bluuebunny/crossref_metadata_2025 huggingface dataset:
Holocene paleointensities: Thellier Experiments on submarine basaltic glass from the East Pacific Rise
Authors: Pick Thomas , Tauxe Lisa
Published: February 2008

A complete description of the geomagnetic field requires both paleodirectional and paleointensity data. Although the paleointensity data base has grown steadily over the last three decades, it remains limited in time and space (the majority of data are of Holocene age and come from Europe). Furthermore, it has been difficult to assess the reliability of the paleointensity determinations. Here we present paleointensity determinations on precisely dated Holocene (0 to 3500 years old) submarine basaltic glass from the East Pacific Rise (15°S to 22°S). Although hysteresis measurements and low‐temperature isothermal remanent magnetization (IRM) acquisition experiments document a significant contribution of superparamagnetic grains, high blocking temperatures (above 400°C) and Curie temperatures between 490°C and 550°C indicate a single‐domain low‐Ti magnetite as the carrier of the remanent magnetization. This notion is further supported by the fact that saturation of remanence is achieved in moderate fields of about 200–300 mT. Submarine basaltic glass proves to be nearly ideal for paleointensity determinations in that it produces a high success rate for Thellier experiments. Twenty‐six out of 30 samples resulted in acceptable paleointenisty determinations. Multiple experiments on splits from the same sample show good reproducibility. The paleointensities for zero age glasses correspond precisely with the present field intensity at the site of recovery. The results of the remaining samples range from 16.7 to 53.9 μT with corresponding virtual axial dipole moments (VADM) of 3.61 ×1022 to 11.9× 1022 A m2. The intensities vary rapidly with time excluding a westward drifting nondipole component as the source for these fluctuations. Basaltic glass is frequently recovered in both dredged and drilled material from the ocean floor. The availability of submarine basaltic glass throughout the world oceans therefore holds great potential for a better distribution of paleointensity data through time and space.
Structures in segment boundary zones of the Lost River and Lemhi Faults, east central Idaho
Authors: Janecke Susanne U.
Published: February 2008

Geologic mapping and structural analysis of the Lost River and Lemhi normal faults in east central Idaho suggest that many rupture segment boundaries, previously inferred from geomorphic data coincide with two classes of structural complexities: (1) zones of distributed faults and steps or jogs in the range front faults, and (2) intersections with preexisting Eocene to Oligocene(?) cross faults at 10–15 km depth. Segment boundaries contain as many as 13 fault splays and are as much as 13 km long and 5–6 km wide (in plan view). In contrast, simple fault traces characterize the range front faults in the interiors of segments. Thus segment boundaries of normal faults are broad zones of intersecting faults, not discrete points. The structural data further imply that segment boundaries contain large volumes of fractured and faulted rock and that zones of distributed normal faults have influenced rupture propagation and arrest along these two normal faults. Intersections of preexisting cross faults and range front faults may also have influenced the locations of rupture boundaries along the Lost River and Lemhi faults. Downdip projections of preexisting Eocene to Oligocene(?) normal faults in the footwall of the Lost River and Lemhi faults may coincide with four or five segment boundary zones at about 15 km depth, the depth at which large earthquakes nucleate in the Basin and Range province. Structural analyses also show that some cross faults, which coincide with segment boundary zones at the surface, are distant from these zones at depth. Conversely, some potentially significant structures present at depth are far removed from the segment boundary zone at the surface. Thus a close spatial relationship between cross faults and segment boundary zones at the surface cannot prove or disprove a genetic relationship between these features because the near‐surface geometry of segment boundary zones may differ dramatically from the subsurface geometry of the zones. Where cross faults define segment boundaries, the enhanced fracture toughness of preexisting fault planes and their damage zones may impede or arrest rupture on younger crosscutting faults.
Fluid evolution in submarine magma‐hydrothermal systems at the Mid‐Atlantic Ridge
Authors: Kelley Deborah S. , Gillis Kathryn M. , Thompson Geoff
Published: February 2008

Fluid inclusions in a suite of gabbro, quartz‐breccia, and metabasalt samples recovered from the MARK area on the Mid‐Atlantic Ridge are the product of a complex hydrothermal history involving late stage magmatic fluids at temperatures >700°C and penetration by modified seawater at 300–400°C. The evolution of volatiles during the early stages of solidification and cooling of magma bodies near the ridge‐transform intersection is marked by exsolution of a CO2 fluid, entrapped within primary inclusions in fluorapatites. Attendant with progressive melt fractionation, residual evolved melts reached water saturation, and locally, supercritical CO2+H2O+NaCl±Fe brines (>50 wt % NaCl) and cogenetic H2O+CO2‐rich vapors (1–2 wt % NaCl) were exsolved as immiscible phases. Concomitant or subsequent fracturing, perhaps in response to volatile exsolution from the melts, allowed migration of these fluids along microfracture networks at >700°C. Trondhjemitic‐hosted inclusions, which homogenize by halite dissolution, indicate that the last fluids exsolved from the melts may have been 35–40 wt % brines. The transition from magmatic to seawater‐dominated hydrothermal conditions in the gabbros is marked by initial penetration of lower salinity fluids (1–7 wt % NaCl) at temperatures in excess of 400°C, with the general cessation of fluid flow occurring at minimum temperatures of ≈ 250°C. The relative enrichment and depletion of NaCl with respect to seawater in these fluids may record supercritical phases separation of seawater or boiling of hydrothermal fluids enriched in NaCl. Migration along microfracture networks of CH4‐rich, 350°C fluids, may reflect deeper seated hydrothermal processes involving hydration of underlying mantle material in response to fluid flow along deeply penetrating fault systems. In shallow crustal rocks, circulation of seawater‐derived fluids occurred at temperatures up to 400°C, with subsequent collapse of the active hydrothermal system at minimum temperatures of 200–250°C. In fault‐related upflow zones, multiple hydrothermal pulses involving 180–340°C and 3.5–10 wt % NaCl fluids, pervasively altered bounding wall rocks, forming chlorite‐rich, pyrite‐ and chalcopyrite‐bearing breccias. At shallow crustal depths, fluids reached temperatures of 150–300°C and contained salinities of 3.8–6.9 wt % NaCl. Following collapse of the axial‐related hydrothermal system, the plutonic and shallow crustal rocks were uplifted and emplaced as allocthonous blocks attending formation of the ridge‐transform intersection massif.
Normal faulting and the topographic roughness of mid‐ocean ridge flanks
Authors: Malinverno Alberto , Cowie Patience A.
Published: February 2008

The ocean floor on the flanks of mid‐ocean ridges is covered by abyssal hills, topographic features elongated perpendicularly to the direction of relative plate motion. These topographic features are interpreted as normal fault blocks and/or volcanic constructions that originated near the ridge axis and were later rafted onto the ridge flanks by seafloor spreading. The purpose of this paper is to quantify the topographic roughness of a profile perpendicular to the strike of a number of normal faults, given a fault population and a mechanical model for the response of the lithosphere to faulting. We obtain expressions for the variation in root‐mean‐square roughness with profile length and for the power spectral density of a profile given three parameters: a fault density (number of faults per unit length crossed by the profile), an average squared fault scarp height, and a characteristic length of flexure. To keep matters simple, we make a number of assumptions and approximations, namely, that the lithosphere behaves as an elastic plate, that faults have an infinite length and a vertical dip, that the response of topography to a number of faults is simply the sum of the responses to each fault, and that faults have random locations and scarp heights independently chosen from some statistical distribution. The theory predicts that the roughness‐length relationship/power spectral density should follow power laws for scales/wavelengths less than a characteristic scale proportional to the length scale of flexure. We compare the predictions of the theory with actual measurements of mid‐ocean ridge flank roughness, and find good first‐order agreement. In particular, we use independent estimates of fault densities, average squared fault scarp heights, and flexural length scales to predict the topographic roughness of the East Pacific Rise, and we find that normal faulting can explain all the observed topographic roughness. Nevertheless, there are some differences between predictions and observations. These differences are likely to be due to processes other than faulting that create topographic relief (e.g., volcanism) and to spatial correlations of fault scarp heights. Despite these shortcomings, the approach presented here provides a first step in understanding the topographic roughness signal by quantifying the contribution of the geological processes that generate surface relief.
Petrology and geochemistry of the Galápagos Islands: Portrait of a pathological mantle plume
Authors: White William M. , McBirney Alexander R. , Duncan Robert A.
Published: February 2008

We report new major element, trace element, isotope ratio, and geochronological data on the Galápagos Archipelago. Magmas erupted from the large western volcanos are generally moderately fractionated tholeiites of uniform composition; those erupted on other islands are compositionally diverse, ranging from tholeiites to picritic basanitoids. While these volcanos do not form a strictly linear age progressive chain, the ages of the oldest dated flows on any given volcano do form a reasonable progression from youngest in the west to oldest in the east, consistent with motion of the Nazca plate with respect to the fixed hotspot reference frame. Isotope ratios in the Galápagos display a considerable range, from values typical of mid‐ocean ridge basalt on Genovesa (87Sr/86Sr: 0.70259, ϵNd: +9.4, 206pb/204Pb: 18 44), to typical oceanic island values on Floreana (87Sr/86Sr: 0.70366, ϵNd: +5.2, 206pb/204Pb: 20.0). La/SmN ranges from 0.45 to 6.7; other incompatible element abundances and ratios show comparable ranges. Isotope and incompatible element ratios define a horseshoe pattern with the most depleted signatures in the center of the Galápagos Archipelago and the more enriched signatures on the eastern, northern, and southern periphery. These isotope and incompatible element patterns appear to reflect thermal entrainment of asthenosphere by the Galápagos plume as it experiences velocity shear in the uppermost asthenosphere. Both north‐south heterogeneity within the plume itself and regional variations in degree and depth of melting also affect magma compositions. Rare earth systematics indicate that melting beneath the Galápagos begins in the garnet peridotite stability field, except beneath the southern islands, where melting may occur entirely in the spinel peridotite stability field. The greatest degree of melting occurs beneath the central western volcanos and decreases both to the east and to the north and south. Si8.0, Fe8.0, and Na8.0 values are generally consistent with these inferences. This suggests that interaction between the plume and surrounding asthenosphere results in significant cooling of the plume. Superimposed on this thermal pattern produced by plume‐asthenosphere interaction is a tendency for melting to be less extensive and to occur at shallower depths to the south, presumably reflecting a decrease in ambient asthenospheric temperatures away from the Galápagos Spreading Center.
Statistical physics model for the spatiotemporal evolution of faults
Authors: Cowie Patience A. , Vanneste Christian , Sornette Didier
Published: February 2008

A statistical physics model is used to simulate antiplane shear deformation and rupture of a tectonic plate with heterogeneous material properties. Rupture occurs when the chosen state variable reaches a threshold value. After rupture, broken elements are instantaneously healed and retain the original material properties. We document the spatiotemporal evolution of the rupture pattern in response to a constant velocity boundary condition. A fundamental feature of this model is that ruptures become strongly correlated in space and time, leading to the development of complex fractal structures. These structures, or “faults,” are simply defined by the loci where deformation accumulates. Repeated rupture of a fault occurs in events (“earthquakes”) which themselves exhibit both spatial and temporal clustering. Furthermore, we observe that a fault may be active for long periods of time until the locus of activity spontaneously switches to a different fault. The formation of the faults and the temporal variation of rupture activity is due to a complex interplay between the random small‐scale structure, long‐range elastic interactions, and the threshold nature of rupture physics. The characteristics of this scalar model suggest that spontaneous self‐organization of active tectonics does not result solely from the tensorial nature of crustal deformation; that is, kinematic compatibility is not required for complex fault pattern formation. Furthermore, the localization of the deformation is a dynamical effect rather than a consequence of preexisting structure or preferential weakening of faults compared to the surrounding medium. We present an analysis of scaling relationships exhibited by the fault pattern and the earthquakes in this model.
Self potential generated by two‐phase flow in a porous medium: Experimental study and volcanological applications
Authors: Antraygues Philippe , Aubert Maurice
Published: February 2008

In order to characterize the relationships between self‐potential generation and hydrothermal convection, laboratory measurements of electric potential and temperature are made along a vertical cylindrical column of porous material where a two‐phase flow (wet steam) occurs. For steady state convection, the vertical distributions of vapor and water flow rates are calculated from thermal balance. At the initiation of convection, a positive electrical charge flux is related to the convective front. For isothermal and steady state columns, a positive electric potential gradient is observed along the vapor flow direction. These electric potentials are mainly a function of the vapor flow rates and of the medium permeability. A sudden and large increase in the vapor flow rate and in the volume fraction of vapor can induce a large and long‐lived increase in the potential differences along the vapor flow direction. An electrokinetic effect related to the saturated vapor flow is the best candidate for this electric potential generation. The experimental results obtained in the present study are applied to self‐potential generation in rising two‐phase convective cells on active volcanoes. The observed positive self‐potential anomalies close to active fissures depend on the electrical charge flux related to the upward saturated vapor flow. These results also demonstrate the value of self‐potential monitoring in the early stages preceding a volcanic eruption.
Passive microwave remote sensing of thin sea ice using principal component analysis
Authors: Wensnahan Mark , Maykut Gary A. , Grenfell Thomas C. , Winebrenner Dale P.
Published: February 2008

Time sequences of surface based measurements of passive microwave emission from growing saline ice reported by Wensnahan et al. (1993) are used to explore the possibility of developing a satellite based sea ice concentration algorithm which solves for the presence of thinner ice. It is shown that two classes of thinner ice can be distinguished from mixtures of open water (OW), first‐year (FY) ice, and multiyear (MY) ice. The two classes do not necessarily correspond to specific World Meteorological Organization ice types; rather, newly formed ice represents a brief transition spectrum between OW and thin ice. Newly formed ice appears to be optically thick at 37 and 90 GHz and has a relatively dry surface. The thin ice spectrum occurs when the ice is greater than 4 cm thick and appears to result from the accumulation of brine at the surface of the ice. Thin ice has a relatively stable spectrum characterized by high brightness temperatures, a near‐zero spectral gradient at vertical polarization, and a large difference between vertical and horizontal polarizations. Supervised principal component analysis (PCA) was done of laboratory data using 10 channels of passive data: vertical and horizontal polarization at 6.7, 10, 19, 37, and 90 GHz. Analyses were also done on subsets of the laboratory data at 6.7 to 37 GHz as well as 19 to 90 GHz, representing the scanning multichannel microwave radiometer (SMMR) and special sensor microwave imager (SSM/I) satellite frequencies, respectively. Using all of the channels or the SMMR subset makes it possible to solve for mixtures of OW and FY, MY, newly formed and thin ice but with large errors. However, any four of these scene types can be distinguished with reasonable accuracy. The SSM/I frequencies allow determination of at most four of these scene types but with moderate errors. PCA was used in a case study of SSM/I data from the Bering Sea for April 2, 1988. Winds from the north formed thin ice areas which the NASA Team algorithm interprets as large amounts of OW and MY ice. With PCA, these same areas are interpreted as 20–30% OW near the lee shores but otherwise as consisting almost entirely of thin ice. We conclude that thin ice can be detected using satellite data. However, questions remain as to how the thin ice spectrum varies with environmental conditions, how it evolves to that of FY, and how this evolution affects the predicted concentrations of thin ice.
Eddies in the North Brazil Current retroflection region observed by Geosat altimetry
Authors: Didden Norbert , Schott Friedrich
Published: February 2008

Mesoscale fluctuations in the western tropical Atlantic are analyzed in Geosat altimetry sea surface height (SSH) and geostrophic velocity anomalies to investigate the role of eddies in the North Brazil Current (NBC) retroflection zone. The detachment of anticyclonic eddies from the NBC retroflection is observed during November through January, when the NBC retroflection into the North Equatorial Countercurrent (NECC) weakens and finally breaks down. These eddies are traced over more than 2 months between 50° and 60°W on their way toward the Caribbean, at average speeds of 15 cm s−1. In one case an apparent merger of two anticyclonic eddies occurs, one detached from the retroflection zone and one detached from the NECC. Cyclonic eddies are also observed but are generally less persistent. Mesoscale SSH variance just west of the retroflection increases by a factor of 2 from early summer to winter, mainly because of the anticyclonic eddies. Interhemispheric water mass transfer associated with the eddy flux out of the NBC retroflection may amount to an average transport of 3 Sv.
An Arctic source for the great salinity anomaly: A simulation of the Arctic ice‐ocean system for 1955–1975
Authors: Häkkinen Sirpa
Published: February 2008

A fully prognostic Arctic ice‐ocean model is used to study the interannual variability of the sea ice during the period 1955–1975 and to explain the large variability of the ice extent in the Greenland and Iceland seas during the late 1960's. In particular, the model is used to test the conjecture of Aagaard and Carmack (1989) that the Great Salinity Anomaly (GSA) was a consequence of the anomalously large ice export in 1968. The objective here is to explore the high‐latitude ice‐ocean circulation changes due to wind field changes. In the simulations the ice extent in the Greenland Sea increased during the 1960's, reaching a maximum in 1968, as observed, and maxima in ice extent were always preceded by large pulses of ice export through the Fram Strait. The ice export event of 1968 was the largest in the simulation, being about twice as large as the average and corresponding to 1600 km3 of excess fresh water. The simulated upper water column in the Greenland Sea has a salinity minimum in the fall of 1968, followed by very low winter salinities. The simulations suggest that, besides the above average ice export to the Greenland Sea, there was also fresh water export to support the larger than average ice cover. Three low‐salinity anomalies, which are created by the variability in ice production/melt, exited through the Fram Strait in 1963–1965, 1966, and 1967–1969, the later two events being associated with a net freshwater export of about 900 km3.The total simulated freshwater input of 2500 km3 to the Greenland Sea compares well with the estimated total freshwater excess of the GSA of about 2200 km3 as it passed through the Labrador Sea (Dickson et al., 1988). Considering the uncertainties in the model, it is possible that the ice export could account for even larger portion of the freshwater excess. However, the main conclusion is that these model results show the origin of the GSA to be in the Arctic, as suggested by Aagaard and Carmack (1989) and support the view that the Arctic may play an active role in climate change.
Experimental determination of the diffusion coefficient of dimethylsulfide in water
Authors: Saltzman E. S. , King D. B. , Holmen K. , Leck C.
Published: February 2008

Estimates of the sea‐to‐air flux of dimethylsulfide (DMS) are based on sea surface concentration measurements and gas exchange calculations. Such calculations are dependent on the diffusivity of DMS (DDMS), which has never been experimentally determined. In this study the diffusivity of DMS in pure water was measured over a temperature range of 5°–30°C. The measurements were made using a dynamic diffusion cell in which the diffusing gas flows over one side of an agar gel membrane and the inert gas flows over the other side. The diffusion coefficient can be estimated from either time dependent or steady state analysis of the data, with an estimated uncertainty of less than 8% (1σ) in each measurement. A best fit to all the experimental results yields the equation DDMS (in cm2 sec−1) = 0.020 exp (−18.1/RT), where R = 8.314 × 10−3 kJ mole−1 K−1 and T is temperature in kelvin. The values of DDMS obtained in this study were 7–28% larger than estimates from the empirical formula of Hayduk and Laudie (1974) which has previously been used for DMS in gas exchange calculations. Applying these values to seawater results in an increase of less than 5% in the global oceanic flux of DMS.
Development, persistence, and variability of upwelling filaments off the Atlantic coast of the Iberian Peninsula
Authors: Haynes R. , Barton E. D. , Pilling I.
Published: February 2008

The development, persistence, and variability of upwelling filaments off the Atlantic coast of the Iberian Peninsula are examined by means of advanced very high resolution radiometer infrared imagery observed between 1982 and 1990. These observations indicate that the regime is broadly similar to that observed in the California Current system and is closely related to the large scale wind climatology of the subtropical gyre. Upwelling generally starts in late May or early June and persists until late September or early October. In May or June, a narrow band of cold water of quite uniform width is observed along much of the west coast of the Iberian Peninsula. This band has a “fringed” appearance; that is it consists of many narrow “fingers” of cool water extending 20–30 km offshore. The major filament structures generally do not begin to form until late July or August. The filaments appear first as bulges in the upwelling front. These bulges grow offshore to form filaments that reach their maximum length (200–250 km) in September. The lengths of the filaments gradually decrease until the filaments become relatively rare in late October.

Typically, five or six fully developed filaments are observed off the Iberian Peninsula late in the upwelling season. Most of these are associated with major topographic features of the region, in particular the large capes which are common to the north and south of the peninsula. It is therefore postulated that the dominant dynamical processes related to filament formation off Iberia is topographic forcing. The exceptions are two major filaments commonly observed along the more regular coastline of northern Portugal. It is hypothesized that these filaments are formed by flow instabilities resulting in meandering of the southward flowing jet. These instabilities may possibly be initiated by the large capes of northern Spain.
Atmospheric‐hydrospheric mechanisms of climate anomalies in the western equatorial Indian Ocean
Authors: Hastenrath Stefan , Nicklis Achim , Greischar Lawrence
Published: February 2008

Atmosphere‐ocean mechanisms of rainfall anomalies at the coast of eastern Africa are studied using long‐term ship observations in the Indian Ocean, surface current measurements, subsurface casts, upper air analyses by the European Centre for Medium Range Weather Forecasts, rain gauge series in eastern Africa and India, and an index of the Southern Oscillation (SO), the high‐SO phase being defined by anomalously high/low pressure at Tahiti/Darwin. The causalities of precipitation anomalies at the coast of eastern Africa differ for the two rainy seasons centered on April‐May and October‐November, and only the latter is strongly related to the SO. In the high‐SO phase, April‐May pressure is low over the entire Indian Ocean domain, whereas in October‐November, pressure is high in the west and low in the east. Concomitantly, surface waters are anomalously cold in the west, and strong westerlies sweep the equatorial zone of the Indian Ocean. Eastern African rainfall anomalies are related to the SO through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern Africa. (1) Equatorial westerly winds are conducive to lower tropospheric divergence over equatorial East Africa, and in the high‐SO phase these are accelerated, especially in October‐November, owing to the anomalous eastward pressure gradient. (2) The equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere, which entails cold‐water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds. (3) In addition, cold‐water anomalies in the western Indian Ocean, most pronounced in October‐November during the high‐SO phase, also suppress convection. (4) In the high‐SO phase, the Indian summer monsoon tends to be strong, leaving behind an anomalously cold western Indian Ocean, which in turn feeds into mechanisms 1 to 3. The eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern Africa.
Temperature and salinity beneath Ronne Ice Shelf, Antarctica
Authors: Nicholls Keith W. , Jenkins Adrian
Published: February 2008

Salinity and temperature measurements have been made beneath Ronne Ice Shelf, Antarctica. Access to the seawater was gained by hot‐water drilling through 562 m of ice at a site 300 km from the ice front. The ice column is composed of 516 m of meteoric‐origin ice, underlain by 31 m of consolidated saline ice, with a further 15 m of unconsolidated slush at the base. The 360‐m‐deep water column beneath the ice shelf consists broadly of a 210‐m layer of Ice Shelf Water (ISW) at a potential temperature of −2.30°C and salinity 34.53, overlying a 100‐m‐thick layer of modified Western Shelf Water (−2.03°C, 34.64). The layers are separated by a weak 50‐m‐thick pycnocline. Isotopic analyses of water samples support this interpretation of the water types. Comparison of the data with published oceanographic observations from the shore lead indicate that the source of the water is either the western Berkner shelf or north of the ice front in the Ronne Depression. The measurements from the drill site broadly match predictions from a simple plume model of the sub‐ice‐shelf water circulation. In that model the basal slope of the ice shelf plays a central role in determining the properties of the ISW plume. With the exception of the upper few tens of meters the temperature and salinity of the entire water column vary by about 0.04°C and 0.03 on tidal timescales, and there is evidence for a longer‐term drift, presumably connected with shifts in the larger‐scale circulation. There is also much variability from internal wave activity. The salinity and deuterium budgets indicate that the meltwater fraction in the ISW observed at the drill site was between 5.3 and 6.5‰, in reasonable agreement with values previously found for ISW in the open ocean.
Simultaneous ozone and polar stratospheric cloud observations at South Pole station during winter and spring 1991
Authors: Rosen James M. , Kjome Norman T. , Oltmans Samuel J.
Published: February 2008

Simultaneous polar stratospheric cloud (PSC) and ozone measurements were made over South Pole Station using a two‐wavelength backscattersonde. This instrument produces aerosol profiles similar to those obtained with a ground‐based lidar system but with higher vertical resolution. In one sounding, depolarization of the PSCs was also measured. The backscattersondes were supplemented with occasional frost point soundings. The measurements made before the appearance of PSCs do not show clear evidence of particle deliquescence, suggesting that the background sulfate particles may be frozen solids rather than liquids. PSCs began appearing at ∼20 km when the temperature at that altitude dropped to −80°C (193 K). Initially, there was apparent evidence of supersaturation (with respect to nitric acid trihydrate) associated with some type I PSCs, while other examples indicated that the condensation of nitric acid was in quantitative agreement with that expected from the saturation vapor pressure and available nitric acid vapor. The apparent supersaturated layers (which occurred within the first 2 weeks of the onset of PSCs) can alternatively be interpreted as denitrified regions. The wavelength dependence of the backscatter is used to deduce rough particle sizes, and in particular, type Ia and Ib population types can be readily identified by the backscattersonde when not occurring as mixed systems. The mode radius of the first observed PSCs of the season was ∼0.5 μm. In the polarization sensitive sounding, two varieties of type I PSCs were observed, one of which exhibited significant depolarization and another which produced very little depolarization. This observation would be consistent with the classification of types Ia and Ib, respectively. At the precise time that sunlight was returning to the stratosphere near South Pole Station, a strong inverse correlation in the structure of PSCs and ozone mixing ratio was observed. Using trajectory analysis, it is argued that the effect is probably the result of chemical depletion rather than transport processes. This chance observation is consistent with enhanced ozone depletion occurring in association with sunlit PSCs during the early spring.
Long‐term change in the biogeochemical cycling of atmospheric selenium: Deposition to plants and soil
Authors: Haygarth P. M. , Cooke A. I. , Jones K. C. , Harrison A. F. , Johnston A. E.
Published: February 2008

Retrospective analysis of archived soil and herbage samples from Rothamsted Experimental Station, southeast England, has determined the long‐term changes in selenium deposition over the last century. Three out of four soils (those under permanent grassland, or growing wheat and barley) accumulated Se at a rate of circa 0.15% yr−1 (rate based on Se concentration, normalized to the earliest date circa 100 years earlier), with a net flux in the order 60–90 μg m−2 yr−1. The increase in soil growing root crops was smaller, with an increase of only 0.07% yr−1, possibly reflecting larger volatilization losses from this soil. Herbage samples were sensitive to changes in air composition. In the earlier half of the twentieth century there was an increase in the selenium content of herbage, probably from increased atmospheric deposition following increased use of fossil fuels. However, following the Clean Air Act (1956) the atmospheric loading of Se at this UK site appears to have declined, with contemporary Se concentrations in herbage considerably lower than they were in the 1970s, probably reflecting a change in fossil fuel usage from coal to oil and gas. The atmosphere has been a significant source of Se to plants and therefore grazing livestock. If the decline in the atmospheric input of selenium to herbage continues, selenium deficiency in livestock may become more prevalent in areas where soil concentrations are marginal.
The poleward dispersal of Mount Pinatubo volcanic aerosol
Authors: Trepte Charles R. , Veiga Robert E. , McCormick M. Patrick
Published: February 2008

Using the SAGE II 1‐μm stratospheric aerosol extinction ratio observations, the dispersal of Mount Pinatubo aerosol within two transport regimes during the first 10 months after the eruption is displayed in meridional cross sections. Maximum aerosol extinction ratio values were contained in a tropical reservoir bounded by strong gradients in the subtropics. The detrainment of aerosol from the equatorial reservoir occurred in episodic synoptic scale events and two examples are presented: (1) transport into the boreal summer hemisphere in a lower regime just above the tropopause associated with upwardly decaying tropospheric disturbances and (2) dispersal into the austral winter hemisphere in an upper transport regime near 30 hPa associated with planetary wave activity in the southern subtropics.
A two‐dimensional model with coupled dynamics, radiation, and photochemistry: 1. Simulation of the middle atmosphere
Authors: Ko Malcolm K. W. , Schneider Hans R. , Shia Run‐Lie , Weisenstein Debra K. , Sze Nien‐Dak
Published: February 2008

Spatial and seasonal distribution of ozone and other trace gases are simulated using the interactive two‐dimensional model of the stratosphere (Schneider et al., 1989) updated to include full gas phase chemistry. The model consists of a primitive equation dynamics module, a full radiative transfer scheme, and a comprehensive gas phase chemistry module. The circulation is derived from heating rates in the stratosphere that are calculated using model‐generated ozone. In the troposphere, parameterized heating rates are adopted. The eddy momentum flux divergence in the zonal mean momentum equation is given by the eddy fluxes of potential vorticity. Eddy fluxes of potential vorticity and tracers are parameterized using a set of predetermined diffusion coefficients. The adopted values for Kyy, which are based on values derived by Newman et al. (1988), show a hemispherical asymmetry in that the values in the lower stratosphere are consistently smaller in the southern hemisphere. The asymmetry in Kyy and in the parameterization of the tropospheric heating rate results in an asymmetry in the circulation giving rise to unique signatures in the trace gas distributions. The model successfully simulates the observed asymmetry in the column abundance of the springtime ozone maxima between northern and southern hemisphere. Results for other trace gases are in agreement with the gross observed features although specific differences exist.
Biogenic sulfur aerosol in the Arctic troposphere: 2. Trends and seasonal variations
Authors: Li Shao‐Meng , Barrie Leonard A. , Sirois Alain
Published: February 2008

An 11‐year record of tropospheric aerosol methanesulfonate (MSA) in the Canadian high Arctic at Alert, Northwest Territories, from 1980 to 1991 shows marked seasonal and long‐term variations. By using spectral‐analysis techniques, the seasonal cycles and long‐term variations have been quantified. There are two distinctly different seasonal peaks with levels of 12–23 ng m−3 in early May and 6–18 ng m−3 in July‐August. Over the 11.3 years, the amplitudes of the MSA seasonal cycles and therefore the annual means of MSA concentrations decreased significantly by 33% (3% annually). So far there is no evidence of experimental artifact in this trend, but such a possibility will be further examined. Analysis of MSA monthly anomalies suggests that in spring they are weakly but significantly related to sea surface temperature anomalies (SSTA) in the North Atlantic west of the coast of continental Europe. In summer, MSA monthly anomalies are significantly correlated with SSTA in oceanic regions further north in the North Atlantic off the coast of Norway and in the northeast North Pacific.
Numerical simulations of tropical cyclone‐ocean interaction with a high‐resolution coupled model
Authors: Bender Morris A. , Ginis Isaac , Kurihara Yoshio
Published: February 2008

The tropical cyclone‐ocean interaction was investigated using a high‐resolution tropical cyclone ocean coupled model. The model design consisted of the NOAA Geophysical Fluid Dynamics Laboratory tropical cyclone prediction model which was coupled with a multilayer primitive equation ocean model. Coupling between the hurricane and the ocean models was carried out by passing into the ocean model the wind stress, heat, and moisture fluxes computed in the hurricane model. The new sea surface temperature (SST) calculated by the ocean model was then used in the tropical cyclone model. A set of idealized numerical experiments were performed in which a tropical cyclone vortex was embedded in both easterly and westerly basic flows of 2.5, 5, and 7.5 m s−1 with a fourth experiment run with no basic flow specified initially. The profile of the tangential wind for Hurricane Gloria at 1200 UTC 22, September 1985 was used as the initial condition of the tropical cyclone for each of the experiments. The model ocean was initially horizontally homogenous and quiescent. To clarify the impact of the ocean response to the hurricane's behavior, analogous experiments were also carried out with the SST kept constant (control cases). The experiments indicated that the cooling of the sea surface induced by the tropical cyclone resulted in a significant impact on the ultimate storm intensity due to the reduction of total heat flux directed into the tropical cyclone above the regions of decreased SST. The sea surface cooling produced by the tropical cyclones was found to be larger when the storms moved slower. In the experiments run without an initial basic flow, the maximum SST anomaly was about −5.6°C with a resulting difference in the minimum sea level pressure and maximum surface winds of 16.4 hPa and −7 m s−1, respectively. In contrast, in the experiments run with the 7.5 m s −1 basic flow, the maximum SST anomalies ranged from about 2.6° to 3.0°C with a difference in the minimum sea level pressure and maximum surface winds of about 7.3 hPa and −2.7 m s−1. The tropical cyclone‐ocean coupling significantly influenced the storm track only for the case with no basic flow and the 2.5 m s−1 easterly flow. In these cases the storm with the ocean interaction turned more to the north and east (no basic flow) or the north (2.5 m s−1 easterly flow) of the experiments with constant SST. In the first case, the storm by 72 hours was located over 70 km to the east‐southeast of the control case. A possible explanation for this track deviation is related to a systematic weakening of the mean tangential flow at all radii of the storm due to the interaction with the ocean and resulting alteration of the beta drift.
Fourier transform‐infrared studies of thin H2SO4/H2O films: Formation, water uptake, and solid‐liquid phase changes
Authors: Middlebrook Ann M. , Iraci Laura T. , McNeill Laurie S. , Koehler Birgit G. , Wilson Margaret A. , Saastad Ole W. , Tolbert Margaret A. , Hanson David R.
Published: February 2008

Fourier transform‐infrared (FTIR) spectroscopy was used to examine films representative of stratospheric sulfuric acid aerosols. Thin films of sulfuric acid were formed in situ by the condensed phase reaction of SO3with H2O. FTIR spectra show that the sulfuric acid films absorb water while cooling in the presence of water vapor. Using stratospheric water pressures, the most dilute solutions observed were >40 wt % before simultaneous ice formation and sulfuric acid freezing occurred. FTIR spectra also revealed that the sulfuric acid films crystallized mainly as sulfuric acid tetrahydrate (SAT). Crystallization occurred either when the composition was about 60 wt % H2SO4or after ice formed on the films at temperatures 1–4 K below the ice frost point. Finally, we determined that the melting point for SAT depended on the background water pressure and was 216–219 K in the presence of 4 × 10−4Torr H2O. Our results suggest that once frozen, sulfuric acid aerosols in the stratosphere are likely to melt at these temperatures, 30 K colder than previously thought.
Light ion flow in the nightside ionosphere of Venus
Authors: Hartle R. E. , Grebowsky J. M.
Published: February 2008

The flow characteristics of the light ions H+ and He+ have been studied in the midnight region of the ionosphere of Venus. Measurements of ion composition, electron and ion temperatures and magnetic fields by instruments onboard the Pioneer Venus Orbiter have been used in the electron and ion equations of conservation of mass and momentum to derive the vertical flow velocities of H+ and He+. When average height profiles of the measured quantities were used, H+ was found to flow upward, accelerating to speeds of almost 1 km s−1 at the ion‐exobase. In a similar fashion, He+ was found to flow downward into the neutral atmosphere where it is readily quenched by charge transfer reactions. The polarization electric field played an important role in forcing H+ upward, but did not contribute enough to the He+ force balance to produce upward flow. At the ion‐exobase, the outward electric polarization force on H+ was shown to be five times the gravitational force. Using an analogy with the terrestrial ion‐exosphere, H+ was inferred to flow upward into the ionotail of Venus and accelerate to escape speeds. A planet averaged escape flux of 1.4 × 107 cm−2 s−1 was calculated, which is comparable to hydrogen loss rates estimated by other investigators.
Helium in the Martian atmosphere
Authors: Krasnopolsky Vladimir A. , Chakrabarti Supriya , Gladstone G. Randall
Published: February 2008

A simple two‐reservoir degassing model for the Earth decribes rather well the current degassing rate of 4He which is equal to (3±1)×106 cm −2s−1 according to data for the helium polar wind and the measured 3He/4He ratio. This value of the helium degassing and loss rate provides an important constraint in modeling of noble gases, and some recent models do not fit this constraint. Scaling this value to the known amounts of 40Ar in the atmospheres of Mars and the Earth and to U/K ratios in their surface rocks, this results in a crude estimate of the helium degassing rate on Mars, which is equal to 2.2×105 cm−2 s−1. Nonthermal escape of He is calculated using the daytime mean atmospheric models for low, mean, and high solar activity with He mixing ratio ƒHe = 1 ppm. Three processes contribute to He escape: (1) electron impact ionization and photoionization above the ionopause followed by solar wind sweeping away of the ions formed (1.3×105 cm−2s−1), (2) collisions with hot oxygen atoms formed mainly by recombination of O2+ (2.8×104 cm−2s−1), (3) charge exchange of He+ and CO2, N2, and CO between the exobase and ionopause (6×103 cm−2s−1). The derived mixing ratio for He is ƒHe = 1.4 ppm for the adopted degassing rate. The He 584 Å airglow intensity is equal to 37 R and 67 R at low and high solar activity, respectively. The intensities are 105 R and 240 R for ƒHe = 7 ppm, and 10 R and 16 R for ƒHe = 0.3 ppm. These brightnesses should be detectable using the Extreme Ultraviolet Explorer satellite. The measurement of helium on Mars is important for the determination of the planetary abundance of U and Th, element differentiation in the primordial nebula (by comparison with the Earth and Venus) and in the interior of Mars (by comparison with the U and Th fraction in the surface rocks on Mars measured by the Mars 5 and Phobos orbiters). This measurement makes possible an estimate of the radiogenic heat flux which is important for the thermal balance of Mars' interior.
Duration and rates of discharge: Maja Valles, Mars
Authors: de Hon R. A. , Pani E. A.
Published: February 2008

The 1600‐km‐long Maja Valles outflow system of Mars consists of three major divisions including the upper valley on Lunae Planum, the canyon section across Xanthe Terra, and the lower valley across western Chryse Planitia. Although water released from the source in Juventae Chasma could reach the terminus of the present day valley system within 40 hours, the original outflow did not traverse the Martian surface as an uninterrupted flow, nor was the source basin emptied instantaneously. The initial flood ponded on northern Lunae Planum and near the western edge of Chryse Planitia, significantly prolonging the lifetime of surface flow. Calculation of pond volumes and maximum discharge rates through various parts of the channel system, assuming full‐bank discharge, allows an estimate of minimum duration of water flow through the outflow complex. About four (terrestrial) months are required to empty Juventae Chasma, and another 10 months are required to drain the impoundment on Lunae Planum. Thus, the history of erosion and sedimentation in the lower reaches of the drainage system is potentially much more complex than that of outflow from the source region.
Formation of Venusian canali: Considerations of lava types and their thermal behaviors
Authors: Gregg Tracy K. P. , Greeley Ronald
Published: February 2008

Because liquid water is unstable at present venusian surface conditions, the discovery of channels (termed “canali”) on Venus thousands of kilometers long was not predicted. Low‐viscosity lavas that remain fluid for several thousand kilometers are considered to be the canali‐forming agents; possible compositions of venusian canali‐forming lavas include komatiite and high‐Fe‐Ti “lunar”‐type basalts. Results of analytical and numerical models of these lavas reveal that total cooling is more efficient on Venus than on Earth, suggesting that venusian lavas rapidly form insulating crusts, and, thus, that the canali lavas were essentially “tube‐fed.” The models also reveal that thermal erosion should be less efficient on Venus than on Earth, suggesting that venusian channels are either the product of mechanical (rather than thermal) erosion or constructional processes.