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At least 667 records · Page 37Linked to original sources

USGS1991

Effect of phosphorus deficiency on spectral reflectance and morphology of soybean plants

Soybean plants were grown in hydroponic solutions having three concentration levels of phosphorus. Spectral reflectance changes included higher reflectance in the green and yellow portions of the electromagnetic spectrum in phosphorus-deficient plants and a difference in position of the long wavelength edge (the red edge) of the chlorophyll absorption band centered near 0.68 ??m. Plants having the least phosphorus in the growing medium did not show the normal shift of the red edge to longer wavelength which occurs as leaves mature. Shoot and root biomass were significantly lower in the phosphorus-deficient plants. These results are consistent with those obtained when soybean plants are dosed with elevated concentrations of metallic elements. We hypothesize that nutrient imbalances or anomalous metal concentrations in the soil set up physiological conditions at the soil/root interface that are responsible for the reflectance differences observed in laboratory and field studies of plants growing in substrates enriched in metallic elements.

Remote Sensing of Environment↗

USGS1988

Pin stripe lamination: A distinctive feature of modern and ancient eolian sediments

Pin stripe laminations are a distinctive feature of modern and ancient eolian sediments. In sets of eolian ripple (or translatent) strata they represent deposition of silt and very fine sand in the troughs of the advancing wind ripples. In sets of avalanche strata they probably result from the downward settling of fine sand and silt within the moving avalanche to the interface of moving and unmoving sands. Wind tunnel experiments suggest that pin stripe laminations can also form in grainfall deposits. The textural segregation associated with deposition of the fine layers in most cases leads to early cementation along and near the finest sand and silt comprising the pin stripe lamination. The pin stripe effect seen in outcrops is usually due to resistance to weathering along such cemented zones. The cementation of the pin stripe laminations can occur early in the history of diagenesis and thus may provide clues to the post-depositional history of the rock. Pin stripe laminations in many instances represent the sequestering of the small population of ultrafine sediment present in most eolian depositional systems. They may prove useful in the recognition of ancient eolian sediments.

Sedimentary Geology↗

USGS1983

Accumulation of fossil fuels and metallic minerals in active and ancient rift lakes

A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H 2 S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes. Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys.

Tectonophysics↗

USGS1994

Seismic structure of the uppermost mantle beneath the Kenya rift

A major goal of the Kenya Rift International Seismic Project (KRISP) 1990 experiment was the determination of deep lithospheric structure. In the refraction/wide-angle reflection part of the KRISP effort, the experiment was designed to obtain arrivals to distances in excess of 400 km. Phases from interfaces within the mantle were recorded from many shotpoints, and by design, the best data were obtained along the axial profile. Reflected arrivals from two thin (< 10 km), high-velocity layers were observed along this profile and a refracted arrival was observed from the upper high-velocity layer. These mantle phases were observed on record sections from four axial profile shotpoints so overlapping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05-8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disappears near the base of the crust. The deeper layer has velocities ranging from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, has a similar dip as the upper one, and is found at depths of 60-65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amounts of extrusive volcanics associated with the rift suggest compositional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly associated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotropy probably due to the preferred orientation of olivine crystals. The seismic model is consistent with hot mantle material rising beneath the Kenya dome in the southern Kenya rift and north-dipping shearing along the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lithosphere due to stretching in the north.

Tectonophysics↗

USGS1991

UDATE1: A computer program for the calculation of uranium-series isotopic ages

UDATE1 is a FORTRAN-77 program with an interface for an Apple Macintosh computer that calculates isotope activities from measured count rates to date geologic materials by uranium-series disequilibria. Dates on pure samples can be determined directly by the accumulation of 230Th from 234U and of 231Pa from 235U. Dates for samples contaminated by clays containing abundant natural thorium can be corrected by the program using various mixing models. Input to the program and file management are made simple and user friendly by a series of Macintosh modal dialog boxes. ?? 1991.

Computers & Geosciences↗

USGS1996

Determination of nitroaromatic explosives and their degradation products in unsaturated-zone water samples by high-performance liquid chromatography with photodiode-array, mass spectrometric, and tandem mass spectrometric detection

Mass spectrometry and tandem mass spectrometry, coupled by a thermospray interface to a high-performance liguid chromatography system and equipped with a photodiode array detector, were used to determine the presence of nitroaromatic explosives and their degradation products in USA unsaturated-zone water samples. Using this approach, the lower limits of quantitation for explosives determined by mass spectrometry in this study typically ranged from 10 to 100 ng/l.

TrAC - Trends in Analytical Chemistry↗

USGS1996

The roles of organic matter in the formation of uranium deposits in sedimentary rocks

Because reduced uranium species have a much smaller solubility than oxidized uranium species and because of the strong association of organic matter (a powerful reductant) with many uranium ores, reduction has long been considered to be the precipitation mechanism for many types of uranium deposits. Organic matter may also be involved in the alterations in and around tabular uranium deposits, including dolomite precipitation, formation of silicified layers, iron-titanium oxide destruction, dissolution of quartz grains, and precipitation of clay minerals. The diagenetic processes that produced these alterations also consumed organic matter. Consequently, those tabular deposits that underwent the more advanced stages of diagenesis, including methanogenesis and organic acid generation, display the greatest range of alterations and contain the smallest amount of organic matter. Because of certain similarities between tabular uranium deposits and Precambrian unconformity-related deposits, some of the same processes might have been involved in the genesis of Precambrian unconformity-related deposits. Hydrologic studies place important constraints on genetic models of various types of uranium deposits. In roll-front deposits, oxidized waters carried uranium to reductants (organic matter and pyrite derived from sulfate reduction by organic matter). After these reductants were oxidized at any point in the host sandstone, uranium minerals were reoxidized and transported further down the flow path to react with additional reductants. In this manner, the uranium ore migrated through the sandstone at a rate slower than the mineralizing ground water. In the case of tabular uranium deposits, the recharge of surface water into the ground water during flooding of lakes carried soluble humic material to the water table or to an interface where humate precipitated in tabular layers. These humate layers then established the chemical conditions for mineralization and related alterations. In the case of Precambrian unconformity-related deposits, free thermal convection in the thick sandstones overlying the basement rocks carried uranium to concentrations of organic matter in the basement rocks.

Ore Geology Reviews↗

USGS1988

Nitrogen cycling between sediment and the shallow-water column in the transition zone of the Potomac River and estuary. I. Nitrate and ammonium fluxes

A three-year study of seasonal variation in water-column and sediment nitrogen species was conducted in the transition zone of the Potomac River 35 m from the Virginia shore at a site with an average water-column depth of approximately 1 m over sandy sediment. A diffusion-controlled sampler was used to collect water samples from the water column, at the interface between the water column and sediment, and at several tens of centimeters into the sediment. Nitrate was the predominant dissolved nitrogen species in the water column. The importance of denitrification was inferred by nitrate fluxes which were directed into the sediment from the water column during approximately 75% of the sampling periods and ranged from 0·02 to 0·69 mmol m −2 day −1 . Flux of nitrate from the sediment into the water column, ⩾0·1 mmol m −2 day −1 , due possibly to nitrification in surficial sediment, occurred during one spring and two summer sampling periods. Ammonium fluxes were less than 0·1 mmol m −2 day −1 during 90% of the sampling periods. Of the ammonium fluxes that were >0·05 mmol m −2 day −1 , all were fluxes into the sediment during sampling periods when sediment resuspension occurred, and all were into the water column during periods of calm. The mean value of ammonium flux (0·005 ± 0·05 mmol m −2 day −1 ) from the sandy, shallow-water sediments was two orders of magnitude less than the ammonium fluxes from the deeper, silty channel sediments in the same reach of the river. Diffusive flux calculations suggest that approximately one order of magnitude more nitrate than ammonium is cycled between the shallow-water column and the sandy sediment in the transition zone of the Potomac River.

Maryland, Virginia↗

USGS1989

Nitrogen cycling between sediment and the shallow-water column in the transition zone of the Potomac River and Estuary. II. The role of wind-driven resuspension and adsorbed ammonium

During periods of sediment resuspension, desorption of ammonium from sediment solids can be the major pathway for enriching the water column with the ammonium that is produced by bacterial degradation of organic matter in the bottom material. This hyopthesis is based on a three-year study of diffusive flux in the transition zone of the Potomac River at a site 35 m from the Virginia shore where the average water-column depth is approximately 1 m over sandy sediment. A diffusion-controlled sampler was used to collect water samples at the interface between the water column and sediment and at several tens of centimeters into the sediment. Interstitial water concentration gradients showed that diffusive flux of ammonium from the sandy shallow-water sediments was approximately 1% of the diffusive flux of ammonium from the silty channel sediments in the same zone of the Potomac River. Organic nitrogen and bound or adsorbed ammonium were the predominant nitrogen forms in the sediment. Adsorbed ammonium concentrations ranged from nondetectable to 3·7 μmol g −1 of sediment. Concentrations of adsorbed ammonium per gram of sediment were one to three orders of magnitude more than interstitial water ammonium concentrations. Desorption of ammonium from sediment solids appeared to be the controlling factor in the degree of water-column ammonium enrichment. In laboratory experiments that simulated sediment resuspension, 40–80% of the adsorbed ammonium predicted to desorb did so after approximately 30 min of mixing. Based on calculations for 1 m 2 to a depth of 4 cm, one resuspenion event lasting minutes could mix more ammonium into the water column from desorption of ammonium from sediment solids than could be delivered to the water column by diffusive flux from shallow-water sediments in 10–1000 days and would be comparable to enrichment by ammonium diffusive flux for 5–50 days from channel sediments in the same river zone.

Maryland, Virginia↗

USGS1983

Distribution of submersed aquatic macrophytes in the tidal Potomac River

Results of a 3-year survey (1978?1980) and review of historic trends have shown a major decline in the number of species and the distribution of submersed aquatic macrophytes in the tidal Potomac River since the early 1900's. The freshwater tidal river is essentially devoid of plants and only very sparse populations remain in the mesohaline section of the estuary. Present plant populations are largely confined to the transition-zone region where salinity instability at the fresh-to-brackish water interface is believed to reduce biotic stress on submersed vegetation. Many factors may be implicated in the loss of vegetation over major regions of the tidal Potomac River; however, long-term conditions of excessive nutrients appear to be primarily responsible for the present distribution.

Aquatic Botany↗

USGS1984

Production induced boiling and cold water entry in the Cerro Prieto geothermal reservoir indicated by chemical and physical measurements

Chemical and physical data suggest that the relatively shallow, western part of the Cerro Prieto reservoir is bounded below by low permeability rocks, and above and at the sides by an interface with cooler water. There is no continuous permeability barrier around or immediately above the reservoir. Permeability within the reservoir is dominantly intergranular. Mixture with cooler water rather than boiling is the dominant cooling process in the natural state, and production causes displacement of hot water by cooler water, not by vapour. Local boiling occurs near most wells in response to pressure decreases, but no general vapour zone has formed. ?? 1984.

Geothermics↗

USGS1993

The hydrogeology of Kilauea volcano

The hydrogeology of Kilauea volcano and adjacent areas has been studied since the turn of this century. However, most studies to date have focused on the relatively shallow, low-salinity parts of the ground-water system, and the deeper hydrothermal system remains poorly understood. The rift zones of adjacent Mauna Loa volcano bound the regional ground-water flow system that includes Kilauea, and the area bounded by the rift zones of Kilauea and the ocean may comprise a partly isolated subsystem. Rates of ground-water recharge vary greatly over the area and discharge is difficult to measure, because streams are ephemeral and most ground-water discharges diffusely at or below sea level. Hydrothermal systems exist at depth in Kilauea's cast and southwest rift zone, as evidenced by thermal springs at the coast and wells in the lower east-rift zone. Available data suggest that dike-impounded, heated ground water occurs at relatively high elevations in the upper east-and southwest-rift zones of Kilauea, and that permeability at depth in the rift zones (probably < 10-15 m2) is much lower than that of unaltered basalt flows closer to the surface (> 10 10 m2). Substantial variations in permeability and the presence of magmatic heat sources influence the structure of the fresh water-salt water interface, so the Ghyben-Herzberg model will often fail to predict its position. Numerical modeling studies have considered only subsets of the hydrothermal system, because no existing computer code solves the coupled fluid-flow, heat- and solute-transport problem over the temperature and salinity range encountered at Kilauea. ?? 1993.

Geothermics↗

USGS1979

Uranium transport in the Walker River Basin, California and Nevada

During the summer of 1976 waters from tributaries, rivers, springs and wells were sampled in the Walker River Basin. Snow and sediments from selected sites were also sampled. All samples were analyzed for uranium and other elements. The resulting data provide an understanding of the transport of uranium within a closed hydrologic basin as well as providing a basis for the design of geochemical reconnaissance studies for the Basin and Range Province of the Western United States. Spring and tributary data are useful in locating areas containing anomalous concentrations of uranium. However, agricultural practices obscure the presence of known uranium deposits and render impossible the detection of other known deposits. Uranium is extremely mobile in stream waters and does not appear to sorb or precipitate. Uranium has a long residence time (2500 years) in the open waters of Walker Lake; however, once it crosses the sediment-water interface, it is reduced to the U(IV) state and is lost from solution. Over the past two million years the amount of uranium transported to the terminal point of the Walker River system may have been on the order of 4 × 10 8 kg . This suggests that closed basin termini are sites for significant uranium accumulations and are, therefore, potential sites of uranium ore deposits.

Journal of Geochemical Exploration↗

USGS1982

Guidelines for finding concretionary Mn-Fe oxides in streams

Concretionary Mn-Fe oxides in streams form at interfaces between oxidizing and reducing environments. A reducing environment produces waters high in dissolved Mn and Fe, and an oxidizing environment causes precipitation. Mineralogical, microprobe, and optical studies of concretionary Mn-Fe oxides may further our understanding of the role of Mn-Fe oxides in determining the trace-element geochemistry of stream sediments.

Journal of Geochemical Exploration↗

USGS1991

Fluid inclusion gas chemistry as a potential minerals exploration tool: Case studies from Creede, CO, Jerritt Canyon, NV, Coeur d'Alene district, ID and MT, southern Alaska mesothermal veins, and mid-continent MVT's

Recent advances in instrumentation now permit quantitative analysis of gas species from individual fluid inclusions. Fluid inclusion gas data can be applied to minerals exploration empirically to establish chemical (gas composition) signatures of the ore fluids, and conceptually through the development of genetic models of ore formation from a framework of integrated geologic, geochemical, and isotopic investigations. Case studies of fluid inclusion gas chemistry from ore deposits representing a spectrum of ore-forming processes and environments are presented to illustrate both the empirical and conceptual approaches. We consider epithermal silver-gold deposits of Creede, Colorado, Carlin-type sediment-hosted disseminated gold deposits of Jerritt Canyon, Nevada, metamorphic silver-base-metal veins of the Coeur d'Alene district, Idaho and Montana, gold-quartz veins in accreted terranes of southern Alaska, and the mid-continent base-metal sulfide deposits of Mississippi Valley-Type (MVT's). Variations in gas chemistry determine the redox state of the ore fluids, provide compositional input for gas geothermometers, characterize ore fluid chemistry (e.g., CH 4 CO 2 , H 2 SSO 2 , CO 2 /H 2 S, organic-rich fluids, gas-rich and gas-poor fluids), identify magmatic, meteoric, metamorphic, shallow and deep basin fluids in ore systems, locate upwelling plumes of magmatic-derived volatiles, zones of boiling and volatile separation, interfaces between contrasting fluids, and important zones of fluid mixing. Present techniques are immediately applicable to exploration programsas empirical studies that monitor fluid inclusion gas threshold concentration levels, presence or absence of certain gases, or changes in gas ratios. We suggest that the greater contribution of fluid inclusion gas analysis is in the integrated and comprehensive chemical dimension that gas data impart to genetic models, and in the exploration concepts based on processes and environments of ore formation derived from these genetic models.

Journal of Geochemical Exploration↗

USGS1986

Role of aquitards in hydrogeochemical systems: A synopsis

Aquitards exert significant influence on the hydrogeochemistry of aquifer systems. This influence is manifested somewhat differently depending on the relative position of aquitards within a system. In the deeper regimes, they are influential in the origin and distribution of brines and the development of geopressured zones. In intermediate regimes, they form multi-layered aquifer systems and provide a source of reactive minerals and exchangeable ions. In shallow regimes, aquitards can influence the topography and drainage patterns; this influences the relationship between the water table and the potentiometric surface of confined aquifers, controls the rates of infiltration and discharge, and controls whether the geochemical system is open or closed to exchange of carbon dioxide gas. In coastal areas, aquitards can determine the depth of the saltwater-freshwater interface, its distance from the shoreline, and the position of the mixing zone that causes geochemical alteration of minerals and development of porosity. ?? 1986.

Applied Geochemistry↗

USGS1991

Radon-222 and its parent radionuclides in groundwater from two study areas in New Jersey and Maryland, U.S.A.

A study of groundwater chemistry and radionuclide mobility in New Jersey and Maryland was conducted to investigate natural processes that control the mobility of radionuclides in the water-rock system. Groundwater was sampled from two geological units in New Jersey and from six in Maryland. The water sampled was from aquifiers in fractured metamorphic rocks of varying composition and metamorphic grade. In both areas, groundwater chemistry was affected most by aquifier mineralogy and lithology; concentrations of total dissolved U, 226Ra and 222Rn were similarly affected. In evey sample for which measurements were made, dissolved Utotal and 226Ra were present in much lower concentrations than 222Rn when expressed in terms of their radioactivity. On the other hand, the total amount of 222Rn that could be produced in these rocks, given their U contents, is much higher than the concentrations observed in groundwater. Thus, the emanating efficiencies of the aquifer rocks studied must be near 10% or less. Such low emanating efficiencies require that a fraction of the 226Ra in the rock be located close to the water-rock interface so that 222Rn, when produced, can be rapidly and efficiently transferred to the aqueous phase. This condition is established when a similar fraction of the U is in a readily leachable position. No known U or Ra solids were supersaturated in any of the samples. Thus, adsorption processes probably play a role in limiting mobilities of Utotal and 226Ra. Concentrations of Utotal and 226Ra found in the water samples are comparable to those found in experimental studies of adsorption onto mineral surfaces. ?? 1991.

Maryland, New Jersey↗

USGS1992

Gypsum accumulation on carbonate stone

The accumulation of gypsum on carbonate stone has been investigated through exposure of fresh samples of limestone and marble at monitored sites, through examination of alteration crusts from old buildings and through laboratory experiments. Several factors contribute to gypsum accumulation on carbonate stone. Marble or limestone that is sheltered from direct washing by rain in an urban environment with elevated pollution levels is likely to accumulate a gypsum crust. Crust development may be enhanced if the stone is porous or has an irregular surface area. Gypsum crusts are a surficial alteration feature; gypsum crystals form at the pore opening-air interface, where evaporation is greatest.

Conference Paper↗