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USGS2012

Mixed sediment beach processes: Kachemak Bay, Alaska

Mixed sediment beaches are morphologically distinct from and more complex than either sand or gravel only beaches. Three digital imaging techniques are employed to quantify surficial grain size and bedload sediment transport rates along the mixed sediment beaches of Kachemak Bay, Alaska. Applying digital imaging procedures originally developed for quickly and efficiently quantifying grain sizes of sand to coarse sediment classes gives promising results. Hundreds of grain size estimates lead to a quantitative characterization of the region's sediment at a significant reduction in cost and time as compared to traditional techniques. Both the sand and coarse fractions on this megatidal beach mobilize into self-organized bedforms that migrate alongshore with a seasonally reflecting the temporal pattern of the alongshore component of wave power. In contrast, the gravel bedforms also migrate in the cross-shore without significant seasonally suggesting that swash asymmetry is sufficient to mobilize the gravel even during low energy summer conditions.

Conference Paper↗

USGS1991

Seasonal patterns of prey availability and the foraging behavior of arctic foxes ( Alopex lagopus ) in a waterfowl nesting area

The foraging behavior of arctic foxes was observed in a waterfowl nesting area on the Yukon–Kuskokwim Delta of Alaska in 1985–1986. Observations were made during peak fox activity from two towers, 3 m high, located in different community types. Data were collected continuously for individual foxes on specific activities, the community in which activities occurred, and the type of food obtained. After migratory birds started nesting in the area, the food potentially available to foxes changed from microtines, old caches, and carrion to include eggs and birds. This change was reflected in the foraging behavior of the foxes as they switched to predation on eggs. After nesting began, the search success rate of foxes increased (from <30% to >50%) and search duration decreased (mean 19.7 s before nest initiation versus mean 9.4 s in mid-incubation) as the rate of food acquisition increased. Over 80% of the eggs taken by foxes were cached rather than eaten immediately, which extended the availability of this temporally limited resource to foxes. Eggs were the primary prey of arctic foxes during the nesting stages in both years, even though microtine populations were high in one year (1985) and low in the other (1986).

Alaska↗

USGS2020

Focus areas for data acquisition for potential domestic resources of 11 critical minerals in the conterminous United States, Hawaii, and Puerto Rico—Aluminum, cobalt, graphite, lithium, niobium, platinum-group elements, rare earth elements, tantalum, tin, titanium, and tungsten

In response to a need for information on potential domestic sources of critical minerals, the Earth Mapping Resources Initiative (Earth MRI) was established to identify and prioritize areas for acquisition of new geologic mapping, geophysical data, and elevation data to improve our knowledge of the geologic framework of the United States. Phase 1 of Earth MRI concentrated on those geologic terranes favorable for hosting the rare earth elements (REEs). Phase 2 continued to address the REEs and also identified focus areas for potential domestic sources of 10 more of the 35 critical minerals on the U.S. critical minerals list (aluminum, cobalt, graphite, lithium, niobium, platinum-group elements, tantalum, tin, titanium, tungsten). This report describes the methodology, data sources, and summary results for mineral systems that host these 11 critical minerals in the conterminous United States, Hawaii, and Puerto Rico; Alaska is covered in a separate report. The mineral systems framework adopted for this study links critical mineral commodities to families of genetically related mineral deposit types. The mineral systems approach is an efficient approach, providing a simultaneous evaluation of geologic terranes through aggregation of genetically related mineral deposit types that are much larger than individual ore deposits. Geologic, geochemical, topographic, and geophysical mapping provided by Earth MRI will document geologic features that reflect the extent of individual mineral systems and provide information about critical mineral deposits that may not have been recognized previously. Each critical mineral commodity is discussed in terms of importance to the Nation’s economy, modes of occurrence, mineral systems, and deposit types along with maps and tables listing examples of focus areas for each critical mineral. Important mineral systems for these critical minerals include chemical weathering systems for aluminum (bauxite); placer systems for titanium and REEs; metamorphic systems for graphite; mafic magmatic systems for platinum-group elements and cobalt; lacustrine evaporite and porphyry tin systems for lithium; and copper-molybdenum-gold (Cu-Mo-Au) systems for tungsten. REEs occur in many different mineral systems. Focus areas were developed by scientists from the U.S. Geological Survey in collaboration with scientists from State geological surveys and other institutions. This first national-scale compilation of focus areas represents an initial step in addressing the Nation’s critical mineral needs by screening areas for acquisition of new data to provide the geologic framework necessary for identifying domestic sources of critical minerals.

Open-File Report↗

USGS2012

Cambrian–Ordovician sedimentary rocks of Alaska

Cambrian-Lower Ordovician carbonate rocks that likely formed as part of the Laurentian continental margin, and may thus have been part of the Cambrian-Ordovician great American carbonate bank, occur in east-central Alaska in the Nation Arch area. These strata accumulated on the southwestern margin (present-day coordinates) of the Yukon stable block, a broad area of early Paleozoic carbonate platform deposition in the northern Yukon Territory, and constitute two successions. The first consists of approximately 900 m (∼2950 ft) of shallow-water limestone and dolostone that are in part silicified, laminated, oolitic, and pisolitic, and make up the lower member of the Jones Ridge Limestone. Conodonts, trilobites, archaeo-cyathids, and brachiopods indicate an age of Early Cambrian to early Early Ordovician (Tremadoc; Ibexian) and have Laurentian biogeographic affinities. Upper Ordovician bio-clastic limestone (the upper member of the Jones Ridge Limestone) unconformably overlies these strata. A roughly coeval, but somewhat deeper water, succession crops out near the Jones Ridge Limestone and consists of, in ascending order, the Funnel Creek Limestone, Adams Argillite, and Hillard Limestone. The Funnel Creek (15-400 m [50-1310 ft] thick) is mainly nonfossilif-erous, extensively silicified, commonly oolitic limestone and dolostone and is assumed to be Lower Cambrian in age. It is overlain by argillite, siltstone, cross-laminated quartzite, and oolitic to sandy limestone of the Adams Argillite (90-180 m [295-550 ft] thick). This unit contains the trace fossil Oldhamia and Lower Cambrian archaeocyathids and trilobites that have Siberian affinities. The Hillard (30-150 m [100-490 ft] thick) is chiefly limestone, with local ooids, edgewise and boulder conglomerate, and phosphatic horizons, and likely formed in a platform-margin setting. Trilobites and brachiopods from this unit are Early Cambrian to earliest Ordovician in age and have mainly Laurentian affinities. Slope and/or basinal rocks of the Road River Formation that are as old as Early Ordovician (early middle Arenig; Ibexian) unconformably overlie the Hillard Limestone. Abrupt facies transitions between the two Nation Arch area carbonate successions may reflect relatively steep paleoslopes and/or telescoping of facies by imbricate thrust faults. Carbonate strata of Cambrian–Ordovician age are also found north of the Nation Arch area in the Porcupine terrane. These rocks have been little studied, and their precise Stratigraphic succession and paleogeographic setting are uncertain. The few fossil collections indicate mainly Laurentian affinities and include Cambrian(?) trilobites and Lower and Middle Ordovician conodonts. Lower Paleozoic strata of the Porcupine terrane probably formed at or near the northwestern edge (present-day coordinates) of the Yukon stable block. Cambrian–Ordovician carbonate strata occur widely in northern Alaska (parts of the Arctic Alaska, York, and Seward terranes) and interior Alaska (Farewell terrane). These rocks share distinctive lithologic and faunal features and were deposited in a range of shallow-shelf to basinal environments. Carbonate platform successions in northern and interior Alaska include fossils of both Laurentian and Siberian biotic provinces and may have formed on a single crustal fragment that rifted away from the Siberian craton during the late Proterozoic. These Alaskan strata were most likely in faunal exchange with, but not physically attached to, the great American carbonate bank. Lower–Middle Ordovician carbonate and siliciclastic rocks are also found in the White Mountains, Livengood, and Ruby terranes of interior Alaska, the Alexander terrane in southeastern Alaska, and the Goodnews terrane in southwestern Alaska. These successions were likely not attached to Laurentia during their deposition, although some authors have proposed Laurentian origins for the White Mountains and Livengood terranes. Little detailed information is available on the resource potential of Cambrian–Ordovician successions in Alaska. Most have low porosity and are too thermally mature to be prospective for oil and gas, although a few units in east-central and northern Alaska may have some potential as petroleum source and reservoir rocks. Strata of this age have potential for metallic mineral resources; strata-bound Zn-Pb ± Ag occurrences are known in the Funnel Creek Limestone in east-central Alaska, as well as several units of possible Cambrian and/or Ordovician age in northern and interior Alaska.

Alaska↗

USGS2009

Model-Based Predictions of the Effects of Harvest Mortality on Population Size and Trend of Yellow-Billed Loons

Yellow-billed loons (Gavia adamsii) breed in low densities in northern tundra habitats in Alaska, Canada, and Russia. They migrate to coastal marine habitats at mid to high latitudes where they spend their winters. Harvest may occur throughout the annual cycle, but of particular concern are recent reports of harvest from the Bering Strait region, which lies between Alaska and Russia and is an area used by yellow-billed loons during migration. Annual harvest for this region was reported to be 317, 45, and 1,077 during 2004, 2005, and 2007, respectively. I developed a population model to assess the effect of this reported harvest on population size and trend of yellow-billed loons. Because of the uncertainty regarding actual harvest and definition of the breeding population(s) affected by this harvest, I considered 25 different scenarios. Predicted trends across these 25 scenarios ranged from stability to rapid decline (24 percent per year) with halving of the population in 3 years. Through an assessment of literature and unpublished satellite tracking data, I suggest that the most likely of these 25 scenarios is one where the migrant population subjected to harvest in the Bering Strait includes individuals from breeding populations in Alaska (Arctic coastal plain and the Kotzebue region) and eastern Russia, and for which the magnitude of harvest varies among years and emulates the annual variation of reported harvest during 2004-07 (317, 45, and 1,077 yellow-billed loons). This scenario, which assumes no movement of Canadian breeders through the Bering Strait, predicts a 4.6 percent rate of annual population decline, which would halve the populations in 15 years. Although these model outputs reflect the best available information, confidence in these predictions and applicable scenarios would be greatly enhanced by more information on harvest, rates of survival and reproduction, and migratory pathways.

Open-File Report↗

USGS1978

Stamukhi shoals of the Arctic - some observations from the Beaufort Sea

A number of linear shoals, representing pronounced topographic anomalies on the surface of the Arctic shelf, have been studied in the Prudhoe Bay area. These shoals have been referred to in several previous studies. Based on seismic reflection records, Reimnitz et al., (1972a), stated that the shoals are constructional features younger than the post-Wisconsin transgression. Large chunks of grounded ice have frequently been seen on the linear shoals of the inner shelf. These ice chunks form barriers parallel to the shore (Reimnitz, et al. 1972b). Reimnitz and Barnes (1974) considered the lack of gravel concentrations on the shoals to be evidence that the stream of pack ice drifting past northern Alaska carries very little gravel today. Lewellen (1977) referred to the linear shoals as submerged barrier islands, while Reimnitz, et al., (1977b) pointed out that, although similar in shape to barrier islands, the linear shoals are very different in composition and do not appear to represent drowned harrier islands. They show that the shoals localize the formation of major shear and pressure events in the ice, which in turn cause the formation of linear belts of deformed and grounded ice. Today the shoals appear to be migrating under the influence of ice-bottom interaction, and indeed may have formed in response to ice-bottom interaction within the "stamukhi zone". The shoals migrate rather slowly and retain their shapes over periods of 25 years, yet control the location and stabilize the outer edge of the floating fast ice zone, and provide shelter for the inner shelf and coast. Reimnitz et al. (1977b) surmised that similar artificial structures might be used to modify the ice environment on the arctic shelf.

Alaska↗

USGS2009

The Mackenzie River magnetic anomaly, Yukon and Northwest Territories, Canada-Evidence for Early Proterozoic magmatic arc crust at the edge of the North American craton

We characterize the nature of the source of the high-amplitude, long-wavelength, Mackenzie River magnetic anomaly (MRA), Yukon and Northwest Territories, Canada, based on magnetic field data collected at three different altitudes: 300 m, 3.5 km and 400 km. The MRA is the largest amplitude (13 nT) satellite magnetic anomaly over Canada. Within the extent of the MRA, source depth estimates (8–12 km) from Euler deconvolution of low-altitude aeromagnetic data show coincidence with basement depths interpreted from reflection seismic data. Inversion of high-altitude (3.5 km) aeromagnetic data produces an average magnetization of 2.5 A/m within a 15- to 35-km deep layer, a value typical of magmatic arc complexes. Early Proterozoic magmatic arc rocks have been sampled to the southeast of the MRA, within the Fort Simpson magnetic anomaly. The MRA is one of several broad-scale magnetic highs that occur along the inboard margin of the Cordillera in Canada and Alaska, which are coincident with geometric changes in the thrust front transition from the mobile belt to stable cratonic North America. The inferred early Proterozoic magmatic arc complex along the western edge of the North American craton likely influenced later tectonic evolution, by acting as a buttress along the inboard margin of the Cordilleran fold-and-thrust belt.

Yukon Territory, Northwest Territories↗

USGS2004

Earthquake nucleation by transient deformations caused by the M = 7.9 Denali, Alaska, earthquake

The permanent and dynamic (transient) stress changes inferred to trigger earthquakes are usually orders of magnitude smaller than the stresses relaxed by the earthquakes themselves, implying that triggering occurs on critically stressed faults. Triggered seismicity rate increases may therefore be most likely to occur in areas where loading rates are highest and elevated pore pressures, perhaps facilitated by high-temperature fluids, reduce frictional stresses and promote failure. Here we show that the 2002 magnitude M = 7.9 Denali, Alaska, earthquake triggered wide-spread seismicity rate increases throughout British Columbia and into the western United States. Dynamic triggering by seismic waves should be enhanced in directions where rupture directivity focuses radiated energy, and we verify this using seismic and new high-sample GPS recordings of the Denali mainshock. These observations are comparable in scale only to the triggering caused by the 1992 M = 7.4 Landers, California, earthquake, and demonstrate that Landers triggering did not reflect some peculiarity of the region or the earthquake. However, the rate increases triggered by the Denali earthquake occurred in areas not obviously tectonically active, implying that even in areas of low ambient stressing rates, faults may still be critically stressed and that dynamic triggering may be ubiquitous and unpredictable.

Nature↗

USGS2013

Effect of ultramafic intrusions and associated mineralized rocks on the aqueous geochemistry of the Tangle Lakes Area, Alaska

Stream water was collected at 30 sites within the Tangle Lakes area of the Delta mineral belt in Alaska. Sampling focused on streams near the ultramafic rocks of the Fish Lake intrusive complex south of Eureka Creek and the Tangle Complex area east of Fourteen Mile Lake, as well as on those within the deformed metasedimentary, metavolcanic, and intrusive rocks of the Specimen Creek drainage and drainages east of Eureka Glacier. Major, minor, and trace elements were analyzed in aqueous samples for this reconnaissance aqueous geochemistry effort. The lithologic differences within the study area are reflected in the major-ion chemistry of the water. The dominant major cation in streams draining mafic and ultramafic rocks is Mg 2+ ; abundant Mg and low Ca in these streams reflect the abundance of Mg-rich minerals in these intrusions. Nickel and Cu are detected in 84 percent and 87 percent of the filtered samples, respectively. Nickel and Cu concentrations ranged from Ni <0.4 to 10.1 micrograms per liter (mg/L), with a median of 4.2 mg/L, and Cu <0.5 to 27 mg/L, with a median of 1.2 mg/L. Trace-element concentrations in water are generally low relative to U.S. Environmental Protection Agency freshwater aquatic-life criteria; however, Cu concentrations exceed the hardness-based criteria for both chronic and acute exposure at some sites. The entire rare earth element (REE) suite is found in samples from the Specimen Creek sites MH5, MH4, and MH6 and, with the exception of Tb and Tm, at site MH14. These samples were all collected within drainages containing or downstream from Tertiary gabbro, diabase, and metagabbro (Trgb) exposures. Chondrite and source rock fractionation profiles for the aqueous samples were light rare earth element depleted, with negative Ce and Eu anomalies, indicating fractionation of the REE during weathering. Fractionation patterns indicate that the REE are primarily in the dissolved, as opposed to colloidal, phase.

Alaska↗

USGS2021

Life-history attributes of Arctic-breeding birds drive uneven responses to environmental variability across different phases of the reproductive cycle

Animals exhibit varied life-history traits that reflect adaptive responses to their environments. For Arctic-breeding birds, traits related to diet, egg nutrient allocation, clutch size, and chick growth are predicted to be under increasing selection pressure due to rapid climate change and increasing environmental variability across high-latitude regions. We compared four migratory birds (black brant [ Branta bernicla nigricans ], lesser snow geese [ Chen caerulescens caerulescens ], semipalmated sandpipers [ Calidris pusilla ], and Lapland longspurs [ Calcarius lapponicus ]) with varied life histories at an Arctic site in Alaska, USA, to understand how life-history traits help moderate environmental variability across different phases of the reproductive cycle. We monitored aspects of reproductive performance related to the timing of breeding, reproductive investment, and chick growth from 2011 to 2018. In response to early snowmelt and warm temperatures, semipalmated sandpipers advanced their site arrival and bred in higher numbers, while brant and snow geese increased clutch sizes; all four species advanced their nest initiation dates. During chick rearing, longspur nestlings were relatively resilient to environmental variation, whereas warmer temperatures increased the growth rates of sandpiper chicks but reduced growth rates of snow goose goslings. These responses generally aligned with traits along the capital-income spectrum of nutrient acquisition and altricial–precocial modes of chick growth. Under a warming climate, the ability to mobilize endogenous reserves likely provides geese with relative flexibility to adjust the timing of breeding and the size of clutches. Higher temperatures, however, may negatively affect the quality of herbaceous foods and slow gosling growth. Species may possess traits that are beneficial during one phase of the reproductive cycle and others that may be detrimental at another phase, uneven responses that may be amplified with future climate warming. These results underscore the need to consider multiple phases of the reproductive cycle when assessing the effects of environmental variability on Arctic-breeding birds.

Alaska↗

USGS2016

Seasonal flows of international British Columbia-Alaska rivers: The nonlinear influence of ocean-atmosphere circulation patterns

The northern portion of the Pacific coastal temperate rainforest (PCTR) is one of the least anthropogenically modified regions on earth and remains in many respects a frontier area to science. Rivers crossing the northern PCTR, which is also an international boundary region between British Columbia, Canada and Alaska, USA, deliver large freshwater and biogeochemical fluxes to the Gulf of Alaska and establish linkages between coastal and continental ecosystems. We evaluate interannual flow variability in three transboundary PCTR watersheds in response to El Niño-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Arctic Oscillation (AO), and North Pacific Gyre Oscillation (NPGO). Historical hydroclimatic datasets from both Canada and the USA are analyzed using an up-to-date methodological suite accommodating both seasonally transient and highly nonlinear teleconnections. We find that streamflow teleconnections occur over particular seasonal windows reflecting the intersection of specific atmospheric and terrestrial hydrologic processes. The strongest signal is a snowmelt-driven flow timing shift resulting from ENSO- and PDO-associated temperature anomalies. Autumn rainfall runoff is also modulated by these climate modes, and a glacier-mediated teleconnection contributes to a late-summer ENSO-flow association. Teleconnections between AO and freshet flows reflect corresponding temperature and precipitation anomalies. A coherent NPGO signal is not clearly evident in streamflow. Linear and monotonically nonlinear teleconnections were widely identified, with less evidence for the parabolic effects that can play an important role elsewhere. The streamflow teleconnections did not vary greatly between hydrometric stations, presumably reflecting broad similarities in watershed characteristics. These results establish a regional foundation for both transboundary water management and studies of long-term hydroclimatic and environmental change.

Alaska, British Columbia↗

USGS1989

Terrain, vegetation, and landscape evolution of the R4D research site, Brooks Range Foothills, Alaska

Maps of the vegetation and terrain of a 22 km2 area centered on the Department of Energy (DOE) R4D (Response, Resistance, Resilience to and Recovery from Disturbance in Arctic Ecosystems) study site in the Southern Foothills Physiographic Province of Alaska were made using integrated geobotanical mapping procedures and a geographic-information system. Typical land forms and surface f orms include hillslope water tracks, Sagavanirktok-age till deposits, nonsorted stone stripes, and colluvial-basin deposits. Thirty-two plant communities are described; the dominant vegetation (51% of the mapped area) is moist tussock-sedge, dwarf-shrub tundra dominated by Eriophorum vaginatum or Carex bigelowii. Much of the spatial variation in the mapped geobotanical characters reflects different-aged glaciated surfaces. Shannon-Wienerin dices indicate that the more mature landscapes, represented by retransported hillslope deposits and basin colluvium, are less heterogeneous than newer landscapes such as surficial till deposits and floodplains. A typical toposequence on a mid-Pleistocene-age surface is discussed with respect to evolution of the landscape. Thick Sphagnum moss layers occur on lower hillslopes, and the patterns of moss-layer development, heat flux, active layer thickness, and ground-ice are seen as keys to developing thermokarst-susceptibility maps.

Alaska↗

USGS2013

Integrated geophysical imaging of a concealed mineral deposit: a case study of the world-class Pebble porphyry deposit in southwestern Alaska

We combined aeromagnetic, induced polarization, magnetotelluric, and gravity surveys as well as drillhole geologic, alteration, magnetic susceptibility, and density data for exploration and characterization of the Cu-Au-Mo Pebble porphyry deposit. This undeveloped deposit is almost completely concealed by postmineralization sedimentary and volcanic rocks, presenting an exploration challenge. Individual geophysical methods primarily assist regional characterization. Positive chargeability and conductivity anomalies are observed over a broad region surrounding the deposit, likely representing sulfide minerals that accumulated during multiple stages of hydrothermal alteration. The mineralized area occupies only a small part of the chargeability anomaly because sulfide precipitation was not unique to the deposit, and mafic rocks also exhibit strong chargeability. Conductivity anomalies similarly reflect widespread sulfides as well as water-saturated glacial sediments. Mineralogical and magnetic susceptibility data indicate magnetite destruction primarily within the Cu-Au-Mo mineralized area. The magnetic field does not show a corresponding anomaly low but the analytic signal does in areas where the deposit is not covered by postmineralization igneous rocks. The analytic signal shows similar lows over sedimentary rocks outside of the mineralized area, however, and cannot uniquely distinguish the deposit. We find that the intersection of positive chargeability anomalies with analytic signal lows, indicating elevated sulfide concentrations but low magnetite at shallow depths, roughly delineates the deposit where it is covered only by glacial sediments. Neither chargeability highs nor analytic signal lows are present where the deposit is covered by several hundred meters of sedimentary and volcanic rocks, but a 3D resistivity model derived from magnetotelluric data shows a corresponding zone of higher conductivity. Gravity data highlight geologic features within the deposit, including shallow diorite sills that locally contain higher-grade mineralization. The results thus show ways in which an integrated survey approach might be used to distinguish zones of potentially economic mineralization.

Alaska↗

USGS2013

Two flysch belts having distinctly different provenance suggest no stratigraphic link between the Wrangellia composite terrane and the paleo-Alaskan margin

The provenance of Jurassic to Cretaceous flysch along the northern boundary of the allochthonous Wrangellia composite terrane, exposed from the Lake Clark region of southwest Alaska to the Nutzotin Mountains in eastern Alaska, suggests that the flysch can be divided into two belts having different sources. On the north, the Kahiltna flysch and Kuskokwim Group overlie and were derived from the Farwell and Yukon-Tanana terranes, as well as smaller related terranes that were part of the paleo-Alaskan margin. Paleocurrent indicators for these two units suggest that they derived sediment from the north and west. Sandstones are predominantly lithic wacke that contain abundant quartz grains, lithic rock fragments, and detrital mica, which suggest that these rocks were derived from recycled orogen and arc sources. Conglomerates contain limestone clasts that have fossils matching terranes that made up the paleo-Alaskan margin. In contrast, flysch units on the south overlie and were derived from the Wrangellia composite terrane. Paleocurrent indicators for these units suggest that they derived sediment from the south. Sandstones are predominantly feldspathic wackes that contain abundant plagioclase grains and volcanic rock fragments, which suggest these rocks were derived from an arc. Clast compositions in conglomerate south of the boundary match rock types of the Wrangellia composite terrane. The distributions of detrital zircon ages also differentiate the flysch units. Flysch units on the north average 54% Mesozoic, 14% Paleozoic, and 32% Precambrian detrital zircons, reflecting derivation from the older Yukon-Tanana, Farewell, and other terranes that made up the paleo-Alaskan margin. In comparison, flysch units on the south average 94% Mesozoic, 1% Paleozoic, and 5% Precambrian zircons, which are consistent with derivation from the Mesozoic oceanic magmatic arc rocks in the Wrangellia composite terrane. In particular, the flysch units on the south contain a large proportion of zircons ranging from 135 to 175 Ma, corresponding to the age of the Chitina magmatic arc in the Wrangellia terrane and the plutons of the Peninsular terrane, which are part of the Wrangellia composite terrane. Flysch units on the north do not contain significant numbers of zircons in this age range. The flysch overlying the Wrangellia composite terrane apparently does not contain detritus derived from rocks of the paleo-Alaska margin, and the flysch overlying the paleo-Alaskan margin apparently does not contain detritus derived from the Wrangellia composite terrane. The provenance difference between the two belts helps to constrain the location of the northern boundary of the Wrangellia composite terrane. Geophysical models place a deep, through-going, crustal-scale suture zone in the area between the two flysch belts. The difference in the provenance of the two belts supports this interpretation. The youngest flysch is Late Cretaceous in age, and structural disruption of the flysch units is constrained to the Late Cretaceous, so it appears that the Wrangellia composite terrane was not near the paleo-Alaskan margin until the Late Cretaceous.

Alaska↗

USGS2016

Development of ion-exchange collectors for monitoring atmospheric deposition of inorganic pollutants in Alaska parklands

Between 2010 and 2014, the U.S. Geological Survey completed a series of laboratory and field experiments designed to develop methodology to support the National Park Service’s long-term atmospheric pollutant monitoring efforts in parklands of Arctic Alaska. The goals of this research were to develop passive sampling methods that could be used for long-term monitoring of inorganic pollutants in remote areas of arctic parklands and characterize relations between wet and dry deposition of atmospheric pollutants to that of concentrations accumulated by mosses, specifically the stair-step, splendid feather moss, Hylocomium splendens. Mosses and lichens have been used by National Park Service managers as atmospheric pollutant biomonitors since about 1990; however, additional research is needed to better characterize the dynamics of moss bioaccumulation for various classes of atmospheric pollutants. To meet these research goals, the U.S. Geological Survey investigated the use of passive ionexchange collectors (IECs) that were adapted from the design of Fenn and others (2004). Using a modified IEC configuration, mulitple experiments were completed that included the following: (a) preliminary laboratory and development testing of IECs, (b) pilot-scale validation field studies during 2012 with IECs at sites with instrumental monitoring stations, and (c) deployment of IECs in 2014 at sites in Alaska having known or suspected regional sources of atmospheric pollutants where samples of Hylocomium splendens moss also could be collected for comparison. The targeted substances primarily included ammonium, nitrate, and sulfate ions, and certain toxicologically important trace metals, including cadmium, cobalt, copper, nickel, lead, and zinc. Deposition of atmospheric pollutants is comparatively low throughout most of Alaska; consequently, modifications of the original IEC design were needed. The most notable modification was conversion from a single-stage mixed-bed column to a two-stage arrangement. With the modified IEC design, ammonium, nitrate, and sulfate ions were determined with a precision of between 5 and 10 percent relative standard deviation for the low loads that happen in remote areas of Alaska. Results from 2012 field studies demonstrated that the targeted ions were stable and fully retained on the IEC during field deployment and could be fully recovered by extraction in the laboratory. Importantly, measurements of annual loads determined by combining snowpack and IEC sampling at sites near National Atmospheric Deposition Program monitoring stations was comparable to results obtained by the National Atmospheric Deposition Program. Field studies completed in 2014 included snowpack and IEC samples to measure depositional loads; the results were compared to concentrations of similar substances in co-located moss samples. Analyses of constituents in snow and IECs included ammonium, nitrate, and sulfate ions; and a suite of trace metals. Constituent measurements in Hylocomium splendens moss included total nitrogen, phosphorous, and sulfur, and trace metals. To recover ammonium ions and metal ions from the upper cation-exchange column, a two-step extraction procedure was developed from laboratory spiking experiments. The 2014 studies determined that concentrations of certain metals, nitrogen, and sulfur in tissues of Hylocomium splendens moss reflected differences in presumptive deposition from local atmospheric sources. Moss tissues collected from two sites farthest from urban locales had the lowest levels of total nitrogen and sulfur, whereas tissues collected from three of the urban sites had the greatest concentrations of many of the trace metals. Moss tissue concentrations of three trace metals (cobalt, chromium, and nickel) were strongly (positively) Spearman’s rank correlated ( p <0.05) with annual depositional loads of those metals. In addition, moss sulfur concentrations were positively rank correlated with annual depositional loads of sulfate ( p <0.07). Exploratory models indicated linear uptake of the three metals by Hylocomium splendens moss and nonlinear uptake of sulfur from sulfate. Our results provided useful preliminary models for several of the targeted substances; however, our ability to characterize relations between concentrations in moss and loadings for many of the metals was precluded by several factors. The few test sites, small concentration gradients, and generally low concentrations hampered model developments. In addition, the weather was unusually warm throughout Alaska during the winter of 2013–14, which caused intermittent melting of the snowpack at some of the test sites; consequently, our measurements of overwinter loads based on snowpack samples (obtained in late March) probably underestimated the actual loads. Regardless of these potential limitations, these studies have established a foundation to support further studies that can improve our understanding of how mosses accumulate inorganic substances and ultimately how mosses might be used as biomonitors of atmospheric pollutants; moreover, the successful development and validation of the IECs during this research documents how the methodology can be used for future monitoring efforts in remote regions of Alaska and elsewhere.

Alaska↗

USGS2015

Testing the use of bulk organic δ 13 C, δ 15 N, and C org :N tot ratios to estimate subsidence during the 1964 great Alaska earthquake

During the M w 9.2 1964 great Alaska earthquake, Turnagain Arm near Girdwood, Alaska subsided 1.7 ± 0.1 m based on pre- and postearthquake leveling. The coseismic subsidence in 1964 caused equivalent sudden relative sea-level (RSL) rise that is stratigraphically preserved as mud-over-peat contacts where intertidal silt buried peaty marsh surfaces. Changes in intertidal microfossil assemblages across these contacts have been used to estimate subsidence in 1964 by applying quantitative microfossil transfer functions to reconstruct corresponding RSL rise. Here, we review the use of organic stable C and N isotope values and C org :N tot ratios as alternative proxies for reconstructing coseismic RSL changes, and report independent estimates of subsidence in 1964 by using δ 13 C values from intertidal sediment to assess RSL change caused by the earthquake. We observe that surface sediment δ 13 C values systematically decrease by &sim;4&permil; over the &sim;2.5 m increase in elevation along three 60- to 100-m-long transects extending from intertidal mud flat to upland environments. We use a straightforward linear regression to quantify the relationship between modern sediment δ 13 C values and elevation ( n = 84, R 2 = 0.56). The linear regression provides a slope–intercept equation used to reconstruct the paleoelevation of the site before and after the earthquake based on δ 13 C values in sandy silt above and herbaceous peat below the 1964 contact. The regression standard error (average = ±0.59&permil;) reflects the modern isotopic variability at sites of similar surface elevation, and is equivalent to an uncertainty of ±0.4 m elevation with respect to Mean Higher High Water. To reduce potential errors in paleoelevation and subsidence estimates, we analyzed multiple sediment δ 13 C values in nine cores on a shore-perpendicular transect at Bird Point. Our method estimates 1.3 ± 0.4 m of coseismic RSL rise across the 1964 contact by taking the arithmetic mean of the differences ( n = 9) between reconstructed elevations for sediment above and below the 1964 earthquake subsidence contact. This estimate compares well with independent subsidence estimates derived from post-earthquake leveling in Turnagain Arm, and from microfossil transfer functions at Girdwood (1.50 ± 0.32 m). While our results support the use of bulk organic δ 13 C for reconstructing RSL change in southern Alaska, the variability of stable isotope values in modern and buried intertidal sediment required the analysis of multiple samples to reduce error.

Alaska↗

USGS2016

A submarine landslide source for the devastating 1964 Chenega tsunami, southern Alaska

During the 1964 Great Alaska earthquake (M w 9.2), several fjords, straits, and bays throughout southern Alaska experienced significant tsunami runup of localized, but unexplained origin. Dangerous Passage is a glacimarine fjord in western Prince William Sound, which experienced a tsunami that devastated the village of Chenega where 23 of 75 inhabitants were lost – the highest relative loss of any community during the earthquake. Previous studies suggested the source of the devastating tsunami was either from a local submarine landslide of unknown origin or from coseismic tectonic displacement. Here we present new observations from high-resolution multibeam bathymetry and seismic reflection surveys conducted in the waters adjacent to the village of Chenega. The seabed morphology and substrate architecture reveal a large submarine landslide complex in water depths of 120–360 m. Analysis of bathymetric change between 1957 and 2014 indicates the upper 20–50 m ( &sim; 0.7 km 3 ) of glacimarine sediment was destabilized and evacuated from the steep face of a submerged moraine and an adjacent &sim; 21 km 2 perched sedimentary basin. Once mobilized, landslide debris poured over the steep, 130 m-high face of a deeper moraine and then blanketed the terminal basin ( &sim; 465 m water depth) in 11 ± 5 m of sediment. These results, combined with inverse tsunami travel-time modeling, suggest that earthquake- triggered submarine landslides generated the tsunami that struck the village of Chenega roughly 4 min after shaking began. Unlike other tsunamigenic landslides observed in and around Prince William Sound in 1964, the failures in Dangerous Passage are not linked to an active submarine delta. The requisite environmental conditions needed to generate large submarine landslides in glacimarine fjords around the world may be more common than previously thought.

Alaska↗

USGS2011

Pore fluid geochemistry from the Mount Elbert Gas Hydrate Stratigraphic Test Well, Alaska North Slope

The BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well was drilled and cored from 606.5 to 760.1 m on the North Slope of Alaska, to evaluate the occurrence, distribution and formation of gas hydrate in sediments below the base of the ice-bearing permafrost. Both the dissolved chloride and the isotopic composition of the water co-vary in the gas hydrate-bearing zones, consistent with gas hydrate dissociation during core recovery, and they provide independent indicators to constrain the zone of gas hydrate occurrence. Analyses of chloride and water isotope data indicate that an observed increase in salinity towards the top of the cored section reflects the presence of residual fluids from ion exclusion during ice formation at the base of the permafrost layer. These salinity changes are the main factor controlling major and minor ion distributions in the Mount Elbert Well. The resulting background chloride can be simulated with a one-dimensional diffusion model, and the results suggest that the ion exclusion at the top of the cored section reflects deepening of the permafrost layer following the last glaciation (∼100 kyr), consistent with published thermal models. Gas hydrate saturation values estimated from dissolved chloride agree with estimates based on logging data when the gas hydrate occupies more than 20% of the pore space; the correlation is less robust at lower saturation values. The highest gas hydrate concentrations at the Mount Elbert Well are clearly associated with coarse-grained sedimentary sections, as expected from theoretical calculations and field observations in marine and other arctic sediment cores.

Alaska↗