黄土和红粘土中宇宙成因核素定年的潜力: 10 Be浓度与化学成分的关系*

宇宙成因核素¹⁰ Be(半衰期1.5Ma)不但是古环境变化的示踪剂,而且具有确定地质年龄的能力。80个全岩样品的¹⁰ Be和化学分析数据表明,黄土高原约6Ma以来的红粘土和黄土序列中年龄校正的¹⁰ Be浓度与沉积物粒度和风化程度的化学指标具有显著的关系。根据这种关系建立的¹⁰ Be浓度与化学指标的经验回归线性模型,可以估计红粘土和黄土形成时的¹⁰ Be浓度,确定红粘土和黄土形成的¹⁰ Be放射性年龄。     

Very high-resolution climate simulation over Fiji using a global variable-resolution model

A high-resolution climate model simulation has been performed for the first time for Fiji’s climatology. The simulation involved a numerical experiment for a 10-year period (1975–1984), and was conducted at a horizontal resolution of 8 km in a stretched-grid configuration, which is currently the highest resolution at which a global climate model has been applied for regional climatological simulations. Analysis of model-generated data demonstrates a fairly good skill of the CSIRO Conformal-Cubic Atmospheric Model (C-CAM) in the simulation of the annual cycles of maximum and minimum temperatures and rainfall at selected locations in Fiji. The model has also successfully reproduced the pattern of maximum and minimum surface air temperatures between the western and central divisions of Fiji. Model simulation of spatial and temporal distribution of monthly total rainfall (10-year mean) over the main island of Viti Levu in Fiji shows that it reproduces the observed intraseasonal and interannual variability; the influence of the El Niño phenomena has also been captured well in the model-simulated rainfall.     

Distributions of O3, CO and NMHCs over the rural sites in central India

The surface level measurements of O₃, CO, CH₄ and light NMHCs were made at eight different rural sites in the central part of India during February, 2004. The online analyzer was used for in-situ measurement of O₃ while air samples were collected for the analyses of CO, CH₄ and NMHCs using the gas chromatography techniques. The average mixing ratios of O₃, which were in the range of 60–90 ppbv, are significantly higher compared to the typical values reported for urban sites of India. The increase rates of O₃ in the forenoon hours were estimated to be in the range of about 8.8–10 ppbv h⁻¹. The slopes of ∆O₃/∆CO, which is an indicator of the efficiency of photochemical production, were in the range of 0.24–0.33 ppbv ppbv⁻¹. However, levels of primary pollutants e.g., NMHCs, CO, etc. at these sites were much lower than urban sites, but higher compared to previously observed values surrounding marine region of India. The estimated ratios of NMHCs and CO indicate fossil fuel combustion process as the dominant source of primary pollutants in this corridor.     

Assessing the utility of 210Pb geochronology for estimating sediment accumulation rates on river floodplains in Fiji

Low‐energy gamma ray spectroscopy has been employed to estimate floodplain sedimentation rates using measurements of ²¹⁰Pb in floodplain alluvium. The utility of the technique is assessed through the analysis of excess (unsupported) ²¹⁰Pb profiles in three sediment cores taken from the floodplain of the Labasa River on Vanua Levu in northern Fiji. A low‐energy germanium spectrometer (LEGe) was used for the nondestructive determination of excess ²¹⁰Pb in a region cultivated intensively with sugarcane. Measured average historical (c. 25 years) vertical accretion rates are between 2.2 and 4.4 cm yr^?1. The findings are broadly comparable with published sedimentation rates from analyses of radionuclide profiles elsewhere in the tropical South Pacific Islands, but the rates are higher than those measured previously at the same Labasa River sites using ¹³⁷Cs profiles. Accelerated soil erosion owing to cane burning and land tillage seems to be largely responsible for sediment production, although flood‐related effects such as channel accretion by coarse bedload and the emplacement of large organic debris also influence floodplain sedimentation. However, application of the ²¹⁰Pb technique in Fiji (and perhaps neighbouring island countries) is found to have serious drawbacks compared to the more robust ¹³⁷Cs method, owing principally to the low ²¹⁰Pb concentrations in the sandy alluvial sediment tested.     

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

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.     

Emission characteristic of ozone related trace gases at a semi-urban site in the Indo-Gangetic plain using inter-correlations

Measurements of surface O₃, CO, NO_x and light NMHCs were made during December 2004 at Hissar, a semi-urban site in the state of Haryana in north-west region of the Indo-Gangetic Plain (IGP). The night-time O₃ values were higher when levels of CO, NO and NO₂ were lower but almost zero values were observed during the episodes of elevated mixing ratios of CO (above 2000 ppbv) and NO_x (above 50 ppbv). Slopes derived from linear fits of O₃ versus CO and O₃ versus NO_x scatter plots were also negative. However, elevated levels of O₃ were observed when CO and NO_x were in the range of 200–300 ppbv and 20–30 ppbv, respectively. Slope of CO-NO_x of about 33 ppbv/ppbv is much larger than that observed in the US and Europe indicating significant impact of incomplete combustion processes emitting higher CO and lesser NO_x. Correlations and ratios of these trace gases including NMHCs show dominance of recently emitted pollutants mostly from biomass burning at this site.     

Cosmogenic10Be: A critical view on its widespread dominion in geosciences

The radionuclide¹⁰Be (half-life = 1.5my), produced naturally in the Earth’s atmosphere by nuclear interactions of cosmic rays, was sought in ocean sediments in the late fifties, considering its potential usefulness as a radiotracer for dating sediments.¹⁰Be was discovered independently by two groups, one in India and the other in the USA, and used only for dating marine sediments and manganese nodules until the seventies. Subsequently, as a result of a technical advance resulting in the improvement in the sensitivity of measurement of¹⁰Be by about a factor of 10⁶, there was a global rush to measure this nuclide in most materials participating in the physical, chemical and biological processes in the dynamic geosphere. This paper outlines the reasons for this “isotope rush”, and the lessons learned from these studies. I also present my personal views of the special attractive features of this nuclide on the one hand, and on the other, the pitfalls or the wrong message this nuclide could convey!     

Potential of Desertification Control to Sequester Carbon and Mitigate the Greenhouse Effect

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.