United States Geological Survey

About: United States Geological Survey is a government organization based out in Reston, Virginia, United States. It is known for research contribution in the topics: Population & Groundwater. The organization has 17899 authors who have published 51097 publications receiving 2479125 citations. The organization is also known as: USGS & US Geological Survey.

Natural seepage of crude oil into the marine environment

Abstract: Recent global estimates of crude-oil seepage rates suggest that about 47% of crude oil currently entering the marine environment is from natural seeps, whereas 53% results from leaks and spills during the extraction, transportation, refining, storage, and utilization of petroleum. The amount of natural crude-oil seepage is currently estimated to be 600,000 metric tons per year, with a range of uncertainty of 200,000 to 2,000,000 metric tons per year. Thus, natural oil seeps may be the single most important source of oil that enters the ocean, exceeding each of the various sources of crude oil that enters the ocean through its exploitation by humankind.

Mechanical properties of sand, silt, and clay containing tetrahydrofuran hydrate

Abstract: [1] The mechanical behavior of hydrate-bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes, drilling and coring operations, and the analysis of certain small-strain properties of these sediments (for example, seismic velocities). This study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 MPa confining pressure on sand, crushed silt, precipitated silt, and clay specimens with closely controlled concentrations of synthetic hydrate. The results show that the stress-strain behavior of hydrate-bearing sediments is a complex function of particle size, confining pressure, and hydrate concentration. The mechanical properties of hydrate-bearing sediments at low hydrate concentration (probably 50% of pore space), the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination, cementing particles together, and filling the pore space. The cementation contribution to the shear strength of hydrate-bearing sediments decreases with increasing specific surface of soil minerals. The lower the effective confining stress, the greater the impact of hydrate formation on normalized strength.

The world at your feet: desert biological soil crusts

Abstract: Desert soil surfaces are generally covered with biological soil crusts, composed of a group of organisms dominated by cyanobacteria, lichens, and mosses. Despite their unassuming appearance, these tiny organisms are surprisingly important to many processes in past and present desert ecosystems. Cyanobacteria similar to those seen today have been found as 1.2 billion-year-old terrestrial fossils, and they probably stabilized soils then as they do now. Biological crusts are vital in creating and maintaining fertility in otherwise infertile desert soils. They fix both carbon and nitrogen, much of which is leaked to the surrounding soils. They also capture nutrient-rich dust, and can stimulate plant growth. These organisms are able to tolerate extreme temperatures, drought, and solar radiation, despite having relatively little wet time for metabolic activity. Under most circumstances, they are extremely vulnerable to climate change and disturbances such as off-road vehicles and grazing livestock. Recovery tim...

Tectonic erosion along the Japan and Peru convergent margins

Abstract: The volume of material removed by subduction erosion can be estimated quantitatively if the position of the volcanic arc, the position of the paleotrench axis, and a paleo-depth reference surface are known. Estimates based on these parameters along the Japan and Peru Trenches indicate rates of erosion comparable to well-known rates of accretion. Proposed erosional mechanisms along the plate boundary, where horsts on the lower plate abrade the upper one, appear insufficient to handle the minimum volumes of eroded material. Some mechanisms of tectonic erosion at the base of the trench slope can be observed at colliding seamounts and ridges where structures are large enough to be seismically imaged. Local tectonic erosion of the lower slope of the Japan Trench resulted when seamounts entered the subduction zone, uplifted the slope, and oversteepened it. The oversteepened slope failed, debris slumped into the trench axis, and much of it was then subducted. Where a seamount was subducted, a large re-entrant was left in the slope, which filled rapidly by local accretion of abundant sediment. Subduction of the oblique-trending Nazca Ridge off Peru produced many similar structures. Erosion is dominated by uplift and breakup of the lower slope, with subduction of the debris rather than abrasion under high-stress conditions. Another form of tectonic erosion occurs along the base of the upper plate. Its magnitude is indicated by massive subsidence along the margin; however, because of deep burial, the structure resulting from basal erosion is rarely imaged in seismic records. The volume of material eroded along the base of the upper plate exceeds that eroded from the front of the lower slope.

Variations in Sr, Rb, K, Na, and Initial Sr87/Sr86 in Mesozoic Granitic Rocks and Intruded Wall Rocks in Central California

Abstract: Initial Sr87/Sr86 of granitic rocks which are exposed north of the Garlock fault in California, and which represent the entire 130-m.y. time span of emplacement during the Mesozoic, ranges mainly from 0.7031 to 0.7082, with one value of 0.7094. A systematic areal variation, independent of age, exists for initial Sr87/Sr86 in these granitic rocks and is the same as the areal variation in initial Sr87/Sr86 of superjacent upper Cenozoic basalts and andesites. Two values of initial Sr87/Sr86, 0.7040 and 0.7060, mark natural separations of granitic rock data on K-Rb, K-Sr, and Rb/Sr-Rb variation diagrams, and also, when contoured, seem to represent geographic markers of paleo-geographic, geochemical, and physiographic significance. Upper Precambrian sedimentary and metamorphic rocks in California crop out only in the region where initial Sr87/Sr86 of granitic rocks is greater than 0.7060. A line of initial Sr87/Sr86 = 0.7060 is approximately coincident with the boundary between Paleozoic eugeosynclinal and miogeosynclinal rocks. Granitic rocks intruded into Paleozoic miogeosynclinal rocks have initial Sr87/Sr86 greater than 0.7060, whereas those intruded into eugeosynclinal Paleozoic rocks have initial Sr87/Sr86 less than 0.7060. The line of initial Sr87/Sr86 = 0.7040 is the eastern limit of principal exposures of ultramafic rocks, the western limit of Cretaceous granitic rocks, and is coincident with an abrupt change in “topographic expression” on the Bouguer gravity map of California. Correlation of the isotopic variations with these major crustal features suggests that there has been a sharp lateral contrast in crust-mantle chemistry across the region of study that has been fixed in position from the Precambrian to the present time. The chemical and isotopic variations observed are best explained if the parent magmas of the majority of granitic rocks investigated were derived in a region that was laterally variable in composition and in a zone of melting that intersected both upper mantle and lower crust. However, some igneous rocks, such as Jurassic volcanic rocks in wall rocks and roof pendants and some granitic rocks with high strontium concentrations and low Rb-Sr ratios, suggest that deeper sources are also involved in the total spectrum of igneous rocks in the region.