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 31 毫秒
1.
High-sensitivity stepped extraction reveals two isotopically distinct forms of carbon in submarine basalt glasses: an isotopically light carbon component released by combustion from 200 to 600°C and an isotopically heavy CO2 liberated from vesicles (magmatic carbon) from 600 to 1200°C. The δ13CPDB of the low release temperature carbon varies from −24 to −30‰ and is believed to be surficial organic contamination. A survey of various types of oceanic glasses demonstrates that the δ13C of magmatic CO2 varies from −4.2 to −7.5‰ in mid-ocean ridge basalt (MORB), from −2.8 to −6.7‰ in glasses from Hawaii and Explorer Seamount and from −7.7 to −16.3‰ in glasses from the Scotia Sea and Mariana Trough. Magmatic CO2 in back-arc basin basalts (BABB) is on average 5‰ lighter than equivalent CO2 in MORB and can be explained by the mixing in the source regions for BABB magmas of juvenile (MORB-like) CO2 with an organic carbon component from subducted pelagic sediments. It is inferred that significant amounts of pelagic carbonate carbon (δ13C 0‰) must be recycled into the mantle.  相似文献   

2.
A series of Co-60 compounds, considered as potentially useful tracers of groundwater movement, were prepared and evaluated in soil column and batch studies. The compounds were: potassium hexacyanocobaltate (III), K3Co(CN)6; potassium ethylenediamminetetraacetato-cobaltate (III), KCo(EDTA); ammonium 12-tungstodicobaltoate (III), (NH4)8[Co(II)Co(II)W12O42] · 20 H2O; chloraquotetrammine cobalt (III) chloride, [Co(NH3)4(H2O)Cl]Cl2; and sodium hexanitrocobaltate (III), Na3Co(NO2)6. The performance of K3 60Co(CN)6 and K 60Co EDTA were dependent on their environment and more especially on the type and amount of clay present. The EDTA cobalt complex cannot be used in soils that are basic or where a large fraction of the clay is saturated with Na+ and/or Ca++. Of the other radioactive tracers tested, none were suitable for groundwater tracing because of instability in aqueous solution (Na3Co(NO2)6), variable anionic structure under acid conditions ((NH3)8[Co(II)Co(II)W12O42] · 20 H2O), or preferential uptake by the clay fraction ([Co(NH3)4(H2O)Cl]Cl2).  相似文献   

3.
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   

4.
The solubility of CO2CO fluids in a mid-ocean ridge basalt (morb) has been measured at 1200°C, 500–1500 bar, and oxygen fugacities between NNO and NNO-4. High oxygen fugacities, and thus CO2-rich fluids, were produced by using a starting material equilibrated at NNO, and Ag2C2O4 as the fluid source. Low oxygen fugacities were achieved by using graphite capsules, and MgCO3 as the fluid source. These graphite-saturated fluids have the lowest possibleC/O2CO ratio for a given pressure and temperature.

Experiments were run in a rapid-quench internally heated pressure vessel. Fluid compositions were measured using a simple vacuum technique and by Raman spectroscopy of fluid inclusions. The two techniques yielded comparable results. Fourier transform micro-infrared spectroscopy was used to identify and measure concentrations of dissolved volatiles in double-polished wafers of the quenched glasses. Carbonate was the only carbon-bearing species identified. Raman spectroscopic analysis of inclusion-free areas of glass confirmed the absence of dissolved molecular CO2, CO and carbon. The measured concentrations of dissolved CO2 in the glasses were proportional to the fugacity of CO2 during the experiments, calculated from the measured fluid compositions. The data were fit to the equationXCO2melt(ppm)= 0.492 fCO2 (bar).

The insolubility of CO, compared to CO2, may be related to the fact that dissolution of CO requires reduction of another species in the melt, whereas dissolution of CO2 does not. Due to the fact that CO will be an important component of natural CO fluids at low pressures and low oxygen fugacities, equilibrium dissolved CO2 contents will be less than calculated assuming pure CO2 fluids, but as theC/O2CO ratio in a pure CO fluid at fixed pressure and temperature is a direct function of oxygen fugacity, measurement of the oxygen fugacity of quenched glasses or trapped fluids in natural samples should allow saturation concentrations to be calculated. Dissolved CO2 contents of somemorb are less than expected if they were in equilibrium with pure CO2. These samples must, therefore, have been more reduced than average if they were fluid-saturated. Together with results from other studies of CO2 and H2O solubilities in basalt, the results of this study provide a comprehensive framework for modelling CO2 solution inmorb.  相似文献   


5.
利用北半球40°N～50°N纬度带上HALOE实验测量的O3和H2O廓线资料,根据示踪成分O3和H2O空间分布的化学寿命以及输运特征时间常数等性质,在等熵坐标中构建了对流层顶附近及最低平流层300～390 K等熵面间,O3/H2O混合关系的结构形态和季节特征.结果表明: (1) 在对流层顶转换层的320～380 K等熵面间O3混合比廓线的斜率具有空间转折"突变",而H2O混合比廓线的斜率则出现空间渐变转折.在对流层顶附近O3和H2O的源分别是平流层与对流层,使O3混合比和H2O混合比在320～380 K等熵面的两侧显现出截然不同的垂直分布梯度.(2) 在对流层顶附近O3/H2O达到最小二乘意义上的最佳拟合时,两者参考关系的对流层支与平流层支呈现出非规则"L"结构形态的季节与季节内变化,其中对流层支的斜率为负,而平流层支的斜率可随季节出现正负变化.同时,由"L"形态的转角处可确定随季节变化的化学对流层顶(chemopause)特征.(3) 由O3/H2O混合关系反映出对流层不同区域空气携带的物质成分分别与平流层空气混合而形成混合层,而且可使混合层的混合线不恒定.混合层的表现在2003年、2005年1月和2003年4月的混合程度相当,混合的等熵厚度大约是30 K,即在320～350 K等熵面间.2005年11月的混合高度有所增高,进入平流层的H2O混合比要比2003年和2005年1月的小,混合的等熵厚度大约为30 K,在330～360 K等熵面间.不同季节混合的等熵厚度变化较小,但高度可随季节而变化.O3/H2O混合关系的平流层支随季节的变化很明显,1月最低平流层空气脱水是引起平流层支季节变化的重要原因.  相似文献   

6.
新保安—沙城断裂带土壤气地球化学特征分析   总被引：3，自引：0，他引：3  
测量断裂带土壤气浓度变化, 是监测断裂活动性和地震危险性的重要途径之一。 2014年5月, 我们在怀来地区新保安-沙城断裂测量了断层土壤气H2、 He、 CO2、 Rn、 Hg浓度以及CO2、 Rn、 Hg的通量。 测量结果显示: H2、 He、 CO2、 Rn、 Hg浓度变化范围分别为, H2: (0.4～34.2)×10-6、 He: (2.8～7)×10-6、 CO2: (0.051～1.19)%、 Rn: (0.31～18.22) kBq/m3、 Hg: (2～54) ng/m3。 土壤气H2和Rn是揭示断层位置的有效参数之一, 在断层附近以及有陡坎的地区有明显高值异常。 通过对怀来地区新保安-沙城断裂土壤气浓度及通量地球化学研究初步确定断裂带有较高的脱气强度, 与此区域地震活动性有一定的关系。  相似文献   

7.
Chemical and isotopic ratio (He, C, H and O) analysis of hydrothermal manifestations on Pantelleria island, the southernmost active volcano in Italy, provides us with the first data upon mantle degassing through the Sicily Channel rift zone, south of the African–European collision plate boundary. We find that Pantelleria fluids contain a CO2–He-rich gas component of mantle magmatic derivation which, at shallow depth, variably interacts with a main thermal (100°C) aquifer of mixed marine–meteoric water. The measured 3He/4He ratios and δ13C of both the free gases (4.5–7.3 Ra and −5.8 to −4.2‰, respectively) and dissolved helium and carbon in waters (1.0–6.3 Ra and −7.1 to −0.9‰), together with their covariation with the He/CO2 ratio, constrain a 3He/4He ratio of 7.3±0.1 Ra and a δ13C of ca. −4‰ for the magmatic end-member. These latter are best preserved in fluids emanating inside the active caldera of Pantelleria, in agreement with a higher heat flow across this structure and other indications of an underlying crustal magma reservoir. Outside the caldera, the magmatic component is more affected by air dilution and, at a few sites, by mixing with either organic carbon and/or radiogenic 4He leached from the U–Th-rich trachytic host rocks of the aquifer. Pantelleria magmatic end-member is richer in 3He and has a lower (closer to MORB) δ13C than all fluids yet analyzed in volcanic regions of Italy and southern Europe, including Mt. Etna in Sicily (6.9±0.2 Ra, δ13C=−3±1‰). This observation is consistent with a south to north increasing imprint of subducted crustal material in the products of Italian volcanoes, whose He and C (but also O and Sr) isotopic ratios gradually evolve towards crustal values northward of the African–Eurasian plate collision boundary. Our results for Pantelleria extend this regional isotopic pattern further south and suggest the presence of a slightly most pristine or ‘less contaminated’, 3He-richer mantle source beneath the Sicily Channel rift zone. The lower than MORB 3He/4He ratio but higher than MORB CO2/3He ratio of Pantelleria volatile end-member are compatible with petro-geochemical evidence that this mantle source includes an upwelling HIMU–EM1-type asthenospheric plume component whose origin, according to recent seismic data, may be in the lower mantle.  相似文献   

8.
怀安盆地北缘断裂东段土壤气体地球化学特征   总被引：5，自引：0，他引：5  
活动断裂带的土壤气体地球化学测量在活动断裂危险性评价方面起着重要作用。 本文讨论了怀安盆地北缘断裂东段的土壤气体地球化学特征及其构造意义。 我们于2012年7月和2013年6月在怀安盆地北缘断裂东段的羊窖沟以及张家窑两地现场测量了土壤气体组分的浓度和通量。 测量结果表明: ① 断裂带土壤气H2、 Rn、 CO2和Hg浓度空间分布曲线呈现双峰模式, 且断裂下盘土壤气的Rn、 CO2和Hg浓度较上盘的高; H2、 Rn、 CO2和Hg浓度平均值分别是4.61 ppm、 4.81 k Bqm-3、 0.38%和9 ngm-3; 土壤气Rn和CO2的通量平均值分别是6718 Bqm-2d-1和49.83 gm-2d-1; ② 土壤气Rn和CO2在两个测区脱气强烈, 羊窖沟地区土壤气CO2、 Rn和Hg浓度平均值以及土壤气Rn和CO2通量值较张家窑地区高。 这些结果可能是两个测区断裂活动性差异引起的。  相似文献   

9.
Benthic foraminiferal magnesium/calcium ratios were determined on one hundred and forty core-top samples from the Atlantic Ocean, the Norwegian Sea, the Indian Ocean, the Arabian Sea and the Pacific Ocean, mostly at sites with bottom water temperatures below 5 °C. Mg/Ca ratios are consistently lower, by  0.2 mmol/mol, in samples cleaned using oxidative and reductive steps than using oxidative cleaning. Differences between Cibicidoides species have been identified: Mg/Ca of Cibicidoides robertsonianus > Cibicidoides kullenbergi > Cibicidoides wuellerstorfi. Comparison with bottom water temperatures support observations of lowered Mg/Ca of C. wuellerstorfi at temperature below  3 °C compared with values predicted by published calibrations and from other Cibicidoides species. Hydrographic data shows that carbonate ion saturation (Δ[CO32−]) decreases rapidly below this temperature. An empirical sensitivity of Δ[CO32−] on Mg/Ca has been established for C. wuellerstorfi of 0.0086 ± 0.0006 mmol/mol/μmol/kg. A novel application using modern temperatures and Last Glacial Maximum temperatures derived via pore fluid modelling supports a carbonate ion saturation state effect on Mg incorporation. This may significantly affect calculated δ18Oseawater obtained from foraminiferal δ18O and Mg/Ca temperature.  相似文献   

10.
The oxygen fugacity (f(O2)) values recorded by diamondiferous peridotite and eclogite xenoliths from Siberia indicate that the redox state of the ancient lithosphere is heterogeneous on a scale of at least four log units, mainly in the range between the wüstite-magnetite (WM) and iron-wüstite (IW) oxygen buffers. Highly reduced peridotites can be interpreted as relict from earlier lower f(O2). The f(O2) values recorded by ‘fertile’ and less modified spinel peridotites from Mongolia, Baikal and Tien-Shan show that the redox state of the lithosphere beneath central Asia and Tien-Shan is heterogeneous on a scale of 2–3 log units, mainly in the range between the WM and IW + 1 oxygen buffers. These data provide evidence for the presence of a lower-f(O2) regime of carbon-bearing mantle beneath the Baikal rift zone and Tien-Shan, and the oxidation of diapirs ascending from the asthenosphere. The ‘dry’ xenoliths from Mongolia primarily reflect closed system behavior in the upper mantle, the f(O2) of which is buffered by ferric-ferrous redox equilibrium. The observed evolution of f(O2) values is closely linked to the distribution of volatile species in the mantle. H2O and CO2 are the dominant volatiles for the more depleted and oxidized part of peridotites, and CH4 for the more reduced and less modified part. It is proposed that the upper mantle was originally more reduced and has become progressively more oxidized, resulting perhaps largely from the preferential loss of hydrogen and carbon during melting. The oxygen budget of the upper mantle results from the opposing contributions of crustal recycling and transfer of carbon-bearing material from the deep mantle.  相似文献   

11.
Four species of marine calcifying algae, the coccolithophores Calcidiscus leptoporus, Helicosphaera carteri, Syracosphaera pulchra and Umbilicosphaera foliosa were grown in laboratory cultures under temperatures varying between 14 and 23 °C, and one species, C. leptoporus, under varying [CO32−], ranging from 105 to 219 μmol/kg. Calcium isotope compositions of the coccoliths resemble in both absolute fractionation and temperature sensitivity previous calibrations of marine calcifying species e.g. Emiliania huxleyi (coccolithophores) and Orbulina universa (planktonic foraminifera) as well as inorganically precipitated CaCO3, but also reveal small species specific differences. In contrast to inorganically precipitated calcite, but similar to E. huxleyi and O. universa, the carbonate ion concentration of the medium has no statistically significant influence on the Ca isotope fractionation of C. leptoporus coccoliths; however, combined data of E. huxleyi and C. leptoporus indicate that the observed trends might be related to changes of the calcite saturation state of the medium. Since coccoliths constitute a significant portion of the global oceanic CaCO3 export production, the Ca isotope fractionation in these biogenic structures is important for defining the isotopic composition of the Ca sink of the ocean, one of the key parameters for modelling changes to the marine Ca budget over time. For the present ocean our results are in general agreement with the previously postulated and applied mean value of the oceanic Ca sink (Δsed) of about − 1.3‰, but the observed inter- and intra-species differences point to possible changes in Δsed through earth history, due to changing physico-chemical conditions of the ocean and shifts in floral and faunal assemblages.  相似文献   

12.
In 1995–1998, Han 11 km terrestrial surge of Kuannersuit Glacier, an outlet glacier of the largest ice cap on Disko Island, West Greenland, affected the catchment dramatically. In order to estimate solute fluxes and provenances, bulk meltwaters were sampled at the main subglacial outlet during the initial part of the quiescent phase. The hydrochemistry is significantly influenced by a subglacial basaltic weathering regime with absence of carbonate minerals. The results show that marine and aerosol derived solutes have minimal contribution to the total ion content, whereas sequestration of atmospheric CO2 associated with carbonation of Ca-rich feldspar and reactive volcanic glass is more dominant than previously reported from glacierized catchments. Application of a sampling strategy dividing water samples into four groups to determine the content of dissolved HCO3− and CO32− shows that the cationic equivalent weathering rate range is 683–860 Σmeq+ m−2 a−1 and solute flux ranges between 76 and 98 t km−2 a−1. The crustal denudation rate is estimated to 26 t km−2 a−1, and the transient CO2 drawdown amounts to 8500–13700 kg C km−2 a−1.  相似文献   

13.
  相似文献   

14.
Magma degassing at Soufrière Hills Volcano (SHV) is characterised by an almost permanent SO2 flux and a HCl production rate which mainly depends on dome growth rate. Degassing processes have been studied through textural, H2O and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits. Cl, Br and I are strongly depleted in melts during H2O degassing with no significant Cl–Br–I fractionation, whereas F is almost unaffected. All magmas erupted at SHV have followed a multi-step degassing path from the magma chamber up to a shallow depth ( 1 km, P  20 MPa). From that depth, however, effusive and explosive paths are distinct; vulcanian eruptions are the result of closed system degassing (CSD), while effusive dome growth is the result of CSD up to a very shallow depth (≤ 200 m, P  5–2 MPa) followed by open system degassing (OSD). CSD is modelled using the H2O solubility law, the perfect gas law and partition coefficients of halogens between a rhyolitic melt and H2O vapour (dv − li). Gas loss characteristic of OSD is modelled using a Rayleigh law. Degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates, which ranges from 150–500. dv − lCl for OSD ranges between 50–300, increasing with melt Cl content. For CSD, the lower effective dv − lCl ( 20) is attributed to kinetic effects.

Dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because OSD is much more efficient at extracting halogens. The model shows that HCl flux is a good proxy for the dome forming eruption rate. Comparison between model and measured gas compositions suggests a high HBr–BrO conversion rate (BrO/Total Br  1/3) in the SHV gas plume.

The degassing behaviour of Cl, Br and I implies similar Cl/Br ( 160) and Br/I ( 90) in initial melts, volcanic clasts and high temperature gases. The low Cl/Br at SHV compared to other island arcs ( 250–300) is attributed to a shallow, pre-eruptive Br enrichment. The almost permanent dome extrusion at SHV since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume.  相似文献   


15.
The ground state of the core-mantle conductivity system is defined as a step function transition from the constant non-zero conductivity of the core to zero conductivity in the mantle. This ground state is reached by letting the thickness of a transition region Δ → 0, where Δ appears as a parameter in the model conductivity of the system. By a transformation of the equation governing the behavior of the electrostatic potential φ, it is shown that the function V(x) = (1/σ1/2)(d2σ1/2/dx2) acts as a potential barrier in the quantum mechanical sense, and that for certain conductivity profiles σ(x), where x is the usual Cartesian coordinate, the electrostatic potential is screened out in regions where σ(x) → 0 as Δ → 0. Consequently, E = −φ also vanishes in these regions. The results generalize to the time-dependent case. Conditions that the conductivity function σ(x) must satisfy to qualify as the ground-state conductivity are defined and an example is provided.  相似文献   

16.
Negative carbon-isotope excursions have been comprehensively studied in the stratigraphic record but the discussion of causal mechanisms has largely overlooked the potential role of biomass burning. The carbon-isotopic ratios (δ13C) of vegetation, soil organic matter and peat are significantly lower than atmospheric carbon dioxide (CO2), and thereby provide a source of low 13C CO2 when combusted. In this study, the potential role of biomass burning to generate negative carbon isotope excursions associated with greenhouse climates is modeled. Results indicate that major peat combustion sustained for 1000 yr increases atmospheric CO2 from 2.5× present atmospheric levels (PAL) to 4.6× PAL, and yields a pronounced negative δ13C excursion in the atmosphere ( 2.4‰), vegetation ( 2.4‰) and the surface ocean ( 1.2‰), but not for the deep ocean ( 0.9‰). Release of CO2 initiates a short-term warming of the atmosphere (up to 14.4 °C, with a duration of 1628 yr), which is consistent with the magnitude and length of an observed Toarcian excursion event. These results indicate that peat combustion is a plausible mechanism for driving negative δ13C excursions in the rock record, even during times of elevated pCO2.  相似文献   

17.
Pre-eruptive conditions and degassing processes of the AD 79 plinian eruption of Mt. Vesuvius are constrained by systematic F and Cl measurements in melt inclusions and matrix glass of pumice clasts from a complete sequence of the pumice-fallout deposits. The entire ‘white pumice’ (WP) magma and the upper part of the ‘grey pumice’ (GP) magma were saturated relative to sub-critical fluids (a Cl-rich H2O vapour phase and a brine), with a Cl melt content buffered at ~ 5300 ppm, and a mean H2O content of ~ 5%. The majority of the GP magma was not fluid-saturated. From these results it can be estimated that the WP magma chamber had a low vertical extent (< 500 m) and was located at a depth of ~ 7.5 km while the GP magma reservoir was located just beneath the WP one, but its vertical extent cannot be constrained. This is approximately two times deeper than previous estimates. H2O degassing during the WP eruption followed a typical closed-system evolution, whereas GP clasts followed a more complex degassing path. Contrary to H2O, Cl was not efficiently degassed during the plinian phase of the eruption.

This study shows that F and Cl behave as incompatible elements in fluid-undersaturated phonolitic melts. H2O saturation is necessary for a significant partitioning of Cl into the fluid phase. However, Cl cannot be extracted in significant quantity from phonolitic melts during rapid H2O degassing, e.g. during plinian eruptions, due to kinetics effects. Halogen contents are better preserved in volcanic glass (melt inclusions or matrix glass) than H2O, therefore the combined analysis of both volatile species is required for reliable determination of pre-eruptive conditions and syn-eruptive degassing processes in magmas stored at shallow depths.  相似文献   


18.
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   

19.
The stable isotopic composition of materials such as glacial ice, tree rings, lake sediments, and speleothems from low-to-mid latitudes contains information about past changes in temperature (T) and precipitation amount (P). However, the transfer functions which link δ18Op to changes in T or P, dδ18Op/dT and dδ18Op/dP, can exhibit significant temporal and spatial variability in these regions. In areas affected by the Southeast Asian monsoon, past variations in δ18O and δD of precipitation have been attributed to variations in monsoon intensity, storm tracks, and/or variations in temperature. Proper interpretation of past δ18Op variations here requires an understanding of these complicated stable isotope systematics. Since temperature and precipitation are positively correlated in China and have opposite effects on δ18Op, it is necessary to determine which of these effects is dominant for a specific region in order to perform even qualitative paleoclimate reconstructions. Here, we evaluate the value of the transfer functions in modern precipitation to more accurately interpret the paleorecord. The strength of these transfer functions in China is investigated using multiple regression analysis of data from 10 sites within the Global Network for Isotopes in Precipitation (GNIP). δ18Op is modeled as a function of both temperature and precipitation. The magnitude and signs of the transfer functions at any given site are closely related to the degree of summer monsoon influence. δ18Op values at sites with intense summer monsoon precipitation are more dependent on the amount of precipitation than on temperature, and therefore exhibit more negative values in the summer. In contrast, δ18Op values at sites that are unaffected by summer monsoon precipitation exhibit strong relationships between δ18Op and temperature. The sites that are near the northern limit of the summer monsoon exhibit dependence on both temperature and amount of precipitation. Comparison with simple linear models (δ18Op as a function of T or P) and a geographic model (δ18Op as a function of latitude and altitude) shows that the multiple regression model is more successful at reproducing δ18Op values at sites that are strongly influenced by the summer monsoon. The fact that the transfer function values are highly spatially variable and closely related to the degree of summer monsoon influence suggests that these values may also vary temporally. Since the Southeast Asian monsoon intensity is known to exhibit large variations on a number of timescales (annual to glacial–interglacial), and the magnitude and sign of the transfer functions is related to monsoon intensity, we suggest that as monsoon intensity changes, the magnitude and possibly even the sign of the transfer functions may vary. Therefore, quantitative paleoclimate reconstructions based on δ18Op variations may not be valid.  相似文献   

20.
We have developed techniques to determine238U,234U and232Th concentrations in seawater by isotope dilution mass spectrometry. U measurements are made using a233U236U double spike to correct for instrumental fractionation. Measurements on uranium standards demonstrate that234U/238U ratios can be measured accurately and reproducibly.234U/238U can be measured routinely to ± 5‰ (2σ) for a sample of 5 × 109 atoms of234U (3 × 10−8 g of total U, 10 ml of seawater). Data acquisition time is 1 hour. The small sample size, high precision and short data acquisition time are superior to-counting techniques.238U is measured to ± 2‰ (2σ) for a sample of 8 × 1012 atoms of238U ( 3 × 10−9 g of U, 1 ml of seawater).232Th is measured to ± 20‰ with 3 × 1011232Th atoms (10−10 g232Th, 1 1 of seawater). This small sample size will greatly facilitate investigation of the232Th concentration in the oceans. Using these techniques, we have measured238U,234U and232Th in vertical profiles of unfiltered, acidified seawater from the Atlantic and238U and234U in vertical profiles from the Pacific. Determinations of234U/238U at depths ranging from 0 to 4900 m in the Atlantic (7°44′N, 40°43′W) and the Pacific (14°41′N, 160°01′W) Oceans are the same within experimental error (± 5‰,2σ). The average of these234U/238U measurements is 144 ± 2‰ (2σ) higher than the equilibrium ratio of 5.472 × 10−5. U concentrations, normalized to 35‰ salinity, range from 3.162 to 3.281 ng/g, a range of 3.8%. The average concentration of the Pacific samples (31°4′N, 159°1′W) is 1% higher than that of the Atlantic (7°44′N, 40°43′W and 31°49′N, 64°6′W).232Th concentrations from an Atlantic profile range from 0.092 to 0.145 pg/g. The observed constancy of the234U/238U ratio is consistent with the predicted range of234U/238U using a simple two-☐ model and the residence time of deep water in the ocean determined from14C. The variation in salinity-normalized U concentrations suggests that U may be much more reactive in the marine environment than previously thought.  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