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At least 667 records · Page 37Linked to original sources
USGS1994

Interstadial climatic cycles: A link between western North America and Greenland?

During the interval 33.6 to 26.1 ka, Searles Lake in southeastern California went through six major cycles of expansion and contraction. A comparison of U/Th ages for these events with the chronologies for quasi-cyclic interstadial episodes in the ice core from Summit, Greenland, suggests that the episodes of low water at Searles Lake are synchronous with the interstadial episodes at Summit. The two phenomena may be linked by variations in the strength of the global hydrological cycle, driven by oscillations in the Atlantic Ocean thermohaline circulation.

California↗
USGS2014

Volcanoes of the passive margin: The youngest magmatic event in eastern North America

The rifted eastern North American margin (ENAM) provides important clues to the long-term evolution of continental margins. An Eocene volcanic swarm exposed in the Appalachian Valley and Ridge Province of Virginia and West Virginia (USA) contains the youngest known igneous rocks in the ENAM. These magmas provide the only window into the most recent deep processes contributing to the postrift evolution of this margin. Here we present new 40 Ar/ 39 Ar ages, geochemical data, and radiogenic isotopes that constrain the melting conditions and the timing of emplacement. Modeling of the melting conditions on primitive basalts yielded an average temperature and pressure of 1412 ± 25 °C and 2.32 ± 0.31 GPa, corresponding to a mantle potential temperature of ∼1410 °C, suggesting melting conditions slightly higher than average mantle temperatures beneath mid-ocean ridges. When compared with magmas from Atlantic hotspots, the Eocene ENAM samples share isotopic signatures with the Azores and Cape Verde. This similarity suggests the possibility of a large-scale dissemination of similar sources in the upper mantle left over from the opening of the Atlantic Ocean. Asthenosphere upwelling related to localized lithospheric delamination is a possible process that can explain the intraplate signature of these magmas that lack evidence of a thermal anomaly. This process can also explain the Cenozoic dynamic topography and evidence of rejuvenation of the central Appalachians.

Geology↗
USGS2019

Detrital zircon geochronology along a structural transect across the Kahiltna assemblage in the western Alaska Range: Implications for emplacement of the Alexander-Wrangellia-Peninsular terrane against North America

The Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular (AWP) terrane to the south, and the Farewell and other peri-cratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard peri-cratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the AWP terrane and mid to Late Cretaceous arc related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at about 81 Ma and they were deformed together by about 80 Ma by southeast-verging fold-and-thrust style deformation interpreted to represent final accretion of the AWP terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the backarc flank of an active, south-facing island arc system (AWP terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench-Nankai Trough on the east side of central Japan.

Alaska↗
USGS1991

Genotypic and phenotypic variability of Zostera marina on the west coast of North America

The relation between environmental factors and leaf morphology of Zostera marina L. have long been unclear, primarily because the species is intrinsically variable. The common-garden method was used to determine the genetic, environmental, and interaction components of leaf size variation. Zostera marina consists of several ecotypes with a wide range of phenotypic plasticity. Variation in the morphology of Z . marina was of three types: genetic, accounting for 14% across the localities studied; environmental (phenotypic plasticity along temporal and spatial gradients), accounting for 32%; and interaction between genotype and environmental, acounting for 35%. Five ecotypes were described for the North American Pacific coast: Z . marina L. var. izembekensis Backman, Z . marina L. var. typica Setchell, Z . marina L. var. phillipsii Backman, Z . marina L. var. latifolia Morong, Z . marina L. var. atàm Backman. Temporal variation was due to seasonal phenotypic changes in ecotypes. Zostera marina var. izembekensis showed little seasonal morphological changes; Z . marina var. typica demonstrated minor increase in leaf size in spring and summer. Zostera marina var. phillipsii and Z . marina var. latifolia behaved similarly in that leaf size of both increased markedly in spring through early summer. Zostera marina var. phillipsii is adapted to Hood Canal and Puget Sound while Z . marina var. latifolia occupies the outer coast. Zostera marina var. atàm exhibits sexual reproduction exclusively and is specifically adapted to the Gulf of California.

Canadian Journal of Botany↗
USGS2013

Bokan Mountain peralkaline granitic complex, Alexander terrane (southeastern Alaska): evidence for Early Jurassic rifting prior to accretion with North America

The circular Bokan Mountain complex (BMC) on southern Prince of Wales Island, southernmost Alaska, is a Jurassic peralkaline granitic intrusion about 3 km in diameter that crosscuts igneous and metasedimentary rocks of the Alexander terrane. The BMC hosts significant rare metal (rare earth elements, Y, U, Th, Zr, and Nb) mineralization related to the last stage of BMC emplacement. U–Pb (zircon) and 40 Ar/ 39 Ar (amphibole and whole-rock) geochronology indicates the following sequence of intrusive activity: ( i ) a Paleozoic basement composed mainly of 469 ± 4 Ma granitic rocks; ( ii ) intrusion of the BMC at 177 ± 1 Ma followed by rapid cooling through ca. 550 °C at 176 ± 1 Ma that was synchronous with mineralization associated with vertical, WNW-trending pegmatites, felsic dikes, and aegirine–fluorite veins and late-stage, sinistral shear deformation; and ( iii ) intrusion of crosscutting lamprophyre dikes at >150 Ma and again at ca. 105 Ma. The peralkaline nature of the BMC and the WNW trend of associated dikes suggest intrusion during NE–SW rifting that was followed by NE–SW shortening during the waning stages of BMC emplacement. The 177 Ma BMC was synchronous with other magmatic centres in the Alexander terrane, such as (1) the Dora Bay peralkaline stock and (2) the bimodal Moffatt volcanic suite located ∼30 km north and ∼100 km SE of the BMC, respectively. This regional magmatism is interpreted to represent a regional extensional event that precedes deposition of the Late Jurassic – Cretaceous Gravina sequence that oversteps the Wrangellia and Alexander exotic accreted terranes and the Taku and Yukon–Tanana pericratonic terranes of the Canadian–Alaskan Cordillera.

Alaska↗
USGS2024

Differences in life history patterns of American shad, Alosa sapidissima, populations between ancestral, Atlantic coast, and non-native, Pacific coast rivers of North America

Organisms naturalized outside their native range can reveal new life history patterns in new environments. Here, we compare life history patterns of American shad, Alosa sapidissima, from five rivers along the U.S. Pacific coast (introduced range) with contemporary data from the Atlantic coast source populations. The Pacific coast fish grew slower, matured at a younger age, and were less often iteroparous than conspecifics from the Hudson River and Susquehanna River sources. Differences among the Pacific coast populations indicated that some combination of phenotypic plasticity and genetic adaptation has occurred since the transplants in the 1870s. Microchemical analysis of otoliths from returning adults demonstrated an immature, extended freshwater life history form, locally known as “mini-shad”. These fish remain in the river for at least 1 year or enter salt water briefly and then return to fresh water for several months prior to subsequent seaward migration and return as adults. This and other forms of variation (extent of estuary use and size at sea water entry) expand the understanding of Alosine migration and life history diversity.

Canadian Journal of Fisheries and Aquatic Sciences↗
USGS2007

Nucularcidae: A new family of palaeotaxodont Ordovician pelecypods (Mollusca) from North America and Australia

The new Ordovician palaeotaxodont family Nucularcidae and the new genus Nucularca are described. Included in Nucularca are four previously described species that have taxodont dentition: N. cingulata (Ulrich) (the type species), N. pectunculoides (Hall), N. lorrainensis (Foerste), and N. gorensis (Foerste). All four species are of Late Ordovician (Cincinnatian; Katian) age and occur in eastern Canada and the northeastern USA. Ctenodonta borealis Foerste is regarded as a subjective synonym of Nucularca lorrainensis. No new species names are proposed. The Nucularcidae includes the genera Nucularca and Sthenodonta Pojeta and Gilbert-Tomlinson (1977). Sthenodonta occurs in central Australia in rocks of Middle Ordovician (Darriwilian) age. The 12 family group names previously proposed for Ordovician palaeotaxodonts having taxodont dentition are reviewed and evaluated in the Appendix. ?? 2007 NRC Canada.

Canadian Journal of Earth Sciences↗
USGS1973

Fish viruses: Isolation and identification of infectious hematopoietic necrosis in eastern North America

Infectious hematopoietic necrosis virus (IHNV) was isolated from diseased fingerling rainbow trout ( Salmo gairdneri ) from a hatchery in West Virginia. Clinical signs, histopathologic findings, and origin of eggs provided a basis for diagnosis, and virus was isolated and presumptively identified by plaque characteristics. Serum neutralization tests provided positive identification of the agent as IHNV, and electronmicroscopy showed its rhabdovirus morphology. Experimental infections resulted in signs of IHNV and death; test fish had characteristic histopathologic alterations and appropriate virus titers. This is the first completely documented occurrence of IHNV beyond the Pacific Northwest.

Journal of the Fisheries Research Board of Canada↗
USGS2004

Migration and stopover strategies of individual Dunlin along the Pacific coast of North America

We radio-marked 18 Dunlin, Calidris alpina (L., 1758), at San Francisco Bay, California, and 11 Dunlin at Grays Harbor, Washington, and relocated 90% of them along the 4200 km long coastline from north of San Francisco Bay to the Yukon-Kuskokwim Delta, Alaska. The Copper River Delta, Alaska, was the single most important stopover site, with 79% of the marked birds detected there. Our second most important site was the Willapa Bay and Grays Harbor complex of wetlands in Washington. The mean length of stay past banding sites ranged from 1.0 to 3.8 days. Controlling for date of departure, birds banded at San Francisco Bay had higher rates of travel to the Copper River Delta than those banded at Grays Harbor. The later a bird left a capture site, the faster it traveled to the Copper River Delta. Length of stay at the Copper River Delta was inversely related to arrival date. We did not find any effect of sex on travel rate or length of stay. Combining the results of this study with our previous work on Western Sandpipers, Calidris mauri (Cabanis, 1875), reveals variation of migration strategies used within and among shorebird species along the eastern Pacific Flyway. ?? 2004 NRC.

Canadian Journal of Zoology↗
USGS2012

Phenology and duration of remigial moult in Surf Scoters ( Melanitta perspicillata ) and White-winged Scoters ( Melanitta fusca ) on the Pacific coast of North America

By quantifying phenology and duration of remigial moult in Surf Scoters ( Melanitta perspicillata (L., 1758)) and White-winged Scoters ( Melanitta fusca (L., 1758)), we tested whether timing of moult is dictated by temporal optima or constraints. Scoters (n = 3481) were captured during moult in Alaska, British Columbia, and Washington, and remigial emergence dates were determined. We provide evidence for a pre-emergence interval of 7.3 days that occurs after old primaries are shed and before new ones become visible. All age and sex classes of both scoter species exhibited a wide range of emergence dates (Surf Scoters: 26 June to 22 September; White-winged Scoters: 6 July to 21 September) suggestive of a lack of strong temporal optima for remigial moult. For both species, timing of moult was influenced by site, year, age, and sex. Relative to other waterfowl species, scoters have typical remigial growth rates (Surf Scoters: 3.9 mm·day –1 ; White-winged Scoters: 4.3 mm·day –1 ) but a long flightless period (34–49 days), in part because their relatively high wing-loading requires a greater proportion of feather regrowth to regain flight. Our data suggest that moulting scoters are not under strong selective pressure to complete moult quickly.

Alaska;British Columbia;Washington↗
USGS2024

Recent advances in characterizing the crustal stress field and future applications of stress data: Perspectives from North America

The stress field controls patterns of crustal deformation, including which faults are likeliest to cause earthquakes or transmit fluids. Since the 1950s, maps of maximum horizontal stress ( S Hmax ) orientations have advanced dramatically, and the style of faulting (relative principal stress magnitudes) has recently been mapped in some regions as well. This perspectives paper summarizes developments in characterizing stress orientations and (relative) magnitudes, including new seismic and borehole methods, as well as progress in identifying the causes of stress variations. Despite these advances, adding far more spatiotemporal detail would allow geoscientists to address many of today's key challenges regarding natural hazards, energy development, and geodynamics. In particular, it is critically important to characterize stress heterogeneity at multiple scales while also recognizing the coherent variability of the stress field. The second part of the paper considers how more detailed stress datasets could prove essential to addressing some of the grand questions in geoscience, including deciphering the poorly understood feedbacks between crustal dynamics and surface processes, improving earthquake and eruption forecasts, and determining the origins and shared properties of plate boundaries.

Geological Society, London, Special Publications↗
USGS2014

Rhynchelmis subgenus Sutroa Eisen new rank, with two new species from western North America (Annelida, Clitellata, Lumbriculidae)

The lumbriculid Rhynchelmis subgenus Sutroa Eisen, 1888 new rank is defined for a group of Nearctic species having multiple diverticula originating at the spermathecal ducts and eversible penial bulbs. Characters are confirmed in specimens of the type species, Rhynchelmis (Sutroa) rostrata (Eisen, 1888), collected from the type locality. Rhynchelmis (Sutroa) klamathensis Fend n. sp. is described from open water benthic habitats in Upper Klamath Lake, Oregon, USA. It resembles other R. (Sutroa) species in the paired spermathecal diverticula, the spermathecal and penial bulbs, the histological structure of the atria, the nonfunctional anterior male funnels, and the arrangement of blood vessels. Rhynchelmis klamathensis differs from all Nearctic Rhynchelmis in lacking a filiform proboscis. The combination of large body size, the elongate spermathecal ducts with paired and usually unbranched diverticula, the highly contorted atria, and the complex male pores with conical penes also distinguish typical R. klamathensis from other Rhynchelmis species. Smaller specimens with otherwise similar morphology, from the Sacramento River Delta, California, are also assigned to this species. Rhynchelmis (Sutroa) diespluviae Fend n. sp. is described from several stream sites, mostly in northern Idaho, USA. Rhynchelmis diespluviae differs from closely related species in morphology of the conical penes, and in the structure and anterolateral position of the paired spermathecae.

Oregon↗
USGS2005

A review of the genus Eclipidrilus (Annelida: Clitellata: Lumbricul-idae), with description of a new species from western North America

Eclipidrilus pacificus n. sp., from northwestern USA, is distinguished from other Eclipidrilus by having a single atrium with midventral male pore, small penis, prostomium without a proboscis, and non-spiraling atrial musculature. Most populations also differ from congeners in having paired spermathecal pores anterior to the ventral chaetae, but specimens from the Columbia River have variable number and placement of the spermathecae. An unusual spiral orientation of the atrial musculature has previously been used to define Eclipidrilus. However, orientation of the atrial muscle layers varies among the previously described species, and does not provide a distinctive synapomorphy for the genus as a whole. Consequently, the genus is redefined to include species having elongate-petiolate atria with an inner, near-transverse muscle layer and an outer, longitudinal to spiral muscle layer.

Idaho, Montana, Oregon, Washington↗
USGS2018

Climate model assessment of changes in winter-spring streamflow timing over North America

Over regions where snow-melt runoff substantially contributes to winter-spring streamflows, warming can accelerate snow melt and reduce dry-season streamflows. However, conclusive detection of changes and attribution to anthropogenic forcing is hindered by brevity of observational records, model uncertainty, and uncertainty concerning internal variability. In this study, a detection/attribution of changes in mid-latitude North American winter-spring streamflow timing is examined using nine global climate models under multiple forcing scenarios. In this study, robustness across models, start/end dates for trends, and assumptions about internal variability is evaluated. Marginal evidence for an emerging detectable anthropogenic influence (according to four or five of nine models) is found in the north-central U.S., where winter-spring streamflows have been coming earlier. Weaker indications of detectable anthropogenic influence (three of nine models) are found in the mountainous western U.S./southwestern Canada and in extreme northeastern U.S./Canadian Maritimes. In the former region, a recent shift toward later streamflows has rendered the full-record trend toward earlier streamflows only marginally significant, with possible implications for previously published climate change detection findings for streamflow timing in this region. In the latter region, no forced model shows as large a shift toward earlier streamflow timing as the detectable observed shift. In other (including warm, snow-free) regions, observed trends are typically not detectable, although in the U.S. central plains we find detectable delays in streamflow, which are inconsistent with forced model experiments.

Journal of Climate↗
USGS1996

Regional aridity in North America during the middle Holocene

Increased aridity throughout the Great Plains and Rocky Mountain region during the middle Holo cene has been documented from pollen records, aeolian proxy variables in lake cores, and active sand dune migration. Varve calibration provided by a continuously varved record of the Holocene from a core from Elk Lake, northwestern Minnesota, shows that the influx of aeolian clastic material increased beginning about 8 ka and ended about 3.8 ka, with peak aeolian activity at about 6 ka. If aeolian influx to Elk Lake corresponds in time to aeolian influx in other lakes and to maximum dune activity in Minnesota dune fields, then the varve calibration in Elk Lake provides precise time calibration of periods of peak aeolian activity in Minnesota. Palaeowind studies from the Minnesota dune fields show that the dominant wind direction when the dunes were active was from the northwest, the same as the dominant wind direction in dune fields throughout the Great Plains and Rocky Mountains. If the mid-Holocene aeolian activity in Minnesota was driven by an increase in westerly zonal winds, then the varve calibration can be extended to more precisely determine the timing of activity of dunes over a much broader area. We suggest that an increase in the westerly zonal wind field might have a solar-geomagnetic cause.

Holocene↗
USGS1997

Complex response of a midcontinent north America drainage system to late Wisconsinan sedimentation

The geomorphic evolution of Mud Creek basin in eastern Iowa, U.S.A. serves to illustrate how geomorphic influences such as sediment supply, valley gradient, climate, and vegetation are recorded in the alluvial stratigraphic record. Sediment supply to the fluvial system increased significantly during the late Wisconsinan through a combination of periglacial erosion and loess accumulation. Subsequent evolution of the Holocene alluvial stratigraphic record reflects long-term routing of the late Wisconsinan sediment through the drainage basin in a series of cut-and-fill cycles whose timing was influenced by hydrologic response to change in climate and vegetation. When viewed in a regional context, the alluvial stratigraphic record appears to reflect a long-term complex response of the fluvial system to increased sediment supply during the late Wisconsinan. Hydrologic and sediment-supply changes accompanying the spread of Euroamerican agriculture to the basin in the 1800s dramatically upset trends in sedimentation and channel behavior established during the Holocene.

Journal of Sedimentary Research↗
USGS2014

Heterogeneous occupancy and density estimates of the pathogenic fungus Batrachochytrium dendrobatidis in waters of North America

Biodiversity losses are occurring worldwide due to a combination of stressors. For example, by one estimate, 40% of amphibian species are vulnerable to extinction, and disease is one threat to amphibian populations. The emerging infectious disease chytridiomycosis, caused by the aquatic fungus Batrachochytrium dendrobatidis ( Bd ), is a contributor to amphibian declines worldwide. Bd research has focused on the dynamics of the pathogen in its amphibian hosts, with little emphasis on investigating the dynamics of free-living Bd . Therefore, we investigated patterns of Bd occupancy and density in amphibian habitats using occupancy models, powerful tools for estimating site occupancy and detection probability. Occupancy models have been used to investigate diseases where the focus was on pathogen occurrence in the host. We applied occupancy models to investigate free-living Bd in North American surface waters to determine Bd seasonality, relationships between Bd site occupancy and habitat attributes, and probability of detection from water samples as a function of the number of samples, sample volume, and water quality. We also report on the temporal patterns of Bd density from a 4-year case study of a Bd -positive wetland. We provide evidence that Bd occurs in the environment year-round. Bd exhibited temporal and spatial heterogeneity in density, but did not exhibit seasonality in occupancy. Bd was detected in all months, typically at less than 100 zoospores L −1 . The highest density observed was ∼3 million zoospores L −1 . We detected Bd in 47% of sites sampled, but estimated that Bd occupied 61% of sites, highlighting the importance of accounting for imperfect detection. When Bd was present, there was a 95% chance of detecting it with four samples of 600 ml of water or five samples of 60 mL. Our findings provide important baseline information to advance the study of Bd disease ecology, and advance our understanding of amphibian exposure to free-living Bd in aquatic habitats over time.

California, Colorado, Idaho, Maine, Minnesota, Nev↗