• Beach ridge geomorphology of Kotzebue Sound: Implications for paleoclimatology and archaeology

      Mason, Owen Kenneth (1990)
      Beach ridges occur on all continents and record the horizontal addition of shoreface beyond the reach of storms. Improved cartographic methods in the nineteenth century allowed British historians to link shoreline changes with abandoned villages. This scientific trajectory was paralleled in the Bering Strait region from the 1880's to the 1930's. In the 1950's J. L. Giddings formalized "beach ridge archaeology" as a survey strategem using relative position to infer relative cultural chronology in northwest Alaska. Modern researchers use archaeological dates and data to document past climates or environments. At Cape Espenberg, on Seward Peninsula, my use of archaeological, stratigraphic, pedological, granulometric and photogrammetric data allows the delineation of 4000 years of coastal evolution. Four chronostratigraphic units are distinguished, using archaeological dates as minimum age assignments. Dune ridges formed in discrete intervals: 3300 to 2000 BP and from 1200 BP to the present; while low, berm ridges are predominant 4000-3300 and from 2000-1200 BP. The two different types of ridges correspond to variable climatic conditions: dune ridges formed after higher storm surges and winter winds while the lower berm ridges are related to less intense storm surges. Coastal dunes at Cape Espenberg are soon altered by plant succession processes with distance from the beach. As primary dunes are eroded, a complex blowout topography results. Erosional processes in blowouts were monitored during 1987-1989, revealing substantial vertical changes, up to 10 cm of erosion per yr. These rapid changes have considerable influence on archaeological site stability. Studies of the gravel ridge systems confirm the proxy storm record apparent in the coastal dunes atop the beach ridges on the Seward Peninsula. The geoarchaeological methodology allows correlations between depositional units within nine of the principal beach ridge and chenier complexes of northwest Alaska. The onset of deposition was at 4000-3500 BP. The complexes at Cape Espenberg and Choris Peninsula contain elevated, broader transgressive ridge sets 3300-2000 BP and from 1100-200 BP, connected with increased storm activity in the North Pacific. Erosional disconformities between successive sets of beach ridges occur at Cape Krusenstern at ca. 3000 BP and before 2000 BP. Between 2000-1000 BP extensive progradation occurred at nearly all complexes, indicating that less stormy conditions predominated.
    • Distant vistas: Bradford Washburn, expeditionary science and landscape, 1930-1960

      Sfraga, Michael P.; Pearson, Roger (1997)
      Bradford Washburn is primarily known for his Alaskan mountaineering accomplishments and mountain photography. Between 1930 and 1960, Washburn led 19 expeditions to Alaska and Canada's Yukon Territory on which he surveyed, photographed and mapped some of the last unexplored mountain regions in North America. This study, however, analyzes Washburn's lesser known role in directing interdisciplinary field research involving high altitude physics, glaciology, cartography and geology, which he accomplished by linking such disparate entities as the motion picture industry, geographic organizations, the U.S. military, and prominent U.S. scientists. Washburn's career can be viewed as an intersection of nineteenth and twentieth century geographic traditions. He combined emerging technologies with new and innovative vehicles of exploration to more accurately study geological, geographical and environmental phenomenon in mountainous regions. During the Second Great Age of Discovery, which began with the Renaissance, explorers ventured into the heart of the world's continents by utilizing various vehicles of exploration such as canoes and pack animals. This style continued into the middle of the twentieth century when the present day Third Great Age of Discovery, characterized by the use of remote sensing platforms and space age satellites, allows for a more accurate geographic study and inventory of our planet. Washburn's interdisciplinary field work reflects the fundamental goals and patterns of expeditionary science found in both ages of discovery. In this study three important themes are examined: Washburn's role as innovative field scientist; geography as a disciplinary bridge; and the work of the independent geographer. By analyzing Washburn's work in the pre World War Two and Cold War era, we gain an understanding of the ways in which expeditionary science was funded and carried out within two fundamentally different political and economic frameworks. Moreover, this study provides an important window into our understanding of interdisciplinary earth sciences in the mid twentieth century. It also explores the often unappreciated link between environmental science and geography in the American context.
    • Extent, timing, and paleogeographic significance of multiple Pleistocene glaciations in the Bering Strait region

      Heiser, Patricia Anne; Hopkins, David M. (1997)
      This study utilizes a multidisciplinary approach to the investigation of the extent, timing, and potential effects of repeated Pleistocene glaciation in Bering Strait region. A major focus of this study was directed toward testing the hypothesis that a continental-scale ice sheet existed in Beringia during the Late Wisconsin glacial period. Satellite synthetic aperture radar (SAR) imagery was used to compile a map of glacial moraines in Chukotka, Russia, and to attempt preliminary correlations with the glacial record in Alaska. Geophysical modelling of the solid-earth response to postulated glacial loading, and the reconstruction of regional snowline were combined with the results of the SAR investigation to test the ice sheet hypothesis. Finally, a detailed study of the Quaternary stratigraphy and surficial geology of St. Lawrence Island was used to correlate the glacial and sea level histories of western Alaska and Chukotka, Russia. The sequences of moraines in Chukotka, mapped from SAR imagery, are similar in morphology and position to moraine sequences described in Alaska, recording a succession of glacial events that most likely began in the middle Pleistocene and ended with the Late Wisconsin. The record of repeated mountain glaciation, characterized by radial flow out of high topographic areas provides strong evidence against the existence of a southward-flowing, continental-scale ice sheet in Beringia at any time in the latter part of the Pleistocene. Geophysical modelling of the solid-earth response to glacial loading predicted relative sea level changes on the scale of meters to tens of meters (rising or falling depending on forebulge effect) around the shores of present-day Bering Strait if a large ice sheet had, indeed, occupied the Beringia during Late Wisconsin time. There is no evidence of these predicted sea level changes anywhere in the region. The reconstruction of Late Wisconsin snowlines in Russian and Alaska show that the paleoclimatic conditions needed to 'grow' the hypothesized ice sheet did not exist. Field mapping and stratigraphic work on St. Lawrence Island revealed that ice advanced onto the island twice in the late Pleistocene, once in the Middle Pleistocene and once after the Last Interglacial, probably during the Early Wisconsin. The record of glaciers advancing from Chukotka onto the island provides an important 'Rosetta Stone' for correlating the glacial histories of northeast Siberia and Alaska.
    • Fluvial and hillslope geomorphology of Hoseanna Creek Watershed, central Alaska

      Wilbur, Stephen Crawford (1995)
      Hoseanna Creek Watershed is rapidly eroding and provides excellent opportunities to describe and quantify hillslope and fluvial processes in the subarctic and in discontinuous permafrost terrain. High landslide and badland densities occur due to asymmetric geologic structure and weakly consolidated lithologies. Late Quaternary regional glaciofluvial processes and tectonism have changed local base level at least 100 m, inducing headward incision through weak lithologies and yielding high rates of sediment production. Earthflows, translational blocks, rotational blocks, lateral spreads or complex landslide types form in coal-bearing formations in response to lateral corrasion of toes by avulsing streams or to undermining of foot areas by headward incising streams. Slides undergo episodic resurgent activity when new lithostatic or hydrostatic thresholds are reached. Average horizontal displacement rates of seven slides monitored between 8/85 and 9/88 ranged from 0.2 m/yr in rotational blocks to 48 m/yr in the earthflows. Although unique sliding mechanisms are not apparent, permafrost and subarctic climate generate delays or catalysts for failure atypical of warmer climates. Freezing/thawing fronts affect soil strength and permeability; break-up/freeze-up processes affect the timing of water supply to the slide mass and affect development of aufeis-related ground-water pore pressures. Aspectual and lithologic variations combine to yield three geohydrologic subbasin types which govern discharge ranges. Regressions were performed on multiple sets of sediment-discharge (Ts-Q) data. Regression variance (r$\sp2$) was found to have a maximum natural threshold indicative of intrinsic variability. Wide ranges in Q (0.001 to 2350 cfs) and Ts (0.005 to 1600 g/l) necessitated log-log scales and power functions. Each geohydrologic subbasin has a unique Ts-Q relationship termed here the mean sediment concentration potential Cp. Systematic differences in regression parameters indicate that variations in spatial conditions define Cp, while systematic changes in Ts-Q regression residuals R (termed here the maintenance rate R$\sp*$) describe the temporal variability of Ts through time with respect to Cp. 50-95% of the annual sediment load is transferred during less than 3% of the year. Erosion rate indexes were established from peak load estimates; Nenana Gravel basins are eroding 260 times faster than schistose basins and ten times coal-bearing basins.
    • Ice -wedge networks and "whale-hole" ponds in frozen ground

      Plug, Lawrence J.; Hopkins, David M. (2000)
      The patterns of ice-wedge networks and of whale-hole ponds in frozen ground self-organize by strong interactions between pattern elements. Mechanisms for the consistent spacing (15--25 m) and orientation between ice wedges are examined in a model encapsulating the opening of fractures under a combination of thermally-induced tensile stress, stress reduction near open fractures, and heterogeneity of frozen ground and insulating snow. Modeled networks are similar to ice-wedge networks on the Espenberg coastal plain, Bering Land-Bridge National Park, Alaska, at the level of variation among Espenberg networks, as indicated by: (i) comparisons of distributions of relative orientation and spacing between wedges; and (ii) application of nonlinear spatial forecasting to modeled and Espenberg network patterns. Spacing in modeled networks is sensitive to fracture depth and weakly sensitive to thermally-induced tensile stress and substrate strength, consistent with the narrow range of spacing between natural ice wedges in different regions. In an extended model that includes recurring fractures over thousands of winters, networks similar to natural ice-wedge networks form. The annual pattern of fractures diverges from the ice-wedge pattern, with only ½--¾ of wedges fracturing in a single year at a steady-state reached after approximately 103 y. Short-lived sequences of extreme stress from cooling can permanently alter the spacing between and the fracture frequency of modeled ice wedges, suggesting that the existence and characteristics of existing and relic natural ice-wedge networks reflect extreme, not mean, climate conditions. Ponds on the Espenberg beach-ridge plain, approximately 2 m across and 1 m deep and surrounded by raised rings of ice-rich permafrost 2 m across and 0.5 m high, form through an interplay between localized bacterial decomposition of peat, thawing of frozen ground and frost heaving of peat in rings. Groups of hundreds of ponds at Espenberg assemble through time because new ponds are favored to form adjacent to raised rings around existing ponds. The nonlinear behavior that results from strong interactions in patterns of ice-wedge networks and in ponds suggests general limitations in the application of linear approaches to inferring the response of geomorphic systems to changes in forcing, such as climate change.
    • Landscape Control Of Thunderstorm Development In Interior Alaska

      Dissing, Dorte; Verbyla, David (2003)
      General Circulation Models suggest a future climate of warmer and possibly drier summers in the boreal forest region, which could change fire regimes in high latitudes. Thunderstorm development is a dominant factor in the continental boreal forest fire regime, through its influence as a fire starting mechanism. Global Climate Change research has identified the land-atmosphere interface as a vital area of a needed research in order to improve our predictions of climate change. This dissertation has focused on the development of thunderstorms and lightning strike activity in a boreal forest region in Interior Alaska and on how the underlying surface can influence their development. I have examined the distributions and correlations between lightning strikes, thunderclouds, thunderstorm indices (CAPE and LI), elevation, and vegetation variables in Alaska. The relationships were examined at scales ranging from the Interior region of the state to individual wildfire burn scars, and at temporal scales ranging from the annual to daily. The objective is to understand the influential factors and processes responsible for thunderstorm development in Alaska, such that we may produce well-founded predictions on future thunderstorm regimes caused by a changing climate. The scale-related studies of this dissertation show that both processes and important variables for development of thunderstorms and lightning activity vary within and between the scales. It appears that on the larger scales, the combined effects of boreal forest and elevation on increased lightning strike activity were more prevalent than at the smallest scale (local). When the scale gets too small for the boundary layer to be affected (<10km), land surface effects on lightning cannot be. My results suggest that the underlying surface (in the form of areal forest coverage and vegetation) has more of an influence on convective development on days with airmass storms than on days with synoptic storms.
    • The Weathering Of Placer Gold And The Quaternary Geology Of Valdez Creek, Clearwater Mountains, Alaska

      Teller, Steven D.; Hopkins, David M. (1995)
      Placer gold grains collected from six paleochannels in the Valdez Creek drainage, south-central Alaska, were deposited during successive interglacial/interstadial intervals since the mid-Early Pleistocene. Statistical analysis of grain size, shape, grain surface characteristics, and the gold content of the interior and exterior of the gold grains determined by electron microprobe analysis demonstrates that the grains were affected by both mechanical and chemical weathering, and that the weathering increased with time. Etch pits, observed under a scanning electron microscope, are a ubiquitous feature of the grain surfaces. Grain surfaces average 26.7% richer in gold than the interior of the grains. The gold content of the surface of the grains increases with age. No high gold fineness rims were observed in cross section on the grains. This evidence indicates that the gold grains experienced corrosion. <p>