bluuebunny/crossref_metadata_2025 huggingface dataset:
A comparison of the Stratospheric Aerosol and Gas Experiment II tropospheric water vapor to radiosonde measurements
Authors: Larsen J. C. , Chiou E. W. , Chu W. P. , McCormick M. P. , McMaster L. R. , Oltmans S. , Rind D.
Published: February 2008

Upper tropospheric Stratospheric Aerosol and Gas Experiment II (SAGE II) water vapor observations are compared to correlative radiosonde observations and radiosonde based climatologies. The SAGE II 1987 monthly zonal mean water vapor climatology is compared to both the Global Atmospheric Circulation Statistics (1963–1973) climatology and to the 1987 radiosonde climatology. The clear sky SAGE II climatology is found to be approximately half the level of both the clear/cloudy sky radiosonde climatologies. To determine whether this is realistic for these two different climatologies or includes additional observational and instrumental biases, we took the 1987 radiosonde data set and identified approximately 800 correlative profile pairs. The observational biases inherent to SAGE II and the radiosondes produce a set of profile pairs characteristic of clear sky, land conditions. A critical review of the radiosonde measurement capability was carried out to establish the operating range and accuracy in the upper troposphere. We show that even with tight coincidence criterion, the quality of the profile pair comparisons varies considerably because of strong water vapor variability occurring on small time and space scales. Annual zonal means calculated from the set of profile pairs again finds SAGE II significantly drier in many latitude bands. Resolving the radiosonde data base by hygrometer type shows this to be true for all hygrometers except for the thin film capacitive type (Vaisala Humicap). For this hygrometer, between 4.5 and 6.5 km SAGE II is drier by approximately 25.%, and from 8.5 to 11.5 km they are nearly equivalent when global annual means are compared. The good agreement with the Vaisala Humicap, currently the most accurate and responsive hygrometer in operational use, suggests existing radiosonde climatologies contain a significant moist bias in the upper troposphere.
Estimation of surface heat and moisture fluxes over a prairie grassland: 3. Design of a hybrid physical/remote sensing biosphere model
Authors: Smith Eric A. , Cooper Harry J. , Crosson William L. , Heng‐Yi Weng
Published: February 2008

This is the third in a series of papers describing a measurement program and modeling approach to the estimation of surface heat and moisture fluxes over a tallgrass prairie. We describe the design and formulation of an experimental biosphere model (Ex‐BATS) which follows the development of Dickinson's biosphere‐atmosphere transfer scheme (BATS). Ex‐BATS has been designed to incorporate in situ measurements and satellite parameterizations of certain canopy variables which are slowly varying in the course of a growing season. All components of a multistage biosphere process model used to simulate the exchange of heat and moisture between the canopy and the atmosphere are presented here. The remote sensing aspects of the model are described in a companion paper. The procedures used to optimize the model and the validation of the model against First ISLSCP Field Experiment (FIFE) observations taken during 1987 are described. Validation was carried out for three of the four intensive field campaigns (IFCs): 1, 2 and 3. The validation intercomparison shows that the Ex‐BATS model reproduces the diurnal behavior of the surface fluxes very closely. The rms differences between in situ measurements of sensible and latent heat fluxes and their model counterparts are of the order of 35 W m−2. Biases over the three IFCs, which ranged in duration from 10 to 17 days, are typically less than 20 W m −2. The overall bias for the 44 days within the first three IFCs is less than 10 W m −2. The biases and rms differences are of the same order as the accuracy and precision uncertainties of the measured fluxes, therefore the model provides a useful experimental tool to explain radiative, hydrological, and physiological controls on surface fluxes over vegetated canopies and the explicit sources of water flux to the atmosphere from transpiration, foliage evaporation, and soil evaporation.
Greenhouse gases in the stratosphere
Authors: Zhong Wenyi , Haigh J. D. , Pyle J. A.
Published: February 2008

The potential radiative forcing in the stratosphere of changing concentrations of ozone, methane, nitrous oxide and chlorofluorocarbons 11 and 12 is assessed. Significant changes in heating rate in the lower stratosphere are found. The response of a fully interactive radiative‐photochemical‐dynamical two‐dimensional model to such changes in gaseous concentrations is investigated. The inclusion of CH4, N2O and the CFCs in the radiation scheme causes a small (1 K) decrease in temperature throughout the stratosphere after 50 model years with a resulting increase in ozone column up to 1% in summer high latitudes. An experiment in which lower stratospheric ozone concentrations were forcibly reduced in line with recent satellite observations results in significant (several degrees) temperature decrease in this region. Such decreases may be very significant in maintaining polar ozone loss.
Antarctic pressure and temperature anomalies surrounding the minimum in the Southern Oscillation index
Authors: Smith Shawn R. , Stearns Charles R.
Published: February 2008

The purpose of our research is to analyze surface pressure and temperature patterns in Antarctica for relationships with the El Niño/Southern Oscillation (ENSO). Using monthly anomalies of surface pressure and temperature in Antarctica, lag correlations are made to the Southern Oscillation index (SOI) and annual composites are created for years where ENSO is in a warm phase. Comparisons are made to annual composites for nonwarm phase years and to six individual ENSO warm phases occurring between 1957 and 1984. We found a sharp change in the sign of the pressure and temperature anomalies between the year before and the year after the minimum in the SOI. We hypothesize that the temperature and pressure anomaly patterns are related, through a thickness argument, to the 500‐mbar southern hemispheric flow and the Australian branch of the Southern Oscillation.
Measurement of partial vapor pressure of ammonia over acid ammonium sulfate solutions by an integral method
Authors: Koutrakis P. , Aurian‐Blăjeni B.
Published: February 2008

We present a simple, integral, passive method for measuring partial vapor pressure. Integral methods are useful tools when dealing with very low concentrations because collection over extended periods increases the analytical sensitivity. Passive methods have the advantage of not introducing constraints external to the system. The principle of the method used here is to selectively react the substance in the atmosphere over a solution with an immobilized coating on an appropriate support. The reaction product is not volatile, but is soluble and can be extracted in an appropriate solvent and analyzed. The method has been applied to measuring the vapor pressure of ammonia over aqueous solutions. The vapor pressure over ammonium sulfate solutions depends on the acidity of the solutions as well as on the salt concentration. The dependence can be explained with a simple model. Furthermore, using the same model, we calculated the ammonia vapor pressure above different ammonium sulfate/sulfuric acid aqueous solutions as a function of sulfate molarity and percentage of sulfuric acid. The results from the calculations suggest that for ambient ammonia concentrations less than 10 ppb, acid sulfate aerosols are not completely neutralized.
Ground‐based instrumentation for measurements of atmospheric conduction current and electric field at the South Pole
Authors: Byrne G. J. , Benbrook J. R. , Bering E. A. , Few A. A. , Morris G. A. , Trabucco W. J. , Paschal E. W.
Published: February 2008

We have constructed instruments to measure the atmospheric conduction current and the atmospheric electric field: two fundamental parameters of the global‐electric circuit. The instruments were deployed at the Amundsen‐Scott South Pole Station in January 1991 and are designed to operate continuously for up to one year without operator intervention. The atmospheric current is measured by a sensor that uses a split‐hemispheric conducting shell of 17.8‐cm radius, separated by a thin Teflon insulating disk. The detection electronics are inside the sphere. In principle, the atmospheric current flows into one hemisphere, through the electronics where it is measured, and out the other hemisphere. The electric field is measured by a field mill of the rotating dipole type. The electric field sensing elements are two 30‐cm‐long antennas, driven to rotate in the vertical plane at 1800 rotations per minute. Two arrays of identical instruments have been deployed, separated by 600 m, in order to distinguish between atmospheric electrical signals of local and global origin. The separation distance of the arrays was determined by the climatology of the Antarctic plateau. Sample data from the first days of operation at the South Pole indicate variations in the global circuit over time scales from minutes, to hours, to days.
Control of near‐surface winds over an Antarctic ice shelf
Authors: King J. C.
Published: February 2008

Simple models are used to assess the factors controlling near‐surface winds over an Antarctic ice shelf. Observations from Halley Station and an automatic weather station situated close to the coastal slopes adjoining the Brunt Ice Shelf are presented. These suggest that katabatic flows originating over the continental slopes adjust more quickly to the regional easterly flow over the ice shelf than is predicted by these simple models. It is suggested that nonlinear mechanisms such as “hydraulic jumps” or internal gravity wave radiation could lead to flow adjustment on the short space and time scales observed. In contrast with the present observations, katabatic flows have been observed to propagate across the Ross Ice Shelf for great distances with little modification. However, in this latter region the topography favors the formation of intense, channeled katabatic flow, while the katabatic flow onto the Brunt Ice Shelf is unconfined and consequently much weaker.
The Earth's radiation budget and its relation to atmospheric hydrology: 3. Comparison of observations over the oceans with a GCM
Authors: Stephens Graeme L. , Randall David A. , Wittmeyer Ian L. , Dazlich Donald A. , Tjemkes Stephen
Published: February 2008

This paper reports on a study that seeks to examine a very limited set of interactions between the hydrological cycle and the radiative processes that take place on Earth and attempt to test how well these interactions are simulated by a general circulation model. Two broad types of tests of the model are introduced in this paper. The first focuses on comparing various sensitivity relationships established by the model with those observed on Earth. The specific relationships examined in this paper include the sensitivities of column‐integrated water vapor, the clear sky greenhouse effect, cloud albedo, and cloud radiative forcing to sea surface temperature. The second type of test focuses on comparison of simulated and observed seasonal cycles of the greenhouse effect and cloud radiative forcing. The main results of this study suggest that the model studied, which we take to be representative of the current generation of global climate models, is able to simulate some aspects of the observed sensitivities fairly well. Qualitative successes of the simulations include realistic variations of column vapor with sea surface temperature, the clear sky greenhouse parameter and its variation with both column vapor and sea surface temperature, and cloud longwave radiative forcing with sea surface temperature, as well as the seasonal changes of the greenhouse effect and the cloud radiative forcing. However, there exist many serious quantitative differences between the model and the observations. These include problems in the simulations of the column vapor in the tropics and an excessively strong clear‐sky greenhouse effect in the mid‐latitudes. These differences combine in such a way that the model underestimates the sensitivity of the clear‐sky greenhouse to changes in sea surface temperature compared to that derived from observations. Significant differences between the simulated and the observed radiative properties of clouds include the overprediction of the longwave cloud radiative forcing over warm tropical oceans and a significant underestimate of the cloud reflection in mid‐latitudes during the summer season.
Radon measurements in the lower tropical stratosphere: Evidence for rapid vertical transport and dehydration of tropospheric air
Authors: Kritz Mark A. , Rosner Stefan W. , Kelly Kenneth K. , Loewenstein Max , Chan K. Roland
Published: February 2008

During the tropical experiment of NASA's Stratosphere‐Troposphere Exchange Program (STEP), in situ radon and other trace constituent measurements were made aboard a NASA ER‐2 high‐altitude research aircraft to investigate the mechanisms of irreversible transfers from the troposphere into the tropical stratosphere. Observations made in and downwind of the cirrus shields of three large tropical cyclones and downwind of the cirrus anvil of a large cumulonimbus cloud cluster showed several clear instances of elevated radon activity occurring simultaneously with low total water mixing ratios. These observations are unambiguous evidence of an effective dehydration process, capable of reducing total water vapor mixing ratios to less than 2.5 ppmv, occurring in conjunction with troposphere‐to‐stratosphere transport and indicate that rapid localized convection, rather than slow regional mean motions, was responsible for the observed transports and associated with the accompanying dehydration. Radon activities measured in regions of active or recent troposphere‐to‐stratosphere transport were consistent with the 17 pCi/scm mean value needed to support the observed abundance of stratospheric 210Pb.
Feasibility of determining surface emissions of trace gases using an inverse method in a three‐dimensional chemical transport model
Authors: Hartley Dana , Prinn Ronald
Published: February 2008

We investigate the feasibility of using an inverse method based on a linear Kalman filter to determine regional surface fluxes through comparisons between observations and predictions in a three‐dimensional atmospheric transport model. The ability of the present ALE/GAGE observation sites to quantify the regional fluxes of anthropogenic trace gases is studied also. These investigations are done in the low‐resolution spectral model of Golombek and Prinn (1986) using CFCl3 as the test tracer since its sources are relatively well known. The first of these investigations is done with the transport model being perfect in the sense that the “observations” were produced by running the model with the CFCl3 emissions derived from industry data. The inverse method used is capable in this case of accurately determining regional surface fluxes using the present ALE/GAGE sites and to converge to the correct solution within a year or two even using initial conditions very different from the final solution. We also investigate how well the Kalman filter approach works with a less than perfect chemistry circulation model by using the ALE/GAGE observations Of CFCl3 for the inversion. The success of this inversion depends largely on the ability of the model circulation to predict observed concentrations of CFCl3 since its chemistry is reasonably well understood. The larger the difference between the model and the observed values using the real (industry) emissions then the larger the bias will be in the estimated emissions. Such studies can help to understand the inherent biases in the model when used in an inverse scheme before trying to use the model to estimate unknown surface fluxes such as those for methane, nitrous oxide, and carbon dioxide. We also investigate where additional observational stations could be placed to enhance the capability of the present ALE/GAGE network for determining regional net fluxes. If we are prepared to accept the circulation in the spectral low‐resolution model as sufficiently realistic, it appears that Hateruma (24N, 123E) and to a lesser extent Kamchatka (51N, 156E) are very promising locations for new stations and that Hateruma is superior to the ALE/GAGE Oregon station in providing information about Asian sources. This type of analysis done with a realistic circulation model can aid the process of choosing observation sites by addressing how well each site contributes to the different goals for use of the data.
Shortwave cloud‐radiative forcing at the top of the atmosphere at the surface and of the atmospheric column as determined from ISCCP C1 data
Authors: Laszlo Istvan , Pinker Rachel T.
Published: February 2008

To understand the role of clouds in the atmospheric circulation and in the modulation of energy available at the surface, their effect on the the atmospheric and surface absorption should be determined. The C1 data of the International Satellite Cloud Climatology Project, along with the Satellite Algorithm for Shortwave Radiation Budget, are used to estimate the shortwave cloud effects in terms of the cloud‐radiative forcing at the top of the atmosphere (TOA), at the surface and of the atmospheric column on a global scale for the July months of 1983–1985. Global means of TOA cloud forcing range from −43.6 (1983) to −39.1 Wm−2 (1985). The cloud forcing for July 1985 is underestimated, by about 8 Wm−2, compared with that obtained from the Earth Radiation Budget Experiment. The cloud forcing at the surface is almost identical to that at the TOA, indicating that the effect of clouds on the shortwave energy budget of the surface‐atmosphere system is such that most of the cooling is at the surface. Regression analysis of the computed fluxes shows a strong linear correlation between the TOA and surface cloud forcing. The monthly averaged regional values of the atmospheric cloud forcing are generally less than the estimated uncertainty of 20 Wm−2. Assuming that the uncertainties cancel, the global mean of the atmospheric cloud forcing is between 1 and 2 Wm−2, suggesting a slight warming owing to the presence of clouds.
Annual and semiannual temperature oscillations at the mesopause levels over Trivandrum (8.5°N, 78°E)
Authors: Reddi C. Raghava , Rajeev K. , Ramkumar Geetha
Published: February 2008

The ambipolar diffusion coefficient (D) of the atmosphere, obtained from the exponential amplitude decay of 62,081 underdense radio meteor trails, recorded by the meteor wind radar operated at Trivandrum, was analyzed to obtain the annual (AO) and semiannual oscillations (SAO) of the monthly mean “D”. The amplitudes of AO and SAO of the mean D were interpreted in terms of AO and SAO of the mesopause temperature. The results show excellent agreement with the AO and SAO of mesopause temperature reported earlier using rocket data in low latitudes. While the COSPAR International Reference Atmosphere (CIRA) 1972 models reproduce the SAO observed in the present study, the more recent CIRA 1986 models do not reproduce it. Both CIRA 1972 and CIRA 1986 do not reproduce AO of mesopause temperature at low latitudes. It is of significance to note that AO of mesopause temperature is much stronger than SAO.
Quasi‐stationary planetary waves in total ozone and their correlation with lower stratospheric temperature
Authors: Wirth Volkmar
Published: February 2008

Hemispheric maps of monthly mean total ozone display distinct waves of low zonal wavenumbers during winter months. In addition, there is a strong and statistically significant spatial correlation between the total ozone perturbation and the temperature perturbation in the lower stratosphere. Previous attempts at an explanation of the correlation appear somewhat unsatisfactory. In the current paper the climatological behavior for southern hemisphere October is investigated. Specifying the dynamics from observations in the linearized stationary ozone continuity equation, one can simulate the lowest wavenumbers of the hemispheric total ozone perturbation. Both vertical and horizontal advection substantially contribute to the total ozone perturbation, with significantly different patterns each. A conceptual picture for the correlation is developed, which accounts for both advection contributions. The slopes of lower stratospheric zonal mean potential temperature and ozone mixing ratio turn out to be important. Furthermore, the feasibility of a Matsuno‐type model for the waves in total ozone is shown.
The tropopause cold trap in the Australian monsoon during STEP/AMEX 1987
Authors: Selkirk Henry B.
Published: February 2008

The relationship between deep convection and tropopause cold trap conditions is examined for the tropical northern Australia region during the 1986–1987 summer monsoon season. Particular attention is paid to the Australian Monsoon Experiment (AMEX) period from January 10 through February 14 when the NASA Stratosphere‐Troposphere Exchange Project (STEP) was conducting aircraft investigations at Darwin. Time mean values of the saturation mixing ratio at the local temperature minimum or cold point at stations in the AMEX radiosonde network were everywhere below the global lower stratospheric average of ∼3.5 ppmv and in places substantially so, thus fulfilling the first of two conditions necessary for stratospheric dehydration. The cold point potential temperature Θcp in the region was anticorrelated both spatially and temporally with mesoscale convection. However, there was no direct connection between surface layer moist entropy and Θcp. Time mean Θcp ranged from 366 to 374 K at stations in the monsoon trough zone along the northern coast and in the northern Gulf of Carpentaria, while the time mean of the average equivalent potential temperature in the surface layer Θe was no greater than 356 K. This difference of at least 10 K between trough mean Θcp values and the maximum potential temperature achieved in undilute moist ascent from the surface layer indicates that rapid incorporation of tropospheric air into the stratosphere was occurring in monsoonal convective systems through a combination of cumulonimbus overshooting and anvil radiative heating. Over the continent, where mean Θe was 6–10 K lower, stratosphere‐troposphere exchange was less widespread but more vigorous. Here the average Θcp approached 380 K and was likely due to deeper tropopause penetrations produced by continental convective systems. This elevation of Θcp in continental systems relative to monsoon trough systems is evident in the Θcp of tropopause layers which had never passed over the continental interior: these were 8–13 K lower than their counterparts of continental origin. This elevating tendency accounts for the upward trends in cold point temperature, saturation mixing ratio and Θcp in the trough zone following monsoon onset in mid‐January. Before onset, tropopause air in the north coastal zone beyond the immediate vicinity of convective systems appeared to have originated in the tropics upstream of the continent. With the establishment of the monsoon continental‐scale upper‐level anticyclone, tropopause air entering the monsoon trough zone had more often been processed by systems of the continental interior. Nonetheless, the necessary conditions for stratospheric dehydration, i.e., cold point saturation mixing ratios below the global average and significant stratosphere‐troposphere exchange, remained in force throughout the monsoon period.
Ocean color and atmospheric dimethyl sulfide: On their mesoscale variability
Authors: Matrai Patricia A. , Balch William M. , Cooper David J. , Saltzman Eric S.
Published: February 2008

The mesoscale variability of dimethyl sulfide (DMS) and ocean color is explored to determine the feasibility of a predictive relationship. During NASA's Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation (GTE/CITE 3), simultaneous shipboard and aircraft studies were carried out in the North Atlantic, followed by aircraft studies in the South Atlantic. Surface concentrations of chlorophyll a were measured with an airborne spectroradiometer, the Ocean Data Acquisition System (ODAS), with simultaneous determinations of tropospheric DMS. Shipboard measurements of DMS in air and water as well as in situ chlorophyll a were taken in the North Atlantic. No relation was observed between shipboard aquatic DMS and chlorophyll a or primary productivity. Higher levels of aqueous DMS were not always reflected by atmospheric DMS, although shipboard and aircraft measurements of atmospheric DMS agreed very well. A significant relationship between atmospheric DMS and ocean color was seen once at low altitudes in both the North and South Atlantic only under clean air conditions. Atmospheric DMS levels during the North Atlantic experiment were probably lowered by the presence of mostly polluted air masses in the study area and were, overall, probably not representative of the in situ sea‐to‐air flux of DMS. Changes in concentration of aircraft‐sensed chlorophyllous pigments were not reflected by atmospheric DMS. If a predictive algorithm is to be found, phytoplankton blooms should probably be the first place to study an ocean color‐DMS relationship.
Characteristics of gravity waves in the mesosphere observed with the middle and upper atmosphere radar: 1. Momentum flux
Authors: Nakamura T. , Tsuda T. , Yamamoto M. , Fukao S. , Kato S.
Published: February 2008

We observed wind motions from 60‐ to 90‐km altitudes with the MU radar (35°N, 136°E) in the four observation periods: October 1986, June 1987, July 1990, and January/February 1991. Mean wind profiles were fairly consistent with results of Kyoto meteor radar observations (35°N, 136°E) collected in 1983–1985 at 80‐ to 110‐km altitudes, and zonal mean winds generally agreed well with CIRA 1986 except for the profiles below 70 km in January/February 1991, although discrepancy of amplitudes sometimes ranged up to 10 to 20 m/s. Characteristics of frequency spectra of radial wind velocities were basically similar among results determined in four observation periods. That is, oblique spectra had a logarithmic slope of about −5/3 in the frequency range lower than 4–5 × 10−4 s−1. Gravity waves with periods longer than 20–50 min to 5 hours (the lowest limit of the spectral analysis) were found to carry a large part of the zonal momentum flux, while a dominant frequency component was not detected for the meridional flux. Zonal and meridional drag of mean winds induced by the gravity wave with periods from 5 min to 8–10 hours were +51 m/s/d and −4 m/s/d in June at 75 km and −4.0 m/s/d and +7.4 m/s/d in October at 70 km, respectively. The major part of the drag force was also found in the wave component with periods larger than 30 min. The day‐to‐day variation in the zonal momentum flux showed a good correlation with the vertical shear of the zonal mean wind, which suggests effects of the gravity wave activities on mean wind fields, or the effect of shear variation on the gravity wave activity.
High‐pressure melting of carbonaceous chondrite
Authors: Agee Carl B.
Published: February 2008

Phase equilibria of carbonaceous chondrite and phase density relations at high pressure govern a simple model for solidification of an initially molten chondritic Earth. The solidification can be described by differentiation stages, each characterized by a particular, experimentally predicted, petrologic process that dominates the distribution of chemical components during a specific evolution interval. The stages, though expressed here as discrete events for simplicity, may actually be overlapping segments in the cooling and crystallization continuum of Earth formation. We focus on five important differentiation stages of a chondritic Earth in the following chronological order: (1) sulfide liquid‐silicate liquid immiscibility leading to protocore formation; (2) magnesiowüstite fractionation at the base of the early mantle; (3) perovskite flotation and fractionation in the midmantle; (4) olivine flotation in the shallow mantle; and (5) garnet flotation in the prototransition zone. Compositional layering produced by the five experimentally predicted differentiation stages obeys mass balance and is consistent with a shallow mantle source for peridotite xenoliths, a transition zone rich in garnet at ∼540 km, a lower mantle with a superchondritic Si/Mg, and a core of Fe‐Ni‐S‐O.
Oxygen ionization rates at Mars and Venus: Relative contributions of impact ionization and charge exchange
Authors: Zhang M. H. G. , Luhmann J. G. , Nagy A. F. , Spreiter J. R. , Stahara S. S.
Published: February 2008

Oxygen ion production rates above the ionopauses of Venus and Mars are calculated for photoionization, charge exchange, and solar wind electron impact ionization processes. The latter two require the use of the Spreiter and Stahara (1980) gas dynamic model to estimate magnetosheath velocities, densities, and temperatures. The results indicate that impact ionization is the dominant mechanism for the production of O+ ions at both Venus and Mars. This finding might explain both the high ion escape rates measured by Phobos 2 and the greater mass loading rate inferred for Venus from the bow shock positions.
Geomorphic evolution of the Martian highlands through ancient fluvial processes
Authors: Craddock Robert A. , Maxwell Ted A.
Published: February 2008

Craters in the Martian highlands are preserved in various stages of degradation. As a result of an erosional process active from the Middle Noachian (4.40–3.92 b.y.) through the Hesperian (3.55–1.8 b.y.), ejecta associated with fresh impact craters became etched, hummocky, and dissected by runoff channels. With time, interior gullies became deeply incised and ejecta deposits were entirely removed. Infilling of the craters followed until, in some instances, the craters were completely buried. Only fluvial processes explain these morphologic variations, the size range of affected craters, and the size‐frequency distribution curves associated with these crater populations. Based on the number of superposed fresh impact craters, fluvial processes affecting the highlands ceased entirely by the end of the Hesperian. No correlation between cessation of degradation and latitude exists. However, a strong correlation exists between cessation of degradation and elevation. Degradation ended at higher elevations (e.g., 3–4 km; N [5]=∼200, Late Noachian) before lower elevations (e.g., 1–2 km; N[5]=∼180, Early Hesperian), suggesting that cessation was coupled to desiccation of the volatile reservoir and degassing of a 5–20 bar primordial atmosphere. Volatiles released to the surface by runoff channel formation and seepage may have been part of a complex hydrologic cycle that included periodic, heavy amounts of precipitation. Rainfall was principally responsible for degrading the highlands, eroding impact craters, and redistributing sediments. Rainfall also recharged the highland aquifers, allowing sapping and seepage to continue for hundreds of millions of years. As the primordial atmosphere was lost, cloud condensation, and thus rainfall and aquifer recharge, occurred at progressively lower elevations. Based on estimates on the amount of material removed and duration of degradation, denudation rates averaged 0.0001–0.005 mm/yr. These rates are equivalent to those in terrestrial periglacial environments.
The surface of Syrtis Major: Composition of the volcanic substrate and mixing with altered dust and soil
Authors: Mustard John F. , Erard S. , Bibring J‐P. , Head J. W. , Hurtrez S. , Langevin Y. , Pieters C. M. , Sotin C. J.
Published: February 2008

Syrtis Major is an old, low relief volcanic plateau near the equatorial regions of Mars. It is a persistent low‐albedo feature on the planet and is thought to contain a high abundance of exposed bedrock and/or locally derived surface material and debris. Spatially resolved variations in surface spectral properties, and therefore composition, are investigated with data from the Imaging Spectrometer for Mars (ISM) instrument. ISM obtained 128 wavelength channel spectra from 0.76 to 3.16 μm for contiguous pixels approximately 22 × 22 km in size across much of the plateau. The value and spatial distribution of four primary spectral variables (albedo, continuum slope, wavelength of the ferric‐ferrous band minimum, area of the ferric‐ferrous absorption) are mapped and coregistered to Viking digital photomosaics. Analysis of these maps shows that although there is a high degree of overall spectral variability on the plateau, the key indicators of mafic mineralogy are relatively homogeneous. Detailed examination of reflectance spectra from representative areas across the plateau indicate the volcanic surface is dominated by augite‐bearing basalts and the pyroxene composition in the basalts is estimated to be 0.275± 0.075 Ca/(Ca+Fe+Mg) and 0.3± 0.1 Fe/(Fe+Ca+Mg). Additional mineral components may include olivine, feldspar, and glass. Most of the spectral variability on the plateau is interpreted to result from mixing of volcanic bedrock and/or locally derived surface material and debris with highly altered dust and soil. In western Syrtis Major the altered material is a transient component on the surface or occurs in large spatially coherent patches (e.g., crater rims). In eastern Syrtis Major it is apparent that the dust components are firmly fixed to the basaltic substrate as a stable oxide rind or coating.
Hard turbulent thermal convection and thermal evolution of the mantle
Authors: Yuen D. A. , Hansen U. , Zhao W. , Vincent A. P. , Malevsky A. V.
Published: February 2008

This article summarizes the results of hard turbulent convection obtained in laboratory experiments and numerical simulations. Its applications to mantle convection are illustrated by two‐dimensional numerical solutions to (1) Newtonian, (2) non‐Newtonian convection and (3) Newtonian convection with multiple phase transitions. In Newtonian mantle convection the transition from soft to hard turbulence is marked by the appearance of disconnected plumes. Spectral analysis of the time series of the Nusselt number reveals the presence of a spectral scaling subrange for hard turbulence but not for soft turbulence. In hard turbulence there is correspondence between the spectra in frequency and wavenumber domains. The slope of the seismic wave spectra measured from seismology suggests that the mantle convection today is strongly time‐dependent. The transition to hard‐turbulence takes place at much lower Nusselt numbers for non‐Newtonian than for Newtonian rheology. For the mantle this would have important ramifications. Non‐Newtonian plumes behave quite differently from Newtonian ones in that large curvatures are developed in their trajectories in the hard turbulent regime. Mantle convection with phase transitions tends to become more layered with increasing Rayleigh numbers. The style of mantle convection might have changed from a layered to a more whole mantle type of flow with time. Catastrophic overturns associated with strong gravitational instabilities in the transition zone could be responsible for superplume events.
Quasi‐random narrow‐band model fits to near‐infrared low‐temperature laboratory methane spectra and derived exponential‐sum absorption coefficients
Authors: Baines Kevin H. , West Robert A. , Giver Lawrence P. , Moreno Fernando
Published: February 2008

Near‐infrared 10‐cm−1 resolution spectra of methane obtained at various temperatures, pressures, and abundances are fit to a quasi‐random narrow‐band model. Exponential‐sum absorption coefficients for three temperatures (112 K, 188 K, and 295 K), and 20 pressures from 0.0001 to 5.6 bars, applicable to the cold environments of the major planets, are then derived from the band model for the 230 wavelengths measured from 1.6 to 2.5 μm. Root‐mean‐square deviations between the laboratory and the exponential‐sum synthetic transmissions are reported for the best fitting 50 wavelengths. Deviations relevant to broadband, 1% spectral resolution observations such as planned to be acquired by Galileo/Near Infrared Mapping Spectrometer (NIMS)and Cassini/Visual and Infrared Mapping Spectrometer are also presented. The validity of exponential‐sum coefficients derived from broadband (10 cm−1) transmission data is demonstrated via direct comparison with line‐by‐line calculations. The complete atlas of coefficients is available from the Planetary Data System‐Planetary Atmospheres Discipline Node.
Correction to “Mercury's atmospheric bright spots and potassium variations: A possible cause” by A. L. Sprague
Authors: Sprague A. L.
Published: February 2008

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.
Early and Middle Miocene stable isotopes: Implications for Deepwater circulation and climate
Authors: Wright James D. , Miller Kenneth G. , Fairbanks Richard G.
Published: February 2008

The middle Miocene δ18O increase represents a fundamental change in the ocean‐atmosphere system which, like late Pleistocene climates, may be related to deepwater circulation patterns. There has been some debate concerning the early to early middle Miocene deepwater circulation patterns. Specifically, recent discussions have focused on the relative roles of Northern Component Water (NCW) production and warm, saline deep water originating in the eastern Tethys. Our time series and time slice reconstructions indicate that NCW and Tethyan outflow water, two relatively warm deepwater masses, were produced from ∼20 to 16 Ma. NCW was produced again from 12.5 to 10.5 Ma. Another feature of the early and middle Miocene oceans was the presence of a high δ13C intermediate water mass in the southern hemisphere, which apparently originated in the Southern Ocean. Miocene climates appear to be related directly to deepwater circulation changes. Deep‐waters warmed in the early Miocene by ∼3°C (∼20 to 16 Ma) and cooled by a similar amount during the middle Miocene δ18O increase (14.8 to 12.6 Ma), corresponding to the increase (∼20 Ma) and subsequent decrease (∼16 Ma) in the production of NCW and Tethyan outflow water. Large (>0.6 ‰), relatively rapid (∼0.5 m.y.) δ18O increases in both benthic and planktonic foraminifera (i.e., the Mi zones of Miller et al. (1991a) and Wright and Miller (1992a)) were superimposed in the long‐term deepwater temperature changes; they are interpreted as reflecting continental ice growth events. Seven of these m.y. glacial/interglacial cycles have been recognized in the early to middle Miocene. Two of these glacial/interglacial cycles (Mi3 and Mi4) combined with a 2° to 3°C decrease in deepwater temperatures to produce the middle Miocene δ18O shift.
Downhole Logging as a Paeoceanographic Tool on Ocean Drilling Program Leg 138: Interface Between High‐Resolution Stratigraphy and Regional Syntheses
Authors: Lyle Mitchell , Mayer Larry , Pisias Nicklas , Hagelburg Terri , Dadey Kathy , Bloomer Stephen
Published: February 2008

On Ocean Drilling Program (ODP) Leg 138, standard shipboard procedures were modified to allow for the real‐time monitoring of several laboratory core‐scanning systems that provide centimeter‐scale measurements of saturated bulk density, magnetic susceptibility and digital color reflectance. These continuous, high‐resolution data sets were used to ensure the proper offset of multiple holes and to splice together complete sedimentary sections. Typically, the spliced, continuous sediment sections were found to be about 10% longer than the section drilled, as measured by the length of the drill string. While the source of this elongation is not yet fully understood, it must be compensated for in order to property determine sediment fluxes and mass accumulation rates. Downhole logging, in conjunction with inverse correlation techniques provided a means to determine where the distortion occurred and to correct back to true in situ depths.

Downhole logging also provides a means, through the generation of synthetic seismograms, of precisely relating the paleoceanographic events found in the core record to the high‐resolution seismic record. Once correlated to the seismic record, the spatial and temporal extent of paleoceanographic events can be traced well beyond the borehole. Most seismic events in the equatorial Pacific are related to rapid changes in carbonate content that, in turn, are related to both productivity events (often expressed as monospecific laminated diatom oozes) and times of enhanced dissolution. While many of these events may have oceanwide extent, others, like the absence of carbonate in the late‐Miocene to Recent in the Guatemala Basin have been shown to be regional and confined to only the deeper portions of the Guatemala Basin. As we identify and trace specific paleoceanographic events in the seismic record, we can begin to explore the response of the ocean through gradients of latitude, productivity, and depth.
On the Structure and Origin of Major Glaciation Cycles 1. Linear Responses to Milankovitch Forcing
Authors: Imbrie J. , Boyle E. A. , Clemens S. C. , Duffy A. , Howard W. R. , Kukla G. , Kutzbach J. , Martinson D. G. , McIntyre A. , Mix A. C. , Molfino B. , Morley J. J. , Peterson L. C. , Pisias N. G. , Prell W. L. , Raymo M. E. , Shackleton N. J. , Toggweiler J. R.
Published: February 2008

Time series of ocean properties provide a measure of global ice volume and monitor key features of the wind‐driven and density‐driven circulations over the past 400,000 years. Cycles with periods near 23,000, 41,000, and 100,000 years dominate this climatic narrative. When the narrative is examined in a geographic array of time series, the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles. We argue that the 23,000‐ and 41,000‐year cycles of glaciation are continuous, linear responses to orbitally driven changes in the Arctic radiation budget; and we use the phase progression in each climatic cycle to identify the main pathways along which the initial, local responses to radiation are propagated by the atmosphere and ocean. Early in this progression, deep waters of the Southern Ocean appear to act as a carbon trap. To stimulate new observations and modeling efforts, we offer a process model that gives a synoptic view of climate at the four end‐member states needed to describe the system's evolution, and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems.

“Solutions to problems involving systems of such complexity are not born full grown like Athena from the head of Zeus. Rather they evolve slowly, in stages, each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step.”

—Victor P. Starr
Pliocene Transgression in the Western Mediterranean Sea: Strontium Isotopes from Cuevas Del Almanzora (Se Spain)
Authors: Müller Daniel W.
Published: February 2008

The deposition of marine sediments in the deep Mediterranean basins following the Messinian salinity crisis (“Zanclean Deluge”) began at 4.9 Ma and continued until normal marine conditions prevailed at 4.6 Ma. The Betic Strait of southeast Spain, the Rif Strait of northern Morocco, and the Strait of Gibraltar have been postulated as possible entry ways for the marine waters into the Mediterranean. Two possible explanations have been offered for the timing of the deposition of marine hemipelagic marls in the Vera Basin (Cuevas del Almanzora section) within the Betic Strait: (1) early Pliocene, representing the “Zanclean Deluge” (Cita et al., 1980), (2) Messinian (∼5.7 Ma), representing the last marine stage before the Messinian salinity crisis (Benson and Rakic‐El Bied, 1991; Benson et al., 1991). The strontium isotopic ratio on mixed assemblages of foraminifers from these marine marls was determined in order to assess their ages. The average 87Sr/86Sr (0.709033±24) of the seven measured samples is, compared to the base of the Miocene/Pliocene boundary stratotype at Capo Rossello, Sicily, higher by 38 × 10−6 and yields an age of 4.6±0.5 Ma when compared to the established open ocean Sr seawater curve. The determined age indicates that (1) the first 3.6 m of marine deposits of the Cuevas del Almanzora section sensu Cita et al. (1980) including the passage zone are Pliocene, (2) the earliest marine Pliocene might not be represented at Cuevas del Almanzora indicating that this marginal basin at the edge of the Mediterranean Sea, yet within the former Betic Strait, was filled progressively by a marine transgression originating from the Mediterranean Sea around 4.6 Ma. The absence of lowermost Pliocene sediments could suggest that the Betic Strait did not serve as a conduit for the water masses entering the Mediterranean basins during the early Pliocene; and (3) the presence of an early evolutionary stage of Globorotalia margaritae in the Moroccan Bou Regreg section (approximately 5.8–5.6 Ma) and in the Spanish Cuevas del Almanzora section (later than 4.6±0.5 Ma) are diachronous suggesting that straightforward correlations of micropaleontological events within the Mediterranean area with micropaleontologic events of open marine sections should be tested thoroughly by independent methods like Sr isotopes.