is considered. The global temperature and the CO2 content increased on this time interval. In this case the 14C absolute content in the atmosphere increased on this time interval, even though the 14С to 12С isotope concentration ratio (as described by the Δ14С parameter) decreased. These variations in the radiocarbon absolute content can be caused by its redistribution between natural reservoirs. It has been indicated that such a redistribution is possible only when the rate of carbon exchange between the ocean and atmosphere depends on temperature. The values of the corresponding temperature coefficient for the 17–10 ka BC time interval, which make it possible to describe the carbon redistribution between the ocean and atmosphere, have been obtained.  相似文献   

5.
Modern to Holocene tropical Pacific stalagmites are commonly difficult to date with the U-series, the most commonly used dating method for speleothems. When U-series does not provide robust age models, due to multiple sources of 230Th or little U, radiocarbon is, potentially, the best alternative. The 14C content of two stalagmites (Pu17 and Nu16) collected from Pouatea and Nurau caves in the Cook Island Archipelago of the South Pacific were measured to obtain accurate chronology for their most modern parts. The bomb-pulse soil continuum modelling indicates that bomb radiocarbon in Pu17 onsets in 1956 and reaches its maximum in 1966 CE, suggesting a fast transfer of atmospheric carbon to the stalagmite of <1 year. The modelling for Pu17 suggests a 20% contribution from C1 - an instantaneous carbon source, which renders possible an immediate transfer of atmospheric signal into the cave. Nu16 shows a slower transfer of atmospheric carbon to the stalagmite than Pu17, with bomb radiocarbon onsetting in 1957 CE and peaking in 1972 CE. The less negative δ13C values in Nu16 than Pu17, and also the modelling corroborated this, which points out no contribution from the instantaneous carbon source. The radiocarbon age models and laminae counting age models were then spliced to achieve a single master chronology for the top part of each stalagmite. This study is an example of 14C age modelling combined with visible physical and chemical laminae counting and how it can improve the accuracy and precision of dating for otherwise hard-to-date tropical Pacific speleothems. Such accurate and precise age models pave the way to obtain sub-annually resolved paleoclimate records by further improving the calibration of climate proxy data with the current and instrumental weather parameters.  相似文献   

6.
Lacustrine carbonate deposits (tufa) record variations in terrestrial hydrology and are preserved in many now-arid regions of the world, but are challenging to date with precision and accuracy. Many contain detritus and/or unsupported thorium (Th) that degrades or prevents the measurement of precise uranium-series (U-series) ages, and radiocarbon ages are frequently affected by both the reservoir (hard-water) effect and contamination with younger atmospheric carbon. The usual method of testing the accuracy of carbonate 14C measurements, comparison with U/Th ages or organic carbon dates, does not separate the reservoir and modern contamination effects, allowing for only relatively imprecise age estimates in samples undatable by U/Th.We have separated the modern contamination problem and the reservoir effect using a step dissolution technique on a variety of carbonate materials from Mono Lake, California, a long-lived closed-basin lake sensitive to regional precipitation variability. New dissolution experiments focus on the deglacial sediments of the Wilson Creek Formation, which preserve ostracodes and fans and mounds of thinolite, a cold-water, hydrated calcium carbonate (CaCO3.6H2O). Stepped-dissolution ages of thinolite crystals and dense calcites increased by 500–1000 years over the bulk age, and produced a plateau of analytically indistinguishable ages, indicating nearly complete removal of modern carbon. Repeated experiments on ostracodes from a single sample showed an increase of >3500 years over the bulk age and >6500 years from first to last step, but ages increased up to the last ∼5% of the CO2 evolved, without forming a plateau. This may be due to the extremely large surface area-to-volume ratio of ostracodes, and inhomogeneous dissolution of the hundreds of individual shells required for the experiments. Further experiments are planned to test the effects of modern carbon on tufa, gastropods, and other shells, with the goal of systematically testing the precision and reliability of chronologies for dramatic changes in lake level in arid regions of the world.  相似文献   

7.
Over the past decade, a number of speleothem studies have used radiocarbon (14C) to address a range of palaeoclimate problems. These have included the use of the bomb pulse 14C to anchor chronologies over the last 60 years, the combination of U-Th and 14C measurements to improve the radiocarbon age-calibration curve, and linking atmospheric 14C variations with climate change. An issue with a number of these studies is how to constrain, or interpret, variations in the amount of radioactively dead carbon (i.e. the dead carbon fraction, or DCF) that reduces radiocarbon concentrations in speleothems. In this study, we use 14C, stable-isotopes, and trace-elements in a U-Th dated speleothem from Flores, Indonesia, to examine DCF variations and their relationship with above-cave climate over the late Holocene and modern era. A strong association between the DCF and hydrologically-controlled proxy data suggests that more dead carbon was being delivered to the speleothem during periods of higher cave recharge (i.e. lower δ18O, δ13C and Mg/Ca values), and hence stronger summer monsoon. To explore this relationship, we used a geochemical soil-karst model coupled with 14C measurements through the bomb pulse to disentangle the dominant components governing DCF variability in the speleothem. We find that the DCF is primarily controlled by limestone dissolution associated with changes in open- versus closed-system conditions, rather than kinetic fractionation and/or variations in the age spectrum of soil organic matter above the cave. Therefore, we infer that periods of higher rainfall resulted in a higher DCF because the system was in a more closed state, which inhibited carbon isotope exchange between the karst water dissolved inorganic carbon and soil-gas CO2, and ultimately led to a greater contribution of dead carbon from the bedrock.  相似文献   

8.
Radiocarbon dating based on geomorphological, archaeological and biological data is widely used in geomorphological studies to reconstruct sequences of climatic variations and coastal evolution during the Holocene. The coastal area of Southern Italy is characterized by aeolian dune belts shaped during the Holocene that crop out along the present shoreline. Archaeological data and previous radiocarbon results suggest three aeolian morphogenetic phases. The first phase corresponds to the end of the rapid post-glacial transgression (7.0–6.0 ka BP); the second to the aeolian sand deposition during the “Greek–Roman” period (2.5–1.9 ka BP); and the most recent phase occurred in the period from the Middle Ages to the present time. The reconstruction of the sequence of the morphogenetic phases was mainly based on radiocarbon analyses carried out on both terrestrial gastropods and marine bivalves. The reliability of the radiocarbon analyses on terrestrial gastropod shells has been questioned by several Authors and a closer understanding of the carbon uptake mechanism in this kind of organisms is needed.A systematic study was carried out by performing Accelerator Mass Spectrometry (AMS) 14C dating on the shells of terrestrial gastropods sampled alive in different geomorphological settings along the Adriatic and Ionian coasts of Southern Italy. The results show significant anomalies in the radiocarbon content and in the carbon stable isotopic ratio. This can be due to the ingestion of 14C-depleted calcium carbonate in the diet of these organisms. We also calculated the carbon fraction from air Xa (between 16% and 48%), plants Xv (between 36% and 73%) and limestone Xc (between 3% and 23%) giving insight to the origin of the age anomalies.  相似文献   

9.
Uptake of atmospheric CO2 during sample collection and analysis, and consequent lowering of estimated ages, has rarely been considered in radiocarbon dating of groundwater. Using field and laboratory experiments, we show that atmospheric CO2 can be easily and rapidly absorbed in hyperalkaline solutions used for the extraction of dissolved inorganic carbon, resulting in elevated 14C measurements. Kinetic isotope fractionation during atmospheric CO2 uptake may also result in decrease of δ13C, leading to insufficient corrections for addition of dead carbon by geochemical processes. Consequently, measured 14C values of groundwater should not be used for age estimation without corresponding δ13C values, and historical 14C data in the range of 1 to 10% modern Carbon should be re‐evaluated to ensure that samples with atmospheric contamination are recognized appropriately. We recommend that samples for 14C analysis should be collected and processed in the field and the laboratory without exposure to the atmosphere. These precautions are considered necessary even if 14C measurements are made with an accelerator mass spectrometer.  相似文献   

10.
Hu W  Liu H  Sun H  Shen O  Wang X  Lam MH  Giesy JP  Zhang X  Yu H 《Marine pollution bulletin》2011,62(11):2356-2361
Methoxylated brominated diphenyl ethers (MeO-BDEs) in aquatic environments have been found to be primarily of natural origin in the marine environment and not from biotransformation of synthetic PBDEs. Two of the eight MeO-PBDEs (2′-MeO-BDE-68 and 6-MeO-BDE-47) that were detected in anchovy from the Yangtze River Delta, were natural products from marine organisms. So 2′-MeO-BDE-68 and 6-MeO-BDE-47 were chosen to study the potential to modulate androgen, estrogen, or thyroid hormone receptor- (AR, ER, ThR) mediated responses by use of reporter gene assays. 2′-MeO-BDE-68 was antiandrogenic at 50 μM, estrogenic at 10 μM and antiestrogenic at 10 and 50 μM (IC50 = 4.88 μM). 2′-MeO-BDE-68 enhanced luciferase expression by 5 nM T3 at 50 μM. 6-MeO-BDE-47 exhibited potent antiandrogenicity at 1 μM and greater (IC50 = 41.8 μM) and possessed estrogenic activity at 10 μM and antiestrogenic activity at 10 and 50 μM (IC50 = 6.02 μM).  相似文献   

11.
Data on variations in the content of the 14C cosmogenic isotope in tree rings and the Earth’s atmosphere (Δ14C) make it possible to study the behavior of solar activity (SA) in previous centuries and millenniums. The latter is related to the fact that SA temporal variations result in a change in the IMF (Interplanetary Magnetic Field) parameters and, as a consequence, in the galactic cosmic ray (GCR) flux, under the action of which the 14C isotope is produced in the Earth’s atmosphere. This makes it possible to study SA history based on data on the 14C isotope content in tree rings. However, in this case we have several difficulties related to climate change. Climate changes result in carbon redistribution between natural reservoirs, which is reflected in radiocarbon data and results in solar signal distortion. The effect of variations in the global temperature and carbon dioxide concentration on the reconstruction of the heliospheric modulation potential and Wolf numbers from the late 14th century to the early 19th century is considered. It has been shown that the radiocarbon data do not make it possible to conclude that SA during the Maunder minimum was extremely low as compared to SA during the Dalton minimum.  相似文献   

12.
Reliable chronological frameworks are crucial to paleoenvironmental studies, and high precision 14C dating is the foundation, but many factors, such as dating materials, surficial deposition (influenced by nuclear bomb), and the 14C age plateau, will affect the reliability of the 14C ages and chronology frameworks. In this paper, we present 87 14C dates of different peat fractions from three peat sites in Xinjiang, China. Plant macrofossils, rootlets, the fine fraction of <90 μm, the mid-size fraction of 90–250 μm and the coarse fraction of >250 μm from selected peat samples were measured to investigate the alternative suitable fraction for dating except for plant macrofossils. We discovered that the 90–250 μm component of peat can provide alternative and reliable results in case of plant macrofossils are not available. Additionally, more dating samples from surficial peat deposition were collected, and accurate surface chronological control points were produced by comparing 14C results of plant macrofossils with atmospheric 14C bomb data. Furthermore, multiple data sets with wiggle matching were used along the radiocarbon age plateau to minimize calibrated errors when dates on the 14C age plateau were shown. Finally, radiocarbon chronology frameworks in peat profiles were optimized. In conclusion, we not only focus on the reliable dating materials, but also highlight that the importance of surficial deposition (after 1950AD) and the anomalous 14C dates when establishing the dating framework in peat profiles. Furthermore, we propose that the obtaining chronological control points of surficial peat is an important part of the establishing and improving of peat chronological framework in future research.  相似文献   

13.
This study is devoted to the production of 14C by the secondary cosmic radiation in polar ice. The radiocarbon production in the reactions caused by the nuclear-active and muon components is considered. The data on 14C from the Vostok and Taylor Dome Antarctic boreholes are analyzed. The 14C concentration values at depths larger than the firn—ice boundary by a factor of 2—3 can be explained by a deep production of radiocarbon in the reactions caused by the cosmic radiation muon component.  相似文献   

14.
Radiocarbon: A chronological tool for the recent past   总被引：2，自引：2，他引：0  
The past few hundred years have seen large fluctuations in atmospheric 14C concentration. In part, these have been the result of natural factors, including the climatic changes of the Little Ice Age, and the Spörer and Maunder solar activity minima. In addition, however, changes in human activity since the middle of the 19th century have released 14C-free CO2 to the atmosphere. Moreover, between c. 1955 and c. 1963, atmospheric nuclear weapon testing resulted in a dramatic increase in the concentration of 14C in the atmosphere. This was followed by a significant decrease in atmospheric 14C as restrictions on nuclear weapon testing began to take effect and as rapid exchange occurred between the atmosphere and other carbon reservoirs. The large fluctuations in atmospheric 14C that occurred prior to 1955 mean that a single radiocarbon date may yield an imprecise calibrated age consisting of several possible age ranges. This difficulty may be overcome by obtaining a series of 14C dates from a sequence and either wiggle-matching these dates to a radiocarbon calibration curve or using additional information on dated materials and their surrounding environment to narrow the calibrated age ranges associated with each 14C date. For the period since 1955 (the bomb-pulse period), significant differences in atmospheric 14C levels between consecutive years offer the possibility of dating recent samples with a resolution of from one to a few years. These approaches to dating the recent past are illustrated using examples from peats, lake and salt marsh sediments, tree rings, marine organisms and speleothems.  相似文献   

15.
Lakes over the inner Tibetan Plateau (TP) are very sensitive to the regional environmental transformations and climate changes. Well-preserved lake sediments around these lakes provide critical geomorphological and sedimentary evidence that can be used to infer the past hydroclimate changes. In this study, a lacustrine section from a sandy shoreline (∼74 m above the modern lake) situated to the northwest of modern Dawa Co in the inner TP was investigated using both luminescence and radiocarbon dating methods. Our results demonstrated: (1) the quartz optically simulated luminescence (OSL) dating yielded much younger ages (∼4 ka) than that of the post-infrared IRSL (pIRIR) dating of the K-feldspar fraction; (2) fading test showed g-values ranging between 1.34 and 4.46%/decade for quartz OSL signals, which is considered to be responsible for the underestimation of the corresponding ages; (3) the AMS 14C age of the charcoal sample from the section is in line with the K-feldspar pIRIR225 ages, confirming the reliability of the pIRIR225 dates and the underestimation of the quartz OSL ages. The anomalous fading of quartz OSL signals and the consequent age underestimation have been reported in several other lakes on the TP, we presented here for the first time firm evidence of the phenomenon with the help of a robust independent control of AMS 14C age of the charcoal. Based on the pIRIR225 and AMS 14C ages, we conclude that Dawa Co underwent a prominent highstand during the early Holocene (∼9–7 ka), which was probably controlled by the large amounts of glacial meltwater input and the increased monsoonal precipitation.  相似文献   

16.
Surface sediments and porewater from 12 sites within Xiamen offshore areas and organisms from a heavily contaminated site Yuandang Lagoon were sampled and analyzed for eight polybrominated diphenyl ethers (PBDEs) congeners (-28, -47, -99, -100, -153, -154, -183 and -209). The total concentrations of eight PBDEs (∑8PBDEs) and BDE-209 in sediments ranged from 0.27 to 76.54 ng/g with an average of 16.31 ng/g and from 0.10 to 70.11 ng/g with an average of 14.94 ng/g, respectively. Concentrations of ∑8PBDEs in porewater ranged from 2.5 to 34.1 ng/L, with a mean value of 15.3 ng/L. In this study, the partition coefficients (log ) of PBDE congeners (without -209) were significantly correlated with their octanol-water partition coefficients (log KOW) (r2 = 0.74, P < 0.01). ∑8PBDE concentrations ranged from 0.33 to 1.26 ng/g (lipid weight) in marine organisms, and PBDE congener patterns were significantly different between fish and clam, crab.  相似文献   

17.
In lake sediments where terrestrial macrofossils are rare or absent, AMS radiocarbon dating of pollen concentrates may represent an important alternative solution for developing a robust and high resolution chronology suitable for Bayesian modelling of age-depth relationships. Here we report an application of the heavy liquid density separation approach (Vandergoes and Prior, Radiocarbon 45:479–492, 2003) to Holocene lake sediments from karstic Lake Sidi Ali, Morocco. In common with many karstic lakes, a significant lake 14C reservoir effect of 450–900 yr is apparent, evidenced by paired dates on terrestrial macrofossils and either aquatic (ostracod) or bulk sediment samples. AMS dating of 23 pollen concentrates alongside laboratory standards (bituminous coal, anthracite, IAEA C5 wood) was undertaken. Concentrates were prepared using a series of sodium polytungstate (SPT) solutions of progressively decreasing density (1.9–1.15 g/cm3) accompanied by microscopic analysis of the resulting residues to allow quantification of the terrestrial pollen content. The best fractions (typically precipitating at 1.4–1.2 g/cm3) yielded dateable samples of 0.5–5 mg (from sediment samples of ∼15 g), with C content typically ∼50% by weight. Terrestrial pollen purity ranges from 29% to 88% (μ = 67%), reflecting the challenge of isolating pollen grains from common aquatic algae, e.g. Pediastrum and Botryococcus. A Poisson-process Bayesian depositional model incorporating radiocarbon (pollen and macrofossil) and 210Pb/137Cs data is employed. As all pollen samples incorporate some non-terrestrial organic matter, we assume an exponential outlier distribution treating each pollen concentrate datum as an old outlier and terminus post quem. This approach yields strong data-model agreement, and differences between the prior and posterior age distributions are furthermore consistent with theoretical offsets anticipated for the known reservoir ages and sample-specific terrestrial content. This application of the pollen concentrate dating approach reinforces the importance of microscopic inspection of the residues during the separation and sieving stages. Sample specific differences mean that the pollen concentrate preparation cannot be reduced to a simplistic “black box” protocol, and dating and subsequent age-model development must be supported by detailed analysis of the microfossil content of the sediments.  相似文献   

18.
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19.
Bivalves, crabs, fishes, seawater, and sediment collected from the inner part of Tokyo Bay, Japan, were measured for 20 polybrominated diphenyl ether (PBDE) and 5 polychlorinated biphenyl (PCB) congeners. To determine the trophic levels of the organisms, carbon and nitrogen stable isotope ratios (δ13C and δ15N) were also measured. Bioconcentration factors of PBDE and PCB congeners increased as the octanol-water partition coefficient (Kow) rose to log Kow = 7, above which they decreased again. Biomagnification of PCBs and several PBDE congeners (BDE47, 99, 100, 153 and 154) up the trophic ladder was confirmed by a positive correlation between their concentrations and δ15N. Other PBDE congeners showed a negative or no correlation, suggesting their biotransformation through metabolism. The more hydrophobic congeners of both PBDEs (Br = 2-6) and PCBs (Cl = 6-9) were biomagnified more. It thus appears that PBDEs are less biomagnified than PCBs.  相似文献   

20.
Man-made polybrominated diphenyl ethers (PBDEs) used as flame retardants in various consumer products may be harmful to marine organisms. Larvae of some marine invertebrates, especially invasive species, can develop resistance to PBDEs through altered protein expression patterns or proteome plasticity. This is the first report of a proteomics approach to study BDE-47 induced molecular changes in the invasive limpet Crepidula onyx. Larvae of C. onyx were cultured for 5 days (hatching to metamorphosis) in the presence of BDE-47 (1 μg L−1). Using a 2-DE proteomics approach with triple quadrupole and high-resolution TOF-MS, we showed that BDE-47 altered the proteome structure but not the growth or metamorphosis of C. onyx larvae. We found eight significant differentially expressed proteins in response to BDE-47, deemed the protein expression signature, consisting of cytoskeletal, stress tolerance, metabolism and energy production related proteins. Our data suggest C. onyx larvae have adequate proteome plasticity to tolerate BDE-47 toxicity.  相似文献   

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