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USGS1987

Crustal structure beneath exposed accreted terranes of Southern Alaska

The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8-9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.

Alaska↗

USGS2015

Relationships among seismic velocity, metamorphism, and seismic and aseismic fault slip in the Salton Sea Geothermal Field region

The Salton Sea Geothermal Field is one of the most geothermally and seismically active areas in California and presents an opportunity to study the effect of high-temperature metamorphism on the properties of seismogenic faults. The area includes numerous active tectonic faults that have recently been imaged with active source seismic reflection and refraction. We utilize the active source surveys, along with the abundant microseismicity data from a dense borehole seismic network, to image the 3-D variations in seismic velocity in the upper 5 km of the crust. There are strong velocity variations, up to ~30%, that correlate spatially with the distribution of shallow heat flow patterns. The combination of hydrothermal circulation and high-temperature contact metamorphism has significantly altered the shallow sandstone sedimentary layers within the geothermal field to denser, more feldspathic, rock with higher P wave velocity, as is seen in the numerous exploration wells within the field. This alteration appears to have a first-order effect on the frictional stability of shallow faults. In 2005, a large earthquake swarm and deformation event occurred. Analysis of interferometric synthetic aperture radar data and earthquake relocations indicates that the shallow aseismic fault creep that occurred in 2005 was localized on the Kalin fault system that lies just outside the region of high-temperature metamorphism. In contrast, the earthquake swarm, which includes all of the M > 4 earthquakes to have occurred within the Salton Sea Geothermal Field in the last 15 years, ruptured the Main Central Fault (MCF) system that is localized in the heart of the geothermal anomaly. The background microseismicity induced by the geothermal operations is also concentrated in the high-temperature regions in the vicinity of operational wells. However, while this microseismicity occurs over a few kilometer scale region, much of it is clustered in earthquake swarms that last from hours to a few days and are localized near the MCF system.

Salton Sea↗

USGS1995

Digital aeromagnetic anomaly data from eastern-most Guyana

The Center for Inter-American Mineral Resource Investigations (CIMRI) supported distribution and analysis of geoscientific and mineral resource related information concerning Latin America. CIMRI staff digitized aeromagnetic data for eastern-most Guyana as part of a preliminary regional assessment of minerals in the Guyana Shield, South America. The data were digitized from 145 aeromagnetic contour maps at a scale of 1:50,000 and merged into a single digital data set. The data were used to examine the Precambrian shield, greenstone belts, and other tectonic boundaries as well as explore ideas concerning mineral deposits within the area. A subset of these digital data were presented to the Guyanan government during early 1995 (Pierce, 1994). This Open-File report, consisting of this text and seven (7) 3.5" IBM-PC compatible ASCII magnetic disks, makes the digital data available to the public. Information regarding the source of data and subsequent processing is included below. The data were collected in Guyana by two contractors at different times. The first data were collected from 1962 to 1963. These data are several aeromagnetic surveys covering parts of 12 quadrangles funded by the United Nations and flown by Aero Service Corporation. The second and more extensive data set was collected from 1971 to 1972 by the Canadian International Development Agency flown by Terra Surveys Ltd. under a contract with the Geological Survey of Guyana. The Guyana Government published the data as contour maps that are available in Georgetown through the Guyana Government. Coverage extends from about 2°45*N to 8°30*N latitude and from 60°0'W to 57°0'W longitude (see Figure 1.). The contour maps were digitized at points where the magnetic contours intersect the flight lines. The data files include XYZ ASCII files, XYZ binary files, ASCII grids, and binary "standard USGS" grids. There are four grids consisting of the following data types: unprotected raw data grid unprotected residual or International Geomagnetic Reference Field (IGRF) removed grid UTM projected residual (IGRF removed) grid UTM projected residual with a second order surface removed These data files were transferred to 3.5" 1.44 megabyte floppy disks readable on IBM-compatible personal computers. These data are also available from the Department of Commerce National Geophysical Data Center.

Open-File Report↗

USGS2021

Climatic controls on soil carbon accumulation and loss in a dryland ecosystems

Arid and semiarid ecosystems drive year-to-year variability in the strength of the terrestrial carbon (C) sink, yet there is uncertainty about how soil C gains and losses contribute to this variation. To address this knowledge gap, we embedded C-depleted soil mesocosms, containing litter or biocrust C inputs, within an in situ dryland ecosystem warming experiment. Over the course of one year, changes in microbial biomass and total soil organic C pools were monitored alongside hourly measurements of soil CO 2 flux. We also developed a biogeochemical model to explore the mechanisms that gave rise to observed soil C dynamics. Field data and model simulations demonstrated that water exerted much stronger control on soil biogeochemistry than temperature, with precipitation events triggering large CO 2 pulses and transport of litter- and biocrust-derived C into the soil profile. We expected leaching of organic matter would result in steady accumulation of C within the mineral soil over time. Instead, the size of the total organic C pool fluctuated throughout the year, largely in response to microbial growth: increases in the size of microbial biomass were negatively correlated with the quantity of C residing in the top 2 cm, where most biogeochemical changes were observed. Our data and models suggest that microbial responses to precipitation events trigger rapid metabolism of dissolved organic C inputs, which strongly limit accumulation of autotroph-derived C belowground. Accordingly, changes in the magnitude and/or frequency of precipitation events in this dryland ecosystem could have profound impacts on the strength of the soil C sink.

Utah↗

USGS2011

A Bayesian network to predict coastal vulnerability to sea level rise

Sea level rise during the 21st century will have a wide range of effects on coastal environments, human development, and infrastructure in coastal areas. The broad range of complex factors influencing coastal systems contributes to large uncertainties in predicting long-term sea level rise impacts. Here we explore and demonstrate the capabilities of a Bayesian network (BN) to predict long-term shoreline change associated with sea level rise and make quantitative assessments of prediction uncertainty. A BN is used to define relationships between driving forces, geologic constraints, and coastal response for the U.S. Atlantic coast that include observations of local rates of relative sea level rise, wave height, tide range, geomorphic classification, coastal slope, and shoreline change rate. The BN is used to make probabilistic predictions of shoreline retreat in response to different future sea level rise rates. Results demonstrate that the probability of shoreline retreat increases with higher rates of sea level rise. Where more specific information is included, the probability of shoreline change increases in a number of cases, indicating more confident predictions. A hindcast evaluation of the BN indicates that the network correctly predicts 71% of the cases. Evaluation of the results using Brier skill and log likelihood ratio scores indicates that the network provides shoreline change predictions that are better than the prior probability. Shoreline change outcomes indicating stability (-1 < rate < 1 m/yr) or erosion (rate < -1 m/yr) tend to occur for two sets of input scenarios. Stable shoreline change rates occur mainly for low rates of relative sea level rise and occur in low-vulnerability geomorphic settings. Rates indicating erosion result for cases where the rate of relative sea level rise is high and moderate-to-high vulnerability geomorphic settings occur. In contrast, accretion (rate > 1 m/yr) was not well predicted. We find that BNs can assimilate important factors contributing to coastal change in response to sea level rise and can make quantitative, probabilistic predictions that can be applied to coastal management decisions. Copyright ?? 2011 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗

USGS2018

Ice wedge degradation and stabilization impacts water budgets and nutrient cycling in Arctic trough ponds

Trough ponds are ubiquitous features of Arctic landscapes and an important component of freshwater aquatic ecosystems. Permafrost thaw causes ground subsidence, creating depressions that gather water, creating ponds. Permafrost thaw also releases solutes and nutrients, which may fertilize these newly formed ponds. We measured water budget elements and chloride, ammonium, and dissolved organic nitrogen (DON) across a chronosequence of trough ponds representing different stages of ice wedge degradation and stabilization. We developed a coupled hydrologic and biogeochemical model to explore how ice wedge degradation affects hydrology and nutrient availability in trough ponds in the advanced degradation stages (DAs), which are characterized by deep troughs with warmer temperatures relative to the other stages. DAs experienced greater evaporation than the other stages, and subsurface inflows entered the DAs from a wide area. Chloride accumulated in the ponds with time since thaw, implying that subsurface fluxes are delivering solutes from the thawing permafrost. Ammonium accumulated at high rates in the initial degradation stage and was seasonally depleted over the summer in all degradation stages. Ammonium trends in the DAs were consistent with high concentration inflows and in‐pond assimilation at rates between 0.37 and 2.0 mg N m −2 day −1 . Seasonal DON trends indicated that the accumulation of recalcitrant organic matter may eventually limit aquatic ecosystem production and foster pond infilling. These results provide direct evidence of nutrient release from thawing permafrost and the utilization of these nutrients by Arctic trough pond ecosystems and highlight infilling as a mechanism by which Arctic surface waters may be lost

Journal of Geophysical Research: Biogeosciences↗

USGS1994

Remote sensing and airborne geophysics in the assessment of natural aggregate resources

Natural aggregate made from crushed stone and deposits of sand and gravel is a vital element of the construction industry in the United States. Although natural aggregate is a high volume/low value commodity that is relatively abundant, new sources of aggregate are becoming increasingly difficult to find and develop because of rigid industry specifications, political considerations, development and transporation costs, and environmental concerns, especially in urban growth centers where much of the aggregate is used. As the demand for natural aggregate increases in response to urban growth and the repair and expansion of the national infrastructure, new sources of natural aggregate will be required. The USGS has recognized the necessity of developing the capability to assess the potential for natural aggregate sources on Federal lands; at present, no methodology exists for systematically describing and evaluating potential sources of natural aggregate. Because remote sensing and airborne geophysics can detect surface and nearsurface phenomena, these tools may useful for detecting and mapping potential sources of natural aggregate; however, before a methodology for applying these tools can be developed, it is necessary to understand the type, distribution, physical properties, and characteristics of natural aggregate deposits, as well as the problems that will be encountered in assessing their potential value. There are two primary sources of natural aggregate: (1) exposed or near-surface igneous, metamorphic, and sedimentary bedrock that can be crushed, and (2) deposits of sand and gravel that may be used directly or crushed and sized to meet specifications. In any particular area, the availability of bedrock suitable for crushing is a function of the geologic history of the area - the processes that formed, deformed, eroded and exposed the bedrock. Deposits of sand and gravel are primarily surficial deposits formed by the erosion, transportation by water and ice, and deposition of bedrock fragments. Consequently, most sand and gravel deposits are Tertiary or Quaternary in age and are most common in glaciated areas, alluvial basins, and along rivers and streams. The distribution of potential sources of natural aggregate in the United States is closely tied to physiography and the type of bedrock that occurs in an area. Using these criteria, the United States can be divided into 12 regions: western mountain ranges, alluvial basins, Columbia Plateau, Colorado Plateau and Wyoming basin, High Plains, nonglaciated central region, glaciated central region, Piedmont Blue Ridge region, glaciated northeastern and Superior uplands, Atlantic and Gulf coastal plain, Hawaiian Islands, and Alaska. Each region has similar types of natural aggregate sources within its boundary, although there may be wide variations in specific physical and chemical characteristics of the aggregates within a region. Conventional exploration for natural aggregate deposits has been largely a ground-based operation (field mapping, sampling, trenching and augering, resistivity), although aerial photos and topographic maps have been extensively used to target possible deposits for sampling and testing. Today, the exploration process also considers other factors such as the availability of the land, space and water supply for processing purposes, political and environmental factors, and distance from the market; exploration and planning cannot be separated. There are many physical properties and characteristics by which aggregate material is judged to be acceptable or unacceptable for specific applications; most of these properties and characteristics pertain only to individual aggregate particles and not to the bulk deposit. For example, properties of crushed stone aggregate particles such as thermal volume change, solubility, oxidation and hydration reactivity, and particle strength, among many others, are important consi

Open-File Report↗

USGS2024

Exploring the dynamic interactions between the Southern San Andreas Fault and a normal fault under the Salton Sea

We investigate the dynamic interactions between the Southern San Andreas Fault (SSAF) and a proximal normal fault (NF) beneath the Salton Sea in southern California. The NF, positioned near the SSAF terminus at Bombay Beach, exhibits 11–15 displacement events across 14 stratigraphic sequences, with a range of 0.2–1.4 m of vertical offset since ∼2–3 ka. Notably, four of these events may align temporally with SSAF earthquakes, raising questions about the possible interplay between the two faults. Utilizing dynamic rupture models, we analyze the coseismic interactions between the SSAF and NF, addressing under what conditions the SSAF induces slip on the NF. Our findings reveal that a suite of SSAF ruptures, particularly those propagating from north to south, can trigger slip on the normal fault and replicate observed vertical offsets. If the SSAF extends beneath the Salton Sea, earthquakes originating south of the NF intersection are less likely to trigger normal fault slip, although we cannot exclude this possibility. Some SSAF ruptures do not trigger discernible slip on the NF, rendering such events undetectable in the stratigraphic record. Our research contributes toward discussions regarding the seismic hazard in southern California, shedding light on the interplay between the SSAF and NF.

California↗

USGS1969

The use of Schlumberger and equatorial soundings in groundwater investigations near El Paso, Texas

Electrical soundings using the symmetric AMNB Schlumberger and the bipole-dipole equatorial arrays were made along two profiles near El Paso, Texas, in support of a groundwater exploration program which included seismic refraction and gravity surveys. Electrode spacings (AB/2 or R) reaching 12,000 ft allowed exploration to depths of about 7000 ft. Geoelectrical information on the subsurface materials was augmented by sounding with the bilateral equatorial configuration and by transforming Schlumberger curves into dipole-polar sounding curves with formulas developed by Al'pin and by Tsekov. The bilateral equatorial sounding curves were found useful for detecting the direction of dip of highly resistive bedrocks whereas transformed sounding curves were used to evaluate the average longitudinal resistivity, and hence the depth, to the "electric basement." A few of the Schlumberger sounding curves were either clearly or subtly distorted by nonhorizontal geologic structures. The interpretation of these sounding curves illustrates the requirement for careful analysis in processing electrical prospecting data obtained over complex geologic conditions. The interpretation of a combined Schlumberger-equatorial sounding curve, which did not agree with the preliminary interpretation of seismic refraction data, was confirmed to be correct by data from a test well drilled to a depth of 4363 ft. The application of electrical sounding data in the El Paso area furnished valuable information on the depth to fresh-water-saltwater interfaces and on the depth to highly resistive impervious bedrocks.

Texas↗

USGS2025

Timescales of surface faulting preservation in low-strain intraplate regions from landscape evolution modeling and the geomorphic and historical record

Large surface-rupturing intraplate earthquakes in stable continental regions (SCRs) are uncommon globally and have recurrence intervals of thousands to hundreds of thousands of years based on the paleoseismic and geomorphic record, challenging accurate active fault identification in these regions. To constrain the timescales of preservation for scarps created by surface ruptures from dip-slip earthquakes, we use a two-dimensional scarp diffusion model for typical intraplate settings and explore which parameters influence fault scarp preservation. These parameters include the coseismic vertical surface offset, the recurrence interval of similar magnitude earthquakes, diffusivity (as a proxy for mean annual precipitation rate), and the erodibility of the surficial material. We constrain parameter ranges from a compilation of historical surface ruptures in intraplate settings in a variety of climates, including the Central and Eastern United States, Australia, Europe, Central Asia (Mongolia, China), India, and West Africa. The timescales of scarp preservation from landscape evolution modeling agree well with observations of scarp preservation in low-strain SCR and intraplate tectonic settings, with some notable exceptions for Australian scarps. We find that the erodibility of the surficial material and earthquake recurrence interval have a stronger effect on the timescales of scarp preservation than diffusivity or coseismic vertical surface offset. Our model results may aid in identifying and characterizing subtle, slow-moving active faults in low-strain SCR and intraplate tectonic settings for different tectonic, geomorphic, and climatic characteristics. Accurate fault locations and characterization from the landscape record has implications for both probabilistic seismic and fault displacement hazard analyses.

Journal of Geophysical Research Solid Earth↗

USGS2016

Probabilistic assessment of erosion and flooding risk in the northern Gulf of Mexico

We assess erosion and flooding risk in the northern Gulf of Mexico by identifying interdependencies among oceanographic drivers and probabilistically modeling the resulting potential for coastal change. Wave and water level observations are used to determine relationships between six hydrodynamic parameters that influence total water level and therefore erosion and flooding, through consideration of a wide range of univariate distribution functions and multivariate elliptical copulas. Using these relationships, we explore how different our interpretation of the present-day erosion/flooding risk could be if we had seen more or fewer extreme realizations of individual and combinations of parameters in the past by simulating 10,000 physically and statistically consistent sea-storm time series. We find that seasonal total water levels associated with the 100 year return period could be up to 3 m higher in summer and 0.6 m higher in winter relative to our best estimate based on the observational records. Impact hours of collision and overwash—where total water levels exceed the dune toe or dune crest elevations—could be on average 70% (collision) and 100% (overwash) larger than inferred from the observations. Our model accounts for non-stationarity in a straightforward, non-parametric way that can be applied (with little adjustments) to many other coastlines. The probabilistic model presented here, which accounts for observational uncertainty, can be applied to other coastlines where short record lengths limit the ability to identify the full range of possible wave and water level conditions that coastal mangers and planners must consider to develop sustainable management strategies.

Journal of Geophysical Research C: Oceans↗

USGS2024

Iron oxyhydroxide-rich hydrothermal deposits at the high-temperature Fåvne vent field, Mohns Ridge

The recently discovered Fåvne vent field, located at 3,040 m depth on the slow-spreading Mohns mid-ocean ridge between Greenland and Norway, is a high-temperature (≥250°C) vent field that is characterized by Fe oxyhydroxide-rich and S-poor chimneys and mounds. The vent field is located on both the hanging wall and footwall of a normal fault with a ∼1.5 km throw that forms the western edge of the ∼20 km wide ridge axial valley. Data collected during exploration of the site using a remotely operated vehicle as well as mineralogical and geochemical analyses of rock samples and sediments are used to characterize the geological setting of the vent field and composition of the hydrothermal deposits. The chimney walls are highly porous and lack defined chalcopyrite lined conduits, typical of high-temperature chimneys. Overall, abundant Fe oxyhydroxide precipitation at high-temperature vents at Fåvne reflects an excess of Fe over reduced S in the fluid, leading to precipitation of Fe oxide and oxyhydroxide minerals at high to moderate temperature vents (>100°C), and as microbially mediated and abiotic precipitation of Fe oxyhydroxide minerals at low-temperature diffuse vents (<100°C). The mounds and chimneys exhibit low base metal and reduced S concentrations relative to globally averaged seafloor deposits and suggest subseafloor mixing of hydrothermal fluid with seawater, causing metal sulfide precipitation. Cobalt enrichment at Fåvne may reflect a subsurface influence of an ultramafic substrate on circulating fluids, although ultramafic rocks are absent on the seafloor and no other elements typical of ultramafic deposits are present.

Geochemistry, Geophysics, Geosystems↗

USGS2017

The relative effectiveness of empirical and physical models for simulating the dense undercurrent of pyroclastic flows under different emplacement conditions

High concentration pyroclastic density currents (PDCs) are hot avalanches of volcanic rock and gas and are among the most destructive volcanic hazards due to their speed and mobility. Mitigating the risk associated with these flows depends upon accurate forecasting of possible impacted areas, often using empirical or physical models. TITAN2D, VolcFlow, LAHARZ, and Δ H/L or energy cone models each employ different rheologies or empirical relationships and therefore differ in appropriateness of application for different types of mass flows and topographic environments. This work seeks to test different statistically- and physically-based models against a range of PDCs of different volumes, emplaced under different conditions, over different topography in order to test the relative effectiveness, operational aspects, and ultimately, the utility of each model for use in hazard assessments. The purpose of this work is not to rank models, but rather to understand the extent to which the different modeling approaches can replicate reality in certain conditions, and to explore the dynamics of PDCs themselves. In this work, these models are used to recreate the inundation areas of the dense-basal undercurrent of all 13 mapped, land-confined, Soufrière Hills Volcano dome-collapse PDCs emplaced from 1996 to 2010 to test the relative effectiveness of different computational models. Best-fit model results and their input parameters are compared with results using observation- and deposit-derived input parameters. Additional comparison is made between best-fit model results and those using empirically-derived input parameters from the FlowDat global database, which represent “forward” modeling simulations as would be completed for hazard assessment purposes. Results indicate that TITAN2D is able to reproduce inundated areas well using flux sources, although velocities are often unrealistically high. VolcFlow is also able to replicate flow runout well, but does not capture the lateral spreading in distal regions of larger-volume flows. Both models are better at reproducing the inundated area of single-pulse, valley-confined, smaller-volume flows than sustained, highly unsteady, larger-volume flows, which are often partially unchannelized. The simple rheological models of TITAN2D and VolcFlow are not able to recreate all features of these more complex flows. LAHARZ is fast to run and can give a rough approximation of inundation, but may not be appropriate for all PDCs and the designation of starting locations is difficult. The Δ H/L cone model is also very quick to run and gives reasonable approximations of runout distance, but does not inherently model flow channelization or directionality and thus unrealistically covers all interfluves. Empirically-based models like LAHARZ and Δ H/L cones can be quick, first-approximations of flow runout, provided a database of similar flows, e.g., FlowDat, is available to properly calculate coefficients or Δ H/L . For hazard assessment purposes, geophysical models like TITAN2D and VolcFlow can be useful for producing both scenario-based or probabilistic hazard maps, but must be run many times with varying input parameters. LAHARZ and Δ H/L cones can be used to produce simple modeling-based hazard maps when run with a variety of input volumes, but do not explicitly consider the probability of occurrence of different volumes. For forward modeling purposes, the ability to derive potential input parameters from global or local databases is crucial, though important input parameters for VolcFlow cannot be empirically estimated. Not only does this work provide a useful comparison of the operational aspects and behavior of various models for hazard assessment, but it also enriches conceptual understanding of the dynamics of the PDCs themselves.

Frontiers in Earth Science↗

USGS2008

Meteorites on Mars observed with Mars Exploration Rovers

Reduced weathering rates due to the lack of liquid water and significantly greater typical surface ages should result in a higher density of meteorites on the surface of Mars compared to Earth. Several meteorites were identified among the rocks investigated during Opportunity's traverse across the sandy Meridiani plains. Heat Shield Rock is a IAB iron meteorite and has been officially recognized as 'Meridiani Planum.' Barberton is olivine-rich and contains metallic Fe in the form of kamacite, suggesting a meteoritic origin. It is chemically most consistent with a mesosiderite silicate clast. Santa Catarina is a brecciated rock with a chemical and mineralogical composition similar to Barberton. Barberton, Santa Catarina, and cobbles adjacent to Santa Catarina may be part of a strewn field. Spirit observed two probable iron meteorites from its Winter Haven location in the Columbia Hills in Gusev Crater. Chondrites have not been identified to date, which may be a result of their lower strengths and probability to survive impact at current atmospheric pressures. Impact craters directly associated with Heat Shield Rock, Barberton, or Santa Catarina have not been observed, but such craters could have been erased by eolian-driven erosion.

Journal of Geophysical Research E: Planets↗

USGS2003

Climate change and Arctic ecosystems: 2. Modeling, paleodata-model comparisons, and future projections

Large variations in the composition, structure, and function of Arctic ecosystems are determined by climatic gradients, especially of growing-season warmth, soil moisture, and snow cover. A unified circumpolar classification recognizing five types of tundra was developed. The geographic distributions of vegetation types north of 55°N, including the position of the forest limit and the distributions of the tundra types, could be predicted from climatology using a small set of plant functional types embedded in the biogeochemistry-biogeography model BIOME4. Several palaeoclimate simulations for the last glacial maximum (LGM) and mid-Holocene were used to explore the possibility of simulating past vegetation patterns, which are independently known based on pollen data. The broad outlines of observed changes in vegetation were captured. LGM simulations showed the major reduction of forest, the great extension of graminoid and forb tundra, and the restriction of low- and high-shrub tundra (although not all models produced sufficiently dry conditions to mimic the full observed change). Mid-Holocene simulations reproduced the contrast between northward forest extension in western and central Siberia and stability of the forest limit in Beringia. Projection of the effect of a continued exponential increase in atmospheric CO 2 concentration, based on a transient ocean-atmosphere simulation including sulfate aerosol effects, suggests a potential for larger changes in Arctic ecosystems during the 21st century than have occurred between mid-Holocene and present. Simulated physiological effects of the CO 2 increase (to >700 ppm) at high latitudes were slight compared with the effects of the change in climate.

Journal of Geophysical Research D: Atmospheres↗

USGS2007

Geology and Mineral Resources of the East Mojave National Scenic Area, San Bernardino County, California

From our evaluations that largely used model-based criteria, we conclude that much of the East Mojave National Scenic Area (EMNSA) contains significant indications of epigenetic mineralization of various types. Economically significant concentrations of many metals may possibly remain to be discovered in many parts of the EMNSA (see also Wetzel and others, 1992). We have discussed specific types of metallic deposits that are known to be present in the EMNSA. Some mountain ranges that have widespread occurrences are the Providence Mountains, Clark Mountain Range, Ivanpah Mountains, and New York Mountains; the area of Hackberry Mountain is included in a tract that is judged to be favorable for the discovery of epithermal, volcanic-hosted gold deposits (pl. 2). These ranges make up a broad, roughly north-south-trending region in the central part of the EMNSA. Much less endowed with known occurrences of all of the various types of deposits considered above are the Granite Mountains, the central parts of the Piute Range, the Fenner Valley area, the general area of Cima Dome, the Cima volcanic field, and areas west to Soda Lake. We have attempted to make some judgments concerning the gravel-covered areas in the EMNSA (pl. 3), including the areal extent of bedrock apparently covered only by thin veneers of gravel. But few data are available to us for the overwhelming bulk of the covered areas. The presence of any mineralization, the type of mineralization, and the extent and intensity of mineralization in the covered areas is essentially unknown. The likelihood is high, however, that those areas in the EMNSA covered only by a thin cap of gravels could host mineralization similar to that known in the adjoining mountain ranges. Most buried epigenetic-mineral deposits do not respond to standard geophysical methods, particularly at the coarse spacing of the data-collection points available for our evaluation. Restricting judgments concerning the presence of undiscovered metal resources in the EMNSA only to currently known types of deposits and to regionally representative tonnages for such deposits would undoubtedly yield small estimates for volumes of many metals that might be exploited. Metals from most newly discovered, base- and ferrous-metal deposits of the types presently known in the EMNSA probably would be insignificant from the standpoint of national needs. For example, copper from a newly discovered skarn deposit in the EMNSA would have roughly a 25 percent chance of being in excess of approximately 10,000 tonnes contained Cu, if the grade-and-tonnage distribution curves of Jones and Menzie (1986b) for copper skarns are applicable to copper skarn in the EMNSA. Most copper in the United States is produced in the Southwest from much larger open-pit operations than those associated with the typical copper skarn; the former operations exploit large-tonnage porphyry-type systems. Historically, the EMNSA has been the site of minor production of many metals from a large number of sites. Since 1985, however, a small number of sites in the EMNSA whose gold production and reserves are much greater than that of the preceding discoveries have been developed (see U.S. Bureau of Mines, 1990a). Nonetheless, widespread distribution of numerous types of deposits (including copper skarn, lead-zinc skarn, tin-tungsten skarn, polymetallic vein, gold-silver quartz-pyrite vein, low-fluorine porphyry molybdenum, gold breccia pipe, and volcanic-hosted gold) that are petrogenetically associated with igneous rock in many parts of the EMNSA is indicative of a metallogenic environment that may be the site of future discoveries of mineral-deposit types that are not now recognized by the exploration community. The science, art, and, yes, even luck of exploration procedures continually evolve, and this evolution is one of the most important aspects of currently employed methods of exploration (Bailly, 1981; Hutchinson and Grauch, 1991).

Miscellaneous Field Studies Map↗

USGS2024

Likely ferromagnetic minerals identified by the Perseverance rover and implications for future paleomagnetic analyses of returned Martian samples

Although Mars today does not have a core dynamo, magnetizations in the Martian crust and in meteorites suggest a magnetic field was present prior to 3.7 billion years (Ga) ago. However, the lack of ancient, oriented Martian bedrock samples available on Earth has prevented accurate estimates of the dynamo's intensity, lifetime, and direction. Constraining the nature and lifetime of the dynamo are vital to understanding the evolution of the Martian interior and the potential habitability of the planet. The Perseverance rover, which is exploring Jezero crater, is providing an unprecedented opportunity to address this gap by acquiring absolutely oriented bedrock samples with estimated ages from ∼2.3 to >4.1 Ga. As a first step in establishing whether these samples could contain records of Martian paleomagnetism, it is important to determine their ferromagnetic mineralogy, the grain sizes of the phases, and the forms of any natural remanent magnetization. Here, we synthesize data from various Perseverance instruments to achieve those goals and discuss the implications for future laboratory paleomagnetic analyses. Using the rover's instrument payload, we find that cored samples likely contain iron oxides enriched in Cr and Ti. The relative proportions of Fe, Ti, and Cr indicate that the phases may be titanomagnetite or Fe-Ti-Cr spinels that are ferromagnetic at room temperature, but we cannot rule out the presence of non-ferromagnetic ulvöspinel, ilmenite, and chromite due to signal mixing. Importantly, the inferred abundance of iron oxides in the samples suggests that even <1 mm-sized samples will be easily measurable by present-day magnetometers.

Journal of Geophysical Research: Planets↗

USGS2006

Body-wave traveltime and amplitude shifts from asymptotic travelling wave coupling

We explore the sensitivity of finite-frequency body-wave traveltimes and amplitudes to perturbations in 3-D seismic velocity structure relative to a spherically symmetric model. Using the approach of coupled travelling wave theory, we consider the effect of a structural perturbation on an isolated portion of the seismogram. By convolving the spectrum of the differential seismogram with the spectrum of a narrow window taper, and using a Taylor's series expansion for wavenumber as a function of frequency on a mode dispersion branch, we derive semi-analytic expressions for the sensitivity kernels. Far-field effects of wave interactions with the free surface or internal discontinuities are implicitly included, as are wave conversions upon scattering. The kernels may be computed rapidly for the purpose of structural inversions. We give examples of traveltime sensitivity kernels for regional wave propagation at 1 Hz. For the direct SV wave in a simple crustal velocity model, they are generally complicated because of interfering waves generated by interactions with the free surface and the Mohorovic??ic?? discontinuity. A large part of the interference effects may be eliminated by restricting the travelling wave basis set to those waves within a certain range of horizontal phase velocity. ?? Journal compilation ?? 2006 RAS.

Geophysical Journal International↗