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1.
Viscoelastic or creep behaviour can have a significant influence on the load transfer (t–z) response at the pile–soil interface, and thus on the pile load settlement relationship. Many experimental and theoretical models for pile load transfer behaviour have been presented. However, none of these has led to a closed‐form expression which captures both non‐linearity and viscoelastic behaviour of the soil. In this paper, non‐linear viscoelastic shaft and base load transfer (t–z) models are presented, based on integration of a generalized viscoelastic stress–strain model for the soil. The resulting shaft model is verified through published field and laboratory test data. With these models, the previous closed‐form solutions evolved for a pile in a non‐homogeneous media have been readily extended to account for visco‐elastic response. For 1‐step loading case, the closed‐form predictions have been verified extensively with previous more rigorous numerical analysis, and with the new GASPILE program analysis. Parametric studies on two kinds of commonly encountered loading: step loading, ramp (linear increase followed by sustained) loading have been performed. Two examples of the prediction of the effects of creep on the load settlement relationship by the solutions and the program GASPILE, have been presented. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

2.
Measured settlements of buildings on the weathered Keuper Marl appeared to be much smaller than calculated settlements, which were based on stiffness modulus from standard oedometer tests. Therefore, both special triaxial K 0-tests and oedometer tests were carried out for an accurate determination of stiffness moduli. Modulus obtained in the triaxial K 0-tests were at least two to three times the values obtained in the oedometer tests. To verify observations from the laboratory tests, the loads and the settlements of two single footings on weathered Keuper mudstone have been measured during construction of a building over 1 year. Also, a large scale footing load test with measurements of deformations were conducted on the weathered Keuper mudstone. The measured settlements of the two single footings and the tested foundation were compared with the settlements based on conventional calculations with moduli from oedometer tests and triaxial K 0-tests. Up to a foundation pressure of σv = 500 kN/m2 the calculated settlement based on E S-modulus obtained from triaxial K 0-tests was found to correspond well to the measured deformation. For foundation pressure beyond 500 kN/m2, the foundation response was highly non-linear and it could not be described any more with the linear-elastic model. Therefore the footing load test was also simulated by FEM analyses.  相似文献   

3.
In this paper, a simple semi‐analytical method has been developed to solve the one‐dimensional non‐linear consolidation problems by considering the changes of compressibility and permeability of the soil layer, subjected to complicated time‐dependent cyclic loadings at the ground surface. The solution presented here takes into account e ～ lg kv and e ～ lg σ′ linear responses. With ck the slope of the e ～ lg kv line and cc as the slope of the e ～ lg σ′ line, the identified parameter cc/ck is found to control the rate of consolidation. Using the solutions obtained, some diagrams are prepared and the relevant behaviours of one‐dimensional non‐linear consolidation of saturated soft soil under cyclic loadings are discussed. The method in this paper does not require any special data; conventional oedometer data can be used. Therefore, the method is particularly efficient and convenient for engineering practice. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

4.
Stability of landslide dams and development of knickpoints   总被引：2，自引：0，他引：2  
The Wenchuan earthquake triggered many landslides and numerous avalanches and created 100 odd quake lakes. The quake lakes may be removed or preserved. The removal strategy was applied to several large landslide dams, which were dangerous because massive amounts of water pooled up in the quake lakes. The dams could eventually fail under the action of dam outburst flooding, potentially endangering the lives of people in the downstream reaches. This paper studied the stability of landslide dams and the development of knickpoints by field investigations and experiments, and analyzing satellite images. The study concluded that if landslide dams were preserved, they would develop into knickpoints and act as a primary control of riverbed incision and, thus, reduce the potential of new landslide. The stability of landslide dams depends mainly on the development of the step-pool system and stream power of the flood flow. If a landslide dam consists of many boulders, a step-pool system may develop on the spillway channel of the dam, which would maximize the resistance, consume most of the flow energy and consequently protect the dam from incision. The development degree of the step-pool system is represented by a parameter S p, which was measured with a specially designed instrument. A preservation ratio of landslide dams is defined as the ratio of preserved height after flood scouring to the original height of the dam. For streams with peak flood discharge lower than 30 m3/s, the preservation ratio is linearly proportional to S p. For rivers with a peak flood discharge higher than 30 m3/s (30–30,000 m3/s), the minimum S p value for stable channel increases with log p, in which p is the unit stream power. For a landslide dam with a poorly developed step-pool system, S p is smaller than the minimum value and the outburst flood incises the spillway channel and causes failure of the dam. For preserved landslide dams, sediment deposits in the quake lakes. A landslide dam may develop into a knickpoint if it is stabilized by long-term action of the flow. Large knickpoints can totally change the fluvial processes and river morphology. Uplift of the Qinghai–Tibetan Plateau has caused extensive channel bed incision along almost all rivers. For many rivers, the incision has been partly controlled by knickpoints. Upstream reaches of a knickpoint have a new and unchanging base level. This brings about a transition from degradation to aggradation and from vertical bed evolution to horizontal fluvial process. Multiple and unstable channels are prominent in the reaches, upstream of the knickpoints. If hundreds of landslide dams occurred simultaneously on a reach of a mountain river, the potential energy of bank failure and the slope erosion would be greatly reduced and sediment yield from the watershed may be reduced to nearly zero. The quake lakes may be preserved long term and become beautiful landscapes. Streams with long-term unfilled quake lakes have good aquatic ecology.  相似文献   

5.
Stratotectonic and morphotectonic data from the two principal exposed domains (pre‐Adelaidean rocks) of the Gawler sub‐province are used to characterize the Proterozoic Olarian orogeny and to distinguish its effects from those of the later Phanerozoic Delamerian orogeny.

The principal metasedimentary sequences in the Gawler domain and in the Willama domain are inferred to have been deposited in a single broad zone of early Proterozoic shallow‐water sedimentation on older (presumed Archaean) continental crust. The sequence becomes more pelitic upwards and may be interpreted as a transgressive sequence with more distal facies to the east.

Three main phases of deformation are recognized, and each phase has similar characteristics and age in both domains. D 1 2nd D2 can be dated between 1850 and 1650 Ma, while D3 appears to be about 1650–1540 Ma.

In high grade rocks, D1 gave rise to a layer‐parallel schistosity, while D 2 is characterized by tight folds with a high‐grade axial‐plane schistosity. The whole sub‐province was characterized by high geothermal gradients so that medium‐ to high‐grade metamorphism affected the lower parts of the succession before and during the D1 and D2 deformation episodes. No distinct tectonic zones can be recognized but large‐scale stratigraphic inversions (i.e. nappe tectonics) during D 1 have been recognized only in the east of the Willyama domain. The higher parts of the stratigraphic succession are generally less deformed and exhibit only low‐grade metamorphism.

D 3 produced relatively open, upright macroscopic folds and was characteristically associated with retrogression, but was demonstrably of pre‐Adelaidean age. The Gawler domain exhibits D 3 structures although it lies in the platform west of the Adelaide Geosyncline and was not affected by deformation during Adelaidean sedimentation or by the subsequent Delamerian orogeny. A network of retrograde shear zones is the principal expression of post‐Olarian deformation in the Willyama domain which forms part of the basement to the Adelaide Geosyncline.

The trends of D 2 and D 3 folding in the two domains are similar and it is shown therefore that no large‐scale rotations of one domain relative to the other has been produced by the Delamerian orogeny. Large‐scale translations on discrete faults or on broad zones of simple shear in the basement are not easily ruled out, but if they exist, are probably largely of pre‐Adelaidean age. However, a significant relationship between Olarian structures and variable Adelaidean fold trends has been deduced.

The Olarian orogeny may have occurred in close proximity to a continental margin to the east and may thus be related to subduction processes. It differs from linear gneissic belts in Phanerozoic orogenies since it occurs in a more stable stratotectonic environment and over a wider area.  相似文献   

6.
The article presents a numerical inversion method for estimation of quality Q factor and phase velocity in linear, viscoelastic, isotropic media using reconstruction of relaxation spectrum from measured or computed complex velocity or modulus of the medium. Mathematically, the problem is formulated as an inverse problem for reconstruction of relaxation spectrum in the analytic Stieltjes representation of the complex modulus using rational approximation. A rational (Padé) approximation to the relaxation spec trum is derived from a constrained least squares minimization problem with regularization. The recovered stress-strain relaxation spectrum is applied to numerical calculation of frequency-dependent Q factor and frequency-dependent phase velocity for known analytical models of a standard linear viscoelastic solid (Zener) model as well as a nearly constant-Q model which has a continuous spectrum. Numerical results for these analytic models show good agreement between theoretical and predicted values and demonstrate the validity of the algorithm. The proposed method can be used for evaluating relaxation mechanisms in seismic wavefield simulation of viscoelastic media. The constructed lower order Padé approximation can be used for determination of the internal memory variables in time-domain finite difference numerical simulation of viscoelastic wave propagation.  相似文献   

7.
Experimental observations clearly show that the relative humidity (hr) conditions influence significantly the creep behavior of cement‐based materials, indicating that the water present within these materials plays a crucial role. This work presents a creep model for hardened cement pastes (HCP), based on a multiscale homogenization approach. It takes into account both free and adsorbed water contained in the porosity and investigates their effects on the HCP macroscopic creep behavior. The calcium silicate hydrate phase is assumed to be linear viscoelastic, and the Mori–Tanaka scheme is applied in the Laplace–Carson space to the composite formed of porosity, calcium silicate hydrate, and the other main hydrated compounds (which behavior is linearly elastic) by making use of the correspondence principle. With this model, estimations of the evolution of the macroscopic creep behavior of HCP submitted to constant external loading are examined under different hr and compared with available experimental data. Finally, a method for implementing the model in a finite element code is proposed, and simulations of standard creep tests are performed to assess its validity. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

8.
Raman peaks of various hydrates in the H20-NaCl-CaCl2 system have been previously identified, but a quantitative relationship between the Raman peaks and XNaCl （i.e.,NaCl/ （NaCl＋CaCl2）） has not been established, mainly due to the difficulty to freeze the solutions. This problem was solved by adding alumina powder to the solutions to facilitate nucleation of crystals. Cryogenic （-185℃） Raman spectroscopic studies of alumina-spiced solutions indicate that XNaCl is linearly correlated with the total peak area fraction of hydrohalite. Capsules of solutions made from silica capillary were prepared to simulate fluid inclusions. Most of these artificial fluid inclusions could not be totally frozen even at temperatures as low as -185℃, and the total peak area fraction of hydrohalite is not correlated linearly with XNaCI. However, the degree of deviation （△XNaCl） from the linear correlation established earlier is related to the amount of residual solution, which is reflected by the ratio （r） of the baseline ＂bump＂ area, resulting from the interstitial unfrozen brine near 3435 cm^-1, and the total hydrate peak area between 3350 and 3600 cm^-1. A linear correlation between △XNaCl and r is established to estimate XNaCl from cryogenic Raman spectroscopic analysis for fluid inclusions.  相似文献   

9.
This paper presents a new simplified method, based on Hypothesis B, for calculating the consolidation settlements of double soil layers exhibiting creep. In the new simplified Hypothesis B method, different stress–strain states including over‐consolidation and normal consolidation states can be considered with the help of the ‘equivalent time’ concept. Zhu and Yin method and US Navy method are adopted to calculate the average degree of consolidation for a double soil layer profile. This new simplified Hypothesis B method is then used to calculate the consolidation settlements of double soil layers, which have two different total thicknesses of soil layer (4 m and 8 m) and three different OCR values (Over‐Consolidation Ratio, OCR = 1, 1.5 and 2). The accuracy and verification of this new simplified method are examined by comparing the calculated results with simulation results from a fully coupled finite element (FE) program using a soft soil creep model. Four cases of double layer soil profiles are analyzed. Hypothesis A method with US Navy method for the average degree of consolidation has also been used to for calculating consolidation settlements of the same cases. For Case I(4m) and Case III(8m), it is found that curves of the new simplified Hypothesis B method using both Zhu and Yin method and US Navy method are very close to the results from FE simulations with the relative errors within 8.5%. For Case II(4m) and Case IV(8m), it is found that curves of the new simplified Hypothesis B method using Zhu and Yin method agree better with results from FE simulations with the relative errors within 11.7% than curves of the new simplified Hypothesis B method adopting US Navy method with the relative error up to 36.1%. Curves of Hypothesis A method adopting US Navy method have the relative error up to 55.0% among all four cases. In overall, the new simplified Hypothesis B method is suitable for calculation of consolidation settlements of double soil layers exhibiting creep, in which, Zhu and Yin method is recommended to obtain the average degree of consolidation. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

10.
By using the upper bound finite‐elements limit analysis, with an inclusion of single and two horizontal layers of reinforcements, the ultimate bearing capacity has been computed for a rigid strip footing placed over (i) fully granular, (ii) cohesive‐frictional, and (iii) fully cohesive soils. It is assumed that (i) the reinforcements are structurally strong so that no axial tension failure can occur, (ii) the reinforcement sheets have negligible resistance to bending, and (iii) the shear failure can take place between the reinforcement and soil mass. It is expected that the different approximations on which the analysis has been based would generally remain applicable for reinforcements in the form of geogrid sheets. A method has been proposed to incorporate the effect of the reinforcement in the analysis. The efficiency factors, ηc and ηγ, to be multiplied with Nc and Nγ , for finding the bearing capacity of reinforced foundations, have been established. The results have been obtained (i) for different values of ? in case of fully granular and cohesive‐frictional soils, and (ii) for different rates at which the cohesion increases with depth for a fully cohesive soil. The optimum positions of the reinforcements' layers have also been determined. The effect of the reinforcements' length on the results has also been analyzed. As compared to cohesive soils, the granular soils, especially with higher values of ?, cause a much greater increase in the bearing capacity. The results compare reasonably well with the available theoretical and experimental data from literature. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

11.
A sequence of prograde isograds is recognized within the Dalradian Inzie Head gneisses where pelitic compositions have undergone variable degrees of partial melting via incongruent melting reactions consuming biotite. Three leucosome types are identified. At the lowest grades, granitic leucosomes containing porphyroblasts of cordierite (CRD‐melt) are abundant. At intermediate grades, CRD‐melt mingles with garnetiferous leucosomes (GT‐melt). At the highest grades, CRD‐melt coexists with orthopyroxene‐bearing leucosomes (OPX‐melt), while garnet is conspicuously absent. The prograde metamorphic field gradient is constrained to pressures of 2–3 kbar below the CRD‐melt isograd, and no greater than 4.5 kbar at the highest grade around Inzie Head. A petrogenetic grid, calculated using thermocalc , is presented for the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system for the phases orthopyroxene, garnet, cordierite, biotite, sillimanite, H2O and melt with quartz and K‐feldspar in excess. For the implied field gradient, the reaction sequence predicted by the grid is consistent with the successive prograde development of each leucosome type. Compatibility diagrams suggest that, as anatexis proceeded, bulk compositions may have been displaced towards higher MgO content by the removal of (relatively) ferroan granitic leucosome. An isobaric (P = 4 kbar) T–aH2O diagram shows that premigmatization fluids must have been water‐rich (aH2O > 0.85) and suggests that, following the formation of small volumes of CRD‐melt, the system became fluid‐absent and melting reactions buffered aH2O to lower values as temperatures rose. GT‐ and OPX‐melt formed by fluid‐absent melting reactions, but a maximum of 7–11% CRD‐melt fraction can be generated under fluid‐absent conditions, much less than the large volumes observed in the field. There is strong evidence that the CRD‐melt leucosomes could not have been derived by buoyantly aided upwards migration from levels beneath the migmatites. Their formation therefore required a significant influx of H2O‐rich fluid, but in a quantity insufficient to have exhausted the buffering capacity of the solid assemblage plus melt. Fluid : rock ratios cannot have exceeded 1 : 30. The fluid was channelled through a regionally extensive shear zone network following melt‐induced failure. Such an influx of fluid at such depths has obvious consequences for localized crustal magma production and possibly for cordierite‐bearing granitoids in general.  相似文献   

12.
 Torsional forced-oscillation and microcreep methods have been employed in a study of the viscoelastic behaviour of fine-grained polycrystalline olivine at high temperatures (to 1300 °C), seismic frequencies and low strain amplitudes. The Fo90 specimens are of low porosity and low dislocation density. They vary in mean grain size from 8 to 150 μm and contain only trace amounts (≪0.1 vol%) of quenched melt glass. For T ≤ 900 °C, their behaviour is essentially elastic and the shear modulus G closely approaches that expected for a dense polycrystal from single-crystal elasticity data – confirming the suppression of thermal microcracking in␣this study. At higher temperatures, pronounced absorption-band dissipation and associated dispersion␣of the shear modulus provide evidence of linear viscoelastic behaviour. Both recoverable (anelastic) and permanent (viscous) strains are involved and the proportion of the latter increases with increasing temperature and decreasing frequency. Comparison of the results for the three specimens provides a clear indication that the viscoelastic behaviour, attributed to diffusional processes, is grain-size-sensitive with the dissipation and associated dispersion increasing with decreasing grain size. Both elastically accommodated and diffusionally accommodated grain-boundary sliding appear to be implicated. Received: 29 September 2000 / Accepted: 7 May 2001  相似文献   

13.
Statistical learning algorithms provide a viable framework for geotechnical engineering modeling. This paper describes two statistical learning algorithms applied for site characterization modeling based on standard penetration test (SPT) data. More than 2700 field SPT values (N) have been collected from 766 boreholes spread over an area of 220 sqkm area in Bangalore. To get N corrected value (Nc), N values have been corrected (Nc) for different parameters such as overburden stress, size of borehole, type of sampler, length of connecting rod, etc. In three‐dimensional site characterization model, the function Nc=Nc (X, Y, Z), where X, Y and Z are the coordinates of a point corresponding to Nc value, is to be approximated in which Nc value at any half‐space point in Bangalore can be determined. The first algorithm uses least‐square support vector machine (LSSVM), which is related to a ridge regression type of support vector machine. The second algorithm uses relevance vector machine (RVM), which combines the strengths of kernel‐based methods and Bayesian theory to establish the relationships between a set of input vectors and a desired output. The paper also presents the comparative study between the developed LSSVM and RVM model for site characterization. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

14.
This paper presents the analysis of creep settlement of pile groups for line pile groups, square pile groups, and rectangular pile groups undergoing creep settlements over a period of time. The soil is treated as a viscoelastic material and is modeled using a three-parameter viscoelastic model. The damping component (dashpot) takes care of the permanent time-dependent deformations in three-parameter viscoelastic model. An approach suggested by Mindlin has been employed to calculate the stress distribution along the pile length in a group. The viscoelastic problem is converted into an elastic problem by the application of Laplace transform. Results in the form of variation of interaction factors for parameters such as pile length to diameter ratio, pile spacing, Poisson's ratio, and modulus ratio have been presented. Comparison has been made between interaction factors for piles groups undergoing immediate settlements and creep settlements. Finally, a typical predictive example has been presented for a 3 × 3 pile group showing creep settlement. The load rearrangement due to creep settlements causes about 5% to 35% increase in base resistance over time. Interaction factors for pile groups (2 × 1, 3 × 1, 2 × 2, and 3 × 2) undergoing creep settlement is about 15% to 55% higher than the interaction factors considering only the immediate settlements for pile group spacing less than or equal to 5d.  相似文献   

15.
The method of stress characteristics has been used for computing the ultimate bearing capacity of strip and circular footings placed on rock mass. The modified Hoek‐and‐Brown failure criterion has been used. Both smooth and rough footing‐rock interfaces have been modeled. The bearing capacity has been expressed in terms of nondimensional factors Nσ0 and Nσ, corresponding to rock mass with (1) γ = 0 and (2) γ ≠ 0, respectively. The numerical results have been presented as a function of different input parameters needed to define the Hoek‐and‐Brown criterion. Slip line patterns and the pressure distribution along the footing base have also been examined. The results are found to compare generally well with the reported solutions.  相似文献   

16.
The thermodynamic properties of silicate minerals can be described as a linear combination of the fractional properties of their constituent polyhedra. In contrast, given the thermodynamic properties of these polyhedra, the thermodynamic properties of minerals can be estimated, where only the crystallography of the mineral needs to be known. Such estimates are especially powerful for hypothetical mineral end‐members or for minerals where experimental determination of their thermodynamic properties is difficult. In this contribution the fractional enthalpy, entropy and molar volume for 35 polyhedra have been determined using weighted multiple linear regression analysis on a data set of published mineral thermodynamic properties. The large number of polyhedra determined, allows calculation of a much larger variety of phases than was previously possible and the larger set of minerals used provides more confident fractional properties. The OH‐bearing minerals have been described by partial and total hydroxide coordinated components, which gives better results than previous models and precludes the need of a S–V term to improve estimates of entropy. However, the fractional thermodynamic properties only give adequate results for silicate minerals and double oxides, and should therefore not be used to estimate the properties of other minerals. The thermodynamic properties of ‘new’ minerals are calculated from a linear stoichiometric combination of their constituent polyhedra, resulting in estimates generally with associated uncertainty of <5%. The quality of such data appears to be of sufficient accuracy for thermodynamic modelling as shown for meta‐bauxites from the Alps and the Aegean, where the effect of Zn on the P–T stability of staurolite can be both qualitatively and quantitatively reproduced.  相似文献   

17.
Non‐associated flow rule is essential when the popular Mohr–Coulomb model is used to model nonlinear behavior of soil. The global tangent stiffness matrix in nonlinear finite element analysis becomes non‐symmetric when this non‐associated flow rule is applied. Efficient solution of this large‐scale non‐symmetric linear system is of practical importance. The standard Krylov solver for a non‐symmetric solver is Bi‐CGSTAB. The Induced Dimension Reduction [IDR(s)] solver was proposed in the scientific computing literature relatively recently. Numerical studies of a drained strip footing problem on homogenous soil layer show that IDR(s = 6) is more efficient than Bi‐CGSTAB when the preconditioner is the incomplete factorization with zero fill‐in of global stiffness matrix Kep (ILU(0)‐Kep). Iteration time is reduced by 40% by using IDR(s = 6) with ILU(0)‐Kep. To further reduce computational cost, the global stiffness matrix Kep is divided into two parts. The first part is the linear elastic stiffness matrix Ke, which is formed only once at the beginning of solution step. The second part is a low‐rank matrix Δ, which is re‐formed at each Newton–Raphson iteration. Numerical studies show that IDR(s = 6) with this ILU(0)‐Ke preconditioner is more time effective than IDR(s = 6) with ILU(0)‐Kep when the percentage of yielded Gauss points in the mesh is less than 15%. The total computation time is reduced by 60% when all the recommended optimizing methods are used. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

18.
Chloritoid–glaucophane‐bearing rocks are widespread in the high‐pressure belt of the north Qilian orogen, NW China. They are interbedded and cofacial with felsic schists originated from greywackes, mafic garnet blueschists and low‐T eclogites. Two representative chloritoid–glaucophane‐bearing assemblages are chloritoid + glaucophane + garnet + talc + quartz (sample Q5‐49) and chloritoid + glaucophane + garnet + phengite + epidote + quartz (sample Q5‐12). Garnet in sample Q5‐49 is coarse‐, medium‐ and fine‐grained and shows two types of zonation patterns. In pattern I, Xgrs is constant as Xpy rises, and in pattern II Xgrs decreases as Xpy rises. Phase equilibrium modelling in the NC(K)MnFMASH system with Thermocalc 3.25 indicates that pattern I can be formed during progressive metamorphism in lawsonite‐stable assemblages, while pattern II zonation can be formed with further heating after lawsonite has been consumed. Garnet growth in Q5‐49 is consistent with a continuous progressive metamorphic process from ～14.5 kbar at 470 °C to ～22.5 kbar at 560 °C. Garnet in sample Q5‐12 develops with pattern I zonation, which is consistent with a progressive metamorphic process from ～21 kbar at 540 °C to ～23.5 kbar at 580 °C with lawsonite present in the whole garnet growth. The latter sample shows the highest P–T conditions of the reported chloritoid–glaucophane‐bearing assemblages. Phase equilibrium calculation in the NCKFMASH system with a recent mixing model of amphibole indicates that chloritoid + glaucophane paragenesis does not have a low‐pressure limit of 18–19 kbar as previously suggested, but has a much larger pressure range from 7–8 to 27–30 kbar, with the low‐pressure part being within the stability field of albite.  相似文献   

19.
Organic carbon isotopes in sediments have been frequently used to identify the source of organic matter.Here we present a study of organic δ~(13)C on two sediment profiles influenced by guano from Guangjin and Jinqing islands in the Xisha Archipelago,South China Sea.Organic matter from ornithogenic coral sand sediments has two main sources,guano pellets and plant residues,and their organic δ~(13)C(δ~(13)C_(OM)) are significantly different.Organic carbon δ~(13)C_(guano) is much higher thanδ~(13)C_(plants),and δ~(13)C_(OM)of bulk samples is intermediate.Based on a two-end-member mixing model,the proportions of guano-and plant-derived organic matter in the bulk samples were reconstructed quantitatively.The results showed that seabirds began to inhabit the islands around approximately1200-1400 AD,and that guano pellets have been an important source of soil organic matter since then.With the accumulation of guano-derived nutrients,plants began to develop prosperously on the islands in the last 200 years,which is reflected by the significant increase of plant-derived organic matter in the upper sediment layer.However,guano-derived organic matter decreased greatly in recent decades,indicating a rapid decrease in seabird population.Our results show that organic δ~(13)C can be effectively used to quantitatively determine different source contributions of OM to bulk ornithogenic coral sand sediments.  相似文献   

20.
Eclogite facies metarodingites occur as deformed dykes in serpentinites of the Zermatt‐Saas ophiolite (Western Alps). They formed during the subduction of the Tethys oceanic lithosphere in the Early Tertiary. The metarodingites developed as a consequence of serpentinization of the oceanic mantle. Three major types of metarodingites (R1, R2 & R3) can be distinguished on the basis of their mineralogical composition. All metarodingites contain vesuvianite, chlorite and hydrogrossular in high modal amounts. In addition they contain: R1 – diopside, tremolite, clinozoisite, calcite; R2 – hydroandradite, diopside, epidote, calcite; and R3 – hydroandradite. Both garnets contain a small but persistent amount of hydrogarnet component. The different metarodingites reflect different original dyke rocks in the mantle. In each group of metarodingite, textural relations suggest that reactions adjusted the assemblages along the P–T path travelled by the ophiolite during subduction and exhumation. Reactions and phase relations derived from local textures in metarodingite can be modelled in the eight‐component system: SiO2‐Al2O3‐Fe2O3‐FeO‐MgO‐CaO‐CO2‐H2O. This permits the analysis of redox reactions in the presence of andradite garnet and epidote in many of the rocks. Within this system, the phase relations in eclogite facies metarodingites have been explored in terms of T–XCO2, T–μ(SiO2), μ(Cal)–μ(SiO2) and P–T sections. It was found that rodingite assemblages are characterized by low μ(SiO2) and low XCO2 conditions. The low SiO2 potential is externally imposed onto the rodingites by the large volume of antigorite‐forsterite serpentinites enclosing them. Moreover, μ(SiO2) decreases consistently from metarodingite R1 to R3. The low μ(SiO2) enforced by the serpentinites favours the formation of hydrogarnet and vesuvianite. Rodingite formation is commonly associated with hydrothermal alteration of oceanic lithosphere at the ocean floor, in particular to ocean floor serpentinization. Our analysis, however, suggests that the metarodingite assemblages may have formed at high‐pressure conditions in the subduction zone as a result of serpentinization of oceanic mantle by subduction zone fluids.  相似文献   

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