Origin of Life     

Ashwini Kumar Lal 《Astrophysics and Space Science》2008,317(3-4):267-278

The evolution of life has been a big enigma despite rapid advancements in the field of astrobiology, microbiology and genetics in recent years. The answer to this puzzle is as mindboggling as the riddle relating to evolution of the universe itself. Despite the fact that panspermia has gained considerable support as a viable explanation for origin of life on the earth and elsewhere in the universe, the issue, however, remains far from a tangible solution. This paper examines the various prevailing hypotheses regarding origin of life-like abiogenesis, RNA world, iron–sulphur world and panspermia, and concludes that delivery of life-bearing organic molecules by the comets in the early epoch of the earth alone possibly was not responsible for kick-starting the process of evolution of life on our planet.  相似文献   

Marine biogeochemistries of Be and Al: A study based on cosmogenic10Be, Be and Al in marine calcite, aragonite, and opal     

Weiquan Dong  Devendra Lal  Barbara Ransom  Wolfgang Berger  Marc W Caffee 《Journal of Earth System Science》2001,110(2):95-102

The geochemical behaviors of Be and Al in ocean waters have been successfully studied in recent years using natural, cosmogenic, radioactive¹⁰Be and²⁶Al as tracers. The present day dissolved concentrations and distribution of the stable and radioactive isotopes of Be and Al in ocean waters have revealed their short residence times and appreciable effects of exchange fluxes at the coastal and ocean-sediment interfaces. It follows that concentrations of these particle-active elements must have varied in the past with temporal changes in climate, biological productivity and aeolian flux of continental detritus to the oceans. We therefore investigated the feasibility of extending the measurements of Be and Al isotope concentrations in marine systems to the 10³–10⁶ y BP time scale. We report here the discovery of significant amounts of intrinsic Be and Al in marine foraminiferal calcite and coral aragonite, and of Al in opal (radiolarians) and aragonite (coral), which makes it possible to determine¹⁰Be/Be and²⁶Al/Al in oceans in the past. We also report measured¹⁰Be/⁹Be in foraminiferal calcite in Pacific Ocean cores, which reveal that the concentrations and ratios of the stable and cosmogenic isotopes of Be and Al have varied significantly in the past 30 ky. The implications of these results are discussed.  相似文献   

Potential of Desertification Control to Sequester Carbon and Mitigate the Greenhouse Effect   总被引：16，自引：0，他引：16  

R. Lal 《Climatic change》2001,51(1):35-72

There is a strong link between desertification of the drylands and emission of CO₂ from soil and vegetation to the atmosphere. Thus, there is a strong need to revisit the desertification process so that its reversal can lead to C sequestration and mitigation of the accelerated greenhouse effect. Drylands of the world occupy 6.31 billion ha (Bha) or 47% ofthe earth's land area distributed among four climates: hyper-arid (1.0 Bha), arid (1.62 Bha), semi-arid (2.37 Bha) and dry sub-humid (1.32 Bha). Principal soils of drylands are Aridisols (1.66 Bha), Entisols (1.92 Bha), Alfisols (0.38 Bha), Vertisols (0.21 Bha) and others (1.27 Bha). Drylands occur in all continents covering 2.01 Bha in Africa, 2.00 Bha in Asia, 0.68Bha in Australia, 1.32 Bha in the Americas and 0.30 Bha in Europe. Desertification, degradation of soil and vegetation in drylands resulting from climatic and anthropogenic factors, affects about 1.137 Bha of soils and an additional 2.576 Bha of rangeland vegetation. The rate of desertification is estimated at 5.8 million hectares (Mha) per year. Desertification is a biophysical process (soil, climate and vegetation) driven by socio-economic and political factors. The principal biophysical processes involved, accelerated soil erosion by water and wind and salinization, reduce soil quality and effective rooting depth, decrease vegetal cover, reduce biomass productivity, and accentuate vagaries of climateespecially low and variable rainfall. Major consequences of desertification include reduction in the total soil C pool and transfer of C from soil to the atmosphere. Total historic loss of C due to desertification may be 19 to 29 Pg. The rate of C emission from drylands due to accelerated soil erosion is estimated at 0.227 to 0.292 Pg C y^–1. Therefore, desertification control and restoration of degraded soils and ecosystems would improve soil quality, increase the pool of C in soil and biomass, and induce formation of secondary carbonates leading to a reduction of C emissions to the atmosphere. Desertification control and soil restoration are affected by establishing vegetative cover with appropriate species, improving water use efficiency, using supplemental irrigation including water harvesting, developing a strategy of integrated nutrient management for soil fertility enhancement, and adopting improved farming systems. Adoption of these improved practices also have hidden carbon costs, especially those due to production and application of herbicides and nitrogen fertilizers, pumping irrigation water etc. Restoration of eroded and salt-affected soils is important to C sequestration. Total potential of C sequestration in drylands through adoption of these measures is 0.9 to 1.9Pg C y^–1 for a 25- to 50-year period beyond which the rate of sequestration is often too low to be important. In addition to enhancing productivity and food security, C sequestration in soils and ecosystem has numerous ancillary benefits. Therefore, identification and implementation of policies is important to facilitate adoption of recommended practices and for commodification of carbon.  相似文献   

Chemographic relationships in sapphirine-bearing rocks from Sonapahar,Assam, India     

R. K. Lal  D. Ackermand  F. Seifert  S. K. Haldar 《Contributions to Mineralogy and Petrology》1978,67(2):169-187

Silica-deficient rocks in which sapphirine coexists with cordierite, gedrite, chondrodite, spinel, corundum and phlogopite in various parageneses, occur in association with cordierite-garnet-sillimanitebearing granulitic gneisses in a Precambrian metamorphic terrain displaying amphibolite-granulite transition facies. Brucite coexists with spinel and corundum and its unusual stability in this environment has been attributed to its significant fluorine content. Kornerupine occurs as armoured relics within cordierite in one sample of the sapphirine-spinel-cordierite-phlogopite-bearing rock. Sapphirines (X Mg 0.77 to 0.92) plot near the 793 composition.X Mg varies in the order: chondrodite and brucite>gedrite>cordierite >phlogopite>sapphirine>spinel.The consistent topology of the observed mineral assemblages with reference to the system MgO-FeO-Al₂O₃-SiO₂-H₂O, systematic partitioning of Mg and Fe in the coexisting phases, and textural relations are in accord with attainment of chemical equilibrium during the regional metamorphism. The sequence of metamorphic reactions has been deduced as a function of changing MgO/(MgO+FeO) ratio in the bulk composition and it has been shown that the topology in the SiO₂-(FeO+MgO)-Al₂O₃ diagram for more magnesian compositions is consistent with experimental data in the system MgO-SiO₂-Al₂O₃-H₂O. The stable occurrence of the spinel-cordierite and spinel-sillimanite joins in less magnesian bulk compositions suggests that these tie lines are stable to higher pressures and lower temperatures respectively as compared to the Fe-free synthetic system.Using experimental and theoretical data on garnet-cordierite, garnet-biotite, garnet-plagioclase and orthopyroxene-clinopyroxene equilibria, PT conditions have been estimated to be 750 °±50 °C/ 5±0.5 Kb for the thermal peak of metamorphism.It is suggested that the unusual bulk composition of the sapphirine-bearing rocks viz. higher Al₂O₃, MgO and MgO/(MgO+FeO) and lower SiO₂, Na₂O and CaO compared to the parent cordierite-bearing granulitic gneisses may be attributed to partial melting and removal of granitic melt. This is in agreement with the variation of oxides in the restites and parent rock. The restites consisted of the assemblage cordierite-spinel—corundum which then reacted to form sapphirine. This reaction, which in the synthetic Fe-free system is confined to extremely low-pressures (300 bars), is likely to occur at higher pressures with MgFe substitution.  相似文献   

Comments on some aspects of particulate transport in the oceans     

D. Lal 《Earth and Planetary Science Letters》1980,49(2):520-527

Recent studies of dissolved and particulate concentrations of trace elements and radionuclides amply demonstrate the importance of particulate transport in the case of several elements. A significant in-situ addition (J-flux) or removal (J-efflux) occurs in the case of a number of elements. However, to date it is not clear how the particulate processes occur and how the particles themselves are transported. Some of the problems are outlined briefly.It is shown that whereas a substantial flux in the case of some elements is due to transport by consolidated fecal particles, this transport does not generally lead to any substantial in-situ addition to deep waters. Changes in the dissolved concentrations of elements within the oceans occur due to small particles (1–10 μm) which sink stochastically with a mean speed of ～10^?3 cm/s. The larger particles sinking at higher Stokes' velocities impact and carry along the small particles. The smaller particles, ～1 μm size are thus transported down rapidly by the larger particles by apiggy-back mechanism. Simple theoretical calculations are consistent with the measured vertical transport rates based on studies of radionuclides.  相似文献   

Fission track etching studies of different planes of epidote     

K. D. Bal  Nand Lal  K. K. Nagpaul 《Physics and Chemistry of Minerals》1982,8(4):158-160

Etching characteristics of different planes of epidote indicate that the bulk etching rate V _g for the plane (100) is a minimum and also that no angular track orientation anisotropy is observable on this or on the basal plane. The significance of these studies in the light of fission track dating of the mineral has also been discussed.  相似文献   

Annual fallout of32Si,210Pb,22Na,35S and7Be in rains in India     

D Lal  V N Nijampurkar  G Rajagopalan  B L K Somayajulu 《Journal of Earth System Science》1979,88(1):29-40

The concentration of radioisotopes⁷Be,³⁵S were measured in Bombay since 1956 and²²Na,²¹⁰Pb,³²Si since 1963. In Khandala and other stations such measurements have been made at irregular periods since 1961. In addition several measurements especially that of³²Si were made in 1970. Data available todate from Indian stations is summarised and critically analysed. We conclude that appreciable amounts of³⁵S,²²Na and³²Si, over and above their production by cosmic rays, were produced during the high yield Russian tests as evidenced by their fallout between 1962–66. Based on the bomb produced excess the half period for their removal from the stratosphere is deduced to be less than 1 year. The ‘excess’ contribution of³²Si due to bombs is, however, small; about 1% of its inventory in the oceans. The present study shows that for stations where orogeny is the principal mechanism of precipitation, the annual fallout is independent of the annual rainfall.  相似文献   

Rocket measurements of ozone concentrations in the stratosphere and mesosphere over Thumba     

B H Subbaraya  Shyam Lal 《Journal of Earth System Science》1981,90(2):173-187

A rocket-borne solar middle ultraviolet photometer has been developed at the Physical Research Laboratory, Ahmedabad for the measurement of ozone concentrations at stratospheric and mesospheric heights. The instrument has now been flown successfully several times from thumba and ozone concentrations determined over an altitude range of 15 to 80 km. This paper describes the instrumentation, data analysis technique as well as the laboratory calibration procedures. Also presented are the results from four successful rocket experiments conducted during equinoctial months under an Indo-USSR collaborative programme for strato-mesospheric studies. The results show that at Thumba peak ozone concentrations vary between 2·2 and 3·1×10¹² molecules per cc and the peak altitude varies from 25 to 29 km from flight to flight. In the altitude region above about 40 km the ozone concentrations over Thumba are lower than the standard mid-latitude model values, by a factor lying between 1·5 and 2·5.  相似文献   

Large-volume in-situ filtration of deep Pacific waters: Mineralogical and radioisotope studies     

S. Krishnaswami  D. Lal  B.L.K. Somayajulu  R.F. Weiss  H. Craig 《Earth and Planetary Science Letters》1976,32(2):420-429

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Determination of atmospheric turbidity parameters over the north and central India     

Murari Lal  Prof. Dr. H. S. Rathor 《Theoretical and Applied Climatology》1971,19(3):297-306

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