Tracing Nile sediment sources by Sr and Nd isotope signatures (Uganda, Ethiopia, Sudan)     

Marta Padoan  Yehudit Harlavan 《Geochimica et cosmochimica acta》2011,75(12):3627-3644

Strontium and neodymium isotopes, measured on diverse mud and sand fractions of sediment in transit along all major Nile branches, identify detritus sourced from Precambrian basements, Mesozoic strata, and Tertiary volcanic rocks exposed along the shoulders of the East African rift and in Ethiopian highlands. Sr and Nd isotopic ratios reflect the weighted average of detrital components generated in different catchments, allowing us to discriminate provenance, calculate sediment budgets, and investigate grain-size and hydraulic-sorting effects.⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd range, respectively, from as high as 0.722 and as low as 0.5108 for sediment derived from Archean gneisses in northern Uganda, to 0.705 and 0.5127 for sediment derived from Neoproterozoic Ethiopian and Eritrean basements. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd, ranging 0.705-0.709 and 0.5124-0.5130 for Blue Nile tributaries, are 0.704-0.705 and 0.5127-0.5128 for largely volcaniclastic sediments of River Tekeze-Atbara, and 0.705-0.706 and 0.5126-0.5127 for main Nile sediments upstream Lake Nasser.Model mantle derivation ages (t_DM), oldest in Uganda where sediment is principally derived from the Congo Craton (3.4-3.0 Ga for Victoria and Albert Nile), progressively decrease northward across the Saharan Metacraton, from 2.6 Ga (Bahr el Jebel in South Sudan), to 2.4-2.2 Ga (Bahr ez Zeraf across the Sudd), and finally 1.6-1.3 Ga (White Nile upstream Khartoum). Instead, t_DM ages of Sobat mud increase from 0.9 to 1.5 Ga across the Machar marshes. T_DM ages are younger for sediments shed by Ethiopian (1.2-0.7 Ga) and Eritrean basements (1.5-1.2 Ga), and youngest for sediments shed from Ethiopian flood basalts (0.3-0.2 Ga).Integrated geochemical, mineralogical, and settling-equivalence analyses suggest influence on the Nd isotopic signal by volcanic lithic grains and titanite rather than by LREE-rich monazite or allanite. Because contributions by ultradense minerals is subordinate, intrasample variability of Sr and Nd ratios is minor. In Blue Nile, Atbara and main Nile sediments of mixed provenance, however, the Nd ratio tends to be higher and t_DM ages lower in largely volcaniclastic mud than in mixed volcaniclastic/metamorphiclastic sand.The complete geochemical database presented here, coupled with high-resolution bulk-petrography and heavy-mineral data, provides a key to reconstructing erosion patterns and detrital fluxes across the whole Nile basin, and to investigate and understand how sources of sediment have changed in the historical and pre-historical past in relation to shifting climatic zones across arid northern Africa.  相似文献   

Porphyry to epithermal transition in the Agua Rica polymetallic deposit, Catamarca, Argentina: An integrated petrologic analysis of ore and alteration parageneses   总被引：3，自引：0，他引：3  

Marta Franchini  Agnes Impiccini  David Lentz  Francisco Javier Ríos  Sol O'Leary  Josefina Pons  Abel Isidoro Schalamuk 《Ore Geology Reviews》2011,41(1):49-74

Agua Rica (27°26′S–66°16′O) is a world class Cu–Au–Mo deposit located in Catamarca, Argentina. In the E–W 6969400 section examined, the Seca Norte and the Trampeadero porphyries that have intruded the metasedimentary rock are cut by interfingered igneous and hydrothermal heterolithic and monolithic breccias, and sandy dikes. Relic biotite and K-feldspar of the early potassic alteration (370° to > 550 °C) with Cu (Mo–Au) mineralization are locally preserved and encapsulated in a widespread, white mica + quartz + rutile or anatase halo (phyllic alteration) with pyrite + covellite that suggests fluids with temperatures ≤ 360 °C and high f(S₂). The Trampeadero porphyry and the surrounding metasedimentary rock with phyllic alteration have molybdenite in stringers and B-type quartz veinlets and the highest Mo grades (> 1000 ppm).Multistage advanced argillic alteration overprinted the earlier stages. Early andalusite ± pyrite ± quartz is preserved in the roots of the argillic halo rimmed by an alumina–silica material and white micas. This alteration assemblage is considered to have been formed at temperatures ≥ 375 °C from condensed magmatic vapor. At higher levels, pyrophyllite replaces muscovite and illite in clasts of hydrothermal breccias in the center and east sector of the study section, suggesting temperatures of 280 to 360 °C. Clasts of vuggy silica in the uppermost levels of the central breccia, indicates that at lower temperatures (< 250 °C), fluids reached very low pH (pH < 2). In this early stage of the advanced argillic alteration, hydrothermal fluids seem to have not precipitated sulfides or sulfosalts.Hydrothermal brecciation was concurrent with fluid exsolution (↑? V), which precipitated intermediate-temperature advanced argillic alunite (svanbergite + woodhouseite) ± diaspore ± zunyite as breccia cement along with abundant covellite + pyrite + enargite ± native sulfur ± kuramite at intermediate depths and in lateral transitional zones to unbrecciated rocks. This mineral assemblage indicates temperatures near 300 °C, oxidized and silica-undersaturated hydrothermal fluids with high sulfur fugacity to prevent gold precipitation. Multiple generations of pyrite, emplectite, colusite, Pb- and Bi-bearing sulfosalts, and native sulfur with Au and Ag, accompanied by alunite introduction in the upper level breccias, probably occurred at lower temperatures, but still high sulfur and oxygen activity. An independent Zn and Pb (as galena) mineralization stage locally coincides with Au–Ag and sulfosalts, and advanced at depth, controlled by fractures and overprinting much of the previous mineralization. A later paragenesis of veinlets of alunite + woodhouseite + svanvergite + pyrite ± enargite that cut the phyllic halo suggests temperatures ~ 250 °C and without woodhouseite + svanvergite, temperatures ~ 200 °C. Kaolinite occurs in the phyllic halo as a late mineral in clots and in veinlets thus, in this zone, the fluid had cooled enough for its formation.  相似文献   

Physical properties of sediment from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope   总被引：1，自引：0，他引：1  

William Winters  Michael WalkerRobert Hunter  Timothy CollettRay Boswell  Kelly RoseWilliam Waite  Marta TorresShirish Patil  Abhijit Dandekar 《Marine and Petroleum Geology》2011,28(2):361-380

This study characterizes cored and logged sedimentary strata from the February 2007 BP Exploration Alaska, Department of Energy, U.S. Geological Survey (BPXA-DOE-USGS) Mount Elbert Gas Hydrate Stratigraphic Test Well on the Alaska North Slope (ANS). The physical-properties program analyzed core samples recovered from the well, and in conjunction with downhole geophysical logs, produced an extensive dataset including grain size, water content, porosity, grain density, bulk density, permeability, X-ray diffraction (XRD) mineralogy, nuclear magnetic resonance (NMR), and petrography.This study documents the physical property interrelationships in the well and demonstrates their correlation with the occurrence of gas hydrate. Gas hydrate (GH) occurs in three unconsolidated, coarse silt to fine sand intervals within the Paleocene and Eocene beds of the Sagavanirktok Formation: Unit D-GH (614.4 m-627.9 m); unit C-GH1 (649.8 m-660.8 m); and unit C-GH2 (663.2 m-666.3 m). These intervals are overlain by fine to coarse silt intervals with greater clay content. A deeper interval (unit B) is similar lithologically to the gas-hydrate-bearing strata; however, it is water-saturated and contains no hydrate.In this system it appears that high sediment permeability (k) is critical to the formation of concentrated hydrate deposits. Intervals D-GH and C-GH1 have average “plug” intrinsic permeability to nitrogen values of 1700 mD and 675 mD, respectively. These values are in strong contrast with those of the overlying, gas-hydrate-free sediments, which have k values of 5.7 mD and 49 mD, respectively, and thus would have provided effective seals to trap free gas. The relation between permeability and porosity critically influences the occurrence of GH. For example, an average increase of 4% in porosity increases permeability by an order of magnitude, but the presence of a second fluid (e.g., methane from dissociating gas hydrate) in the reservoir reduces permeability by more than an order of magnitude.  相似文献   

In situ experimental decomposition studies in estuaries: A comparison of Phragmites australis and Fucus vesiculosus     

Marta Lobo Lopes  Patrícia Martins  Fernando Ricardo  Ana Maria Rodrigues  Victor Quintino 《Estuarine, Coastal and Shelf Science》2011,92(4):497-580

The decomposition rates of Phragmites australis and Fucus vesiculosus were experimentally determined in an estuarine system using the leaf-bag technique. The study was conducted in fifteen sites arranged in five areas, extending from freshwater, outside the tidal range, to the marine environment, near the mouth of the estuary. The leaf-bags (5 mm mesh), were set up with 3.0 g of dried substrate, submerged in the experimental sites at day 0 and collected at days 3, 7, 15, 30 and 60, to follow biomass loss. The biomass loss through the leaching phase (day 3) was about 16% for Phragmites australis and 33% for Fucus vesiculosus and was independent of salinity for both substrates. The difference in the remaining biomass between the two species increased with time and the decomposition rates differed along the salinity gradient. For F. vesiculosus, the decomposition rate was highest near the mouth of the estuary, corresponding to the preferential distribution area of the algae, and decreased towards freshwater. For Phragmites australis, the fastest decay was observed in the mid estuary, where Phragmites australis occurs naturally, confirming previous studies. The decomposition rates measured at different time intervals (0–15, 0–30 and 0–60 days) were always higher for the algae and decreased with time for both species. These results indicate that the use of decomposition rates as a measure of ecosystem integrity or quality status in transitional waters will not be straightforward and must take into account, among others, the test species, the study area positioning along the estuarine gradient, and the time interval for the calculation of the decomposition rate.  相似文献   

Seasonal distribution of organic matter and copper under stratified conditions in a karstic,marine, sulfide rich environment (Rogoznica Lake,Croatia)     

Marta Plavšić  Irena CiglenečkiSlađana Strmečki  Elvira Bura-Nakić 《Estuarine, Coastal and Shelf Science》2011

Closed, isolated small systems, as the representatives of a “unique-environmental feature”, are valuable natural laboratories for studying different biogeochemical processes. The saline Rogoznica Lake (“Dragon Eye”), situated on the Eastern Adriatic coast is such a system (10 276 m², 15 m deep) typical of many stratified, sulfide rich water bodies. The depth of mixolimnion changes seasonally and it is greatly influenced by meteorological conditions, i.e. temperature and rainfall. Vertical mixing usually occurs during winter when cold, oxygen-rich water from the surface sinks downwards.  相似文献   

An ocean observation system for monitoring the affects of climate change on the ecology and sustainability of pelagic fisheries in the Pacific Ocean     

Simon J. Nicol  Valerie Allain  Graham M. Pilling  Jeff Polovina  Marta Coll  Johann Bell  Paul Dalzell  Peter Sharples  Robert Olson  Shane Griffiths  Jeffrey M. Dambacher  Jock Young  Antony Lewis  John Hampton  Jesus Jurado Molina  Simon Hoyle  Karine Briand  Nic Bax  Patrick Lehodey  Peter Williams 《Climatic change》2013,119(1):131-145

Climate change presents an emerging challenge to the sustainable management of tuna fisheries, and robust information is essential to ensure future sustainability. Climate and harvest affect tuna stocks, populations of non-target, dependent species and the ecosystem. To provide relevant advice we need an improved understanding of oceanic ecosystems and better data to parameterise the models that forecast the impacts of climate change. Currently ocean-wide data collection in the Pacific Ocean is primarily restricted to oceanographic data. However, the fisheries observer programs that operate in the region offer an opportunity to collect the additional information on the mid and upper trophic levels of the ecosystem that is necessary to complement this physical data, including time-series of distribution, abundance, size, composition and biological information on target and non-target species and mid trophic level organisms. These observer programs are in their infancy, with limited temporal and spatial distribution but recent international and national policy decisions have been made to expand their coverage. We identify a number of actions to initiate this monitoring including: consolidating collaborations to ensure the use of best quality data; developing consistency between sub-regional observer programmes to ensure that they meet the objectives of ecosystem monitoring; interrogating of existing time series to determine the most appropriate spatial template for monitoring; and exploring existing ecosystem models to identify suitable indicators of ecosystem status and change. The information obtained should improve capacity to develop fisheries management policies that are resilient and can be adapted to climate change.  相似文献   

The hydrogen Balmer,Paschen, and Brackett series lines in quiescent prominences     

Marta G. Rovira  Juan Manuel Fontenla 《Solar physics》1986,106(2):315-333

To calculate the Balmer, Paschen, and Brackett line intensities, we solved the statistical equilibrium equations for a twenty level plus continuum atom of hydrogen.From the temperature, ionization, and the first three level populations of the prominence models deduced in a previous work we have calculated the populations of the twenty bound levels and the integrated intensities corresponding to the series mentioned above. The method was also applied to the Heasley and Mihalas theoretical models.Since the Lyman series are optically thick, we have worked out two different options: (a) assume radiative balance in these lines and (b) correct the radiative rates by multiplying them with an integro-exponential function EI2 which depends on the line optical depth at the thread center. The first option is shown to be the more consistent.The integrated line intensities from the Balmer series have been compared with the observations and a clear difference was noted between quiescent and active prominences in the sense that the active prominence case can not be well fitted with the available models.To evaluate the influence of the pressure, the temperature, the thermal conduction coefficient and the turbulence, velocity on the spectrum we have compared the results from different models.From this study we conclude that only the lines arising from the lower levels up to 8–10 can give us information about the physical parameters that characterized the solar prominences since the intensities from the higher members of the series depend only on atomic properties because of the small departures from LTE of the upper levels involved.Member of the Carrera del Investigador, CONICET, Argentina.  相似文献   

Strike-slip faults parallel to crustal spreading axes: data from Iceland and the Afar Depression     

Pietro Passerini  Marta Marcucci  Giovanni Sguazzoni  Leonardo Zan  Abdourahman Omar Haga 《地学学报》1991,3(6):607-618

Slickenside studies in regions of crustal spreading such as Iceland and the Afar Depression, East Africa, reveal that a significant number of faults parallel and close to rift axes are strike-slip rather than normal. Therefore, the pattern of brittle deformation in these regions does not conform to the classic two-dimensional schemes of oceanic tectonics and pre-oceanic rifting. Dip-slip and strike-slip faulting presumably alternated along or in the vicinity of spreading axes, indicate a varying stress field and a combination of transverse and longitudinal movements. In Iceland, strike-slip faults parallel to rifts are observed both west and east of the rift system as well as in a median area between overlapping rifts; the mechanisms proposed for their origin include accommodation of oblique convergence or divergence of crustal sections due to variations of spreading directions along axis and the interaction of overlapping rifts. In the Afar Depression this kind of fault is recorded west of the rift of Asal and can be imputed to reflect an interaction among rifts in the vicinity of the Afar triple junction. Rift-parallel strike-slip faults cannot however be assumed to be a feature of all crustal spreading axes due to the peculiarity of the examined regions: both of them are hot-spot areas and the Afar Depression lies at a triple junction.  相似文献   

Formation of supermassive black holes by direct collapse in pre-galactic haloes     

Mitchell C. Begelman  Marta Volonteri  Martin J. Rees 《Monthly notices of the Royal Astronomical Society》2006,370(1):289-298

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The flares of April 1980     

Marcos E. Machado  Boris V. Somov  Marta G. Rovira  Cornelis De Jager 《Solar physics》1983,85(1):157-184

We discuss the spatial and temporal characteristics of X-ray flares occurring in the active region NOAA2372 from April 6 to 13, 1980. The flares are seen to extend in most cases across the whole active complex, involving several magnetic features. They originate in an intermediate bipole, between the two main sunspots of the active region, where high magnetic shear was detected. A rapid expansion is seen in some cases, in conjunction with the start of the impulsive hard X-ray bursts. We also detect, in the late phases of some of the events, a large soft X-ray structure overlying the whole active region, which also shows up as a noise storm region at metric wavelengths. These large loops cool by heat conduction but, in some cases, Hα condensations seem to appear, probably as a result of magnetic compression and a condensation mode of the thermal instability. The topological aspects of the field configuration are discussed, in the context of flare models invoking magnetic reconnection at the site of the primary energy release. In such a model, the intermediate bipole is the natural site of initial magnetic reconnection, particle acceleration and heating. In one particular case of a flare observed at the limb, we find possible evidence of particle acceleration in a neutral sheet at the boundary between two clearly defined magnetic structures.  相似文献