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At least 577 records · Page 32Linked to original sources
USGS2020

Sphalerite oxidation in seawater with covellite: Implications for seafloor massive sulfide deposits and mine waste

Metal sulfide minerals exist in several marine environments and are in thermodynamic disequilibrium with oxygenated seawater from the time of their formation. Oxidation is both ubiquitous and heterogeneous, as observational and experimental evidence demonstrates that sulfide minerals may oxidize completely on decadal timescales (hydrothermal plumes) or incompletely in billions of years (mineral deposits); however, the processes, rates, and interactions among minerals as oxidative dissolution occurs are not well understood. Added impetus to understanding these processes exists due to the potential for mining of seafloor massive sulfide deposits and potential environmental impacts of that activity. Here, we present a laboratory-based experimental study on the galvanic oxidation of sphalerite and synthesized zinc sulfide and coupled to covellite. We find that, in contrast to single-mineral reactions, coupled mineral reactions are at least 2 orders of magnitude more rapid, light independent, and have a lower apparent activation energy for oxidation. These results begin to provide insight into observed differences between laboratory and environmentally observed oxidation rates and are a step in the direction of more accurately predicting environmental rates as well as any changes to those rates from anthropogenic disturbances.

ACS Earth and Space Chemistry↗
USGS2009

Multiple origins of linear dunes on Earth and Titan

Dunes with relatively long and parallel crests are classified as linear dunes. On Earth, they form in at least two environmental settings: where winds of bimodal direction blow across loose sand, and also where single-direction winds blow over sediment that is locally stabilized, be it through vegetation, sediment cohesion or topographic shelter from the winds. Linear dunes have also been identified on Titan, where they are thought to form in loose sand. Here we present evidence that in the Qaidam Basin, China, linear dunes are found downwind of transverse dunes owing to higher cohesiveness in the downwind sediments, which contain larger amounts of salt and mud. We also present a compilation of other settings where sediment stabilization has been reported to produce linear dunes. We suggest that in this dune-forming process, loose sediment accumulates on the dunes and is stabilized; the stable dune then functions as a topographic shelter, which induces the deposition of sediments downwind. We conclude that a model in which Titan's dunes formed similarly in cohesive sediments cannot be ruled out by the existing data.

Nature Geoscience↗
USGS2012

Perils of correlating CUSUM-transformed variables to infer ecological relationships (Breton et al. 2006; Glibert 2010)

We comment on a nonstandard statistical treatment of time-series data first published by Breton et al. (2006) in Limnology and Oceanography and, more recently, used by Glibert (2010) in Reviews in Fisheries Science. In both papers, the authors make strong inferences about the underlying causes of population variability based on correlations between cumulative sum (CUSUM) transformations of organism abundances and environmental variables. Breton et al. (2006) reported correlations between CUSUM-transformed values of diatom biomass in Belgian coastal waters and the North Atlantic Oscillation, and between meteorological and hydrological variables. Each correlation of CUSUM-transformed variables was judged to be statistically significant. On the basis of these correlations, Breton et al. (2006) developed "the first evidence of synergy between climate and human-induced river-based nitrate inputs with respect to their effects on the magnitude of spring Phaeocystis colony blooms and their dominance over diatoms."

Limnology and Oceanography↗
USGS2020

Global challenges for nitrogen science-policy interactions: Towards the International Nitrogen Management System (INMS) and improved coordination between multi-lateral environmental agreements

Human interference with the nitrogen cycle has doubled reactive nitrogen inputs to the global biosphere over the past century, leading to changes across multiple environmental issues that require urgent action. Nitrogen fertilizers and biological nitrogen fixation have allowed benefits of increased crop harvest and livestock production, while in some areas there is insufficient nitrogen to fertilize crops. Whether in excess or deficit, nitrogen losses from its inefficient use are causing a combination of freshwater and marine pollution, air pollution, alteration of climate balance, stratospheric ozone loss, biodiversity loss and reduction of soil quality. The resulting nitrogen pollution affects human health, well-being and livelihoods. Scientific efforts have begun to bring these issues together. However, there is still a high degree of fragmentation between research on the different benefits and threats of reactive nitrogen and between the respective policy frameworks, especially at the global scale. We argue that a more joined-up approach to managing the global nitrogen cycle is needed to develop the ‘gravity of common cause’ between nitrogen issues and to avoid policy trade-offs. We describe how a coherent system for science evidence provision is being developed to support policy development through the ‘International Nitrogen Management System’ (INMS). There is now a matching challenge to bring together the multiple policy agreements relevant for nitrogen as a foundation to address synergies/trade-offs and to set priorities. Based on review of existing frameworks, we outline the concept for an Interconvention nitrogen coordination mechanism. This could make a major contribution to multiple Sustainable Development Goals by stimulating the next generation of international nitrogen strategies: maximizing the benefits of efficient nitrogen use, while minimizing its many environmental threats.

Book chapter↗
USGS2019

Reproductive failure in the Eastern Migratory Population: The interaction of research and management

The reintroduction of the Eastern Migratory Population of Whooping Cranes ( Grus americana ) has shown the most promise of any effort to date toward the establishment of a self-sustaining population. However, reproduction – including both nest success and chick survival – has been a major challenge. Here, we review the research and management efforts deployed to identify and address this challenge. While there is evidence that a proximal cause of nest failure is harassment by blood-feeding black flies ( Simulium spp. ), this is not likely to be the only contributing factor, and we present alternative hypotheses. There is less understanding of the causes of poor chick survival, and we present hypotheses for factors that may be contributing to chick mortality, including environmental, genetic, and behavioral factors. For both nest success and chick survival, we carried out a value of information analysis, using expert judgment, which allowed us to prioritize the alternative hypotheses for evaluation based on the expected management benefit of addressing each hypothesis. On the horizon are multiple opportunities to explore hypotheses about environmental factors as well as genetic and behavioral characteristics of the birds themselves that may contribute to poor reproduction. If bird-specific characteristics, such as genetics or rearing methods, are hindering reproduction, this has important implications for all efforts to restore Whooping Crane populations.

Book chapter↗
USGS2011

Critical steps for the continuing advancement of hydrogeophysics

Special hydrogeophysics issues published by hydrology and geophysics journals, special sessions and workshops at conferences, and an increasing number of short courses demonstrate the growing interest in the use of geophysics for hydrologic investigations. The formation of the hydrogeophysics technical subcommittee of AGU's Hydrology section adds further evidence of the recognized significance of this growing interdisciplinary field. Given the clear value of nondestructive and nonintrusive imaging for subsurface investigations, we believe the advances in the adoption of existing geophysical methods, the development of novel methods, and the merging of geophysical and other data made in hydrogeophysics could be applied to a wide range of geological, environmental, and engineering applications.

Eos Science News↗
USGS2010

Use of cover habitat by bull trout, Salvelinus confluentus, and lake trout, Salvelinus namaycush, in a laboratory environment

Lacustrine-adfluvial bull trout, Salvelinus confluentus , migrate from spawning and rearing streams to lacustrine environments as early as age 0. Within lacustrine environments, cover habitat provides refuge from potential predators and is a resource that is competed for if limiting. Competitive interactions between bull trout and other species could result in bull trout being displaced from cover habitat, and bull trout may lack evolutionary adaptations to compete with introduced species, such as lake trout, Salvelinus namaycush . A laboratory experiment was performed to examine habitat use and interactions for cover by juvenile (i.e., <80 mm total length) bull trout and lake trout. Differences were observed between bull trout and lake trout in the proportion of time using cover ( F 1,22.6 = 20.08, P < 0.001) and bottom ( F 1,23.7 = 37.01, P < 0.001) habitat, with bull trout using cover and bottom habitats more than lake trout. Habitat selection ratios indicated that bull trout avoided water column habitat in the presence of lake trout and that lake trout avoided bottom habitat. Intraspecific and interspecific agonistic interactions were infrequent, but approximately 10 times greater for intraspecific interactions between lake trout. Results from this study provide little evidence that juvenile bull trout and lake trout compete for cover, and that species-specific differences in habitat use and selection likely result in habitat partitioning between these species.

Environmental Biology of Fishes↗
USGS2018

Evaluating long-term patterns of decreasing groundwater discharge through a lake-bottom permeable reactive barrier

Identifying and quantifying groundwater exchange is critical when considering contaminant fate and transport at the groundwater/surface-water interface. In this paper, areally distributed temperature and point seepage measurements are used to efficiently assess spatial and temporal groundwater discharge patterns through a glacial-kettle lakebed area containing a zero-valent iron permeable reactive barrier (PRB). Concern was that the PRB was becoming less permeable with time owing to biogeochemical processes within the PRB. Patterns of groundwater discharge over an 8-year period were examined using fiber-optic distributed temperature sensing (FO-DTS) and snapshot-in-time point measurements of temperature. The resulting thermal maps show complex and uneven distributions of temperatures across the lakebed and highlight zones of rapid seepage near the shoreline and along the outer boundaries of the PRB. Repeated thermal mapping indicates an increase in lakebed temperatures over time at periods of similar stage and surface-water temperature. Flux rates in six seepage meters permanently installed on the lakebed in the PRB area decreased on average by 0.021 md -1 (or about 4.5 percent) annually between 2004 and 2015. Modeling of diurnal temperature signals from shallow vertical profiles yielded mean flux values ranging from 0.39 to 1.15 md -1 , with stronger fluxes generally related to colder lakebed temperatures. The combination of an increase in lakebed temperatures, declines in direct seepage, and observations of increased cementation of the lakebed surface provide in situ evidence that the permeability of the PRB is declining. The presence of temporally persistent rapid seepage zones is also discussed.

Journal of Environmental Management↗
USGS1993

Terrestrial and marine records of climatic and environmental changes during the Pliocene in subtropical Florida

Pollen, ostracode, and benthic foraminifer assemblages deposited during sea-level high-stands in subtropical Florida record a climate change during the period 4.5-1.0 Ma. Before 3.5 Ma, open-shelf marine faunas and pollen assemblages with abundant Pinus , Quercus , Fagus , Carya , and nonarboreal pollen were present, indicating cooler conditions than today. From ∼3.5 to 1.0 Ma, marine and terrestrial records indicate warmer conditions, similar to those existing in south Florida today. Combined with evidence for much warmer than modern conditions at high latitudes, these data suggest that increased poleward oceanic heat transport, possibly related to the emergence of the Central American isthmus between ∼3.5 and 2.5 Ma, was a major influence on mid-Pliocene warmth.

Geology↗
USGS1999

Terrestrial predator alarm vocalizations are a valid monitor of stress in captive brown capuchins (Cebus apella)

The vocal behavior of captive animals is increasingly exploited as an index of well-being. Here we show that the terrestrial predator alarm (TPA) vocalization, a robust and acoustically distinctive anti-predation vocal response present in many mammal and bird species, offers useful information on the relative well-being and stress levels of captive animals. In a 16-week experiment evaluating the effects of varying levels of physical environmental enrichment (control < toys < foraging box < foraging box and toys) in the cages of eight singly housed adult male brown capuchins, we quantified the 1) emission rate of TPAs, 2) proportions of normal and abnormal behavior sample intervals, and 3) fecal and plasma cortisol levels. Variation in TPA emission across the experimental conditions was significant. We found significant reductions in the mean TPA production rate by the group in the enriched (toys, foraging box, and foraging box and toys) compared to the control condition; pre- and post-experimental conditions, however, did not differ from the control condition. Mean TPA production by the group was also significantly positively correlated to mean group levels of fecal cortisol and proportion of abnormal behavior sample intervals, and significantly negatively correlated to the average proportion of normal behavior sample intervals in the group. Based on group means, plasma cortisol levels were positively, but not significantly, related to increasing TPA rate. At the level of the responses of an individual subject, however, the covariation between the vocal and non-vocal behavioral measures and the cortisol assays seldom attained significance. Nevertheless, the direction of the relationships among these parameters within individual subjects typically mirrored those correlations based on group means. At both the group mean and individual levels, our results are consistent with the interpretation that in conditions of low environmental enrichment the study subjects were more stressed, and therefore more reactive to the presence of a threatening terrestrial stimulus (human observer), than when in more enriched conditions. We suggest that protocols to evaluate the effectiveness of enrichment for captive species other than brown capuchins could also profitably exploit TPAs as a first-line monitor or as corroboratory evidence of current well-being.

Zoo Biology↗
USGS2020

From pools to flow: The PROMISE framework for new insights on soil carbon cycling in a changing world

Soils represent the largest terrestrial reservoir of organic carbon, and the balance between soil organic carbon (SOC) formation and loss will drive powerful carbon‐climate feedbacks over the coming century. To date, efforts to predict SOC dynamics have rested on pool‐based models, which assume classes of SOC with internally homogenous physicochemical properties. However, emerging evidence suggests that soil carbon turnover is not dominantly controlled by the chemistry of carbon inputs, but rather by restrictions on microbial access to organic matter in the spatially heterogeneous soil environment. The dynamic processes that control the physicochemical protection of carbon translate poorly to pool‐based SOC models; as a result, we are challenged to mechanistically predict how environmental change will impact movement of carbon between soils and the atmosphere. Here, we propose a novel conceptual framework to explore controls on belowground carbon cycling: P robabilistic R epresentation of O rganic M atter I nteractions within the S oil E nvironment (PROMISE). In contrast to traditional model frameworks, PROMISE does not attempt to define carbon pools united by common thermodynamic or functional attributes. Rather, the PROMISE concept considers how SOC cycling rates are governed by the stochastic processes that influence the proximity between microbial decomposers and organic matter, with emphasis on their physical location in the soil matrix. We illustrate the applications of this framework with a new biogeochemical simulation model that traces the fate of individual carbon atoms as they interact with their environment, undergoing biochemical transformations and moving through the soil pore space. We also discuss how the PROMISE framework reshapes dialogue around issues related to SOC management in a changing world. We intend the PROMISE framework to spur the development of new hypotheses, analytical tools, and model structures across disciplines that will illuminate mechanistic controls on the flow of carbon between plant, soil, and atmospheric pools.

Global Change Biology↗
USGS1986

Organochlorine contaminants in passeriformes and other avian prey of the peregrine falcon in the western United States

About 1,150 individuals of 38 species of Passeriformes and other small birds were collected in 1980 at 36 locations in eight western U.S. states for organochlorine (OC) chemical analysis. The bird carcasses (less beak, tarsi, gastrointestinal tract, and feathers) were combined by species and location (4 to 15 specimens per sample) into 124 composited samples for analysis. Also analyzed were 77 single specimens of four species, to assess variation in residue concentrations in individual birds. The chemicals detected (greater than 0.05 ppm) were the following (in order of frequency): DDE, polychlorinated biphenyls (PCBs), hexachlorocyclohexane (HCH), heptachlor epoxide, oxychlordane, dieldrin and toxaphene. DDE accounted for 72% of total OC concentrations (ppm DDE/ppm total OCs) overall and PCBs accounted for 3%. DDE mean (geometric) residues were highest in tree swallows ( Tachycineta bicolor Vieillot), 12.0 ppm; killdeer ( Charadrius vociferus L.), 5.9 ppm; and Brewer's blackbirds ( Euphagus cyanocephalus Wagler), 2.7 ppm. Eight migratory species showed mean DDE residues 13 times higher than those in four resident species (1.3 vs. 0.1 ppm, p < 0.05), but PCB residues were similar (0.03 vs. 0.02 ppm). Insectivorous species showed higher DDE, PCB and total OC residues than did either omnivores or granivores ( p < 0.05). Males of some species showed higher residues of DDE, PCBs and total OCs than did females ( p < 0.05). DDE residues, but not PCBs, in killdeer, Brewer's blackbirds and violet-green swallows ( Tachycineta thalassina Swainson) were significantly related (0.01 < p < 0.08) to latitude and longitude of origin. Concentrations of DDE in the carcass fat of some individual tree swallows and killdeer would be in the lethal range if 15 to 20% of the stored DDE were rapidly mobilized to the brain. Samples of 13 species contained DDE concentrations (greater than 3 ppm) considered sufficient to inhibit the normal reproduction of avian predators feeding on them. The evidence suggests that potentially harmful OC concentrations are present in certain western migrants and pose an even greater hazard to avian predators such as the peregrine falcon ( Falco peregrinus Tunstall).

Environmental Toxicology and Chemistry↗
USGS2024

Potential hazards of polycyclic aromatic hydrocarbons in Great Lakes tributaries using water column and porewater passive samplers and sediment wquilibrium partitioning

The potential for polycyclic aromatic hydrocarbon (PAH)-related effects in benthic organisms is commonly estimated from organic carbon-normalized sediment concentrations based on equilibrium partitioning (EqP). Although this approach is useful for screening purposes, it may overestimate PAH bioavailability by orders of magnitude in some sediments, leading to inflated exposure estimates and potentially unnecessary remediation costs. Recently, passive samplers have been shown to provide an accurate assessment of the freely dissolved concentrations of PAHs, and thus their bioavailability and possible biological effects, in sediment porewater and overlying surface water. We used polyethylene passive sampling devices (PEDs) to measure freely dissolved porewater and water column PAH concentrations at 55 Great Lakes (USA/Canada) tributary locations. The potential for PAH-related biological effects using PED concentrations were estimated with multiple approaches by applying EqP, water quality guidelines, and pathway-based biological activity based on in vitro bioassay results from ToxCast. Results based on the PED-based exposure estimates were compared with EqP-derived exposure estimates for concurrently collected sediment samples. The results indicate a potential overestimation of bioavailable PAH concentrations by up to 960-fold using the EqP-based method compared with measurements using PEDs. Even so, PED-based exposure estimates indicate a high potential for PAH-related biological effects at 14 locations. Our findings provide an updated, weight-of-evidence–based site prioritization to help guide possible future monitoring and mitigation efforts.

Illinois, Indiana, Michigan, Minnesota, New York, ↗
USGS2023

Aging, climate, and invasions threaten reservoirs in the Mississippi basin

Reservoirs in the Mississippi River basin are facing three momentous threats. The first two, aging and climate change, are relatively slow moving and their signal can be hard to discern given their stretched temporal scales. The third, species invasions, is faster paced and discernable within shorter temporal scales and restricted spatial scales. Aging and climate directly affect reservoir environments and indirectly affect their biotic communities. Climate change is expected to interact with aging to speed up and, in some instances, slow down aging. Conversely, invasions primarily imperil biotic communities but can also impact environmental elements. This triple jeopardy is expected to transform reservoir environs and their biotic assemblages in various, often uncertain, ways. I take a broad view of these threats within the Mississippi River basin. A basin-scale perspective, in contrast to a single reservoir or regional scale, may enhance awareness of reservoirs at a larger level and produce understanding less evident at local levels, hence possibly offering a wider range of choices for confronting threats. My aim is to (1) provide a synopsis of the assemblage of reservoirs and their attributes over the Mississippi River basin, (2) describe the issues related to aging, climate change, and invasions, and (3) consider a conservation framework for confronting these pressures. Given the overlapping temporal, spatial, and ecological effects of these threats, it is essential to address their effects simultaneously.

Mississippi River basin↗
USGS2014

Toxicity of Pb-contaminated soil to Japanese quail (Coturnix japonica) and the use of the blood-dietary Pb slope in risk assessment

This study relates tissue concentrations and toxic effects of Pb in Japanese quail ( Coturnix japonica ) to the dietary exposure of soil-borne Pb associated with mining and smelting. From 0% to 12% contaminated soil, by weight, was added to 5 experimental diets (0.12 to 382 mg Pb/kg, dry wt) and fed to the quail for 6 weeks. Benchmark doses associated with a 50% reduction in delta-aminolevulinic acid dehydratase activity were 0.62 mg Pb/kg in the blood, dry wt, and 27 mg Pb/kg in the diet. Benchmark doses associated with a 20% increase in the concentration of erythrocyte protoporphyrin were 2.7 mg Pb/kg in the blood and 152 mg Pb/kg in the diet. The quail showed no other signs of toxicity (histopathological lesions, alterations in plasma–testosterone concentration, and body and organ weights). The relation of the blood Pb concentration to the soil Pb concentration was linear, with a slope of 0.013 mg Pb/kg of blood (dry wt) divided by mg Pb/kg of diet. We suggest that this slope is potentially useful in ecological risk assessments on birds in the same way that the intake slope factor is an important parameter in risk assessments of children exposed to Pb. The slope may also be used in a tissue-residue approach as an additional line of evidence in ecological risk assessment, supplementary to an estimate of hazard based on dietary toxicity reference values.

Missouri↗
USGS2021

Sage-grouse population dynamics are adversely impacted by overabundant feral horses

In recent decades, feral horse ( Equus caballus ; horse) populations increased in sagebrush ( Artimesia spp.) ecosystems, especially within the Great Basin, to the point of exceeding maximum appropriate management levels (AML max ), which were set by land administrators to balance resource use by feral horses, livestock, and wildlife. Concomitantly, greater sage-grouse ( Centrocercus urophasianus ; sage-grouse) are sagebrush obligates that have experienced population declines within these same arid environments as a result of steady and continued loss of seasonal habitats. Although a strong body of research indicates that overabundant populations of horses degrade sagebrush ecosystems, empirical evidence linking horse abundance to sage-grouse population dynamics is missing. Within a Bayesian framework, we employed state-space models to estimate population rate of change ( λ ) using 15 years (2005–2019) of count surveys of male sage-grouse at traditional breeding grounds (i.e., leks) as a function of horse abundance relative to AML max and other environmental covariates (e.g., wildfire, precipitation, % sagebrush cover). Additionally, we employed a post hoc impact-control design to validate existing AML max values as related to sage-grouse population responses, and to help control for environmental stochasticity and broad-scale oscillations in sage-grouse abundance. On average, for every 50% increase in horse abundance over AML max , our model predicted an annual decline in sage-grouse abundance by 2.6%. Horse abundance at or below AML max coincided with sage-grouse λ estimates that were consistent with trends at non-horse areas elsewhere in the study region. Thus, AML max , as a whole, appeared to be set adequately in preventing adverse effects to sage-grouse populations. Results indicated 76%, 97%, and >99% probability of sage-grouse population decline relative to controls when horse numbers are 2, 2.5, and ≥3 times over AML max , respectively. As of 2019, horse herds exceeded AML max in Nevada, USA, by >4 times on average across all horse management areas. If feral horse populations continue to grow at current rates unabated, model projections indicate sage-grouse populations will be reduced within horse-occupied areas by >70.0% by 2034 (15-year projection), on average compared to 21.2% estimated for control sites. A monitoring framework that improves on estimating horse abundance and identifying responses of sage-grouse and other key indicator species (plant and animal) would be beneficial to guide management decisions that promote co-occurrence of horses with sensitive wildlife and livestock within landscapes subjected to multiple uses. Published 2021. This article is a U.S. Government work and is in the public domain in the USA. The Journal of Wildlife Management published by Wiley Periodicals LLC on behalf of The Wildlife Society.

Nevada↗
USGS2021

Directional selection shifts trait distributions of planted species in dryland restoration

The match between species trait values and local abiotic filters can restrict community membership. An often-implicit assumption of this relationship is that abiotic filters select for a single locally optimal strategy, though difficulty in isolating effects of the abiotic environment from those of dispersal limitation and biotic interactions has resulted in few empirical tests of this assumption. Similar constraints have made it difficult to assess whether the type and intensity of abiotic filters shift along gradients of environmental harshness, as predicted by the stress-dominance hypothesis. We planted 9,216 plants of 29 perennial grass and forb species that had a range of functional trait values and were assigned to a warm, intermediate or cool temperature tolerance pool across eight sites on the Colorado Plateau. We compared the distributions of traits of surviving individuals to null distributions to evaluate whether there were shifts in trait means and variation. Borrowing from phenotypic selection concepts in evolutionary biology, we assessed support for stabilizing, directional and disruptive abiotic filtering of trait distributions and whether these types of filtering varied with initial species pool. Functional composition was significantly different from null distributions for nearly all traits at all sites, with trait variation more restricted in harsher abiotic conditions, supporting the stress-dominance hypothesis. Contrary to expectations, we primarily found evidence for directional selection, which increased in frequency in warm species pools while disruptive selection was found more often in cool and intermediate species pools. Synthesis . This study provides a controlled experimental approach to test the effect of the abiotic environment on plant trait filtering. We found that opportunistic strategies allowing for rapid water acquisition during favourable periods improved survival at warmer sites. Species with these strategies may be expected to benefit from increasing aridity and may be selected for active management efforts. More generally, the prevalence of directional selection may have important implications for dynamic vegetation models that rely on trait distributions for translating environmental variation into ecosystem processes.

Journal of Ecology↗
USGS1983

Late Holocene ice wedges near Fairbanks, Alaska, USA: Environmental setting and history of growth

Test trenches excavated into muskeg near Fairbanks in 1969 exposed a polygonal network of active ice wedges. The wedges occur in peat that has accumulated since about 3500 yr BP and have grown episodically as the permafrost table fluctuated in response to fires, other local site conditions and perhaps regional climatic changes. Radiocarbon dates suggest one or two episodes of ice-wedge growth between about 3500 and 2000 yr BP as woody peat accumulated at the site. Subsequent wedge truncation evidently followed a fire that charred the peat. Younger peat exhibits facies changes between sedge-rich components that filled troughs over the ice wedges and woody bryophytic deposits that formed beyond the troughs. A final episode of wedge development took place within the past few hundred years. Pollen data from the site indicate that boreal forest was present throughout the past 6000 yr, but that it underwent a gradual transition from a predominantly deciduous to a spruce-dominated assemblage. This change may reflect either local site conditions or a more general climatic shift to cooler, moister summers in late Holocene time. The history of ice-wedge growth shows that wedges can form and grow to more than 1 m apparent width under mean annual temperatures that probably are close to those of the Fairbanks area today (-3.5°C) and under vegetation cover similar to that of the interior Alaskan boreal forest. The commonly held belief that ice wedges develop only below mean annual air temperatures of -6 to -8°C in the zone of continuous permafrost is invalid.

Alaska↗