• Characterization Of Permafrost Development By Isotopic And Chemical Analysis Of Soil Cores Taken From The Copper River Basin And An Upland Loess Deposit In Interior Alaska

      Oliver, Lola Kay; Romanovsky, Vladimir (2012)
      Chemical and isotopic analyses of pore water from permafrost cores taken from the dry lake bed of ancient Lake Atna in the Copper River Basin and from an upland loess deposit northeast of Fairbanks, Alaska reveal information about the local past environments not available by other means. Thawed core samples from both sites were analyzed for delta18O and deltaD values using an isotope ratio mass spectrometer. Water content was determined as well, and subsamples of the cores were analyzed for nitrogen and carbon content. Water extracts of the core samples were analyzed for cations (Ca, Mg, K and Fe), as well as pH, electrical conductivity, and bicarbonate. Magnetic susceptibility was determined on samples from the Fairbanks site. Data from samples taken from the Lake Atna site revealed a sequence of events that occurred in the basin after the lake drained about 10,000 years ago. At one location, oxygen isotopes show that permafrost formed continuously down through the lakebed. At the other location, 400 m distant, oxygen isotopes show that after permafrost formed, a thaw lake was produced on top of the permafrost. After the thaw lake had drained at least one wildfire passed through the area, large enough to thaw the surface permafrost to a depth of over 2 m at both core sites. The surface permafrost refroze, and currently the permafrost at the research site extends from 1 m depth to between 45 and 55 m depth. At the Fairbanks site chemical analyses showed indications of pedogenesis at several depths in the loess profile of the permafrost core. Water isotope data from a site where the permafrost table is at less than 1 m depth show repeated episodes of thawing and refreezing. Charcoal and ash in several layers of soil in the area and from the sample core suggest that fire may be the cause for the thawing events. The primary core water isotope analyses also show several thawing and refreezing events, but the depths of the signatures in the core indicate that these episodes happened thousands of years ago when the ground surface was much lower than it is today.
    • 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.