Eastern Alaska Research Articles

Persistent long-term ( c. 24 Ma) exhumation in the Eastern Alaska Range constrained by stacked thermochronology

Abstract To address Miocene–present episodic v. persistent exhumation, we utilize a simple graphical procedure that vertically stacks spatially diverse K-feldspar 40 Ar/ 39 Ar multi-domain diffusion (MDD) models from the length of the approximately 100 km-long high-peak region of the Eastern Alaska Range. We supply additional constraints with 40 Ar/ 39 Ar mica dating because the higher closure-temperature-window places limits on the initiation of rapid Eastern Alaska Range exhumation. We also provide a broad 40 Ar/ 39 Ar K-feldspar minimum closure age data set to add more detail on spatial patterns in the regional exhumation history for the Eastern Alaska Range. We find that rapid and persistent exhumation has occurred in the Eastern Alaska Range since about 24 Ma at a long-term rate of approximately 0.9 km/Ma, but that this rapid exhumation is spatially variable through time. Onset of rapid Eastern Alaska Range exhumation is coincident with the initiation of rapid exhumation in SW Alaska, the Western Alaska Range and the Chugach–Saint Elias Range at around 25 Ma, implying a region-wide deformational response to a change in tectonic forcing. The initiation of highly coupled flat-slab subduction of the Yakutat microplate is probably responsible for this prolonged period of rapid exhumation in southern Alaska. Supplementary material: Sample locations from the Eastern Alaska Range, and 40 Ar/ 39 Ar data tables and age spectrum figures are available at www.geolsoc.org.uk/SUP18603 .

Parmelina yalungana resurrected and reported from Alaska, China and Russia

Parmelina quercina is a well-studied foliose macro-lichen found on rocks and trees in the Northern Hemisphere. Recent studies support multiple species within P. quercina based on material from Europe, North America and western Asia. The identities of Parmelina quercina s.lat. reported from eastern Asia and Alaska remain unknown. We compared DNA sequences, secondary chemistry and morphological traits of Parmelina from Alaska, Russia and China. These data support the resurrection of Parmelina yalungana to accommodate eastern Asian and Alaskan material. Parmelina yalungana differs from congeners in ascospore dimensions, geographic range and molecular data from three gene loci. We place P. yalungana in the phylogenetic context of the P. quercina group using DNA from Alaskan and Russian material.

Food Web Structure of the Alaskan Nearshore Shelf and Estuarine Lagoons of the Beaufort Sea

The eastern Alaska Beaufort Sea coast is characterized by numerous shallow (2–5 m) estuarine lagoons, fed by streams and small rivers that drain northward from the Brooks Range through the arctic coastal plain, and bounded seaward by barrier islands and shoals. Millions of birds from six continents nest and forage during the summer period in this region using the river deltas, lagoons, and shoreline along with several species of anadromous and marine fish. We examined biogeochemical processes linking the benthic community to the overall food web structure of these poorly studied but pristine estuaries, which are largely covered by 1.8 m of ice for 10 months annually. In summer, these lagoons are relatively warm with brackish salinities (5–10°C, S = 10–25) compared to more open coastal waters (0–5°C, S > 27). The stable isotopic composition of organic materials in sediments (i.e., benthic particulate organic matter) and water column suspended particulate organic matter from both streams and lagoons are largely indistinguishable and reflect strong terrestrial contributions, based upon δ13C and δ15N values (−25.6‰ to −27.4‰ and 1.4‰ to 3.3‰, respectively). By comparison, shifts toward more heavy isotope-enriched organic materials reflecting marine influence are observed on the adjacent coastal shelf (−24.8‰ to −25.4‰ and 3.4‰ to 5.3‰, respectively). The isotopic composition of lagoon fauna is consistent with a food web dominated by omnivorous detritovores strongly dependent on microbial processing of terrestrial sources of carbon. Biomagnification of 15N in benthic organisms indicate that the benthic food web in lagoons support up to four trophic levels, with carnivorous gastropod predators and benthic fishes (δ15N values up to 14.4‰) at the apex.

The role of rheology and slab shape on rapid mantle flow: Three‐dimensional numerical models of the Alaska slab edge

Away from subduction zones, the surface motion of oceanic plates is well correlated with mantle flow direction, as inferred from seismic anisotropy. However, this correlation breaks down near subduction zones, where shear wave splitting studies suggest the mantle flow direction is spatially variable and commonly non‐parallel to plate motions. This implies local decoupling of mantle flow from surface plate motions, yet the magnitude of this decoupling is poorly constrained. We use 3D numerical models of the eastern Alaska subduction‐transform plate boundary system to further explore this decoupling, in terms of both direction and magnitude. Specifically, we investigate the role of the slab geometry and rheology on the mantle flow velocity at a slab edge. The subducting plate geometry is based on Wadati‐Benioff zone seismicity and tomography, and the 3D thermal structure for both the subducting and overriding plates, is constrained by geologic and geophysical observations. In models using the composite viscosity, a laterally variable mantle viscosity emerges as a consequence of the lateral variations in the mantle flow and strain rate. Spatially variable mantle velocity magnitudes are predicted, with localized fast velocities (greater than 80 cm/yr) close to the slab where the negative buoyancy of the slab drives the flow. The same models produce surface plate motions of less than 10 cm/yr, comparable to observed plate motions. These results show a power law rheology, i.e., one that includes the effects of the dislocation creep deformation mechanism, can explain both observations of seismic anisotropy and decoupling of mantle flow from surface motion.

Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: Stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat-North America collision zone

New geochronologic, geochemical, sedimentologic, and compositional data from the central Wrangell volcanic belt (WVB) document basin development and volcanism linked to subduction of overthickened oceanic crust to the northern Pacific plate margin. The Frederika Formation and overlying Wrangell Lavas comprise >3 km of sedimentary and volcanic strata exposed in the Wrangell Mountains of south-central Alaska (United States). Measured stratigraphic sections and lithofacies analyses document lithofacies associations that reflect deposition in alluvial-fluvial-lacustrine environments routinely influenced by volcanic eruptions. Expansion of intrabasinal volcanic centers prompted progradation of vent-proximal volcanic aprons across basinal environments. Coal deposits, lacustrine strata, and vertical juxtaposition of basinal to proximal lithofacies indicate active basin subsidence that is attributable to heat flow associated with intrabasinal volcanic centers and extension along intrabasinal normal faults. The orientation of intrabasinal normal faults is consistent with transtensional deformation along the Totschunda-Fairweather fault system. Paleocurrents, compositional provenance, and detrital geochronologic ages link sediment accumulation to erosion of active intrabasinal volcanoes and to a lesser extent Mesozoic igneous sources. Geochemical compositions of interbedded lavas are dominantly calc-alkaline, range from basaltic andesite to rhyolite in composition, and share geochemical characteristics with Pliocene–Quaternary phases of the western WVB linked to subduction-related magmatism. The U/Pb ages of tuffs and 40 Ar/ 39 Ar ages of lavas indicate that basin development and volcanism commenced by 12.5–11.0 Ma and persisted until at least ca. 5.3 Ma. Eastern sections yield older ages (12.5–9.3 Ma) than western sections (9.6–8.3 Ma). Samples from two western sections yield even younger ages of 5.3 Ma. Integration of new and published stratigraphic, geochronologic, and geochemical data from the entire WVB permits a comprehensive interpretation of basin development and volcanism within a regional tectonic context. We propose a model in which diachronous volcanism and transtensional basin development reflect progressive insertion of a thickened oceanic crustal slab of the Yakutat microplate into the arcuate continental margin of southern Alaska coeval with reported changes in plate motions. Oblique northwestward subduction of a thickened oceanic crustal slab during Oligocene to Middle Miocene time produced transtensional basins and volcanism along the eastern edge of the slab along the Duke River fault in Canada and subduction-related volcanism along the northern edge of the slab near the Yukon-Alaska border. Volcanism and basin development migrated progressively northwestward into eastern Alaska during Middle Miocene through Holocene time, concomitant with a northwestward shift in plate convergence direction and subduction collision of progressively thicker crust against the syntaxial plate margin.

Associations between the Autumn Arctic Sea Ice and North American Winter Precipitation

Associations between the autumn Arctic sea ice concentrations (SICs) and North American winter precipitation were examined using singular value decomposition. The results show that a reduced SIC in the majority of the Arctic is accompanied by dry conditions over the Great Plains, the southern United States, Mexico, eastern Alaska, and southeastern Greenland, and by wet conditions over the majority of Canada, the northeastern United States, and the majority of Greenland. Atmospheric circulation anomalies associated with the SIC variability show a wave train structure that is persistent from autumn to winter and is responsible for the covariability between the autumn Arctic SICs and North American winter precipitation. This relationship suggests a potential long-term out-look for the North American winter precipitation.

Arm in the Ice

In this article the author discusses how fingerprint- and DNA-identification techniques helped to solve a mystery from a 60-year-old airplane crash. She provides an overview of the March 12, 1948 crash of Northwest Airlines flight 4422 into Mount Sanford in the Wrangell Mountains in eastern Alaska that killed 24 passengers, including merchant marines returning to the U.S. from Shanghai, China and Northwest crew members. Topics include the discovery of an arm in 1999 by former U.S. Air Force pilots Kevin McGregor and Marc Millican and the development of a DNA-identification technique by researcher Odile Loreille of the Armed Forces DNA Identification Laboratory in Rockville, Maryland, that uses a demineralization process to dissolve bone matrix for mitochondrial DNA (mtDNA) analysis.

Vowels of Upper Tanana Athabascan

This paper provides a detailed phonetic, synchronic description of the oral vowels of Upper Tanana, an Athabascan language spoken in eastern interior Alaska and the western Yukon Territory. Upper Tanana has, depending on the dialect, six or seven oral monophthongs that can be modified for (low) tone, nasalization, and length. Previous accounts are auditory. They disagree regarding the articulation of two of the vowels (which we represent as /ɘ/ and /ʌ/), and raise the question of whether ‘length’ indicates a quantity or a quality distinction. In contrast to earlier accounts, we provide acoustic measurements of F1, F2, and the duration of the vowels of two Alaskan dialects of Upper Tanana: Northway and Tetlin. We find that the phonetic realization of one of the vowels is quite different from the earlier auditory descriptions. For the other vowels and for the quantity distinction, our findings support earlier descriptions while supplying new details. Several co-articulation effects, previously undescribed, are also identified.

Pteropod time-series from the NE Pacific

AbstractMackas, D. L., and Galbraith, M. D. 2012. Pteropod time-series from the NE Pacific. – ICES Journal of Marine Science, 69: 448–459. Pteropods are marine planktonic molluscs that play important roles as broad-spectrum microplankton grazers, and as prey for fish, squid, and other plankton. Most species (e.g. Limacina, Clio) form aragonite shells. Others (e.g. Clione) lack shells as adults but are narrow-spectrum predators that rely on shelled pteropods as their primary or exclusive prey. The entire group is therefore potentially threatened by increasing ocean acidification, which in some regions (including the NE Pacific) is now approaching the solubility threshold for aragonite. Despite the grounds for ecological concern, there are few long-term time-series of pteropod populations. Time-series of pteropod biomass anomalies off the Vancouver Island continental margin and in the eastern Alaska Gyre (Line P) are analysed. Off both southern and northern Vancouver Island, Limacina (the dominant Subarctic thecate pteropod) has declined notably. Continental margin trends for Clione (the dominant athecate) are mostly positive but not significant. Occurrence rate and quantity of Clio (a subtropical species) have increased greatly. The shorter (13–14 year) Line P time-series as yet shows no overall trends for any of the species, although there are positive annual anomalies of Clio in the same years in both continental margin and oceanic regions.

Population fragmentation causes randomly fixed genotypes in populations of Arabidopsis kamchatica in the Japanese Archipelago

Populations of arctic alpine plants likely disappeared and re-colonised several times at the southern edge of their distributions during glacial and interglacial cycles throughout the Quaternary. Range shift and population fragmentation after a glacial period would affect the genetic structure of such plants in southernmost populations. We aimed to elucidate how climatic oscillations influenced the population subsistence of alpine plants in the Japanese Archipelago as one of the southernmost populations, by inferring the genetic structure of Arabidopsis kamchatica subsp. kamchatica and the intraspecific littoral taxon, subsp. kawasakiana. We identified genotypes based on the haplotypes of five nuclear genes and two chloroplast DNA spacers for 164 individuals from 24 populations. Most populations harboured only one private genotype, whereas few polymorphisms were found in each population. Two genetic genealogies were found, suggesting that northern Japanese populations of alpine subsp. kamchatica, subsp. kawasakiana and the northerly subsp. kamchatica in eastern Russia and Alaska clustered and differentiated from populations in central Honshu, western Japan and Taiwan. During climatic oscillations, the genetic structure of extant southernmost populations would have been shaped by strong genetic drift under population fragmentation and randomly fixed to a single genotype among their ancestral polymorphisms.

Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

40 Ar/ 39 Ar, apatite fission-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from ∼33 Ma to ∼18 Ma for 40 Ar/ 39 Ar biotite, ∼18 Ma to ∼6 Ma for K-feldspar minimum closure ages, and ∼15 Ma to ∼1 Ma for apatite fission-track ages, and apatite (U-Th)/He cooling ages range from ∼4 Ma to ∼1 Ma. There has been at least ∼11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite 40 Ar/ 39 Ar thermochronology. Variations in exhumation history along and across the strike of the fault are influenced by both far-field effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least ∼22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (∼past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.

A 16 000‐year record of vegetational change in south‐western Alaska as inferred from plant macrofossils and pollen

AbstractPollen and macrofossil analyses of a sediment core from Beaver Pond (60° 37′ 14″ N, 154° 19′ W, 579 m a.s.l.) reveal a record of regional and local postglacial vegetation change in south‐western Alaska. The chronology is based on five AMS (accelerator mass spectrometry) 14C ages obtained from terrestrial plant macrofossils. Pollen and macrofossil records suggest that open herb and shrub tundra with e.g. Poaceae, Cyperaceae, Artemisia, Vaccinium and Salix prevailed on the landscape before ca. 14 000 cal a BP. The shift from herb‐ to shrub‐dominated tundra (Salix, subsequent Betula expansion) possibly reflects climatic warming at the beginning of the Bølling period at ca. 14 700–14 500 and around 13 500 cal a BP. Vegetation (Betula shrub tundra) remained relatively stable until the early Holocene. Macrofossil influx estimates provide evidence for greater biomass in Betula shrub tundra during the early postglacial period than today. Charcoal accumulation rates suggest tundra fire activity was probably greater from ca. 12 500 to 10 500 cal a BP, similar to results from elsewhere in Alaska. The pollen and macrofossil records of Beaver Pond suggest the prevalence of low shrub tundra (shrub Betula, Betula nana, Vaccinium, Ledum palustre, Ericaceae) and tall shrub tundra (Alnus viridis ssp. crispa, Salix) between 10 000 and 4000 cal a BP. This Holocene vegetation type is comparable with that of the modern treeless wet and moist tundra in south‐western Alaska. The expansion of Picea glauca occurred ∼4000 cal a BP, much later than that of A. viridis (ssp. crispa), whereas in central and eastern Alaska Picea glauca expanded prior to or coincident with Alnus (viridis). At sites located only 200–400 km north‐east of Beaver Pond (Farewell and Wien lakes), Picea glauca and Betula forests expanded 8000–6000 cal a BP. Unfavourable climatic conditions and soil properties may have inhibited the expansion and establishment of Picea across south‐west Alaska during the mid and late Holocene. Copyright © 2011 John Wiley & Sons, Ltd.

Upper plate proxies for flat-slab subduction processes in southern Alaska

The timing of initiation of flat-slab subduction beneath southern Alaska and the upper plate record of this process are not well understood. We explore the record of flat-slab subduction in southern Alaska by integrating stratigraphic, provenance, geochronologic, and thermochronologic data from the region directly above and around the perimeter of ongoing flat-slab subduction. These datasets document a change from regional Paleocene–Oligocene subduction-related magmatism and basin development to an absence of magmatism and initiation of rock exhumation that continues to today. We infer that initiation of flat-slab subduction prompted crustal shortening, exhumation, inversion of sedimentary basins, and cessation of magmatism above and around the area of ongoing flat-slab subduction. Surface uplift and erosion above the flat slab resulted in deposition of thick, clastic wedges in sedimentary basins located along the western and northern perimeters of the flat-slab region. Along the eastern perimeter, northwestward-propagating Oligocene–Quaternary slab-edge volcanism and transtensional basin development along dextral strike-slip faults record progressive northwestward insertion of a shallow slab against the curved continental margin of eastern Alaska. Collectively, these geologic data indicate that flat-slab subduction was shaping southern Alaska by late Eocene–early Oligocene time, much earlier than previous models infer. Upper plate processes related to subduction of a flat slab in Alaska are similar to those documented in other modern flat-slab regions. These processes include: 1) shortening and exhumation of the upper plate several hundred kilometers inboard from the plate margin, 2) cessation of subduction-related magmatism within ten million years of the onset of shallow subduction, 3) shoaling or inversion of sedimentary basins above the flat-slab region, 4) deformation and/or erosion of the accretionary prism during flat-slab subduction, and 5) in some settings, reestablishment of sedimentation in the accretionary prism after the shallow slab has migrated laterally. Unlike other flat-slab margins, strike-slip-related volcanism and basin development characterizes one edge of the flat-slab region in Alaska, a consequence of oblique insertion of the flat slab into the corner of a curved continental margin.

Chapter 34 Geology and petroleum potential of the rifted margins of the Canada Basin

Abstract Three sides of the Canada Basin are bordered by high-standing, conjugate rift shoulders of the Chukchi Borderland, Alaska and Canada. The Alaska and Canada margins are mantled with thick, growth-faulted sediment prisms, and the Chukchi Borderland contains only a thin veneer of sediment. The rift-margin strata of Alaska and Canada reflect the tectonics and sediment dispersal systems of adjacent continental regions whereas the Chukchi Borderland was tectonically isolated from these sediment dispersal systems. Along the eastern Alaska–southern Canada margin, termed herein the ‘Canning–Mackenzie deformed margin’, the rifted margin is deformed by ongoing Brooks Range tectonism. Additional contractional structures occur in a gravity fold belt that may be present along the entire Alaska and Canada margins of the Canada Basin. Source-rock data inboard of the rift shoulders and regional palaeogeographic reconstructions suggest three potential source-rock intervals: Lower Cretaceous (Hauterivian–Albian), Upper Cretaceous (mostly Turonian) and Lower Palaeogene. Burial history modelling indicates favourable timing for generation from all three intervals beneath the Alaska and Canada passive margins, and an active petroleum system has been documented in the Canning–Mackenzie deformed margin. Assessment of undiscovered petroleum resources indicates the greatest potential in the Canning–Mackenzie deformed margin and significant potential in the Canada and Alaska passive margins.

The Browning of Alaska’s Boreal Forest

We used twelve Landsat scenes from the 1980s–2009 and regional 2000–2009 MODIS data to examine the long-term trend in the normalized difference vegetation index (NDVI) within unburned areas of the Alaskan boreal forest. Our analysis shows that there has been a declining trend in NDVI in this region, with the strongest “browning trend” occurring in eastern Alaska where the climate during the growing season is relatively dry and warm. Possible reasons for the "browning trend" are decreased vegetation due to temperature-induced drought stress and increased infestations of insect pests.

Linkages between large-scale climate patterns and the dynamics of Arctic caribou populations

Recent research has linked climate warming to global declines in caribou and reindeer (both Rangifer tarandus) populations. We hypothesize large-scale climate patterns are a contributing factor explaining why these declines are not universal. To test our hypothesis for such relationships among Alaska caribou herds, we calculated the population growth rate and percent change of four arctic herds using existing population estimates, and explored associations with indices of the Arctic Oscillation (AO) and the Pacific Decadal Oscillation (PDO). The AO, which more strongly affects eastern Alaska, was negatively associated with the population trends of the Porcupine Caribou Herd and Central Arctic Herd, the easternmost of the herds. We hypothesize that either increased snowfall or suboptimal growing conditions for summer forage plants could explain this negative relationship. Intensity of the PDO, which has greatest effects in western Alaska, was negatively associated with the growth rate of the Teshekpuk Caribou Herd in northwestern Alaska, but the Western Arctic Herd in western Alaska displayed the opposite trend. We suggest that the contrasting patterns of association relate to the spatial variability of the effects of the PDO on western and northwestern Alaska. Although predation and winter range quality have often been considered the primary causes of population variation, our results show that large-scale climate patterns may play an important role in caribou population dynamics in arctic Alaska. Our findings reveal that climate warming has not acted uniformly to reduce caribou populations globally. Further research should focus on the relative importance of mechanisms by which climate indices influence caribou population dynamics.

Interannual Sea Level Variability in the North Pacific Ocean and Its Mechanisms

AbstractWe use the 15 years (1993~2007) data of monthly‐averaged sea surface height anomalies (SSHA) to analyze their interannual spatial and temporal variability and their thermodynamic and dynamic mechanisms. The result shows that the prominent interannual variability occurred in the Kuroshio Extension and the western Pacific warm pool. According to decomposition using the EOF method, the first mode of interannual SSHA is more likely zonal, while the second is much more meridional. The steric SSHA induced by the heat flux explains more than 40% of interannual SSHA in the middle‐latitudinal northeastern Pacific Ocean. The time‐varying barotropic Sverdrup balance can account for 20%~40% in the western subarctic gyre, whereas their interannual variations are indistinctive. Among the baroclinic mechanisms, the first baroclinic Rossby waves model forced by large‐scale wind stress could explain the interannual SSHA 20%~60% in the tropical Pacific, 20%~40% in the central middle‐latitudes, and 20%~60% in the eastern Alaska gyre and western subarctic gyre, respectively. The interannual SSHA induced by the local Ekman pumping has a local structure, which could explain more than 40% of the observed changes in the northeast Pacific Ocean, likewise in the Bering sea and central tropical Pacific Ocean. The westward propagating Rossby waves, derived from the difference between the Rossby waves model and Ekman pumping model simulating SSHA, could explain 20%–60% of the interannual SSH observations in the central and western subtropical gyre and east of the Hawaiian Islands.

Satellite based analysis of northern ET trends and associated changes in the regional water balance from 1983 to 2005

Summary We developed an evapotranspiration (ET) algorithm driven by satellite remote sensing inputs, including AVHRR GIMMS NDVI, MODIS land cover and NASA/GEWEX solar radiation and albedo, and regionally corrected NCEP/NCAR Reanalysis daily surface meteorology. The algorithm was used to assess spatial patterns and temporal trends in ET over the pan-Arctic basin and Alaska from 1983 to 2005. We then analyzed associated changes in the regional water balance defined as precipitation (P) minus ET, where monthly P was defined from Global Precipitation Climatology Project (GPCP) and Global Precipitation Climatology Center (GPCC) sources. Monthly ET results derived from both in situ meteorological measurements and coarse resolution model reanalysis inputs agreed well (RMSE = 5.1–6.3 mm month −1 ; R 2 = 0.91–0.92) with measurements from eight independent flux towers representing regionally dominant land cover types. ET showed generally positive trends over most of the pan-Arctic domain, though negative ET trends occurred over 32% of the region, primarily in boreal forests of southern and central Canada. Generally positive trends in ET, P and available long-term river discharge measurements imply that the pan-Arctic terrestrial water cycle is intensifying despite uncertainty in regional P and associated water balance estimates. Increasing water deficits in eastern Alaska, Canadian Yukon and western Prairie Provinces, and Northern Mongolia agree with regional drought records and recent satellite observations of vegetation browning and productivity decreases. Our results indicate that the pan-Arctic water balance is responding to a warming climate in complex ways with direct links to terrestrial carbon and energy cycles.

Foraging behavior of adult female Steller sea lions during the breeding season in Southeast Alaska

AbstractDuring the 1990s, the Steller sea lion (Eumetopias jubatusSchreber) Western Alaska stock (WS) suffered steep population decline while the Eastern Alaska stock (ES) steadily increased. One bottom‐up forcing hypothesis explaining this decline predicted lactating adult female foraging behavior would be different between stocks. To investigate this effect, we monitored 11 ES females at two breeding rookeries using satellite dive recorders (SDR) during the early breeding seasons of 1992–1993, examined their behavior with respect to prey, physiological limitations, and habitat, and made limited comparisons to observations of WS female behavior reported in the literature. ES females were not operating at the extremes of ability, with most diving within the limits of aerobic metabolism, less than one‐quarter of possible foraging time during trips spent submerged and most foraging trips requiring less than one‐half the lipid store fasting ability of dependent pups. Thus, females may have some capacity to alter behavior to accommodate future changes in foraging conditions, but the extent of this plasticity is unknown. Because recent work suggests WS recovery is impeded by low natality, future studies should test differences between reproductive and non‐reproductive mature females in order to properly assess the contribution of foraging ecology to SSL population dynamics.

Deformation rates estimated from earthquakes in the northern Cordillera of Canada and eastern Alaska

We estimate deformation rates from earthquake catalog statistics to investigate current strain distribution resulting from the collision of the Yakutat block with North America. The collision is accommodated over a broad region of Alaska and NW Canada in a variety of tectonic regimes. Our estimates have significant uncertainties, but the results are generally in agreement with other deformation rate estimates (GPS and geological data) and provide useful first‐order constraints on local long‐term tectonic deformation and seismic hazard. We estimate that 50–75% of relative plate motion is taken up by earthquakes on the Yakutat/North America boundaries, with lateral crustal extrusion, other far‐field strain, and possibly aseismic creep accommodating the remainder. We calculate dextral strike slip of 4.5–10.2 mm/a on the Denali fault system in Alaska, remarkably similar to late Pleistocene/Holocene rates from geomorphic offsets. Right‐lateral motion essentially bypasses the Denali fault in easternmost Alaska and westernmost Yukon, passing instead along the Totschunda and Duke River fault systems. Right‐lateral transpression of 5–10 mm/a suggests that the Alaskan fore arc does not have fully block‐like behavior. Between the Denali and Tintina‐Kaltag faults in Alaska, 10–20 mm/a of dextral shear is accommodated by several NE trending “bookshelf” faults. Significant seismic deformation (in cases higher than expected from GPS data) is indicated in the NE Cordillera, where preexisting faults have a strong control on the pattern and mechanisms of current deformation.