bluuebunny/crossref_metadata_2025 huggingface dataset:
First observations of intraseasonal oscillations in the equatorial mesosphere and lower thermosphere
Authors: Eckermann Stephen D. , Vincent Robert A.
Published: February 2004

We present the first evidence of strong intraseasonal (10–100 day) variability of the equatorial mesosphere and lower thermosphere, drawn from spectral analysis of three years of radar wind measurements of the region at Christmas Island (2°N, 157°W). Oscillations in the zonal wind as large as ±20 m s−1 are observed with periods ∼60 days at 85–95 km. Amplitudes attenuate near 80 km then reintensify near 75 km, where they are usually of longer period and are out of phase with the upper‐level oscillations. Oscillations with periods ∼35–40 days are also observed at 85–95 km. These oscillations are transient and usually appear when the background flow is westward. Similar signals are not observed in the meridional winds. These facts suggest that they could be Kelvin waves, similar to intraseasonal variability in the equatorial lower atmosphere associated with The Madden‐Julian Oscillation. Somewhat different time‐height activity at ∼20 days could be related to the (1,3) Rossby normal mode or the 20–25 day tropospheric oscillation. As it is unlikely that any of these oscillations could propagate to these heights from the lower atmosphere, we postulate that they arise due to variations in gravity‐wave activity and transmissivity induced by intraseasonal oscillations lower down, which is transferred into the mean flow when these gravity waves dissipate in the mesosphere. This may mean that this intraseasonal activity is a time‐varying mean wind rather than a forced planetary wave.
Electron density changes in the nighttime D region due to heating by very‐low‐frequency transmitters
Authors: Rodriguez Juan V. , Inan Umran S.
Published: February 2004

Modification of the nighttime D region electron density (Ne) due to heating by very‐low‐frequency (VLF) transmitters is investigated theoretically using a four‐species model of the ion chemistry. The effects of a 100 kW, a 265 kW, and a 1000 kW VLF transmitter are calculated for three ambient Ne profiles. Results indicate that Ne is reduced by up to 26% at ∼80 km altitude over a 1000 kW transmitter.
Correlation between stratospheric temperature, total ozone, and tropospheric weather systems
Authors: Petzoldt K. , Naujokat B. , Neugebohren K.
Published: February 2004

The unusual dynamical situation over Europe in the winter 1991/92 caused an unusual behaviour of the total ozone. The strongest negative deviations from the long‐term monthly means occured in January. A very cold middle stratosphere developed at the edge of the polar vortex above a warm anticyclonic block in the troposphere. The corresponding low temperature in the tropopause region was located just beneath the coldest air in the stratosphere.

A high positive correlation between the temperature and ozone partial pressure was derived at the edge of the polar stratospheric vortex for the tropopause region and for the middle stratosphere. The physical background was vertical motions changing temperature as well as the ozone content. That means unusually low ozone values can be expected when an elevated tropopause is combined with the adiabatically cooled end of the upwelling branch of an enhanced planetary wave in the middle stratosphere. It is shown that in February 1990, when extremely low total ozone was also observed over Scandinavia during a short anticyclonic blocking, this event was caused by the same process. However, during transient events the tropopause temperature is often anticorrelated with the middle stratospheric temperature (a tropospheric ridge reaching into the stratosphere has a cold high tropopause, but a warm middle stratosphere). The forced vertical motions result in extreme ozone columns only when they are in the same direction in both layers.

The enhanced wave activity was connected with a strong polar warming in the upper stratosphere in both winters. At 30 hPa in the middle stratosphere the warm center was situated over Eastern Siberia and the coldest part shifted towards Northern Europe.

The same situation was available in 11 cases during the fourteen years series of TOMS data (1979–1992), when total ozone reached values below 225 DU at the edge of the stratospheric polar vortex: enhanced wave activity in the middle stratosphere, polar warming in the upper stratosphere, shift of the coldest part of the polar vortex towards northern Europe over a cold high tropopause of a tropospheric anticyclone.
Lidar observations of polar stratospheric clouds at Andøya, Norway, in January 1992
Authors: Schäfer H. J. , Scheuch P. , Langer M. , Fricke K. H. , von Zahn U. , Knudsen B. M.
Published: February 2004

A Rayleigh‐Raman lidar on Andøya (69°N, 16°E), in northern Norway, was used to monitor aerosols during the Arctic winter 1991/92. Polar stratospheric clouds (PSCs) were detected on four days in January 1992. The observations on Jan. 5 show a PSC apparently at an early stage of development. Fully‐developed PSCs were seen on Jan. 8 and 9, covering an altitude range from 18 to 24 km; the cross‐polarized returns were high and the parallel returns low, characteristic of type 1a PSCs. The temperature histories of appropriate air parcels indicates that PSCs were mixed into the Pinatubo aerosol at lower altitudes on Jan. 19.
Intergranular basaltic melt is distributed in thin, elogated inclusions
Authors: Faul Ulrich H. , Toomey Douglas R. , Waff Harve S.
Published: February 2004

We describe a method to analyze the melt distribution in experimentally produced ultramafic partial melts. It is shown that the melt inclusions can be approximated by ellipses in two dimensions and by penny‐shaped ellipsoids in three dimensions. The aspect ratios of these ellipses (the ratio of the minor to the major axis) can in turn be used to calculate bulk physical properties of partial melts. We apply this method to two olivine‐basalt samples with 3.2% and 0.75% melt fraction. In the samples analyzed approximately 75% of the melt is contained in inclusions with much smaller aspect ratios than triple junction tubules. The reduction of P‐wave velocities calculated for this melt distribution is twice as large as for melt distributed solely in triple junction tubules.
The transition from porous to channelized flow due to melt/rock reaction during melt migration
Authors: Daines M. J. , Kohlstedt D. L.
Published: February 2004

The effect of melt/rock reaction on the mechanisms and kinetics of melt migration is investigated through a series of melt migration experiments in which melt saturated only in olivine infiltrates synthetic peridotites containing different proportions of olivine and Ca‐poor pyroxene. The microstructures that develop are significantly different from those formed during melt migration in a chemically equilibrated system. A reaction zone develops in which all pyroxene is dissolved, the melt fraction is increased, and olivine grain size is increased. The reaction front is unstable so that finger‐like structures or channels with higher melt content develop. The morphology of the instability depends on the initial pyroxene content of the infiltrated rock. This reactive‐infiltration instability with the resulting formation of melt fingers could facilitate the transition from porous to channelized flow in the mantle.
Two dimensional stratospheric aerosol distributions during EASOE
Authors: Wirth M. , Ehret G. , Mörl P. , Renger W.
Published: February 2004

LIDAR data of stratospheric aerosol layers from the Mount Pinatubo eruption, taken on long range flights with a Transall aircraft, have been investigated to study the exchange of air‐masses across the polar vortex boundary. From these measurements we found that the center region of the stratospheric polar vortex was free of volcanic aerosol above PT values of 485 K throughout the whole winter, with the exception of small isolated clouds appearing at the end of Jan. 92. Below 420 K the aerosol layers are very homogeneous on an isentropic surface and do not show much correlation with the vortex.
On a period with very low ozone concentrations within the lower stratosphere
Authors: Wege K. , Claude H.
Published: February 2004

The Hohenpeissenberg ozone data reveals a conspicuous ozone decrease during the winter of 1991/92. Compared with the long‐term mean, a decrease of about 10% could be observed in total ozone. A decrease of about 25% in the lower stratosphere was also seen. The possible causes for this ozone reduction were examined. At first the existing ozone trend which is considered to be caused by CFCs was eliminated. Because of contemporary, frequent high‐pressure situations over Europe, this dynamic influence was also estimated and eliminated. There still remains an ozone deficit, showing that there must be additional reasons. Here the aerosols from Pinatubo should be considered.
Freezing of stratospheric aerosol droplets
Authors: Luo Beiping , Peter Thomas , Crutzen Paul
Published: February 2004

Theoretical calculations are presented for homogeneous and heterogeneous freezing of sulfuric acid droplets under stratospheric conditions, based on classical nucleation theory. In contrast to previous results it is shown that a prominent candidate for freezing, sulfuric acid tetrahydrate (SAT ≡ H2SO4·4H2O), does not freeze homogeneously. The theoretical results limit the homogeneous freezing rate at 200 K to much less than 1 cm−3s−1, a value that may be estimated from bulk phase laboratory experiments. This suggests that the experimental value is likely to be a measure of heterogeneous, not homogeneous nucleation. Thus, under statospheric conditions, freezing of SAT can only occur in the presence of suitable nuclei; however, even for heterogeneous nucleation experimental results impose strong constraints. Since a nitric acid trihydrate (NAT) embryo probably needs a solid body for nucleation, these results put an important constraint on the theory of NAT formation in polar stratospheric clouds.
On the shape of the subducting slab beneath the Peloponnese, Greece
Authors: Hatzfeld Denis
Published: February 2004

The Mediterranean sea floor is subducted beneath the western Hellenic arc at a gentle angle for about 200 km and then steepens beneath the Gulf of Corinth. The seismic zone extends to a depth of 150 km, but the subducted intact lithosphere could reach 600 km. The present relative motion between Africa and Aegea can be considered as the sum of 2 motions: 1) slow (1 cm/y) relative motion between Africa and Europe, which describes the rate at which the deep slab penetrates the asthenosphere and which started during the upper Cretaceous time when Africa started to converge toward Europe and 2) fast (5 cm/y) relative motion between the southern Aegea and Europe, manifest by internal deformation within the Aegean, which may have been active at present rates only since Pliocene time. I suggest that the increase in the total convergent rates which began about 5 my ago is responsible for the wide, flat slab beneath the Peloponnese.
Anisotropy in Kuril‐Kamchatka Subduction Zone structure
Authors: Fischer Karen M. , Yang Xiaoping
Published: February 2004

To constrain the orientation and extent of seismic velocity anisotropy in the Kuril‐Kamchatka subduction zone, we have developed a technique that isolates anisotropy at shallow depths by differencing splitting observed in teleseismic shear‐wave phases. Olivine a‐axis orientations inferred from 29 sS‐S phase pairs and two local S phases vary between the major tectonic components of the trench/back‐arc system. Although the sS paths do not uniquely constrain a single model of anisotropy, a‐axes are consistent with finite strain patterns predicted by plate convergence and back‐arc spreading and shearing.
Elastic wave velocities in isochemical granulite and amphibolite: Origin of a low‐velocity layer at the slab/mantle wedge interface
Authors: Tatsumi Y. , Ito K. , Goto A.
Published: February 2004

Simultaneous measurements of compressional and shear wave velocities, Vp and Vs, in granulite and amphibolite with identical chemistry were carried out from room temperature to 850°C at 1 GPa. Both Vp and Vs in granulite and amphibolite decreased linearly with increasing temperature. The values of Vp and Vs at 700°C are 7.75 km/s and 4.37 km/s for granulite, and 7.40 km/s and 4.21 km/s for amphibolite, respectively. 3–4 % lowering of elastic wave velocities in hydrous amphibolite compared with anhydrous granulites may be consistent with seismological observations indicating the presence of a low‐velocity layer at the slab/mantle wedge interface in subduction zones.
Balloon‐borne mass spectrometer measurements of HNO3 and HCN in the winter Arctic stratosphere — Evidence for HNO3‐processing by aerosols
Authors: Spreng S. , Arnold F.
Published: February 2004

Altitude profiles of the stratospheric trace gases HNO3 and HCN were simultaneously measured by a balloon‐borne mass spectrometer, which was launched on 13 February, 1992 at Kiruna (68°N, 20°E, Northern Sweden). The HCN‐profile is nearly constant, around 0.18 ppbv, which is similar to previously measured midlatitude data. The HNO3‐profile measured below 24 km on the descent part of the balloon flight is similar to NOy (≲20ppbv), and becomes smaller than NOy above 24 km. Probably, the large HNO3, below 24 km, reflect heterogenous HNO3‐formation by H2SO4‐H2O aerosols from volcano Pinatubo. On ascent HNO3 was substantially decreased around 18 to 24 km, which was possibly due to HNO3‐dissolution in H2SO4‐H2O aerosols.
Constraints on North Atlantic upper mantle anisotropy from S and SS phases
Authors: Yang Xiaoping , Fischer Karen M.
Published: February 2004

Several analyses of SSH‐SH differential travel‐times disagree on whether these data contain evidence for azimuthal anisotropy in the North Atlantic upper mantle. To explore possible explanations for these discrepant results, we calculated the azimuthal variation in SSH‐SH travel‐times predicted by an anisotropic upper mantle model. Theoretical travel‐times were obtained by cross‐correlating S and SS phases on transverse‐component reflectivity synthetic seismograms. We determined that predicted azimuthal travel‐time patterns are sensitive to the relative amplitudes of radial and transverse particle motions incident on the anisotropic medium. If a large variation in the relative incident amplitudes exists in a dataset, the overlap of different azimuthal travel‐time patterns may obscure anisotropic signatures.

We analyzed shear‐wave splitting in a dataset of 12 SS and 10 S phase observations to constrain the presence of anisotropy in the North Atlantic upper mantle. A comparison of SS splitting parameters with S and SKS particle motions suggests that shear wave splitting apparent in the SS phases is at least partially due to anisotropy in the vicinity of their bounce‐points, although contributions from anisotropy in the source and station regions cannot be ruled out. Observed SS splitting is consistent with an olivine model in which horizontal a‐axes are aligned at an average of N24°W in the central North Atlantic (15°–40°N), and at an average of N23°E in the northern North Atlantic (40°–50°N).
Lidar observations of ice crystals associated with noctilucent clouds at middle latitudes
Authors: Thomas L. , Marsh A. K. P. , Wareing D. P. , Hassan M. A.
Published: February 2004

Lidar observations at Aberystwyth (52.4°N, 4.1°W) have shown layers of enhanced backscatter at heights just below the mesopause during four nights in late June and early July, 1993. The characteristics of these layers and their association with noctilucent clouds indicate that they are comprised of ice crystals. An examination is made of the temperature prior to the formation of a layer, the rates of descent of layers, and their integrated backscatter coefficients. The results are discussed in terms of the size of the crystals giving rise to backscatter, the ambient water mixing ratio, and the equivalent water vapour represented by these crystals.
Data‐dependent non‐double‐couple components of shallow earthquake source mechanisms: Effects of waveform inversion instability
Authors: Kuge Keiko , Lay Thorne
Published: February 2004

Seismic moment tensors are routinely determined for global earthquakes using waveform inversions, and the resulting double‐couple and non‐double‐couple characteristics are extensively used in tectonic analyses. For shallow events, several moment tensor components are intrinsically poorly resolved, especially for surface wave inversions, which can lead to artificial non‐double‐couple components or can obscure actual non‐double‐couple components in the source representations. The relatively greater instability of surface wave inversions can cause differences between inversions based on only body waves and joint inversions of body and surface waves. The Harvard CMT catalog exhibits systematic variations in non‐double‐couple components with seismic moment, which can be attributed to use of only body waves in inversions for small events and combined body and surface waves for large events. Systematic behavior of non‐double‐couple components with regional strain environment is significantly less dependent on seismic moment when solutions influenced by less stable surface waves are omitted.
Transition to whistler mediated magnetic reconnection
Authors: Mandt M. E. , Denton R. E. , Drake J. F.
Published: February 2004

The transition in the magnetic reconnection rate from the resistive magnetohydrodynamic (MHD) regime where the Alfvén wave controls reconnection to a regime in which the ions become unmagnetized and the whistler wave mediates reconnection is explored with 2‐D hybrid simulations. In the whistler regime the electrons carry the currents while the ions provide a neutralizing background. A simple physical picture is presented illustrating the role of the whistler in driving reconnection and the rate of whistler mediated reconnection is calculated analytically. The development of an out‐of‐plane component of the magnetic field is an observable signature of whistler driven reconnection.
Strain accumulation 1986–1992 across the Reykjanes Peninsula Plate Boundary, Iceland, determined from GPS measurements
Authors: Sturkell Erik , Sigmundsson Freysteinn , Einarsson Páll , Bilham Roger
Published: February 2004

GPS geodetic observations 1986–1992 spanning the oblique plate boundary on the Reykjanes Peninsula, SW Iceland, show that left‐lateral shear strain is accumulating in the area. The principal strain rates are: έ1=0.255 ± 0.055 µstrain/yr (extension) at N121 ± 6°E, and έ2=‐0.190 ± 0.053 µstrain/yr (contraction) at N31 ± 6°E. The principal axis of extension is perpendicular to the strike of fractures within volcanic systems on the Peninsula. The direction of maximum left‐lateral shear strain is N76 ±6°E, the same as the trend of the seismic zone on the Peninsula. Tensor shear strain in this direction is −0.222±0.054 µstrain/yr. Continuous aseismic slip of the North American‐Eurasian plates below a 5–11 km locking depth on the Peninsula can explain the observed depth of earthquakes, the width of the deformation zone determined from recent fissuring and faulting, and the GPS‐measurements of strain.
Compositional vs. thermal buoyancy and the evolution of subducted lithosphere
Authors: Gaherty James B. , Hager Bradford H.
Published: February 2004

We formulate 2‐D Cartesian finite element models that explore the fate of compositionally defined lithosphere as it encounters a viscosity increase at the boundary between the upper and lower mantle. Subducted lithosphere is represented as a cold, stiff, layered composite of denser eclogite underlain by more buoyant harzburgite. Slabs impinging on a lower mantle 30 and 100 times more viscous than the upper mantle thicken and fold strongly as they penetrate the lower mantle. Approximately a factor of two thickening occurs via pure shear just above the discontinuity, with additional enhancement due to folding by over a factor of two. No separation of the individual slab components occurs at the discontinuity, and direct comparison with models in which compositional buoyancy is explicitly ignored indicates that slab evolution is largely controlled by the thermal buoyancy. These results are at odds with hypotheses about slab evolution in which the compositional buoyancy contributions lead to component separation and the formation of slab megaliths or a compositionally layered upper mantle.
CF3 chemistry: Potential implications for stratospheric ozone
Authors: Ko M. K. W. , Sze N.‐D. , Rodríguez J. M. , Weistenstein D. K. , Heisey C. W. , Wayne R. P. , Biggs P. , Canosa‐Mas C. E. , Sidebottom H. W. , Treacy J.
Published: February 2004

Previous evaluations of the impact of fluorine chemistry on stratospheric ozone have concluded that the role of fluorine compounds in catalytic ozone removal is negligible. However, recent investigations of the degradation pathways for compounds containing CF3 groups indicates that if the reaction of CF3O with O3 is sufficiently fast, there may be an ozone impact. Some recent measurements indicate that the reaction rate constant of CF3O+O3 is sufficiently low that the ozone impact is likely to be small. However, it is not possible a‐priori to rule out significant ozone removal without additional kinetic data on other reactions. We present calculations to illustrate how different key reactions affect the calculated stratospheric concentrations of the CF3X species (CF3, CF3O, CF3O2, CF3OH, CF3OOH, CF3ONO2, CF3O2NO2, CF3OOCl) and their ability to remove stratospheric ozone. We utilize our results to suggest kinetic measurements that could substantially reduce the uncertainties in CF3 chemistry relevant to the determination of ozone depletion potential of CF3‐bearing compounds.
Measurements of the J(O¹D) actinic flux within and above stratiform clouds and above snow surfaces
Authors: Junkermann Wolfgang
Published: February 2004

Photochemical reactions in the troposphere are strongly dependent on the amount of radiation available for radical production. Albedo changes affect significantly the vertical distribution of the actinic radiation fluxes in the planetary boundary layer. Actinic fluxes over and within cloud layers or snow surfaces with very high albedo are of special interest. Flight experiments were performed in an altitude range of up to 1000 m above ground with and without stratiform cloud layer, and above fresh snow surfaces. The photolysis rate J(O¹D) was measured from a hangglider with two 2π sensitive photoelectric detectors separately providing the upward and downward radiation components with a high vertical resolution. The actinic flux shows a very strong contribution of reflected or backscattered radiation within the planetary boundary layer. The albedo contributions seem to be more dependent on atmospheric optical thickness than are assumed in current model calculations.
Relativistic electron acceleration and decay time scales in the inner and outer radiation belts: SAMPEX
Authors: Baker D. N. , Blake J. B. , Callis L. B. , Cummings J. R. , Hovestadt D. , Kanekal S. , Klecker B. , Mewaldt R. A. , Zwickl R. D.
Published: February 2004

High‐energy electrons have been measured systematically in a low‐altitude (520 × 675 km), nearly polar (inclination = 82°) orbit by sensitive instruments onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX). Count rate channels with electron energy thresholds ranging from 0.4 MeV to 3.5 MeV in three different instruments have been used to examine relativistic electron variations as a function of L‐shell parameter and time. A long run of essentially continuous data (July 1992–July 1993) shows substantial acceleration of energetic electrons throughout much of the magnetosphere on rapid time scales. This acceleration appears to be due to solar wind velocity enhancements and is surprisingly large in that the radiation belt “slot” region often is filled temporarily and electron fluxes are strongly enhanced even at very low L‐values (L ∼ 2). A superposed epoch analysis shows that electron fluxes rise rapidly for 2.5 ≲ L ≲ 5. These increases occur on a time scale of order 1–2 days and are most abrupt for L‐values near 3. The temporal decay rate of the fluxes is dependent on energy and L‐value and may be described by J = Ke‐t/to with to ≈ 5–10 days. Thus, these results suggest that the Earth's magnetosphere is a cosmic electron accelerator of substantial strength and efficiency.
The importance of subduction for the modern stress field in the Tyrrhenian area
Authors: Bassi Gianna , Sabadini Roberto
Published: February 2004

The Tyrrhenian sea was formed by lithospheric extension in a context of continental convergence between Africa and Eurasia. At present, a compilation of strain and stress indicators in this region by Rebai et al. (1992) shows that an extensive regime is still predominant in the Tyrrhenian sea while northern Italy and southern Sicily are undergoing compression. This tectonic configuration is the result of two major contributions which act on a very heterogeneous lithosphere: the Africa‐Eurasia collision, on the one hand, and the subduction along the Calabrian arc, on the other. The latter is expected to create a ‘trench suction force’ on the overriding Tyrrhenian plate. The objective of this paper is to investigate the relative importance of these contributions using a numerical model developed by P. Bird (1989) based on an improved version of the ‘thin plate’ approach which models lithospheric deformation in an horizontal plane. The results of this model demonstrate the necessity of considering the forces related to subduction to understand the modern stress and strain field. An extrusion mechanism alone cannot explain present‐day observations. The salient features of the current tectonic regime are reproduced by the model which, however, predicts more limited extension than expected. Further work will be needed to find a better tuning of the parameters and ascertain the quality of available data.
Unprecedented 1993 ozone decrease over the United States from Dobson spectrophotometer observations
Authors: Komhyr W. D. , Grass R. D. , Evans R. D. , Leonard R. K. , Quincy D. M. , Hofmann D. J. , Koenig G. L.
Published: February 2004

Dobson spectrophotometer observations conducted since the early‐to‐mid 1960s at Bismarck, North Dakota; Caribou, Maine; Boulder, Colorado; Wallops Island, Virginia; Nashville, Tennessee; and at Fresno, California, since 1983, have revealed record low total ozone values during 1993. The tendency toward the low ozone values began in May 1992, but accelerated in early 1993. During January–August 1993, ozone monthly means at the stations were more than 2 standard deviations below long‐term normal monthly means 72% of the time and more than 3 standard deviations below normals 42% of the time. On average, the January–April 1993 ozone values were 12.6% below normal, with ozone deficits as large as 18% observed at Caribou and Wallops Island in January. Of particular concern are unusually low ozone values that occur in summertime when solar ultraviolet insolation is high. Such record lows occurred at four of the six stations (Caribou, Wallops Island, Fresno, and Nashville). During May–August 1993, ozone was on average 8.5% below normal at these sites. Monthly means at these stations were, furthermore, lower on average by 3.7% than corresponding lowest values observed there in the past. The ozone decrease of 12.6% below normal at the six continental Dobson instrument stations during the winter and spring months of 1993 implies a possible average increase in UV erythemal radiation at that time of 16–25% above normal. The 8.5% decrease in ozone at Caribou, Wallops Island, Fresno, and Nashville, implies that on average, UV erythemal radiation may have been higher than normal at these stations during the summer of 1993 by 11–17%.
In situ observations of dissolved iron and manganese in hydrothermal vent plumes, Juan de Fuca Ridge
Authors: Chin Carol S. , Coale Kenneth H. , Elrod Virginia A. , Johnson Kenneth S. , Massoth Gary J. , Baker Edward T.
Published: February 2004

In situ measurements of dissolved manganese and total dissolved iron were conducted in hydrothermal plumes over the Juan de Fuca Ridge using a submersible chemical analyzer (Scanner). The Scanner was deployed as part of a CTD/transmissometer/rosette instrument package on both tow‐yos and vertical casts during the VENTS Leg I cruise in 1989. Dissolved manganese and total dissolved iron concentrations, along with temperature and light attenuation anomalies, were determined over the ridge crest every 5 s. Discrete samples for laboratory analyses of dissolved iron II, total dissolved iron II+III and manganese were also collected. Metal to heat ratios (Me:Q) measured in situ were extremely variable in one steady state plume, while an event plume had constant Me:Q. Uniform values of Mn:Q in the event plume demonstrate that Mn behaves conservatively in the near‐field plume. Variability in the Mn: Q ratios in a steady state plume indicated the presence of at least two hydrothermal sources with distinct Me:Q values. A simple mixing model shows that the contribution of Mn from high Me:Q sources, with a composition characteristic of black smoker vents, varies between 1% and 99% within the core of the steady state plume with an average value of 55%. On average, over 50% of the excess heat within the plume originates from low Me:Q ratio sources, with a composition characteristic of low‐temperature, diffuse flow vent fluids. Less than 4% of the volume of hydrothermal fluids in the plume originates from black smokers. The Fe II concentrations were used to provide an estimate of plume age on a transect across the ridge axis. Plume ages were about 2.5 days on axis and >12 days off axis. These plume ages were modeled to provide estimates of plume transport and horizontal diffusion and show excellent agreement with ages determined using 222Rn.
Trace metals in hydrothermal solutions from Cleft segment on the southern Juan de Fuca Ridge
Authors: Trefry John H. , Butterfield David B. , Metz Simone , Massoth Gary J. , Trocine Robert P. , Feely Richard A.
Published: February 2004

Concentrations of trace metals in Fe‐ and Cl‐rich hydrothermal solutions from the southern Juan de Fuca Ridge (SJFR) have been determined and corrected for residual precipitates formed in the sampler. Precipitate corrections for Ag, Cd, Cu, Mo, Pb, Sb, and Zn were variable, contributing an average of 20% to total solution concentrations whereas corrections for Co averaged less than 5% and essentially no corrections were required for Fe and Mn. Values for Cu, Co, and Mo in these solutions showed a strong dependence on temperature with sharp decreases in concentrations as temperatures decreased to less than 320°C. In addition, and unlike most other metals studied, all vent fluids from the SJFR were almost completely depleted in Mo relative to seawater values of about 110 nmol kg−1. In contrast to the Cu‐Co‐Mo group, concentrations of Ag, Cd, Sb, and Pb correlated well with those for Zn and are presumed to follow a distribution that is influenced less by temperature over the 246 to 332°C range encountered in this study and more by the combined chemical processes that control Zn levels along the transport pathway from the deep reaction zone to the vent orifice.
Three‐dimensional gravity study of the Mid‐Atlantic Ridge: Evolution of the segmentation between 28° and 29°N during the last 10 m.y.
Authors: Rommevaux C. , Deplus C. , Patriat P. , Sempéré J.‐C.
Published: February 2004

During the high‐resolution survey SARA (Segmentation Ancienne de la Ride Atlantique), Sea Beam bathymetry, magnetic, gravity, and seismic reflection data were collected on the flanks of the Mid‐Atlantic Ridge, between 28° and 29°N. This survey was designed to provide off‐axis information (up to approximately 10 m.y.) and to complement a detailed on‐axis survey carried out in 1988 and 1989 (Sempéré et al., 1990) between the Atlantis and Kane fracture zones. A previous gravity study had revealed the existence of “bulls‐eye” shaped gravity lows centered on the axial segments and gravity highs centered on the non‐transform discontinuities (Lin et al., 1990). We carried out a three‐dimensional calculation of the mantle Bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center. The off‐axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities. After correcting for the gravity effect of lithospheric cooling away from the ridge, the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry. Off‐axis bathymetry is characterized by southward pointing, V‐shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously. However, the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows: northward on the western flank and southward on the eastern flank. The sense of offset between the gravity and bathymetric traces appears to be related to the right‐stepping nature of the axial discontinuities.
The evolution of deformation and topography of high elevated plateaus: 1. Model, numerical analysis, and general results
Authors: Wdowinski Shimon , Bock Yehuda
Published: February 2004

A temperature dependent viscoplastic flow model of continental lithosphere is used to investigate the evolution of deformation and topography of high elevated plateaus. Such plateaus are products of both continent‐continent collision (Tibetan Plateau) and ocean‐continent collision (the Altiplano) and develop in the overriding continental plate. In this study we emphasize the mechanically simpler case of oceanic collision, because it does not involve mass transfer between the two plates. The lithosphere is deformed in response to tectonic and buoyancy forces. The tectonic forces arise from subduction of an oceanic plate (or underthrusting of continental lithosphere) that horizontally indents and vertically shears the overriding lithosphere. The buoyancy forces arise in response to horizontal density variations and tend to relax existing topography or thick crust. The time evolution of the deformation and topography is investigated using a finite element technique that solves for the flow field in the overriding lithosphere. The model produces dynamically supported near‐trench topography and inland mountain topography that is isostatically supported by a thick crust. A finite region of localized deformation, thick crust, and high topography develops only if the model includes a horizontal thermal perturbation or an initially thick crust; however, only thermally perturbed lithosphere generates a plateau topography. The shape and size of the calculated plateau depend on the wavelength of the thermal perturbation, Grashof number, and density contrast between the crust and mantle. The time evolution of the deformation shows a significant change in the deformation pattern as the high elevated plateau evolves. During early stages, compressional deformation of the crust and mantle are localized in the thermally perturbed weak zone. At later stages, as the crust thickens, buoyancy forces of larger magnitude resist further thickening of the crust and the locus of compressional crustal deformation migrates inland. This migration does not affect the location of the mantle deformation, which remains in the thermally weak region, but it is accompanied by a significant shear deformation in the weak lower crust. The separation of the crustal locus from the mantle locus of deformation emphasizes the importance of vertically dependent deformation in the formation of high elevated plateaus. This demonstrates the limitations of models that ignore changes of deformation with depth, such as plane stress or thin sheet models.
Three‐dimensional modeling of plume‐lithosphere interaction
Authors: Ribe N. M. , Christensen U. R.
Published: February 2004

In order to understand better the dynamics of hot spots such as Hawaii, we present a three‐dimensional numerical model for the interaction of a thermal plume with a moving lithosphere. The model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure. The lithosphere is represented by a layer of cold, highly viscous fluid moving with an imposed horizontal velocity U in the x direction, and a thermal plume is generated by a circular temperature anomaly on the bottom of the box. The steady flow is determined numerically using a hybrid spectral/finite difference technique. The flow is characterized by a “stagnation streamline” of width y ∼ x1/5 that represents the edge of the spreading plume material. We illustrate the detailed behavior of the model using the example of the Hawaiian plume. Our best fitting Hawaiian model is obtained by adjusting the plume buoyancy flux B until the predicted topography anomaly matches the observed Hawaiian swell topography; we find B = 4100 kg s−1, which implies a plume radius of 90 km for an assumed plume/mantle temperature contrast of 300°C. The predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion. We therefore conclude that the classical “lithospheric reheating” model is unable to account for hotspot swells. The horizontal flux of buoyancy associated with the swell exceeds B by up to 80%, suggesting that current estimates of B for mantle plumes are too high. Empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on B and U that agree well with those predicted by the “refracted plume” model of Olson (1990). The principal weakness of the model is that the predicted geoid/topography ratio of 0.010 for the Hawaiian swell is about twice the observed value of 0.004–0.006.
The evolution of deformation and topography of high elevated plateaus: 2. Application to the central Andes
Authors: Wdowinski Shimon , Bock Yehuda
Published: February 2004

The central Andes form a wide elevated plateau flanked in the west by a steep slope toward the deep Chilean Trench and in the east by a gentle slope that subsides gradually toward the Brazilian Shield. The low elevated trench topography is dynamically supported, whereas the high Andean mountain topography is mostly isostatically supported by a thick crust. The last mountain building phase, which thickened the crust and formed the present‐day Andes, began 26 m.y. ago, in the late Oligocene, with the increase of the convergence rate between the Nazca and the South American plates. We investigate the time evolution of the Andean deformation and topography by applying a temperature dependent viscoplastic flow model of continental lithosphere to the South American plate. The model predicts the observed present‐day topographic profile across the central Andes, from the trench across the high Altiplano plateau to the Brazilian Shield. Our numerical results, combined with observations of the spatial and temporal distribution of igneous activity in the central Andes, lead us to conclude that the Altiplano developed and extended to its present width of 400 km as a result of thermal weakening of the lithosphere since late Oligocene until present. The model also predicts the observed eastward migration of the locus of the Andean crustal deformation with time. At early stages, both the crustal and mantle loci of deformation lie in the thermally weak region, which results in crustal thickening in this finite region. At later stages, as the crust thickens, it induces increased buoyancy forces, which resist crustal thickening beyond 65 km. As a result, the locus of crustal deformation migrates eastward. The detachment of the crustal locus of deformation from that of the mantle can explain the observed change in deformation pattern from thick‐skinned tectonism during early stages of the deformation to thin‐skinned tectonism during the more recent stages.
Gravity model improvement using the DORIS tracking system on the SPOT 2 satellite
Authors: Nerem R. S. , Lerch F. J. , Williamson R. G. , Klosko S. M. , Robbins J. W. , Patel G. B.
Published: February 2004

A high‐precision radiometric satellite tracking system, DORIS, has recently been developed by the French space agency, Centre National d'Etudes Spatiales (CNES). DORIS was designed to provide tracking support for missions such as the joint United States/French TOPEX/Poseidon. As part of the flight testing process, a DORIS package was flown on the French SPOT 2 satellite. A substantial quantity of geodetic quality tracking data was obtained on SPOT 2 from an extensive international DORIS tracking network. These data were analyzed to assess their accuracy and to evaluate the gravitational modeling enhancements provided by these data in combination with the GEM‐T3 gravitational model. These observations have noise levels of 0.4 to 0.5 mm/s, with few residual systematic effects. Although the SPOT 2 satellite experiences high atmospheric drag forces, the precision and global coverage of the DORIS tracking data have enabled more extensive orbit parameterization to mitigate these effects. As a result, the SPOT 2 orbital errors have been reduced to an estimated radial accuracy in the 10–20 cm RMS range. The addition of these data, which encompass many regions heretofore lacking in precision satellite tracking, has significantly improved GEM‐T3 and allowed greatly improved orbit accuracies for Sun‐synchronous satellites like SPOT 2 (such as ERS 1 and EOS). Comparison of the ensuing gravity model with other contemporary fields (GRIM‐4C2, TEG2B, and OSU91 A) provides a means to assess the current state of knowledge of the Earth's gravity field. Thus, the DORIS experiment on SPOT 2 has provided a strong basis for evaluating this new orbit tracking technology and has demonstrated the important contribution of the DORIS network to the success of the TOPEX/Poseidon mission.
Physical state of the western U.S. upper mantle
Authors: Humphreys Eugene D. , Dueker Kenneth G.
Published: February 2004

Using observed P wave images of the western U.S. upper mantle, which show lateral variations of up to 8%, and existing scaling relations, we infer that the low‐velocity mantle is hot and partially molten to depths of 100–200 km, and that the high‐velocity upper mantle is subsolidus. Most the high‐velocity upper mantle within a few hundred kilometers of the coastline appears to be relatively dense, suggesting that it is relatively cool (i.e., a thermal lithosphere). This is expected for features associated with the subducting Juan de Fuca and Gorda slabs, and the high velocity upper mantle beneath the Transverse Ranges has been attributed to the sinking of negatively buoyant mantle lithosphere. Other high‐velocity mantle structures near the continental margin are consistent with this interpretation. In contrast, the generally high elevations of the continental interior imply a buoyant upper mantle there, an inference that holds for both the high‐ and the low‐velocity upper mantle. The only reasonable way to produce the high‐velocity low‐density upper mantle is through basalt depletion, thereby creating mantle of increased solidus temperature and decreased density. We distinguish a marginal domain, within ∼250 km of the Pacific coast, from an interior domain. This is based on the inferred upper mantle compositional difference and regional associations: beneath the marginal domain, upper mantle structures trend parallel to the surface physiography and young tectonic structures, whereas upper mantle structures beneath the continental interior trend northeasterly. This northeast orientation is discordant with the young tectonic structures, but aligns with young volcanic activity. The high lateral gradients in observed upper mantle seismic structure found throughout the western United States imply high lateral gradients in the: associated temperature or partial melt fields. Because these fields diffuse on time scales of less than a few tens of millions of years, the imaged upper mantle structure is young. The following upper mantle processes are hypothesized to account for these findings and inferences. Away from the plate margin, small‐scale upper mantle convection driven by partial melt‐induced buoyancy of hot upper mantle leads to the production and segregation of melt and the creation of compositional variations. The heterogeneous upper mantle P wave structure of the elevated continental interior is largely a consequence of partial meltvariations that are modulated by the compositional variations, and throughout this region we infer high temperatures and low densities. Near the plate margin, relative plate motions force upper mantle flow, although upper mantle flow driven by the positive buoyancy of melt and the negative buoyancy lithosphere is important locally.
Off‐axis hydrothermal circulation: Parametric tests of a refined model of processes at Deep Sea Drilling Project/Ocean Drilling Program site 504
Authors: Fisher A. T. , Becker K. , Narasimhan T. N.
Published: February 2004

We have developed a refined model of coupled heat and fluid flow to numerically simulate off‐axis hydrothermal circulation through the upper oceanic crust. The new representation includes mesh elements with curved sides and noncentered nodes. These curvilinear elements allow improved simulation of topography at the seafloor and within underlying sediment and basaltic layers. Curvilinear simulations of circulation at Deep Sea Drilling Project/Ocean Drilling Program site 504 confirm the broad conclusions derived from the rectilinear simulations of Fisher et al. (1990) but also suggest that permeability within the upper few hundred meters of crust around hole 504B must be concentrated within several narrow zones. This interpretation is consistent with core observations and wireline logs from hole 504B. Within these highly permeable zones, absolute permeability may be several orders of magnitude higher than the bulk permeability measured for the upper crust as a whole. A series of detailed parametric simulations was conducted to determine the quantitative importance of basal heat flow, seafloor bathymetry, basement relief, and differential sediment thickness in modifying the geometry and intensity of off‐axis hydrothermal circulation. These studies reveal that basement relief and differential sediment thickness are more important than seafloor bathymetry in enhancing off‐axis convection within the seafloor. Observed natural correlations between seafloor bathymetry and heat flow may exist because bathymetry is a proxy for the other two important parameters, basement relief and differential sediment thickness. Off‐axis convection at geochemically significant velocities is possible even with heat input appropriate for some of the oldest oceanic crust. A thick sediment layer over older crust may cause enough conductive refraction at the seafloor to mask the variations in seafloor heat flow often associated with off‐axis hydrothermal convection.