Wisconsinan Late-glacial environmental change in southern New England: A regional synthesis     

D. M. Peteet  R. Daniels  L. E. Heusser  J. S. Vogel  J. R. Southon  D. E. Nelson 《第四纪科学杂志》1994,9(2):151-154

A synthesis of vegetational and climatic history during the Wisconsinan Late-glacial (13-9 ka BP) is presented for the southern New England region. The interpretations are based on a number of pollen-stratigraphical investigations supported by plant macrofossil data. Chronology is based partly on AMS dates of plant macrofossils. Reference is made briefly to the controversial nature of the evidence for deglaciation in the region. A curve representing mean July temperature changes in the region during the period 13-9 ka BP is provided.  相似文献   

Sulfur flows of Ra Patera,Io     

D.C. Pieri  S.M. Baloga  R.M. Nelson  Carl Sagan 《Icarus》1984,60(3):685-700

Voyager 1 imaging data have been used to investigate the color and morphology of several radial flow-like features at Ra Patera, a broad volcanic structure at approximately 8° latitude and 325° longitude on the Galilean satellite Io (J1). It was found that downstream progressions of flow color and morphology are consistent with lava of a predominately sulfur composition cooling radiatively and erupting in the range of 470 to 520°K at effusion rates at 10¹⁰ to 10¹¹ cm³/sec. This implies global resurfacing rates by volcanic flows on Io of the order of 1 cm/year. Calculated energy content and effusion rates for flows at Ra Patera, using the physical parameters of sulfur, are of the order of the largest known terrestial basaltic eruptions and are consistent with calculations of globally available energy.  相似文献   

Sedimentary, tectonic, and sea-level controls on submarine fan and slope-apron turbidite systems     

Dorrik A. V. Stow  David G. Howell  C. Hans Nelson 《Geo-Marine Letters》1984,3(2-4):57-64

To help understand factors that influence submarine fan deposition, we outline some of the principal sedimentary, tectonic, and sea-level controls involved in deep-water sedimentation, give some data on the rates at which they operate, and evaluate their probable effects. Three depositional end-member systems, two submarine fan types (elongate and radial), and a third nonfan, slope-apron system result primarily from variations in sediment type and supply. Tectonic setting and local and global sea-level changes further modify the nature of fan growth, the distribution of facies, and the resulting vertical stratigraphic sequences. Margin setting represents fan and/or source area  相似文献   

Flood pattern and weather determine Populus leaf litter breakdown and nitrogen dynamics on a cold desert floodplain     

D.C. Andersen  S.M. Nelson 《Journal of Arid Environments》2006,64(4):626-650

Patterns and processes involved in litter breakdown on desert river floodplains are not well understood. We used leafpacks containing Fremont cottonwood (Populus deltoides subsp. wislizenii) leaf litter to investigate the roles of weather and microclimate, flooding (immersion), and macroinvertebrates on litter organic matter (OM) and nitrogen (N) loss on a floodplain in a cool-temperate semi-arid environment (Yampa River, northwestern Colorado, USA). Total mass of N in fresh autumn litter fell by 20% over winter and spring, but in most cases there was no further N loss prior to termination of the study after 653 days exposure, including up to 20 days immersion during the spring flood pulse. Final OM mass was 10–40% of initial values. The pattern of OM and N losses suggested most N would be released outside the flood season, when retention within the floodplain would be likely. The exclusion of macroinvertebrates modestly reduced the rate of OM loss (by about 10%) but had no effect on N dynamics over nine months. Immersion in floodwater accelerated OM loss, but modest variation in litter quality did not affect the breakdown rate. These results are consistent with the concept that decomposition on desert floodplains progresses much as does litter processing in desert uplands, but with periodic bouts of processing typical of aquatic environments when litter is inundated by floodwaters. The strong dependence of litter breakdown rate on weather and floods means that climate change or river flow management can easily disrupt floodplain nutrient dynamics.  相似文献   

Current state and trends in Canadian Arctic marine ecosystems: II. Heterotrophic food web, pelagic-benthic coupling, and biodiversity     

Gérald Darnis  Dominique Robert  Corinne Pomerleau  Heike Link  Philippe Archambault  R. John Nelson  Maxime Geoffroy  Jean-éric Tremblay  Connie Lovejoy  Steve H. Ferguson  Brian P. V. Hunt  Louis Fortier 《Climatic change》2012,115(1):179-205

As part of the Canadian contribution to the International Polar Year (IPY), several major international research programs have focused on offshore arctic marine ecosystems. The general goal of these projects was to improve our understanding of how the response of arctic marine ecosystems to climate warming will alter food web structure and ecosystem services provided to Northerners. At least four key findings from these projects relating to arctic heterotrophic food web, pelagic-benthic coupling and biodiversity have emerged: (1) Contrary to a long-standing paradigm of dormant ecosystems during the long arctic winter, major food web components showed relatively high level of winter activity, well before the spring release of ice algae and subsequent phytoplankton bloom. Such phenological plasticity among key secondary producers like zooplankton may thus narrow the risks of extreme mismatch between primary production and secondary production in an increasingly variable arctic environment. (2) Tight pelagic-benthic coupling and consequent recycling of nutrients at the seafloor characterize specific regions of the Canadian Arctic, such as the North Water polynya and Lancaster Sound. The latter constitute hot spots of benthic ecosystem functioning compared to regions where zooplankton-mediated processes weaken the pelagic-benthic coupling. (3) In contrast with another widely shared assumption of lower biodiversity, arctic marine biodiversity is comparable to that reported off Atlantic and Pacific coasts of Canada, albeit threatened by the potential colonization of subarctic species. (4) The rapid decrease of summer sea-ice cover allows increasing numbers of killer whales to use the Canadian High Arctic as a hunting ground. The stronger presence of this species, bound to become a new apex predator of arctic seas, will likely affect populations of endemic arctic marine mammals such as the narwhal, bowhead, and beluga whales.  相似文献   

Unstable climates: Exploring the statistical and social constructions of ‘normal’ climate     

Mike Hulme  Suraje Dessai  Irene Lorenzoni  Donald R. Nelson 《Geoforum》2009,40(2):197-206

The idea of climate has both statistical and social foundations. Both of these dimensions of climate change over time: climate, as defined by meteorological statistics, changes for both natural and anthropogenic reasons; and our expectations of future climate also change, as cultures, societies and knowledge evolves. This paper explores the interactions between these different expressions of climate change by focusing on the idea of ‘normal’ climates defined by statistics. We show how this idea came into being in meteorological circles and then review how this idea of climatic normality gets entangled with cultural and psychological processes. Using data from historical and predicted climates in the UK, we illustrate the significance of choosing different baseline ‘normals’ for retrospective and prospective interpretations of climate change. Since the choice of these statistical ‘normals’ reflects cultural, political and psychological preferences and practices as much as scientific ones, we argue that expectations of the climatic future are influenced by social as well as statistical norms. Seeing climate as co-constructed between the psycho-cultural constraints of society and the physical constraints of the material world offers a different way of thinking about the instabilities of climate and the ways we adapt to them.  相似文献   

Distribution of icy particles across Enceladus' surface as derived from Cassini-VIMS measurements     

R. Jaumann  K. Stephan  R.N. Clark  R.H. Brown  S.F. Newman  G. Filacchione  D.P. Cruikshank  C.A. Hibbitts  R.M. Nelson  C. Sotin 《Icarus》2008,193(2):407-419

The surface of Enceladus consists almost completely of water ice. As the band depths of water ice absorptions are sensitive to the size of particles, absorptions can be used to map variations of icy particles across the surface. The Visual and Infrared Mapping Spectrometer (VIMS) observed Enceladus with a high spatial resolution during three Cassini flybys in 2005 (orbits EN 003, EN 004 and EN 011). Based on these data we measured the band depths of water ice absorptions at 1.04, 1.25, 1.5, and 2 μm. These band depths were compared to water ice models that represent theoretically calculated reflectance spectra for a range of particle diameters between 2 μm and 1 mm. The agreement between the experimental (VIMS) and model values supports the assumption that pure water ice characterizes the surface of Enceladus and therefore that variations in band depth correspond to variations in water ice particle diameters. Our measurements show that the particle diameter of water ice increases toward younger tectonically altered surface units with the largest particles exposed in relatively “fresh” surface material. The smallest particles were generally found in old densely cratered terrains. The largest particles (∼0.2 mm) are concentrated in the so called “tiger stripes” at the south pole. In general, the particle diameters are strongly correlated with geologic features and surface ages, indicating a stratigraphic evolution of the surface that is caused by cryovolcanic resurfacing and impact gardening.  相似文献   

Pilot study experiments sourcing quartzite,Gunnison Basin,Colorado     

Bonnie L. Pitblado  Carol Dehler  Hector Neff  Stephen T. Nelson 《Geoarchaeology》2008,23(6):742-778

This paper reports the results of pilot‐study efforts to develop methods to profile quartzite, a rock type to which geochemical and other sourcing techniques have only rarely been applied. The long‐term goal of the research is to fingerprint sources of quartzite in the Gunnison Basin, southwest Colorado, used by Paleoindian people ca. 11,500–8,000 years ago to make stone tools. Success would facilitate reconstruction of Paleoindian mobility in the Southern Rocky Mountains and potentially anywhere prehistoric people used quartzite. The goals of this paper are more modest: to demonstrate that a small‐scale exploration of sourcing techniques suggests reason for optimism that quartzites may be amenable to source discrimination. For the same twenty Gunnison Basin quartzite samples, this study evaluated petrography, ultraviolet fluorescence (UVF), wavelength dispersive X‐ray fluorescence (WD‐XRF), instrumental neutron activation analysis (INAA), and inductively coupled plasma mass spectrometry—both acid‐digestion (AD‐ICP‐MS) and laser ablation (LA‐ICP‐MS)—as means to differentiate among the specimens and the sources they represent. Although more testing is needed to verify and refine our results, the study suggests there is potential for petrography, INAA, and both versions of ICP‐MS to discriminate among quartzites from different source localities in the Gunnison Basin. The greatest potential for discriminating among different sources of quartzite in the Gunnison Basin may lie in a methodology combining petrographic analysis and LA‐ICPMS. Future testing is required to evaluate this two‐fold approach. © 2008 Wiley Periodicals, Inc.  相似文献   

PERMAFROST ZONATION AND CLIMATE CHANGE IN THE NORTHERN HEMISPHERE: RESULTS FROM TRANSIENT GENERAL CIRCULATION MODELS   总被引：6，自引：1，他引：5  

OLEG A. Anisimov  FREDERICK E. Nelson 《Climatic change》1997,35(2):241-258

Numerous studies have demonstrated that both global patterns and local details of permafrost distribution are highly responsive to climatic fluctuations, at several temporal and spatial scales. Permafrost currently underlies about one fourth of the land area of the northern hemisphere, and many qualitative predictions have been made for a severe reduction of this area in response to global warming. A map of permafrost distribution compiled using the frost index, a dimensionless number that can be related to the zonal arrangement of permafrost, shows very good correspondence with a recently published empirical map. The frost index was used in conjunction with three transient general circulation models to compile maps of permafrost zonation for conditions in the mid-21st century. Although regional patterns and local details differ substantially between the three scenarios, all result in reductions in the area occupied by each permafrost zone. Localized expansions of the area underlain by permafrost are apparent from two of the scenarios. Reductions in the areal extent of equilibrium permafrost predicted from two of the three transient models are much less than those indicated by runs using 2 × CO models.  相似文献   

Hydrodynamic simulations of propagating warps and bending waves in accretion discs     

Richard P. Nelson  John C. B. Papaloizou 《Monthly notices of the Royal Astronomical Society》1999,309(4):929-940

We present the results of a study of propagating warp or bending waves in accretion discs. Three-dimensional hydrodynamic simulations were performed using smoothed particle hydrodynamics (SPH), and the results are compared with calculations based on the linear theory of warped discs.
We examine the response of a gaseous disc to an initially imposed warping disturbance under a variety of physical conditions. We consider primarily the physical regime in which the dimensionless viscosity parameter α < H r , where H r is the disc aspect ratio, so that bending waves are expected to propagate. We also performed calculations for disc models in which α > H r , where the warps are expected to evolve diffusively. Small-amplitude (linear) perturbations are studied in both Keplerian and slightly non-Keplerian discs, and we find that the results of the SPH calculations can be reasonably well fitted by those of the linear theory. The main results of these calculations are: (i) the warp in Keplerian discs when α < H r propagates with little dispersion, and damps at a rate expected from estimates of the code viscosity; (ii) warps evolve diffusively when α > H r ; (iii) the slightly non-Keplerian discs lead to a substantially more dispersive behaviour of the warps, which damp at a similar rate to the Keplerian case, when α < H r .
Initially imposed higher amplitude, non-linear warping disturbances were studied in Keplerian discs. The results indicate that non-linear warps can lead to the formation of shocks, and that the evolution of the warp becomes less wave-like and more diffusive in character.
This work is relevant to the study of the warped accretion discs that may occur around Kerr black holes or in misaligned binary systems, and is mainly concerned with discs in which α < H r . The results indicate that SPH can model the hydrodynamics of warped discs, even when using rather modest numbers of particles.  相似文献