bluuebunny/crossref_metadata_2025 huggingface dataset:
Intercomparison of Stratospheric Aerosol and Gas Experiment (SAGE) with Umkehr[64] and Umkehr[92] ozone profiles and time series: 1979–1991
Authors: Newchurch M. J. , Cunnold D. M. , Cao J.
Published: February 2004

Analyzing coincident observations of ozone profiles at 15 Umkehr stations with Stratospheric Aerosol and Gas Experiment (SAGE) I and SAGE II measurements within 1000 km and 12 hours in low‐aerosol conditions between 1979 and 1991, we find improved agreement between Umkehr and SAGE retrievals using the new (1992) Umkehr algorithm compared to the previous (1964) algorithm, but some significant differences remain. The column ozone amounts in layers 4 through 10 for both old Umkehr[64] (after adjustment for the scale change from the International Ozone Commission/World Meteorological Organization 1968 scale to the Bass and Paur [1985] scale) and new Umkehr[92] retrievals are approximately 5–6% lower than SAGE column amounts. The structure of the aerosol‐corrected Umkehr[92] profiles compares much more favorably to SAGE than did the Umkehr[64] profiles in layers 4 to 7 (20–35 km), with considerable consistency in the vertical structure of differences across most sites. The layer‐ozone differences, however, increase from zero in layer 4 to −15% (Umkehr[92] low) in layer 8. Belsk and Sapporo are the only 2 sites of the 15 analyzed here that exhibit somewhat dissimilar vertical structure in their differences versus SAGE. The Umkehr[92] a priori climatology contains less ozone in the lower layers (2–5) than does SAGE and somewhat more in the upper layers (7–9). However, these differences in a priori climatology do not significantly affect the broad altitude structure in the SAGE‐Umkehr layer‐mean ozone differences. On average, the Umkehr[92] profiles possess a correlation between 0.3 and 0.5 (higher at some stations) with the SAGE‐measured ozone in individual layers 4 to 8. The time series of the Umkehr[92] and SAGE measurements typically exhibit similar trends except for discontinuous changes noted at Mauna Loa and Kagoshirna.
Synergistic algorithm for estimating vegetation canopy leaf area index and fraction of absorbed photosynthetically active radiation from MODIS and MISR data
Authors: Knyazikhin Y. , Martonchik J. V. , Myneni R. B. , Diner D. J. , Running S. W.
Published: February 2004

A synergistic algorithm for producing global leaf area index and fraction of absorbed photosynthetically active radiation fields from canopy reflectance data measured by MODIS (moderate resolution imaging spectroradiometer) and MISR (multiangle imaging spectroradiometer) instruments aboard the EOS‐AM 1 platform is described here. The proposed algorithm is based on a three‐dimensional formulation of the radiative transfer process in vegetation canopies. It allows the use of information provided by MODIS (single angle and up to 7 shortwave spectral bands) and MISR (nine angles and four shortwave spectral bands) instruments within one algorithm. By accounting features specific to the problem of radiative transfer in plant canopies, powerful techniques developed in reactor theory and atmospheric physics are adapted to split a complicated three‐dimensional radiative transfer problem into two independent, simpler subproblems, the solutions of which are stored in the form of a look‐up table. The theoretical background required for the design of the synergistic algorithm is discussed.
Non‐sea‐salt‐sulphate formation in sea‐salt aerosol
Authors: Clegg Nicola A. , Toumi Ralf
Published: February 2004

By comparing the observed non‐sea‐salt‐sulphate:sodium (nss‐SO42−:Na+) ratios with model calculations the main sources of nss‐SO42− observed in sea‐salt aerosol could be determined. As heterogeneous production appears to be responsible for at most half of the measured nss‐SO42− [Clegg and Toumi, 1997; Chameides and Stelson, 1992] other mechanisms have been examined here. Processes such as the presence of oxidizers other than hydrogen peroxide and ozone and alternative sources of sulphur appear to make little if any difference to calculated ratios. Contamination by “pure” sulphate particles is deemed unlikely, as is a sufficient increase in the alkalinity of the sea‐salt particles. However, observed ratios can be achieved in the presence of even a small amount of cloud processing. While observed ratios may possibly be due to experimental uncertainties the sulphate production produced by cloud processing is the most likely explanation. Predictions also show that smaller sea‐salt particles will have much greater nss‐SO42−:Na+ ratios than larger sea‐salt particles. The presence of large quantities of submicron sea‐salt particles which could also act as cloud condensation nuclei would make sea salt the largest sink for sulphur dioxide in the marine boundary layer.
Dissipation and mixing of a small‐scale stratospheric intrusion in the upper troposphere
Authors: Langford A. O. , Reid S. J.
Published: February 2004

Ground‐based lidar measurements of ozone mixing ratio and aerosol backscatter profiles above Fritz Peak Observatory near Boulder, Colorado (39.9° N, 105.3°W), are used in conjunction with nearby radiosonde profiles and GOES‐9 satellite water vapor imagery to describe the mixing of a dry, ozone‐rich streamer of stratospheric air into the upper troposphere over the western United States on June 30 and July 1, 1997. The satellite images show that the streamer extended over more than 2000 km from the eastern Pacific Ocean to the central United States beneath the subtropical jet with a width of ∼200 km over Colorado. The streamer persisted with little change for more than 6 hours of lidar observations and then abruptly disappeared to be replaced by a region of unusually low ozone. The disappearance of the streamer is attributed to localized convective activity induced by the incursion of a moist, ozone‐poor subtropical air mass between the streamer and the convective boundary layer. This activity disrupted the streamer and, together with shear‐induced mixing beneath the descending subtropical jet, merged ∼6.5 × 1031 molecules of ozone into the free troposphere over Colorado between 8 and 10 km. These results illustrate the effectiveness of small‐scale processes for the irreversible transfer of stratospheric air into the upper troposphere.
Airborne lidar characterization of power plant plumes during the 1995 Southern Oxidants Study
Authors: Senff Christoph J. , Hardesty R. Michael , Alvarez Raul J. , Mayor Shane D.
Published: February 2004

One of the objectives of the 1995 Southern Oxidants Study was to assess the extent to which fossil fuel power plants contribute to high ozone episodes that often occur in the Nashville area during summer. Among other instruments, the National Oceanic and Atmospheric Administration airborne ozone and aerosol lidar was used to investigate power plant plumes in the vicinity of Nashville, Tennessee. Owing to its ability to characterize the two‐dimensional structure of ozone and aerosols below the aircraft, the airborne lidar is well suited to document the evolution of the size and shape of a power plant plume as well as its impact on ozone concentration levels as the plume is advected downwind. We report on two case studies of the Cumberland power plant plume that were conducted on July 7 and 19, 1995. The meteorological conditions on these 2 days were distinctly different and had a significant impact on the plume characteristics. On July 7, the Cumberland plume was shaped symmetrically and confined to the boundary layer, while on July 19 the plume had an irregular shape and showed two cores, one above and the other within the boundary layer. Close to the Cumberland power plant, we found that ozone in the plume was destroyed at a rate of 5 to 8 ppbv h−1 due to titration at high NO levels. Farther downwind, where plume NOx reacts with ozone precursor gases to form ozone, we measured plume‐averaged ozone production rates of 1.5 to 4 ppbv h−1. The results of these two case studies are compared to aircraft in situ measurements of the same power plant plume.
Effects of snow cover on UV irradiance and surface albedo: A case study
Authors: McKenzie R. L. , Paulin K. J. , Madronich S.
Published: February 2004

A heavy snowfall, followed by several days of cloudless skies before significant snow melt had occurred, enabled a quantitative study of the effects of snow on downwelling UV spectral irradiances at the National Institute for Water and Atmospheric Research UV measurements site in Lauder, New Zealand. The largest UV enhancements (>70%) were seen during partly cloudy conditions immediately after the snowfall. A radiative transfer model was used to quantify the enhancements due to the snow cover and the spectral albedo of the snow under clear‐sky conditions. The first cloudless day on which the radiative transfer model could be used with confidence occurred 7 days after the snowfall. By this time, the maximum enhancements due to snow at solar zenith angle (SZA) 70° were approximately 22% in the UV‐A region. In the UV‐B region, the enhancements were approximately 28% and tended to increase slightly at larger SZA. The corresponding surface albedo was 0.62±0.08, and comparison with supplementary measurements indicated that the albedo decayed with time. Any spectral or SZA dependencies in the enhancements were below the measurement uncertainties in the UV region. Comparisons with supplementary data indicated that the albedo immediately after the snow was greater than 0.8.
Annual accumulation at two sites in northwest Greenland during recent centuries
Authors: Anklin Martin , Bales Roger C. , Mosley‐Thompson Ellen , Steffen Konrad
Published: February 2004

During summer 1995, 150‐m firn and ice cores were drilled to determine annual accumulation rates at two Greenland sites, 73.84°N, 49.49°W (NASA‐U site) and 78.53°N, 56.83°W (Humboldt glacier site). Annual layers were identified in the cores using multiple parameters: δ18O and concentrations of dust, H2O2, NH4+, Ca2+, and NO3−. Using all parameters together to define annual layers resulted in a 350‐year record for the NASA‐U core with no dating uncertainty. For the lower‐accumulation Humboldt core, the dating uncertainty is about 5 years over the 852‐year period of record, with no uncertainty over the past 200 years. Annual accumulation over the periods of record at the two sites averaged about 0.34 and 0.14 m water equivalent, respectively. A set of 20‐m firn cores drilled near the main 150‐m cores showed that interannual variability of accumulation exceeded spatial variability at NASA‐U. The Humboldt cores showed equal spatial and interannual variability. The accumulation rates at both sites showed a low‐frequency variation of about 100 years, and both sites showed 200‐year cumulative fluctuations of about 2 m from mean accumulation rates. Compared to central Greenland and to NASA‐U, the Humboldt core showed higher annual accumulation rates around 1760–1810, possibly indicating a changed circulation pattern for the more northern part of Greenland in that period.
Seasonal ozone variations in the isentropic layer between 330 and 380 K as observed by SAGE II: Implications of extratropical cross‐tropopause transport
Authors: Wang Pi‐Huan , Cunnold Derek M. , Zawodny Joseph M. , Pierce R. Bradley , Olson Jennifer R. , Kent Geoffrey S. , Skeens Kristi M.
Published: February 2004

To provide observational evidence on the extratropical cross‐tropopause transport between the stratosphere and the troposphere via quasi‐isentropic processes in the middleworld (the part of the atmosphere in which the isentropic surfaces intersect the tropopause), this report presents an analysis of the seasonal variations of the ozone latitudinal distribution in the isentropic layer between 330 K and 380 K based on the measurements from the Stratospheric Aerosol and Gas Experiment (SAGE) II. The results from SAGE II data analysis are consistent with (1) the buildup of ozone‐rich air in the extratropical middleworld through the large‐scale descending mass circulation during winter, (2) the spread of ozone‐rich air in the isentropic layer from midlatitudes to subtropics via quasi‐isentropic transport during spring, (3) significant photochemical ozone removal and the absence of an ozone‐rich supply of air to the layer during summer, and (4) air mass exchange between the subtropics and the extratropics during the summer monsoon period. Thus the SAGE II observed ozone seasonal variations in the middle‐world are consistent with the existing model calculated annual cycle of the diabatic circulation as well as the conceptual role of the eddy quasi‐adiabatic transport in the stratosphere‐troposphere exchange reported in the literature.
Correlations of stratospheric abundances of NOy, O3, N2O, and CH4 derived from ATMOS measurements
Authors: Michelsen H. A. , Manney G. L. , Gunson M. R. , Zander R.
Published: February 2004

Correlations are presented for [NOy] relative to [N2O] and [O3] derived from measurements from the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument from a wide range of altitudes and latitudes, including the tropics for which previous analyses have not extended above ∼20 km. Relationships for [O3] versus [N2O] are also given. The results are shown to be in good agreement with aircraft‐ and balloon‐based observations. Distinct correlations are observed for the tropics, the springtime polar vortex, and the extratropics‐extravortex regions. These correlations demonstrate rapid production of NOy and O3 in the tropical middle stratosphere and episodic export of air from this region to higher latitudes. Isolation of air within the developing polar vortices in the fall is also shown. Arctic vortex data from April 1993 appear to indicate denitrification of 25–30%, which is evident as a 3.0–4.5 ppb deficit in [NOy] when the vortex [NOy]:[N2O] correlation is compared with the extravortex correlation. A mixture of air descended from above 40 km with air from lower altitudes can fully account for this deficit in [NOy], in addition to approximately half of an apparent Arctic ozone loss of 50–60%, as inferred by comparison of the vortex and extravortex [O3]:[N2O] correlations. Comparison of Antarctic vortex and extravortex correlations from November 1994 similarly show a 60–80% deficit in [NOy] and 80–100% deficit in [O3]; at least half of this apparent denitrification and ozone loss can be attributed to mixing of air descended from higher altitudes with air from lower altitudes.
Venus: Crater distribution and plains resurfacing models
Authors: Hauck Steven A. , Phillips Roger J. , Price Maribeth H.
Published: February 2004

Detailed analysis of the distribution of craters on Venus using Mth nearest neighbor analysis, coupled with models based upon surface morphology constraints, indicates that the hypothesis of complete spatial randomness (CSR) cannot be rejected, but is not a unique model of the observed crater distribution. Based on morphologic mapping, the extensive volcanic plains can be divided into four units that have a spread in age of the order of 0.5T (the mean surface age of the planet). This four‐unit plains model, along with its derivatives, produce test statistics that indicate such models also cannot be rejected. Further, the probability of obtaining a result at least as extreme as the observed test statistic given that the null hypothesis (model corresponds to Venus) is true is lowest for the CSR model. There is no particular reason to pick a CSR model (along with its implications for catastrophic resurfacing) as a constraint on the evolution of Venus, and there are geological reasons to choose the multiage models. We find that we cannot distinguish statistically among models that have two, three, or four distinct production ages within the plains. However, the hypothesis that the variation in crater density within all of the plains is due to a single random process can be rejected for two reasons. First, the binomial probability that such a process could exist within each of the plains units is ≤0.05 except the smallest and youngest unit, PL1. Second, using a chi‐squared statistic to test the hypothesis that four plains units have the same age gives a p value of 10−4, indicating confident rejection of the hypothesis. Thus CSR cannot be used as a constraint on models of resurfacing or planetary evolution of Venus because of the non‐uniqueness in matching such a model to the observed crater distribution and the strong indication of distinct ages within the plains with a significant spread in age. Geological and geophysical constraints provide our best clues for understanding Venus.
Tectonic evolution of Bell Regio, Venus: Regional stress, lithospheric flexure, and edifice stresses
Authors: Rogers Patricia G. , Zuber Maria T.
Published: February 2004

In order to understand the relationship between volcanic and tectonic processes and the stress state in the lithosphere of Venus, we analyzed the stress environments and resulting tectonic features associated with the major volcanic edifices in Bell Regio, using Magellan synthetic aperture radar (SAR) images and altimeter measurements of topography. The major volcanoes of Bell Regio, Tepev Mons and Nyx Mons, exhibit tectonic characteristics that are unique relative to other volcanic edifices on Venus. The most prominent distinction is the lack of large rift zones within the overall highland uplift, which characterize many other highland rises on Venus. Also, previous studies have determined that many large Venus volcanoes exhibit radial tectonic structures on their flanks but generally lack the circumferential graben which surround volcanoes on Earth and Mars. Tepev and Nyx Montes exhibit both the radial tectonic features associated with other Venusian edifices and numerous concentric graben. Nyx Mons implies a more distributed magmatic system by its broad shape, radial chains of pit craters, and expansive flow fields, whereas Tepev Mons is a more centralized volcanic system, with limited associated long flows. We investigate the regional stresses associated with Bell Regio and structural features believed to be a consequence of lithospheric flexure due to volcanic loading, modeling both Nyx Mons and Tepev Mons as axisymmetric loads with Gaussian mass distributions on an elastic plate. The relationship between the tectonic features surrounding Tepev Mons and stresses associated with magma chamber inflation are also examined through finite element analysis. Using topography data to model the shape of the volcano, we determine that a horizontally ellipsoidal or tabular reservoir at a range of depths from approximately 20 to 40 km can satisfy the locations of graben formation observed in Magellan images. These results imply a shift in volcanic style within Bell Regio from an early phase of broad, low shield formation to later steep‐sided, more centralized edifice development. Such changes are consistent with an increase in the thickness of the lithosphere over time.
Growth of large volcanoes on Venus: Mechanical models and implications for structural evolution
Authors: McGovern Patrick J. , Solomon Sean C.
Published: February 2004

The structure, tectonics, and evolution of large volcanoes on Venus, as revealed by data from the Magellan mission, appear to be distinct from those on Earth and Mars. To determine the conditions and processes that account for these differences, we model the evolution of stress and deformation in growing volcanic edifices on Venus using the finite element method. Large volcanoes on Venus are characterized by topographically prominent conical edifices surrounded by relatively flat flow aprons. The surfaces of both edifice and apron consist dominantly of radially oriented flows. Tectonic features, usually the surface expression of shallowly intruded dikes, also have predominantly radial orientations. Features similar to Hawaiian‐style linear rift zones or large‐scale flank failure, common on large volcanoes on Earth and Mars, are absent. By comparing predictions of faulting for models with detached and welded basal boundary conditions with the observed tectonic features, we determine that a welded condition is more likely. Horizontal compressive stresses transmitted into edifices across welded basal boundaries are inversely proportional to elastic lithosphere thickness Te. Contravening stress increments from magma chamber expansion, differential thermal contraction, or buoyant loading from beneath the crust or lithosphere are required to reorient principal stresses in the edifice so that magma ascent and radial dike formation are favored. Incremental volcano growth results in stress distributions that decrease with height in the edifice. Dike intrusion is easiest in the uppermost (youngest) layers, consistent with the observed shallow radial dikes on Venus volcanoes. Small values of Te greatly inhibit the formation of large shields, in that conical edifice topography cannot be supported and large contravening stresses are required for further growth. Large volcano formation is much more likely at large Te, where more moderate contravening stresses are sufficient for growth. The near‐surface manifestation of mantle upwelling on Venus may thus depend on Te in that the response to upwellings beneath thin or nonexistent lithosphere is likely to be dominantly ductile, leading to the formation of coronae, but a thick elastic lithosphere is required to support the growth of the largest volcanoes. The transitional value of Te between these modes of evolution likely depends on the horizontal scale of the upwelling.
Phase transitions and the three‐dimensional planform of thermal convection in the Martian mantle
Authors: Harder Helmut
Published: February 2004

Previous convection models for Earth have shown that the endothermic phase transition from spinel to perovskite plus magnesiowüstite at 660 km depth has a strong effect on mantle convection. For Mars, the depth of the main phase transitions is estimated using a set of 32,000 density models with randomly perturbed input parameters. It is found that, on average, the depth of the spinel to perovskite transformation is 1910 km. Thus a perovskite layer exists in the Martian mantle only if the core radius is less than 0.45 of the planetary radius. Recent results on the density of FeS at pressure and temperature conditions relevant to the Martiam core indicate that such a small core size is consistent with the cosmochemical estimate of around 14% sulphur content of the core. Numerical simulations of three‐dimensional spherical convection demonstrate that the presence of the endothermic phase transition close to the core‐mantle boundary is the crucial factor determining the style of convection. In addition to the endothermic phase boundary, the simulations assume an isoviscous mantle, which is heated both from below and within, a rigid upper boundary, and Rayleigh numbers of the order of 106. Under these conditions, it is shown that the natural planform of mantle convection exhibits only one or two strong mantle plumes. The number of plumes decreases with time during planetary evolution. For high Rayleigh numbers, the plumes start to oscillate about a mean position and pulsate in time. Strongly localized mantle upwellings are obtained even when the proportion of basal heating is only 10%. These results are robust in the sense that they do not critically depend on the parameters of the phase transition, the Rayleigh number, or the rate of basal heating. This planform of mantle convection is consistent with the volcanic history of Mars, where early volcanism was widespread, but later concentrated in two provinces, Tharsis and Elysium, and finally in Tharsis alone.
Planetary protection, sample return missions and Mars exploration: History, status, and future needs
Authors: DeVincenzi Donald L. , Race Margaret S. , Klein Harold P.
Published: February 2004

As the prospect grows for a Mars sample return mission early in the next millennium, it will be important to ensure that appropriate planetary protection (PP) controls are incorporated into the mission design and implementation from the start. The need for these PP controls is firmly based on scientific considerations and backed by a number of national and international agreements and guidelines aimed at preventing harmful cross contamination of planets and extraterrestrial bodies. The historical precedent for the use of PP measures on both unmanned and manned missions traces from post‐Sputnik missions to the present, with periodic modifications as new information was obtained. In consideration of the anticipated attention to PP questions by both the scientific/technical community and the public, this paper presents a comprehensive review of the major issues and problems surrounding PP for a Mars Sample Return (MSR) mission, including an analysis of arguments that have been raised for and against the imposition of PP measures. Also discussed are the history and foundations for PP policies and requirements; important research areas needing attention prior to defining detailed PP requirements for a MSR mission; and legal and public awareness issues that must be considered with mission planning.
Basalt thickness in Mare Humorum: The crater excavation method
Authors: Budney Charles J. , Lucey Paul G.
Published: February 2004

Basalt thicknesses in mare basins have been determined using assumptions about the premare topography of partly buried craters and by comparison to the Orientale basin. Differences in those assumptions have led to a factor of 4 difference in mare thickness estimates. Further, knowledge of thickness is restricted to areas in which buried craters are present. Using mixing models applied to multispectral images acquired by the Clementine spacecraft, we have shown that craters in the mare sometimes excavate highland material from below the mare cover. Using such craters and assumptions about their depth of excavation, we obtain independent estimates of basalt thickness. Our results are in agreement with the lowest of previous thickness estimates. We derive a volume of 40,000 km3 for basalts in Mare Humorum, considerably less than the ∼110,000 km3 from previous estimates. We also confirm a diameter of 425 km for the Humorum multiring basin based on the Clementine gridded global topography, assuming an original morphology like Orientale.
Ammonia abundance in Jupiter's atmosphere derived from the attenuation of the Galileo probe's radio signal
Authors: Folkner W. M. , Woo R. , Nandi S.
Published: February 2004

The radio signal from the Galileo probe to the orbiter experienced attenuation due to ammonia in Jupiter's atmosphere during the probe descent. A profile of the ammonia content as a function of depth in the atmosphere has been derived from the measurements of the attenuation. The derived ammonia abundance rises to a molar fraction of 700±100 parts per million for pressures greater than 7 bars, about 4 times that expected based on the solar abundance of nitrogen.
The search for life on Mars: The role of rovers
Authors: Stoker Carol
Published: February 2004

The search for evidence of life on Mars will require highly capable rovers. The strategy for the search for life divides into four major objectives: understanding the history of the Martian climate, finding evidence of chemical evolution, searching for evidence of past life, and searching for evidence of living organisms. To achieve these objectives, rovers will need to traverse distances of the order of 10 km to study a variety of relevant features (e.g., fluvial channels, shorelines, eskers, hydrothermal deposits) in sufficient detail to diagnose their formation processes. Requirements for rovers include color imaging at a range of spatial scales (wide angle for geologic context, high resolution for resolving millimeter‐to centimeter‐scale textures, submillimeter resolution of selected areas); in situ capabilities to perform chemical, mineralogical, and organic analysis; and the ability to collect samples of rocks, soils, and subsurface samples for return to Earth for detailed laboratory analysis. Simulations of rover missions to Mars and the Moon were performed for 3 days each in February 1995 using the Marsokhod rover deployed on Kilauea volcano, Hawaii, and operated via satellite communications from Moffett Field, California. During the simulations, science teams analyzed images taken by the Marsokhod and deduced the geologic setting and history of the field site. Imaging for the Mars mission simulation used mast‐mounted monochrome stereo cameras with 30° field of view, 1.4 mrad/pixel resolution, 1.7 m height, 1 m horizontal spacing, and fixed pointing. A camera mounted on a manipulator arm was capable of resolving 1 mm features. During 18 hours of operation, the rover traversed 800 m of terrain, made observations at eight science targets, and obtained 52 stereo images and 18 arm camera images. For the lunar mission simulation, stereo cameras were mounted on a mast with a pointable platform at 1.5 m height and 10 cm horizontal spacing. Stereo color video was transmitted to mission operations. Command cycles with 1.5 s time delay allowed live teleoperation. In 15 hours of operation the rover traversed 1.2 km, and nine science targets were studied in detail. These experiments show that rovers can be used to successfully perform field geology on other planets.
Rocky 7 prototype Mars rover field geology experiments: 1. Lavic Lake and Sunshine Volcanic Field, California
Authors: Arvidson R. E. , Acton C. , Blaney D. , Bowman J. , Kim S. , Klingelhöfer G. , Marshall J. , Niebur C. , Plescia J. , Saunders R. S. , Ulmer C. T.
Published: February 2004

Experiments with the Rocky 7 rover were performed in the Mojave Desert to better understand how to conduct rover‐based, long‐distance (kilometers) geological traverses on Mars. The rover was equipped with stereo imaging systems for remote sensing science and hazard avoidance and 57Fe Mössbauer and nuclear magnetic resonance spectrometers for in situ determination of mineralogy of unprepared rock and soil surfaces. Laboratory data were also obtained using the spectrometers and an X ray diffraction (XRD)/XRF instrument for unprepared samples collected from the rover sites. Simulated orbital and descent image data assembled for the test sites were found to be critical for assessing the geologic setting, formulating hypotheses to be tested with rover observations, planning traverses, locating the rover, and providing a regional context for interpretation of rover‐based observations. Analyses of remote sensing and in situ observations acquired by the rover confirmed inferences made from orbital and simulated descent images that the Sunshine Volcanic Field is composed of basalt flows. Rover data confirmed the idea that Lavic Lake is a recharge playa and that an alluvial fan composed of sediments with felsic compositions has prograded onto the playa. Rover‐based discoveries include the inference that the basalt flows are mantled with aeolian sediment and covered with a dense pavement of varnished basalt cobbles. Results demonstrate that the combination of rover remote sensing and in situ analytical observations will significantly increase our understanding of Mars and provide key connecting links between orbital and descent data and analyses of returned samples.
Simulations of two Last Glacial Maximum ocean states
Authors: Bigg Grant R. , Wadley Martin R. , Stevens David P. , Johnson John A.
Published: February 2004

Two stable thermohaline circulation states were produced in simulations of the last glacial maximum (LGM) ocean, both forced by the same prescribed atmosphere. Direct application of this atmosphere led to an equilibrium state with strong North Atlantic Deep Water formation (northern sinking state (NSS)). Upon applying a weak freshwater flux anomaly to the North Atlantic for a short time a state was entered with little North Atlantic, but significant Southern Ocean, Deep Water formation (southern sinking state (SSS)). A fully dynamic and thermodynamic iceberg trajectory model was developed, and icebergs were seeded into both states. Distinctly different pathways were found, particularly in the eastern Atlantic where the SSS icebergs tend to move south along the European coast while they move north in the NSS. While neither ocean state is claimed to be a definitive model for the LGM ocean, paleoclimate data are more consistent with the characteristics of the NSS.
The impact of paleogeographic evolution on the surface oceanic circulation and the marine environment within the Mid‐Cretaceous tethys
Authors: Poulsen Chris J. , Seidov Dan , Barron Eric J. , Peterson William H.
Published: February 2004

The concept of a stable, westward flowing circumglobal current throughout the Cretaceous Tethys has become the subject of extensive debate. Results from a series of oceanic general circulation model experiments, using the NCAR Parallel Ocean Climate Model, are presented which oppose this concept and suggest a more complicated circulation pattern dominated by a clockwise gyre in the Mediterranean Tethys. Although a narrow westward flowing current is simulated hugging the southern margin of Tethys, ocean model experiments reveal the large sensitivity of this current to paleogeographic evolution. Small modifications to continental geometry, representing sea level change or tectonic plate movement during the Cretaceous, alter the direction and strength of the Tethyan circulation and, consequently, lead to regional changes in seawater properties (i.e., temperature and salinity). These oceanic general circulation model experiments indicate that geographic evolution is an important mechanism of climatic and environmental change. Since the Cretaceous experienced rapid tectonic changes these results imply large surface circulation variations in the Tethys Ocean, supporting the idea of a dynamic tropical oceanic environment. In fact, a simulation of sea‐level rise during the mid‐Cretaceous reproduces the fluctuations in seawater properties recorded by fauna in the Caribbean Sea.
High‐resolution δ13C stratigraphy through the Early Aptian “Livello selli” of the Alpine tethys
Authors: Menegatti Alessio P. , Weissert Helmut , Brown Robert S. , Tyson Richard V. , Farrimond Paul , Strasser André , Caron Michèle
Published: February 2004

Very similar high‐resolution δ13Ccarb and δ13Corg curves are reported for the early Aptian “Livello Selli” (LS, oceanic anoxic subevent 1a) at two pelagic successions of the Alpine Tethys: “Roter Sattel” (Swiss Préalpes), deposited along the northwest margin, and “Cismon” (southern Alps of northern Italy), deposited along the southeast margin. The δ13Ccarb and δ13Corg curves are both divided into eight segments, six of which occur within the early Aptian Globigerinelloides blowi foraminiferal zone, indicating significant subzonal resolution. Most of the LS coincides with a chemostratigraphically defined “Selli event”: a period of constant isotopic values (mean values of δ13Ccarb and δ13Corg ≈ +2.6‰ and −25‰ respectively) lasting 500 kyr to 1 Myr, rather than of positive excursion as previously supposed. This uniformity may reflect an equilibrium between Corg burial and increased recycling rates of 12C and nutrient‐rich intermediate water resulting from the intensification of oceanic thermohaline circulation.
Measurement of ocean wind vector by an airborne, imaging polarimetric radiometer
Authors: Skou Niels , Laursen Brian
Published: February 2004

Airborne measurements of the sea surface have been carried out with an imaging polarimetric 16‐GHz radiometer system, aimed at determining the wind direction. The radiometer system features a high‐speed digital correlator, and it measures all four parameters of the brightness temperature Stokes vector simultaneously. Preliminary experiments have confirmed the directional signatures of the sea brightness temperature as reported by other researchers and have led to development of improved instrumentation with the intention of determining the wind vector pixel by pixel in the radiometer imagery.
Reconstruction of a three‐dimensional ionosphere from backscatter and vertical ionograms measured by over‐the‐horizon radar
Authors: Fridman S. V.
Published: February 2004

A method for inversion of backscatter ionograms is described. Source data for the method consist of leading edge of backscatter ionograms from all azimuthal sectors of the backscatter sounder and the quasi vertical incidence (QVI) ionogram. Such data are routinely collected at over‐the‐horizon radar (OTHR) installations operated by the U.S. Navy. The output is a three‐dimensional model of the ionosphere for the coverage area of the sounder. The model produced is consistent with all the input data. The method is based on the Newton‐Kontorovich method for nonlinear operator equations and Tikhonov's regularization technique. Theory of the method and principles of its numerical realization are described. The method is applied to data acquired by the backscatter sounder operating at the Chesapeake, Virginia, OTHR facility. Maps of plasma frequency at a constant altitude over a geographic region that covers a 64°–80° azimuthal sector out to distances of more than 2000 km are presented. The backscatter ionogram inversion technique suggested makes it possible for OTHR backscatter sounders to provide real‐time monitoring of the ionosphere over a large geographical area.
Demonstration of an efficient boundary layer parameterization for unbiased propagation estimation
Authors: Rogers L. Ted
Published: February 2004

Experimental results are presented indicating that only a few parameters describing capping inversion of the marine atmospheric boundary layer (MABL) need to be considered deterministically for accurate estimation of the MABL's effects on propagation loss, provided that (1) the right deterministic parameters are chosen and (2) other parameters are accounted for statistically. To perform this experiment, the refractivity structure associated with the capping inversion of the MABL is modeled using shifted empirical orthogonal functions (SEOFs) developed from the second‐order statistics of radiosondes. Propagation estimates calculated using the SEOFs as well as radiosondes and the heuristic trilinear model are compared with measured propagation. Using only two deterministic parameters, the results obtained using the SEOFs are essentially as good as can be obtained with a fading channel. Why this result is important in data assimilation for refractivity assessments is discussed.
Uplift and erosion of the San Bernardino Mountains associated with transpression along the San Andreas fault, California, as constrained by radiogenic helium thermochronometry
Authors: Spotila James A. , Farley Kenneth A. , Sieh Kerry
Published: February 2004

Apatite helium thermochronometry provides new constraints on the tectonic history of a recently uplifted crystalline mass adjacent to the San Andreas fault. By documenting aspects of the low‐temperature (40°–100°C) thermal history of the tectonic blocks of the San Bernardino Mountains in southern California, we have placed new constraints on the magnitude and timing of uplift. Old helium ages (64–21 Ma) from the large Big Bear plateau predate the recent uplift of the range and show that only several kilometers of exhumation has taken place since the Late Cretaceous period. These ages imply that the surface of the plateau may have been exposed in the late Miocene and was uplifted only ∼1 km above the Mojave Desert in the last few Myr by thrusting on the north and south. A similar range in helium ages (56–14 Ma) from the higher San Gorgonio block to the south suggests that its crest was once contiguous with that of the Big Bear block and that its greater elevation represents a localized uplift that the Big Bear plateau did not experience. The structure of the San Gorgonio block appears to be a gentle antiform, based on the geometry of helium isochrons and geologic constraints. Young ages (0.7–1.6 Ma) from crustal slices within the San Andreas fault zone indicate uplift of a greater magnitude than blocks to the north. These smaller blocks probably experienced ≥3–4 km of uplift at rates ≥1.5 mm/yr in the past few Myr and would stand ≥2.5 km higher than the Big Bear plateau if erosion had not occurred. The greater uplift of tectonic blocks adjacent to and within the San Andreas fault zone is more likely the result of oblique displacement along high‐angle faults than motion along the thrust fault that bounds the north side of the range. We speculate that this uplift is the result of convergence and slip partitioning associated with local geometric complexities along this strike‐slip system. Transpression thus appears to have been accommodated by both vertical displacement within the San Andreas fault zone and thrusting on adjacent structures.
Crustal structure and orogenic material budget in the west central Pyrenees
Authors: Teixell Antonio
Published: February 2004

Surface and subsurface data are combined to construct a crustal‐scale cross section of the western central Pyrenees (France and Spain) at the boundary between the European and Iberian plates. The position of Moho reflections in the ECORS‐Arzacq reflection profile suggests a tectonic wedge of European crust and upper mantle had indented the Iberian plate at lower crustal levels. The European wedge is overlain by an upper, Iberian wedge thus constituting a double (stacked) wedge geometry. The upper wedge was delaminated and deformed giving rise to the Pyrenean orogenic prism, manifested as a bivergent fan in upper crustal levels. The underthrust lower Iberian plate has been imaged to depths of 55–60 km, but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km. Total orogenic contraction calculated from surface structures is about 75–80 km, which was accomplished between the latest Cretaceous and the early Miocene at an averaged rate of 1.2 mm/yr. Consistent with these moderate values, exhumation of the orogen is much less than in the more shortened eastern parts of the range. Paleozoic basement and preorogenic Mesozoic rocks are little eroded, and much of the exhumation involved cannibalization of early foreland basins, which once covered the entire, poorly emergent orogen at this transect. This resulted in a continuous process of sediment recycling and, coupled with a considerable lateral arrival of material, a bulk negative erosion‐sedimentation budget.
Structural analysis of the Carolina‐Inner Piedmont terrane boundary: Implications for the age and kinematics of the central Piedmont suture, a terrane boundary that records Paleozoic Laurentia‐Gondwana interactions
Authors: West Thomas E.
Published: February 2004

New field mapping along the Carolina‐Inner Piedmont terrane boundary in South Carolina and eastern Georgia reveals preaccretionary, synaccretionary, and postaccretionary faults. The dextral strike‐slip Lowndesville shear zone is adjacent to a ∼50‐km‐long segment of the terrane boundary. However, the Lowndesville shear zone is correlated eastward with the dextral strike‐slip Beaver Creek shear zone, which is within the Carolina terrane and predates ∼415 Ma. The Lowndesville shear zone is overprinted by a dextral strike‐slip phyllonite zone, named the Deal Creek shear zone. The Deal Creek shear zone is correlated eastward with the Gold Hill‐Silver Hill shear zone which is also within the Carolina terrane and records dextral strike‐slip motion between ∼400 and ∼325 Ma. The Cross Anchor and Mulberry Creek faults both truncate the Lowndesville and the Deal Creek shear zones and form the terrane boundary. The Mulberry Creek fault, probably of Triassic‐Jurassic age, juxtaposes the Lowndesville shear zone adjacent to the Carolina‐Inner Piedmont terrane boundary west of Waterloo, South Carolina. The Cross Anchor fault is the terrane boundary east of Waterloo, South Carolina, and forms the southeastern boundary of the Whitmire reentrant. Crosscutting relationships indicate that the Cross Anchor fault is the oldest fault which juxtaposes the Carolina and Inner Piedmont terranes in the study area. These structural interpretations and available geochronological data indicate that the Cross Anchor fault is a ∼325 Ma thrust fault and may be the central Piedmont suture. An early Alleghanian suture resolves the problem of inserting the Carolina terrane into the western Iapetus Ocean.
Miocene‐Pliocene tectonic evolution of the Slovenian Periadriatic fault: Implications for Alpine‐Carpathian extrusion models
Authors: Fodor László , Jelen Bogomir , Márton Emő , Skaberne Dragomir , Čar Jože , Vrabec Marko
Published: February 2004

The Periadriatic Line (PAL) is a remarkable, several hundred kilometer long fault system of the Alpine orogen. Its dextral character was documented by several authors using diverse criteria, but detailed kinematics and timing of movements had not been investigated along its whole length. Structural and paleomagnetic measurements, mapping, and stratigraphic and sedimentological studies have helped to unravel the Miocene‐Pliocene evolution of the Slovenian segment of the PAL. Brittle deformation was characterized by NW‐SE to N‐S compression and perpendicular tension. Deformation has resulted in dextral strike‐slip faulting, folding, and tilting of beds. The first transpressional event corresponds to the first phase of lateral extrusion of the East Alpine‐Western Carpathian‐Northern Pannonian block in the early Miocene (24–17.5 Ma). After a short period of transtension during the Karpatian (17.5–16.5 Ma), dextral transpression reoccurred during the middle Miocene to Pliocene and lasted up to the Quaternary. Middle Miocene dextral slip can be connected to the second phase of extrusion. The highly deformed rocks within the dextral shear zones show variable clockwise, sometimes counterclockwise, rotations. The mechanism of rotation seems to be complex, ranging from regional rotation to local folding due to pure or simple shear (domino‐type rotation).
Kinematic history of the Laramide orogeny in latitudes 35°–49°N, western United States
Authors: Bird Peter
Published: February 2004

The kinematic history of the Rocky Mountain foreland and adjacent areas is computed back to 85 Ma, using virtually all the structural, paleomagnetic, and stress data in the literature. A continuous velocity field is fit to the data in each time step by weighted least squares, and this velocity is integrated back through time. As proposed by Hamilton [1981], the net movement of the Colorado Plateau was a clockwise rotation about a pole in northern Texas; but the rotation was less (3°) than some have inferred from paleomagnetism. The Laramide orogeny occurred during 75–35 Ma, with peak Colorado Plateau velocities of 1.5 mm yr−1 during 60–55 Ma. The mean azimuth of foreland velocity and mean direction of foreland shortening was stable at 40° for most of the orogeny, increasing to 55° in 50–40 Ma; the counterclockwise rotation of shortening directions proposed by some previous authors is incorrect. Comparing the computed histories of foreland flow speed and direction with the known motions of the Kula and Farallon plates confirms that the Laramide orogeny had a different mechanism from the early Sevier orogeny: it was driven by basal traction during an interval of horizontal subduction, not by edge forces due to coastal subduction or the spreading of the western cordillera or by accretion of terranes to the coast. Tentatively, a minor clockwise rotation of shortening directions at 50 Ma may record the passage of an active Kula‐Farallon transform within the subducted slab.