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Sökning: L773:1752 0894 OR L773:1752 0908 > (2008-2009)

  • Resultat 1-8 av 8
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  • Battin, Tom J., et al. (författare)
  • The boundless carbon cycle
  • 2009
  • Ingår i: Nature Geoscience. - 1752-0894. ; 2:9, s. 598-600
  • Tidskriftsartikel (refereegranskat)
  • Christensen, Torben (författare)
  • Climate Science Patchy Peat
  • 2009
  • Ingår i: Nature Geoscience. - : Nature Publishing Group. - 1752-0908. ; 2:3, s. 163-164
  • Tidskriftsartikel (övrigt vetenskapligt)
  • Engwall, Erik, et al. (författare)
  • Earth’s ionospheric outflow dominated by hidden cold plasma
  • 2009
  • Ingår i: Nature Geoscience. - : Nature Publishing Group. - 1752-0894. ; 2:1, s. 24-27
  • Tidskriftsartikel (refereegranskat)abstract
    • The Earth constantly loses matter, mostly in the form of H+and O+ ions, through various outflow processes from the upper atmosphere and ionosphere. Most of these ions are cold (below 1 eV in thermal energy), but can still escape and travel farther out along the magnetic field lines into the magnetospheric tail lobes. The outflow has previously beenmeasured close to the Earth. To understand what fraction does not return but instead escapes, the measurements should be conducted at larger geocentric distances. However, at high altitudes the cold ions are normally invisible to spacecraft measurements, because the potential of a sunlit spacecraft exceeds the equivalent energy of the ions. Here we show that cold ions dominate in both flux and density in the distant magnetotail lobes, using a new measurement technique on the Cluster spacecraft. The total loss of cold hydrogen ions from the planet is inferred to be of the order of 1026 s−1, which is larger than the previously observed more energetic outflow. Quantification and insight of the loss processes of the Earth’s atmosphere and ionosphere are also important for understanding the evolution of atmospheres on other celestial bodies.
  • Kleman, Johan (författare)
  • Geomorphology : Where glaciers cut deep
  • 2008
  • Ingår i: Nature Geoscience. - 1752-0894. ; 1:6, s. 343-344
  • Tidskriftsartikel (populärvet., debatt m.m.)abstract
    • Stunning images of fjords are familiar to geologists, but their origins are less well known. A simple model suggests that topographic steering of ice and erosion proportional to ice discharge are sufficient to explain fjord formation during the Quaternary period.
  • Kolesnikov, Anton, et al. (författare)
  • Methane-derived hydrocarbons produced under upper-mantle conditions
  • 2009
  • Ingår i: Nature geosicence. - 1752-0894. ; 2:8, s. 566-570
  • Tidskriftsartikel (refereegranskat)abstract
    • There is widespread evidence that petroleum originates from biological processes(1-3). Whether hydrocarbons can also be produced from abiogenic precursor molecules under the high-pressure, high-temperature conditions characteristic of the upper mantle remains an open question. It has been proposed that hydrocarbons generated in the upper mantle could be transported through deep faults to shallower regions in the Earth's crust, and contribute to petroleum reserves(4,5). Here we use in situ Raman spectroscopy in laser-heated diamond anvil cells to monitor the chemical reactivity of methane and ethane under upper-mantle conditions. We show that when methane is exposed to pressures higher than 2 GPa, and to temperatures in the range of 1,000-1,500 K, it partially reacts to form saturated hydrocarbons containing 2-4 carbons (ethane, propane and butane) and molecular hydrogen and graphite. Conversely, exposure of ethane to similar conditions results in the production of methane, suggesting that the synthesis of saturated hydrocarbons is reversible. Our results support the suggestion that hydrocarbons heavier than methane can be produced by abiogenic processes in the upper mantle.
  • Muscheler, Raimund, et al. (författare)
  • Tree rings and ice cores reveal C-14 calibration uncertainties during the Younger Dryas
  • 2008
  • Ingår i: Nature Geoscience. - : Nature Publishing Group. - 1752-0908. ; 1:4, s. 263-267
  • Tidskriftsartikel (refereegranskat)abstract
    • The Younger Dryas interval during the Last Glacial Termination was an abrupt return to glacial-like conditions punctuating the transition to a warmer, interglacial climate. Despite recent advances in the layer counting of ice-core records of the termination, the timing and length of the Younger Dryas remain controversial. Also, a steep rise in the concentration of atmospheric radiocarbon at the onset of the interval, recorded primarily in the Cariaco Basin, has been difficult to reconcile with simulations of the Younger Dryas carbon cycle. Here we discuss a radiocarbon chronology from a tree-ring record covering the Late Glacial period that has not been absolutely dated. We correlate the chronology to ice-core timescales using the common cosmic production signal in tree-ring C-14 and ice-core Be-10 concentrations. The results of this correlation suggest that the Cariaco record may be biased by changes in the concentration of radiocarbon in the upper ocean during the early phase of the Younger Dryas climate reversal in the Cariaco basin. This bias in the marine record may also affect the accuracy of a widely used radiocarbon calibration curve over this interval. Our tree-ring-based radiocarbon record is easily reconciled with simulated production rates and carbon-cycle changes associated with reduced ocean ventilation during the Younger Dryas.
  • van de Schootbrugge, B., et al. (författare)
  • Floral changes across the Triassic/Jurassic boundary linked to flood basalt volcanism
  • 2009
  • Ingår i: Nature Geoscience. - : Nature Publishing Group. - 1752-0908. ; 2:8, s. 589-594
  • Tidskriftsartikel (refereegranskat)abstract
    • One of the five largest mass extinctions of the past 600 million years occurred at the boundary of the Triassic and Jurassic periods, 201.6 million years ago. The loss of marine biodiversity at the time has been linked to extreme greenhouse warming, triggered by the release of carbon dioxide from flood basalt volcanism in the central Atlantic Ocean. In contrast, the biotic turnover in terrestrial ecosystems is not well understood, and cannot be readily reconciled with the effects of massive volcanism. Here we present pollen, spore and geochemical analyses across the Triassic/Jurassic boundary from three drill cores from Germany and Sweden. We show that gymnosperm forests in northwest Europe were transiently replaced by fern and fern-associated vegetation, a pioneer assemblage commonly found in disturbed ecosystems. The Triassic/Jurassic boundary is also marked by an enrichment of polycyclic aromatic hydrocarbons, which, in the absence of charcoal peaks, we interpret as an indication of incomplete combustion of organic matter by ascending flood basalt lava. We conclude that the terrestrial vegetation shift is so severe and wide ranging that it is unlikely to have been triggered by greenhouse warming alone. Instead, we suggest that the release of pollutants such as sulphur dioxide and toxic compounds such as the polycyclic aromatic hydrocarbons may have contributed to the extinction.
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  • Resultat 1-8 av 8

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