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At least 1,495 records · Page 83Linked to original sources

USGS1995

Survival of lake trout eggs on reputed spawning grounds in Lakes Huron and Superior: In situ incubation, 1987-1988

Lake trout reproduce widely in Lake Superior but little in Lake Huron. We examined whether survival of lake trout eggs and fry in either lake was reduced by physical disturbances and swim-up mortality. Eggs were collected from feral lake trout in Lake Superior and placed in 108 plastic incubators. A total of 48 incubators was set at Partridge Island Reef in southern Lake Superior, 48 were set at Port Austin Reef in southern Lake Huron, and 12 were held as controls inflowing well water at a laboratory. Survival-to-hatching of these eggs at Partridge Island Reef (18%) was significantly different from that at Port Austin Reef (43%) and significantly different in the laboratory (88%) from that at either reef (P < 0.05). During egg-fry incubation from 28 October 1987 to 5 May 1988, 11–18 cm of sediment accumulated in sediment traps placed on the reefs but < 1 cm of sediment was present on each reef in May 1988. Analysis showed that 44% of the eggs at Port Austin Reef and 28% of those at Partridge Island Reef were buried and killed by sediments. During the first week after deployment, mean wave energy was 90% higher at Partridge Island Reef and significantly different from that at Port Austin Reef. Wave energy may be a habitat condition that makes Partridge Island Reef less suitable than Port Austin Reef for incubation of lake trout eggs. Fry from eggs incubated at all three sites experienced no swim-up mortality. We conclude that in 1987–88 habitat conditions required for survival of lake trout eggs were more suitable at Port Austin Reef than at Partridge Island Reef.

Journal of Great Lakes Research↗

USGS2004

Chapter 4 The meade peak member of the phosphoria formation: Temporal and spatial variations in sediment geochemistry

Variations in the geochemistry of rocks from the Meade Peak Member of the Phosphoria Formation were examined using ratios of elements associated with either the +terrigenous or marine sediment fractions. Inter-element relationships in the terrigenous fraction appear useful for chemo-stratigraphic correlation. A sharp decrease upsection in K 2 O/AI 2 O 3 ratios occurs in the lower half of all but the most northeasterly section, wherein an offset is still evident in average and minimum values. These offsets correspond closely to the lower Guadalupian Series boundary as defined by conodont zonations, coincident with a change from major low-stand to transgressive conditions. The offsets are possibly the result of increased transport distances or flooding of source areas related to transgres- sion of the Phosphoria sea on the Wyoming shelf. A series of intervals displaying high Fe 2 O 3 /Al 2 O 3 , Ba/Al 2 O 3 and Sc/Al 2 O 3 ratios occur in the upper beds of the easternmost sections. The intervals do not appear to reflect amplified marine signals, but rather the introduction of terrigenous sediment from a secondary source, or, simply, reworking of sediments under higher energy conditions. The westernmost section, presumably repre- senting the deepest parts of the Phosphoria basin, contains intervals with high Ba/Al 2 O 3 . We suggest these horizons represent periods of low sediment accumulation during maxi- mum flooding and high-stand conditions. Inter-element relationships in the marine-derived sediment fraction indicate that bottom waters of the Phosphoria basin were dominantly denitrifying (suboxic). Ratios of Cd and Mo to Zn and Cu closely approach those in modern plankton in most of the sections, implying a major biogenic source for these elements. Exceptions occur through- out the westernmost (distal) section, possibly due to changes in the dominant plankton populations and relative nutrient uptakes, and in the upper part of the most northeasterly (shoreward, ramp) section, which we suggest is due to increased oxygen levels. Relatively thick phosphatic layers occur in basinal areas due largely to lack of terrig- enous dilution during deposition. These basinal deposits appear to have lower concentra- tions of many trace elements than more shoreward deposits. This may reflect deposition away from areas of peak primary production. Alternatively, biogenic detritus in these areas may have been derived from differing populations of primary producers with differing nutrient requirements. Both mid-shelf (middle ramp) and marginal environments were sites of accumulation of rich phosphatic units with high concentrations of trace elements. Deposits from marginal areas have the most varied geochemistry, largely because they experienced greater variability in terrigenous sediment influx. Even moderate changes in sea level may have dramatically altered energy levels, sediment mixing, and the amount of organic detritus reaching the sediment surface in these shallower marginal areas.

Colorado, Idaho, Montana, Nebraska, North Dakota, ↗

USGS2019

Formation of pedestalled, relict lakes on the McMurdo Ice Shelf, Antarctica

Surface debris covers much of the western portion of the McMurdo Ice Shelf and has a strong influence on the local surface albedo and energy balance. Differential ablation between debris-covered and debris-free areas creates an unusual heterogeneous surface of topographically low, high-ablation, and topographically raised (‘pedestalled’), low-ablation areas. Analysis of Landsat and MODIS satellite imagery from 1999 to 2018, alongside field observations from the 2016/2017 austral summer, shows that pedestalled relict lakes (‘pedestals’) form when an active surface meltwater lake that develops in the summer, freezes-over in winter, resulting in the lake-bottom debris being masked by a high-albedo, superimposed, ice surface. If this ice surface fails to melt during a subsequent melt season, it experiences reduced surface ablation relative to the surrounding debris-covered areas of the ice shelf. We propose that this differential ablation, and resultant hydrostatic and flexural readjustments of the ice shelf, causes the former supraglacial lake surface to become increasingly pedestalled above the lower topography of the surrounding ice shelf. Consequently, meltwater streams cannot flow onto these pedestalled features, and instead divert around them. We suggest that the development of pedestals has a significant influence on the surface-energy balance, hydrology and flexure of the ice shelf.

Journal of Glaciology↗

USGS2026

Importance of fish in the diet of Adélie penguins across multiple life stages

Over recent decades, the Adélie penguin ( Pygoscelis adeliae ) population has grown across most of its range, the exception being the northern coast of the western Antarctic Peninsula (WAP). In the Ross Sea, the very large Cape Crozier colony grew to reach ~9% of the global population. Demographic factors driving the Cape Crozier trend have yet to be identified. However, low chick fledging mass - a negative influence in the northern WAP where the diet is mostly less-energy-dense krill - is not showing an effect in the Cape Crozier colony. Previously, we hypothesized that Cape Crozier fledglings, and post-breeding adults, must be finding sufficient higher-quality prey once free of the prey-depleted colony foraging area, with subsequent higher survival. Here we explore penguin diet within and outside the colony foraging area using stable isotope analysis (SIA) of feathers: those grown by post-breeding adults outside the foraging area, prior to moult, compared to those of near-fledged chicks raised on meals obtained within the foraging area. Second, we explore whether a major difference occurs between breeding and post-breeding diet of adults from a small, nearby colony (Cape Royds) that exploits a smaller, little-depleted preyscape. Finally, we compare the results to SIA analyses published by others to explore whether the pre-moult diet of Ross Island adults, mostly foraging within the north-eastern Ross Sea, compares with that of adults nesting in the southern WAP (Bellingshausen Sea coast). At the latter, populations are not decreasing, with diet containing an appreciable contribution of fish. The results confirmed that fish contributed greatly to chick diets at both Ross Island colonies (δ 15 N 10.6–11.4). Once adults were released from central-place foraging, SIA levels (δ 15 N 9.7–12.9) were similar to those of the southern WAP. Foraging on energy-dense fish in coastal Antarctic waters could be one reason Adélie penguins are among the most abundant and therefore most ecologically successful penguin species.

Antarctic Science↗

USGS2010

Hubble Space Telescope observations of Europa in and out of eclipse

Europa is a prime target for astrobiology and has been prioritized as the next target for a National Aeronautics and Space Administration flagship mission. It is important, therefore, that we advance our understanding of Europa, its ocean and physical environment as much as possible. Here, we describe observations of Europa obtained during its orbital eclipse by Jupiter using the Hubble Space Telescope. We obtained Advanced Camera for Surveys Solar Blind Channel far ultraviolet low-resolution spectra that show oxygen line emission both in and out of eclipse. We also used the Wide-Field and Planetary Camera-2 and searched for broad-band optical emission from fluorescence of the surface material, arising from the very high level of incident energetic particle radiation on ices and potentially organic substances. The high-energy particle radiation at the surface of Europa is extremely intense and is responsible for the production of a tenuous oxygen atmosphere and associated FUV line emission. Approximately 50% of the oxygen emission lasts at least a few hours into the eclipse. We discuss the detection limits of the optical emission, which allow us to estimate the fraction of incident energy reradiated at optical wavelengths, through electron-excited emission, Cherenkov radiation in the ice and fluorescent processes. ?? 2010 Cambridge University Press.

Conference Paper↗

USGS2008

Comparison of two U.S. power-plant carbon dioxide emissions data sets

Estimates of fossil-fuel CO2 emissions are needed to address a variety of climate-change mitigation concerns over a broad range of spatial and temporal scales. We compared two data sets that report power-plant CO 2 emissions in the conterminous U.S. for 2004, the most recent year reported in both data sets. The data sets were obtained from the Department of Energy's Energy Information Administration (EIA) and the Environmental Protection Agency's eGRID database. Conterminous U.S. total emissions computed from the data sets differed by 3.5% for total plant emissions (electricity plus useful thermal output) and 2.3% for electricity generation only. These differences are well within previous estimates of uncertainty in annual U.S. fossil-fuel emissions. However, the corresponding average absolute differences between estimates of emissions from individual power plants were much larger, 16.9% and 25.3%, respectively. By statistical analysis, we identified several potential sources of differences between EIA and eGRID estimates for individual plants. Estimates that are based partly or entirely on monitoring of stack gases (reported by eGRID only) differed significantly from estimates based on fuel consumption (as reported by EIA). Differences in accounting methods appear to explain differences in estimates for emissions from electricity generation from combined heat and power plants, and for total and electricity generation emissions from plants that burn nonconventional fuels (e.g., biomass). Our analysis suggests the need for care in utilizing emissions data from individual power plants, and the need for transparency in documenting the accounting and monitoring methods used to estimate emissions.

Environmental Science & Technology↗

USGS1998

Partition characteristics of polycyclic aromatic hydrocarbons on soils and sediments

The partition behavior was determined for three polycyclic aromatic hydrocarbons (PAHs), i.e., naphthalene, phenanthrene, and pyrene, from water to a range of soil and sediment samples. The measured partition coefficients of the individual PAHs between soil/sediment organic matter (SOM) and water (i.e., K(oc) values) are relatively invariant either for the 'clean' (uncontaminated) soils or for the clean sediments; however, the mean K(oc) values on the sediments are about twice the values on the soils. This disparity is similar to the earlier observation for other nonpolar solutes and reflects the compositional differences between soil and sediment organic matters. No significant differences in K(oc) are observed between a clean coastal marine sediment and freshwater sediments. The coastal sediments that are significantly impacted by organic contaminants exhibit higher K(oc) values. At given K(ow) values (octanol-water), the PAHs exhibit much higher K(oc) values than other relatively nonpolar solutes (e.g., chlorinated hydrocarbons). This effect is shown to result from the enhanced partition of PAHs to SOM rather than from lower K(ow) values of PAHs at given supercooled liquid solute solubilities in water. The enhanced partition of PAHs over other nonpolar solutes in SOM provides an account of the markedly different correlations between log K(oc) and log K(ow) for PAHs and for other nonpolar solutes. The improved partition of PAHs in SOM stems apparently from the enhanced compatibility of their cohesive energy densities with those of the aromatic components in SOM. The approximate aromatic fraction in soil/sediment organic matter has been assessed by solid-state 13C-NMR spectroscopy.The partition behavior was determined for three polycyclic aromatic hydrocarbons (PAHs), i.e., naphthalene, phenanthrene, and pyrene, from water to a range of soil and sediment samples. The measured partition coefficients of the individual PAHs between soil/sediment organic matter (SOM) and water (i.e., Koc values) are relatively invariant either for the `clean' (uncontaminated) soils or for the clean sediments; however, the mean Koc values on the sediments are about twice the values on the soils. This disparity is similar to the earlier observation for other nonpolar solutes and reflects the compositional differences between soil and sediment organic matters. No significant differences in Koc are observed between a clean coastal marine sediment and freshwater sediments. The coastal sediments that are significantly impacted by organic contaminants exhibit higher Koc values. At given Kow values (octanol-water), the PAHs exhibit much higher Koc values than other relatively nonpolar solutes (e.g., chlorinated hydrocarbons). This effect is shown to result from the enhanced partition of PAHs to SOM rather than from lower Kow values of PAHs at given supercooled liquid solute solubilities in water. The enhanced partition of PAHs over other nonpolar solutes in SOM provides an account of the markedly different correlations between log Koc and log Kow for PAHs and for other nonpolar solutes. The improved partition of PAHs in SOM stems apparently from the enhanced compatibility of their cohesive energy densities with those of the aromatic components in SOM. The approximate aromatic fraction in soil/sediment organic matter has been assessed by solid-state 13C-NMR spectroscopy.

Environmental Science & Technology↗

USGS1998

Application of the surface complexation concept to complex mineral assemblages

Two types of modeling approaches are illustrated for describing inorganic contaminant adsorption in aqueous environments: (a) the component additivity approach and (b) the generalized composite approach. Each approach is applied to simulate Zn2+ adsorption by a well-characterized sediment collected from an aquifer at Cape Cod, MA. Zn2+ adsorption by the sediment was studied in laboratory batch experiments with a range of pH and Zn(II) concentrations selected to encompass conditions observed in the aquifer. In the generalized composite approach, one, and two-site surface complexation model parameters were calibrated with the experimental data using FITEQL. The pH dependence of Zn2+ adsorption was simulated without explicit representation of electrostatic energy terms. Surface acidity constants and ion pair formation by major electrolyte ions were also not required in the model thereby minimizing the number of fitted parameters. Predictions of Zn2+ adsorption with the component additivity modeling approach did not simulate the experimental data adequately without manipulation of surface area or site density parameter values. To apply the component additivity approach to environmental sorbents, further research is needed to better characterize the composition of sediment surface coatings. The generalized composite modeling approach requires less information and can be viewed as more practical for application within solute transport models. With only three adjustable parameters, this approach could simulate Zn2+ adsorption over a range of chemical conditions that would cause several orders of magnitude variation in the distribution coefficient (K(d)) for Zn2+ within the aquifer.Two types of modeling approaches are illustrated for describing inorganic contaminant adsorption in aqueous environments: (a) the component additivity approach and (b) the generalized composite approach. Each approach is applied to simulate Zn2+ adsorption by a well-characterized sediment collected from an aquifer at Cape Cod, MA. Zn2+ adsorption by the sediment was studied in laboratory batch experiments with a range of pH and Zn(II) concentrations selected to encompass conditions observed in the aquifer. In the generalized composite approach, one- and two-site surface complexation model parameters were calibrated with the experimental data using FITEQL. The pH dependence of Zn2+ adsorption was simulated without explicit representation of electrostatic energy terms. Surface acidity constants and ion pair formation by major electrolyte ions were also not required in the model, thereby minimizing the number of fitted parameters. Predictions of Zn2+ adsorption with the component additivity modeling approach did not simulate the experimental data adequately without manipulation of surface area or site density parameter values. To apply the component additivity approach to environmental sorbents, further research is needed to better characterize the composition of sediment surface coatings. The generalized composite modeling approach requires less information and can be viewed as more practical for application within solute transport models. With only three adjustable parameters, this approach could simulate Zn2+ adsorption over a range of chemical conditions that would cause several orders of magnitude variation in the distribution coefficient (Kd) for Zn2+ within the aquifer.

Environmental Science & Technology↗

USGS2014

Methane hydrates in nature - Current knowledge and challenges

Recognizing the importance of methane hydrate research and the need for a coordinated effort, the United States Congress enacted the Methane Hydrate Research and Development Act of 2000. At the same time, the Ministry of International Trade and Industry in Japan launched a research program to develop plans for a methane hydrate exploratory drilling project in the Nankai Trough. India, China, the Republic of Korea, and other nations also have established large methane hydrate research and development programs. Government-funded scientific research drilling expeditions and production test studies have provided a wealth of information on the occurrence of methane hydrates in nature. Numerous studies have shown that the amount of gas stored as methane hydrates in the world may exceed the volume of known organic carbon sources. However, methane hydrates represent both a scientific and technical challenge, and much remains to be learned about their characteristics and occurrence in nature. Methane hydrate research in recent years has mostly focused on: (1) documenting the geologic parameters that control the occurrence and stability of methane hydrates in nature, (2) assessing the volume of natural gas stored within various methane hydrate accumulations, (3) analyzing the production response and characteristics of methane hydrates, (4) identifying and predicting natural and induced environmental and climate impacts of natural methane hydrates, (5) analyzing the methane hydrate role as a geohazard, (6) establishing the means to detect and characterize methane hydrate accumulations using geologic and geophysical data, and (7) establishing the thermodynamic phase equilibrium properties of methane hydrates as a function of temperature, pressure, and gas composition. The U.S. Department of Energy (DOE) and the Consortium for Ocean Leadership (COL) combined their efforts in 2012 to assess the contributions that scientific drilling has made and could continue to make to advance our understanding of methane hydrates in nature. COL assembled a Methane Hydrate Project Science Team with members from academia, industry, and government. This Science Team worked with COL and DOE to develop and host the Methane Hydrate Community Workshop, which surveyed a substantial cross section of the methane hydrate research community for input on the most important research developments in our understanding of methane hydrates in nature and their potential role as an energy resource, a geohazard, and/or as an agent of global climate change. Our understanding of how methane hydrates occur in nature is still growing and evolving, and it is known with certainty that field, laboratory, and modeling studies have contributed greatly to our understanding of hydrates in nature and will continue to be a critical source of the information needed to advance our understanding of methane hydrates.

Journal of Chemical and Engineering Data↗

USGS1997

Vertical accretion and shallow subsidence in a mangrove forest of southwestern Florida, U.S.A

Simultaneous measurements of vertical accretion from artificial soil marker horizons and soil elevation change from sedimentation-erosion table (SET) plots were used to evaluate the processes related to soil building in range, basin, and overwash mangrove forests located in a low-energy lagoon which recieves minor inputs of terregenous sediments. Vertical accretion measures reflect the contribution of surficial sedimentation (sediment deposition and surface root growth). Measures of elevation change reflect not only the contributions of vertical accretion but also those of subsurface processes such as compaction, decomposition and shrink-swell. The two measures were used to calculate amounts of shallow subsidence (accretion minus elevation change) in each mangrove forest. The three forest types represent different accretionary envrionments. The basin forest was located behind a natural berm. Hydroperiod here was controlled primarily by rainfall rather than tidal exchange, although the basin flooded during extreme tidal events. Soil accretion here occurred primarily by autochthonous organic matter inputs, and elevation was controlled by accretion and shrink-swell of the substrate apparently related to cycles of flooding-drying and/or root growth-decomposition. This hydrologically-restricted forest did not experience an accretion or elevation deficit relative to sea-level rise. The tidally dominated fringe and overwash island forests accreted through mineral sediment inputs bound in place by plant roots. Filamentous turf algae played an important role in stabilizing loose muds in the fringe forest where erosion was prevalent. Elevation in these high-energy environments was controlled not only by accretion but also by erosion and/or shallow subsidence. The rate of shallow subsidence was consistently 3-4 mm y-1 in the fringe and overwash island forests but was negligible in the basin forest. Hence, the vertical development of mangrove soils was influenced by both surface and subsurface processes and the procces controlling soil elevation differed among forest types. The mangrove ecosystem at Rookery Bay has remained stable as sea level has risen during the past 70 years. Yet, lead-210 accretion data suggest a substantial accretion deficit has occurred in the past century (accretion was 10-20 cm < sea-level rise from 1930 to 1990) in the fringe and island forests at Rookery Bay. In contrast, our measures of elevation change mostly equalled the estimates of sea-level rise and shallow subsidence. These data suggest that (1) vertical accretion in this system is driven by local sea-level rise and shallow subsidence, and (2) the mangrove forests are mostly keeping pace with sea-level rise. Thus, the vulnerability of this mangrove ecosystem to sea-level rise is best described in terms of an elevation deficit (elevation change minus sea-level rise) based on annual measures rather than an accretion deficit (accretion minus sea-level rise) based on decadal measures.

Mangroves and Salt Marshes↗

USGS1997

The rupture process of the Manjil, Iran earthquake of 20 June 1990 and implications for intraplate strike-slip earthquakes

In terms of seismically radiated energy or moment release, the earthquake of 20 January 1990 in the Manjil Basin-Alborz Mountain region of Iran is the second largest strike-slip earthquake to have occurred in an intracontinental setting in the past decade. It caused enormous loss of life and the virtual destruction of several cities. Despite a very large meizoseismal area, the identification of the causative faults has been hampered by the lack of reliable earthquake locations and conflicting field reports of surface displacement. Using broadband data from global networks of digitally recording seismographs, we analyse broadband seismic waveforms to derive characteristics of the rupture process. Complexities in waveforms generated by the earthquake indicate that the main shock consisted of a tiny precursory subevent followed in the next 20 seconds by a series of four major subevents with depths ranging from 10 to 15 km. The focal mechanisms of the major subevents, which are predominantly strike-slip, have a common nodal plane striking about 285°–295°. Based on the coincidence of this strike with the dominant tectonic fabric of the region we presume that the EW striking planes are the fault planes. The first major subevent nucleated slightly south of the initial precursor. The second subevent occurred northwest of the initial precursor. The last two subevents moved progressively southeastward of the first subevent in a direction collinear with the predominant strike of the fault planes. The offsets in the relative locations and the temporal delays of the rupture subevents indicate heterogeneous distribution of fracture strength and the involvement of multiple faults. The spatial distribution of teleseismic aftershocks, which at first appears uncorrelated with meizoseismal contours, can be decomposed into stages. The initial activity, being within and on the periphery of the rupture zone, correlates in shape and length with meizoseismal lines. In the second stage of activity the aftershock zone expands and appears to cluster about the geomorphic and geologic features several tens of kilometres from the rupture zone. The activity is interpreted as a regional response to quasistatic stress migration along zones of tectonic weakness. The radiated energy of the main shock and the estimate of seismic moment yields an apparent stress of 20 bars. High apparent stress may be typical of strike slip earthquakes occurring in intracontinental environments undergoing continental collision.

Studia Geophysica et Geodaetica↗

USGS1999

Formation of natural gas hydrates in marine sediments 1. Conceptual model of gas hydrate growth conditioned by host sediment properties

The stability of submarine gas hydrates is largely dictated by pressure and temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of deep marine sediments may also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our conceptual model presumes that gas hydrate behaves in a way analogous to ice in a freezing soil. Hydrate growth is inhibited within fine-grained sediments by a combination of reduced pore water activity in the vicinity of hydrophilic mineral surfaces, and the excess internal energy of small crystals confined in pores. The excess energy can be thought of as a "capillary pressure" in the hydrate crystal, related to the pore size distribution and the state of stress in the sediment framework. The base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature (nearer to the seabed) than would be calculated from bulk thermodynamic equilibrium. Capillary effects or a build up of salt in the system can expand the phase boundary between hydrate and free gas into a divariant field extending over a finite depth range dictated by total methane content and pore-size distribution. Hysteresis between the temperatures of crystallization and dissociation of the clathrate is also predicted. Growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, and lenses in muds; cements in sands) can largely be explained by capillary effects, but kinetics of nucleation and growth are also important. The formation of concentrated gas hydrates in a partially closed system with respect to material transport, or where gas can flush through the system, may lead to water depletion in the host sediment. This "freeze-drying" may be detectable through physical changes to the sediment (low water content and overconsolidation) and/or chemical anomalies in the pore waters and metastable presence of free gas within the normal zone of hydrate stability.

Journal of Geophysical Research B: Solid Earth↗

USGS2000

Preface [to special section on recent Loch Vale Watershed research]

Catchment-scale intensive and extensive research conducted over the last decade shows that our understanding of the biogeochemical and hydrologic processes in subalpine and alpine basins is not yet sufficiently mature to model and predict how biogeochemical transformations and surface water quality will change in response to climatic or human-driven changes in energy, water, and chemicals. A better understanding of these processes is needed for input to decision-making regulatory agencies and federal land managers. In recognition of this problem the National Research Council [1998] has identified as a critical research need an improved understanding of how global change will affect biogeochemical interactions with the hydrologic cycle and biogeochemical controls over the transport of water, nutrients, and materials from land to freshwater ecosystems. Improved knowledge of alpine and subalpine ecosystems is particularly important since high-elevation catchments are very sensitive to small changes in the flux of energy, chemicals, and water. Furthermore, alpine ecosystems may act as early warning indicators for ecosystem changes at lower elevations.

Colorado↗

USGS2000

Fracture process zone in granite

In uniaxial compression tests performed on Aue granite cores (diameter 50 mm, length 100 mm), a steel loading plate was used to induce the formation of a discrete shear fracture. A zone of distributed microcracks surrounds the tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samples. Propagation velocity of the process zone is varied by using the rate of acoustic emissions to control the applied axial force. The resulting velocities range from 2 mm/s in displacement-controlled tests to 2 μm/s in tests controlled by acoustic emission rate. Wave velocities and amplitudes are monitored during fault formation. P waves transmitted through the approaching process zone show a drop in amplitude of 26 dB, and ultrasonic velocities are reduced by 10%. The width of the process zone is ∼9 times the grain diameter inferred from acoustic data but is only 2 times the grain size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acoustic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic events show similar orientation of nodal planes on both sides of the shear fracture. The ratio of the process zone width to the fault length in Aue granite ranges from 0.01 to 0.1 inferred from crack data and acoustic emissions, respectively. The fracture surface energy is estimated from microstructure analysis to be ∼2 J. A lower bound estimate for the energy dissipated by acoustic events is 0.1 J.

Journal of Geophysical Research B: Solid Earth↗

USGS2000

A compartmentalized solute transport model for redox zones in contaminated aquifers: 1. Theory and development

This paper, the first of two parts [see Abrams and Loague , this issue], takes the compartmentalized approach for the geochemical evolution of redox zones presented by Abrams et al . [1998] and embeds it within a solute transport framework. In this paper the compartmentalized approach is generalized to facilitate the description of its incorporation into a solute transport simulator. An equivalent formulation is developed which removes any discontinuities that may occur when switching compartments. Rate‐limited redox reactions are modeled with a modified Monod relationship that allows either the organic substrate or the electron acceptor to be the rate‐limiting reactant. Thermodynamic constraints are used to inhibit lower‐energy redox reactions from occurring under infeasible geochemical conditions without imposing equilibrium on the lower‐energy reactions. The procedure used allows any redox reaction to be simulated as being kinetically limited or thermodynamically limited, depending on local geochemical conditions. Empirical reaction inhibition methods are not needed. The sequential iteration approach (SIA), a technique which allows the number of solute transport equations to be reduced, is adopted to solve the coupled geochemical/solute transport problem. When the compartmentalized approach is embedded within the SIA, with the total analytical concentration of each component as the dependent variable in the transport equation, it is possible to reduce the number of transport equations even further than with the unmodified SIA. A one‐dimensional, coupled geochemical/solute transport simulation is presented in which redox zones evolve dynamically in time and space. The compartmentalized solute transport (COMPTRAN) model described in this paper enables the development of redox zones to be simulated under both kinetic and thermodynamic constraints. The modular design of COMPTRAN facilitates the use of many different, preexisting solute transport and geochemical codes. The companion paper [ Abrams and Loague , this issue] presents examples of the application of COMPTRAN to field‐scale problems.

Water Resources Research↗

USGS2001

Stress drop with constant, scale independent seismic efficiency and overshoot

To model dissipated and radiated energy during earthquake stress drop, I calculate dynamic fault slip using a single degree of freedom spring-slider block and a laboratory-based static/kinetic fault strength relation with a dynamic stress drop proportional to effective normal stress. The model is scaled to earthquake size assuming a circular rupture; stiffness varies inversely with rupture radius, and rupture duration is proportional to radius. Calculated seismic efficiency, the ratio of radiated to total energy expended during stress drop, is in good agreement with laboratory and field observations. Predicted overshoot, a measure of how much the static stress drop exceeds the dynamic stress drop, is higher than previously published laboratory and seismic observations and fully elasto-dynamic calculations. Seismic efficiency and overshoot are constant, independent of normal stress and scale. Calculated variation of apparent stress with seismic moment resembles the observational constraints of McGarr [1999].

Geophysical Research Letters↗

USGS2001

Moss and lichen cover mapping at local and regional scales in the boreal forest ecosystem of central Canada

Mosses and lichens are important components of boreal landscapes [Vitt et al., 1994; Bubier et al., 1997]. They affect plant productivity and belowground carbon sequestration and alter the surface runoff and energy balance. We report the use of multiresolution satellite data to map moss and lichens over the BOREAS region at a 10 m, 30 m, and 1 km scales. Our moss and lichen classification at the 10 m scale is based on ground observations of associations among soil drainage classes, overstory composition, and cover type among four broad classes of ground cover (feather, sphagnum, and brown mosses and lichens). For our 30 m map, we used field observations of ground cover-overstory associations to map mosses and lichens in the BOREAS southern study area (SSA). To scale up to a 1 km (AVHRR) moss map of the BOREAS region, we used the TM SSA mosaics plus regional field data to identify AVHRR overstory-ground cover associations. We found that: 1) ground cover, overstory composition and density are highly correlated, permitting inference of moss and lichen cover from satellite-based land cover classifications; 2) our 1 km moss map reveals that mosses dominate the boreal landscape of central Canada, thereby a significant factor for water, energy, and carbon modeling; 3) TM and AVHRR moss cover maps are comparable; 4) satellite data resolution is important; particularly in detecting the smaller wetland features, lakes, and upland jack pine sites; and 5) distinct regional patterns of moss and lichen cover correspond to latitudinal and elevational gradients. Copyright 2001 by the American Geophysical Union.

Journal of Geophysical Research D: Atmospheres↗

USGS2004

Crystallographic controls on the frictional behavior of dry and water-saturated sheet structure minerals

We compare the frictional strengths of 17 sheet structure mineral powders, measured under dry and water-saturated conditions, to identify the factors that cause many of them to be relatively weak. The dry coefficient of friction μ ranges upward from 0.2 for graphite, leveling off at 0.8 for margarite, clintonite, gibbsite, kaolinite, and lizardite. The values of μ (dry) correlate directly with calculated (001) interlayer bond strengths of the minerals. This correlation occurs because shear becomes localized along boundary and Riedel shears and the platy minerals in them rotate into alignment with the shear planes. For those gouges with μ (dry) < 0.8, shear occurs by breaking the interlayer bonds to form new cleavage surfaces. Where μ (dry) = 0.8, consistent with Byerlee's law, the interlayer bonds are sufficiently strong that other frictional processes dominate. The transition in dry friction mechanisms corresponds to calculated surface energies of 2–3 J/m 2 . Adding water causes μ to decrease for every mineral tested except graphite. If the minerals are separated into groups with similar crystal structures, μ (wet) increases with increasing interlayer bond strength within each group. This relationship also holds for the swelling clay montmorillonite, whose water-saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure. Water in the saturated gouges forms thin, structured films between the plate surfaces. The polar water molecules are bonded to the plate surfaces in proportion to the mineral's surface energy, and μ (wet) reflects the stresses required to shear through the water films.

Journal of Geophysical Research B: Solid Earth↗