• 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.
    • Depositional Systems, Paleoclimate, And Provenance Of The Late Miocene To Pliocene Beluga And Sterling Formations, Cook Inlet Forearc Basin, Alaska

      Mongrain, Jacob; McCarthy, Paul; Fowell, Sarah; Helmold, Ken; Layer, Paul; Swenson, Robert (2012)
      The sedimentary record of forearc basins provides critical clues to the complex geologic history of subduction zones. This study focuses on Cook Inlet forearc basin, part of a larger forearc basin complex in southern Alaska. Specifically, I investigated the sedimentology, paleoclimate, and provenance of the Beluga and Sterling formations, comprising the late Miocene to Pliocene Cook Inlet basin record. These interpretations are used to reconstruct the Miocene-Pliocene basin history and better understand forearc development. Before ~11 Ma anabranching/single channel depositional systems of the Beluga Fm. deposited sediment on the western and eastern margins of Cook Inlet. Sandstone compositional data suggest sediment from the eastern margin was sourced from the accretionary prism. Between ~11 Ma and ~8 Ma deposition of the Beluga Fm. waned and sandstone compositional data indicate increases in volcanic lithic fragments derived from the volcanic arc to the northwest. Deposition by the southward-directed, sandy braided fluvial systems of the Sterling Fm. started on the western margin of the basin and migrated eastward, reaching the eastern margin ~8 Ma. By ~6 Ma, sandstone compositional data suggest that the volcanic arc was the dominant sediment contributor to the basin. Palynological results suggest that forests were predominately confined to coal swamps, and the surrounding floodplains were occupied by shrubs, herbs, and dispersed tree taxa. Thermophilic taxa persisted until at least ~6 Ma. Mean annual precipitation (MAP), estimated from delta13C values, ranged from 420 to 3900 mm a-1 with the greatest variability ~8 Ma. This ~8 Ma event correlates with a decline in sea surface temperatures of the Alaska Gyre and a North Pacific climate optimum. Climate likely played a minor role in fluvial style change. The dramatic change in depositional style between the Beluga and Sterling fms. is attributed to a change in sediment flux from the accretionary prism to the volcanic arc and western Alaska Range, most likely due to orogen-scale tectonic processes driven by far-afield flat-slab subduction of the Yakutat microplate. The change in fluvial style and sediment flux starting ~11 Ma suggests a previously unrecognized exhumation in the western Alaska Range at this time.
    • Micromorphology, Site Spatial Variation And Patterning, And Climate Change At The Mead Site (Xbd-071): A Multi-Component Archaeological Site In Interior Alaska

      Gilbert, Phoebe J.; Potter, Ben; Irish, Joel D.; Bigelow, Nancy H.; Beget, Jim E. (2011)
      The Mead Site, located in the Tanana River Valley in Interior Alaska, is a deeply buried archaeology site with multiple occupations and excellent preservation. The site provides a rare opportunity to study the human/climate relationship in prehistory. Magnetic susceptibility, micromorphology, geochemical and spatial analysis were utilized to (I) determine the amount of post-depositional disturbance at the site, (2) see if there are detectable buried surfaces that indicate cultural occupation in the upper sratigraphic layers and, (3) investigate the paleosols at the site and determine if the occupations at the site correlate with ameliorating climate. The results show that the upper three cultural zones are heavily disturbed by taphonomic processes to the point that assignment of the remains to cultural zones is suspect. The lower two components have also been affected by post-depositional disturbance, but the patterning of cultural remains in these zones is primarily a reflection of the original depositional context. No buried surfaces were detected in the upper stratigraphic layers, and the paleosols are natural in origin but are anthropogenically enhanced. The cultural zones at the site are more closely associated with cool episodes than with periods of ameliorating climate.