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Articles 2671 - 2700 of 4541

Full-Text Articles in Earth Sciences

Sources Of Ancient Maize Found In Chacoan Great Houses, Larry Benson Jan 2004

Sources Of Ancient Maize Found In Chacoan Great Houses, Larry Benson

United States Geological Survey: Staff Publications

Between the 9th and 12th centuries A.D., Chaco Canyon, located near the middle of the high-desert San Juan Basin of north-central New Mexico (fig. 1), was the focus of an unprecedented construction effort by pre-Columbian Native Americans. It has been estimated that from 2,000 to 6,000 people occupied Chaco Canyon during its heyday (Windes, 1984; Drager, 1976). One indication of Chaco’s regional importance is a network of roads that linked Chaco Canyon with other great houses and communities spread throughout a region covering at least 60,000 km2 (fig. 2). At the height of its cultural florescence in the 11th century, …

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Stress Field Variations In The Swiss Alps And The Northern Alpine Foreland Derived From Inversion Of Fault Plane Solutions, Ulrike Kastrup, Mary Lou Zoback, Nicholas Deichmann, Keith Evans, Domenico Giardini, Andrew J. Michael Jan 2004

Stress Field Variations In The Swiss Alps And The Northern Alpine Foreland Derived From Inversion Of Fault Plane Solutions, Ulrike Kastrup, Mary Lou Zoback, Nicholas Deichmann, Keith Evans, Domenico Giardini, Andrew J. Michael

United States Geological Survey: Staff Publications

This study is devoted to a systematic analysis of the state of stress of the central European Alps and northern Alpine foreland in Switzerland based on focal mechanisms of 138 earthquakes with magnitudes between 1 and 5. The most robust feature of the results is that the azimuth of the minimum compressive stress, S3, is generally well constrained for all data subsets and always lies in the NE quadrant. However, within this quadrant, the orientation of S3 changes systematically both along the structural strike of the Alpine chain and across it. The variation in stress along the mountain …

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Arsenic In Nebraska's Groundwater And Public Water Supplies, David Gosselin, Lynne Klawer, Angela Noe Jan 2004

Arsenic In Nebraska's Groundwater And Public Water Supplies, David Gosselin, Lynne Klawer, Angela Noe

Department of Earth and Atmospheric Sciences: Faculty Publications

Because arsenic (As) in drinking water is considered a primary contributor to cancer in humans, the U.S. Environmental Protection Agency (EPA) recently lowered the maximum contaminant level (MCL) for arsenic from 50 micrograms per liter (μg/L) to 10 μg/L (1 μg/L ~ I part per billion: ppb). This MCL will become effective in 2006. On a national scale, EPA has estimated that of the 74,000 public water supply systems regulated by this MCL, approximately 4,000 systems will have to make changes to comply with it. Of the affected systems, 97 percent are small systems that serve fewer than 10,000 people …

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Sedimentology And Geochemistry Of An Urban Coastal Lake System: Coombabah Lake Nature Reserve, Gold Coast, Queensland, Tracy D. Frank, Christopher R. Fielding Jan 2004

Sedimentology And Geochemistry Of An Urban Coastal Lake System: Coombabah Lake Nature Reserve, Gold Coast, Queensland, Tracy D. Frank, Christopher R. Fielding

Department of Earth and Atmospheric Sciences: Faculty Publications

A study was initiated to address environmental concerns associated with changes in land use in the catchment area of Coombabah Lake, a brackish coastal lake system located in southeast Queensland. Sedimentological and geochemical data derived from a series of cores that penetrate the ca. 0–6000 year-old lacustrine sequence indicate that throughout much of its history, Coombabah Lake has remained a quiet, shallow, water body fed by fine-grained sediment dropped from suspension. Discrete and laterally continuous, shelly horizons form the basis for the stratigraphy developed for the lake sequence. A lithological transition in the upper 50 cm of the sediment column, …

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Semianalytical Solutions For Stream Depletion In Partially Penetrating Streams, Xunhong Chen, Yanfeng Yin Jan 2004

Semianalytical Solutions For Stream Depletion In Partially Penetrating Streams, Xunhong Chen, Yanfeng Yin

Conservation and Survey Division: Faculty and Staff Publications

No abstract provided.

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Review Of Geologic Factors Affecting Underground Limestone Mining In The U.S. And An Overview Of Underground Limestone Mining In Nebraska, R. M. Joeckel Jan 2004

Review Of Geologic Factors Affecting Underground Limestone Mining In The U.S. And An Overview Of Underground Limestone Mining In Nebraska, R. M. Joeckel

Conservation and Survey Division: Faculty and Staff Publications

No abstract provided.

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Federal Listing Of Prairie Grouse: Lessons From The Attwater’S Prairie Chicken, Michael Morrow, Terry A. Rossignol, Nova J. Silvy Jan 2004

Federal Listing Of Prairie Grouse: Lessons From The Attwater’S Prairie Chicken, Michael Morrow, Terry A. Rossignol, Nova J. Silvy

United States Geological Survey: Staff Publications

Considerable controversy has often surrounded proposals to confer official status (i.e., list) species under the authority of the Endangered Species Act (ESA) of 1973 as amended or its precursors. Recent proposals to list the lesser prairie-chicken (Tympanuchus pallidicinctus), the western sage grouse (Centrocercus urophasianus phaios), and the Gunnison sage grouse (C. minimus) were met with strong opposition by those concerned with prospects of an increased regulatory environment associated with such an action. The Attwater's prairie-chicken (T. cupido attwateri) was one of the first species listed under The Endangered Species Conservation Act of …

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History Of Greater Sage-Grouse In The Dakotas: Distribution And Population Trends, Joe T. Smith, Lester D. Flake, Kenneth F. Higgins, Gerald D. Kobriger Jan 2004

History Of Greater Sage-Grouse In The Dakotas: Distribution And Population Trends, Joe T. Smith, Lester D. Flake, Kenneth F. Higgins, Gerald D. Kobriger

United States Geological Survey: Staff Publications

The greater sage-grouse (Centrocercus urophasianus) has declined throughout its range and its status is of major concern to federal, state, and provincial wildlife agencies. We collected information on current and historical greater sage-grouse distribution and lek activity in western North and South Dakota. A steady decline in lek attendance by males occurred over the entire recorded period in North Dakota (1951-2002) and South Dakota (1972-2002). There was no apparent change in numbers of known active leks due to discovery of new leks, but there was an abandonment of regions once occupied by active leks.

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The History Of Recent Limnological Changes And Human Impact On Upper Klamath Lake, Oregon, J. Platt Bradbury, Steven M. Colman, Richard L. Reynolds Jan 2004

The History Of Recent Limnological Changes And Human Impact On Upper Klamath Lake, Oregon, J. Platt Bradbury, Steven M. Colman, Richard L. Reynolds

United States Geological Survey: Staff Publications

Hypereutrophic Upper Klamath Lake has been studied for almost 50 years to evaluate the nature, cause, and effects of its very productive waters. Mitigation of undesirable effects of massive cyanobacterial blooms requires understanding their modern causes as well as their history. Knowledge of the pre-settlement natural limnology of this system can provide guidelines for lake restoration and management of land and water use strategies to maximize the benefits of this aquatic resource. This investigation uses a paleolimnological approach to document the nature and chronology of limnological and biological changes in Upper Klamath Lake for the past 200 years, covering the …

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Geochemical Cycles In Sediments Deposited On The Slopes Of The Guaymas And Carmen Basins Of The Gulf Of California Over The Last 180 Years, Walter E. Dean, Carol Prideb, Robert Thunell Jan 2004

Geochemical Cycles In Sediments Deposited On The Slopes Of The Guaymas And Carmen Basins Of The Gulf Of California Over The Last 180 Years, Walter E. Dean, Carol Prideb, Robert Thunell

United States Geological Survey: Staff Publications

Sediments deposited on the slopes of the Guaymas and Carmen Basins in the central Gulf of California were recovered in two box cores. Q-mode factor analyses identified detrital-clastic, carbonate, and redox associations in the elemental composition of these sediments. The detrital-clastic fraction appears to contain two source components, a more mafic component presumably derived from the Sierra Madre Occidental along the west coast of Mexico, and a more felsic component most likely derived from sedimentary rocks (mostly sandstones) of the Colorado Plateau and delivered by the Colorado River. The sediments also contain significant siliceous biogenic components and minor calcareous biogenic …

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A Mechanical Model Of The San Andreas Fault And Safod Pilot Hole Stress Measurements, Jean Chéry, Mark D. Zoback, Stephen Hickman Jan 2004

A Mechanical Model Of The San Andreas Fault And Safod Pilot Hole Stress Measurements, Jean Chéry, Mark D. Zoback, Stephen Hickman

United States Geological Survey: Staff Publications

Stress measurements made in the SAFOD pilot hole provide an opportunity to study the relation between crustal stress outside the fault zone and the stress state within it using an integrated mechanical model of a transform fault loaded in transpression. The results of this modeling indicate that only a fault model in which the effective friction is very low (<0.1) through the seismogenic thickness of the crust is capable of matching stress measurements made in both the far field and in the SAFOD pilot hole. The stress rotation measured with depth in the SAFOD pilot hole (~28 ° ) appears to be a typical feature of a weak fault embedded in a strong crust and a weak upper mantle with laterally variable heat flow, although our best model predicts less rotation (15 ° ) than observed. Stress magnitudes predicted by our model within the fault zone indicate low shear stress on planes parallel to the fault but a very anomalous mean stress, approximately twice the lithostatic stress.

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A Holocene Pollen Record Of Persistent Droughts From Pyramid Lake, Nevada, Usa, Scott A. Mensing, Larry Benson, Michaele Kashgarian, Steve Lund Jan 2004

A Holocene Pollen Record Of Persistent Droughts From Pyramid Lake, Nevada, Usa, Scott A. Mensing, Larry Benson, Michaele Kashgarian, Steve Lund

United States Geological Survey: Staff Publications

Pollen and algae microfossils preserved in sediments from Pyramid Lake, Nevada, provide evidence for periods of persistent drought during the Holocene age. We analyzed one hundred nineteen 1-cm-thick samples for pollen and algae from a set of cores that span the past 7630 years. The early middle Holocene, 7600 to 6300 cal yr B.P., was found to be the driest period, although it included one short but intense wet phase. We suggest that Lake Tahoe was below its rim for most of this period, greatly reducing the volume and depth of Pyramid Lake. Middle Holocene aridity eased between 5000 and …

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Mortality Of An Adult Cougar Due To A Forest Fire, Dorothy M. Fecske, Jonathan A. Jenks, Frederick D. Lindzey Jan 2004

Mortality Of An Adult Cougar Due To A Forest Fire, Dorothy M. Fecske, Jonathan A. Jenks, Frederick D. Lindzey

United States Geological Survey: Staff Publications

Mortality from natural causes has been documented throughout the range of the cougar (Puma concolor), including California (Beier and Barrett 1993), Canada (Ross et al. 1995), Florida (Taylor et al. 2002), Idaho (Hornocker 1970), Nevada (Ashman et al. 1983), New Mexico (Logan and Sweanor 2001), and Utah (Gashwiler and Robinette 1957, Lindzey et al. 1988). Intraspecific killing by adult males is the most common cause of death in unhunted populations; adult males have killed kittens, subadults, and adults of both sexes (Beier and Barrett 1993, Maehr 1997, Logan and Sweanor 2001, Taylor et al. 2002). Other mortality factors …

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High Resolution Paleoceanography Of The Guaymas Basin, Gulf Of California, During The Past 15 000 Years, John A. Barron, David Bukry, James L. Bischoff Jan 2004

High Resolution Paleoceanography Of The Guaymas Basin, Gulf Of California, During The Past 15 000 Years, John A. Barron, David Bukry, James L. Bischoff

United States Geological Survey: Staff Publications

Deep Sea Drilling Project Site 480 (27°54.10’N, 111°39.34’W; 655 m water depth) contains a high resolution record of paleoceanographic change of the past 15 000 years for the Guaymas Basin, a region of very high diatom productivity within the central Gulf of California. Analyses of diatoms and silicoflagellates were completed on samples spaced every 40-50 yr, whereas ICP-AES geochemical analyses were completed on alternate samples (sample spacing 80-100 yr). The Bolling-Allerod interval (14.6-12.9 ka) (note, ka refers to 1000 calendar years BP throughout this report) is characterized by an increase in biogenic silica and a decline in calcium carbonate relative …

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Paleolimnology And Paleoclimate Studies In Upper Klamath Lake, Oregon, Steven M. Colman, J. Platt Bradbury, Joseph G. Rosenbaum Jan 2004

Paleolimnology And Paleoclimate Studies In Upper Klamath Lake, Oregon, Steven M. Colman, J. Platt Bradbury, Joseph G. Rosenbaum

United States Geological Survey: Staff Publications

The subsiding Upper Klamath Lake Basin contains sediments that were continuously deposited in a shallow, freshwater lake for more than 40 000 years. Well dated by radiometric methods and containing volcanic ashes of known age, these sediments constitute a valuable paleoclimate record. Sediment constituents and properties that reflect past climatic conditions in the area include pollen, diatoms, sediment geochemistry, and sediment magnetic properties. Many of these proxy measurements are also useful for comparing natural conditions in the lake to conditions following human settlement. Because of its location, the paleoclimate record from Upper Klamath Lake is valuable for comparisons to offshore …

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Quaternary Sea-Level History Of The United States, Daniel R. Muhs, John F. Wehmiller, Kathleen R. Simmons, Linda L. York Jan 2004

Quaternary Sea-Level History Of The United States, Daniel R. Muhs, John F. Wehmiller, Kathleen R. Simmons, Linda L. York

United States Geological Survey: Staff Publications

In the past 30 years, there have been tremendous advances in our understanding of Quaternary sea-level history, due directly to developments in Quaternary dating methods, particularly uranium-series disequilibrium and amino acid racemization. Another reason for this progress is that coastline history can now be tied to the oxygen-isotope record of foraminifera in deep-sea cores. Furthermore, both records have been linked to climate change on the scale of glacial-interglacial cycles that are thought to be forced by changes in Earth-Sun geometry, or “orbital forcing” (Milankovitch, 1941).

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Genetic Identification Of Spotted Owls, Barred Owls, And Their Hybrids: Legal Implications Of Hybrid Identity, Susan M. Haig, Thomas D. Mullins, Eric D. Forsman, Pepper W. Trail, Liv Wennerberg Jan 2004

Genetic Identification Of Spotted Owls, Barred Owls, And Their Hybrids: Legal Implications Of Hybrid Identity, Susan M. Haig, Thomas D. Mullins, Eric D. Forsman, Pepper W. Trail, Liv Wennerberg

United States Geological Survey: Staff Publications

Recent population expansion of Barred Owls (Strix varia) into western North America has led to concern that they may compete with and further harm the Northern Spotted Owl (S. occidentalis caurina), which is already listed as threatened under the U.S. Endangered Species Act (ESA). Because they hybridize, there is a legal need under the ESA for forensic identification of both species and their hybrids. We used mitochondrial control-region DNA and amplified fragment-length polymorphism (AFLP) analyses to assess maternal and biparental gene flow in this hybridization process. Mitochondrial DNA sequences (524 base pairs) indicated large divergence between …

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Subspecific Relationships And Genetic Structure In The Spotted Owl, Susan M. Haig, Thomas D. Mullins, Eric D. Forsman Jan 2004

Subspecific Relationships And Genetic Structure In The Spotted Owl, Susan M. Haig, Thomas D. Mullins, Eric D. Forsman

United States Geological Survey: Staff Publications

Hierarchical genetic structure was examined in the three geographically-defined subspecies of spotted owl (Strix occidentalis) to define relationships among subspecies and quantify variation within and among regional and local populations. Sequences (522 bp) from domains I and II of the mitochondrial control region were analyzed for 213 individuals from 30 local breeding areas. Results confirmed significant differences between northern spotted owls and the other traditional geographically defined subspecies but did not provide support for subspecific level differences between California and Mexican spotted owls. Divergence times among subspecies estimated with a 936 bp portion of the cytochrome b gene …

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An Association Of Benthic Foraminifera And Gypsum In Holocene Sediments Of Estuarine Chesapeake Bay, Usa, John Cann, Thomas Cronin Jan 2004

An Association Of Benthic Foraminifera And Gypsum In Holocene Sediments Of Estuarine Chesapeake Bay, Usa, John Cann, Thomas Cronin

United States Geological Survey: Staff Publications

Two cores of Holocene sediments recovered from the Cape Charles Channel of Chesapeake Bay yielded radiocarbon ages of about 6.8 to 5.8 ka for the lower intervals. Fossil foraminifera preserved in these lower sediments are dominated by species of Elphidium, which make up about 90% of the assemblage throughout, and probably signify deposition in hypersaline waters. Buccella frigida and Ammonia beccarii are the only other species commonly present. Hypersalinity of bottom waters seems to have been maintained by water-density stratification in a basin-like section of the channel. In core PTXT -4-P-I transition to modem Chesapeake conditions, in which numbers …

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Fine-Scale Structure Of The San Andreas Fault Zone And Location Of The Safod Target Earthquakes, C. Thurber, S. Roecker, H. Zhang, S. Baher, W. Ellsworth Jan 2004

Fine-Scale Structure Of The San Andreas Fault Zone And Location Of The Safod Target Earthquakes, C. Thurber, S. Roecker, H. Zhang, S. Baher, W. Ellsworth

United States Geological Survey: Staff Publications

We present results from the tomographic analysis of seismic data from the Parkfield area using three different inversion codes. The models provide a consistent view of the complex velocity structure in the vicinity of the San Andreas, including a sharp velocity contrast across the fault. We use the inversion results to assess our confidence in the absolute location accuracy of a potential target earthquake. We derive two types of accuracy estimates, one based on a consideration of the location differences from the three inversion methods, and the other based on the absolute location accuracy of ‘‘virtual earthquakes.’’ Location differences are …

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Streaks, Multiplets, And Holes: High-Resolution Spatio-Temporal Behavior Of Parkfield Seismicity, F. Waldhauser, W. L. Ellsworth, D. P. Schaff, A. Cole Jan 2004

Streaks, Multiplets, And Holes: High-Resolution Spatio-Temporal Behavior Of Parkfield Seismicity, F. Waldhauser, W. L. Ellsworth, D. P. Schaff, A. Cole

United States Geological Survey: Staff Publications

Double-difference locations of ~8000 earthquakes from 1969–2002 on the Parkfield section of the San Andreas Fault reveal detailed fault structures and seismicity that is, although complex, highly organized in both space and time. Distinctive features of the seismicity include: 1) multiple recurrence of earthquakes of the same size at precisely the same location on the fault (multiplets), implying frictional or geometric controls on their location and size; 2) sub-horizontal alignments of hypocenters along the fault plane (streaks), suggestive of rheological transitions within the fault zone and/or stress concentrations between locked and creeping areas; 3) regions devoid of microearthquakes with typical …

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Introduction To Special Section: Preparing For The San Andreas Fault Observatory At Depth, Stephen Hickman, Mark D. Zoback, William Ellsworth Jan 2004

Introduction To Special Section: Preparing For The San Andreas Fault Observatory At Depth, Stephen Hickman, Mark D. Zoback, William Ellsworth

United States Geological Survey: Staff Publications

The San Andreas Fault Observatory at Depth (SAFOD) is a comprehensive project to drill into the hypocentral zone of repeating M ~ 2 earthquakes on the San Andreas Fault at a depth of about 3 km. The goals of SAFOD are to establish a multi-stage geophysical observatory in close proximity to these repeating earthquakes, to carry out a comprehensive suite of downhole measurements in order to study the physical and chemical conditions under which earthquakes occur and to exhume rock and fluid samples for extensive laboratory studies. In the vicinity of SAFOD, the San Andreas is moving through a combination …

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Stress Orientations And Magnitudes In The Safod Pilot Hole, Stephen Hickman, Mark D. Zoback Jan 2004

Stress Orientations And Magnitudes In The Safod Pilot Hole, Stephen Hickman, Mark D. Zoback

United States Geological Survey: Staff Publications

Borehole breakouts and drilling-induced tensile fractures in the 2.2-km-deep SAFOD pilot hole at Parkfield, CA, indicate significant local variations in the direction of the maximum horizontal compressive stress, SHmax, but show a generalized increase in the angle between SHmax and the San Andreas Fault with depth. This angle ranges from a minimum of 25 ± 10 ° at 1000–1150 m to a maximum of 69 ± 14 ° at 2050–2200 m. The simultaneous occurrence of tensile fractures and borehole breakouts indicates a transitional strike-slip to reverse faulting stress regime with high horizontal differential stress, although there is considerable uncertainty in …

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Stress-Induced, Time-Dependent Fracture Closure At Hydrothermal Conditions, N. M. Beeler, S. H. Hickman Jan 2004

Stress-Induced, Time-Dependent Fracture Closure At Hydrothermal Conditions, N. M. Beeler, S. H. Hickman

United States Geological Survey: Staff Publications

Time-dependent closure of fractures in quartz was measured in situ at 22–530 °C temperature and 0.1–150 MPa water pressure. Unlike previous crack healing and rock permeability studies, in this study, fracture aperture is monitored directly and continuously using a windowed pressure vessel, a long-working-distance microscope, and reflectedlight interferometry. Thus the fracture volume and geometry can be measured as a function of time, temperature, and water pressure. Relatively uniform closure occurs rapidly at temperatures and pressures where quartz becomes significantly soluble in water. During closure the aperture is reduced by as much as 80% in a few hours. We infer that …

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In Situ Stress, Fracture, And Fluid Flow Analysis In Well 38c-9: An Enhanced Geothermal System In The Coso Geothermal Field, Judith M. Sheridan, Stephen H. Hickman Jan 2004

In Situ Stress, Fracture, And Fluid Flow Analysis In Well 38c-9: An Enhanced Geothermal System In The Coso Geothermal Field, Judith M. Sheridan, Stephen H. Hickman

United States Geological Survey: Staff Publications

Geoscientists from the Coso Operating Company, EGI-Utah, GeoMechanics International, and the United States Geological Survey are cooperating in a multi-year study to develop an Enhanced Geothermal System (EGS) in the Coso Geothermal Field. Key to the creation of an EGS is an understanding of the relationship among natural fracture distribution, fluid flow, and the ambient tectonic stresses that exist within the resource in order to design a hydraulic and thermal stimulation of an east-flank injection well, the first step in the creation of a heat exchanger at depth. Well datasets from the east flank of the Coso Geothermal Field are …

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Persistence Of Pharmaceutical Compounds And Other Organic Wastewater Contaminants In A Conventional Drinking-Water-Treatment Plant, Paul E. Stackelberg, Edward T. Furlong, Michael T. Meyer, Steven D. Zaugg, Alden K. Henderson, Dori B. Reissman Jan 2004

Persistence Of Pharmaceutical Compounds And Other Organic Wastewater Contaminants In A Conventional Drinking-Water-Treatment Plant, Paul E. Stackelberg, Edward T. Furlong, Michael T. Meyer, Steven D. Zaugg, Alden K. Henderson, Dori B. Reissman

United States Geological Survey: Staff Publications

In a study conducted by the US Geological Survey and the Centers for Disease Control and Prevention, 24 water samples were collected at selected locations within a drinking-water-treatment (DWT) facility and from the two streams that serve the facility to evaluate the potential for wastewater-related organic contaminants to survive a conventional treatment process and persist in potable-water supplies. Stream-water samples as well as samples of raw, settled, filtered, and finished water were collected during low-flow conditions, when the discharge of effluent from upstream municipal sewage-treatment plants accounted for 37–67% of flow in stream 1 and 10–20% of flow in stream …

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Uranium-Series Coral Ages From The Us Atlantic Coastal Plain–The ‘‘80 Ka Problem’’ Revisited, John F. Wehmiller, Kathleen R. Simmons, Hai Cheng, R. Lawrence Edwards, Jamie Martin-Mcnaughton, Linda L. York, David E. Krantz, Chuan-Chou Shen Jan 2004

Uranium-Series Coral Ages From The Us Atlantic Coastal Plain–The ‘‘80 Ka Problem’’ Revisited, John F. Wehmiller, Kathleen R. Simmons, Hai Cheng, R. Lawrence Edwards, Jamie Martin-Mcnaughton, Linda L. York, David E. Krantz, Chuan-Chou Shen

United States Geological Survey: Staff Publications

Uranium series coral ages for emergent units from the passive continental margin US Atlantic Coastal Plain (ACP) suggest sea level above present levels at the end of marine oxygen isotope stage (MIS) 5, contradicting age-elevation relations based on marine isotopic or coral reef models of ice equivalent sea level. We have reexamined this problem by obtaining high precision 230Th/238U and 231Pa/235U thermal ionization mass spectrometric ages for recently collected and carefully cleaned ACPcorals, many in situ. We recognize samples that show no evidence for diagenesis on the basis of uranium isotopic composition and age …

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Testing Density-Dependent Groundwater Models: Two-Dimensional Steady State Unstable Convection In Infinite, Finite And Inclined Porous Layers, Douglas Weatherill, Craig T. Simmons, Clifford I. Voss, Neville I. Robinson Jan 2004

Testing Density-Dependent Groundwater Models: Two-Dimensional Steady State Unstable Convection In Infinite, Finite And Inclined Porous Layers, Douglas Weatherill, Craig T. Simmons, Clifford I. Voss, Neville I. Robinson

United States Geological Survey: Staff Publications

This study proposes the use of several problems of unstable steady state convection with variable fluid density in a porous layer of infinite horizontal extent as two-dimensional (2-D) test cases for density-dependent groundwater flow and solute transport simulators. Unlike existing density-dependent model benchmarks, these problems have well-defined stability criteria that are determined analytically. These analytical stability indicators can be compared with numerical model results to test the ability of a code to accurately simulate buoyancy driven flow and diffusion. The basic analytical solution is for a horizontally infinite fluid-filled porous layer in which fluid density decreases with depth. The proposed …

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Holocene Loess Deposition And Soil Formation As Competing Processes, Matanuska Valley, Southern Alaska, Daniel R. Muhs, John P. Mcgeehin, Jossh Beann, Eric Fisher Jan 2004

Holocene Loess Deposition And Soil Formation As Competing Processes, Matanuska Valley, Southern Alaska, Daniel R. Muhs, John P. Mcgeehin, Jossh Beann, Eric Fisher

United States Geological Survey: Staff Publications

Although loess–paleosol sequences are among the most important records of Quaternary climate change and past dust deposition cycles, few modern examples of such sedimentation systems have been studied. Stratigraphic studies and 22 new accelerator mass spectrometry radiocarbon ages from the Matanuska Valley in southern Alaska show that loess deposition there began sometime after ~6500 14C yr B.P. and has continued to the present. The silts are produced through grinding by the Matanuska and Knik glaciers, deposited as outwash, entrained by strong winds, and redeposited as loess. Over a downwind distance of ~40 km, loess thickness, sand content, and sand-pluscoarse- …

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Great Plains Ecosystems: Past, Present, And Future, Fred B. Samson, Fritz L. Knopf, Wayne Ostlie Jan 2004

Great Plains Ecosystems: Past, Present, And Future, Fred B. Samson, Fritz L. Knopf, Wayne Ostlie

United States Geological Survey: Staff Publications

Little questions exists that the main bodies of North American prairie (i.e., the tall-grass, mixed, and shortgrass) are among the most endangered resources on the continent. The purpose of this paper is to provide a past and present biological base- line by which to understand North American prairies and to provide a platform for future conservation. Events both immediate to the end of the Pleistocene and historic suggest that the present grassland conditions are different from those within which most of the grassland organisms evolved. Our analysis suggests that few grassland landscapes remain adequate in area and distribution to sustain …

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