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171.
Abstract We review the carbon‐isotope data for finely disseminated carbonates from bioaltered, glassy pillow rims of basaltic lava flows from in situ slow‐ and intermediate‐spreading oceanic crust of the central Atlantic Ocean (CAO) and the Costa Rica Rift (CRR). The δ13C values of the bioaltered glassy samples from the CAO show a large range, between ?17 and +3‰ (Vienna Peedee belemnite standard), whereas those from the CRR define a much narrower range, between ?17‰ and ?7‰. This variation can be interpreted as the product of different microbial metabolisms during microbial alteration of the glass. In the present study, the generally low δ13C values (less than ?7‰) are attributed to carbonate precipitated from microbially produced CO2 during oxidation of organic matter. Positive δ13C values >0‰ likely result from lithotrophic utilization of CO2 by methanogenic Archaea that produce CH4 from H2 and CO2. High production of H2 at the slow‐spreading CAO crust may be a consequence of fault‐bounded, high‐level serpentinized peridotites near or on the sea floor, in contrast to the CRR crust, which exhibits a layer‐cake pseudostratigraphy with much less faulting and supposedly less H2 production. A comparison of the δ13C data from glassy pillow margins in two ophiolites interpreted to have formed at different spreading rates supports this interpretation. The Jurassic Mirdita ophiolite complex in Albania shows a structural architecture similar to that of the slow‐spreading CAO crust, with a similar range in δ13C values of biogenic carbonates. The Late Ordvician Solund–Stavfjord ophiolite complex in western Norway exhibits structural and geochemical evidence for evolution at an intermediate‐spreading mid‐ocean ridge and displays δ13C signatures in biogenic carbonates similar to those of the CRR. Based on the results of this comparative study, it is tentatively concluded that the spreading rate‐dependent tectonic evolution of oceanic lithosphere has a significant control on the evolution of microbial life and hence on the δ13C biosignatures preserved in disseminated biogenic carbonates in glassy, bioaltered lavas. 相似文献
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Sounik K. Banerjee 《Geomechanics and Geoengineering》2018,13(2):104-114
Assessment of tunnel stability has become increasingly crucial as more and more tunnels are built in difficult terrains such as sloping ground. The required support pressure on the tunnel walls associates both tunnel stability and liner design considerations. The present analysis attempts to find a uniform internal pressure which can support a circular tunnel built in a sloping ground with a particular level of stability in cohesive-frictional soils. The lower bound finite element limit analysis has been applied to find the required minimum uniform internal support pressure presented as a non-dimensional term p/c; where p is the minimum normal internal pressure on the tunnel boundary to avoid collapse and c is the cohesion of soil. The variation of p/c is presented for a range of normalised embedment depth of tunnel (H/D), stability number (γD/c), internal friction angle of soil (?) and slope angle (β); where H is the crown depth of the tunnel, D is the tunnel diameter and γ is the unit weight of soil. Appropriate comparisons have been carried out with available literature. Failure patterns of the tunnel have also been studied to understand the extent and the type of failure zone which may generate during the collapse. 相似文献
174.
Geotectonics - The abnormalities in the ionosphere Total Electron Content (TEC) is linked to the deformation, stress accumulation and earthquake preparation processes in the earth’s crust and... 相似文献
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177.
Ashes Banerjee Srinivas Pasupuleti Mritunjay Kumar Singh G.N. Pradeep Kumar 《Acta Geophysica》2018,66(1):81-91
Complexity of the pore geometry and the random nature of flow velocity make it difficult to predict and represent post laminar flow through porous media. Present study experimentally investigates the applicability of Forchheimer and Wilkins equations for post laminar flow where Darcy’s law is invalid due to predominant inertial effect. It is observed that both porosity and media size have significant influence over the coefficients of the Forchheimer coefficients. To incorporate the effect of porosity and media size, behaviour of Forchheimer coefficients are investigated with hydraulic radius as characteristic length. An inversely proportional variation trend is found for all the present and earlier reported data. A new empirical relation between Forchheimer coefficients and hydraulic radius is obtained which can be universally applicable for all media size and porosity. Coefficients of the Wilkins equation are found to be non-deviating for different hydraulic radius in the present study and in the reported literature validating its applicability in predicting the non laminar flow through porous media. Further the Wilkins equation is modified after incorporating the correction factors for better applicability on the field. 相似文献
178.
Arshad Ali Iffat Jabeen Neil R. Banerjee Gordon R. Osinski Ian Nicklin David Gregory Patrick Herrmann 《Meteoritics & planetary science》2018,53(6):1223-1237
Oxygen isotope measurements of olivine in main group (MG) pallasites by traditional laser fluorination method are associated with some uncertainties including terrestrial weathering, incomplete olivine reaction, and sample state. We improved our laser fluorination approach by pretreating olivine grains with acid to remove terrestrial weathering products and by modifying the sample holder for an efficient and complete laser reaction. Our experiments on Brahin olivine demonstrate that acid-washing successfully removes the terrestrial weathering with <0.1‰ variation in δ18O value and, at the same time, improving the ∆17O value significantly. We also achieved a complete olivine fluorination by employing a custom-designed sample holder with “V”-shaped profile having rounded bottom because incomplete/partial reaction of olivine gives comparatively lighter δ18O values. Using these new techniques, we present precise triple oxygen isotope data (N = 72) of 25 olivine samples separated from main group pallasites. The data are, on average, ~0.5‰ heavier in δ18O relative to the values published in the literature for the same samples. Critically, the ∆17O values of MG pallasites and to some extent their Fo-contents suggest that there are at least two populations of olivine. Based on our improved data set, we propose that MG pallasites potentially have high-∆17O- and low-∆17O-bearing subgroups that are statistically distinct. The subgroups present average ∆17O values of −0.166 ± 0.003 (2SE; N = 16) and −0.220 ± 0.003 (2SE; N = 9), respectively. Furthermore, the high-∆17O-bearing subgroup samples trend toward lower Fo-contents compared to the other subgroup. Taken together, our data provide evidence that argues against a single parent body origin for MG pallasites. 相似文献
179.
The equation of state and the adiabatic index of thermally dissociated matter composed of nucleons, electrons, positrons, neutrinos, antineutrinos and photons are calculated in the density and temperature ranges, 109q(g cm–3)1013, 2×1010T K5×1011, respectively. The interaction between nucleons is explicitly included. This leads to a softening of the equation of state. The implications of the results for the problem of supernova collapse are discussed. 相似文献
180.
新疆黄土-古土壤序列环境磁学参数的变化机理及其气候意义仍存在争议.本文选择天山北麓的中梁黄土剖面,系统开展了低温和常温下环境磁学参数的测试与研究,测量包括室温的磁化率与饱和磁化强度,以及磁化率与饱和剩余磁化强度的低温变化.结果发现,该剖面黄土和古土壤样品的磁性矿物主要由磁铁矿与磁赤铁矿组成,不含任何粒级成壤形成的超顺磁矿物颗粒,其磁化率信号主要记录了粉尘磁性矿物含量变化,较高的磁化率指示较强的风动力状况或者较近的风尘源区,新疆黄土的这种环境磁学"风尘输入模式"可用来重建干旱区的风动力强弱变化. 相似文献