• Climate, seasonal snow cover and permafrost temperatures in Alaska north of the Brooks Range

      Zhang, Tingjun; Osterkamp, T. E.; Bowling, S. A.; Goering, D.; Gosink, J.; Hopkins, D.; Wendler, G. (1993)
      Climatological data, active layer and permafrost measurements, and modeling were used to investigate the response of permafrost temperatures to changes in climate in Alaska north of the Brooks Range. Mean annual air temperature (MAAT) from 1987 to 1991 within about 110 km from the Arctic Coast was ${-12.4}\pm0.3\sp\circ C,$ while the mean annual permafrost surface temperature (MAPST) ranged from ${-9.0}\sp\circ C$ along the coast to ${-5.2}\sp\circ C$ inland. Air temperature changes alone can not explain the permafrost warming from the coast to inland. Measurements show that MAPST are about $3\sp\circ C$ to $6\sp\circ C$ warmer than MAAT in the region. The interaction of local microrelief and vegetation with snow appears to change the insulating effect of seasonal snow cover and may be the major factor which controls the permafrost temperature during the winter and thus the MAPST. Sensitivity analyses show that for the same MAAT conditions, changes in seasonal snow cover parameters can increase or decrease the MAPST about $7\sp\circ C.$ Snowfall was greater during the cold years and less during the warm years and was poorly correlated between stations. These results suggest that the effects of changes in air temperatures on permafrost temperatures historically may also have been modified by changes in snow cover. A numerical model was used to investigate the effect of changes in initial permafrost temperature conditions, MAAT, seasonal snow cover and thermal properties of soils on the permafrost temperatures. Permafrost may have started warming about the same time as the atmosphere did in the late 1800's, and the long term mean surface temperature of the permafrost may have been established prior to this time. Variations in the penetration depth of the warming signal may be related to differences in thermal properties of permafrost. Variations in the magnitude of the permafrost surface warming may be due to the effect of local factors such as soil type, vegetation, microrelief, soil moisture, and seasonal snow cover. The effect of the interaction of vegetation and snow cover may amplify the signal of temperature change in the permafrost.
    • Critical Parameters In Magmatic Degassing

      Mongrain, Joanna; Larsen, Jessica (2008)
      Decompression experiments conducted at pressures up to 200MPa and temperatures of 825�C-880�C on hydrated K-phonolite and rhyolite melts were used to explore the critical parameters controlling nucleation, exsolution and degassing behavior. Experiments on the low viscosity/surface tension K-Phonolite melt highlighted the role of melt properties. Although the sample porosities deviated below equilibrium values for pressures less than ~40MPa, the melt exsolved water in equilibrium over all the pressures and decompression rates studied. Melt shearing is proposed to have caused bubble deformation and alignment, lowering the porosity at which extensive permeability develops and significant degassing occurs compared to rhyolite. Experiments on a rhyolite melt decompressed slowly from 100 MPa and then held at 10 MPa for up to 900 s highlighted the critical parameters controlling the formation and stability of a highly vesicular magma: bubble number density, bubble size distribution and porosity. The porosity of the interconnected, highly vesicular network decreased during "Stage I" degassing and the bubble size distribution evolved from a unimodal population to include a population of much larger bubbles. During Stage II degassing, the network collapsed. Pre-collapse and collapse degassing rates were obtained and a coalescence-induced coalescence model proposed to explain the rapid destabilization. The ability of a melt to efficiently exsolve volatiles and the ease of bubble coalescence are both a function of the initial distribution of nucleated bubbles. The development of a new method for quantifying this distribution using spatial statistics will allow future researchers to explore the underlying controls on nucleation such as melt structure and the occurrence of a prior nucleation event. To investigate the critical parameters controlling shallow dike intrusion and therefore magmatic ascent rate, the fracture mechanics of intrusion into homogeneous and layered (weak sandstone/strong granite) particle models under lithostatic, compressive and extensional regimes were examined. Although the scale of the model intrusions were an order of magnitude greater than field observations, extensive microfracturing across the weaker layers, parallel dike jointing in the stronger layers and a length scale dependence to fracture toughness were observed suggesting that the use of a particle code is a promising approach to intrusion modeling.