Peter L. Guth

Global carbon dioxide emissions from inland waters

Carbon dioxide (CO2) transfer from inland waters to the atmosphere, known as CO2 evasion, is a component of the global carbon cycle. Global estimates of CO2 evasion have been hampered, however, by the lack of a framework for estimating the inland water surface area and gas transfer velocity and by the absence of a global CO2 database. Here we report regional variations in global inland water surface area, dissolved CO2 and gas transfer velocity. We obtain global CO2 evasion rates of 1.8  petagrams of carbon (Pg C) per year from streams and rivers and 0.32  Pg C yr−1 from lakes and reservoirs, where the upper and lower limits are respectively the 5th and 95th confidence interval percentiles. The resulting global evasion rate of 2.1 Pg C yr−1 is higher than previous estimates owing to a larger stream and river evasion rate. Our analysis predicts global hotspots in stream and river evasion, with about 70 per cent of the flux occurring over just 20 per cent of the land surface. The source of inland water CO2 is still not known with certainty and new studies are needed to research the mechanisms controlling CO2 evasion globally.

Geomorphometry from SRTM: Comparison to NED

The Shuttle Radar Topography Mission (SRTM) produced near-global 1� and 3� DEMs. The cartographically-derived National Elevation Dataset (NED) provides a mechanism to assess SRTM quality. We compared 12 geomorphometric parameters from SRTM to NED for about 500,000 sample areas over the continental United States. For basic parameters like average elevation or relief, the two data sets correlate very highly. For more derived measures, such as curvature and higher moments (skewness and kurtosis), the correlations are much lower, with some parameters essentially uncorrelated between the two DEMs. Correlations improve after restricting analysis to region with average slopes greater than 5 percent, and the SRTM data set compares more closely to simulated 2� NED than to 1� NED. SRTM has too much noise in flat areas, increasing average slope, while in high relief areas SRTM over smoothes topography and lowers average slopes. The true resolution of 1� SRTM DEMs proves to be no better than 2�.

Magnitude of crustal extension in the southern Great Basin

Strike-slip faults in the southern Great Basin separate areas of Cenozoic upper crustal extension from relatively stable tectonic blocks. Linear geologic features, offset along the Garlock fault, Las Vegas Valley shear zone, and Lake Mead fault system, allow reconstruction of the southern Great Basin to a pre-extension configuration. The Sierra Nevada, Mojave Desert, Spring Mountains, and Colorado Plateau are treated as stable, unextended blocks that have moved relative to each other in response to crustal extension, with the Spring Mountains held fixed to the Mojave block. Our reconstruction indicates a minimum of 65% extension (140 km) between the southern Sierra Nevada and Colorado Plateau.

Tertiary extension north of the Las Vegas Valley shear zone, Sheep and Desert Ranges, Clark County, Nevada

Detailed mapping reveals the presence of high-angle extensional faults and low-angle gravity slides on the west side of the Sheep Range. Three major high-angle faulting events each account for 20° of eastward rotation and accommodate extension between the Las Vegas Range and the Desert Range. Low-angle faults represent surficial slides in response to topography produced by extension on the high-angle faults. Faulting took place during the Miocene, synchronously with deposition of the Horse Spring Formation and with displacement on the Las Vegas Valley shear zone. The extension in the Sheep Range took place without volcanism, intrusion, or metamorphism of the Paleozoic sedimentary rocks. Offset thrust faults suggest that the area west of the Sheep Range extended almost 100% during the Miocene, while the corresponding area south of the Las Vegas Valley shear zone did not extend significantly. The shear zone bounded the extending terrane on the south, acting as a transform fault. This extension west of the Sheep Range may in part balance that mapped by Anderson (1971) in the Eldorado Mountains. The Las Vegas Valley shear zone and the Lake Mead fault system may have acted together to compensate for areas of localized extension between the Colorado Plateau and the vicinity of the Specter Range.

Microcomputer program for manipulating large digital terrain models

Abstract The MicroDEM program manipulates large (90,000 elevations per data set) digital terrain models to produce contour and slope maps. topographic profiles. and oblique views. The program runs on industry standard MS-DOS microcomputers. The modular source code, written in Turbo Pascal, allows easy modification and customization. Alternatively, the executable code will run without modification and requires little user understanding of the computer and its operating system. Readily available data covers the entire United States.

Military Geology in War and Peace

In warfare, military geologists pursue five main categories of work: tactical and strategic terrain analysis, fortifications and tunneling, resource acquisition, defense installations, and field construction and logistics. In peace, they train for wartime operations and may be involved in peace-keeping and nation-building exercises. The classic dilemma for military geology has been whether support can best be provided by civilian technical-matter experts or by uniformed soldiers who routinely work with the combat units. In addition to the introductory paper this volume includes 24 papers, covering selected aspects of the history of military geology from the early 19th century through the recent Persian Gulf war, military education and operations, terrain analysis, engineering geology in the military, use of military geology in diplomacy and peace keeping, and the future of military geology.

The Geometry of Line-of-Sight and Weapons Fan Algorithms

Intervisibility algorithms, when applied to digital elevation models, are used to compute line-of-sight, weapons fans or viewsheds, and accurate three- dimensional perspective views. Whereas data quality, atmospheric effects, vegetation, and buildings contribute to the final result, the geometric model has a major impact. Seven geometric parameters should be explicitly defined: Viewer and target locations, interpretation of viewshed, point interpolation, point selection along radials, viewshed creation, vertical earth curvature, and horizontal earth curvature. The importance of horizontal earth curvature - the determination of straight line distance between observer and target - has not been sufficiently appreciated. Unless Universal Transverse Mercator approximations are valid, geodetic computations should be used. Because digital elevation models available to the military typically have a geographic- based point spacing, many established procedures that implicitly assume a conformal Universal Transverse Mercator grid introduce errors. A spaced radial algorithm produces the best weapons fans.

Quantifying topographic fabric: eigenvector analysis using digital elevation models

Digital elevation models provide an estimate of the topographic fabric (the tendency for topography to have a preferred orientation) of the earths surface. The algorithm extracts the eigenvectors and eigenvalues from a 3 X 3 matrix of the sums of the cross products of the directional cosines of the surface normals computed at each point in the DEM. The ratio of eigenvalues S1 (largest) and S2 measures the ruggedness of the terrain. The ratio of eigenvalues S2 and S3 (smallest) measures the tendency for the terrain to have a preferred orientation, while their orientation reflects the direction of dominant topographic fabric. Sample sizes of about 500 - 2500 points provide robust statistics, allowing sample regions of 1/2 to 1/3 square degree for global data sets and about 600 meters on a side with 30 m US topography. Topographic fabric appears to be a fundamental characteristic of landforms amenable to quantitative study. It should be included in terrain analysis and classification, and may lead to better estimates for cross-country mobility.

Limitations on the role of pore pressure in gravity gliding

A simple model has been developed to evaluate the gravity gliding of Nappes in sedimentary terranes. Three factors play a critical role in the model: (1) the ratio of horizontal to vertical stress, (2) the rate of fluid flow to the detachment horizon, and (3) the permeability of lithologies immediately above that horizon. If the ratio of horizontal to vertical stress is less than one, this ratio limits the maximum attainable pore pressure through hydrofracture. Existing in situ stress measurements suggest that vertical hydrofracturing rather than gravity gliding might be the result of elevated pore pressures unless a low-strength cap rock exists. If the stress ratio exceeds one, a relatively simple equation relates the cap rock permeability and fluid flow necessary for gravity gliding. Effective cap-rock permeabilities less than 10/sup -4/ to 10/sup -5/ MD are required for gravity gliding in a sedimentary basin. Based on available in situ permeability measurements, only shales and evaporites could have sufficiently low permeabilities on a regional scale. 24 references.

Microcomputer techniques and applications Microcomputer software for structural geologists

Abstract Many microcomputer programs allow efficient storage, manipulation and analysis of structural data. These programs can facilitate traditional analyses by accelerating time-consuming graphical and numerical methods as well as allowing new quantitative analytical procedures that would be either impossible or impractical without an easily accessible computer. Excellent programs exist that store and manage structural data, display and analyse orientations, analyse strain in brittle and ductile rocks, and model folded and faulted strata. The essential characteristics of the best of these programs are reviewed and guidelines for software selection suggested.