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251.
Carole Nehme Andrew Farrant Daniel Ballesteros Dominique Todisco Joel Rodet Diana Sahy J. Michael Grappone Jean-Claude Staigre Damase Mouralis 《地球表面变化过程与地形》2020,45(8):1860-1876
Quantifying rates of river incision and continental uplift over Quaternary timescales offer the potential for modelling landscape change due to tectonic and climatic forcing. In many areas, river terraces form datable archives that help constrain the timing and rate of valley incision. However, old river terraces, with high-level deposits, are prone to weathering and often lack datable material. Where valleys are incised through karst areas, caves and sediments can be used to reconstruct the landscape evolution because they can record the elevation of palaeo-water tables and contain preserved datable material. In Normandy (N. France), the Seine River is entrenched into an extensive karstic chalk plateau. Previous estimates of valley incision were hampered by the lack of preserved datable fluvial terraces. A stack of abandoned phreatic cave passages preserved in the sides of the Seine valley can be used to reconstruct the landscape evolution of the region. Combining geomorphological observations, palaeomagnetic and U/Th dating of speleothem and sediments in eight caves along the Lower Seine valley, we have constructed a new age model for cave development and valley incision. Six identified cave levels up to ∼100 m a.s.l. were formed during the last ~1 Ma, coeval with the incision of the Seine River. Passage morphologies indicate that the caves formed in a shallow phreatic/epiphreatic setting, modified by sediment influxes. The valley's maximum age is constrained by the occurrence of late Pliocene marine sand. Palaeomagnetic dating of cave infills indicates that the highest-level caves were being infilled prior to 1.1 Ma. The evidence from the studied caves, complemented by fluvial terrace sequences, indicates that rapid river incision occurred during marine isotope stage (MIS) 28 to 20 (0.8–1 Ma), with maximal rates of ~0.30 m ka−1, dropping to ~0.08 m ka−1 between MIS 20–11 (0.8–0.4 Ma) and 0.05 m ka−1 from MIS 5 to the present time. © 2020 John Wiley & Sons, Ltd. 相似文献
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253.
Diana P. Kjurkchieva 《Astrophysics and Space Science》1989,159(2):333-338
In the framework of the direct photometric problem we obtained analytic expressions describing the light variations of spotted stars in case of arbitrary limb-darkening law. These could be used as a basis for solution of the inverse photometric problem for arbitrary types of rotating stars characterized by surface spots. 相似文献
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256.
Diana Menzel Stefan Schouten Pim F. van Bergen Jaap S. Sinninghe Damst 《Organic Geochemistry》2004,35(11-12):1343
The δ13C values of higher plant wax C27–33 n-alkanes were determined in three, time-equivalent Pliocene (2.943 Ma) sapropels and homogeneous calcareous ooze from three different sites forming an east-west transect in the eastern Mediterranean Basin in order to study the composition of the vegetation on the continents surrounding the Mediterranean Sea. A two-end member mixing model transformed the measured δ13C values into the contribution of C4 plants to the terrestrial vegetation. These calculations indicated a high C4 plant contribution (i.e. 40–50%) in the periods just before and just after sapropel formation. During sapropel deposition the C4 plant contribution increased by up to 20% at all sites. This is interpreted to record the increased overall plant coverage of the Mediterranean borderlands resulting from the change in formerly barren desert areas into C4 grass-dominated savannahs as a response to the wetter climate during sapropel deposition. Enhanced accumulation rates (ARs) of long-chain n-alkanes (C27–33) and n-alkan-1-ols (C26–30) towards the middle of the sapropel in concert with a decrease in the Ti/Al ratio confirm an increased delivery of terrigenous organic matter at all sites. These biomarkers were probably predominantly fluvially transported to the Mediterranean Sea, not only by the Nile but by fossil wadi river systems on the northern African continent. 相似文献
257.
Ernesto Martínez Alfonso Aranda Yolanda Díaz-De-Mera Ana Rodríguez Diana Rodríguez Alberto Notario 《Journal of Atmospheric Chemistry》2004,48(3):283-299
The reaction of Cl with cyclohexanone (1) was investigated, for the first time, as a function of temperature (273–333 K) and at a low total pressure (1 Torr) with helium as a carrier gas using a discharge flow-mass spectrometry technique (DF-MS). The resulting Arrhenius expression is proposed, k
1= (7.7 ± 4.1) × 10–10 exp[–(540 ± 169)/T]. We also report a mechanistic study with the quantitative determination of the products of the reaction of Cl with cyclohexanone. The absolute rate constant derived from this study at 1 Torr of total pressure and room temperature is (1.3 ± 0.2) × 10–10 cm3 molecule–1 s–1. A yield of 0.94 ± 0.10 was found for the H-abstraction channel giving HCl. In relative studies, using a newly constructed relative rate system, the decay of cyclohexanone was followed by gas chromatography coupled with flame-ionisation detection. These relative measurements were performed at atmospheric pressure with synthetic air and room temperature. Rate constant measured using the relative method for reaction (1) is: (1.7 ± 0.3) × 10–10 cm3 molecule–1 s–1. Finally, results and atmospheric implications are discussed and compared with the reactivity with OH radicals. 相似文献
258.
Modelling Carbon Dynamics of Boreal Forest Ecosystems Using the Canadian Land Surface Scheme 总被引:1,自引:1,他引:1
The ecosystem carbon (C) and nitrogen (N) simulations recently implemented in the Canadian Land Surface Scheme (CLASS) are presented. The main calculations include plant photosynthesis, autotrophic respiration, root N uptake, litterfall, plant growth, and soil heterotrophic respiration. Model experiments are made on two boreal forest ecosystems, deciduous (aspen) and conifers (black spruce). Simulated plant, soil, and ecosystem CO2 exchanges are analysed on half-hourly, daily, and annual time scales and compared with tower eddy correlation flux measurements and estimates from various authors. Modeled daily ecosystem CO2 exchange explained86% and 54%, respectively, of the observed variance of eddy correlationflux at the aspen site and at the black spruce site. Annual results show that the aspen ecosystem was simulated as a C sink in both 1994 (+164 g Cm–2) and 1996 (+142 g C m–2), and the black spruce ecosystem wassimulated as a C sink in 1994 (+39 g C m–2) and 1995 (+25 g Cm–2), but as a C source in 1996 (–27 g C m–2). 相似文献
259.
David W. Muenow Diana G. Graham Norman W.K. Liu John R. Delaney 《Earth and Planetary Science Letters》1979,42(1):71-76
High-temperature mass spectrometric studies have been made to determine the distribution of volatiles within glassy rims of submarine pillow basalts dredged from the east rift zone of Kilauea volcano, Hawaii. The CO2/H2O mole ratio for glass-vapor inclusions within olivine phenocrysts in the glassy rims is greater than 30 : 1 compared to 0.06 for matrix glasses. Enclosing matrix glasses contain 0.53–0.74 wt.% H2O, 0.02–0.04 wt.% carbon, 0.08–0.12 wt.% sulfur, 0.012–0.028 wt.% chlorine and 0.012–0.077 wt.% fluorine. 相似文献
260.