Metrics with four conic singularities and spherical quadrilaterals

A spherical quadrilateral is a bordered surface homeomorphic to a closed disk, with four distinguished boundary points called corners, equipped with a Riemannian metric of constant curvature 1, except at the corners, and such that the boundary arcs between the corners are geodesic. We discuss the problem of classification of these quadrilaterals and perform the classification up to isometry in the case that two angles at the corners are multiples of pi. The problem is equivalent to classification of Heun's equations with real parameters and unitary monodromy.Comment: 68 pges, 25 figure

Spherical quadrilaterals with three non-integer angles

We classify spherical quadrilaterals up to isometry in the case when one inner angle is a multiple of pi while the other three are not. This is equivalent to classification of Heun's equations with real parameters and one apparent singularity such that the monodromy consists of unitary transformations.Comment: 38 pages, 16 figures. arXiv admin note: text overlap with arXiv:1409.152

Permafrost, landscape and ecosystem responses to late Quaternary warm stages in Northeast Siberia

Permafrost, landscape and ecosystem responses to late Quaternary warm stages in Northeast Siberia S. Wetterich1, F. Kienast2, L. Schirrmeister1, M. Fritz1, A. Andreev3, P. Tarasov4 1Alfred Wegener Institute for Polar and Marine Research, Department of Periglacial Research, Potsdam, Germany; 2Senckenberg Research Institute and Natural History Museum, Research Station for Quaternary Palaeontology, Weimar, Germany; 3Institute of Geology and Mineralogy, University of Cologne, Germany; 4Institute of Geological Sciences, Free University Berlin, Germany Perennially frozen ground is widely distributed in Arctic lowlands and beyond. Permafrost responds sensitive to changes in climate conditions. Climate-driven dynamics of landscape, sedimentation and ecology in periglacial regions are frequently recorded in permafrost deposits. The study of late Quaternary permafrost can therefore reveal past glacial-interglacial and stadialinterstadial environmental dynamics. One of the most striking processes under warming climate conditions is the extensive thawing of permafrost (thermokarst) and subsequent surface subsidence. Thermokarst basins promote the development of lakes, whose sedimentological and paleontological records give insights into past interglacial and interstadial (warm). In this paper we present results of qualitative and quantitative reconstructions of climate and environmental conditions for the last Interglacial (MIS 5e, Kazantsevo; ca. 130 to 115 ka ago), the lateglacial Allerød Interstadial (ca. 13 to 11 uncal. ka BP), and the early Holocene (ca. 10.5 to 8 uncal. ka BP). The study was performed in course of the IPY project #15 ‘Past Permafrost’ with permafrost deposits exposed at the coasts of the Dmitry Laptev Strait (East Siberian Sea, East Siberia). The reconstruction based on fossil-rich findings of plants (pollen, macro-remains) and invertebrates (beetles, chironomids, ostracods gastropods). Interglacial vegetation dynamics are reflected in the pollen records by changes from early interglacial grass-sedge-tundra to shrub-tundra during the interglacial thermal optimum followed by grass-sedge-tundra vegetation at the end of the Kazantsevo warm period. Terrestrial beetle and plant remains prove the former existence of open forest tundra with Dahurian larch, grey alder and boreal shrubs interspersed with patches of steppes and meadows during the interglacial thermal optimum. Mean temperature reconstructions of the warmest month (MTWA, TJuly) for the interglacial thermal optimum are based on quantitative chironomid transfer functions revealed a TJuly of 12.9 ± 0.9 °C. The TJuly reconstructed by plant macrofossils amounts to 13.2 ± 0.5 °C, and the pollen-based TJuly reaches 14.3 ± 3.3 °C. Low net precipitation is reflected by steppe plants and beetles. The temperature reconstruction based on three independent approaches. Nethertheless, all methods consistently indicate an interglacial TJuly about 10 °C higher than today, which is interpreted as a result of a combination of increased insolation and higher climatic continentality during the last Interglacial. Grass-sedge dominated tundra vegetation occurred during the lateglacial to Holocene transition which was replaced by shrub tundra during the early Holocene. The presence of Salix and Betula pollen reflects temperatures about 4 °C higher than present between 12 to 11 uncal. ka BP, during the Allerød Interstadial, but shrubs disappeared in the following Younger Dryas stadial, reflecting a climate deterioration. Alnus fruticosa, Betula nana, Poaceae and Cyperaceae dominate early Holocene pollen spectra. Pollen-based reconstructions point to TJuly 4 °C warmer than present. Shrubs gradually disappeared from coastal areas after 7.6 uncal. ka BP when vegetation cover became similar to modern wet tundra. Thermokarst acted as response to warming conditions on landscape scale in permafrost regions. Concurrent changes in relief, hydrology and ecosystems are obvious and detectable by analyses of the paleontological record preserved in thermokarst deposits

Fast Traveling-Wave Reactor of the Channel Type

The main aim of this paper is to solve the technological problems of the TWR based on the technical concept described in our priority of invention reference, which makes it impossible, in particular, for the fuel claddings damaging doses of fast neutrons to excess the ~200 dpa limit. Thus the essence of the technical concept is to provide a given neutron flux at the fuel claddings by setting the appropriate speed of the fuel motion relative to the nuclear burning wave. The basic design of the fast uranium-plutonium nuclear traveling-wave reactor with a softened neutron spectrum is developed, which solves the problem of the radiation resistance of the fuel claddings material.Comment: 18 pages, 5 figures, 2 table

Eurasian perspective

Reproducing the tree cover changes throughout the Holocene is a challenge for land surface–atmosphere models. Here, results of a transient Holocene simulation of the coupled climate–carbon cycle model, CLIMBER2-LPJ, driven by changes in orbital forcing, are compared with pollen data and pollen-based reconstructions for several regions of Eurasia in terms of changes in tree fraction. The decline in tree fraction in the high latitudes suggested by data and model simulations is driven by a decrease in summer temperature over the Holocene. The cooler and drier trend at the eastern side of the Eurasian continent, in Mongolia and China, also led to a decrease in tree cover in both model and data. In contrast, the Holocene trend towards a cooler climate in the continental interior (Kazakhstan) is accompanied by an increase in woody cover. There a relatively small reduction in precipitation was likely compensated by lower evapotranspiration in comparison to the monsoon-affected regions. In general the model-data comparison demonstrates that climate-driven changes during the Holocene result in a non-homogeneous pattern of tree cover change across the Eurasian continent. For the Eifel region in Germany, the model suggests a relatively moist and cool climate and dense tree cover. The Holzmaar pollen record agrees with the model for the intervals 8–3 ka and 1.7–1.3 ka BP, but suggests great reduction of the tree cover 3–2 ka and after 1.3 ka BP, when highly developed settlements and agriculture spread in the region