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1.
Li isotopic compositions of magmatic rocks have gained considerable attention recently as probes of mantle-scale processes. However, the concentrations and isotopic composition of Li in mantle minerals from mid-ocean ridges remain relatively unconstrained. This is largely because of the general presence of seawater alteration in abyssal peridotites. Lithium elemental and isotopic compositions for mineral separates of coexisting olivine, clinopyroxene, orthopyroxene and bulk rocks of serpentine-free Gakkel Ridge peridotites were investigated. Bulk rocks have Li contents of 1.6 to 2.7 ppm and δ7Li values of 3 to 5‰, which fall within the range of reported normal pristine “MORB mantle” values. Lithium concentrations vary in the order cpx (2.1–4.7 ppm) > opx (0.9–1.7 ppm) ≥ olivine (0.4–0.9 ppm), the opposite found in “equilibrated” mantle peridotite xenoliths (Seitz and Woodland, 2000). The Li isotopic compositions indicate a systematic mineral variation with δ7Liolivine (7.14‰–15.09‰) > δ7Liopx (1.81‰–3.66‰) > δ7Licpx (?2.43‰ ? ?0.39‰). The δ7Li values of cpx are negatively correlated with their Li concentrations with the lightest value for the most enriched cpx grains. There is a first order negative linear correlation between Δolivine–cpx (δ7Liolivine ? δ7Licpx) and ol/cpxD (Liolivine/Licpx).Numerical simulations indicate that the observed systematic inter-mineral variations of Li concentrations and isotopic compositions could be explained by a cooling driven diffusive redistribution between minerals in a closed system if there is a temperature dependent partitioning of Li between olivine and clinopyroxene. The studied Gakkel Ridge abyssal peridotites may alternatively have cooled under a variable cooling rate with a rapid cooling before the Li system was closed, which is less likely given the tectonic setting. Our calculations confirm that Li systematics in minerals, especially in coexisting mineral phases could potentially be used as a mantle geospeedometer, even for slowly cooled mantle rocks.  相似文献   

2.
Carbonatites are mantle-derived, intraplate magmas that provide a means of documenting isotopic variations of the Earth's mantle through time. To investigate the secular Li isotopic evolution of the mantle and to test whether Li isotopes document systematic recycling of material processed at or near the Earth's surface into the mantle, we analyzed the Li isotopic compositions of carbonatites and spatially associated mafic silicate rocks. The Li isotopic compositions of Archean (2.7 Ga) to Recent carbonatites (δ7Li = 4.1 ± 1.3 (n = 23, 1σ)) overlap the range typical for modern mantle-derived rocks, and do not change with time, despite ongoing crustal recycling. Thus, the average Li isotopic composition of recycled crustal components has not deviated greatly from the mantle value (~ + 4) and/or Li diffusion is sufficiently fast to attenuate significant heterogeneities over timescales of 108 years. Modeling of Li diffusion at mantle temperatures suggests that limited δ7Li variation in the mantle through time reflects the more effective homogenization of Li in the mantle compared to radiogenic isotope systems. The real (but limited) variations in δ7Li that exist in modern mantle-derived magmas as well as carbonatites studied here may reflect isotopic fractionation associated with shallow-level processes, such as crustal assimilation and diffusive isotopic fractionation in magmatic systems, with some of the scatter possibly related to low-temperature alteration.  相似文献   

3.
The South Sandwich volcanic arc is sited on a young oceanic crust, erupts low-K tholeiitic rocks, is characterized by unexotic pelagic and volcanogenic sediments on the down-going slab, and simple tectonic setting, and is ideal for assessing element transport through subduction zones. As a means of quantifying processes attending transfer of subduction-related fluids from the slab to the mantle wedge, boron concentrations and isotopic compositions were determined for representative lavas from along the arc. The samples show variable fluid-mobile/fluid-immobile element ratios and high enrichments of B/Nb (2.7 to 55) and B/Zr (0.12 to 0.57), similar to those observed in western Pacific arcs. δ11B values are among the highest so far reported for mantle-derived lavas; these are highest in the central part of the arc (+ 15 to + 18‰) and decrease toward the southern and northern ends (+ 12 to + 14‰). δ11B is roughly positively correlated with B concentrations and with 87Sr/86Sr ratios, but poorly coupled with other fluid-mobile elements such as Rb, Ba, Sr and U. Peridotites dredged from the forearc trench also have high δ11B (ca. + 10‰) and elevated B contents (38–140 ppm). Incoming pelagic sediments sampled at ODP Site 701 display a wide range in δ11B (+ 5 to ? 13‰; average = ? 4.1‰), with negative values most common. The unusually high δ11B values inferred for the South Sandwich mantle wedge cannot easily be attributed to direct incorporation of subducting slab materials or fluids derived directly therefrom. Rather, the heavy B isotopic signature of the magma sources is more plausibly explained by ingress of fluids derived from subduction erosion of altered frontal arc mantle wedge materials similar to those in the Marianas forearc. We propose that multi-stage recycling of high-δ11B and high-B serpentinite (possibly embellished by arc crust and volcaniclastic sediments) can produce extremely 11B-rich fluids at slab depths beneath the volcanic arc. Infiltration of such fluids into the mantle wedge likely accounts for the unusual magma sources inferred for this arc.  相似文献   

4.
Mylonites display petrographical and geochemical characteristics that can be related to syn-deformation fluid circulation. The South Armorican Shear Zone, a major structural feature of the Armorican Massif (France), is outlined by the presence of mylonitic rocks cropping out mostly in open quarries. These mylonites were essentially formed at the expense of peraluminous granitic bodies. Deformation occurred from ductile conditions in the biotite stability field (>400 °C) down to lower greenschist cataclasis and brecciation, where carbonation developed. U–Pb analyses on zircon and monazite define a minimum duration of 15 Ma for the deformation and hydrothermal history, between 315 Ma and 300 Ma. Fluid circulations are well documented, by way of petrographic observation (chlorite and carbonate crystallization), mineralogical composition analysis (muscovite chemistry), erratic mobility behavior of some elements (As, Sn, U for instance), and stable isotope composition analysis of the infiltrated rocks. High temperature deformation is not accompanied by alteration of the O isotope system, which implies either low fluid/rock ratio and/or the involvement of δ18O crustal fluids with a composition similar to that of the rocks. On the other hand, some low temperature mylonites show a drastic decrease in the δ18O values, which has to be related to the influx of surface derived waters. The heat source necessary for this crustal scale downward infiltration of fluids followed by upward motion was likely provided by the exhumation of lower crustal units in the South Armorican domain.  相似文献   

5.
The stable oxygen and hydrogen isotopic features of precipitation in Taiwan, an island located at the western Pacific monsoon area, are presented from nearly 3,500 samples collected during the past decade for 20 stations. Results demonstrate that moisture sources from diverse air masses with different isotopic signals are the main parameter in controlling the precipitation's isotope characteristics. The air mass from polar continental (Pc) region contributes the precipitation with high deuterium excess values (up to 23‰) and relatively enriched isotope compositions (e.g., ? 3.2‰ for δ18O) during the winter with prevailing northeasterly monsoon. By contrast, air masses from equatorial maritime (Em) and tropical maritime (Tm) supply the precipitation with low deuterium excess values (as low as about 7‰) and more depleted isotope values (e.g., ? 8.9‰ and ? 6.0‰ for δ18O of Tm and Em, respectively) during the summer with prevailing southwesterly monsoon. Thus seasonal differences in terms of δ18O, δD, and deuterium excess values are primarily influenced by the interactions among various precipitation sources. While these various air masses travel through Taiwan, secondary evaporation effects further modify the isotope characteristics of the inland precipitation, such as raindrop evaporation (reduces the deuterium excess of winter precipitation) and moisture recycling (increases the deuterium excess of summer precipitation). The semi-quantitative estimations in terms of evaluation for changes in the deuterium excess suggest that the raindrop evaporation fractions for winter precipitation range 7% to 15% and the proportions of recycling moisture in summer precipitation are less than 5%. Additionally, the isotopic altitude gradient in terms of δ18O for summer precipitation is ? 0.22‰/100 m, greater than ? 0.17‰/100 m of winter precipitation. The greater isotopic gradient in summer can be attributed to a higher temperature vs. altitude gradient relative to winter. The observed spatial and seasonal stable isotopic characteristics in Taiwan's precipitation not only contribute valuable information for regional monsoon research crossing the continent–ocean interface of East Asia, but also can serve as very useful database for local water resources management.  相似文献   

6.
In-situ cosmogenic 3He exposure ages of pyroxene phenocrysts from basalts from the Upper Neostromboli formation in southwest Stromboli date its eruption at 7.0 ± 0.3 ka (1σ, n = 3, Ginostra site) and 6.8 ± 0.2 ka (1σ, n = 10, Timpone del Fuoco site) respectively. Correlation of our new data to previous K/Ar and palaeomagnetic ages from the northwestern Neostromboli phase suggests that it erupted within a confined period between roughly 6 and 14 ka. The low uncertainty on the 3Hecos ages as well as on individual exposure ages (4.4–8.7%) demonstrates that 3Hecos exposure dating is a viable tool for dating Holocene basalt lavas. The ages compare favourably to uncertainties obtained for radiocarbon dating of similar rocks.  相似文献   

7.
High resolution OSL dating back to MIS 5e in the central Sea of Okhotsk   总被引：1，自引：0，他引：1  
Marine sediments contain important archives of past ocean and climate changes, but at high latitudes the absence of carbonate has prevented the construction of accurate chronological models. We have begun a study to (1) determine the accuracy of luminescence ages in deep-sea marine sediments, e.g. by comparison with marine oxygen isotope stratigraphy where possible, (2) describe changes in sedimentation rate through time, and (3) test whether it is possible to date back to marine isotope stage 5e (MIS 5e). We show here that optical dating of fine grains of quartz from the central Sea of Okhotsk is able to provide an accurate and precise chronology for the reconstruction of the palaeoceanic and palaeoclimatic environment at our site. The upper 6.5 m of the 18.42 m long core MR0604-PC07A is believed, based on its magnetic susceptibility and the oxygen isotope (δ18O) records to contain the last ～150 ka. Forty OSL samples were taken from this upper part of the core. The single-aliquot regenerative-dose (SAR) procedure is used for equivalent dose (De) determination. The luminescence characteristics of fine-grained quartz (4–11 μm) extracted from the core are described. The OSL signal is dominated by the fast component and a dose recovery test shows that we can accurately measure a known dose given in the laboratory prior to any heat treatment. Dose rates were determined using high-resolution gamma spectrometry, and vary between 0.4 and 1.6 Gy/ka. The OSL ages from this section lie between ～140 ka and ～15 ka and are in very good agreement with the δ18O stratigraphy up to MIS 5e. A clear change in sedimentation rate is identified: between ～139 and 110 ka, the sedimentation rate was ～0.09 m/ka, but then from ～110 to 15 ka, the sedimentation rate decreases to a constant value of ～0.04 m/ka. Our data confirm that OSL dating using widely distributed fine-grain quartz has great potential for dating deep-sea sediments. Because luminescence methods use clastic materials, they do not depend on the presence of biogenic carbonate. As a result it is now likely that we can establish a chronology in regions of the ocean that were previously undatable.  相似文献   

8.
Sampling and analyses methods for determining the stable isotopic compositions of Hg in an active volcanic system were tested and optimized at the volcanic complex of Vulcano (Aeolian Islands, Italy). Condensed gaseous fumarole Hg(fum)T, plume gaseous elemental Hg(g)0 and plume particulate Hg(p)II were obtained at fumaroles F0, F5, F11, and FA. The average total Hg emissions, based on HgT/SO2 in condensed fumarolic gases and plumes, range from 2.5 to 10.1 kg y? 1, in agreement with published values [Ferrara, R., Mazzolai, B., Lanzillotta, E., Nucaro, E., Pirrone, N., 2000. Volcanoes as emission sources of atmospheric mercury in the Mediterranean Basin. Sci. Total Environ. 259(1–3), 115–121; Aiuppa, A., Bagnato, E., Witt, M.L.I., Mather, T.A., Parello, F., Pyle, D.M., Martin, R.S., 2007. Real-time simultaneous detection of volcanic Hg and SO2 at La Fossa Crater, Vulcano (Aeolian Islands, Sicily). Geophys. Res. Lett. 34(L21307).]. Plume Hg(p)II increases with distance from the fumarole vent, at the expense of Hg(g)0 and indicates significant in-plume oxidation and condensation of fumarole Hg(fum)T.Relative to the NIST SRM 3133 Hg standard, the stable isotopic compositions of Hg are δ202Hg(fum)T = ? 0.74‰ ± 0.18 (2SD, n = 4) for condensed gaseous fumarole Hg(fum)T, δ202Hg(g)0 = ? 1.74‰ ± 0.36 (2SD, n = 1) for plume gaseous elemental Hg(g)0 at the F0 fumarole, and δ202Hg(p)II = ? 0.11‰ ± 0.18 (2SD, n = 4) for plume particulate Hg(p)II. The enrichment of Hg(p)II in the heavy isotopes and Hg(g)0 in the light isotopes relative to the total condensed fumarolic Hg(fum)T gas complements the speciation data and demonstrates a gas-particle fractionation occurring after the gas expulsion in ambient T° atmosphere. A first order Rayleigh equilibrium condensation isotope fractionation model yields a fractionation factor αcond-gas of 1.00135 ± 0.00058.  相似文献   

9.
We have evaluated all parameters for the calculation of cosmogenic 36Cl production rates and thus surface exposure ages in dolomite and limestone. We found that we can use either of both published negative muon stopping rates until more information is available. The largest uncertainty of the age estimation in the upper meter of rock comes from the 36Cl production rate from Ca spallation and, in the case of 50–100 ppm Cl content, from the production rate of epithermal neutrons, which we estimate at 760 ± 150 neutrons/g_air/yr (1σ). For a sample with representative amounts of Ca and Cl (20 wt% Ca and 50 ppm Cl, or 40 wt% Ca and 100 ppm Cl), the age can be calculated with a precision of 7–10% in the top 1.5 m of the depth profile. Further improvement of 36Cl calculations depends on new calibration of 36Cl production from Ca spallation, re-evaluation of 36Cl production by low-energy neutron capture on 35Cl, as well as of the muon flux and muon capture based on the most recent measurement data.  相似文献   

10.
The production rate of cosmogenic 3He in apatite, zircon, kyanite and garnet was obtained by cross-calibration against 10Be in co-existing quartz in glacial moraine boulders from the Nepalese Himalaya. The boulders have 10Be ages between 6 and 16 kyr and span elevations from 3200 to 4800 m. In all of these minerals 3He correlates with 10Be and is dominantly cosmogenic in origin. After modest correction for non-cosmogenic components, 3He/10Be systematics imply apparent sea-level high-latitude (SLHL) apparent production rates for 3He of 226 atoms g? 1 yr? 1 in zircon, 254 atoms g? 1 yr? 1 in apatite, 177 atoms g? 1 yr? 1 in kyanite, and 153 atoms g? 1 yr? 1 in garnet. These production rates are unexpectedly high compared with rates measured elsewhere in the world, and also compared with proposed element-specific production rates. For apatite and zircon, the data are sufficient to conclude that the 3He/10Be ratio increases with elevation. If this reflects different altitudinal scaling between production rates for the two isotopes then the SLHL production rates estimated by our approach are overestimates. We consider several hypotheses to explain these observations, including production of 3He via thermal neutron capture on 6Li, altitudinal variations in the energy spectrum of cosmic-ray neutrons, and the effects of snow cover. Because all of these effects are small, we conclude that the altitudinal variations in production rates of cosmogenic 3He and 10Be are distinct from each other at least at this location over the last ～ 10 kyr. This conclusion calls into question commonly adopted geographic scaling laws for at least some cosmogenic nuclides. If confirmed, this distinction may provide a mechanism by which to obtain paleoelevation estimates.  相似文献   

11.
To date, studies of the stability of subsurface ice in the McMurdo Dry Valleys of Antarctica have been mainly based on climate-based vapor diffusion models. In University Valley (1800 m), a small glacier is found at the base of the head of the valley, and adjacent to the glacier, a buried body of massive ice was uncovered beneath 20–40 cm of loose cryotic sediments and sandstone boulders. This study assesses the origin and stability of the buried body of massive ice by measuring the geochemistry and stable O–H isotope composition of the ice and applies a sublimation and molecular diffusion model that accounts for the observed trends. The results indicate that the buried massive ice body represents an extension of the adjacent glacier that was buried by a rock avalanche during a cold climate period. The contrasting δ18O profiles and regression slope values between the uppermost 6 cm of the buried massive ice (upward convex δ18O profile and SD-18O = 5.1) and that below it (progressive increase in δ18O and SD-18O = 6.4) suggest independent post-depositional processes affected the isotope composition of the ice. The upward convex δ18O profile in the uppermost 6 cm is consistent with the ice undergoing sublimation. Using a sublimation and molecular diffusion model, and assuming that diffusion occurred through solid ice, the sublimation rate needed to fit the measured δ18O profile is 0.2 ? 10? 3 mm yr? 1, a value that is more similar to net ice removal rates derived from 3He data from cobbles in Beacon Valley till (7.0 ? 10? 3 mm yr? 1) than sublimation rates computed based on current climate (0.1–0.2 mm yr?1). We suggest that the climate-based sublimation rates are offset due to potential ice recharge mechanisms or to missing parameters, particularly the nature and thermo-physical properties of the overlying sediments (i.e., temperature, humidity, pore structure and ice content, grain size).  相似文献   

12.
《Marine pollution bulletin》2009,58(6-12):889-894
As basic research for the effect of heavy oil on the fish immune system, in this study, the number of leukocyte was counted in Japanese flounder Paralichthys olivaceus, after exposure to heavy oil at a concentration of 30 g/8 L for 3 days. To compare the numbers of bacteria in the skin mucus between oil-exposed and control fish, viable bacteria were enumerated by counting colony forming unit (CFU). Compared with 5.79 ± 1.88 × 107 leukocytes/mL in the controls, the exposed fish demonstrated higher counts, averaging 1.45 ± 0.45 × 108 cells/mL. The bacterial numbers of control fish were 4.27 ± 3.68 × 104 CFU/g, whereas they were 4.58 ± 1.63 × 105 CFU/g in the exposed fish. The results suggest that immune suppression of the fish occurred due to heavy oil stressor, and bacteria could invade in the mucus, resulting in the increasing leukocyte number to prevent infectious disease.  相似文献   

13.
《Continental Shelf Research》2006,26(17-18):2178-2204
Continental-shelf lithofacies are described from a series of cores collected in the northern Gulf of Alaska, a high-energy paraglacial shelf experiencing rapid rates of sediment accumulation. Short-lived tracers (234Th and chlorophyll-a) indicate that during the annual peak in fluvial sediment input (summer), biologic sediment mixing coefficients in the surficial seabed are generally lower than other coastal environments (<20 cm2 yr−1) and mixing extends downward <10 cm.210Pb geochronology indicates that sediment accumulation rates (time scales of 10–100 yr) are 0.1–3 cm yr−1. The measured bioturbation and accumulation rates lead to predictions of moderate to bioturbated lithofacies, as observed. Primary depositional fabric is preferentially preserved where sediment accumulation rates >2 cm yr−1 and non-steady sediment deposition occurs. Depositional fabric is also observed in strata at 50–100 m water depths and is similar in appearance to beds that may form through deposition of wave-induced fluid-mud flows, which have been observed forming on other shelves with moderate to high wave energy. Five general lithofacies can be identified for the study area: inner-shelf sand facies, interbedded sandy mud facies, moderate-to-well-bioturbated mud facies, gravelly mud facies, and Tertiary bedrock facies. The moderate-to-well-bioturbated mud facies is areally dominant, representing over 50% of the shelf area, although roughly equal volumes (∼0.4 km3) of strata with some preservation of primary fabric are annually accumulating. Lithofacies on this paraglacial shelf generally resemble mid- and low-latitude allochthonous shelf strata to a much greater degree than Holocene glacimarine strata formed on shelves dominated by icebergs and floating ice shelves. Paraglacial strata may be differentiated from non-glacial shelf strata by lower organic carbon concentrations, a relatively lower degree of bioturbation, and increased preservation of primary depositional fabric.  相似文献   

14.
With transmission electron microscopy (TEM) we observed nanometer-sized pores in four ultracataclastic and fractured core samples recovered from different depths of the main bore hole of the San Andreas Fault Observatory at Depth (SAFOD). Cutting of foils with a focused ion beam technique (FIB) allowed identifying porosity down to the nm scale. Between 40 and 50% of all pores could be identified as in-situ pores without any damage related to sample preparation. The total porosity estimated from TEM micrographs (1–5%) is comparable to the connected fault rock porosity (2.8–6.7%) estimated by pressure-induced injection of mercury. Permeability estimates for cataclastic fault rocks are 10? 21–10? 19 m2 and 10? 17 m2 for the fractured fault rock. Porosity and permeability are independent of sample depth. TEM images reveal that the porosity is intimately linked to fault rock composition and associated with deformation. The TEM-estimated porosity of the samples increases with increasing clay content. The highest porosity was estimated in the vicinity of an active fault trace. The largest pores with an equivalent radius > 200 nm occur around large quartz and feldspar grains or grain-fragments while the smallest pores (equivalent radius < 50 nm) are typically observed in the extremely fine-grained matrix (grain size < 1 μm). Based on pore morphology we distinguish different pore types varying with fault rock fabric and alteration. The pores were probably filled with formation water and/or hydrothermal fluids at elevated pore fluid pressure, preventing pore collapse. The pore geometry derived from TEM observations and BET (Brunauer, Emmett and Teller) gas adsorption/desorption hysteresis curves indicates pore blocking effects in the fine-grained matrix. Observations of isolated pores in TEM micrographs and high pore body to pore throat ratios inferred from mercury injection suggest elevated pore fluid pressure in the low permeability cataclasites, reducing shear strength of the fault.  相似文献   

15.
We investigated the provenance of organic matter in the inner fjord area of northern Patagonia, Chile (～44–47°S), by studying the elemental (organic carbon, total nitrogen), isotopic (δ13C, δ15N), and biomarker (n-alkanoic acids from vascular plant waxes) composition of surface sediments as well as local marine and terrestrial organic matter. Average end-member values of N/C, δ13C, and δ15N from organic matter were 0.127±0.010, ?19.8±0.3‰, and 9.9±0.5‰ for autochthonous (marine) sources and 0.040±0.018, ?29.3±2.1‰, and 0.2±3.0‰ for allochthonous (terrestrial) sources. Using a mixing equation based on these two end-members, we calculated the relative contribution of marine and terrestrial organic carbon from the open ocean to the heads of fjords close to river outlets. The input of marine-derived organic carbon varied widely and accounted for 13–96% (average 61%) of the organic carbon pool of surface sediments. Integrated regional calculations for the inner fjord system of northern Patagonia covered in this study, which encompasses an area of ～4280 km2, suggest that carbon accumulation may account for between 2.3 and 7.8×104 ton C yr?1. This represents a storage capacity of marine-derived carbon between 1.8 and 6.2×104 ton yr?1, which corresponds to an assimilation rate of CO2 by marine photosynthesis between 0.06 and 0.23×106 ton yr?1. This rate suggests that the entire fjord system of Patagonia, which covers an area of ～240,000 km2, may represent a potentially important region for the global burial of marine organic matter and the sequestration of atmospheric CO2.  相似文献   

16.
Sulfur compounds in volcanic gases are responsible for the global cooling after explosive eruptions and they probably controlled the early evolution of the Earth's atmosphere. We have therefore studied the oxidation state of sulfur in aqueous fluids under the pressure and temperature conditions and oxygen fugacities typical for magma chambers (0.5–3 kbar, 650–950 °C, Ni–NiO to Re–ReO2 buffer conditions). Sulfur speciation was determined by Raman spectroscopy of quenched fluids trapped as inclusions in quartz. Our results show that sulfur in hydrothermal fluids and volcanic gases is much more oxidized than previously thought and in particular, some explosive eruptions may release a significant fraction of sulfur as SO3 or its hydrated forms. In the pressure range from 500 to 2000 bar, the equilibrium constant K1 of the reaction 2H2S + 3O2 = 2SO2 + 2H2O in aqueous fluids can be described by lnK1 = ?(57.1 ± 7.1) + (173,480 ± 7592)T? 1, where T is temperature in Kelvin. The equilibrium constant K2 for the reaction SO2 + ½O2 = SO3 in aqueous fluids, where SO3 may include hydrated forms, such as H2SO4, was found to be strongly pressure dependent, with lnK2 = ?(5.2 ± 5.7) + (19,243 ± 5993)T? 1 at 1500 bar; lnK2 = ?(11.1 ± 1.3) + (25,383 ± 1371)T? 1 at 2000 bar and lnK2 = ?(22.1 ± 2.2) + (37,082 ± 2248)T? 1 at 2500 bar. Our data imply that volcanoes may directly inject hexavalent sulfur in the form of H2SO4 into the atmosphere, not only on Earth, but possibly also on Venus and on Mars, when it was still tectonically active. Remote measurements from satellites may have underestimated the sulfur yield of some recent eruptions. Moreover, the mechanisms of the interaction of volcanic gases with the stratosphere need to be reconsidered.  相似文献   

17.
The Qujialing site is a representative Neolithic archaeological site in the middle reaches of the Yangtze River, China. Absence of suitable material for radiocarbon dating in this region makes the timing of the similar sites difficult. Here we applied optically stimulated luminescence (OSL–SAR) and thermoluminescence (TL–SAR) techniques to date the archaeological and natural deposits from the Qujialing site with known age, testing the techniques on samples at archaeological sites in this region. The results showed that the luminescence properties of quartz from sediment and baked earth samples are very similar. The quartz OSL ages obtained for a sediment sample and a baked earth sample from the cultural layer are 5.4 ± 0.3 and 5.1 ± 0.3 ka, respectively. The quartz TL age of the baked earth sample is 5.6 ± 0.5 ka. These dates are consistent with the calibrated radiocarbon ages (4.9 ± 0.1 and 5.1 ± 0.1 ka cal BP (±1σ)) of the two charcoal samples from the cultural layer at a nearby locality, and are also in agreement with the age of Qujialing culture period. The results indicate that the OSL dating techniques can be applied to date similar archaeological sites in the middle reaches of the Yangtze River, China.  相似文献   

18.
We present the new 14C extraction line at ETH Zürich. This system is designed to extract in situ-produced cosmogenic 14C from terrestrial quartz samples, and to obtain pure CO2 gas for analysis with a gas ion source Accelerator Mass Spectrometry (AMS) system. Samples are degassed at 1550–1600 °C without the use of a fluxing agent. Gas purification is achieved by a series of cryogenic traps and passage through hot Ag and Cu wool/mesh. Graphitization and, thus, sample dilution is not required. Tests to determine the CO2 recovery after gas extraction and cleaning yielded consistently good recovery rates of >99.8% (n = 7). The 14C blank contribution from the all-metal tubing system is negligible. Our preliminary procedural blank estimate – deriving mostly from the hot extraction furnace – is <5 × 105 14C atoms. Extraction tests on two quartz samples by stepped-heating show a quantitative separation of atmospheric 14C at ≤500 °C from the in situ component above 1200 °C. Based on these data, we estimate to achieve a complete 14C extraction from a quartz sample.  相似文献   

19.
Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ～ 600 ka samples with an uncertainty of better than ± 15 ka (2σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [Villemant B. and Feuillet N. (2003) Dating open systems by the 238U–234U–230Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman M.W., Goldstein S.L., and Speed R.C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of 230Th and 234U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model.  相似文献   

20.
Stable cosmogenic isotopes such as 3He and 21Ne are useful for dating of diverse lithologies, quantifying erosion rates and ages of ancient surfaces and sediments, and for assessing complex burial histories. Although many minerals are potentially suitable targets for 3He and 21Ne dating, complex production systematics require calibration of each mineral–isotope pair. We present new results from a drill core in a high-elevation ignimbrite surface, which demonstrates that cosmogenic 3He and 21Ne can be readily measured in biotite and hornblende. 21Ne production rates in hornblende and biotite are similar, and are higher than that in quartz due to production from light elements such as Mg and Al. We measure 21Nehbl/21Neqtz = 1.35 ± 0.03 and 21Nebio/21Neqtz = 1.3 ± 0.02, which yield production rates of 25.6 ± 3.0 and 24.7 ± 2.9 at g? 1 yr? 1 relative to a 21Neqtz production rate of 19.0 ± 1.8 at g? 1 yr? 1. We show that nucleogenic 21Ne concentrations produced via the reaction 18O(α,n)21Ne are manageably small in this setting, and we present a new approach to deconvolve nucleogenic 21Ne by comparison to nucleogenic 22Ne produced from the reaction 19F(α,n)22Ne in F-rich phases such as biotite. Our results show that hornblende is a suitable target phase for cosmogenic 3He dating, but that 3He is lost from biotite at Earth surface temperatures. Comparison of 3He concentrations in hornblende with previously measured mineral phases such as apatite and zircon provides unambiguous evidence for 3He production via the reaction 6Li(n,α)3H → 3He. Due to the atypically high Li content in the hornblende (~ 160 ppm) we estimate that Li-produced 3He represents ~ 40% of total 3He production in our samples, and must be considered on a sample-specific basis if 3He dating in hornblende is to be widely implemented.  相似文献   

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