bluuebunny/crossref_metadata_2025 huggingface dataset:
A GCM study of the late January 1992 “mini‐hole” event observed during EASOE
Authors: Orsolini Y. , Cariolle D. , Déqué M.
Published: February 2004

An ozone initialization scheme has been developed for short‐term simulations of the northern hemisphere ozone field with the General Circulation Model “Arpège” of Météo‐France; the scheme follows largely the one proposed by Riishojgaard et al. (1992) and uses a combination of meteorological analyses, satellite total ozone measurements and statistical analysis of ozone profiles. We took advantage of the large number of ozonesondes launched during the EASOE campaign to verify and even improve the statistical treatment of ozone profiles. This, in conjunction with the use of potential vorticity to delimit the polar vortex, has allowed a more accurate representation of the initial ozone field inside the polar vortex. The scheme is tested by simulating an ozone “mini‐hole” event which developed over northern Europe in late January 1992. The ozone field forecast by the model is compared with satellite measurements and soundings. The results indicate the occurrence of a strong meridional intrusion of ozone‐poor air coming from subtropical latitudes over northern Europe during the last days of January. The strength and large extent of this poleward advection account for most of the reduction observed in lower stratospheric ozone content.
Airborne measurements during the Arctic Stratospheric Experiment: Observation of O3 and NO2
Authors: Pfeilsticker K. , Platt U.
Published: February 2004

During winter 1991/92 DOAS (Differential Optical Absorption Spectroscopy) measurements were performed from a C130 Transall aircraft within the framework of the EASOE (European Arctic Stratospheric Ozone Experiment) campaign. The campaign extended from Dec. 1991 until March 1992 with a series of flights in the Arctic (60°W to 60°E, 85°N to 48°N). Two spectrometers were operated on the aircraft: an UV‐instrument (see Brandtjen et al., this issue) and a DOAS‐vis instrument to detect O3, O4, NO2 and NO3 in the visible band (363 nm–680 nm). The DOAS‐vis ozone measurements were largely influenced by the presence of the Mt Pinatubo aerosol cloud. A correction of the data by actual Mie‐profiles is described. A NO2 depletion with minimum values of 2.3*1014/cm² in Jan. 92 was observed. Occasionally, OClO could also be detected.
Groundbased measurements of stratospheric OClO, NO2, and O3 at Søndre Strømfjord in winter 1991/92
Authors: Perner D. , Roth A. , Klüpfel T.
Published: February 2004

Experiment (EASOE) campaign Within the scope of the 1991/92 European Arctic Stratospheric Ozone stratospheric chlorine dioxide, OClO, nitrogen dioxide, NO2, bromine monoxide, BrO, and ozone, O3, were measured by groundbased UV‐VIS remote sensing in Søndre Strømfjord, Greenland (50.4°W, 67.0°N). The measurement period was from the middle of November 1991 until the end of January 1992. The anticipated correlation between potential vorticity and OClO appeared less pronounced in the beginning and its abundance was similar inside and outside of the polar vortex. During November 1991 stratospheric temperatures throughout the polar region were still above threshold for polar stratospheric cloud (PSC) formation. During January when NO2 column densities were at their lowest, OClO inside of the vortex appeared to be highly variable. Average vertical column densities reached about 1013 molec cm−2, less than found in previous years.

The chemistry as well as the observations may have been disturbed by the aerosol cloud from Mt. Pinatubo eruption.
Null fields in the outer Jovian magnetosphere: Ulysses observations
Authors: Haynes P. L. , Balogh A. , Dougherty M. K. , Southwood D. J. , Fazakerley A. , Smith E. J.
Published: February 2004

This paper reports on a magnetic field phenomenon, hereafter referred to as null fields, which were discovered during the inbound pass of the recent flyby of Jupiter by the Ulysses spacecraft. These null fields which were observed in the outer dayside magnetosphere are characterised by brief but sharp decreases of the field magnitude to values less than 1 nT. The nulls are distinguished from the current sheet signatures characteristic of the middle magnetosphere by the fact that the field does not reverse across the event. A field configuration is suggested that accounts for the observed features of the events.
Airborne measurements during the European Arctic Stratospheric Ozone Experiment column amounts of HNO3 and O3 derived from FTIR emission sounding
Authors: Blom C. E. , Fischer H. , Glatthor N. , Gulde T. , Höpfner M.
Published: February 2004

During the EASOE (European Arctic Stratospheric Ozone Experiment) winter campaign 1991/92, first measurements were made from an aircraft with the new atmospheric sounder MIPAS‐FT (Michelson Interferometer for Passive Atmospheric Sounding ‐ Flugzeug Transall). The instrument measures infrared radiation emitted by atmospheric trace constituents in the spectral range between 750 and 1250 cm−1. As first results, we present zenith column amounts of O3 and HNO3 retrieved from data recorded between January 30 and February 16, 1992 in the region from 45° west to 45° east and from 55° to 80° north. The column amounts vary considerably, from 1 to 3 × 1016 molecules/cm² for HNO3 and from 180 to 420 DU for O3, respectively. The strong linear correlation between these stratospheric gases indicates that the large variations during that period are mainly due to dynamic processes in the arctic atmosphere.
Stratospheric NO2 observations at the Jungfraujoch Station between June 1990 and May 1992
Authors: Van Roozendael M. , Hermans C. , De Mazière M. , Simon P. C.
Published: February 2004

Observations of nitrogen dioxide have been performed at the International Scientific Station at the Jungfraujoch (46.5°N, 8.0°E) during the winters 1990–1991 and 1991–1992. Vertical abundances of NO2 were obtained by measuring the scattered sunlight at the zenith, during sunset and sunrise, using the differential absorption method in the visible spectrum. The comparison between the two winters shows a NO2 reduction of 15% in March–May 1992, following the eruption of Mt. Pinatubo. This decrease is likely to be due to heterogeneous reactions on the volcanic aerosols converting N2O5 to HNO3. Radiative interferences on ground‐based visible absorption measurements of NO2 caused by the enhanced aerosol layer are considered to be negligible (Perliski and Solomon, 1992). The results are discussed with respect to measurements made at Lauder, New Zealand (45°S, 170°E) and published by Johnston et al. (1992).
The spatial and temporal extent of chlorine activation by polar stratospheric clouds in the northern hemisphere winters of 1988/89 and 1991/92
Authors: MacKenzie A. R. , Knudsen B. , Jones R. L. , Lutman E. R.
Published: February 2004

A trajectory‐based analysis of the northern hemisphere lower stratospheres of 1988/89 and 1991/92, from 30°N to the pole, has been carried out, allowing estimates of the area subject to chlorine activation by polar stratospheric clouds to be made. In both winters a significant area is covered by activated air (i.e., air subject to temperatures cold enough for polar stratospheric cloud formation up to 20 days previously). Activated air in the middle latitudes is a combination of the temporary displacement of vortical air equatorward and in situ activation of extra‐vortical air. Relative to 1988/89, 1991/92 shows increased activation in the middle latitudes. This was due to a displacement of low temperatures towards lower values of potential vorticity by an extreme anticyclonic forcing event.
Temporal development of ozone within the Arctic Vortex during the winter of 1991/92
Authors: Braathen Geir O. , Rummukainen Markku , Kyrö Esko , Schmidt Ulrich , Dahlback Arne , Jørgensen Torben S. , Fabian Rolf , Rudakov Vladimir V. , Gil Manuel , Borchers Reinhardt
Published: February 2004

In this study we address the question of temporal ozone trends on isentropic surfaces within the Arctic polar vortex during EASOE. We have combined ozone sonde data from twelve campaign stations distributed throughout the European sector of the Arctic. The development of ozone at the 425, 475, 550 and 700K levels is presented, using analysed fields of isentropic potential vorticity and isentropic back‐trajectories to separate inner vortex air from air staying outside the vortex. Vertical N2O profiles measured in Kiruna have been used to obtain information on the vertical movement of the polar air mass. Elimination of effects due to diabatic descent leads to a weak, yet significant, chemically induced loss of ozone at 475 K. At 425 K we observe a negative trend in the ozone mixing ratio, but for this level it has not been possible to assess the influence of subsidence. Heterogeneous chemistry on aerosols of volcanic origin from the eruption of Mt. Pinatubo might have contributed to the negative ozone trends.
Accuracy of the isentropic trajectories calculated for the EASOE Campaign
Authors: Knudsen B. M. , Carver G. D.
Published: February 2004

During EASOE 10 day backward trajectories ending on most measurement sites were calculated on 6 isentropic levels every day. The temporal and spatial coarseness of the ECMWF analyses used cause significant errors in the trajectories. These interpolation errors are largest in the direction along the trajectories. The difference between two trajectories of the Polar Vortex Balloon Experiment and calculated trajectories is 8 and 12% of the trajectory length after 1.83 and 2.14 days. This discrepancy is not explainable by interpolation errors alone.
Interpretation of lidar depolarization measurements of the Pinatubo stratospheric aerosol layer during EASOE
Authors: Flesia C. , Mugnai A. , Emery Y. , Godin S. , de Schoulepnikoff L. , Mitev V.
Published: February 2004

During the EASOE campaign in the Arctic, all lidar measurements showed total depolarization values for the Pinatubo stratospheric aerosol layer varying between 2% and 4%. If multiple scattering is negligible, the radiation backscattered by a polydispersion of spheres has the same polarization as the incident light. There is always some depolarization of a lidar return because of the anisotropy of the polarizability of the air molecules, but depolarization in the signal returned from aerosols indicates the presence of non‐spherical particles. In this paper, we utilize scattering calculations for non‐spherical particles to explore interpretations of lidar signals returned by Pinatubo aerosols. Taking into account several particle shapes and lognormal size distributions, calculations show that only slightly‐to‐moderate deformed spheres with equivalent‐mean‐radius less then 0.2 µm yield depolarization values in the range given by the lidar measurements, if such particles form a substantial fraction of the population. The implications of particle non‐sphericity on the retrieval of the aerosol size distribution are discussed.
Thermospheric meridional neutral winds associated with equatorial midnight temperature maximum (MTM)
Authors: Sastri J. Hanumath , Ranganath Rao H. N. , Somayajulu V. V. , Chandra H.
Published: February 2004

Coordinated measurements were made on two nights in December 1992 of thermospheric temperature with a Fabry‐Perot Interferometer (FPI) at Kavalur (dip∼ 9.5°N) and of F‐layer height with ionosondes at Trivandrum (dip∼ 0.6°S), Kodaikanal ( dip 3°N), Sriharikota (dip ∼10.5°N) and Ahmedabad (dip 34°N) in India. On both the nights which are geomagnetically quiet (Ap≤9), the FPI measurements revealed a distinct temperature enhancement around midnight over Kavalur. Neutral winds estimated from h'F data of Trivandrum and Sriharikota showed significant poleward winds in association with the temperature increase over Kavalur. At Ahmedabad a clear‐cut descent of F‐layer followed the temperature enhancement over Kavalur. This pattern is consistently seen on the two nights. The simultaneous thermospheric and ionospheric observations demonstrate for the first time the relationship, in totality, between the equatorial midnight temperature maximum (MTM), poleward reversal of nighttime meridional winds and ‘postmidnight collapse’ of F‐layer at low latitudes.
Trajectory model studies of ClOx activation during the 1991/92 northern hemispheric winter
Authors: Lutman E. R. , Pyle J. A. , Jones R. L. , Lary D. J. , MacKenzie A. R. , Kilbane‐Dawe I. , Larsen N. , Knudsen B.
Published: February 2004

Calculations with a chemical box model along air parcel trajectories between November 1991 to January 1992 show a build up of reactive chlorine in late December and early January, culminating in values of ClOx greater than 2.0 ppbv widespread in the vortex on 9 January 1992. These values are quantitatively comparable to the MLS satellite measurements of ClO. We discuss the chemistry occurring within the vortex, around the vortex edge and outside the vortex.
A broken plate beneath the North Baikal Rift Zone revealed by gravity modelling
Authors: Burov E. B. , Houdry F. , Diament M. , Déverchère J.
Published: February 2004

We modelled a 1200 km long gravimetric profile in the North Baikal rift to assess the mechanical behaviour of the lithosphere, using a numerical model that accounts for realistic brittle‐elasto‐ductile rheology. We use published seismicity and refraction data, a new 5′ × 7.5′ free‐air/Bouguer gravity and topography data set, and a detailed map of faults obtained from high resolution SPOT imagery. Analysis of the gravity field over the North Baikal rift zone indicates significant asymmetry of the mechanical processes governing the deformation of the diverging sides of the rift. These anomalies cannot be explained by a conventional continuous plate undergoing extension beneath the rift zone, whereas a strong mechanical discontinuity (wedge shaped detachment zone beneath the rift axis) is able to reproduce observations. Such a discontinuous model provides a good fit to the gravity and crustal thickness data and explains the deep seismicity reported there.
The 630 nm MIG and the vertical neutral wind in the low latitude nighttime thermosphere
Authors: Herrero F. A. , Meriwether J. W.
Published: February 2004

It is shown that large negative divergences (gradients) in the horizontal neutral wind in the equatorial thermosphere can support downward neutral winds in excess of 20 m/s. With attention to the meridional and vertical winds only, the pressure tendency equation is used to derive the expression Uz0 ≈ 〈∂Uy /∂y〉 H for the vertical wind Uz0 at the reference altitude for the pressure tendency equation; H is the atmospheric density scale height, and 〈∂Uy/∂y〉 is the meridional wind gradient. The velocity gradient associated with the Meridional Intensity Gradient (MIG) of the O(¹D) emission (630 nm) at low latitudes is used to estimate the vertical neutral wind in the MIG region. Velocity gradients derived from MIG data are about 0.5 (m/s)/km) or more, indicating that the MIG region may contain downward neutral winds in excess of 20 m/s. Though direct measurements of the vertical wind are scarce, Fabry‐Perot interferometer data of the equatorial F‐region above Natal, Brazil, showed downward winds of 30 m/s occurring during a strong meridional wind convergence in 1982. In‐situ measurements with the WATS instrument on the DE‐2 satellite also show large vertical neutral winds in the equatorial region.
Local gravity fields from PVO and Magellan data: A comparative study
Authors: Barriot Jean‐Pierre , Balmino Georges
Published: February 2004

The gravity maps locally derived from the PVO and Magellan gravity LOS data present strong correlations, but also some discrepancies. We analyze in detail the probable origin of these discrepancies over a test region on Venus, the Gula Mons zone. We also study the optimal merging of the two data sets.
The effect of tropical instability waves on CO2 species distributions along the equator in the eastern equatorial Pacific during the 1992 ENSO event
Authors: Feely R. A. , Wanninkhof R. , Cosca C. E. , McPhaden M. J. , Byrne R. H. , Millero F. J. , Chavez F. P. , Clayton T. , Campbell D. M. , Murphy P. P.
Published: February 2004

Tropical instability waves have been shown to have a major impact on the variability of temperature and nutrients along the equatorial wave guide. In order to assess the impact of these features on carbon species distributions during an ENSO event, sea surface temperature, salinity, sigma‐t, nitrate, CO2 fugacity, total inorganic carbon, total alkalinity, and pH along the equator were measured from 130°W to 100°W during 8–15 May 1992. Concurrent moored measurements of surface currents and temperature were also made at 0°, 110°W. Results indicate that tropical instability waves, with periods of 15 to 20 days and zonal wavelengths of 700–800 km, controlled the observed spatial variability of the CO2 species, nitrate and hydrographic parameters at the equator.
The lifetime of leewave‐induced ice particles in the Arctic stratosphere: I. Balloonborne observations
Authors: Deshler Terry , Peter Thomas , Müller Rolf , Crutzen Paul
Published: February 2004

During the European Arctic Stratospheric Ozone Experiment (EASOE) in 1991/1992 there were only a few occasions when polar stratospheric clouds (PSCs) were observed. One of these was on 27 January 1992 when PSCs were detected by two balloonborne aerosol instruments released from Esrange near Kiruna, Sweden (68°N). High concentrations of particles were found at altitudes between 19 and 24 km, in a series of many very thin layers. Particles as large as 5 µm were detected. The surface area concentration in the densest layers exceeded 100 µm² cm−3, as estimated from retrieved size distributions. Estimates of the total mass of the particles suggest that ice must have been the main constituent, although the air temperature was 3 K or more above the frost point. The particles were probably formed during leewave cooling over the Norwegian mountains, some hours earlier, and some 300 km upstream of the measurements. A possible explanation for the slow evaporation observed is offered in part II of this paper (Peter et. al., this issue).
Pinatubo volcanic aerosols observed by lidar at Wakkanai, Japan
Authors: Shibata Takashi , Itabe Toshikazu , Mizutani Kohei , Asai Kazuhiro
Published: February 2004

Dense stratospheric aerosol layers originating from the volcanic eruptions of Mt. Pinatubo in the Philippines on June 15, 1991 were observed at the Wakkanai radio observatory (45.4°N, 141.7°E) of the Communications Research Laboratory (CRL) using a Nd:YAG lidar system. These observations started in late August 1991, and the dense layer was first detected in early October 1991. The backscattering of the aerosol layers varied greatly in October and November 1991 then stabilized in December 1991. The integrated backscattering coefficient (IBC) at the beginning of 1992 was smaller than the values observed in October and November 1991. The IBC increased slightly during 1992, but began to decrease in 1993. The estimated surface area density of aerosol particles increased one to two orders of magnitude in late 1991 and early 1992 to about 10−7 cm2/cm3. The IBC and surface area started to decrease sooner at higher altitudes: in May 1992 from 25 to 30 km, in November 1992 from 20 to 25 km, and in February 1993 from 13 to 20 km in altitude. Calculation by Prather [1992] indicates that our estimated surface area correspond to about 30 to 50% less NOx. Almost same degree of decrease in NO2 was observed by Koike et al.[1993].
The stratospheric aerosol content above Spitzbergen during winter 1991/92
Authors: Beyerle Georg , Neuber Roland
Published: February 2004

Lidar investigations of polar stratospheric aerosols were performed at Ny‐Ålesund, Spitsbergen (79°N, 12°E) between December 1991 and March 1992. Backscatter ratios at wavelengths of 353 and 532 nm and volume depolarization at 532 nm in altitudes ranging from the tropopause up to 30 km are presented. Throughout the whole measurement period a layer of aerosols of volcanic origin was observed. During the period of stable vortex conditions we found averaged peak backscatter ratios of 2.3 at 532 nm at altitudes around 14 km. Before the break‐up of the polar vortex in mid‐March no aerosols were detected at altitudes above 20 km. Observed volume depolarization profiles suggest that the main aerosol layer consisted of spherical particles throughout the whole period of observation. Enhanced values of volume depolarization in the lower part of the aerosol cloud directly above the tropopause are possibly caused by scattering on volcanic ash particles of sub‐micron size.
Radiatively forced dispersion of the Mt. Pinatubo volcanic cloud and induced temperature perturbations in the stratosphere during the first few months following the eruption
Authors: Young Richard E. , Houben Howard , Toon Owen B.
Published: February 2004

A combined 3‐dimensional circulation model and aerosol microphysical/transport model is used to simulate the dispersion of the Mt. Pinatubo volcanic cloud in the stratosphere for the first few months following the eruption. Radiative heating of the cloud due to upwelling infrared radiation from the troposphere is shown to be an important factor affecting the transport. Without cloud heating, cloud material stays mostly north of the equator, whereas with cloud heating, the cloud is transported southward across the equator within the first two weeks following the eruption. Generally the simulations agree with TOMS, AVHRR, and SAGE satellite observations showing the latitude distribution of cloud material to be between about 20°S and 30°N within the first few months. Temperature perturbations in the stratosphere induced by the aerosol heating are generally 1–4 K, in the range of those observed.
Methane in groundwater used for Japanese agriculture: Its relationship to other physico‐chemical properties and possible tropospheric source strength
Authors: Watanabe A. , Kimura M. , Kasuya M. , Kotake M. , Katoh T.
Published: February 2004

The concentration of CH4 in 131 groundwater samples used for agriculture in Aichi Prefecture, central Japan, averaged 1.58 mgC l−1 for those water samples above detection (>0.006 mgC l−1), with the highest value of 18.4 mgC l−1. Methane was detected in more than half of the groundwater samples. The amount of CH4 released to the atmosphere because of agricultural groundwater use was estimated to be 2.00 × 107 gC yr−1 for a cultivated area of 8.61 × 104 ha, or about 1.4% of the CH4 production in paddy fields derived from soil organic matter in the same geographic area. Distribution of measurements of redox potential (Eh), chemical oxygen demand (COD), Fe, Mn, NH4‐N, and NO3‐N was clearly different between the CH4‐detected and undetected samples; Eh values and NO3‐N concentrations were lower while the other four factors were higher in the CH4‐detected samples.
Effects of the local interstellar medium magnetic field on the structure of the heliosphere: A laboratory simulation
Authors: Minami Shigeyuki
Published: February 2004

The ‘heliopause’ is the hypothesized boundary separating the outflowing solar wind from the interstellar medium. The outer heliosphere itself is a problem of topical interest because several spacecraft are presently cruising towards the terminal shock of the heliosphere. While there have been theoretical and computer simulation studies of this boundary, there has so far been no laboratory simulation. We have now performed such a simulation, and the results are presented here. The results indicate the possible existence of such a boundary as being highly axiasymmetrical in the case of the low Alfvenic Mach number of the local interstellar medium.
Northern hemisphere total ozone values from 1989–1993 determined with the NOAA‐11 Solar Backscatter Ultraviolet (SBUV/2) instrument
Authors: Planet W. G. , Lienesch J. H. , Miller A. J. , Nagatani R. , McPeters R. D. , Hilsenrath E. , Cebula R. P. , Deland M. T. , Wellemeyer C. G. , Horvath K.
Published: February 2004

Determinations of global total ozone amounts have been made from recently reprocessed measurements with the SBUV/2 on the NOAA‐11 environmental satellite since January 1989. This data set employs a new algorithm and an updated calibration. Comparisons with total ozone amounts derived from a significant subset of the global network of Dobson spectrophotometers shows a 0.3% bias between the satellite and ground measurements for the period January 1989–May 1993. Comparisons with the data from individual stations exhibit differing degrees of agreement which could be due to the matchup procedures and also to the uncertainties in the Dobson data. The SBUV/2 data set discussed here traces the Northern Hemisphere total ozone from 1989 to the present, showing a marked decrease from the average of those years starting in the summer of 1992 and continuing into 1993, with an apparent returning to more normal levels in late 1993.
Hydro‐isostatic deflection and tectonic tilting in the central Andes: Initial results of a GPS survey of Lake Minchin shorelines
Authors: Bills Bruce G. , de Silva Shanaka L. , Currey Donald R. , Emenger Robert S. , Lillquist Karl D. , Donnellan Andrea , Worden Bruce
Published: February 2004

Sufficiently large lake loads provide a means of probing rheological stratification of the crust and upper mantle. Lake Minchin was the largest of the late Pleistocene pluvial lakes in the central Andes. Prominent shorelines, which formed during temporary still‐stands in the climatically driven lake level history, preserve records of lateral variations in subsequent net vertical motions. At its maximum extent the lake was 140 m deep and spanned 400 km N‐S and 200 km E‐W. The load of surficial water contained in Lake Minchin was sufficient to depress the crust and underlying mantle by 20–40 m, depending on the subjacent rheology. Any other differential vertical motions will also be recorded as departures from horizontality of the shorelines. We recently conducted a survey of shoreline elevations of Lake Minchin with the express intent of monitoring the hydro‐isostatic deflection and tectonic tilting. Using real‐time differential GPS, we measured topographic profiles across suites of shorelines at 15 widely separated locations throughout the basin. Horizontal and vertical accuracies attained are roughly 30 and 70 cm, respectively. Geomorphic evidence suggests that the highest shoreline was occupied only briefly (probably less than 200 years) and radiocarbon dates on gastropod shells found in association with the shore deposits constrain the age to roughly 17 kyr. The basin‐wide pattern of elevations of the highest shoreline is composed of two distinct signals: (27±1) m of hydro‐isostatic deflection due to the lake load, and a planar tilt with east and north components of (6.8±0.4) 10−5 and (−5.3±0.3) 10−5. This rate of tilting is too high to be plausibly attributed to steady tectonism, and presumably reflects some unresolved combination of tectonism plus the effects of oceanic and lacustrine loads on a laterally heterogeneous substrate. The history of lake level fluctuations is still inadequately known to allow detailed inferences of crust and mantle rheology. However, it is already clear that the effective elastic plate thickness is closer to 40 km than the 60–70 km crustal thickness in the central Andes and the effective viscosity is less than 5 1020 Pa s.
Changes in the O2/N2 ratio of the atmosphere during recent decades reflected in the composition of air in the firn at Vostok Station, Antarctica
Authors: Bender M. L. , Sowers T. , Barnola J.‐M. , Chappellaz J.
Published: February 2004

Samples of air at various depths in firn were collected at Vostok Station, Antarctica, and analyzed for δ15N of N2, O2/N2 ratio, and CO2. The ultimate objective of this work is to constrain the recent rate of the atmospheric [O2] decrease, thereby providing a direct experimental constraint on net CO2 fluxes into the ocean and the land biosphere. δ15N increases with depth, because of gravitational enrichment, at approximately the rate predicted by the barometric equation. Gravitationally corrected CO2 decreases with depth to 308 ppmV at 101.9 m depth, because deeper air is older and less contaminated with anthropogenic CO2. The gravitationally corrected O2/N2 ratio increases with depth mainly because burning fossil fuel consumes O2. Samples in the top 20 m of the firn have anomalously high CO2 concentrations and anomalously low O2/N2 ratios. Samples below 96.2 m depth have anomalously high O2/N2 ratios. Between 30 and 96.2 m depth, the gravitationally corrected increase in the O2/N2 ratio is nearly equal to that computed from the rate of O2 consumption by combustion of fossil fuels. Our results indicate that the rate of anthropogenic O2 consumption can be accurately constrained by future firn air studies.
Bitter patterns versus hysteresis behavior in small single particles of hematite
Authors: Halgedahl Susan L.
Published: February 2004

To establish links among domain images, volume changes in magnetization, and mechanisms for pseudosingle‐domain (PSD) behavior, we have studied single platelets of natural hematite (Elba, Italy) in order to compare Bitter patterns resulting from high‐field treatments to the hysteresis behavior of the same individual particles. There is excellent agreement between the fields that cause large changes of a particle's Bitter pattern and the fields that cause large changes of a particle's magnetic moment. When in the saturation remanent state, most particles exhibit patterns that suggest a state of near saturation. Such patterns change very little until the application of a critical back field triggers the appearance of a major Bitter line that nearly bisects the grain. Likewise, in the majority of particles the measured ratio of saturation remanence to saturation moment is large (≅ 0.7), and there is little change in saturation remanence until a critical back field equal to the coercive force (Hc) reverses the moment through a large Barkhausen jump. In most particles whose patterns have been compared to their measured behavior, this large jump on the hysteresis loop occurs in the same critical back field that causes the sudden appearance of a major Bitter line. Consequently, we interpret hysteresis to be governed by the nucleation of walls in the majority of hematite particles studied here. In such particles, Hc increases as effective grain size decreases, according to Hc ∝ deff−0.64, where deff ∝ Ms1/3 and Ms equals the particle's saturation moment. This study leads to four conclusions: (1) Bitter patterns do represent volume domains and the magnetization processes that they imply, (2) nucleation is the dominant hysteresis mechanism in PSD particles of this natural hematite, (3) the grain size dependence of nucleation field Hn in hematite is given by Hn = Hc ∝ deff−0.64, and (4) nucleation is a viable mechanism for explaining PSD behavior in other natural magnetic materials, such as magnetite, titanomagnetite, and pyrrhotite.
Lava flows from a mid‐1980s submarine eruption on the Cleft segment, Juan de Fuca Ridge
Authors: Chadwick William W. , Embley Robert W.
Published: February 2004

A series of lava flows with a total volume of 0.05 km3 were erupted in the mid‐1980s along 17 km of the northern Cleft segment of the Juan de Fuca Ridge. Observations from camera tows and submersible dives show that the new flows are all similar in appearance and consist entirely of pillow lava with a mixture of smooth and striated surface textures, suggesting a relatively uniform eruption rate approaching 1 m3/s at point source vents. The flows vary in size from small patches to large steep‐sided ridges and were probably erupted from a dike intruded along the ridge axis because they are aligned along a linear fissure/graben system. Observations at north Cleft show that the physical appearance of new flows changes more rapidly than previously realized and that earlier qualitative dating of young lavas based on sediment cover and glassy surface texture were probably overestimates by an order of magnitude. Sediment accumulation on the lavas is quite variable and locally surprisingly substantial, mainly due to hydrothermal deposits that formed while the lava flows were cooling. Biological vent communities photographed on the new flows in 1989 show that vent animals can colonize new vent sites rapidly but that warm water was still venting only in a few places. Nonvent animals are much slower to colonize the new flows and rates of colonization observed at north Cleft may be useful for making improved age estimates of young (<10 years) lava flows elsewhere. The north Cleft eruption represents about 2% of the estimated average annual volcanic output along the global mid‐ocean ridge, implying that many other submarine eruptions are occurring undetected.
A geophysical‐geological transect of the Silent Canyon Caldera Complex, Pahute Mesa, Nevada
Authors: Ferguson John F. , Cogbill Allen H. , Warren Richard G.
Published: February 2004

Revision of lithological logs for boreholes penetrating the volcanic center at Pahute Mesa, Nevada, has led to a thorough review of the volcanic stratigraphy and geologic structure. We have combined this review with a compilation of old and newly acquired gravity and seismic travel time data, producing a unified interpretation along a northwest to southeast profile. The analysis supports a new interpretation of the Silent Canyon caldera complex. The caldera is found to be more asymmetric than previously suggested, with the southeastern boundary formed by linear, high‐angle normal faults and a more gently sloping northwestern boundary. The total thickness of volcanic units within the caldera complex does not appear to exceed 5 km. The shallow structure at Pahute Mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site. The Silent Canyon caldera complex is actually a set of nested calderas first filled by thick (>1 km) postcaldera lavas and subsequently buried by outflow sheets of the Timber Mountain caldera to the south. Thick, postcaldera lavas filled a half‐graben structure formed west of the West Greeley fault, dropping the tops of the youngest caldera‐forming units to depths in excess of 2 km. Therefore the western boundary of the caldera complex is poorly defined. East of the West Greeley fault, two overlapping calderas are defined, and stratigraphic data suggest the presence of even older calderas. The youngest caldera, the calc‐alkaline Area 20 caldera, is well defined from drill hole data. The Area 20 caldera overlaps the 13.6 Ma peralkaline Grouse Canyon caldera, which is less well defined, but apparently collapsed in trap‐door style along the Almendro fault. For both these calderas, collapse continued after the main caldera‐forming eruption, concurrent with the accumulation of thick (>1 km) lavas within the peripheral collapse zones. The geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the NNE trending Scrugham Peak and Almendro faults, which offset the pre‐Tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 Ma age. Drill hole data show that offsets along these faults increase systematically within older (up to 15 Ma) units, which are commonly rotated eastward in a style similar to units at the surface. Abrupt changes in the subsurface thickness of the caldera‐forming units occur across the faults, indicating that these linear features served as caldera boundaries.
Computation of porosity redistribution resulting from thermal convection in slanted porous layers
Authors: Gouze Philippe , Ribstein Anne Coudrain‐ , Bernard Dominique
Published: February 2004

Unlike fluid displacement due to regional hydraulic head, thermoconvective motions are generally slow. The thermal impacts of such movements are very weak, whereas their chemical impacts may be significant because of their cumulated effects over geologic time. For nonhorizontal thick sedimentary reservoirs, the fluid velocity due to thermal convection can be accurately approximated by an explicit function of the dip of the reservoir, the permeability and the difference in thermal conductivity between the aquifer and the confining beds. The latter parameter controls the rotation direction of the flow and, for clastic reservoirs bounded by impervious clayey media, fluid moves up the slope along the caprock layer. As the fluid velocity is small, the major rock‐forming minerals control the fluid composition by thermodynamic equilibrium. Thus, whereas the volume of redistributed minerals depends on the volume of water circulated, the localization of porosity enhancement is strongly controlled by the reservoir mineralogy. With realistic values of permeability and layer thickness, several per cent of secondary porosity per million years can be created or lost at shallow depth (<2 km), depending on the chlorinity, the set of representative minerals and the temperature. In sandstone reservoirs and high‐chlorinity calcarenite reservoirs, the porosity decreases under the caprock where hydrocarbons can accumulate. In chloride‐depleted carbonate aquifers, the simultaneous control by carbonates, silica and aluminosilicates can produce a decrease of porosity above the bedrock and an enhancement of porosity under the caprock. However, computations show that the quality of the upper part of the reservoir is mainly reduced by the precipitation of silica and clays.
Is Old Faithful a strange attractor?
Authors: Nicholl Michael J. , Wheatcraft Stephen W. , Tyler Scott W. , Berkowitz Brian
Published: February 2004

Geysers are systems that exhibit hydrothermal eruptive behavior at irregular and, occasionally, relatively regular intervals. Because geyser dynamics are governed by strongly nonlinear differential equations, it is reasonable to expect that these systems are capable of displaying chaotic behavior. Geyser dynamics are shown to be analogous to a simple conceptual model known to exhibit chaotic behavior. State space reconstructions of eruption interval data then show that the geyser Old Faithful erupts at chaotic intervals. Establishing the system as chaotic explains the inability to predict long‐term eruptive behavior of Old Faithful.
Simulated annealing of three‐dimensional micromagnetic structures and simulated thermoremanent magnetization
Authors: Thomson Leslie C. , Enkin Randolph J. , Williams Wyn
Published: February 2004

This paper deals with the application of simulated annealing to rock magnetism. The implementation of the algorithm is discussed in detail. It is shown that the technique can be used to determine stable micromagnetic structures by finding very low energy states which are not easily found using other optimization techniques. A modified simulated annealing algorithm is used to incorporate thermal fluctuations into a micromagnetic model in order to simulate the acquisition of a thermoremanent magnetization (TRM). The results obtained agree with Néel's single domain (SD) TRM theory when the magnetization of the model is constrained to rotate coherently. When this constraint is removed, a decrease in the blocking temperature of the model is observed above ≈0.07 μm showing Néel's SD TRM theory to be inaccurate for grains above this size.
Crustal structure and composition of the southern Foothills Metamorphic Belt, Sierra Nevada, California, from seismic data
Authors: Miller Kate C. , Mooney Walter D.
Published: February 2004

The Foothills Metamorphic Belt is an accreted terrane consisting of Paleozoic and Mesozoic metamorphic rocks that separates the Great Valley from the Sierra Nevada batholith in northern and central California. Until recently, the only available geophysical data for this area were reconnaissance refraction surveys, and gravity and magnetic data. New insights into the structure of the deep crust are provided by the interpretation of a seismic reflection profile (CC‐2), acquired in 1984 by the U.S. Geological Survey at the southern end of the Foothills Metamorphic Belt. Our interpretation is constrained by a new seismic velocity model derived from coincident microearthquake data. Earthquake hypocenters that occur at unusually great depths of 12 to 30 km make the data set particularly useful for obtaining deep crustal velocity information. The velocity model shows velocities of 5.2 to 6.3 km s−1 for the upper 12 km of the crust, and 6.7 to 6.8 km s−1 from 12 km to an estimated Moho at 32 km. The upper crustal velocities correspond to metamorphic rocks and serpentinites of the Foothills Metamorphic Belt as well as to diorites and granodiorites of the Sierra Nevada batholith, while the lower crustal velocities are interpreted to represent intermediate to mafic granulites. The majority of the earthquake hypocenters as well as a 6.7 km s−1 layer in the velocity model corresponds in depth to thick zones of west dipping midcrustal reflections that may represent major shear zones formed during the late Jurassic Nevadan orogeny or synbatholithic ductile shear zones that accommodated crustal extension associated with batholith intrusion. These reflections are truncated updip by an inferred subvertical contact that coincides with the western edge of the Sierra Nevada batholith and the southward trace of the Bear Mountains fault zone. The updip truncation of midcrustal shear zones and high lower crustal velocities indicate that strike‐slip faulting and magmatic underplating can be important processes during the docking and welding of an accreted terrane.
Heat flux from black smokers on the Endeavour and Cleft segments, Juan de Fuca Ridge
Authors: Ginster Ursula , Mottl Michael J. , Von Herzen Richard P.
Published: February 2004

We have estimated the heat flux from black smoker vents on the Juan de Fuca Ridge to evaluate their importance for heat transfer from young oceanic crust. The velocity and temperature of smoker effluent were measured from the manned submersible Alvin within a few centimeters of vent orifices, using a turbine flowmeter with an attached temperature probe. Exit velocity was calculated from a simple plume model, and vent orifices were measured in photographs and video records. The estimated power output from smokers alone is 49±13 MW for the Plume site, Vent 1 and Vent 3 on the southern Cleft segment near 45°N; 364±73 MW for the main vent field on the Endeavour Segment near 48°N; and 122±61 MW for the Tubeworm field 2 km north. The estimates for the Cleft and Tubeworm fields could be too low because of undiscovered vents. These values constitute only 4% to 14% of the total advective heat flux estimated for these vent fields from measurements in the nonbuoyant plume and of diffuse flow at the seafloor, indicating that most of the heat advected at these hydrothermal vent sites is carried by diffuse rather than focused flow. Values for individual smokers vary from 0.1 to 94 MW, with an average of 6.2 MW at the Endeavour field and 3.1 MW at the Cleft field. Our estimates agree well at all scales with those of Bemis et al. [1993] based on measurements made during the same dives, in some cases simultaneously, up to 50 m high in the buoyant plume. The good agreement between the two techniques implies that little diffuse flow at either high or low temperature is incorporated into the buoyant plumes generated by smokers at these sites. Velocity‐temperature measurements at vents excavated by Alvin could not be modeled successfully, suggesting that vent structures may grow in equilibrium with the force of the exiting water such that orifice size is determined by volume flux. At the Endeavour field the heat flux is focused by faults.
Seismic structure and anisotropy of the Juan de Fuca Ridge at 45°N
Authors: McDonald Mark A. , Webb Spahr C. , Hildebrand John A. , Cornuelle Bruce D. , Fox Christopher G.
Published: February 2004

A seismic refraction experiment was conducted with air guns and ocean bottom seismometers on the Juan de Fuca Ridge at 45°N, at the northern Cleft segment and at the overlapping rift zone between the Cleft and Vance segments. These data determine the average velocity structure of the upper crust and map the thickness variability of the shallow low‐velocity layer, which we interpret as the extrusive volcanic layer. The experiment is unique because a large number of travel times were measured along ray paths oriented at all azimuths within a small (20 km by 35 km) area. These travel times provide evidence for compressional velocity anisotropy in the upper several hundred meters of oceanic crust, presumed to be caused by ridge‐parallel fracturing. Compressional velocities are 3.35 km/s in the ridge strike direction and 2.25 km/s across strike. Travel time residuals are simultaneously inverted for anisotropy as well as lateral thickness variations in the low‐velocity layer. Extrusive layer thickness ranges from approximately 200 m to 550 m with an average of 350 m. The zone of the thinnest low‐velocity layer is within the northern Cleft segment axial valley, in a region of significant hydrothermal activity. Layer thickness variability is greatest near the Cleft‐Vance overlapping rift zone, where changes of 300 m occur over as little as several kilometers laterally. These low‐velocity layer thickness changes may correspond to fault block rotations in an episodic spreading system, where the low side of each fault block accumulates more extrusive volcanics.
Paleostress adjacent to the Alpine fault: Broader implications from fault analysis near Nelson, South Island, New Zealand
Authors: Pettinga Jarg R. , Wise Donald U.
Published: February 2004

Stress tensor determinations from probable Mio‐Pliocene age minor faults along the edge of the Richmond block, northwest of the Alpine fault system of South Island, show pervasive thrusting rather than strike‐slip motion as proposed in some analyses. This study, with results almost identical to that of Nicol and Wise (1992) on the other side of the Alpine/Marlborough fault system, shows that the present pattern of regional compression at high angles to the Alpine fault has persisted through much of the Neogene over a width of about 150 km normal to the Alpine fault zone. One explanation might be San Andreas‐type models invoking strike‐slip motion concentrated in fault zones by highly lubricating gouge coupled with strain partitioning to allow compression in adjacent regions to act at high angles to strike. Such models are difficult to apply to the New Zealand example which now must include deformed forelands on either side of the zone but which shows little or no strike‐slip motion. Instead, a model is proposed for a crustal scale “flower structure” to accommodate the along‐strike convergence of opposite dipping, subducting plates. A regional surface slab comprising Marlborough and its adjacent forelands is undergoing compression normal to the strike of the system while semidetached, deeper plate motion accommodates most of the fundamental, converging, oblique, strike‐slip displacements.