Late-Quaternary biogeographic scenarios for the brown bear (Ursus arctos), a wild mammal model species     

John Davison  Simon Y.W. Ho  Sarah C. Bray  Marju Korsten  Egle Tammeleht  Maris Hindrikson  Kjartan Østbye  Eivind Østbye  Stein-Erik Lauritzen  Jeremy Austin  Alan Cooper  Urmas Saarma 《Quaternary Science Reviews》2011,30(3-4):418-430

This review provides an up-to-date synthesis of the matrilineal phylogeography of a uniquely well-studied Holarctic mammal, the brown bear. We extend current knowledge by presenting a DNA sequence derived from one of the earliest known fossils of a polar bear (dated to 115 000 years before present), a species that shares a paraphyletic mitochondrial association with brown bears. A molecular clock analysis of 140 mitochondrial DNA sequences, including our new polar bear sequence, provides novel insights into the times of origin for different brown bear clades. We propose a number of regional biogeographic scenarios based on genetic data, divergence time estimates and paleontological records. The case of the brown bear provides an example for researchers working with less well-studied taxa: it shows clearly that phylogeographic models based on patterns of modern genetic variation alone can be substantially improved by including data on historical patterns of genetic diversity in the form of ancient DNA sequences derived from accurately dated samples and by using an approach to divergence-time estimation that suits the data under analysis. Using such approaches it has been possible to (i) establish that the processes shaping modern genetic diversity in brown bears acted recently, within the last three glacial cycles; (ii) distinguish among hypotheses concerning species’ responses to climatic oscillations in accordance with the lack of phylogeographic structure that existed in brown bears prior to the last glacial maximum (LGM); (iii) reassess theories linking monophyletic brown bear populations to particular LGM refuge areas; and (iv) identify vicariance events and track analogous patterns of migration by brown bears out of Eurasia to North America and Japan.  相似文献   

A laboratory study of the biogeochemical cycling of Fe,Mn, Zn and Cu across the sediment-water interface of a productive lake     

John Hamilton-Taylor  William Davison  Keith Morfett 《Aquatic Sciences - Research Across Boundaries》1996,58(3):191-209

Laboratory incubation experiments were carried out on sediment cores collected from Esthwaite Water, U.K., during April 1987, when the sediments displayed a characteristic surface (1.5 to 2 cm) oxide floc. The experiments were undertaken at 10°C, in the dark, under variable redox and pH conditions for periods of ~ 720 h (30 d). In some cases, realistic amounts of decomposing lake algae were added to simulate the deposition of an algal bloom. Pore waters and overlying waters were obtained from the incubated sediment cores at various time intervals and the samples analysed for pH and dissolved Fe, Mn, Zn and Cu by AAS. The results demonstrated that trace metal concentrations at the sediment-water interface can show rapid, pulsed responses to episodic events associated with controlling factors such as algal deposition and mixing conditions. The variations in dissolved Fe and Mn concentrations could generally be explained by their well known redox behaviour. Appreciable loss of Mn from solution under conditions of well-developed anoxia was consistent with adsorption of Mn²⁺ by FeS. Cu and Zn were both rapidly (24 h) released into solution during incubation of sediment cores prior to the development of anoxia in the overlying waters. Their most likely sources were the reductive remobilization of Mn oxides and the decomposition of organic matter. The addition of decomposing algae to a series of cores resulted in even higher interfacial dissolved concentrations of Cu and Zn, probably through acting as a supplementary source of the metals and through increased oxide dissolution. Switching from anoxic to oxic conditions also rapidly increased dissolved Cu and Zn concentrations, possibly due to their release during the oxidation of metal sulphides. The enhanced releases of dissolved Cu and Zn were generally short-lived with removal being attributed to the formation of sulphides during anoxia and to adsorption by Fe and Mn oxides under oxic conditions.  相似文献   

The Determination of Major Oxide and Trace Element Concentrations in Eighteen Chinese Standard Reference Samples by X-ray Fluorescence Spectrometry     

Davison V. VIVIT  Bi-Shia Wang KING 《Geostandards and Geoanalytical Research》1988,12(2):363-370

Eighteen Chinese geologic reference samples (stream sediments GSD 9–12, soils GSS 1–8, and rocks GSR 1–6) were analyzed by wavelength-dispersive X-ray fluorescence spectrometry (XRFS) for major elements Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P, and by energy-dispersive XRFS for trace elements Ba, Ce, Cr, Cu, La, Nb, Ni, Rb, Sr, Y, Zn, and Zr. Major element analysis followed gravimetric determination of loss on ignition, and samples were prepared by fusion with Li₂B₄O₇. A loose-powder sample preparation was used for trace element analysis. The results reported in this study are, generally, in good agreement with concentrations compiled by X. Xie of the Geochemical Standard Reference Group and the Institute of Geophysical and Geochemical Exploration (People's Republic of China).  相似文献   

Manganese chemistry in rivers and streams     

Duncan P.H. Laxen  William Davison  Colin Woof 《Geochimica et cosmochimica acta》1984,48(10):2107-2111

The speciation of Mn has been determined in 15 rivers and streams representing a wide variety of physico-chemical conditions. Using the technique of anodic stripping voltammetry (asv), specific for reduced Mn(II) species, it is found that a major part of the <0.015 μm Mn size fraction is present in a reduced Mn(II), asv-labile, form. In some waters there is also a significant asv inactive Mn fraction considered to be present as a ‘small colloidal’ species. The soluble (<0.015 μm) Mn fraction represents 15–95% of total Mn and does not appear to be dependent upon pH, alkalinity, specific conductance or humic substance concentration in the water. It is argued that under the dynamic, short residence time, conditions that apply in most rivers the paniculate and soluble Mn fractions are decoupled, their respective presence being dependent principally upon the catchment hydrogeological conditions. This contrasts with a previously held view that the paniculate phase is coupled to the dissolved phase by the pH dependent oxidation of dissolved Mn(II) to highly insoluble Mn(IV) species (Graham et al., 1976). Consideration of manganese speciation in waters which were incubated for five months showed that pH becomes the controlling factor when equilibrium is approached.  相似文献   

A numerical assessment of simple airblast models of impact airbursts          下载免费PDF全文

Gareth S. Collins  Elliot Lynch  Ronan McAdam  Thomas M. Davison 《Meteoritics & planetary science》2017,52(8):1542-1560

Asteroids and comets 10–100 m in size that collide with Earth disrupt dramatically in the atmosphere with an explosive transfer of energy, caused by extreme air drag. Such airbursts produce a strong blastwave that radiates from the meteoroid's trajectory and can cause damage on the surface. An established technique for predicting airburst blastwave damage is to treat the airburst as a static source of energy and to extrapolate empirical results of nuclear explosion tests using an energy‐based scaling approach. Here we compare this approach to two more complex models using the iSALE shock physics code. We consider a moving‐source airburst model where the meteoroid's energy is partitioned as two‐thirds internal energy and one‐third kinetic energy at the burst altitude, and a model in which energy is deposited into the atmosphere along the meteoroid's trajectory based on the pancake model of meteoroid disruption. To justify use of the pancake model, we show that it provides a good fit to the inferred energy release of the 2013 Chelyabinsk fireball. Predicted overpressures from all three models are broadly consistent at radial distances from ground zero that exceed three times the burst height. At smaller radial distances, the moving‐source model predicts overpressures two times greater than the static‐source model, whereas the cylindrical line‐source model based on the pancake model predicts overpressures two times lower than the static‐source model. Given other uncertainties associated with airblast damage predictions, the static‐source approach provides an adequate approximation of the azimuthally averaged airblast for probabilistic hazard assessment.  相似文献   

Bridges or fords? Geographical Association branches and higher education     

Richard Yarwood  Teresa Davison 《Area》2007,39(4):544-550

  相似文献   

I. Pulford and H. Flowers: Environmental Chemistry at a Glance     

William Davison 《Journal of Atmospheric Chemistry》2007,57(1):105-105

  相似文献   

Controls on soil pore water solutes: An approach for distinguishing between biogenic and lithogenic processes     

Art F. White  Marjorie S. Schulz  Davison V. Vivit  Alex E. Blum  David A. Stonestrom 《Journal of Geochemical Exploration》2006,88(1-3):363

Spatial and temporal variations in pore water compositions are characterized for a deep regolith profile developed on a marine terrace chronosequence near Santa Cruz California. Variations are resolved in terms of the dominance of either a lithogenic process, i.e. chemical weathering, or a biogenic process, i.e. plant nutrient cycling. The concept of elemental fractionation is introduced describing the extent that specific elements are mobilized and cycled as a result of these processes.  相似文献   

The effect of impact obliquity on shock heating in planetesimal collisions     

Thomas M. Davison  Fred J. Ciesla  Gareth S. Collins  Dirk Elbeshausen 《Meteoritics & planetary science》2014,49(12):2252-2265

Collisions between planetesimals in the early solar system were a common and fundamental process. Most collisions occurred at an oblique incidence angle, yet the influence of impact angle on heating in collisions is not fully understood. We have conducted a series of shock physics simulations to quantify oblique heating processes, and find that both impact angle and target curvature are important in quantifying the amount of heating in a collision. We find an expression to estimate the heating in an oblique collision compared to that in a vertical incidence collision. We have used this expression to quantify heating in the Rhealsilvia‐forming impact on Vesta, and find that there is slightly more heating in a 45° impact than in a vertical impact. Finally, we apply these results to Monte Carlo simulations of collisional processes in the early solar system, and determine the overall effect of impact obliquity from the range of impacts that occurred on a meteorite parent body. For those bodies that survived 100 Myr without disruption, it is not necessary to account for the natural variation in impact angle, as the amount of heating was well approximated by a fixed impact angle of 45°. However, for disruptive impacts, this natural variation in impact angle should be accounted for, as around a quarter of bodies were globally heated by at least 100 K in a variable‐angle model, an order of magnitude higher than under an assumption of a fixed angle of 45°.  相似文献   

Thermal consequences of impacts in the early solar system     

Fred J. Ciesla  Thomas M. Davison  Gareth S. Collins  David P. O'Brien 《Meteoritics & planetary science》2013,48(12):2559-2576

Collisions between planetesimals were common during the first approximately 100 Myr of solar system formation. Such collisions have been suggested to be responsible for thermal processing seen in some meteorites, although previous work has demonstrated that such events could not be responsible for the global thermal evolution of a meteorite parent body. At this early epoch in solar system history, however, meteorite parent bodies would have been heated or retained heat from the decay of short‐lived radionuclides, most notably ²⁶Al. The postimpact structure of an impacted body is shown here to be a strong function of the internal temperature structure of the target body. We calculate the temperature–time history of all mass in these impacted bodies, accounting for their heating in an onion‐shell–structured body prior to the collision event and then allowing for the postimpact thermal evolution as heat from both radioactivities and the impact is diffused through the resulting planetesimal and radiated to space. The thermal histories of materials in these bodies are compared with what they would be in an unimpacted, onion‐shell body. We find that while collisions in the early solar system led to the heating of a target body around the point of impact, a greater amount of mass had its cooling rates accelerated as a result of the flow of heated materials to the surface during the cratering event.  相似文献