Now showing items 657-676 of 1029

• #### Paleobiology of ichthyosaurs: using osteohistology to test hypotheses of growth rates and metabolism in a clade of secondarily aquatic marine tetrapods

Ichthyosaurians (Ichthyosauria) are one of the most prominent groups of secondarily aquatic Mesozoic marine reptiles. Over their 160 million years of evolution, the clade evolved a streamlined body plan with paddle-like limbs, convergent with modern cetaceans. Despite the fact that ichthyosaurians have been studied by paleontologists for over a century, very little is known about aspects of their biology, including quantification of their age structure and growth rates. Multiple lines of evidence, including oxygen isotope, swimming modality, and body shape analyses suggest that ichthyosaurians experienced elevated growth rates and likely maintained an elevated body temperature relative to ambient sea water. In this dissertation, I test these hypotheses using osteohistological methods. In the first manuscript, we describe new material of the small-bodied Upper Triassic ichthyosaurian Toretocnemus from the Nehenta Formation and the Hound Island Volcanics (both Norian, Upper Triassic) of Southeast Alaska. During the Upper Triassic, ichthyosaurians experienced their greatest size disparity, with large-bodied species rivaling the size of modern blue whales (Balaenoptera musculus; 20+ m body length) living alongside small-bodied species (1 m body length) like Toretocnemus. Prior to this study, Toretocnemus was known from Carnian deposits of California and possibly Sonora, Mexico. The referred material described here expands its geographic and temporal range. There are very few known ichthyosaurians from the Norian; thus, this material sheds light on the clade's diversity before the end Triassic extinction event. In the second and third manuscripts, we use osteohistological methods to describe the microstructure of various skeletal elements of two species of Stenopterygius from the Posidonia Shale (Lower Jurassic) of Germany. The Posidonia Shale is a Konservat-Lagerstätten that preserves over 3000 ichthyosaurian specimens, approximately 80 percent of which are referable to Stenopterygius. First, we sampled over 40 skeletal elements from one individual specimen referred to Stenopterygius quadriscissus to 1) describe the mineralized tissues across the skeleton, 2) infer relative growth rate, and 3) identify elements with growth marks. Almost all elements described demonstrate fibrolamellar primary bone, indicative of a rapid growth rate. We also identify growth marks in several elements, including the dentary and premaxilla, that will be used in future growth studies. In the third manuscript, we sample a scleral ossicle from Stenopterygius triscissus to describe its microstructure and investigate the use of ossicles for skeletochronology. The use of scleral ossicles for determining age structure has been documented in extant sea turtles as well as dinosaurs. We sectioned one ossicle in three planes and document conspicuous growth banding in the short axis section. Although this method requires further testing, we tentatively determine a minimum age of 7 years at the time of death for this individual. This dissertation lays critical groundwork for future studies of the paleobiology of ichthyosaurians. We are already in the preliminary stages of using these results to 1) quantify age structure and growth rates of an ichthyosaurian (Stenopterygius quadriscissus) for the first time, and 2) test the use of scleral ossicles for skeletochronology of ichthyosaurians. Through addressing these basic aspects of ichthyosaurian biology, we can begin to investigate how ichthyosaurian development and physiology changed over time and space and develop a greater understanding of this clade's 160 million years of evolution.
• #### Paleoclimate and paleoenvironment of the Prince Creek and Cantwell formations, Alaska: terrestrial evidence of middle Maastrichtian greenhouse event

I studied the paleoclimate and paleoenvironmental conditions of the Prince Creek Formation, North Slope Alaska, and the lower Cantwell Formation in Denali National Park, Alaska. I used data from pollen analysis, clay mineral analysis and stable isotope analysis of clay minerals and organic matter applied to paleosols of the Prince Creek Formation. In the lower Cantwell Formation, I reconstructed the sedimentary environment, produced a terrestrial carbon stable isotope record and obtained a ~ 69.5 Ma radiometric age for the lower Cantwell Formation. Clay analysis in the Prince Creek Formation indicates that the genesis of the paleosols was strongly influenced by the properties of the parent material and that an epiclastic bentonitic source contributed to the development of non-allophanic properties which suggests the presence of Andept-like paleosols. Paleosols formed on the floodplains of the Prince Creek Formation reveal features attributed to wet-dry cycles as a result of seasonal flooding, perhaps due to snow melt in the ancestral Brooks Range. Carbon and oxygen isotope analyses, and the geochemistry of paleosol Bw/Bt horizons indicate mean annual precipitation values between 745.56 and 1426.88 ±221.38 mm/yr and mean annual temperatures of 12 ±4.4 °C. The meteoric water δ¹⁸O value calculated from smectite at a 6.3 °C mean annual temperature is ~-24 ⁰/₀₀. The calculated value is δ¹⁸O-depleted as is expected for high latitudes during the Late Cretaceous. Sedimentary facies analysis suggests that The East Fork measured section of the lower Cantwell Formation was likely deposited in the distal part of an alluvial fan. A new U-Pb age of 69.5 ±0.7 Ma from bentonites and carbon isotope values of bulk sedimentary organic matter and wood fragments indicates that a greenhouse event, known as the mid-Maastrichtian Event (MME), is recorded at the East Fork of the Toklat River Section. A mean annual precipitation value of ~ 517.92 ±134.44 mm/yr was obtained from δ¹⁸O terrestrial organic matter. The coeval nature of the lower Cantwell Formation (~69.5 Ma) and the Prince Creek Formation (~69.2 Ma) suggests that the MME likely affected the Prince Creek Formation as well. In the Prince Creek Formation, the age, precipitation, temperature and meteoric water composition from bentonite smectites are consistent with increased precipitation due to an intensified paleo-arctic hydrological cycle, which may have been the result of increased latent heat transport during the mid-Maastrichtian greenhouse episode.
• #### Paleoecology and ecomorphology of the giant short-faced bear in Eastern Beringia

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.
• #### Paleoecology of Twin Cays: interpretation of palynological, isotopic, and stomatal proxies in a peat core from Belize

Reconstructions of Holocene climate from numerous mid- and high-latitude sites have identified millennial-scale cool and arid intervals at 8,200 and 4,200 yrs. B.P. The global nature of these events can only be established by examination of Holocene climate records from low latitude sites. The Central American island of West Twin Cays was chosen as the study location due to its thick peat deposits, which allow for the reconstruction of Holocene vegetation, sea level, and climate for the Belize coastal region. Rhizophora mangle (red mangrove) dominated the island's vegetation since its formation 8,200 cal. yrs. B.P. Alternating periods dominated by dwarf or tall R. mangle reflect changes in phosphorus and nitrogen availability. Heightened Myrsine-type pollen concentrations between 6,300 and 4,200 cal. yrs. B.P. suggests lower sea levels and drier climates. Regional warming accompanied by increased precipitation in the middle to late Holocene is recorded by an increase in exotic Pinus concentrations. These vegetation shifts correlate with regional Central American climate changes and westem Atlantic sea level shifts at 8,200 and 4,200 cal. yrs. B.P. More importantly, these data link Central American changes to worldwide climate events.
• #### Paleoenvironmental changes at treeline: a 6,500 year long pollen and stable isotope record

Combined pollen, macrofossil and stable-isotope records from two lakes in the sub-alpine zone of the north-central Alaska Range indicate significant changes in vegetation and lake productivity during the past 6̃750 ¹⁴C yrs BP. These changes are associated with neoglacial cooling and climate variations during the Little Ice-Age (LIA). Highest spruce densities occurred during a period from 5,000 ¹⁴C yrs BP and 2,500 ¹⁴C yrs BP and coincided with the onset of cooler and moister climate. The shifts in climate, which resulted in increased effective moisture levels in Central Alaska, possibly shifted the competitive balance towards spruce and against tundra taxa. Lake productivity declined as climate cooled. A brief episode of climate amelioration between 1,500 ¹⁴C yrs BP and 800 ¹⁴C yrs was followed by cooking events of the LIA which resulted in decreased spruce densities in the sub-alpine forest-tundra zone and a possible lowering in treeline at higher elevation.
• #### Paleohydrology of a catastrophic flood release from Okmok caldera and post-flood eruption history at Okmok Volcano, Umnak Island, Alaska

Okmok caldera, located on the northeastern end of Umnak Island, Alaska, contained a 5.8 x 10⁹m³ lake that catastrophically drained as a result of failure of the 2050 yr. B.P. caldera rim between 1560 and 1010 yr. B.P. Flow competence equations, dam-break models, and the Simplified Dam-Break computer model were used to estimate the paleohydrology of the flood. Models indicate that the peak discharge at the breach in the caldera rim was at least 5.8 x 10⁴ m³/s, and the maximum possible discharge was 1.9 x 10⁶ m³/s. A second smaller flood release occurred 190 yr B.P., coinciding with the 1817 A.D. eruption, and destroyed a small Aleut village at Cape Tanak. Stratigraphic analysis reveals that Okmok Volcano has maintained a high level of volcanic activity following the large flood release. Major eruptive events producing air-fall tephra deposits average 1 every 80 years since 1010 yr. B.P.
• #### Paleopedology, palynology, and geochronological interpretation of alluvial paleosols, Cenomanian Dunvegan formation, Alberta, Canada

The Dunvegan Formation is a mid-Cretaceous deltaic deposit exposed in the Rocky Mountain foothills of Alberta and British Columbia along the Peace River Valley. Intrinsic features of Dunvegan Formation paleosols understood through micromorphology, geochemical, and mineralogical analysis provide a paleoclimatic interpretation of warm to cool temperate. Micromorphological and geochemical analysis lead to reconstruction of the depositional and pedogenic histories of the five primary paleosols of this study. Preserved palynomorphs of the paleosols are primarily composed of fern spores with much less abundant conifer and cycad species. The paleoclimate based on the palynomorphs is humid and ranges from cool temperate to subtropical. The overall paleoenvironmental interpretation based on both paleosols and palynology is humid cool to warm temperate. Geochronology using the ⁴⁰Ar/³⁹Ar dating method was implemented in an effort to date pedogenesis. This technique needs further refinement in order to be successful in dating paleosols. The multi-proxy approach of this study lead to a more complete interpretation of the climatic, pedogenic and depositional history and should be used in the future.
• #### Particle dynamics in the plasma sheet

Trajectories of charged particles in the tail region of the earth's magnetosphere are studied using a model magnetic field. The particles form a thin sheet-like structure in the magnetotail called the plasma sheet. It is shown that most trajectories are categorized by two dimensionless parameters. One of them is equal to the ratio of the cross-tail electric force to the magnetic force in the midplane and determines the maximum particle energization. The other parameter is the ratio of the plasma sheet thickness to the particle gyroradius in the midplane and determines the degree to which the particle motion is adiabatic. All previous attempts at studying trajectories in the magnetotail are shown to be applicable only over limited ranges of the two parameters. Hence those studies are combined into a common framework, and those trajectories which have not been studied previously are added for completeness.
• #### Particle simulations of magnetic field reconnection and applications to flux transfer events

Basic plasma processes associated with driven collisionless magnetic reconnection at the Earth's dayside magnetopause are studied on the basis of particle simulations. A two-and-one-half-dimensional (2$1\over2$-D) electromagnetic particle simulation model with a driven inflow boundary and an open outflow boundary is developed for the present study. The driven inflow boundary is featured with a driving electric field for the vector potential, while the open outflow boundary is characterized by a vacuum force free condition for the electrostatic potential. The major findings are as follows. (1) The simulations exhibit both quasi-steady single X-line reconnection (SXR) and intermittent multiple X line reconnection (MXR). The MXR process is characterized by repeated formation and convection of magnetic islands (flux tubes or plasmoids). (2) Particle acceleration in the MXR process occurs mainly in O line regions as particles are trapped within magnetic islands, not in X line regions. The MXR process results in a power law particle energy spectrum of $f(E)\sim E\sp{-4}$. (3) Field-aligned particle heat fluxes and intense plasma waves associated with the collisionless magnetic reconnection process are also observed. (4) When applied to the dayside magnetopause, simulation results show that the MXR process tends to generate a simultaneous magnetic field perturbation on both sides of the dayside magnetopause, resembling the observed features of two-regime flux transfer events (FTEs). (5) An intrusion of magnetosheath plasma bulge into the magnetosphere due to the formation of magnetic islands may lead to the layered structures observed in magnetospheric FTEs. (6) In the current sheet, the enhanced tearing mode instability caused by the driving force applied at the driven inflow boundary creates an energy source at a specific wavenumber range with $k\sb{z}L\sim$ 0.3 in the modal spectrum of the magnetic field $B\sb{x}$ component. An inverse cascade of the modal spectrum of $B\sb{x}$ leads to the formation of the large-scale ordered magnetic island structures observed in the simulations. (7) In addition, the results of a theoretical study show that the tearing mode instability, and hence the magnetic reconnection at the dayside magnetopause, do not exhibit strong dependence on the magnetosheath $\beta$ values.
• #### Patterns of and controls over nitrogen inputs by green alder (Alnus viridis ssp. fruticosa) to a secondary successional chronosequence in interior Alaska

Patterns of and controls over N₂ fixation by green alder (A. viridis) were studied in post-fire, mid-succession, and white spruce upland forests in interior Alaska during 1997-2000, focusing on the hypothesis that ecosystem-level nitrogen (N) inputs decrease with successional development. Across all stands, alder created islands of elevated soil N and carbon, depleted soil phosphorus (P), and more acidic soils, effects which translated to the stand-level in response to greater alder stem density. Rates of N₂ fixation (measured by acetylene reduction = ARA) closely tracked plant phenology during the 1997 (a drought year) and 1998 (a year of normal precipitation) growing seasons. During 1998, stands with higher maximum ARA had higher %N in the O, A, and C soil horizons. N₂-fixation rates were influenced by soil P, as evidenced by the findings that maximum ARA was positively correlated with foliar N:P ratios, and with subcanopy %P in the O and A soil horizons. During the drought year, alder leaf %P and leaf N resorption were lower and leaves were thinner when compared to 1998. Drought effects were most pronounced in mid-succession where alder exhibited reduced ARA ( -76%), leaf %P ( -14%), leaf thickness ( -40%), and lower leaf resorption of P ( -66%) and N ( -78%). Although ARA and nodule biomass did not differ among stand types, increases in alder densities with successional time translated to increasing ecosystem-level N inputs across the chronosequence. These results contradict established theory predicting a decline in N₂-fixation rates and N₂-fixer abundance during successional stand development.
• #### Patterns of behavioral entrainment in mice

The experiments described in this Dissertation were designed to assess the process by which the master circadian clock in the suprachiasmatic nuclei (SCN) can be entrained by normocaloric scheduled feeding. Mouse lines that differ in some circadian rhythm characteristics and features of the SCN were used in these experiments. The initial experiment showed that the lines differ in the percentage of individuals that entrained to scheduled feeding. In mice that entrained to scheduled feeding, both wheel-running activity and the molecular clock in the SCN were synchronized to the time of food provision. The molecular clock in the SCN remained in phase with the free-running light entrainable activity component in mice that did not entrain to scheduled feeding. Subsequent experiments were performed to determine if line-specific differences in entrainment were related to differences in circadian rhythm characteristics. Several circadian characteristics showed line effects, but no differences between mice that entrained and those that did not entrain were found. The next series of experiments assessed the influence of non-photic factors not directly related to food availability and food ingestion. The results indicated that the timing of food availability relative to the phase of the SCN during the initial phase of the experiment, and factors related to disturbance, influenced entrainment. Three factors suggest involvement of the FEO in entrainment to scheduled feeding: 1) it is a circadian oscillator that is entrained by scheduled feeding; 2) entrained mice express robust food anticipatory behavior; and 3) the time of food provision corresponds to the peak of mPER2 expression in the SCN of entrained mice. The results of the disturbance experiments raise the possibility that the entrainment signal may be generated by a system other than the FEO, perhaps one related to arousal state or reward processes. Ultimately, multiple factors influencing entrainment of the SCN and SCN-controlled circadian rhythms by scheduled feeding are likely to be identified. These factors may include the combined influence of all non-photic signals received by the SCN, differences entrainment signal strength, and differences in the sensitivity or responsiveness of the clock mechanism due to genetic variation.
• #### Patterns of genetic variation among various Beringian birds

Every animal has two different sets of genes, the mitochondrial genome and the nuclear genome. They are inherited independently of each other and can have dissimilar evolutionary histories. For this thesis, I examined the correlation of the mitochondrial (using direct sequencing of mitochondrial DNA) and nuclear (using a fragment-based technique, AFLPs) evolutionary histories in Beringian birds. The first study looked at relationships among a within-Beringia radiation of birds, the Aethia auklets. I was able to infer a resolved phylogeny using mitochondrial DNA, but found a polytomy using nuclear DNA. The lack of correlation between the genomes is either the result of incomplete lineage sorting or reflects the fact that AFLPs may not be an appropriate technique to survey nuclear diversity at this level of divergence. In the second study, I examined the correlation between taxonomic status, mitochondrial divergence, and nuclear divergence among nine lineages of trans-Beringian birds. I found no relationship between any of the three variables, suggesting that trans-Beringian birds have complex and discordant genetic and phenotypic histories.
• #### Peeking through a frosty window: molecular insights into the communities of Arctic soil fungi

Fungi are thought to be one of the most diverse groups of organisms in the Arctic. They drive mineral and energy cycles and influence the occurrence of other organisms as mutualists (mycorrhizae, endophytes, lichens), decomposers and pathogens. Nevertheless, information on fungal biodiversity and distribution patterns in relation to environments across the Arctic is still sparse. Molecular methods were used to examine the diversity and community structures of ectomycorrhizal fungi (EMF) associated with two principal arctic host plants, Salix aretica and Dryas integrifolia, as well as total soil fungal communities of adjacent disturbed and undisturbed areas of patterned-ground features across the five bioclimatic subzones (A-E) of the North American Arctic. Key findings include the following: (1) More diverse fungal communities had been observed than previously known. These communities encompass nearly all fungal phyla and included all fungal guilds. However, a few species-rich fungal families dominated these fungal communities. (2) Surprisingly, species richness did not decline with latitude. (3) The most abundant fungal taxa were widely distributed in and beyond the Arctic. Yet root (EMF) and soil fungal communities showed niche preferences in regard to bioclimatic subzones. Furthermore, disturbed and undisturbed patterned ground features harbored different soil fungal communities with the exception of the coldest subzone A. In contrast, EMF community composition was not affected by host plant identity. (4) Fungal communities in the warmest subzone E were distinct from the other arctic subzones and the majority of taxa matched fungi from the boreal forest. (5) Key drivers of fungal community and guild composition along the bioclimatic gradient included regional climate, p.H as well as vegetation composition and productivity across the subzones. At the local scale of patterned-ground features, fungal communities were correlated with vegetation composition and microclimate. With a warming climate, I would expect an enhanced colonization of patterned-ground features by vascular plants that would then affect fungal community structure not only at the species level, but also at the level of fungal guilds. In particular I would expect increases in fungi that are symbiotic with plants and a northward shift of both plant and fungal taxa.
• #### Perchlorate toxicity in fish: trophic transfer, developmental windows, and histological biomarkers

The perchlorate anion is an oxygenated chlorine compound often used by the military as an oxidizer in solid rocket propellant and by industry in numerous other applications. It is a known endocrine disruptor and competitively inhibits the uptake of iodide into thyroid tissue in a concentration dependent manner, effectively reducing production of thyroid hormones. Perchlorate is highly water soluble, kinetically inert and has low adsorption tendency, making it a persistent and mobile aquatic contaminant. Perchlorate has been detected in drinking water sources throughout the United States and is present in many commercially available food and drink products. The objective of this dissertation was to better understand the bioaccumulation, toxicodynamics and morphological changes caused by perchlorate exposure utilizing northern pike (Esox lucius) and the threespine stickleback species (Gasterosteus aculeatus) model. The first research chapter (chapter 2) examines the potential for bioaccumulation and trophic transfer of perchlorate in northern pike exposed to 10 and 100 mg/L perchlorate via ambient water and food. As expected, perchlorate does not biomagnify, but does concentrate in the gastrointestinal tract tissue of pike. At the lower exposure concentration (10 mg/L) for combined water and food exposure, greater than additive tissue concentrations were detected indicating the need for regulatory testing to consider not only contaminated water but the associated contaminated food in the contaminated ecosystem (e.g., some studies provide food that is not contaminated while fish are exposed only to contaminated water). The following two chapters (chapters 3 and 4) examine the morphologic effects of exposure timing and duration on developing stickleback utilizing a unique upshift/downshift exposure regime to determine if critical developmental windows of perchlorate sensitivity exist for two exposure concentrations (30 and 100 mg/L). In chapter three, gross morphology (body size and skeletal armor traits) were quantified in sexually mature fish. The results demonstrate that growth can be suppressed with continuous exposure beginning within the first 14 days post fertilization (dpf). Skeletal armor traits responded variably to perchlorate exposure, with some increasing, some decreasing and others developing normally. The traits measured in this study (excluding standard length) were not sensitive to the timing of exposure (i.e., no definitive critical windows), but responses were concentration-dependent. Chapter four examines thyroid tissue histomorphological endpoints, sex ratio, and gonadal maturity in stickleback exposed to perchlorate (30 and 100 mg/L) for varying times and durations. Thyroid tissue responded with increased follicle hyperplasia, decreased area of colloid, increased angiogenesis, and follicle cell hypertrophy. Within the first 42 dpf, a critical window emerged for follicle hyperplasia and area of colloid. Stickleback rescued (removed from perchlorate contaminated water) anytime up to 305 dpf recovered from follicle hyperplasia and reduced colloid area. Angiogenesis increased in fish exposed to perchlorate and a critical window was detected for fish exposed to 30 mg/L anytime between 7 and 154 dpf. Recovery from angiogenesis did not occur. The ratio of males to females and gonadal development were altered in stickleback exposed to perchlorate continuously beginning within the first 14 dpf. Sex ratio was skewed toward males in a concentration-dependent manner, which could be due either to a masculinizing effect of perchlorate on sexual development or to differential survival of the sexes. Additionally, gonadal maturation was delayed for both sexes as the proportion of late stage testes and oocytes decreased in perchlorate exposed fish. Overall, these results demonstrate that the effects of perchlorate on aquatic vertebrates are complex. Movement within and between organisms is complicated due to the iodide concentrating mechanism of some tissues. Abnormalities of growth and skeletal armor traits are caused by perchlorate exposure and both are important to the survival and reproductive success of stickleback. In addition, as expected, the histomorphology of thyroid tissue is a responsive biomarker of perchlorate exposure in stickleback. Critical windows of sensitivity to perchlorate exist during early development and future research should scrutinize the biochemical mechanisms driving changes in thyroid condition and abnormal development, particularly for reproductive endpoints.
• #### Permafrost and wildlife influences on stream nutrient dynamics and metabolism in boreal forest watersheds of Interior Alaska

In the boreal forest of interior Alaska, both permafrost and wildfire impact stream solute concentrations, but their effect on stream function is unknown. This research focused on the effects of wildfire and permafrost on stream nutrient dynamics and metabolism in the Caribou Poker Creeks Research Watershed (CPCRW) in interior Alaska. Wildfire impact on chemistry was determined through comparison of pre- and post-fire stream chemistry in a control and a burned watershed. We predicted that increased nutrient and decreased dissolved organic carbon (DOC) delivery to streams after fire would stimulate gross primary productivity (GPP) and reduce ecosystem respiration in burned watersheds. Fire resulted in higher stream nitrate, sulfate, and cation concentrations, and lower DOC concentration, at the burned site than at the control site. Streams draining burned watersheds had higher summer GPP (2.4 gO₂ m⁻² day⁻¹) than the unburned sites (1.2 gO₂ m⁻² day⁻¹). Respiration was also higher in burned than unburned watersheds (3.9 and 3.0 gO₂ m⁻² day⁻¹, respectively). Metabolism was not correlated with the increased nutrient concentrations observed after fire. Instead, we suggest GPP was stimulated through increased soluble reactive phosphorus availability after fire, whereas respiration was likely controlled by pre-existing differences in stream physical or chemical characteristics.
• #### Permafrost Distribution Mapping And Temperature Modeling Along The Alaska Highway Corridor, Interior Alaska

An up-to-date permafrost distribution map is critical for making engineering decisions during the planning and design of any engineering project in Interior Alaska. I used a combination of empirical-statistical and remote sensing techniques to generate a high-resolution spatially continuous near-surface (< 1.6 m) permafrost map by exploiting the correlative relationships between permafrost and biophysical terrain parameters. A Binary Logistic Regression (BLR) model was used to establish the relationship between vegetation type, aspect-slope and permafrost presence. The logistic coefficients for each variable class obtained from the BLR model were supplied to respective variable classes mapped from remotely sensed data to estimate permafrost probability for every pixel. The BLR model predicts permafrost presence/absence at an accuracy of 88%. Near-surface permafrost occupies 37% of the total study area. A permafrost map based on the interpretation of airborne electromagnetic (EM) resistivity data shows 22.5 -- 43.5% of the total study area as underlain by permafrost. Permafrost distribution statistics from both the maps suggest near-surface permafrost distribution in the study area is sporadic (10 -- 50 % of the area underlain by permafrost). Changes in air temperature and/or winter snow depth are important factors responsible for permafrost aggradation or degradation. I evaluated the effects of past and recent (1941-2008) climate changes on permafrost and active-layer dynamics at selected locations using the Geophysical Institute Permafrost Laboratory model. Results revealed that active-layer thickness reached 0.58 and 1.0 m, and mean annual permafrost temperature increased by 1.6 and 1.7 �C during 1966-1994 at two sites in response to increased mean annual air temperature, mean summer air temperature and winter snow depth. The study found that active-layer thickness is not only a function of summer air temperature but also of mean annual air temperature and winter snow depth. Model simulation with a projected (2008-2098) climate scenario predicts 0.22 m loss of near-surface permafrost at one site and complete permafrost disappearance at another site by 2098. The contrasting permafrost behaviors at different sites under similar climate scenarios highlight the role of soil type and ground ice volume on permafrost dynamics; these factors determine permafrost resilience under a warming climate.
• #### Permafrost Dynamics In 20Th And 21St Centuries Along The East-Siberian And Alaskan Transects

High latitude ecosystems where the mean annual ground surface temperature is around or below 0�C are highly sensitive to global warming. This is largely because these regions contain vast areas of permafrost, which will begin to degrade when the mean annual ground temperatures will rise above 0�C. The Alaskan and East Siberian transects, centered on the 155� WL and 135� EL, were chosen for evaluation of permafrost---atmosphere interactions. The analysis of measured data shows a significant increase in air and ground temperatures that started from the late 1960s within both these transects and the magnitude of this increase is from 0.2 to 0.5�C per decade. This trend is comparable to trends predicted by majority of global warming scenarios. A simple and accurate model for evaluating the permafrost dynamics was developed in Geophysical Institute Permafrost Lab (GIPL). The GIPL model is a fusion of the modified Kudryavtsev's approach, which is a set of analytical formulas for active layer thickness (ALT) and mean annual ground temperature (MAGT) calculations, with the Geographic Information System (GIS). The evaluation of the GIPL performance showed that when applied to long-term (decadal or longer time scale) averages, this model achieves an accuracy of +/-0.2--0.4�C for the mean annual ground temperatures and +0.1--0.3 m for the active layer thickness calculations. The GIPL model was used for the hindcast of the permafrost dynamics in the 20th century, using a combination of observationally-based and simulated monthly grids of surface air temperature. The results showed that during the 20th century there were a number of relatively cold and warm periods. These climatic variations produced 1 to 3�C changes in MAGT and up to 1 m in the ALT. The forecast for the period of 2000--2100 was performed using climatic parameters from six Global Climate Models provided by Arctic Climate Impact Assessment program. The results showed that by the end of 21st century mean annual ground temperatures will be 2 to 6�C warmer and the ALT from 0.2 to 1 m deeper. During this period, in many areas within both transects the degradation of permafrost from the surface will start. By 2100, the area with actively degrading permafrost will cover about 10--15% of the Siberian transect and up to 30% and more within the Alaskan transect.
• #### Permafrost in a warmer world: net ecosystem carbon imbalance

Arctic tundra and boreal forest have accumulated a vast pool of organic carbon, twice as large as the atmospheric carbon pool and three times as large as the carbon contained by all living things. As the permafrost region warms, more of this carbon will be exposed to decomposition, combustion, and hydrologic export. This permafrost carbon feedback has been described as the largest terrestrial feedback to climate change as well as one of the most likely to occur; however, estimates of its strength vary by a factor of thirty. Models predict that some portion of this release will be offset by increased arctic and boreal biomass, but the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets with serious societal and environmental consequences. In this dissertation I investigate the potential and actual response of Arctic and boreal carbon balance to climate change. First, I present estimates from 98 permafrost-region experts of the response of circumarctic biomass, wildfire, and hydrologic carbon flux to warming over the next several centuries. Because precise estimates of the factors driving arctic and boreal carbon balance are unlikely in the near future, these qualitative estimates provide a holistic summary of current scientific understanding and provide a framework for assessing uncertainty and risk. Assessments indicate that little agreement exists on the magnitude and even sign of change in high-latitude biomass, and that end-of-the-century organic carbon release from arctic rivers and collapsing coastlines could increase three-fold while carbon loss via burning could increase seven-fold. Second, I test the impact of permafrost collapse (thermokarst) on carbon and nutrient release from upland tundra on the North Slope of Alaska. The biogeochemical consequences of thermokarst are not adequately conceptualized or characterized to incorporate into numerical models, though thermokarst may impact a third of the permafrost region by the end of the century. I employ a coupled aquatic and terrestrial experimental design to address this knowledge gap, measuring the displacement of soil organic carbon, surface flux of CO₂, CH₄, and N₂O, and hydrologic export of dissolved carbon and nutrients. Results show that thermokarst can stimulate or suppress ecosystem respiration depending on feature morphology; remove a large portion of ecosystem carbon; mobilize highly biodegradable dissolved organic carbon; disrupt the nitrogen cycle resulting in N₂O production and hydrologic nitrogen losses; and influence offsite organic matter decomposition by the release of labile dissolved organic carbon nitrogen, and other nutrients. Spatial patterns of carbon and nutrient export from thermokarst suggest that upland thermokarst may be a dominant linkage between terrestrial and aquatic ecosystems as the permafrost region warms. I conclude that the strength of the permafrost climate feedback depends largely on coupled carbon and nutrient dynamics, which will interact with disturbance such as wildfire and thermokarst. My results indicate that three-quarters of permafrost carbon release could be avoided if human emissions are actively reduced, though the window of opportunity to keep that carbon in the ground is rapidly closing.