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1.
The transport pathways of fine sediments (fraction <2 μm) along the Brazilian continental shelf from Ceará to the Amazon River
mouth were studied by means of clay mineral analyses. On the continental shelf southeast of the Amazon mouth, fluctuations
in clay mineral compositions reflect simple mixing between the suspended load of the North Brazil Current and sediment from
several smaller rivers. Previously, clay mineral variations west of the Amazon mouth have been explained by variable settling
velocities of different grain-size classes within the <2 μm fraction or by selective coagulation of individual clay mineral
groups. By contrast, our experiments with river bank samples show that selective coagulation does not occur in Amazon River
sediments. A more appropriate explanation for observed variations in clay mineral composition off the Amazon mouth seems to
be, similarly to that for the shelf between Ceará and the Amazon mouth, a mixing of Amazon sediments with suspended material
of the North Brazil Current. This interpretation is supported by data on clay mineral composition east and south of the Amazon
mouth, showing more affinity to sediments of the North Brazil Current than to the suspended load of the Amazon River. Additionally,
relatively low sedimentation rates and low concentrations of fine-grained sediments on the shelf suggest that high riverine
input by the Amazon River does not overprint the sediments of the North Brazil Current in this region. The strong North Brazil
Current shunts the Amazon suspended load in a north-westerly direction along the north-eastern coast of South America. Hence,
stronger sedimentation of Amazon sediments would occur only west of the river mouth. 相似文献
2.
Riparian plants can adapt their water uptake strategies based on climatic and hydrological conditions within a river basin. The response of cold-alpine riparian trees to changes in water availability is poorly understood. The Lhasa River is a representative cold-alpine river in South Tibet and an under-studied environment. Therefore, a 96 km section of the lower Lhasa River was selected for a study on the water-use patterns of riparian plants. Plant water, soil water, groundwater and river water were measured at three sites for δ18O and δ2H values during the warm-wet and cold-dry periods in 2018. Soil profiles differed in isotope values between seasons and with the distance along the river. During the cold-dry period, the upper parts of the soil profiles were significantly affected by evaporation. During the warm-wet period, the soil profile was influenced by precipitation infiltration in the upper reaches of the study area and by various water sources in the lower reaches. Calculations using the IsoSource model indicated that the mature salix and birch trees (Salix cheilophila Schneid. and Betula platyphylla Suk.) accessed water from multiple sources during the cold-dry period, whereas they sourced more than 70% of their requirement from the upper 60–80 cm of the soil profile during the warm-wet period. The model indicated that the immature rose willow tree (Tamarix ramosissima Ledeb) accessed 66% of its water from the surface soil during the cold-dry period, but used the deeper layers during the warm-wet period. The plant type was not the dominant factor driving water uptake patterns in mature plants. Our findings can contribute to strategies for the sustainable development of cold-alpine riparian ecosystems. It is recommended that reducing plantation density and collocating plants with different rooting depths would be conducive to optimal plant growth in this environment. 相似文献
3.
Doklady Earth Sciences - The results of isotope U–Pb dating of zircons from lherzolite and vein olivine orthopyroxenite composing the Roseta ultramafic massif are presented. The zircons... 相似文献
4.
Alatorre-Ibargüengoitia Miguel A. Hernández-Urbina Karina Ramos-Hernández Silvia G. 《Natural Hazards》2021,106(1):1011-1035
Natural Hazards - The 1982 eruption of El Chichón volcano constitutes the worst volcanic disaster in Mexico producing more than 2000 fatalities, thousands of displaced people and severe... 相似文献
5.
Sources of dissolved REE in mountainous streams draining granitic rocks, Sierras Pampeanas (Córdoba, Argentina) 总被引:1,自引:0,他引:1
M. Gabriela García Karina L. Lecomte Andrea I. Pasquini Stella M. Formica Pedro J. Depetris 《Geochimica et cosmochimica acta》2007,71(22):5355-5368
Stream waters draining granitic terrains from the highest part (850 to 2200 m a.s.l.) of Sierras Pampeanas (Córdoba, Argentina, ∼32°S, ∼65°W) were sampled in order to define sources and distribution of dissolved rare earth elements (REE), and to describe the geochemical processes that govern their mobility. The contribution of the regional granite to the dissolved REE pool in stream water is limited due to the physical conditions predominating in the area (i.e., steep slopes and semiarid climate). Therefore, precipitation is considered a seasonally significant source controlling REE concentration in stream water. Dissolved REE concentrations are inversely correlated with monthly precipitation and rainfall frequency. During the rainy season (i.e., the austral summer) REE concentrations in stream water are lower than during the dry season (i.e., austral winter). Such low concentrations reflect the balance between the REE input from precipitation and their removal by adsorption. In contrast, during the dry season, the longer residence time of water within fractures and colluvium determines an increased REE concentration in the base flow. Lower pH values also contribute to raise REE concentration through desorption from mineral surfaces. 相似文献
6.
Girardeau Cornen Beslier Le Gall Monnier Agrinier Dubuisson Pinheiro Ribeiro & Whitechurch 《地学学报》1998,10(6):330-336
The Gorringe Bank corresponds to an upper mantle peridotite ridge enclosing a 500-m thick/ 50-km long laccolith-like body of gabbro, locally cut and poorly covered by tholeiitic rocks. Strain and kinematic analysis of orientated gabbros and peridotites sampled during the GORRINGE diving cruise (1996) provides new kinematic constraints on extensional high-temperature deformation recorded at deep levels during stretching, near an accreting centre axis of a mantle-dominant oceanic lithosphere. It is argued that the Gorringe Bank lithosphere formed at an oceanic ultra-slow, N010°–020°-trending accreting centre, mostly by passive tectonic denudation of the mantle, without any synchronous large magmatism. This peculiar lithosphere may be representative of the Iberia oceanic domain located between the continent and the J anomaly ridge, which likely marks the beginning of true spreading at an oceanic spreading ridge. 相似文献
7.
Karina Marshall-Bowman Dimitri A. Sverjensky Robert M. Hazen 《Geochimica et cosmochimica acta》2010,74(20):5852-5861
The abiotic polymerization of amino acids may have been important for the origin of life, as peptides may have been components of the first self-replicating systems. Though amino acid concentrations in the primitive oceans may have been too dilute for significant oligomerization to occur, mineral surface adsorption may have provided a concentration mechanism. As unactivated amino acid polymerization is thermodynamically unfavorable and kinetically slow in aqueous solution, we studied mainly the reverse reaction of polymer degradation to measure the impact of mineral surface catalysis on peptide bonds.Aqueous glycine (G), diglycine (GG), diketopiperazine (DKP), and triglycine (GGG) were reacted with minerals (calcite, hematite, montmorillonite, pyrite, rutile, or amorphous silica) in the presence of 0.05 M, pH 8.1, KHCO3 buffer and 0.1 M NaCl as background electrolyte in a thermostatted oven at 25, 50 or 70 °C. Below 70 °C, reaction kinetics were too sluggish to detect catalytic activity over amenable laboratory time-scales. Minerals were not found to have measurable effects on the degradation or elongation of G, GG or DKP at 70 °C in solution. At 70 °C pyrite was the most catalytic mineral with detectible effects on the degradation of GGG, although several others also displayed catalytic behavior. GGG degraded ∼1.5-4 times faster in the presence of pyrite than in control reactions, depending on the ratio of solution concentration to mineral surface area. The rate of pyrite catalysis of GGG hydrolysis was found to be saturable, suggesting the presence of discrete catalytic sites on the mineral surface. The mineral-catalyzed degradation of GGG appears to occur via a GGG → DKP + G mechanism, rather than via GGG → GG + G, as in solution-phase reactions. These results are compatible with many previous findings and suggest that minerals may have assisted in peptide synthesis in certain geological settings, specifically by speeding the approach to equilibrium in environments where amino acids were already highly concentrated, but that minerals may not significantly alter the expected solution-phase equilibria. Thus the abiotic synthesis of long peptides may have required activating agents, dry heating at higher temperatures, or some form of phase separation. 相似文献
8.
Luís Pinheiro Branco António Topa Gomes António Silva Cardoso Carla Santos Pereira 《Geotechnical and Geological Engineering》2014,32(4):911-922
The renewal imposed by the Eurocodes regarding the methodologies of safety evaluation requires a statistical analysis of the variability of ground geotechnical parameters. However, the studies published in the reviewed literature do not cover the typical materials from the northeast region of Portugal—residual soils from granite—to which a strong heterogeneity is associated. Hence, a statistical characterization of the natural variability of a granite residual soil from Porto has been made through a significant amount of experimental tests, focusing on its geomechanical properties. In order to provide a database for probabilistic analysis of problems involving this type of soils, an appropriate statistical law has been used to model its variability, which has been quantified by means of coefficients of variation and scales of fluctuation. 相似文献
9.
ABSTRACT This paper presents new geochemical data on gas-hydrate-bearing mud volcanoes discovered for the first time in the Gulf of Cadiz during cruises TTR9 and TTR10 of the R/V Professor Logachev in 1999–2000. The estimated gas hydrate content is 3–16% of sediment volume and 5–31% of pore space volume. Estimated values of the water isotopic composition for the Ginsburg mud volcano are very heavy for δ18 O (up to +53‰) and light for δD (up to − 210‰). Gas released from the hydrates contains 81% of C1 and 19% of C2+ . The inferred source of the gas in the hydrates is enriched in C2 –C6 (≤ 5%), indicating that the gas has a thermogenic origin. Gas hydrate of cubic structure II should be formed from a gas of such composition. It is interpreted that the composition of the mud volcano fluid corresponds to deep oil basins below the Gulf of Cadiz. 相似文献
10.