bluuebunny/crossref_metadata_2025 huggingface dataset:
Gravity waves generated by a tropical cyclone during the STEP tropical field program: A case study
Authors: Pfister L. , Chan K. R. , Bui T. P. , Bowen S. , Legg M. , Gary B. , Kelly K. , Proffitt M. , Starr W.
Published: February 2008

Overflights of a tropical cyclone during the Australian winter monsoon field experiment of the Stratosphere‐Troposphere Exchange Project (STEP) show the presence of two mesoscale phenomena: a vertically propagating gravity wave with a horizontal wavelength of about 110 km and a feature with a horizontal scale comparable to that of the cyclone's entire cloud shield (wavelength of 250 km or greater). The larger feature is fairly steady, though its physical interpretation is ambiguous. The 110‐km gravity wave is transient, having maximum amplitude early in the flight and decreasing in amplitude thereafter. Its scale is comparable to that of 100‐to 150‐km‐diameter cells of low satellite brightness temperatures within the overall cyclone cloud shield; these cells have lifetimes of 4.5 to 6 hours. Aircraft flights through the anvil show that these cells correspond to regions of enhanced convection, higher cloud altitude, and upwardly displaced potential temperature surfaces. A three‐dimensional transient linear gravity wave simulation shows that the temporal and spatial distribution of meteorological variables associated with the 110‐km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400–600 m, a lifetime of 4.5–6 hours and a size comparable to the cells of low brightness temperature. The forcing amplitudes indicate that the zonal drag due to breaking mesoscale transient convective gravity waves is definitely important to the westerly phase of the stratopause semiannual oscillation and possibly important to the easterly phase of the quasi‐biennial oscillation. There is strong evidence that some of the mesoscale gravity waves break below 20 km as well. The effect of this wave breaking on the diabatic circulation below 20 km may be comparable to that of above‐cloud diabatic cooling.
Correlation of ozone with NOy in photochemically aged air
Authors: Trainer M. , Parrish D. D. , Buhr M. P. , Norton R. B. , Fehsenfeld F. C. , Anlauf K. G. , Bottenheim J. W. , Tang Y. Z. , Wiebe H. A. , Roberts J. M. , Tanner R. L. , Newman L. , Bowersox V. C. , Meagher J. F. , Olszyna K. J. , Rodgers M. O. , Wang T. , Berresheim H. , Demerjian K. L. , Roychowdhury U. K.
Published: February 2008

During the summer of 1988, measurements of photochemical trace species were made at a coordinated network of seven rural sites in the eastern United States and Canada. At six of these sites concurrent measurements of ozone and the sum of the reactive nitrogen species, NOy, were made, and at four of the sites a measure for the reaction products of the NOx oxidation was obtained. Common to all sites, ozone, in photochemically aged air during the summer, shows an increase with increasing NOy levels, from a background value of 30–40 parts per billion by volume (ppbv) at NOy mixing ratios below 1 ppbv to values between 70 to 100 ppbv at NOy levels of 10 ppbv. Ozone correlates even more closely with the products of the NOx oxidation. The correlations from the different sites agree closely at mixing ratios of the oxidation products below 5 ppbv, but systematic differences appear at higher levels. Variations in the biogenic hydrocarbon emissions may explain these differences.
Physicochemical properties of aerosols over the northeast Atlantic: Evidence for wind‐speed‐related submicron sea‐salt aerosol production
Authors: O'Dowd Colin D. , Smith Michael H.
Published: February 2008

Physicochemical characteristics of submicron aerosol particles over the Northeast Atlantic (63°N, 8°W) during October/November 1989 have been examined using a thermal analytical technique and are classified according to air mass origin. Aerosol associated with anthropogenically influenced air masses contained typically 80% sulphate particles by number, the remainder being soot carbon and sea salt. For Arctic air masses the contribution of sulphate to the total aerosol was reduced to around 65%, due to low concentrations relative to sea salt which is dependent on wind speed. In situations with clean maritime air and high wind speeds, sulphate aerosol accounted for less than 25% of the total accumulation mode particles, the remainder consisting predominantly of sea salt. Arctic air masses and clean maritime air during periods of high winds were consistently acidic with inferred molar ratios of NH4+/SO4= near 0.2. The continental and modified maritime aerosol encountered was found to have molar ratios of about 0.8. Soot carbon was present in all air masses to a similar degree (5–13%). In clean air masses, submicron sea salt aerosol concentrations showed a strong exponential increase with wind speed (correlation coefficients cc ≥ 0.8), down to a dry particle radius of 0.05 μm. Under these “clean air” conditions and high winds the sea salt aerosol dominated all particle sizes for r > 0.05 μm and accounted for approximately 75% of the total concentration, suggesting that under these conditions, sea salt aerosol would comprise the primary source of cloud condensation nuclei (CCN) in stratiform clouds.
Comparison of Stratospheric Aerosol and Gas Experiment II and balloon‐borne stratospheric water vapor measurements
Authors: Pruvost P. , Ovarlez J. , Lenoble J. , Chu W. P.
Published: February 2008

The Stratospheric Aerosol and Gas Experiment II has one channel at 940 nm related to water vapor. Two inversion procedures were developed independently in order to obtain the water vapor profile: the Chahine method by the Langley Research Center, and the Mill method by the Laboratoire d'Optique Atmosphérique. Comparisons were made between these two algorithms and some results are presented at mid‐latitudes (∼45°N) and tropical latitudes (12°S–25°S). They are compared with in situ frost point hygrometer data provided by balloon experiments from the Laboratoire de Météorologie Dynamique. At ±0.5 ppmv, agreement between the inversion results and the experimental results was obtained in the altitude range from 18–19 to 26–27 km. Below 18–19 km and above 26–27 km the error is larger (sometimes 1 ppmv and more).
Uptake of formaldehyde by sulfuric acid solutions: Impact on stratospheric ozone
Authors: Tolbert Margaret A. , Pfaff Jeanne , Jayaweera Indira , Prather Michael J.
Published: February 2008

Chemical reactions on sulfuric acid aerosols have recently been shown to play an important role in stratospheric chemistry. In particular, these reactions push odd‐nitrogen compounds into HNO3 and thereby enhance the chlorine‐catalyzed destruction of ozone. It has been suggested that our current set of heterogeneous reactions may be incomplete. Indeed we show that formaldehyde, CH2O, is rapidly and irreversibly taken up by stirred sulfuric acid solutions (60 to 75 wt % H2SO4 at −40° to −65°C) with uptake coefficients as large as γ = 0.08. If similar uptake occurs under stratospheric pressures of CH2O (that is, 1000 times lower than used in the present study), then the removal of CH2O from the gas phase can take away a significant source of odd hydrogen in the mid‐ and high‐latitude lower stratosphere. We show here that with the inclusion of this reaction, concentrations of OH and HO2 are reduced by as much as 4% under background levels of aerosols and more than 15% under elevated (volcanic) conditions. Further, the accumulation of CH2O in stratospheric aerosols over a season may alter the composition and reactivity of these sulfuric acid‐water mixtures.
Gravity wave forcing in the middle atmosphere due to reduced ozone heating during a solar eclipse
Authors: Fritts David C. , Luo Zhangai
Published: February 2008

We present an analysis of the gravity wave structure and the associated forcing of the middle atmosphere induced by the screening of the ozone layer from solar heating during a solar eclipse. Fourier integral techniques and numerical evaluation of the integral solutions were used to assess the wave field structure and to compute the gravity wave forcing of the atmosphere at greater heights. Our solutions reveal dominant periods of a few hours, characteristic horizontal and vertical scales of ∼5000 to 10,000 km and 200 km, respectively, and an integrated momentum flux in the direction of eclipse motion of ∼5.6 × 108 N at each height above the forcing level. These results suggest that responses to solar eclipses may be difficult to detect above background gravity wave and tidal fluctuations until well into the thermosphere. Conversely, the induced body forces may penetrate to considerable heights because of the large wave scales and will have significant effects at levels where the wave field is dissipated.
A k distribution method considering centres and wings of atmospheric absorption lines
Authors: Hollweg Heinz‐Dieter
Published: February 2008

The k distribution method is applied to band overlap and pressure as well as temperature dependence of absorption coefficients. The overlap of spectral absorption coefficients of H2O, CO2, O3, N2O, and CH4 is considered by a multivariate frequency distribution of absorption coefficients for a given pressure and temperature. In a first approach the vertical inhomogeneity is taken into account by the separation into the absorption coefficient at a reference state, the temperature dependence of the line strengths, and the dependence of the line profiles on pressure and temperature. If at distinct spectral positions absorption coefficients are changing from line wing absorption in the lower atmosphere to line centerlike absorption in the stratosphere, this is considered by a rough approximation. The known far wing scaling approximation is included, modified as special case. Comparison of infrared radiation flux densities in a standard atmosphere from this k distribution method with those from a line‐by‐line model leads to a difference about −1.8 W/m2 for the upward radiation flux density at the top of the atmosphere and −0.25 W/m2 for the downward directed at the surface. Cooling rate differences remain smaller than |0.1| K/d for the entire troposphere, but reach an intolerable 5 K/d at 50‐km height. In a second approach an extended version of the k distribution model is obtained by separating the spectral absorption coefficients into central and wing contributions. A multivariate frequency distribution which considers both strongly increases accuracy to less than 0.2% relative error for flux densities at all heights. And for cooling rates, a relative error of at most ±15% is found in the upper stratosphere.
Inverse theory for occultation measurements: 1. Spectral inversion
Authors: Kyrölä E. , Sihvola E. , Kotivuori Y. , Tikka M. , Tuomi T. , Haario H.
Published: February 2008

In this paper we investigate methods to solve the inverse problem for the optical occultation measurements from a planet‐orbiting satellite. Our emphasis is on the stellar occultation technique which offers a promising method to monitor the global vertical distributions of the crucial trace gases like ozone in the Earth's stratosphere. The occultation measurement principle is self‐calibrating because the star (or Sun) signal is also measured when the light is not attenuated by the atmosphere. This feature is crucial in developing the long‐term space‐borne monitoring systems of the Earth's trace gases. The inverse problem for an occultation measurement can be divided into a spectral inversion part and into a spatial inversion part. In the former problem we aim at finding the tangential column densities of the different gases which are involved in the attenuating of the starlight traversing through the atmosphere onto our detector. In the latter part we bring together measurements from different altitudes and geographical locations to construct the global distributions of the trace gases in the Earth's atmosphere. This paper deals with the first problem; the second problem will be treated in a subsequent paper.
Interactive coupling of a lake thermal model with a regional climate model
Authors: Hostetler S. W. , Bates G. T. , Giorgi F.
Published: February 2008

A one‐dimensional model of lake temperature, evaporation, and ice has been coupled in an interactive mode with the climate version of the National Center for Atmospheric Research/Pennsylvania State University regional (mesoscale) atmospheric model (MM4). The coupled MM4‐lake model makes possible high‐resolution simulations of climate in the proximity of large water bodies. Atmospheric variables required as input for the lake model are supplied by MM4 and simulated values of lake temperature, evaporation, and ice cover axe supplied to MM4 by the lake model. We have tested the coupled model system with a 60‐day, summertime simulation at Pyramid Lake, Nevada, and with a 10‐day, wintertime simulation of the North American Great Lakes and vicinity. Both simulations were conducted at a 60‐km resolution. Results from these tests indicate the coupled model system produces realistic simulations of lake temperature, evaporation, and ice cover and that the coupled system is applicable to simulations of regional climate change.
Interpretation of planetary radar observations: the relationship between actual and inferred slope distributions
Authors: McCollom Thomas M. , Jakosky Bruce M.
Published: February 2008

We examined the distribution of surface slopes of a variety of terrestrial surfaces by field measurement, representing surfaces formed by a wide range of processes, and compared the results to planetary radar data. Slope distributions of the measured surfaces differed considerably from the distributions assumed by accepted models of radar scattering. The rms slope values for the terrestrial surfaces ranged from 0° to 16.5°, compared to a range of rms slope values of <1° to about 10° for planetary surfaces as inferred from radar observations. While the great majority of planetary surfaces observed by radar have rms slope estimates in the lower end of this range (<5°), nearly all of the terrestrial surfaces we measured have rms slope values greater than 5°. We also used Hagfors' model of radar scattering to predict the return that would be expected from surfaces where two discrete surface types were present within the radar field of view and found that the shapes of the resulting slope distributions differed from those predicted by the Hagfors model for homogeneous surfaces. Additionally, the resulting best fit rms slope was a nonlinear combination of those of the pure surfaces, emphasizing the smoother surface. Together, these results suggest that current methods of determining surface roughness from radar may significantly underestimate the roughness of planetary surfaces and that the derived rms slope can best be used as a qualitative guide to the physical interpretation of actual surface properties.
Behavior of a perched crystal layer in a magma ocean
Authors: Morse S. A.
Published: February 2008

The thermal regime in the neighborhood of a crystal suspension embedded in a hot magma ocean is tied to the crystal‐plus‐liquid equilibrium temperature. Any such internal suspension will tend to melt because it overlies hot magma. Internal cumulates cannot form by crystal flotation because the crystals melt upon rising along the hot adiabat. Transient formation of a perched crystal layer is considered for the upper mande stage of a magma ocean, after fractionation has enriched the liquid in olivine components. Olivine crystals accumulate at a neutral point in the density contrast, aided by two‐phase (solid + liquid) convection in cold plumes from above. The cold plumes extract heat from the region of the neutral point. Crystals can survive there only if forced deposition exceeds the melting rate; the suspension cumulate is intrinsically ephemeral. Heat transfer occurs by addition of mass at the top and melting at the bottom of the layer. The liquid in the lower cell becomes more olivine‐rich and more Fe‐rich because of partial melting, increasing the buoyancy and thermal stability of the crystal layer. However, melting and physical disruption will still occur. Forcing must cease when a subsolidus lid to the magma ocean is established. The perched layer is now destroyed by melting, but the neutral point remains a site of heat transfer between lower and upper cells. The former neutral point becomes the original lithosphere/asthenosphere boundary. Subsequent diapirism leads to the internal generation of ultramafic and then mafic magmas.
Metal‐silicate thermochemistry at high temperature: Magma oceans and the “excess siderophile element” problem of the Earth's upper mantle
Authors: Capobianco Christopher J. , Jones John H. , Drake Michael J.
Published: February 2008

Recent theoretical considerations indicate that Earth should have been at least partly molten at the end of accretion and perhaps sufficiently to produce a magma ocean. Metal‐silicate partition coefficients applicable to a magma ocean (3000 K‐4000 K and up to 130 GPa) would be valuable to test such models against well‐known mantle siderophile element abundances, but they have not been measured. However, an extrapolation of existing low temperature data to these extreme conditions has recently been attempted [Murthy, 1991]. Murthy's results seem to account for many apparent excesses of siderophile elements in the mantle. We examined his extrapolation method and found it to be inconsistent with published temperature dependencies for metal‐silicate partition coefficients. We also attempted an extrapolation to magma ocean temperatures, based on known chemical behavior for several elements, and using published metal‐silicate partition coefficients. If the chemistry quantified by the low‐temperature data is applicable at high temperature, an important assumption, then our results indicate that high temperature alone will not help ameliorate the excess siderophile element problem of the upper mantle, in contrast to the conclusions of Murthy [1991]. For most elements, a modest increase in siderophile behavior is predicted with rising temperature if an iron‐wüstite redox buffer is paralleled. But long‐range extrapolation of experimental data containing even modest experimental errors is hazardous. Extrapolated high‐temperature partition coefficients can differ by orders of magnitude for a given element, even though the input from independent studies is consistent within quoted errors. Direct experimental measurements for at least some of the siderophile elements will be necessary to accurately assess siderophile element behavior in a magma ocean. The excess siderophile element problem of the Earth's upper mantle remains unsolved.
Paleoproductivity and carbon burial across the California Current: The multitracers transect, 42°N
Authors: Lyle Mitchell , Zahn Rainer , Prahl Frederick , Dymond Jack , Collier Robert , Pisias Nicklas , Suess Erwin
Published: February 2008

The Multitracers Experiment studied a transect of water column, sediment trap, and sediment data taken across the California Current to develop quantitative methods for hindcasting paleoproductivity. The experiment used three sediment trap moorings located 120 km, 270 km, and 630 km from shore at the Oregon/California border in North America. We report here about the sedimentation and burial of particulate organic carbon (Corg) and CaCO3. In order to observe how the integrated CaCO3 and Corg burial across the transect has changed since the last glacial maximum, we have correlated core from the three sites using time scales constrained by both radiocarbon and oxygen isotopes. By comparing surface sediments to a two‐and‐a‐half year sediment trap record, we have also defined the modern preservation rates for many of the labile sedimentary materials. Our analysis of the Corg data indicates that significant amounts (20–40%) of the total Corg being buried today in surface sediments is terrestrial. At the last glacial maximum, the terrestrial Corg fraction within 300 km of the coast was about twice as large. Such large fluxes of terrestrial Corg obscure the marine Corg record, which can be interpreted as productivity. When we corrected for the terrestrial organic matter, we found that the mass accumulation rate of marine Corg roughly doubled from the glacial maximum to the present. Because preservation rates of organic carbon are high in the high sedimentation rate cores, corrections for degradation are straightforward and we can be confident that organic carbon rain rate (new productivity) also doubled. As confirmation, the highest burial fluxes of other biogenic components (opal and Ba) also occur in the Holocene. Productivity off Oregon has thus increased dramatically since the last glacial maximum. CaCO3 fluxes also changed radically through the deglaciation; however, they are linked not to CaCO3 production but rather to changes in deepwater carbonate chemistry between 18 Ka and now.
Did glacio‐eustatic sea level drop trigger the Messinian salinity crisis? New evidence from Ocean Drilling Program Site 654 in the Tyrrhenian Sea
Authors: Kastens Kim A.
Published: February 2008

It has long been speculated that glacio‐eustatic sea level drop may have caused or contributed to the isolation and consequent desiccation of the Mediterranean in the late Miocene (the “Messinian salinity crisis”). Ocean Drilling Program site 654 on the Sardinia margin of the Tyrrhenian Sea is the first deep‐sea drill site to penetrate through upper Messinian evaporites into lower Messinian/upper Tortonian open marine sediments, and thus offers a unique opportunity to date the onset of the salinity crisis. A reexamination of the magnetostratigraphic, biostratigraphic, and stable isotope‐stratigraphic constraints on the preevaporite sediments of site 654 has yielded two possible ages for the contact between salinity crisis sediments and the underlying normal marine sediments. One magnetostratigraphic interpretation plus the biostratigraphically determined position of the Tortonian/Messinian boundary imply a date of about 6.2 Ma for the youngest presalinity crisis sediments. An alternative magnetostratigraphic interpretation plus the carbon isotope stratigraphy imply a date of about 5.2 Ma. The younger of these dates coincides with a δ18O spike in open ocean sediments [Keigwin, 1987], which is attributed to increased ice volume.
Oxygen isotope evidence for high‐salinity surface seawater in the Mid‐Cretaceous Gulf of Mexico: Implications for warm, saline deepwater formation
Authors: Woo Kyung‐Sik , Anderson Thomas F. , Railsback L. Bruce , Sandberg Philip A.
Published: February 2008

On the basis of textural and geochemical criteria we have identified a suite of calcitic molluscs and radiaxial fibrous calcite cements in Albian‐Cenomanian limestones from the Gulf Coast region that have not suffered significant diagenetic alteration. The δ18O values of these preserved components (−2.94 to −0.45 versus PDB) are our best estimate for original marine calcite in the rocks, and therefore should reflect the temperature and δ18O of coeval seawater. Isotopic paleotemperatures for average ice‐free seawater (δ18O = −1 per mil versus SMOW) are 14°–24°C, lower than present‐day mean annual temperatures for western Gulf of Mexico surface waters (23.5°–26.5°C) and estimates for mid‐Cretaceous subtropical seawater (T ≥ modern T). The oxygen isotopic composition of original marine calcite yield reasonable paleotemperatures if the 18O/16O ratio and hence the salinity of surface waters in the ancestral Gulf of Mexico were elevated due to evaporation. Applying a model for T‐S‐σt reconstructions from calcite δ18O values predicts salinities of 37–41 ppt and densities (as σt) of 25–26 for contemporaneous seawater. A comparison of model results for Tethyan surface waters to those for mid‐Cretaceous equatorial Pacific waters suggests that Tethyan waters were dense enough to sink through the pycnocline to considerable depths. Our results and interpretations support the proposal (based on other lines of evidence) that warm, saline waters originating in subtropical seas were an important component in deepwater formation during the mid‐Cretaceous.
Application of Ocean Models for the Interpretation of Atmospheric General Circulation Model Experiments on the Climate of the Last Glacial Maximum
Authors: Lautenschlager M. , Mikolajewicz U. , Maier‐Reimer E. , Heinze C.
Published: February 2008

We examined the response of our ocean models of physical circulation and geochemical tracer distributions on atmospheric glacial forcing. The CLIMAP Project Members (1981) sea surface temperature (SST) was taken directly as a boundary condition. The wind stress and freshwater flux were derived from the ice age response of an atmospheric general circulation model (AGCM). Near the surface, the ocean response in temperature and circulation reflects primarily the imposed glacial forcing fields. The simulated deep ocean response, especially in the North Pacific, is in conflict with the observational evidence. The glacial changes in the physical ocean circulation appear to be qualitatively as derived from sediment cores, but, probably, highly overestimated. The misfit between model and data is established clearly in the distribution of δ13C, where a direct comparison with sediment core data is possible. The simulated AGCM freshwater flux, which is linked for example by the evaporation to the imposed SST field, is suspected to be the most probable reason for this conflict.
The geophysical signature of terrestrial impact craters
Authors: Pilkington M. , Grieve R. A. F.
Published: February 2008

A major tool in the initial recognition and study of terrestrial impact craters, ∼20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and outlined with emphasis on its relation to the impact process and as an aid to the recognition of additional impact craters. The most common and conspicuous geophysical signature is a circular gravity low. For simple bowl‐shaped craters, gravity models indicate that the anomaly is largely due to the presence of an interior allochthonous breccia lens. In complex craters, modeling indicates that the main contribution to the gravity anomaly is from fractured parautochthonous target rocks in the floor of the crater. The gravity signature of both simple and complex crater forms can be modeled well, using known morphometric parameters of impact structures. The size of the gravity anomaly generally increases with increasing crater diameter reaching a maximum of ∼20–30 mGal at diameters D of ∼20–30 km. Further increases in D have a negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity signature of a simple low can be modified by target rock and erosional effects, and there is a tendency for larger complex structures ( D > 30 km) to exhibit a relative gravity high restricted to the crater center and extending out to <0.5D. The magnetic signature of craters is more varied. The dominant effect is a magnetic low due to a reduction in susceptibility. Large structures (D > 40 km) tend to exhibit central high‐amplitude anomalies, with dimensions of <0.5D, due to remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat, and chemical alteration. The few studies over craters involving electrical methods indicate resistivity lows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques, particularly reflection surveys, have provided details of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due to brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the structure. From the various geophysical techniques a set of general criteria can be established that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due to impact. Confirmation of an impact origin, however, is based on geologic evidence.
Rock magnetism and the interpretation of anisotropy of magnetic susceptibility
Authors: Rochette P. , Jackson M. , Aubourg C.
Published: February 2008

The conventional rules, derived from empirical and theoretical considerations, for the interpretation of anisotropy of magnetic susceptibility (AMS) in terms of microstructure and deformation are subject to numerous exceptions as a result of particular rock magnetic effects. Unusual relationships between structural and magnetic axes (so‐called inverse or intermediate magnetic fabrics) can occur because of the presence of certain magnetic minerals, either single domain magnetite or various paramagnetic minerals. When more than one mineral is responsible for magnetic susceptibility, various problems appear, in particular the impossibility of using anisotropy to make quantitative inferences on the intensity of the preferred orientation and consequently on strain. In ferromagnetic grains, AMS may also be influenced by the magnetic memory of the grains (including natural remanence). The effect of alternating field or thermal demagnetization on AMS is briefly discussed. Various rock magnetic techniques, specific to AMS interpretation, have to be developed for a better assessment of the geological significance of AMS data. These techniques mainly rely on measurements of susceptibility versus magnetic field and temperature, together with anisotropy of remanence.
Secular variation of the aurora for the past 500 years
Authors: Silverman S. M.
Published: February 2008

Knowledge of solar variability and its effects on the Earth is essential since the Sun affects almost every aspect of our lives. Direct observations of the Sun, usually of sunspots, with some continuity, exist only since about 1700. Understanding of long‐term solar variability must then depend on proxy data, such as visual auroral observations, measurements of magnetic activity, and the radiocarbon record. These also give us information on the interaction between the solar wind, the interplanetary field, and the terrestrial magnetosphere, as well as, for the radiocarbon record, heliospheric conditions. This paper uses a data base of visual auroral observations for a period of about 500 years, from 1450 to 1948, comprising about 45,000 observations, in addition to the well‐known sunspot series and the magnetic activity index aa, from 1868 to 1990. The secular variation of the aurora is examined and compared, where possible, to sunspot data and magnetic activity data. Blackman‐Tukey power spectra are used to determine periodicities. The study confirms the variability of the periodicities in both frequency and amplitude. In particular, the well‐known 11.1‐year cycle disappears during the Maunder minimum and at the end of the eighteenth and beginning of the nineteenth century. While the 11.1‐year period is normally strongly dominant for sunspots, other shorter periods become important, and even dominant, for auroras and magnetic activity. Consequently, the temporal behavior of these three variables differs. Prolonged solar activity minima are clearly evident. In addition to the known Spörer, Maunder, Dalton, and 1901–1913 minima, a previously unrecognized minimum about 1765 is clearly evident in the data. Comparison of the depth of these minima shows that the Dalton minimum may be the deepest, or at least rivals the Maunder minimum in importance. This minimum clearly deserves further study. Combining the polar data base with that of mid‐latitudes provides for the first time a globally comprehensive historical record of auroral occurrence. The data provide confirmation of the anticorrelation of auroral occurrence in the polar regions with sunspot activity, as a result of displacement of the auroral oval with changes in solar and magnetic activity. Assuming the validity of some current models of the solar origin of geomagnetic activity, the data provide a basis for understanding the variation over time of the general magnetic field of the Sun, in particular the polar field.
A simple method to determine evaporation duct height in the sea surface boundary layer
Authors: Musson‐Genon Luc , Gauthier Sylvie , Bruth Eric
Published: February 2008

A formulation to determine the evaporation duct height in the sea surface boundary layer is presented. This formulation is based upon the theory of similarity of Monin Obukhov by using analytical solutions currently used in the field of numerical weather prediction. The proposed solution is simple, coherent with the surface boundary layer parameterization used in the Meteo France and European Centre for Medium‐Range Weather Forecasts weather prediction models and gives good results when compared to more traditional methods.
A ray model for decimetric radiowave propagation in an urban area
Authors: Rossi J.‐P. , Levy A. J.
Published: February 2008

A ray‐tracing model involving multiple reflections and diffractions is proposed to predict wideband decimetric radio wave propagation in an urban area. A computer code was developed in which only buildings close to the vehicle are considered. Wall irregularities and electromagnetic properties of material are taken into account by a constant reflection factor, while diffraction attenuation is computed from geometrical theory of diffraction results. Simulated channel path losses and complex impulse responses are compared with some measurements performed in Paris at a 900‐MHz central frequency.
region real heights and their implications for MF radar‐derived wind and tidal climatologies
Authors: Namboothiri S. P. , Manson A. H. , Meek C. E.
Published: February 2008

region real height determinations for 2.2 MHz has been carried out using standard electron density (N(h)) profiles from Maeda's rocket observations and the International Reference Ionosphere (IRI) models for application to the Saskatoon (52°N, 107°W) MF radar. A comparative study of the virtual and real heights for the totally reflected signals, for both solar maximum and minimum conditions, reveals that these heights increase from solar maximum to minimum. The group retardations from rocket N(h) are found to be lower than that of the IRI models in all the seasons. During summer the noon time virtual heights are very similar for the IRI model and MF radar data, but during the winter the IRI virtual heights are considerably lower than the observed heights. The derived group retardations are added to the mean wind and tidal climatologies for Saskatoon. This enables the prediction of the heights up to which the winds and tidal data can be taken without any correction for group retardation.
Prebreakup geology of the Gulf of Mexico‐Caribbean: Its relation to Triassic and Jurassic rift systems of the region
Authors: Bartok Peter
Published: February 2008

A review of the prebreakup geology of west central Pangea, comprising northern South America, the Gulf of Mexico, and West Africa, combined with a study of the Mesozoic rift trends of the region confirms a relation between the rift systems and the underlying older grain of deformation. The prebreakup analysis focuses attention on the Precambrian, early Paleozoic, and late Paleozoic tectonic events affecting the region and assumes a Pindell fit. Two late Precambrian orogenic belts are observed in west central Pangea. Along the northern South American margin and Yucatan a paleo northeast trending Pan‐African aged fold belt is documented. A second system is observed along West Africa extending from the High Atlas to the Mauritanides and Rockelides. Similar aged orogenies in the Appalachians are compared. During the late Paleozoic, renewed orogenic activity, associated with the Gondwana‐Laurentia suture, affected large segments of west central Pangea. The general trend of the system is northeast‐southwest and essentially parallels the Guayana craton and West African and eastern North American cratons. Mesozoic rifling closely followed either the Precambrian trends or the late Paleozoic orogenic belt. The Triassic component focused along the western portions of the Gulf of Mexico continuing into eastern Mexico and western South America. The Jurassic rift trend followed along the separation between Yucatan and northern South America. At Lake Maracaibo the Jurassic rift system eventually overlaps the Triassic rifts. The Jurassic rift resulted in the “Hispanic Corridor” that permitted Tethyan and Pacific marine faunas to mix at a time when the Gulf of Mexico underwent continental sedimentation.
Early Permian transform margin development of the southern New England Orogen, eastern Australia (eastern Gondwana)
Authors: Aitchison Jonathan C. , Flood Peter G.
Published: February 2008

The southern New England orogen, part of the eastern margin of Gondwana, evolved from a zone of high‐angle plate convergence during the Carboniferous, into either a transform margin or a highly oblique‐convergent margin by the Early Permian. Fault‐bounded, narrow, elongate, rapidly filled sedimentary basins that developed along the Peel‐Manning fault system in the Early Permian provide support for this hypothesis. They are similar to sedimentary basins developed along strike‐slip fault zones worldwide. Strike‐slip dislocations provide the best explanation for juxtaposition of terranes with distinctly different geological histories and discontinuities in regional geological trends across these basins. A close spatial relationship also exists between occurrences of serpentinite‐matrix ophiolitic melange, the Peel‐Manning fault system, and related secondary faults. Early Permian transtension may have facilitated the diapiric rise of serpentinites, which originated from an older underlying ultramafic precursor, up these fault zones. Rise and Early Permian emplacement of the Bundarra Plutonic Suite, an S‐type batholith across which regional geological trends are offset, is also best accommodated in a model that involves a transtensional regime.
Evidence for extensional shear zones in the mantle, offshore Britain, and their implications for the extension of the continental lithosphere
Authors: Reston T. J.
Published: February 2008

Deep seismic profiles recorded in the seas around the British Isles have revealed bands of seismic reflections in the uppermost mantle. On the basis of their appearance, geophysical characteristics, and association with major crustal faults, these features are best interpreted as mantle shear zones. Furthermore, in the North Sea, the mantle reflectors appear to follow an extensional trend and, when interpreted as extensional shear zones, help explain the regional stratigraphy of the overlying basin. The crosscutting relationships between mantle reflections to the west of Scotland and a major strike‐slip fault, the Great Glen Fault (GGF), suggest that the mantle structure postdates the GGF, and hence is probably related to subsequent extension. Furthermore, kinematic analysis of lithospheric deformation to the north of Scotland is consistent with the structure observed on the seismic data if the Flannan mantle reflection is an extensional shear zone. Reconstruction of the lithosphere prior to Permian extension restores the Flannan to the edge of the Rockall Trough, a rift probably initiated in the Carboniferous. Thus the Flannan Fault may have developed during Carboniferous rifting. It is concluded that most of the mantle reflections are some form of extensional shear zone in the mantle, and the role of such structures during lithospheric extension is considered.
Variscan magnetic fabric overprinting in sedimentary and crystalline thrust sheets in the NE Bohemian Massif
Authors: Hrouda František
Published: February 2008

The magnetic fabric of pre‐Cambrian and Devonian crystalline rocks of the Hrubý Jeseník Mountains and of Lower Carboniferous sedimentary rocks of the Nízký Jeseník Mountains is predominantly deformational in origin. The orientations of the tectonic magnetic fabric elements are similar in all rocks and geological units investigated. At least one strong deformation phase affected both the sedimentary and crystalline complexes, overprinting the original depositional magnetic fabric in sedimentary rocks as well as earlier deformational magnetic fabrics in crystalline rocks. The overprinted magnetic fabric is probably of Variscan origin.
Tectonic role of active faulting in central Oregon
Authors: Pezzopane Silvio K. , Weldon Ray J.
Published: February 2008

Geologic and geodetic studies in California indicate that about 1 cm yr−1 of right‐lateral shear occurs across what has been referred to as the Eastern California Shear Zone. Northwest trending zones of dextral, sinistral, and normal faults splay eastward from the San Andreas system, continuing through the Mojave Desert, east of the Sierra Nevada, and northward along the Central Nevada and Walker Lane fault zones. Aerial photography, field investigations, and fault studies in southern and central Oregon, compiled with a comprehensive analysis of previous studies nearby, indicate that latest Pleistocene and Holocene fault activity is concentrated along four zones that stretch northward into the Cascade volcanic arc and across the northwestern edge of the Basin and Range Province. The Oregon zones appear to continue the activity in eastern California and northwestern Nevada northward and provide a connection to seismically active zones in southern and central Washington. Several techniques are applied to fault data from the Oregon zones in an attempt to estimate the overall direction and rate of motion across them. The orientations and styles of faults younger than middle Tertiary are used with models of oblique rifting to estimate that the motion of western Oregon is ∼N60° ± 20°W, relative to North America. Summation of geologic moment tensors from faults with latest Pleistocene and Holocene slip yields a direction ∼N90° ± 30°W at a rate of ∼0.5 mm yr−1. This result is a minimum since many fault scarps have not been preserved or recognized, and additional deformation is recorded as folding and tilting. Crustal strain associated with slip during 76 of the largest crustal earthquakes in the past 120 years located along this broad zone from northern California and Nevada, across Oregon, to Washington and Vancouver Island, indicates motions at rates of 3 ± 1 mm yr−1 in a direction N55° ± 10°W. Although the motion across central Oregon is much slower, its similarity in style with regions to the north and south suggests that the regional averages are meaningful. Oregon fault zones, taken together, may accommodate as much as 6 mm yr−1 oriented ∼N60° to 70°W. A tectonic model of fault activity reveals that this proposed shear zone through Nevada, Oregon, and Washington can account for 10% to 20% of the total Pacific‐North American transform motion and much of the lateral component of relative motion between the Juan de Fuca and North American plates.
Carbon dioxide and methane fluxes from drained peat soils, southern Quebec
Authors: Glenn Shannon , Heyes Andrew , Moore Tim
Published: February 2008

Fluxes of CO2 and CH4 were determined by a static chamber technique at eight drained swamp peatland sites, with crop and forest covers. Over a 6‐ month period (May ‐ October, 1991), CH4fluxes ranged from −5 to 7 mg CH4 m−2 d−1 and were not correlated with either soil temperature or water table position. Integrated seasonal emissions were −0.40 to 0.04 g CH4 m−2 over 147 days; the sites with a forest or grass cover were a small sink of CH4 whereas the sites with horticultural crops showed no significant flux. Laboratory incubations showed that the highest CH4 consumption rates (3 to 9 μg CH4 g−1 d−1) occurred in the least disturbed soils. The results, when compared with CH4 fluxes from nearby swamps which have been unaffected by drainage, suggest that drainage of temperate peatlands has reduced emissions of CH4 to the atmosphere by 0.6 ‐ 1 × 1012g CH4 yr−1. CO2 fluxes ranged from 0 to 16 g CO2 m−2 d−1 and were correlated with the seasonal pattern of temperature in the upper part of the soil profile. Integrated seasonal fluxes for the sites in which root respiration was an unimportant contribution were 0.6 ‐ 0.8 kg CO2 m−2 over 181 days. Aerobic laboratory incubations revealed CO2 production rates of 0.2 ‐ 1.4 mg CO2 g−1 d−1, an average of 5 times the rate under anaerobic conditions. Using bulk density and loss‐on‐ignition data, we found that the seasonal CO2 fluxes translate into surface lowering of the peat of about 2 mm yr−1, whereas the commonly observed lowering in these cultivated peatlands is 20 mm yr−1. These data suggest that processes other than direct oxidation, such as shrinkage and aeolian erosion, are the major contributor to the surface lowering of the peat.
Changes in the δ13C of surface water particulate organic matter across the subtropical convergence in the SW Indian Ocean
Authors: Francois Roger , Altabet Mark A. , Goericke Ralf , McCorkle Daniel C. , Brunet Christian , Poisson Alain
Published: February 2008

We have measured the carbon isotopic composition of particulate organic matter suspended in surface waters (POM) between 59°S and 30°S in the SW Indian Ocean during the austral summer. In an attempt to further document the pattern and causes of covariance between POC‐δ13C and [CO2aq], we concurrently measured surface water pCO2, temperature, salinity, nitrate concentration, POM concentration, chlorophyll a and the δ13C of total dissolved inorganic carbon. While we found the previously reported general negative correlation between POC‐δ13C and [CO2aq], we also observed a prominent maximum in POC‐δ13C in the region immediately north of the Subtropical Convergence, coinciding with a maximum in [POM] and chlorophyll a, and with a minimum in pCO2. The increase in POC‐δ13C between 59°S and the Subtropical Convergence is consistent with the trend expected if [CO2aq] were the main factor controlling the isotopic composition of POM. In contrast, data from the region north of the Subtropical Convergence clearly illustrate that POC‐δ13C can also vary independently of [CO2aq] as a 5 per mil decrease in POC‐δ13C was found in a region characterized by nearly constant [CO2aq]. We review several physiological factors which may account for these observations and discuss their implications for paleoceanographic reconstruction of [CO2aq] from the carbon isotopic composition of sedimentary organic matter.
Tree growth‐climate relationships at the northern boreal forest tree line of North America: Evaluation of potential response to increasing carbon dioxide
Authors: D'Arrigo Rosanne D. , Jacoby Gordon C.
Published: February 2008

Tree growth at the northern limit of the range of boreal forests is primarily limited by temperature‐related factors. Thus the position of this range limit, and the growth rates of trees along the northern forest border, may undergo significant change if predictions of enhanced greenhouse warming at northern latitudes are realized. In this paper we evaluate tree ring width and maximum latewood density chronologies of white spruce for three temperature‐sensitive tree line sites in northern North America: in the Brooks Range, Alaska, the Franklin Mountains, Northwest Territories, and Churchill, Manitoba. The ring width data, which more strongly integrate low‐frequency temperature trends than the density series, show overall enhanced growth and inferred warming during the period of anthropogenic increase in greenhouse gases. The recent growth at these sites equals or exceeds that which has occurred during earlier centuries of more clearly natural climate variability. When the ring width and density variations are estimated using temperature and precipitation data in principal components regression analysis, no substantial residual trends are detected which might require CO2 or other nutrient fertilization as an additional explanation for recent growth changes.