Hydrochemical appraisal of ice‐ and rock‐glacier meltwater in the hyperarid Agua Negra drainage basin,Andes of Argentina     

Karina L. Lecomte  Juan Pablo Milana  Stella M. Formica  Pedro J. Depetris 《水文研究》2008,22(13):2180-2195

The Agua Negra drainage system (30 12′S, 69 50′ W), in the Argentine Andes holds several ice‐ and rock‐glaciers, which are distributed from 4200 up to 6300 m a.s.l. The geochemical study of meltwaters reveals that ice‐glaciers deliver a HCO₃^?? Ca²⁺ solution and rock‐glaciers a SO₄^2?? HCO₃^?? Ca²⁺ solution. The site is presumably strongly influenced by sublimation and dry deposition. The main processes supplying solutes to meltwater are sulphide oxidation (i.e. abundant hydrothermal manifestations), and hydrolysis and dissolution of carbonates and silicates. Marine aerosols are the main source of NaCl. The fine‐grained products of glacial comminution play a significant role in the control of dissolved minor and trace elements: transition metals (e.g. Mn, Zr, Cu, and Co) appear to be selectively removed from solution, whereas some LIL (large ion lithophile) elements, such as Sr, Cs, and major cations, are more concentrated in the lowermost reach. Daily concentration variation of dissolved rare earth elements (REE) tends to increase with discharge. Through PHREEQC inverse modelling, it is shown that gypsum dissolution (i.e. sulphide oxidation) is the most important geochemical mechanism delivering solutes to the Agua Negra drainage system, particularly in rock‐glaciers. At the lowermost reach, the chemical signature appears to change depending on the relative significance of different meltwater sources: silicate weathering seems to be more important when meltwater has a longer residence time, and calcite and gypsum dissolution is more conspicuous in recently melted waters. A comparison with a non‐glacierized semiarid drainage of comparable size shows that the glacierized basin has a higher specific denudation, but it is mostly accounted for by relatively soluble phases (i.e. gypsum and calcite). Meltwater chemistry in glacierized arid areas appears strongly influenced by sublimation/evaporation, in contrast with its humid counterparts. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

Flare model chromospheres and photospheres     

Marcos E. Machado  Jeffrey L. Linsky 《Solar physics》1975,42(2):395-420

Homogeneous plane-parallel model atmospheres for solar flares have been constructed to approximately simulate observations of flares. The wings of the Ca II lines have been used to derive flare upper photosphere models, which indicate temperature increases of ～100 K over the temperature distribution in the pre-existing facula at a height of 300 km above τ₅₀₀₀ = 1. In the case of flares covering sunspots the temperature rise seems to occur much higher in the atmosphere. We solve the transfer and statistical equilibrium equations for a three-level hydrogen atom and a five-level calcium atom in order to obtain the chromospheric flare models. The general properties of flares, including n _e, N ₂, linear thickness, and Lyman continuum intensity are approximately reproduced. We find that with increasing flare importance the height of the upper chromosphere and transition region occur lower in the solar atmosphere, accounting for the factor of 60–600 increase in pressure in these regions relative to the quiet Sun. The Ca II line profiles agree with observations only by assuming a macro-velocity distribution that increases with height. Also the chromospheric parts of flares appear to be highly inhomogeneous. We show that shock and particle heated flare models do not agree with the observations and propose a thermal response model for flares. In particular, it appears that heating in the photosphere is an essential aspect of flares.  相似文献   

The significance of Chara vegetation in the precipitation of lacustrine calcium carbonate     

Mariusz Pełechaty  Andrzej Pukacz  Karina Apolinarska  Aleksandra Pełechata  Marcin Siepak 《Sedimentology》2013,60(4):1017-1035

A significant portion of calcium carbonate is deposited in lake sediments as a result of biological processes related to the photosynthetic activity of phytoplankton in the pelagic realm and, in addition, macrophytes in the littoral zone. Lake Wigry, one of the largest lakes in Poland (north‐east Poland), is characterized by: (i) carbonate sediments with a CaCO₃ content exceeding 80% within the littoral zone; and (ii) large areas of submerged vegetation dominated by charophytes (macroscopic green algae, Characeae family). It is claimed that charophytes are highly effective in utilizing HCO₃^? and forming thick CaCO₃ encrustations. Thus, this study was aimed at evaluating the CaCO₃ production by dense Chara stands overgrowing the lake bottom reaching a depth of 4 m. In late July 2009, the fresh and dry mass of plants, the percentage contribution of calcium carbonate and the production of CaCO₃ per 1 m² were investigated along three transects at three depths (1 m, 2 m and 3 m, with each sample area equal to 0·0625 m²) per transect. The composition and structure of phytoplankton and the physico‐chemical properties of the water analysed in both the littoral and pelagic zones served as the environmental background and demonstrated moderately low fertility in the lake. The greatest dry plant mass exceeded 1000 g m^?2 and CaCO₃ encrustations constituted from 59% to over 76% of the charophyte dry weight. Thus, the maximum and average values of carbonates precipitated by charophytes were 685·5 and 438 g m^?2, respectively, which exceeded previously reported results. A correlation of carbonate production with the depth of Chara stands was detected, and intermediate depths offered the most favourable conditions for carbonate precipitation (589 g m^?2 on average). As precipitated carbonates are ultimately stored in bottom deposits, the results highlight the significance of charophytes in lacustrine CaCO₃ sedimentation.  相似文献   

Natural protected areas of San Luis Potosí, Mexico: ecological representativeness,risks, and conservation implications across scales     

Leonardo Chapa-Vargas  Karina Monzalvo-Santos 《International journal of geographical information science》2013,27(9):1625-1641

Assessments of the conservation status of natural resources have been conducted at large (i.e., global, continental, and countrywide) extents. Studies at finer scales, however, can yield increased detail needed to identify conservation strategies for smaller scales which can contribute to goals at the larger extents. Our study was conducted at the scale of a single state, San Luis Potosí, in Central Mexico. Although the state harbors great biological diversity, large areas covered by natural vegetation communities have been and continue to be destroyed by land use changes, cattle grazing, and mine-generated pollution. Our purposes were to quantify the proportion of distribution within natural protected areas (NPAs) of land cover classes, soil types, ecoregions, and altitudinal steps. In addition, within individual NPAs, we aimed at assessing risks. We conducted our analysis at the 1:1, 1:250,000, and 1:1,000,000 scales depending on layer availability. Our results show that actions within the state could contribute to federal and global conservation goals by protecting sites of globally imperiled ecosystems (cloud forests and natural grasslands). At a finer scale, we identified that piedmont scrubland, medium and low tropical moist forests, wetlands, and low elevation habitats are rare and/or constitute conservation omissions within the state. Similarly, ecoregions with mesquite, semi-deciduous, and moist tropical thorn forest, wetlands, and low elevations (<1200 m.a.s.l.) are conservation omissions or gaps. Finally, at the smallest scale, risks within some protected areas (PAs) include the presence of agriculture, cattle grazing, and mine tailings from mines dedicated to extraction of Au, Ag, Cu, Pb, and Zn. Therefore, management within these PAs should minimize such risks through identification of the most important conservation zones within the PAs, and implementation of local legislation prohibiting such activities at the immediacies of conservation zones. Approaches like ours within similar administrative units will help identify ecosystems that are vulnerable and not well protected. However, to prioritize conservation action it is also important to consider surrogates such as species from different groups and ecosystems, as well as rarity and complementarity of such surrogates.  相似文献   

Saline groundwater seepage zones and their impact on soil and water resources in the Spicers Creek catchment,central west,New South Wales,Australia     

Karina?MorganEmail author  Jerzy?Jankowski 《Environmental Geology》2004,46(2):273-285

Saline seepage zone development and hence dryland salinity is a major environmental problem which many arid to semiarid landscapes in Australia are experiencing. Due to the geological complexity of the regional aquifer system and the heterogeneous nature of the local groundwater system, each groundwater seepage zone in the Spicers Creek catchment, central west, New South Wales, Australia possesses different mechanisms which control its development. Saline seepage zones have formed adjacent to a fault zone, and two experimental sites were established through these groundwater discharge zones to understand geochemical processes which have led to the development of soil sodicity, gully erosion and the flushing of salts into the surface water systems. Seepage zone groundwaters contain a distinctive geochemical signature with elevated concentrations of Na, Cl, HCO₃, Ca, Sr, B, As and Li. The mixing of deep saline groundwaters together with ion exchange processes lead to a distinctive seepage zone groundwater chemistry being developed. Altering the landscape features within this rural groundwater system has developed water toxicity for crops, soil sodicity leading to land degradation, and waterlogging problems.  相似文献   

Relative Response of Biological and Non-Biological Monitors in a Coastal Environment     

A. Machado  M. C. Freitas  A. M. G. Pacheco 《Journal of Atmospheric Chemistry》2004,49(1-3):161-173

A field study was carried out in the spring and summer of 2003 (2 months each), to assess the efficiency of alternative exposure modes of biological monitors – lichen biomass and tree-bark biomass – together with prospective, non-biological monitors – cellulose acetate and Chelex-100 resin – versus conventional transplants of the same species. After exposure to the marine atmosphere of Sines (SW Portugal), saline elements (Cl, Na, K, Mg) contents were determined in all samples by instrumental neutron activation analysis (INAA). Manganese was used as the crustal reference for data normalisation. As a whole, the results pointed to a consistent elemental accumulation regardless of the season (spring, summer), and to some fair reproducibility of data. There were good correlations between Cl and Na in terms of both raw and normalised data, for either the biomonitors or the cellulose acetate. Elemental ratios generally agreed with the average relative composition of bulk seawater. The degree of association between Cl and Na appeared rather unaffected by the normalisation procedure, which suggests that (i) both elements most likely had an almost exclusive marine origin; and (ii) accounting for crustal inputs via Mn did not bias the results. Other sources than the sea-spray might contribute to the Mg levels, though, as only Mn-normalised data showed intrinsic (correlation) and extrinsic (ratio) consistency. The performance of the Chelex-100 resin was rather disappointing when compared to the other monitors.  相似文献   

Evidence for the photospheric origin of the flare optical continuum     

Marcos E. Machado 《Solar physics》1971,17(2):389-391

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Environmental influences on the demersal fish assemblages in the Sepetiba Bay, Brazil   总被引：1，自引：0，他引：1  

Francisco Gerson Araújo  Márcia Cristina Costa de Azevedo  Márico de Araújo Silva  André Luiz Machado Pessanha  Iracema David Gomes  Antônio Gomes da Cruz-Filho 《Estuaries and Coasts》2002,25(3):441-450

The structure of the demersal fish assemblage in Sepetiba Bay, southeast Brazil (22°54′–23°04′S, 43°34′–44°10′W) was described and related to environmental variables. A 3-yr long trawl survey (from July 1993 to June 1996) was conducted using a stratified random sampling strategy at 158 stations. One hundred and seven species of fishes in 80 genera and 44 families were identified. Gerreidae and Ariidae made up 30.5% and 28.6%, and 19.8% and 28.1% of the total number and weight, respectively. Sciaenidae, Engraulididae, Sparidae, and Carangidae were other prominent families. Three assemblages existed in the bay: one inhabited shallow, less saline and less transparent water in the inner zone (depth <5 m, transparency <2 m, salinity <30 psu, and slightly higher temperature); another in the outer zone with the opposite situation (depth >10 m, transparency >3 m, salinity >30 psu, and slightly lower temperature); and a third assemblage in the middle zone likely to prefer average values or show no clear preferences. Most species decreased in abundance over the 3-yr period of study. No seasonal change was detected in the assemblage structure, but many species were rare, indicating high spatial and temporal patchiness. Canonical correlation analyses were used to describe and compare the fish assemblages in the three different habitats. The outer zone had the greatest number of species recorded, lowest abundance, highest diversity and evenness, and was comprised mainly byCynossion leiarchus, Prionotus punctatus, Anchoa tricolor, Haemulon steindachneri, Diplectrum radiale, Etropus crossotus, andSphoeroides greeleyi; the inner zone had the lowest number of species recorded but the greatest number of species per sample, highest abundance, lowest diversity and evenness, being comprised mainly byDiapterus rhombeus, Genidens genidens, Cathrops spixii, Chloroschombrus chrysurus, Sciadeichthyes luniscutis, Cetengraulis edentulus, andArchorsargus rhomboidalis; while the third group had the lowest number of species per sample, being comprised by species widely distributed asGerres aprion, G. genidens, Gerres gula, andMicropogonias furnieri. Depth, followed by transparency and salinity, were the primary factors influencing assemblage distribution.  相似文献   

SERMIN1 and VIDAC18: Two Proposed Reference Materials for Rare Earth Element Determination in Groundwater     

Rui Santos  M.J. Canto Machado  Leopoldo Cortez 《Geostandards and Geoanalytical Research》2008,32(2):181-191

In this work, homogeneous samples of two Portuguese mineral waters, SERMIN1 and VIDAC18, have been analysed for their REE contents. The participation of twenty international laboratories made it possible to evaluate the data by robust statistical procedures. Eighty-one percent of the participants used ICP-QMS for REE determination in these two candidate reference materials. Assigned values are reported for SERMIN1, covering the fourteen naturally occurring lanthanides (La-Lu). For VIDAC18, assigned values are presented only for ten REE, since accurate assigned values for La, Ce, Tb and Eu need further analytical work to improve data quality.  相似文献   

Predictive Mapping of Prospectivity in the Gurupi Orogenic Gold Belt,North–Northeast Brazil: An Example of District-Scale Mineral System Approach to Exploration Targeting     

Leandro Duarte Campos  Sulsiene Machado de  Souza  Diogo Alves de  Sordi  Felipe Mattos Tavares  Evandro Luiz Klein  Elem Cristina dos Santos Lopes 《Natural Resources Research》2017,26(4):509-534

The Gurupi Belt hosts a Paleoproterozoic gold province located in north–northeastern Brazil, at the borders of Pará and Maranhão states. It is considered to be an extension of the prolific West African Craton’s Birimian gold province into South America. Additionally, the belt has been the object of recent mineral exploration programs with significant resource discoveries. This study presents the results of predictive mapping using up-to-date mineral system concepts and recently finished regional-scale geological mapping, stream sediment and airborne geophysical surveys conducted by the Geological Survey of Brazil. We relate gold mineralization to an initially enriched crust, metamorphism, deep fluid pathways, structurally controlled damage zones and hydrothermal alteration. Prospective targets were generated using only regional public datasets and knowledge-driven targeting technique. This work did not incorporate any known gold deposits, yet it predicted the largest known deposits and their satellite targets. Besides, high prospective targets mapped almost 40% of known primary gold occurrences within 7% of the project area. This work allowed considerable search area reduction and identification of new target areas, thus collaborating on reducing costs, time and risk of mineral exploration. Results indicate that we achieved an efficient understanding of the geological processes related to the Gurupi Belt mineral system.  相似文献