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1.
The artificial radionuclides 137Cs, 134Cs and 106Ru have been determined up to 16 cm deep in two sediment cores collected in March 1988 at the River Adige estuary in the northern Adriatic Sea. Interdependent relations between the properties of the sediment levels and the activity of radionuclides have been investigated using multivariate statistical analysis. The interactions between radiocontaminants and the particulate matter or water phase are discussed during their transfer processes from the river to the estuary and the marine environment. Diffusion phenomena within the sedimentary column have also been considered. The vertical distribution of the artificial radionuclides due to Chemobyl fallout in sediment cores from the River Adige mouth, compared to those from similar estuarine environments in the Adriatic Sea, point out a general rapid accumulation of river-borne solid material in coastal zones of the North Adriatic Sea.  相似文献   

2.
Abstract. 40Ar-39Ar analyses of two alunite samples from phreatic craters in the Pliocene Muine volcano in southwest Hokkaido, Japan, were carried out. The alunite with 17.4 permil δ34SV_CDT value in hydrothermal breccia from the Nagaoyama crater and that with 14.3 permil δ34SV_CDT value in silicified andesite from the Konuma crater give total fusion ages of 1.40 ± 0.04 Ma (la uncertainty) and 1.24 ± 0.08 Ma, respectively. However, the spectra of these samples indicate they have been effected by thermal overprinting and/or the existence of excess argon. These preliminary 40Ar-39A analyses suggest that the alunite underwent multiple hydrothermal activity by magmatic gas and vapor subsequent to the main hydrothermal activity.  相似文献   

3.
Dating recent lake sediment records yielding disturbed 210Pb profiles has been a problem of wide interest in palaeoclimatic and palaeoseismic studies over the last few centuries. When applied to an alpine lake sedimentary record, a high-resolution sedimentological study reveals that the 210Pb profile is disturbed by the occurrence of single-event deposits triggered by two different mechanisms: flood events deposits and gravity reworking. Removing disturbed layers from the 210Pb profile yields a logarithmic depth–activity relationship. Using a simple 210Pb decay model (CFCS) provides an assessment of mean accumulation rate of `continuous sedimentation', as opposed to `event-linked sedimentation'. The correlation of the thickest four gravity-reworked deposits with historically known earthquakes permits both validation and refinement of the age–depth relationship. This refinement highlights variations in accumulation rate consistent with post-Little Ice Age climatic variations.  相似文献   

4.
Values of δ13C obtained from conventional bulk sediment radiocarbon dates encompassing the Pleistocene Holocene boundary have been compiled and plotted against 14C age. In all. 286 lake sediment dates from southern Sweden in the range 8.000 to 13.000 BP have been evaluated. A significant decrease in δ13C values, initiated shortly before 10.000 RP and amounting to 5%, is distinguished. This change is accompanied by increased limnic productivity. decreased erosive input and increased organic carbon content of the sediments. A probable explanation for the δ13C decline in organic material is decreased importance of dissolution of silicates at the transition to the Holocene. During the Late Weichselian. extensive weathering of exposed minerogenic material with subsequent input of bicarbonate to the lake water may have caused a relative enrichment of 13C in dissolved inorganic carbon. Furthermore, the early Holocene increase in terrestrial vegetation cover probably led to an increased supply of 13C depleted carbon dioxide to the lake water by root respiration. Altered limnic vegetation, presumably towards increased production of phytoplankton. could also have contributed to the observed decreasing δ13C trend. The importance of these processes compared to other possible influencing factors. mainly endogenic carbonate production and changes in the global carbon cycle. is discussed.  相似文献   

5.
Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution 40Ar/39Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2, S3, and S4) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40Ar/39Ar geochronology. Mean 40Ar/39Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.  相似文献   

6.
For studies of mass-dependent fractionation of calcium isotopes in natural materials, the 48Ca/42Ca ratio is a superior choice to the conventionally measured 44Ca/40Ca ratio for two important reasons. These are (1) mass-dependent fractionation can be determined free from the effects of inherited or ingrown radiogenic 40Ca and (2) this ratio increases the spread of measured isotopic masses by 50%, resulting in statistically better resolution of fractionation, assuming similar precision. A third, though strictly technical, advantage is the inherent ability of a mass spectrometer to measure ratios close to unity (48Ca/42Ca) more precisely than very small or large ratios (44Ca/40Ca). However, because of the very low natural abundance of both 48Ca and 42Ca, their ratio has been very difficult to measure, the only attempt so far being on a high mass resolution MC-ICP-MS with a precision of 0.33%. We report here determination of the 48Ca/42Ca ratio by the more commonly available and user-friendly multi-collector TIMS using a 43Ca-46Ca double-spike, with a significantly better precision of 0.18% (2s). The 48Ca/40Ca or 44Ca/40Ca ratio can also be measured in the same mass spectrometer run to provide complementary information on any radiogenic component.  相似文献   

7.
Data from a series of extended analytical sessions using a range of different zircon reference samples have been used to investigate the periodic derivation of abnormal 207Pb/206Pb ages during microbeam analysis%For the Canberra SHRIMP II, this phenomenon probably results from the presence of a signal at mass 204 that is alien to the Pb spectrum; isotopic fractionation is unlikely to be a significant contributor. In contrast, abnormal 207Pb/206Pb ages obtained from SHRIMP I at the same research centre require a different, but as yet unknown explanation. Assessment of the reasons for atypical 207Pb/206Pb measurements and the means of correcting for them should therefore be independently assessed for individual analytical instruments. The use of reference samples with very different ages is an effective way of doing this, because older reference samples are more useful for documenting isotopic fractionation, whereas younger ones are more sensitive indicators of isobaric interference.  相似文献   

8.
In this paper we address the question of how to determine the period of activity (sedimentation) of fossil (Holocene) fluvial systems in vertically aggrading environments. Our data base consists of almost 100 14C ages (partly AMS) from the Rhine–Meuse delta in the central Netherlands. Radiocarbon samples from the tops of lithostratigraphically correlative organic beds underneath overbank deposits (sample type 1) yield consistent ages, indicating a synchronous onset of overbank deposition over distances of at least up to 20 km along channel belts. Similarly, 14C ages from the base of organic residual channel fills (sample type 3) generally indicate a clear termination of within-channel sedimentation. In contrast, 14C ages from the base of organic beds overlying overbank deposits (sample type 2), commonly assumed to represent the end of fluvial sedimentation, show a large scatter reaching up to 1000 14C years. This setting usually produces 14C ages significantly younger than residual channels of the same fluvial system, indicating the presence of non-depositional unconformities. These usually occur on top of slightly elevated sediment bodies (especially natural levee deposits), which often contain palaeosols. Such hiatuses appear to be much more abundant than hitherto supposed. We conclude that a combination of sample types 1 and 3 generally yields a satisfactory delimitation of the period of activity of a fossil fluvial system. The problems associated with 14C samples of type 2 may also be present in coastal areas with an alternation of tidal deposits and organic beds.  相似文献   

9.
Tree-ring studies carried out on subfossil oak trunk deposits within the Holocene valley fills of the River Main can reconstruct phases of increased fluvial activities. These phases have been dated on the base of two absolutely tree-ring dated chronologies and in addition by 11C-datings of eleven floating tree-ring scries of subfossil oaks.
Geological-pedological investigations reveal an alternation between increased and reduced fluvial activity during the Holocene.
Periods of increased gravel redeposition are dated by dendrochronology, and by 14C and cultural findings. Increased fluvial activity becomes more frequent towards Modern Times with culminations in the Middle Atlantic, the Subboreal, the Iron-Roman Age, the Main Middle Ages till earliest Modern Times, and in the last century.
On the sequence of Holocene river deposits there developed specific soil types as indicators for the age of the river deposits since the Last Glacial.
Among other palaeoecological results an important finding is the correlation between tree-ring width, flood-loam sedimentation, and soil development.  相似文献   

10.
Results from detailed pollen and 18O/16O studies on two sediment profiles from small Swiss lakes are reported. 18O/16O records in lacustrine carbonate contain paleoclimatic information because they reflect mainly the isotope ratio in rain and snow which is correlated to temperature. Several transitions between different climatic periods determined palyno-logically are also indicated by marked changes in the isotope ratios in both profiles, namely the transitions Oldest Dryas - Bøiling and Allerød - Younger Dryas - Preboreal. 18o/16O was 2 to 3 %0 lower during Younger Dryas than during the adjacent periods, corresponding to a temperature drop of a few degrees Centigrade according to a tentative estimate.  相似文献   

11.
The increasing focus on the chronology of environmental and climatic changes of the last glacial-interglacial transition has led to several independent attempts to try to calibrate the 14C time-scale beyond the Holocene. The Late Weichselian Gotiglacial varved clays of the Swedish Time Scale could potentially be used for this purpose. The reliability of the Swedish Time Scale is discussed as well as different ways of using the Swedish varved clays for calibrating the 14C chronology. The strategy and initial results from an ongoing calibration project are presented. They show clearly that, if the right strategy is adopted, varved clay may be dated by accelerator mass spectrometry (AMS) 14C measurement of terrestrial macrofossils. A Late Weichselian 'event stratigraphy', including the Vedde Ash fall-out, is established for south Scandinavia using three dating categories: clay varve measurements, terrestrial macrofossil measurement, and lake sediment (including aquatic mosses) measurements. It suggests that a 14C chronology based on terrestrial organic remains is not consistent with the traditional Late Weichselian chronostratigraphy based on lake-sediment samples, and that 'clay varve years' exceed 'terrestrial 14C years' by c. 900 years at the end of, and by 1100–1200 years at the beginning of the Younger Dryas Chronozone. Further back in time, the time-scales appear to converge. These results are compared with other recently published calibration studies.  相似文献   

12.
In order to evaluate the sensitivity of aquatic and terrestrial ecosystems to climatic changes, lithological (sediment structure and color, grain size, physical properties) and biochemical (TOC, TOC/TN, δ13C of TOC and carbonates) investigations were carried out on an 11.12 m-long sediment core from Lama Lake (Central Siberia, Putorana Plateau). According to the pollen data, the sequence represents the termination of the Pleistocene, and the entire Holocene. It is composed of highly terrigenous and stratified clays and silts. Sediment structure, grain-size distribution, carbonate contents and physical properties of the sediment indicate that glaciers were present in the catchment area of Lama Lake during the period Oldest Dryas to AllerØd. For the same time period, δ13C values of TOC give indications of a perennial ice cover. Since the AllerØd, organic matter accumulation has increased, caused by an increasing input of land vegetation and aquatic primary production as revealed by relations TOC contents, TOC/TN ratios and δ13C values of TOC. During the Holocene climatic optimum, in late Preboreal and Boreal times, biogenic primary production in Lama Lake reached its maximum and the vegetation in the catchment area changed from grassy tundra to a dense forest. From the Atlantic period to the present, small variations in δ13C values of TOC and TOC contents are probably related to the location of Lama Lake on the border between grass and wooded steppe, leading to responses of environmental conditions to even small changes in climate.  相似文献   

13.
The alkalic Scituate Granite was emplaced into crystalline sequences within the New England Esmond–Dedham terrane in the Late Devonian ( c. 370 Ma). Variably recrystallized amphibole (iron-rich, hastingsite–hastingsitic hornblende) from four variably deformed samples of the pluton record south-westerly younging 40Ar/39Ar plateau ages ranging between 276 and 263 Ma. These are interpreted to date diachronous cooling through temperatures appropriate for intracrystalline retention of argon following late Palaeozoic orogenic activity. Iron-rich biotite concentrates from the samples record only slightly younger ages, and therefore suggest relatively rapid post-metamorphic cooling. The 40Ar/39Ar ages indicate that the late Palaeozoic tectonothermal overprint was much more regionally pervasive than was previously considered. The apparent timing of this activity is similar to previous estimates for the chronology of high-grade metamorphism throughout the adjacent Hope Valley terrane and for phases of ductile movement on the intervening Lake Char–Honey Hill fault system.  相似文献   

14.
ABSTRACT This work presents a detailed 87Sr/86Sr isotope curve for the interval 7.5–9.7 Ma obtained by a high-resolution analysis (sampling spacing of about 40 kyr) of an astronomically calibrated land-based sedimentary sequence exposed in the central Mediterranean area (Gibliscemi section, southern Sicily). The main aim is to verify a synchronous response of the Mediterranean seawater Sr isotope record to the oceanic forcing on the basis of multiple comparisons of the Gibliscemi record with published coeval 87Sr/86Sr curves. A good correlation with the 87Sr/86Sr data from the ODP site 926 (equatorial Atlantic ocean), considered to be the Sr chemostratigraphic reference section for the Late Miocene, and from the Pacific DSDP site 590B was registered. Conversely, the comparison of the Gibliscemi Sr isotope data with 87Sr/86Sr ratios from the coeval segment of the land-based Sardella section (eastern Mediterranean) shows important differences highlighting a local control on the seawater Sr isotope changes in semi-isolated subbasins within the Late Miocene Mediterranean.  相似文献   

15.
An exhaustive 14C dating programme of molluscs from the Fossvogur sediments in Reykjavik. Iceland is presented. For the first time all the fossiliferous units of the sediments are dated. The results confirm earlier conclusions of a widespread occurrence of marine sediments of Allerød age in Reykjavik. The set of dates from the Fossvogur sediments shows a narrow 14C age distribution (standard deviation of ±235 years) of molluscs from all localities and from successive marine units in vertical sections. The weighted mean conventional 14C age is 11,400 BP. Assuming a reservoir effect of 400 years. this corresponds to a reservoir-corrected age of I1,000 BP. i.e. the Allerød- Younger Dryas transition for the sampled units, These new 14C dates from Fossvogur confirm the need for a revision of the Upper Pleistocene chronology of the Reykjavik region. They also have a bearing on the Late Weichselian record of glacier readvances and sea-level changes in the area. The dates suggest that the marine units in Fossvogur accumulated within a restricted time-span of a few hundred years. The sediments in Fossvogur are of volcaniclastic origin and are extremely lithified, indicating local geothermal activity soon after their deposition. This may explain anomalously high D/L amino acid ratios measured in molluscs from the Fossvogur sediments. δ13C and δ18O results suggest that temperatures may have ranged up to 60°C.  相似文献   

16.
A direct method for the determination of lead isotopic ratios by laser ablation-inductively coupled plasma-quadrupole mass spectrometry (LA-ICP-QMS) is presented. Samples of lake sediments were ground and pressed as pellets before being analysed. Mass bias was corrected by analysing an external calibration sample manufactured with pure silica doped with NIST SRM 981 solution. The efficiency of the mass bias correction was checked by comparing the ICP-MS data with lead isotopic ratios determined by thermal ionisation mass spectrometry (TIMS). The average long term reproducibilities were 0.40%, 0.40%, 0.20% and 0.30% (RSD) respectively for the 206Pb/204Pb, 207Pb/204Pb, 206Pb/207Pb and 208Pb/206Pb ratios. The method was applied to the study of lake sediment samples, in order to determine the amount and origin of historical contamination by lead.  相似文献   

17.
The ahermatypic coral Lophelia pertusa which produces aragonitic skeletons is widely distributed along the Norwegian coast. Specimens from a number of localities have been analyzed for oxygen and stable carbon isotope composition and 14C age. Stable isotope ratios of recent corals provide information on growth rate and seasonality of oceanographic conditions. Lophelia can be useful in paleoenvironmental reconstructions. 14C dates of fossil Lophelia from Drøak in the Oslofjord, collected from 20 m ahove and 40 m below present day sea-level, indicate regional extinction between 8700 and 7800 years ago. We suggest that the extinction resulted from the cut-off of deep waters by a rising sill in connection with the postglacial shoreline displacement. Radiocarbon dating of coral bushes suggests a fairly rapid growth rate as older and younger parts of recent corals do not reveal any difference in activity despite the short time scale of the history of bomb-produced 14C in the oceans.  相似文献   

18.
This raised delta structure is an ice-contact deltaic complex with a volume of c. 4.4.109 m3, deposited c . 9500 yr BP in a shallow wide 'fjord' during the retreat of the Scandinavian ice cap. The delta plain lies at an altitude of 200–223 m. It aggraded c . 20 m above the contemporaneous sea level during a regional marine regression. The braidplain palaeochannel characteristics indicate a peak meltwater discharge of 7–9 103 m3/s. Calculations based on a glacial ablation model indicate a mid-summer discharge of c . 5.5 103 m3/s. However, the fluvial topset of the delta has an erosive base whose altitude decreases upstream and indicates stream incision by more the 6 m below the contemporaneous sea level. The deep scour is ascribed to episodic floods over the relatively short delta plain, which exceeded direct ablation-associated discharges. The depositional time-span of the delta is assessed to have been 70 years, calculated from coastal gradient and shoreline displacement curves. The average sedimentation rate of the delta is thereby inferred to have been extremely high, c . 6. 107 m3/yr. The sedimentation is thought to reflect 'extreme' ice-margin conditions, where the sediment and water discharge was maximized by full-scale ablation, with simultaneous subglacial, englacial and supraglacial sediment and water supply. These conditions might further coincide with an abundant rainfall in the catchment area or the drainage of dammed waters, initiating episodic floods which eroded deep beneath sea level. As a whole, the study illustrates the hydrological conditions of proglacial sedimentation at the front of the rapidly retreating last Scandinavian ice cap.  相似文献   

19.
A well-preserved moraine on the northern coast of County Donegal, Ireland, has played a critical role in our understanding of the glacial history of this sector of the Irish Ice Sheet (IIS). Because of a lack of numerical dating of the moraine, however, previous interpretations of its age and significance to the glacial history of this region have varied widely. Here we report eight in situ cosmogenic 10Be ages on boulders sampled from the moraine. Two of these ages are outliers, with the remaining six ranging from 18.8±1.0 10Be kyr to 20.9±1.3 10Be kyr, with an uncertainty-weighted mean age of 19.4±0.3 10Be kyr (19.4±1.2 kyr accounting for production rate uncertainty). Our results confirm one previous 10Be age obtained from the moraine, with the combined data ( n =7) constraining the age of initial deglaciation of the IIS from its LGM position on the continental shelf to be 19.3±0.3 10Be kyr (19.3±1.2 kyr accounting for production rate uncertainty). These ages are in excellent agreement with calibrated 14C ages that constrain retreat of the IIS margin from the continental shelf elsewhere in northwestern and western Ireland and the Irish Sea Basin associated with the start of the Cooley Point Interstadial (≥20–≤18.2 cal. kyr BP), suggesting widespread deglaciation of the IIS ∼19.5–20 kyr ago.  相似文献   

20.
New 40Ar/39Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar 40Ar/39Ar ages. Subsequent unroofing at rates of 5–25 km Myr−1 brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by 40Ar/39Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr−1. Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr−1, suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr−1, suggesting relatively shallow crustal depths.  相似文献   

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