Eastern Brooks Range Research Articles

Hydrology Controls Dissolved Organic Carbon and Nitrogen Export and Post‐Storm Recovery in Two Arctic Headwaters

AbstractClimate change is rapidly altering hydrological processes and consequently the structure and functioning of Arctic ecosystems. Predicting how these alterations will shape biogeochemical responses in rivers remains a major challenge. We measured [C]arbon and [N]itrogen concentrations continuously from two Arctic watersheds capturing a wide range of flow conditions to assess understudied event‐scale C and N concentration‐discharge (C‐Q) behavior and post‐event recovery of stoichiometric conditions. The watersheds represent low‐gradient, tundra landscapes typical of the eastern Brooks Range on the North Slope of Alaska and are part of the Arctic Long‐Term Ecological Research sites: the Kuparuk River and Oksrukuyik Creek. In both watersheds, we deployed high‐frequency optical sensors to measure dissolved organic carbon (DOC), nitrate (), and total dissolved nitrogen (TDN) for five consecutive thaw seasons (2017–2021). Our analyses revealed a lag in DOC: stoichiometric recovery after a hydrologic perturbation: while DOC was consistently elevated after high flows, diluted during rainfall events and consequently, recovery in post‐event concentration was delayed. Conversely, the co‐enrichment of TDN at high flows, even in watersheds with relatively high N‐demand, represents a potential “leak” of hydrologically available organic N to downstream ecosystems. Our use of high‐frequency, long‐term optical sensors provides an improved method to estimate carbon and nutrient budgets and stoichiometric recovery behavior across event and seasonal timescales, enabling new insights and conceptualizations of a changing Arctic, such as assessing ecosystem disturbance and recovery across multiple timescales.

Ulungarat Basin: Record of a major Middle Devonian to Mississippian syn-rift to post-rift tectonic transition, eastern Brooks Range, Arctic Alaska

Abstract The Ulungarat Basin of Arctic Alaska is a unique exposed stratigraphic record of the mid-Paleozoic transition from the Romanzof orogeny to post-orogenic rifting and Ellesmerian passive margin subsidence. The Ulungarat Basin succession is composed of both syn-rift and post-rift deposits recording this mid-Paleozoic transition. The syn-rift deposits unconformably overlie highly deformed Romanzof orogenic basement on the mid-Paleozoic regional angular unconformity and are unconformably overlain by post-rift Endicott Group deposits of the Ellesmerian passive margin. Shallow marine strata of Eifelian age at the base of the Ulungarat Formation record onset of rifting and limit age of the Romanzof orogeny to late Early Devonian. Abrupt thickness and facies changes within the Ulungarat Formation and disconformably overlying syn-rift Mangaqtaaq Formation suggest active normal faulting during deposition. The Mangaqtaaq Formation records lacustrine deposition in a restricted down-faulted structural low. The unconformity between syn-rift deposits and overlying post-rift Endicott Group is interpreted to be the result of sediment bypass during deposition of the outboard allochthonous Endicott Group. Within Ulungarat Basin, transgressive post-rift Lower Mississippian Kekiktuk Conglomerate and Kayak Shale (Endicott Group) are older and thicker than equivalents to the north. North of Ulungarat Basin, deformed pre-Middle Devonian rocks were exposed to erosion at the mid-Paleozoic regional unconformity for ∼50 m.y., supplying sediments to the rift basin and broader Arctic Alaska rifted margin beyond. Although Middle Devonian to Lower Mississippian chert- and quartz-pebble conglomerates and sandstones across Arctic Alaska share a common provenance from the eroding ancestral Romanzof highlands, they were deposited in different tectonic settings.

Poor nutrition as a potential cause of divergent tree growth near the Arctic treeline in northern Alaska.

Trees growing near the Arctic treeline have long been used to reconstruct past climates. However, recent studies have shown deterioration of historically strong positive correlations between air temperature and tree growth (known as "divergence"). Divergence has important implications for confidence in paleoclimate reconstructions and ecosystem-atmosphere carbon exchange. Studies in the Brooks Range of northern Alaska showed that white spruce in the west increased growth in response to late 20th century warming, whereas those in the east failed to show a growth increase. In an earlier study across four watersheds in the Brooks Range, we tested and rejected the hypothesis that divergence in the easternmost watershed reflects moisture limitation of growth. Here, using 16 sites distributed across the same four watersheds, we tested an alternative hypothesis, that greater nutrient limitation in the east may have impeded positive growth responses to warming. Climate comparison across the four Brooks Range study watersheds revealed that, although the easternmost watershed generally had a drier growing-season climate, the most consistent difference was that winter air temperature and both winter and summer soil temperatures were much colder in the central and eastern watersheds. Soil nutrient availability, foliar nutrient concentrations, and tree growth were all generally lower in the central and eastern than in the western watersheds. Foliar phosphorus concentration was the best predictor of spatial variation in branch extension growth-a finding that is somewhat inconsistent with the theory that forest productivity on young, glacially derived soils should be strongly nitrogen limited. Experimental fertilization yielded the greatest growth increase in the eastern, an intermediate response in the central, and the smallest growth increase in the western watershed, generally mirroring trends in soil temperature, soil nutrient availability, foliar nutrient concentrations, and growth of control trees. Our results confirm that growth in the easternmost watershed is more nutrient limited and suggest that phosphorus limitation may be at least as important as nitrogen limitation of growth. We hypothesize that cold soil effects on tree access to nutrients might explain divergence in the eastern Brooks Range and elsewhere near the Arctic treeline, particularly in areas with cold winters and widespread permafrost.

Drought-induced stomatal closure probably cannot explain divergent white spruce growth in the Brooks Range, Alaska, USA.

Increment cores from the boreal forest have long been used to reconstruct past climates. However, in recent years, numerous studies have revealed a deterioration of the correlation between temperature and tree growth that is commonly referred to as divergence. In the Brooks Range of northern Alaska, USA, studies of white spruce (Picea glauca) revealed that trees in the west generally showed positive growth trends, while trees in the central and eastern Brooks Range showed mixed and negative trends during late 20th century warming. The growing season climate of the eastern Brooks Range is thought to be drier than the west. On this basis, divergent tree growth in the eastern Brooks Range has been attributed to drought stress. To investigate the hypothesis that drought-induced stomatal closure can explain divergence in the Brooks Range, we synthesized all of the Brooks Range white spruce data available in the International Tree Ring Data Bank (ITRDB) and collected increment cores from our primary sites in each of four watersheds along a west-to-east gradient near the Arctic treeline. For cores from our sites, we measured ring widths and calculated carbon isotope discrimination (δ13C), intrinsic water-use efficiency (iWUE), and needle intercellular CO2 concentration (C(i)) from δ13C in tree-ring alpha-cellulose. We hypothesized that trees exhibiting divergence would show a corresponding decline in δ13C, a decline in C(i), and a strong increase in iWUE. Consistent with the ITRDB data, trees at our western and central sites generally showed an increase in the strength of the temperature-growth correlation during late 20th century warming, while trees at our eastern site showed strong divergence. Divergent tree growth was not, however, associated with declining δ13C. Meanwhile, estimates of C(i) showed a strong increase at all of our study sites, indicating that more substrate was available for photosynthesis in the early 21st than in the early 20th century. Our results, which are corroborated by measurements of xylem sap flux density, needle gas exchange, and measurements of growth and δ13C along moisture gradients within each watershed, suggest that drought-induced stomatal closure is probably not the cause of 20th century divergence in the Brooks Range.

Overwintering Locations, Migrations, and Fidelity of Radio-Tagged Dolly Varden in the Hulahula River, Arctic National Wildlife Refuge, 2007–09

Essential overwintering habitats for anadromous Dolly Varden Salvelinus malma on Alaska’s North Slope appear to be limited to a small number of perennial springs, primarily in eastern Brooks Range drainages. Because future petrochemical development in the region continues to be a possibility, and development would require large quantities of freshwater, we sought to identify and document the overwintering areas used by Dolly Varden in the Hulahula River, eastern Brooks Range. In August 2007, we implanted 52 Dolly Varden with multi-year radio transmitters at a known overwintering area in the lower Hulahula River. Other wintering areas were located during 11 aerial surveys conducted over the next 2.5 years. A stationary receiver located in the lower Hulahula River provided migration timing information. Radio-tagged Dolly Varden used four discrete areas with perennial springs for overwintering in the Hulahula River drainage. The springs, totaling approximately 12 km in stream length, were located between river km 40 and 105. Radio-tagged Dolly Varden migrated downstream on their way to the Beaufort Sea in early June. Most tagged fish known to have survived the summer at sea returned to the Hulahula River during late July and August, but seven fish overwintered in other North Slope drainages. Within the Hulahula River drainage, 15 fish overwintered in more than one area during the three winters of the project, but only the four identified perennial spring areas were used. These data clearly indicate that the perennial springs in the Hulahula River are essential overwintering habitats for Dolly Varden.

Limited phylogeographic structure and genetic variation in Alaska's arctic and alpine endemic, the Alaska marmot

Alpine and arctic environments are thought to be more vulnerable to climate change than other lower-elevation and lower-latitude regions. Being both arctic and alpine distributed, the Alaska marmot (Marmota broweri )i s uniquely suited to serve as a harbinger of the effects of climate change, yet it is the least-studied marmot species in North America. We investigated the phylogeography and genetic diversity of M. broweri throughout its known distribution in northern Alaska using the mitochondrial cytochrome b gene to better understand how post-Pleistocene changes and population fragmentation have structured genetic diversity. Our results show significant, although shallow, geographic structure among Alaska marmot populations. The diversity within and among populations is consistent with 2 phylogeographic hypotheses: Alaska marmots persisted in the eastern Brooks Range, Ray Mountains, and Kokrines Hills during the Pleistocene and have only recently expanded into the western Brooks Range; and the western Brooks Range served as a refugium as well and those populations have undergone a bottleneck resulting in reduced genetic variation in extant populations. Levels of mitochondrial deoxyribonucleic acid diversity are lower in M. broweri than in any other codistributed small mammal species and alpine mammal species with comparable data available. This is the 1st phylogeographic study of any marmot species and provides a baseline measure of the current structure and diversity within M. broweri.

Deformation and the timing of gas generation and migration in the eastern Brooks Range foothills, Arctic National Wildlife Refuge, Alaska

Along the southeast border of the 1002 Assessment Area in the Arctic National Wildlife Refuge, Alaska, an explicit link between gas generation and deformation in the Brooks Range fold and thrust belt is provided through petrographic, fluid inclusion, and stable isotope analyses of fracture cements integrated with zircon fission-track data. Predominantly quartz-cemented fractures, collected from thrusted Triassic and Jurassic rocks, contain crack-seal textures, healed microcracks, and curved crystals and fluid inclusion populations, which suggest that cement growth occurred before, during, and after deformation. Fluid inclusion homogenization temperatures (175–250C) and temperature trends in fracture samples suggest that cements grew at 7–10 km depth during the transition from burial to uplift and during early uplift. CH4-rich (dry gas) inclusions in the Shublik Formation and Kingak Shale are consistent with inclusion entrapment at high thermal maturity for these source rocks. Pressure modeling of these CH4-rich inclusions suggests that pore fluids were overpressured during fracture cementation.Zircon fission-track data in the area record postdeposition denudation associated with early Brooks Range deformation at 64 3 Ma. With a closure temperature of 225–240C, the zircon fission-track data overlap homogenization temperatures of coeval aqueous inclusions and inclusions containing dry gas in Kingak and Shublik fracture cements. This critical time-temperature relationship suggests that fracture cementation occurred during early Brooks Range deformation. Dry gas inclusions suggest that Shublik and Kingak source rocks had exceeded peak oil and gas generation temperatures at the time structural traps formed during early Brooks Range deformation. The timing of hydrocarbon generation with respect to deformation therefore represents an important exploration risk for gas exploration in this part of the Brooks Range fold and thrust belt. The persistence of gas high at thermal maturity levels suggests, however, that significant volumes of gas may have been generated.

Deformation and the timing of gas generation and migration in the eastern Brooks Range foothills, Arctic National Wildlife Refuge, Alaska

Deformation and the timing of gas generation and migration in the eastern Brooks Range foothills, Arctic National Wildlife Refuge, Alaska

The Alaskan Arctic Frontal Zone: Forcing by Orography, Coastal Contrast, and the Boreal Forest

Experiments have been conducted with a regional climate model to indicate the conditions required to generate preferred regions of frontal activity in the Alaskan region. Several objective methods of frontal identification were first investigated. It was found that· the vertical component of relative vorticity,· a thermal front parameter −∇|∇T850| · n, where T850 is the 850-hPa temperature and n is a unit vector in the direction of the 850-hPa temperature gradient, and· a parameter derived from the Q vector as a measure of vertical motionwere useful in combination to determine the occurrence of fronts. The preferred locations for frontal activity were located to the southern side of the eastern Brooks Range and over the Alaska Range. These diagnostics were then used to characterize frontal frequency in a series of experiments removing topographic and vegetation contrasts. It was found that the removal of the treeline contrast and its associated heating gradient had a small effect on frontal frequency in the immediate vicinity of the tree line, but that the largest impact was in response to the removal of topography, greatly reducing frontal frequency. The coastal contrast was found to have a limited role in synoptic frontal activity in the Alaskan region.

Alaska's indigenous muskoxen: a history

Muskoxen (Ovibos moschatus) were widespread in northern and interior Alaska in the late Pleistocene but were never a dominant component of large mammal faunas. After the end of the Pleistocene they were even less common. Most skeletal finds have come from the Arctic Coastal Plain and the foothills of the Brooks Range. Archaeological evidence, mainly from the Point Barrow area, suggests that humans sporadically hunted small numbers of muskoxen over about 1500 years from early Birnirk culture to nineteenth century Thule culture. Skeletal remains found near Kivalina represent the most southerly Holocene record for muskoxen in Alaska. Claims that muskoxen survived into the early nineteenth century farther south in the Selawik - Buckland River region are not substantiated. Remains of muskox found by Beechey's party in Eschscholtz Bay in 1826 were almost certainly of Pleistocene age, not recent. Neither the introduction of firearms nor overwintering whalers played a significant role in the extinction of Alaska's muskoxen. Inuit hunters apparently killed the last muskoxen in northwestern Alaska in the late 1850s. Several accounts suggest that remnant herds survived in the eastern Brooks Range into the 1890s. However, there is no physical evidence or independent confirmation of these reports. Oral traditions regarding muskoxen survived among the Nunamiut and the Chandalar Kutchin. With human help, muskoxen have successfully recolonized their former range from the Seward Peninsula north, across the Arctic Slope and east into the northern Yukon Territory.

Stratigraphic contrasts and tectonic relationships between Carboniferous successions in the Trans‐Alaska Crustal Transect corridor and adjacent areas, northern Alaska

The Carboniferous succession along the Trans‐Alaska Crustal Transect (TACT) corridor in the Atigun Gorge area of the central Brooks Range consists of the Kayak Shale (Kinderhookian) and the Lisburne Group (Kinderhookian through Chesterian). The Kayak Shale is at least 210 m thick; it is chiefly black, noncalcareous shale with several limestone beds of pelmatozoan‐bryozoan packstone and formed in an open‐marine setting. The Lisburne Group is a carbonate rock succession about 650 m thick and consists mainly of skeletal packstone, wackestone, and milestone which contain locally abundant calcispheres, ostracodes, algae, and sponge spicules; it accumulated largely in a shallow water platform environment with restricted circulation. This restriction was probably produced by a coeval belt of skeletal sand shoals recognized 70 km to the west in the Shainin Lake area. Significant and apparently abrupt shifts in the age and lithofacies of Carboniferous strata occur across the central and eastern Brooks Range. These shifts are most marked in a zone roughly coincident with what is interpreted by many workers to be the leading edge of the Endicott Mountains allochthon. Notable lithologie contrasts are also observed, however, between sections in the northern and southern parts of the Endicott Mountains allochthon. This suggests that considerable tectonic shortening has taken place within the allochthon, as well as between it and parautochthonous rocks to the northeast. The Carboniferous section near Mount Doonerak is more similar in age and lithofacies to coeval sections in the central Brooks Range that are considered allochthonous than to parautochthonous sections to the northeast.

Devonian-Mississippian Carbonate Sequence in the Maiyumerak Mountains, Western Brooks Range, Alaska: ABSTRACT

Essentially continuous, dominantly carbonate sedimentation occurred from at least the Early Devonian through the Mississippian in the area that is now the Maiyumerak Mountains, western Brooks Range. This succession is in striking contrast to Paleozoic sequences in the eastern Brooks Range and in the subsurface across northern Alaska, where uppermost Devonian-Mississippian clastic and Carboniferous carbonates unconformably overlie Proterozoic or lower Paleozoic metasedimentary or sedimentary rocks. Conodonts obtained throughout the Maiyumerak Mountains sequence indicate that any hiatus is less than a stage in duration, and there is no apparent physical evidence of unconformity within the succession. The sequence is best exposed northwest of the Eli River, where Emsian-Eifelian dolostones (Baird Group) are conformably overlain by Kinderhookian-Osagian sandy limestones (Utukok Formation) and Osagian-Chesterian fossiliferous limestones (Kogruk Formation) of the Lisburne Group. Conodont species assemblages and sedimentary structures indicate deposition in a range of shallow-water shelf environments. The sequence extends at least 30 km, from the Noatak Quadrangle northeast into the Baird Mountains Quadrangle; its easternmost extent has not been definitively determined. The Ellesmerian orogeny, thought to have produced the extensive middle Paleozoic unconformity seen through much of northern Alaska apparently had little effect on this western Brooks Range sedimentary succession.

Northeastern Brooks Range, Alaska: New Evidence for Complex Thin-Skinned Thrusting: ABSTRACT

Extensive fieldwork has shed new light on the style of deformation in the Franklin, Romanzof, and British Mountains of the northeastern Brooks Range. Bedding-parallel thrusting controls the structure, and two End_Page 676------------------------------ major decollements are recognized. In the mountain belt, the lower one lies in the Lower Carboniferous Kayak Formation but often steps down to the base of the Upper Devonian Kanayut or Lower Carboniferous Kekiktuk Conglomerates. Near the Sadlerochit and Shublik Mountains, it steps down to its deepest level to the base of the Cambrian to Middle Devonian Katakturuk Dolomite. The upper decollement is poorly exposed in the mountains and lies in the Jurassic Kingak Shale. Locally, these are removed by Early Cretaceous erosion and the decollement steps upsequence. The two decollements separate three tectonic sequences that deform differently. First, basement below the lower decollement deforms into a set of thrust duplexes. The core of these is well exposed in the Franklin Mountains. The Sagavanirktok sidewall ramp is a major basement structure that causes the northern swing in the mountain front between the central and northeastern Brooks Range. Second, the lower cover between the two decollements deforms more complexly than basement by both passive drape over the underlying duplexes and by active thrust stacking. Large-scale buckle folding occurs in a shear zone above the Sagavanirktok sidewall ramp. Third, the upper cover above the upper decollement is poorly preserved in the mountains as allochthonous klippe in depressions in the basement a d lower cover duplexes. Crustal shortening across the eastern Brooks Range is estimated by two-dimensional section balancing at over 400 km. This is substantially more than previous estimates and is comparable to those for the western Brooks Range. The inferred lack of relative rotation between the western and eastern Brooks Range does not substantiate a rotational opening for the Arctic Ocean. End_of_Article - Last_Page 677------------

Structural Style--Brooks Range Mountain Front, Alaska: ABSTRACT

The Brooks Range mountain front between the Sagavanirktok River and Kurupa Lake is characterized by thrust sheets of Lisburne rocks, which dominantly have stratigraphic tops to the north and either dip northward or are overturned to the south. The early (Jurassic-Neocomian) thrust belt strikes obliquely (N70°W) into the mountain front and can be traced on seismic sections into the foothills. Individual thrust structures die out westward into overturned folds and ultimately into plunging noses as the thrust belt plunges to the west and successively higher structural levels are exposed. The total thrust displacement remains essentially constant because of transfer of motion to higher thrusts. Most of the west occurs along narrow zones of instant plunge with essentially zero occurring along trend. The present east-west mountain front is oblique to the original thrust vergence (N20°E) of Jurassic-Neocomian age and is due to later (Albian and younger) Brooks Range core uplift, folding, and comparatively minor thrusting. The Lisburne folds at the mountain front respond almost plastically to the late core uplift and gravitationally slide downward on rotated north-dipping thrust faults to form cascading folds with easiest relief upward and northward. Core uplift has rotated original thrust sheets 90° in some instances so that geologic map patterns are plunge-projection cross sections of thrust plates. The pattern at Eskimo Creek (Table Mountain quadrangle, eastern Brooks Range) illustrates how a view can reveal the true structure of the thrust belt. Examples of mountain-front structures are given at Killik River, Kurupa Lake, Kaikshak Hill, Akmagolik Creek, Atigun River, and Ivishak River. End_of_Article - Last_Page 660------------

Water Resources of the North Slope, Alaska: ABSTRACT

Lakes, streams, springs, snow, and ice are the most obvious source of fresh water on the North Slope. However, permafrost and seasonal climatic effects restrict the availability of these sources for water supply. Shallow thaw lakes, ranging from the 315-mi2 (815-km2) Teshekpuk Lake to ponds less than an acre, literally blanket large parts of the coastal plain. Ice-cover formation and thickening on these lakes in winter are accompanied by an increase in dissolved-solids concentration in the remaining water, thus limiting its suitability for water supply. Most of the precipitation occurs as snow, which is stored on the land surface until it melts in late spring and summer. Snow and ice are used to construct temporary roads and airfields, and melted snow and ice are often used as potable water. Most of the annual streamflow occurs during End_Page 678------------------------------ a brief two- to three-week breakup period in late May and early June. Streamflow virtually ceases in all streams, including the largest rivers, during the long, cold winters. At a few locations, ground-water supplies have been obtained from shallow thawed zones adjacent to or underlying streams. Most ground water beneath the permafrost is brackish at best. Large perennial springs such as Shublik and Sadlerochit discharge from carbonate rocks in the central and eastern Brooks Range and foothills and within the southwestern Brooks Range, but are remote from areas of present development. The most successful water-supply developments combine the use and removal of gravel with simultaneous creation of deep surface to store abundant summer streamflow. End_of_Article - Last_Page 679------------

Seismotectonics and Structure of Brooks Range, Alaska: ABSTRACT

Data collected by seismic networks operated by the Geophysical Institute of the University of Alaska-Fairbanks are used to study the seismicity and tectonics of northern Alaska. Microearthquake activity (less than 4.7 ML) is seen as a diffuse band trending north-northeast from Fairbanks to Barter Island and as an easterly trend roughly parallel to, but south of, the crest of the Brooks Range. Depths of the events range from 10 to 25 km. Some clustering occurs, with the most clearly defined feature being a line of epicenters at 157°30^prime that trends north between 66° and 67°N. A crustal velocity structure of the eastern Brooks Range is constrained using refracted phases from earthquakes local to the Barter Island, Fort Yukon, and Fairbank networks, respectively. Focal mechanism solutions from the Brooks Range show normal, thrust, and strike-slip faulting. Common to all of them, however, is an east-striking nodal plane that parallels the regional structural grain, suggesting that the fault planes are on reactivated faults. This is in contrast to the earthquakes in interior Alaska, which show mainly strike-slip focal mechanisms. The orientation of the pressure axes in both areas is consistent with the convergence of the Pacific and North American plates. End_of_Article - Last_Page 664------------

Pre-Mississippian Accreted Terranes of Northeastern Brooks Range, Alaska: ABSTRACT

Low-grade metamorphic rocks (Neruokpuk Formation, sensu lato) underlying a regional angular unconformity in the northeastern Brooks Range have been interpreted as a conformable Precambrian to Devonian stratigraphic succession. The pre-Mississippian rocks include not only miogeoclinal quartzites and carbonates but also a variety of eugeoclinal lithologies such as radiolarian cherts, argillites, and graptolitic shales; mafic to intermediate volcanic rocks; and volcanogenic gray-wackes. Fossils of Cambrian, Ordovician, and Silurian age have been identified in these lithologies. Many of these units are fault-bounded and may be interpreted as pre-Mississippian tectono-stratigraphic terranes. Several of the terranes are depositionally overlapped by Middle(?) Devonian clasti rocks and intruded by plutonic rocks depositionally overlain by Mississippian and younger rocks, but which yield equivocal middle Paleozoic age dates. Similar relationships are exposed in the Doonersk anticlinorium in the central Brooks Range. These features are interpreted to indicate that the lower Paleozoic rocks of the eastern Brooks Range were tectonically assembled by accretionary processes along an active continental margin prior to Middle Devonian time. Subsequent uplift and erosion occurred prior to deposition of the Mississippian to Neocomian (Ellesmerian) passive margin sequence. End_of_Article - Last_Page 670------------

Evidence in the Bering Strait region for differential movement between North America and Eurasia

North-trending folds and regional deflection of Cordilleran trends in the Bering Strait region strongly suggest east-west compression and crustal shortening between the North American and Eurasian continents during late Mesozoic or early Cenozoic time. Along the northwest coast of Alaska, the west- and southwest-trending grain of the Cordilleran belt appears to be deflected by a belt of folds and thrust faults that can be traced from Cape Lisburne southward across the Seward Peninsula and Yukon-Koyukuk province to the Kaltag fault. Fold axes are generally overturned to the east, and locally Paleozoic strata are thrust eastward onto Mesozoic strata. Major geologic trends on the north side of the Kaltag fault appear to be offset as much as 130 km to the east. On the Bering-Chukchi shelf, increasing evidence suggests that Cordilleran trends between Alaska and Siberia are buckled into a tight southward-looping orocline. The Paleozoic and early Mesozoic stratigraphic sequence on St. Lawrence Island is nearly identical with that found in the western Brooks Range, 400 km to the north, and distinctive belts of late Paleozoic and Mesozoic mafic volcanic and intrusive rocks and alkalic intrusive rocks appear to be traceable in an arc stretching southward from the Kobuk-Selawik area to St. Lawrence Island and then northward onto the Chukotsk Peninsula. Oroclinal bending in the Bering Strait region and in the eastern Brooks Range appears to have foreshortened the northern part of the Alaska Cordillera by at least 800 km. As neither of these deflections is reflected in the arcuate trends of the Alaska and Aleutian Ranges, the northern part of Alaska may have been decoupled from the southern part along the Kaltag fault. This compressional event in Late Cretaceous or early Tertiary time may be related to opening of the North Atlantic Ocean basin.

Active faults of Alaska

Geologically young displacements have been observed along 24 faults in an area of Alaska of approximately 624,000 km 2. Active faults of southern Alaska include the Patton Bay and Hanning Bay reverse-slip faults, both reactivated in 1964. The Fairweather strike-slip fault experienced surface faulting in 1958 and possibly in 1899. Three normal-slip faults trending northerly were not reactivated in 1964; these are the Johnstone Bay fault, Ragged Mountain fault, and the Long Glacier fault. The Castle Mountain fault trends northeasterly and is marked by a low, south-facing escarpment. Active faults in the Alaska Range and adjacent areas include those of the Denali fault system, including the Totschunda fault and the McGinnis Glacier fault; inactive faults include the Hines Creek fault. Other active faults include the Donnelly Dome fault, the Granite Mountain fault, and the Healy Creek fault. The north front of the Alaska Range west of Donnelly Dome is a monocline with few associated surface faults. The Clearwater Lake fault, located along the Tanana River east of Delta Junction, appears to be active, although an origin of landforms due to faulting cannot be proven. Active faults in the Tanana—Kuskokwim Lowland and Yukon—Tanana Upland include the Minto, Shaw Creek, Champion Creek, and Tintina faults. The active Kaltag fault, located southwest of Tanana, does not appear to connect by means of an active trace with the Tintina fault, which is 300 km to the east. Active faults of the Kokrine—Hodzana Highlands include the Kanuti and Dall Mountain faults. Active faults west of the Kanuti include the Huslia and a fault along Shoestring Sand Dune, both in the Koyukuk Flats. The active Kobuk—Alatna Hills fault trends westerly south of the Brooks Range. No active faults were observed in the central and eastern Brooks Range, Arctic Foothills, or Arctic Coastal Plain in the area of study.

A Heat-Balance Study on Mccall Glacier, Brooks Range, Alaska: A Contribution to the International Hydrological Decade

On the McCall Glacier, an Arctic glacier in the eastern Brooks Range, northern Alaska, a heat balance study was carried out during the summer of 1970 to investigate quantitatively the relationship between energy transfer, climatic parameters and melting processes. Considering the individual energy balance terms, it was found that radiation is the most important heat source for snow and ice melt. The melting period itself is only 11 weeks long, which is quite short. The evaporation overcompensates for what little condensation occurs, and amounts to about 2% of the total ablation. Evaporation is more important in the spring, but becomes decreasingly so during the summer. The melt water which percolates into the snow-pack and refreezes at a lower level is a more effective way of transporting energy into the ground than conduction, and is of importance during the beginning of the melt period. The summer balance is considerably less energetic than over the tundra north of the Brooks Range. The main difference is a higher surface albedo, and to a lesser extent the protected nature of the glacier in a deep valley on a N–S axis, which reduces the duration of possible sunshine by 39% in summer. These are the important factors in maintaining the McCall Glacier and other similar glaciers in an otherwise low precipitation area.