• Depositional Environments Of The Late Cretaceous (Maastrichtian) Dinosaur-Bearing Prince Creek Formation: Colville River Region, North Slope, Alaska

      Flaig, Peter Paul; McCarthy, Paul J. (2010)
      The Prince Creek Formation contains first-order meandering trunk channels, second-order meandering distributary channels, third-order fixed anastomosed(?) distributary channels, crevasse splays, levees, lakes, ponds, swamps, paleosols, and ashfall deposits. Trampling by dinosaurs is common. Most deposition occurred on crevasse splay-complexes adjacent to trunk channels. Rhythmically-repeating coarse-to fine-grained couplets in inclined heterolithic stratification suggest tidal-influence in channels. Cumulative to compound soils similar to Entisols, Inceptisols, and potential acid sulfate soils formed on levees, point bars, crevasse splays, and on the margins of lakes and swamps. Frequent overbank flooding is evidenced by silt and sand dispersed throughout paleosol profiles and fluctuations with depth in several molecular ratios. Drab colors, organics, siderite, depletion coatings, and zoned peds indicate waterlogged, anoxic conditions while ferruginous and manganiferous features, insect and worm burrows, and rare illuvial clay coatings and infillings suggest drying and oxidation of some soils. Repeated wetting and drying is tied to fluctuating river discharge. Marine influence is evidenced by jarosite, pyrite, and gypsum which become increasingly common up-section near the contact with the shallow-marine Schrader Bluff Formation. Pollen includes Peridinioid dinocysts; algae; projectates; Wodehouseia edmontonicola; pollen from lowland trees, shrubs and herbs; Bisaccates; fern and moss spores; and fungal hyphae and indicates that all strata are Early Maastrichtian and that sediments become progressively younger to the north. 40Ar/39Ar analysis of a tuff returned an age of 69.2 +/- 0.5 Ma. World-class dinosaur bonebeds are encased in muddy overbank alluvium overlying floodplains. No concentration of bone was found in channels. Bonebeds are laterally extensive except where truncated by distributaries. At the Sling Pont, Liscomb, and Byers bonebeds alluvium encasing bone exhibits a bipartite division of flow and a massive mudstone facies containing flow-parallel plant fragments that "float" in a mud matrix suggesting deposition by fine-grained hyperconcentrated flows. Exceptional floods driven by seasonal snowmelt in the Brooks Range increased suspended sediment concentrations, generating hyperconcentrated overbank flows that killed and buried scores of juvenile dinosaurs occupying this high-latitude coastal plain. This unique killing mechanism likely resulted from fluctuating discharge tied to extreme seasonality brought about by the near polar latitude of northern Alaska in the Late Cretaceous.
    • Evidence For A Northern Transitional Continental Margin Flora In The Cretaceous (Campanian To Maastrichtian) Matanuska Formation, Talkeetna Mountains, Southcentral Alaska

      Reid, Sabra Louise; Fowell, Sarah J. (2007)
      The Late Cretaceous Matanuska Formation contains shallow and deep marine sediments and nonmarine sediment derived from the Talkeetna volcanic island arc. The sediment accumulated in the Matanuska Seaway, a tectonically active basin on the southern margin of southcentral Alaska. The Matanuska Seaway was contemporaneous with the Cretaceous Western Interior Seaway (CWIS) of North America. Angiosperm pollen taxa from the CWIS have been used to date units and reconstruct both paleolatitude and paleoclimate (Nichols and Sweet, 1993). Comparison of pollen taxa from the CWIS to assemblages from the Matanuska Formation reveals that outcrops at Mazuma Creek, Granite Creek, Syncline Mountain, and Slide Mountain are Late Maastrichtian, while Hicks Creek outcrops are Campanian. During the Late Maastrichtian, the Matanuska Seaway was located south of 75° N latitude. The presence of ash layers overlain by low-diversity palynofloras with relatively high proportions of spores indicates the presence of volcanic recovery floras within the Matanuska Formation. Palynofloral composition and diversity of the climax vegetation suggest that the Matanuska paleoflora is a northern, transitional, continental margin flora that shares taxa with Late Cretaceous Pacific Rim floras of the Russian Far East and Japan and continental margin floras of western North America. When combined with coeval assemblages from the Alaska Peninsula, southcentral Alaskan palynofloras of the Late Cretaceous contain the most diverse assemblages of Aquilapollenites group taxa known from the North Pacific Rim. The Matanuska Seaway thus represents a coastal dispersal corridor where floras of the North Pacific Rim and western North American mingled. Within Alaska, the diversity of Late Cretaceous Aquilapollenites group taxa increases from north to south. Comparison of the Matanuska Formation palynoflora with assemblages from the interior Lower Cantwell Formation and the Arctic Prince Creek Formation reveals the presence of a north-south paleoecological and paleoclimatic gradient during the Campanian-Maastrichtian. The Matanuska Formation paleoflora suggests that the paleoclimate of southcentral Alaska was warm and humid. Assemblages from the interior Lower Cantwell Formation paleoflora are indicative of a warm, dry paleoclimate, while palynofloras from the Arctic Prince Creek Formation paleoflora record a cooler, more temperate paleoclimate on the North Slope.
    • Paleoecology and ecomorphology of the giant short-faced bear in Eastern Beringia

      Matheus, Paul Edward; Guthrie, R. Dale (1997)
      The short-faced bear (Arctodus simus) was a widespread Tremarctine bear indigenous to North America until its extinction around 11,500 BP. Arctodus inhabited Pleistocene ice-free refugia in Eastern Beringia (the northwestern limit of its range) until at least 20,000 BP. Because of its gracile, long-legged build and extremely large size, most paleontologists believe this bear was a high-speed pursuit predator which had preyed on the largest herbivores of Pleistocene North America. Alternatively, energetic arguments have been used to suggest that Arctodus was too large to be carnivorous and evolved its large size within an herbivorous or omnivorous niche. To test these competing hypotheses, I reconstructed aspects of Arctodus' trophic position and paleodiet by analyzing stable isotope ratios ($\delta\ \sp{13}$C and $\delta\ \sp{15}$N) in bone collagen extracted from east Beringian fossils. Other bears and carnivores from Beringia were analyzed to help interpret the results. Isotopes reveal that Arctodus was highly carnivorous, it fed on herbivores which consumed C3 vegetation, and it did not eat salmon. The herbivore/omnivore hypothesis is thus rejected. Predatory hypotheses predict that we should find certain morphological features in a predatory bear which would enhance one or more of the following skills: top running speed, acceleration, or maneuverability at high speeds. I re-analyzed the postcranial morphology of Arctodus and used data on running speed and bone strength in other large mammals to show that a bear the size of Arctodus with long, gracile limbs would not have been able to endure the extreme dynamic forces incurred during predatory activities. Instead, Arctodus' morphology and body size indicate it had evolved to maximize locomotor efficiency using a pacing gait. I suggest that Arctodus evolved as a specialized scavenger adapted to cover an extremely large home range in order to seek out, procure, and defend large-mammal carcasses from other carnivores. By modeling herbivore populations and their mortality, I show that enough carcass biomass was being produced in Pleistocene Beringia to make this scavenging niche energetically feasible. The model helps show that Arctodus' extinction probably is best tied to a reduction of year-round carcasses on the landscape, a condition which arose in the Holocene when the herbivore fauna became less diverse and began to experience more seasonal mortality.
    • The Development And Application Of Stable Oxygen And Hydrogen Isotope Analyses Of Chironomidae (Diptera) As Indicators Of Past Environmental Change

      Wang, Yiming; Wooller, Matthew (2008)
      Environmental change continues to be of increasing interest to scientists in all disciplines, and there is a paramount need to gain a comprehensive understanding of the impacts of environmental change in the past to better predict the future. A challenge associated with interpreting past change is a lack of reliable proxies to infer past environmental conditions preserved in the fossil record. My research has been dedicated to developing techniques associated with analyzing the stable oxygen and hydrogen isotopic composition (delta18O and deltaD) of subfossil chironomid (Chironomidea: Diptera) headcapsules (primarily composed of chitin) preserved in lake sediments as a new proxy of past hydrological environmental changes. My developments have included: (1) assessing and modeling the potential of contamination sources during sample preparation; and (2) culturing chironomid larvae under controlled, replicated laboratory conditions, to examine the degree to which water and diet influence the delta18O and deltaD of chironomids. My growth experiment demonstrated that 69.0+/-0.4% of oxygen and 30.8+/-2.6% of hydrogen in chironomid larvae are derived from habitat water using a two-end member mixing model. The delta18O of chironomids remains can better constrain past habitat water isotopic changes compared to deltaD, due to 69% of the chironomid oxygen being influenced by habitat water. Having examined these methodological issues I then applied stable oxygen isotope analyses of fossil chironomid remains preserved in a sediment core from Idavain Lake, in southwest Alaska. The core represents the last ~16,000 years to the present and isotope analyses of chironomids from the core showed that the delta18O of past lake water had changed since deglaciation. Large variation in delta18O of chironomids (up to 20‰) are interpreted as alternating shifts in atmospheric flow regimes that are predominant in southwest Alaska, which are consistent with other evidence of past environmental changes at Idavain Lake (i.e. pollen, delta13C, delta15N, C/N). A zonal flow regime appears to have been dominant from 16,000 to 13,800 cal yr BP, 11,000 to 10,500 cal yr BP, 10,000 to 8,000 cal yr BP, and during a majority of the periods from 8,200 to 3,500 cal yr BP and from 2,000 cal yr BP to present. A mixed modern flow regime seems to have been dominant during the periods from 13,000 to 11,000 cal yr BP, 10,500 to 10,000 cal yr BP, 6,000 to 5,500 cal yr BP and 2,500 to 1,800 cal yr BP. These shifts in moisture regime appear to coincide with a series of glacier advances and recessions along the Gulf of Alaska. Thus, stable isotopic analysis of chironomid headcapsules is a promising tool for indicating paleoenvironmental change.