Acidification and metal contamination in Whitepine Lake (Sudbury,Canada): a paleolimnological perspective     

Aruna S. Dixit  Sushil S. Dixit  John P. Smol 《Journal of Paleolimnology》1993,9(2):141-146

Diatom and chrysophyte assemblages from a sediment core from Whitepine Lake were examined to infer changes in lakewater pH, nickel and aluminum concentrations since pre-industrial times, and to help determine the cause of the virtual extirpation of the lake trout population from the lake during the 1960s and 1970s. Our study indicates that acidification started in the 1920s, and that the maximum inferred pH decline (from 6.2 to 5.8) occurred between 1960 and 1970, coincident with the peak in metal mining and smelting activity in the Sudbury basin. Lakewater [Al] and [Ni], as inferred from our diatom transfer functions, increased. It appears that in addition to the pH decline, elevated [Al] may have played an important role in the decline of lake trout from Whitepine Lake in the 1960s and 1970s. Diatom-inferred lakewater pH and [Ni] have recovered slightly in the recent sediments, which coincides with reductions in emissions that have occurred since the mid-1970s.  相似文献   

Evening and morning twilight enhancement of λ5577 Å atomic oxygen line at Allahabad     

N. Shrivastava  S. Shrivastava  S. D. Dixit  A. N. Shrivastava 《Earth, Moon, and Planets》1989,44(2):181-190

A photometer for the observation of twilight airglow emissions has been fabricated. Intensity variation in morning and evening twilight of 5577 Å line of atomic oxygen has been recorded at Allahabad for one year duration from 23rd March, 1987 to 22nd March, 1988. The enhancement in the intensity of the line is found to be irregular. Out of 40 clear evenings and 25 clear mornings from 23rd March, 1987 to 15th June, 1987 enhancement is observed in 24 evenings and 15 mornings. However, no enhancement is observed during the period from 15th September, 1987 to 15th October, 1987. It is found that rise and fall of intensity is exponential during twilight period. Considering different mechanism for the excitation of atomic oxygen green line, it is concluded that none of them are able to explain enhancement. More work is needed to be carried out for the better understanding of the problem.  相似文献   

Floods and Vulnerability: Need to Rethink Flood Management     

Dixit  Ajaya 《Natural Hazards》2003,28(1):155-179

Responses to flooding in the Himalaya-Ganga region have conventionally beensought under the hierarchic mode using the strategy of control. Because it has notgiven due consideration to diverse contexts of the region and thereby the specificapproaches that such context necessitate, the particular approach has not broughtabout security from flooding. Three responses are seen in the terrain of flooddisaster. These are the hierarchic manager, individualistic innovator and theegalitarian social activist. The hierarchies define control as the solution to theproblems of flood: this is the approach preferred by state agencies. At theindividualistic level the sought strategy is flexibility to cope with the situation.The response by social activists is guided by egalitarian critiques of the hierarchicapproach. Each pursues his/her own styles and continuously contests the policyterrain. This paper reviews the nature of flood disaster in the Himalaya-Gangaby focussing on plains Nepal. It argues that conventional approach has not beenable to provide the security envisaged. The paper suggests that vulnerability ofpeople in risk-prone areas must be addressed by enhancing resilience capacity.For this to happen the approach must be pluralistic that gives space to eachmanagement style with varying obligations at varying scales.  相似文献   

Special issue on Flood Problems and Management in South Asia: Preface     

Monirul Qader Mirza  M.  Dixit  Ajaya  Nishat  Ainun 《Natural Hazards》2003,28(1):vii-ix

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Oxygen isotope analysis of multiple,single ostracod valves as a proxy for combined variability in seasonal temperature and lake water oxygen isotopes   总被引：1，自引：0，他引：1  

Yama Dixit  David A. Hodell  Rajiv Sinha  Cameron A. Petrie 《Journal of Paleolimnology》2015,53(1):35-45

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Glassy magnetic cronstedtite signatures in Mukundpura CM2 chondrite based on magnetic and Mössbauer studies     

Ambesh Dixit  R. P. Tripathi  N. Bhandari 《Meteoritics & planetary science》2019,54(12):2902-2907

We have studied the Mukundpura CM2 meteorite for magnetic properties as a function of temperature and magnetic field, as well as its Mössbauer spectrum, at room and low temperatures (up to 5 K). We find that the high temperature paramagnetic phase is followed by two magnetic transitions: a weak transition near 125 K and a strong transition at 8 K. The weak (125 K) magnetic phase can be attributed to complex Fe²⁺–Fe³⁺ constituents present in the meteorite. The absence of the characteristic sextet corresponding to magnetite in Mossbauer spectrum indicates that this magnetic phase is not magnetite, which, if present, must be in insignificant amount. The 8 K magnetic ordering is superimposed with weak ferromagnetic ordering, showing spin‐glass transition. The Mössbauer spectrum taken at 5 K substantiates the observed spin‐glassy nature, as very large hyperfine field ~32 T is recorded, causing localized subordering leading to spin‐glass behavior. The Mössbauer spectra also confirm that iron is mainly present in serpentine‐group minerals, both in ferrous and ferric states. The complete serpentinization of basic silicates indicates aggressive hydrous alteration. These results show that the observed spin‐glass signature is a characteristic feature of the cronstedtite phase in CM meteorites. This feature is unique to carbonaceous CM chondrites and could be used for nondestructive, quick, and independent classification of this rare class of meteorites. Furthermore, the absence of olivine and the presence of cronstedtite in Mossbauer spectra show that the degree of aqueous alteration observed is the most severe in Mukundpura CM2 meteorite, as compared to many other CM2 meteorites. The degree of aqueous alteration in CM2 carbonaceous chondrites increases in the sequence: Paris, Murchison, Murray, Mighei, Nogoya, Cold Bokkeveld, and Mukundpura.  相似文献   

Diatom assemblages from Adirondack lakes (New York,USA) and the development of inference models for retrospective environmental assessment   总被引：5，自引：0，他引：5  

Sushil S. Dixit  Brian F. Cumming  H. J. B. Birks  John P. Smol  John C. Kingston  Allen J. Uutala  Donald F. Charles  Keith E. Camburn 《Journal of Paleolimnology》1993,8(1):27-47

Detrended canonical coreespondence analysis (DCCA) was used to examine the relationships between diatom species distributions and environmental variables from 62 drainage lakes in the Adirondack region, New York (USA). The contribution of lakewater pH, Al_m (monomeric Al), NH₄, maximum depth, Mg, and DOC (dissolved organic carbon) were statistically significant in explaining the patterns of variation in the diatom species composition. Twenty-three and sixteen diatom taxa were identified as potential indicator species for pH and Al_m, respectively (i.e. a taxon with a strong statistical relationship to the environmental variable of interest, a well defined optimum, and a narrow tolerance to the variable of interest). Using weighted-averaging regression and calibration, predictive models were developed to infer lakewater pH (r ²=0.91), Al_m (r ²=0.83), DOC (dissolved organic carbon) (r ²=0.64), and ANC (acid neutralizing capacity; r ²=0.90). These variables are of key importance in understanding watershed acidification processes. These predictive models have been used in the PIRLA-II (Paleoecological Investigation of Recent Lake Acidification-II) project to answer policy-related questions concerning acidification, recovery, and fisheries loss.  相似文献   

Impact of Rainwater Harvesting on Hydrological Processes in a Fragile Watershed of South Asia     

Kaushal K. Garg  K. H. Anantha  A. Venkataradha  Sreenath Dixit  Ramesh Singh  Ragab Ragab 《Ground water》2021,59(6):839-855

Agricultural water management (AWM) interventions play an important role in ensuring sustainable food production and mitigating climate risks. This study was carried out in a watershed located in a low rainfall (400–600 mm) region of western India. The Soil and Water Assessment Tool model was calibrated using surface runoff, soil loss, and reservoir storage levels, between the year 2000 and 2006. The investigation indicated that the various AWM interventions increased groundwater recharge from 30 mm/year to 80 mm/year and reduced surface runoff from 250 mm/year to 100 mm/year. The intervention structures were refilled two to three times during the monsoon season depending on rainfall intensity and duration. The interventions have the advantage of building a resilient system by enhancing groundwater availability even in dry years, stimulating crop intensification and protecting the landscape from severe erosion. The results indicate that soil erosion has been reduced by more than 75% compared to the nonintervention situation. Moreover, the AWM interventions led to the cultivation of 100–150 ha of fallow land with high-value crops (horticulture, vegetables, and fodder). Household income increased by several folds compared to the nonintervention situation. The study showed about 50% reduction in downstream water availability, which could be a major concern. However, there are a number of ecosystem trade-offs such as improved base flow to the stream and reduction in soil loss that should be considered. The study is of great importance to stakeholders to decide on the optimal design for AWM interventions to achieve sustainable development goals.  相似文献   

News and notes     

P. Krishnamurthy  K. S. Godhavari  Manish A. Mamtani  Swati Dixit  Md. Firoze Quamar  Ratan Kar  Sita Bora  Quazi A. Rasool  K. V. Subbarao  S. D. Mukherji  C. V. R. K. Prasad  Kakani Nageswara Rao 《Journal of the Geological Society of India》2013,81(6):849-864

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The relationship between sedimentary diatom assemblages and lakewater pH in 35 Quebec lakes,Canada   总被引：1，自引：0，他引：1  

Aruna S. Dixit  Sushil S. Dixit  R. Douglas Evans 《Journal of Paleolimnology》1988,1(1):23-38

Surface sediment diatoms from 35 soft-water lakes in southern Quebec were studied to examine the relationship with lakewater pH. The lakes ranged in pH from 5.25 to 7.66. The species composition and the relative abundance of diatoms in the study lakes was found to be closely related to pH and/or factors closely associated with pH. Predictive models were developed to infer lakewater pH using simple linear regression equations of index alpha, index B, and multiple regressions using pH preference categories. Among the predictive models examined here, the multiple regression technique provided the highest correlation coefficient (r²=0.88) and the lowest standard error (±0.26 of a pH unit) in computing diatom-inferred pH. This model appears to be the most appropriate to reconstruct lake pH histories in Quebec region.  相似文献