Rapid Holocene hydrologic change along boreal treeline revealed by δ13C and δ18O in organic lake sediments, Northwest Territories, Canada     

Brent B. Wolfe  Thomas W. D. Edwards  Ramon Aravena  Glen M. MacDonald 《Journal of Paleolimnology》1996,15(2):171-181

Analysis of ¹⁸O_cellulose, ¹³C_{organic matter}, and ¹³C_cellulose at about 100 year intervals from organic matter deposited in Toronto Lake, Northwest Territories, Canada, revealed an 8000-year history of rapid, post-glacial hydrologic change at the treeline zone. Several mid-Holocene phases of enriched ¹³C_org and ¹³C_cell, caused by elevated lake productivity, declining [CO_2(aq)], and closed basin conditions, were abruptly terminated by intervals of open hydrology recorded by sharply depleted ¹⁸O_cell. Two of these events, at 5000 and 4500 BP, are correlated with increased total organic content and Picea mariana pollen concentration, which indicate that high levels of productivity were also accompanied by northern treeline advances. A third treeline advance at about 2500 BP is also marked by an apparent outflow event from Toronto Lake, but this was not associated with ¹³C_org/cell enrichment in the sediment record because rapid and substantial lake water renewal probably prevented productivity-driven enrichment of the dissolved inorganic carbon and replenished the CO_2(aq) supply to thriving phytoplankton. However, high sediment organic content during this period suggests increased productivity. Increases in the inflow:evaporation ratio at about 6500 and 3500 BP were also sufficient to cause Toronto Lake to overflow but the prevailing climate during these periods apparently did not favour appreciable northward treeline migration or changes in lake productivity.This is the 14th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers  相似文献   

Holocene Climate and the Development of a Subarctic Peatland near Inuvik, Northwest Territories, Canada     

Sheila R. Vardy  Barry G. Warner  Ramon Aravena 《Climatic change》1998,40(2):285-313

Multi-proxy paleoecological analyses were carried out on cores from a peatland in the continuous permafrost zone, to determine the effect of past climatic change on peatland development. Stratigraphy, bulk density and organic matter content were analyzed for eight cores from Campbell Creek peatland (69°17.3N, 133°15W). Detailed pollen and macrofossil analyses were carried out on two cores, and stable isotope analysis on one of those cores. The results indicate a succession, starting at 9000-9500 BP, from an open water mineral wetland with aquatic plants, to a fen dominated by Drepanocladus mosses and sedges, and then a change to a Sphagnum-dominated ombrotrophic vegetation typical of most of the peatland today. The beginning of the latter transition appears to coincide with the end of the early Holocene warm period. The physical stratigraphies of the other six cores indicate that a similar successional sequence occurred across the peatland, with some local variation. The transition to ombrotrophic conditions may be associated with the aggradation of permafrost in the peatland, in response to regional cooling. The effect of climate cooling on the peatland was probably indirect, through the aggradation of permafrost and associated hydrological changes.  相似文献   

Gas,dust and molecules in the Galaxy     

Ramon J. Quiroga 《Astrophysics and Space Science》1978,53(2):295-333

The relative abundances of cool neutral hydrogen, carbon monoxide and formaldehyde are studied using all the available observational data in the literature. The obtained mean valuesN _{H 1}/ ,N _{H 1}/N _CO,N _CO/ are approximately constant in the dark clouds of the solar neighbourhood and in the distant molecular clouds.The observed correlationsN _CO,A _v and ,A _v show that formaldehyde can also be used as an indicator of molecular hydrogen. The ratioN H₁/A _v depends on densities and decays considerably in the ranges of visual absorptions in which the molecules become detectable (A _v 2 mg); an average of /N _{H 1}10 is calculated for the dense dark clouds.Indications of systematic temperature gradiens T/A _v are found for formaldehyde and neutral hydrogen inside the dark clouds, and qualitative comparisons are made with theoretical quantum mechanics calculations.The observed carbon monoxide and formaldehyde abundances, the free electron layer in the Galaxy, the distribution of neutral hydrogen in different states are only compatible if an ionization rate of 10^–16 is accepted, provided presumably by 2 MeV protons of cosmic radiation.Three main states for neutral hydrogen and dust are identified from different kinds of observational data (21 cm line in emission, absorption in galactic radio sources and self-absorption in the hot gas background): (1) a homogeneous intercloud stratum of tenuous gas and dust with a galactic halfwidth of 350 pc and mean parametersn _H=0.2 atom cm^–3, spin temperatureT _s 10000 K andn _d 0.3 mg kpc^–1; (2) cool gas and dust concentrated in spiral features with a galactic half-width of less than 100 pc, probably forming clouds with diffuse and indefinite limits, with mean parametersn _H2 atom cm^–3,T _s <1100 K (probable average,T _s =135 K) andn _d 3 mg kpc^–1; (3) dense gas and dust clouds with a mean diameter of 7 pc and mean parametersn _H700 atom cm^–3 (90% in a molecular state),T _s 63 K andn _d 1 mg pc^–1 on which molecules as CO and H₂CO are formed.The application of the Jeans criteria for gravitational instability shows that the dense clouds are gravitationally bound while the gas in the intermediate state (2) can be protected against collapse by the total internal energy in the medium increasing due to cosmic rays and the magnetic field in the Galaxy.The observed velocity halfwidths and galacticZ-halfwidths in states (1) and (2) are compatible with a total mass density in the galactic layer of 90M pc^–2 (gas plus stars) according to the barometric equation.The relative abundancesN _{H 1}/N _CO, calculated from C¹²O and C¹³O data and comparisons with studies in the 21 cm emission line, show that the antenna temperatureT _A ⁺ in the 2.6 mm line of C¹²O is a good indicator of the cool gas densities in the Galaxy. The possible application of this for studies in galactic structure is discussed and hypothetical distributions of carbon monoxide in the zones outside the galactic planeB=0° are presented.From a synthesis based on the results obtained, a cycle is postulated for the neutral hydrogen in the Galaxy: condensation and cooling of gas molecular formation gravitational collapse and star formation gas dissipation and heating by cosmic rays and UV radiation.  相似文献   

Vulnerability and adaptation of coastal communities to climate variability and sea-level rise: Their implications for integrated coastal management in Cavite City,Philippines     

Ramon Faustino M. Sales 《Ocean & Coastal Management》2009,52(7):395-404

Coastal areas are vulnerable to the impacts of climate change and sea-level rise. These impacts will exacerbate the risks posed by the continuing environmental degradation confronting the coastal communities.Adopting a participatory research approach, the study examines the vulnerability of socioeconomic groups among the coastal population in Cavite City, Philippines, their current adaptation strategies and their adaptive capacity to cope with the impacts of climate variability and extremes and sea-level rise. Under a future scenario of a 1-m accelerated sea-level rise (ASLR), the study also looks into its potential effects on these urban coastal communities and ecosystems.In the context of poverty reduction and sustainable development, this study suggests a local framework for integrating adaptation strategies and actions into integrated coastal management (ICM) planning. It also recommends appropriate policy and institutional reform, capacity building and improved knowledge management towards increasing the resilience and adaptive capacity of these coastal communities to current and future climate risks.  相似文献   

Uppermost Lower Aptian transgressive records in Mexico and Spain: chronostratigraphic implications for the Tethyan sequences     

Josep A. Moreno‐Bedmar  Telm Bover‐Arnal  Ricardo Barragán  Ramon Salas 《地学学报》2012,24(4):333-338

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Hydrolytic ectoenzyme activity associated with suspended and sinking organic particles within the anoxic Cariaco Basin     

Gordon T. Taylor  Robert Thunell  Ramon Varela  Claudia Benitez-Nelson  Mary I. Scranton 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(8):1266-1283

Ectohydrolase activities of suspended microbiota were compared to those associated with sinking particles (sed-POM) retrieved from sediment traps deployed in the permanently anoxic Cariaco Basin. In shore-based assays, activities of aminopeptidase, β-glucosidase, chitinase and alkaline phosphatase were measured in samples obtained from oxic and anoxic depths using MUF- and MCA-labeled fluorogenic substrate analogs. Hydrolysis potentials for these enzymes in the seston varied widely over the nine cruises sampled (8 Nov 1996–3 May 2000) and among depths (15–1265 m); from <10 to over 1600 nM d^?1 hydrolysate released, generally co-varying with one another and with suspended particulate organic carbon (POC) and particulate nitrogen (PN). Hydrolytic potentials, prokaryotic abundances and POC/PN concentrations in sinking debris were 400–1.3×10⁷ times higher than in comparable volumes of seawater. However when normalized to PN, hydrolytic potentials in sediment trap samples were not demonstrably higher than in Niskin bottle samples. We estimate that PN pools in sediment trap samples were turned over 2–1400 times (medians=7–26x) slower by hydrolysis than were suspended PN pools. Median prokaryotic growth rates (divisions d^?1) in sinking debris were also ～150 times slower than for bacterioplankton. Hydrolytic potentials in surface oxic waters were generally faster than in underlying anoxic waters on a volumetric basis (nM hydrolysate d^?1), but were not significantly (p>0.05) different when normalized to PN or prokaryote abundances. Alkaline phosphatase was consistently the most active ectohydrolase in both sample types, suggesting that Cariaco Basin assemblages were adapted to decomposing phosphate esters in organic polymers. However, phosphorus limitation was not evident from nutrient inventories in the water column. Results support the hypothesis that efficiencies of polymer hydrolysis in anoxic waters are not inherently lower than in oxic waters.  相似文献   

Evolution of fault zones in carbonates with mechanical stratigraphy – Insights from scale models using layered cohesive powder     

Heijn W. van Gent  Marc Holland  Janos L. Urai  Ramon Loosveld 《Journal of Structural Geology》2010,32(9):1375-1391

We present analogue models of the formation of dilatant normal faults and fractures in carbonate fault zones, using cohesive hemihydrate powder (CaSO₄·½H₂O). The evolution of these dilatant fault zones involves a range of processes such as fragmentation, gravity-driven breccia transport and the formation of dilatant jogs. To allow scaling to natural prototypes, extensive material characterisation was done. This showed that tensile strength and cohesion depend on the state of compaction, whereas the friction angle remains approximately constant. In our models, tensile strength of the hemihydrate increases with depth from 9 to 50 Pa, while cohesion increases from 40 to 250 Pa. We studied homogeneous and layered material sequences, using sand as a relatively weak layer and hemihydrate/graphite mixtures as a slightly stronger layer. Deformation was analyzed by time-lapse photography and Particle Image Velocimetry (PIV) to calculate the evolution of the displacement field. With PIV the initial, predominantly elastic deformation and progressive localization of deformation are observed in detail. We observed near-vertical opening-mode fractures near the surface. With increasing depth, dilational shear faults were dominant, with releasing jogs forming at fault-dip variations. A transition to non-dilatant shear faults was observed near the bottom of the model. In models with mechanical stratigraphy, fault zones are more complex. The inferred stress states and strengths in different parts of the model agree with the observed transitions in the mode of deformation.  相似文献   

Major ion and isotope geochemistry of fluids and gases from coalbed methane and shallow groundwater wells in Alberta,Canada     

Katrina Cheung  Patrick Klassen  Bernhard Mayer  Fariborz Goodarzi  Ramon Aravena 《Applied Geochemistry》2010

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On-line access to ground water     

Ragone SE  Llamas MR 《Ground water》2006,44(4):500-500

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Discovery of Earth's quasi‐satellite     

Martin Connors  Christian Veillet  Ramon Brasser  Paul Wiegert  Paul Chodas  Seppo Mikkola  Kimmo Innanen 《Meteoritics & planetary science》2004,39(8):1251-1255

Abstract— The newly discovered asteroid 2003 YN₁₀₇ is currently a quasi‐satellite of the Earth, making a satellite‐like orbit of high inclination with apparent period of one year. The term quasi‐satellite is used since these large orbits are not completely closed, but rather perturbed portions of the asteroid's orbit around the Sun. Due to its extremely Earth‐like orbit, this asteroid is influenced by Earth's gravity to remain within 0.1 AU of the Earth for approximately 10 years (1997 to 2006). Prior to this, it had been on a horseshoe orbit closely following Earth's orbit for several hundred years. It will re‐enter such an orbit, and make one final libration of 123 years, after which it will have a close interaction with the Earth and transition to a circulating orbit. Chaotic effects limit our ability to determine the origin or fate of this object.  相似文献