bluuebunny/crossref_metadata_2025 huggingface dataset:
Mirror modes and fast magnetoacoustic waves near the magnetic pileup boundary of comet P/Halley
Authors: Glassmeier K. H. , Motschmann U. , Mazelle C. , Neubauer F. M. , Sauer K. , Fuselier S. A. , Acuña M. H.
Published: February 2008

Large‐amplitude ultralow‐frequency wave structures observed on both sides of the magnetic pileup boundary of comet P/Halley during the flyby of the Giotto spacecraft have been analyzed using suprathermal electron density and magnetic field observations. Upstream of the boundary, electron density and magnetic field magnitude variations are anticorrelated, while in the pileup region these quantities are clearly correlated. Both in front of and behind the pileup boundary the observed waves are quasi‐perpendicular wave structures as a minimum variance analysis shows. A detailed comparison of our observations in the prepileup region with theoretical and numerical results shows that the mirror mode mode waves may have been generated by a mirror instability driven by the pressure anisotropy of the ring‐type distributions of the heavy (water group) pickup cometary ions. A mean wavelength λ1 = 2071 km is found, which is about 4 times the water group ion thermal gyroradius. For the fast mode wave structures in the pileup region, analysis of wave propagation on both sides of the pileup boundary shows that mode conversion from the mirror mode to fast mode type waves at the boundary is not a possible source mechanism. However, we show that the fast mode waves may be interpreted as stationary wave structures in a multifluid plasma. The wavelength λ2 = 1141 km derived from the observations, when using the stationary wave hypothesis, is in good agreement with the theoretical expected wave length λtheo = (774 ± 506) km computed from available ion observations.
Coseismic slip of two large Mexican earthquakes from teleseismic body waveforms: Implications for asperity interaction in the Michoacan Plate Boundary Segment
Authors: Mendoza Carlos
Published: February 2008

Teleseismic body waves recorded for the October 25, 1981, Playa Azul and September 21, 1985, Zihuatanejo earthquakes in western Mexico were inverted to derive the distributions and depths of coseismic slip. Broadband P wave displacements and intermediate‐period records were included to incorporate a wide frequency band in the analysis. For the Zihuatanejo earthquake, digital SH records located away from the nodal directions were used in the inversion process. Nonnodal SH waveforms were not available for the Playa Azul earthquake, but additional azimuthal coverage was obtained by including selected worldwide analog P wave records. The results for the Playa Azul earthquake indicate that rupture occurred in two separate zones both updip and downdip of the point of initial nucleation with the majority of the slip concentrated in a circular region (15‐km radius) downdip from the hypocenter. The maximum slip in this downdip region exceeds 3.6 m and is at a depth of about 16 km. The computed seismic moment is 7.14 × 1026 dyn cm. Coseismic slip is observed to occur entirely within the area of reduced slip separating the two main shallow sources of the Michoacan earthquake that occurred almost 4 years later on September 19, 1985. For the Zihuatanejo earthquake the P and SH data suggest rupture over a larger area (30‐km radius) with a lower peak slip (2 m) and encompassing depths between 12 and 26 km. Slip is concentrated in an area adjacent to one of the major sources of the Michoacan earthquake and represents the southeastern continuation of rupture along the Cocos‐North America plate boundary. The corresponding seismic moment is 1.35 × 1027 dyn cm. The zones of peak slip observed for the Playa Azul, Zihuatanejo, and Michoacan earthquakes are interpreted as asperity regions that control the cessation and generation of large earthquakes within the Michoacan segment of the plate boundary. The stress drops within each of these asperities are observed to be very similar and may reflect a characteristic asperity strength for this portion of the subduction zone.
Ion microprobe analyses bearing on the composition of the upper mantle beneath the Basin and Range and Colorado Plateau Provinces
Authors: Roden Michael F. , Shimizu Nobumichi
Published: February 2008

Ion microprobe analyses of clinopyroxene from predominantly spinel‐bearing peridotite xenoliths found in volcanic rocks of the Colorado Plateau (Buell Park, Green Knobs localities) and the Basin and Range (Black Canyon, Lunar Crater localities) provinces are highly variable in light and heavy rare earth element (LREE, HREE), Sr, Zr and Ti contents. Abundances of these elements vary by 2–3 orders of magnitude. The LREE are fractionated from the HREE such that relatively LREE‐enriched and LREE‐depleted inclusions occur at all localities. However, LREE‐depleted clinopyroxenes predominate at the Colorado Plateau localities, whereas LREE‐enriched clinopyroxenes or those with chondritic REE patterns predominate at the Basin and Range localities. In addition to fractionation of the LREE from the HREE, the high field strength elements Zr and Ti are generally depleted relative to REE of similar compatibility. Although some of this depletion reflects subsolidus partitioning between orthopyroxene and clinopyroxene, the extremely low Ti abundances (50–500 ppm) of some clinopyroxenes must reflect very low bulk rock contents of Ti. This depletion does not correlate with LREE/HREE ratios. The bulk of the peridotites from the Colorado Plateau are interpreted to be residues from partial melting; extremely low Ce/Sm]cnratios (less than 0.1) are similar to these ratios in clinopyroxenes from abyssal peridotites and require that the melting process in some cases approached ideal fractional melting; the very low Sr content of these clinopyroxenes (less than 10 ppm) suggest that clinopyroxene was not a residual phase. The association of these peridotites with LREE‐depleted websterites which may be crystal segregates from a magma similar to modern mid‐ocean ridge basalts reinforces the link between modern oceanic lithosphere and the upper mantle beneath the Colorado Plateau. In contrast, the clinopyroxenes from the Basin and Range tend to have relatively high Ti contents (greater than 2000 ppm) and only moderate fractionation of the LREE from the HREE or to be LREE‐enriched and have low Ti contents (as low as 100 ppm) but high Sr contents (to 220 ppm). The former clinopyroxenes are derived from fertile peridotites which could yield typical basaltic magmas upon melting or may have equilibrated with such magmas in the upper mantle. The latter clinopyroxenes are from metasomatic peridotites which likely reacted with a carbonatitic melt These features may be a consequence of widespread Cenozoic magmatism of the Basin and Range Province.
Constitutive laws for high‐velocity frictional sliding and their influence on stress drop during unstable slip
Authors: Weeks John D.
Published: February 2008

The Dieterich‐Ruina state‐variable friction laws do a good job describing results of rock friction experiments, and fault models based on them are able to mimic natural seismicity in many respects. To be useful for modeling earthquakes, high velocities must be successfully modelled. Ruina gave a formula for steady state frictional strength with a constant (usually negative) slope on a logarithmic plot that does not agree with recent observations of friction of a variety of materials at velocities greater than 30 to 100 μms−1. This steady state function, termed the “log‐linear function,” with inertia neglected, does not recover from instability and, consequently, cannot give predictions of stress drop or peak velocities during unstable slip. Adding inertia yields stress drops that are too large to match experimental observations. This paper explores the consequences for unstable slip when inertia is considered and when the steady state function is altered at high velocity. Two steady state functions are considered: one that has no dependence on velocity at high slip velocity (“zero‐slope”) and one that has a positive velocity dependence at high velocity (“positive‐slope”). Inclusion of inertia and use of these modified steady state functions improve the results of simulations in terms of qualitatively reproducing many aspects of unstable sliding, but the positive‐slope function yields the best quantitative agreement with experimental observations. Use of the modified steady state functions predicts that stress drop during unstable sliding should decrease with increasing loading velocity and at sufficiently high load point velocity there will be a transition to stable sliding, a result that is observed experimentally.
Seafloor spreading and the ophiolitic sequences of the Troodos Complex: A principal component analysis of lava and dike compositions
Authors: Thy P. , Esbensen K. H.
Published: February 2008

The Troodos complex of Cyprus has been interpreted as a slice of oceanic crust formed from ultramafic magma in a back‐arc basin. Recent investigations, however, have suggested that only the upper gabbros, the sheeted dike complex, and the lower lava sequence formed as a direct consequence of seafloor spreading and that the primary magma was of basaltic composition. According to this model, the upper lava suite and related ultramafic cumulates are not represented in the sheeted dike complex. A multivariate statistical analysis of major element compositions of 480 dikes and 130 volcanic glasses supports this suggestion. Statistically (98%), the dikes are similar to the glasses of the lower lava suite. This supports the model that the seafloor spreading related sequence of the Troodos ophiolite is relatively thin and of basaltic composition. The magma chambers represented by the ultramafic cumulates and associated gabbros and the upper lavas appear to have been emplaced after, or during, the termination of the major spreading events.
Morphotectonic characteristics of a propagating spreading system in the northern Lau Basin
Authors: Wiedicke Michael , Habler Walter
Published: February 2008

Bathymetric Sea Beam mapping of a southward propagating rift system in the Lau Basin (SW Pacific) shows a morphotectonic segmentation of the two competing axes: Over a distance of 70 km the propagating axis develops from a narrow rift graben at the propagator tip, via a leaky graben, into a juvenile volcanic ridge, and finally into a fully developed spreading ridge. The competing dying spreading axis 70 km farther east shows steady deepening towards the north, combined with a change from a small volcanic ridge to a narrow depression before it is buried by sediments. The relay zone between the two axes contains two deep N‐S striking grabens; the eastern one, 3200 m deep, may act as a short‐lived spreading center. A NE‐trending string of slightly offset deep sediment‐filled basins is interpreted as fossil analoges to these grabens, originally generated in the southward migrated relay zone. The string of basins suggests an episodic rather than steady state southward migration. Propagation and opening of grabens led to a rotation of the inherited ridge‐parallel crustal fabric. However, the propagating tip and the grabens cut obliquely through the fabric thus generating small‐scale structural and morphological inhomogeneities of the crust. At the propagating tip, old and new tectonic lineaments closely interfinger.
On the accuracy of digital bathymetric data
Authors: Smith Walter H. F.
Published: February 2008

The global 5‐arcmin gridded topography data ETOPO‐5 are based on contour maps rather than original soundings and have large errors. The artificial statistical distribution generated by digitizing contours makes these data unsuitable for use in regression models for depth‐age variations. Their amplitude spectrum is bounded by a (frequency)−4 power law, so that they should not be used when the gravity‐topography transfer function is important, such as in studies of flexural isostasy and mantle convection. I assess the accuracy of 14,491,069 digital ship soundings in 2253 cruise surveys collected between 1955 and 1992 in the Lamont‐Doherty Earth Observatory on‐line data base by analyzing 329,058 crossover errors (COEs) at intersecting ship tracks. Five percent of cruises with internal COEs yield root‐mean‐square COE amplitudes exceeding 500 m; all of these have errors in digitizing two‐way travel time from analog precision depth recorder traces. Twenty‐eight cruises were found which had errors caused by misinterpretation of the nominal sound velocity used when travel times were reported as nominal depths. Two nominal sound velocities in common use differ by 2.5%, an amount which is often undetectable, producing uncertainties in depth of this magnitude. Ship data have been acquired at different rates over time, with the peak of activity in the early 1970s. Although present technologies can yield very accurate data, these are acquired at a rate which is small with respect to the total available data; the cumulative median global COE has remained constant at 26 m since the late 1970s. Most recent data acquisition has been in the northern hemisphere oceans, and the oldest and least accurate data are in the southern oceans where median COEs are 100–250 m. The majority of the data in the South Pacific were acquired before the advent of satellite navigation.
Magnetite dissolution and neocrystallization during cleavage formation: Paleomagnetic study of the Martinsburg Formation, Lehigh Gap, Pennsylvania
Authors: Housen Bernard A. , van der Pluijm Ben A. , Van der Voo Rob
Published: February 2008

Paleomagnetic study of the shale to slate transition in the Ordovician Martinsburg Formation at Lehigh Gap, Pennsylvania reveals that the remanence of the relatively undeformed shales predates cleavage formation, and that the slates were remagnetized during cleavage development. The shales display two components of remanence. Component 1 (C1) was the only magnetization direction in the majority of the shales, and the first‐removed component in 10 shale specimens. Component 2 (C2) is observed as a single‐component remanence with a low (300–400°C) unblocking temperature range in samples from one shale station and as a second‐removed component in 10 shale specimens. The mean directions are (declination/inclination) 347°/−04° (346°/−47°) in situ (tilt‐corrected) for C1 and 80°/−63° (135°/−40°) for C2. The pencil slates and well‐cleaved slates both display single component remanences. The mean directions are 198°/−12° (201°/26°) for the pencil slates and 199°/24° (225°/54°) for the well‐cleaved slates. Using the relationship between cleavage development and remagnetization in the slates, the timing of cleavage development and folding, and the regional geology, we can constrain the relative ages of the remanences as follows: (1) the shale C1 direction is most likely prefolding and represents a primary upper Ordovician direction, (2) the slate directions are younger in age than the shale directions and are probably of Permo‐Carboniferous (Alleghenian) age. These results show that remagnetization occurred during development of slaty cleavage in these rocks. We interpret the mechanism for this remagnetization to have been strain‐induced dissolution and neocrystallization of magnetite. The additional difference in the characteristic directions of the pencil slates and well‐cleaved slates may be attributed to strain reorientation of the pencil slate remanence after new growth of magnetite. The unknown temporal relationship between dissolution and neocrystallization and strain precludes any quantitative attempt to correct remanence for strain in these samples. Our study shows that paleomagnetic studies of deformed sediments must consider the possibility of dissolution and neocrystallization of remanence carriers, in addition to strain‐induced rotation.
Confidence intervals for motion and deformation of the Juan de Fuca Plate
Authors: Wilson Douglas S.
Published: February 2008

Finite‐rotation parameters for 17 isochrons from anomalies 1‐4A for the Juan de Fuca‐Pacific plate pair are reported. Comparison of confidence intervals for plate rotations derived by different techniques indicates that a single‐plate misfit criterion significantly underestimates the confidence region. Defining the confidence interval by a threshold level of misfit determined by an F statistic gives similar results to the technique of bootstrap resampling for internally consistent data. Bootstrap resampling offers far greater robustness in the case of inconsistent data. Precision of about 1 km in the direction normal to isochrons is available for the younger anomalies. This precision allows confident recognition of 2–5 km of eastward deformation of the Pacific plate, 50–100 km south of the active Sovanco transform. Slight deformation of the northern Juan de Fuca plate adjacent to the Nootka fault zone is also apparent East of the northern Gorda ridge, younger (< 3 Ma) anomalies do not show measurable deformation near the Blanco fracture zone, but older anomalies are up to 30 km east of their position predicted by rigid behavior of the Juan de Fuca plate. Reconstruction of the overlap geometry of past propagators shows no evidence that the extent of overlap has ever exceeded 70 km, even with offset of 75 km in one case and 140 km in another. New techniques for testing for intervals of constant plate motion require at least two changes in instantaneous motion at 99% confidence: a clockwise change in motion direction at about 5 Ma and an increase in the gradient of spreading rate since 2 Ma. Two other changes in gradient since 5 Ma are required at 95% confidence.
A view of the lower crustal component of hydrothermal systems at the Mid‐Atlantic Ridge
Authors: Gillis Kathryn M. , Thompson Geoffrey , Kelley Deborah S.
Published: February 2008

An extensive suite of hydrothermally altered gabbros was recovered by Alvin and dredging from the Mid‐Atlantic Ridge, in the vicinity of the eastern ridge‐transform intersection (RTI) of the Kane Fracture Zone (MARK), where tectonic extension has provided a window into the lower crustal component of hydrothermal cells. Four alteration types are distinguished on the basis of metamorphic assemblage, mineral composition, and deformation textures. A conceptual model is presented that places temperature ‐ fluid/rock ratio relationships, in conjunction with styles of deformation, into a spatial and temporal framework. Exsolution of late stage magmatic fluids in the vicinity of the magma‐hydrothermal interface at temperatures >700°C marks the onset of fluid‐rock interaction in the MARK gabbros (Kelley et al., this issue), with subsequent alteration involving seawater‐derived fluids being strongly influenced by deformation mechanisms. In zones of ductile shear, hydration was initiated at temperatures between 550° and 700°C and moderate fluid/rock ratios. Elsewhere in the lower crust, the onset of seawater penetration occurred at temperatures of 400°–550°C and was facilitated by brittle fracturing. Early vein networks indicate very low initial fluid/rock ratios and are interpreted as being related to the solidification of plutons. Later fractures indicate higher water/rock ratios and may be related to the downward propagation of shallow fault systems. Throughout the crust, fluid‐rock interaction ceased at temperatures 180°–300°C. Subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the RTI massif produced moderate fluid/rock ratios within localized zones. Hydrothermal alteration in the crustal section exposed at MARK was initiated at lower temperatures, and water/rock interaction proceeded to lower temperatures than many other plutonic suites. These differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust.
Interactions between magma chambers and hydrothermal systems: Oceanic and ophiolitic constraints
Authors: Nehlig P.
Published: February 2008

A compilation of published exit temperatures and salinities for seafloor hydrothermal fluids, along with fluid inclusion data on oceanic and ophiolitic rocks, shows that ridge axes hydrothermal systems are generally characterized by maximum temperatures around 300°–360°C, thus clearly being in the one‐phase field. They overlie a lower system marked by the generation of highly saline brines at high temperatures. The brines may have originated either when seawater approached the top of the magma chamber or more probably as exsolution products of the differentiated melts that typify the roof of oceanic magma chambers. In ophiolites, the brines typically occur within the transition zone between the sheeted‐dike complex and the plutonic sequence, i.e., on top of magma chambers. This transition zone, relatively well known from ophiolite studies (Oman, Troodos, Bay of Islands, Josephine), is characterized by mutually intrusive relationships between gabbro, plagiogranite, and dikes and xenoliths of altered diabase within plagiogranite and gabbro. In fast to medium spreading ophiolites (Oman) it is the locus of a major downward decrease in the density of hydrothermal veins and alteration. The vein system is strongly anisotropic and shows a well‐marked preferred vertical along‐strike orientation. The underlying cumulates are almost unaltered and exhibit only a diffuse net of amphibole veins, while the overlying sheeted‐dike complex is densely veined and pervasively altered. This zone is marked by major gradients in the physical parameters over short distances: temperature (several degrees Celsius per meter), pressure (several hundred kilopascals), transition from brittle to ductile. In steady state systems, direct interactions between the magma chamber and the hydrothermal system are restricted to this zone, characterized by exsolution of magmatic volatiles into the hydrothermal system and the incorporation of hydrothermal fluids into the magma chamber by digestion of altered roof rocks. Complications to this steady state model resulting from a non‐steady state magma chamber (e.g., Josephine ophiolite) are mainly the development of late stage faulting and more pronounced retrogressive alteration.
Magnitude‐frequency relations for earthquakes using a statistical mechanical approach
Authors: Rundle John B.
Published: February 2008

At very small magnitudes, observations indicate that the frequency of occurrence of earthquakes is significantly smaller than the frequency predicted by simple Gutenberg‐Richter statistics. Previously, it has been suggested that the dearth of small events is related to a rapid rise in scattering and attenuation at high frequencies (i.e., the controversial “fmax” problem) and the consequent inability to detect these events with standard arrays of seismometers. However, several recent studies have suggested that instrumentation cannot account for the entire effect and that the decline in frequency may be real. Working from this hypothesis, we derive a magnitude‐frequency relation for very small earthquakes that is based upon the postulate that the system of moving plates can be treated as a system not too far removed from equilibrium. As a result, it is assumed that in the steady state, the probability P[E] that a segment of fault has a free energy E is proportional to the exponential of the free energy P ∝ exp[‐ E/ EN]. In equilibrium statistical mechanics this distribution is called the Boltzmann distribution. The probability weight EN is the space‐time steady state average of the free energy of the segment. Earthquakes are then treated as fluctuations in the free energy of the segments. With these assumptions, it is shown that magnitude‐frequency relations can be obtained. For example, previous results obtained by the author can be recovered under the same assumptions as before, for intermediate and large events, the distinction being whether the event is of a linear dimension sufficient to extend the entire width of the brittle zone. Additionally, a magnitude‐frequency relation is obtained that is in satisfactory agreement with the data at very small magnitudes. At these magnitudes, departures from frequencies predicted by Gutenberg‐Richter statistics are found using a model that accounts for the finite thickness of the inelastic part of the fault zone. The inelastic thickness of the fault zone that is obtained is in general agreement with thicknesses found from field observations following earthquakes. Thus, departures from simple Gutenberg‐Richter scaling are apparently due at very large magnitudes to the finite width of the brittle layer, and at very small magnitudes to the finite thickness of the inelastic fault zone.
Equation of state, elastic properties, and stability of CaSiO3 perovskite: First principles (periodic Hartree‐Fock) results
Authors: Sherman David M.
Published: February 2008

The cubic CaSiO3 perovskite phase is possibly the third most abundant mineral in the lower mantle. In addition to its geochemical significance, CaSiO3 perovskite provides a useful system with which to test the application of quantum chemistry to mantle silicates. Here, the electronic structure, equation of state, and elastic properties of cubic CaSiO3 perovskite are calculated using the periodic Hartree‐Fock formalism. Cubic CaSiO3 is found to be stable relative to an orthorhombic structure to at least 106 GPa. Calculations using a moderately extended basis set give an equation of state for CaSiO3 in close agreement with experiment (V0 = 44.96 Å3, K0 = 300 GPa, and K0′ = 4). The aggregate shear modulus of CaSiO3 perovskite is found to be 209 GPa which is comparable to that of MgSiO3 perovskite. The high seismic velocities of CaSiO3 perovskite means it will act as a seismic complement to magnesiowustite in the lower mantle. The free energy, as a function of pressure, of stishovite and B2‐CaO have also been calculated at the same level of theory. From those results, it is found that cubic CaSiO3 perovskite is stable relative to the free oxides at pressures up to 130 GPa (the core‐mantle boundary).
Importance of model parameterization in finite fault inversions: Application to the 1974 Mw 8.0 Peru Earthquake
Authors: Hartzell Stephen , Langer Charley
Published: February 2008

The spatial and temporal slip distributions for the October 3, 1974 (Mw = 8.0), Peru subduction zone earthquake and its largest aftershock on November 9 (Ms = 7.1) are calculated and analyzed in terms of the inversion parameterization and tectonic significance. Teleseismic, long‐period World‐Wide Standard Seismograph Network, P and SH waveforms are inverted to obtain the rupture histories. We demonstrate that erroneous results are obtained if a parameterization is used that does not allow for a sufficiently complex source, involving spatial variation in slip amplitude, risetime, and rupture time. The inversion method utilizes a parameterization of the fault that allows for a discretized source risetime and rupture time. Well‐located aftershocks recorded on a local network have the same general pattern as teleseismically determined hypocenters and help to constrain the geometry of the subduction zone. For the main shock a hinged fault is preferred having a shallow plane with a dip of 11° and a deeper, landward plane with a dip of 30°. The preferred nucleation depth lies between 11 and 15 km. A bilateral rupture is obtained with two major concentrations of slip, one 60 to 70 km to the northwest of the epicenter and a second 80 to 100 km to the south and southeast of the epicenter. For these source regions, risetimes vary from 6 to 18 s. Our estimates of risetimes are consistent with the time for the rupture to traverse the dominant local asperity. The slip distribution for the November 9 aftershock falls within a conspicuous hole in the main shock rupture pattern, near the hypocenter of the main shock. The November 9 event has a simple risetime function with a duration of 2 s. Aftershocks recorded by the local network are shown to cluster near the hypocenter of the impending November 9 event and downdip from the largest main shock source region. Slip during the main shock is concentrated at shallow depths above 15 km and extends updip from the hypocenter to near the plate boundary at the trench axis. The large amount of slip at shallow depths is attributed to the absence of any significant accretionary wedge of sediments, and the relatively young age and high convergence rate of the subducted plate, which results in good seismic coupling near the trench axis.
Tracking mesoscale ocean features in the Caribbean Sea using Geosat altimetry
Authors: Nystuen Jeffrey A. , Andrade Carlos A.
Published: February 2008

Geosat Exact Repeat Mission (ERM) altimetry data, collected during 1987–1988 over the Caribbean Sea, are used to detect and track the movement of mesoscale sea surface height anomalies. Along‐track derivatives are used to estimate the mean sea surface height. Contour maps of sea surface height (SSH) anomaly are generated for each 17‐day exact repeat period. Two anticyclonic features are observed during April‐July, one each year, with SSH signatures that reached +30 cm. These features initially are observed near the Beata Ridge and propagate westward with an average speed of 15 cm/s. No mesoscale anomalies associated with the Caribbean Current are apparent during other seasons. The formation of the anticyclones coincides with the seasonal increase to maximum wind stress curl and water transport in the central Caribbean. A quasi‐permanent cyclonic eddy is detected near the southwestern portion of the Colombian Basin, confirming model predictions.
Spatially correlated depth changes in the nearshore zone during autumn storms
Authors: Hay Alex E. , Bowen Anthony J.
Published: February 2008

Acoustic reflection measurements of seabed elevation were made at Stanhope Beach, Prince Edward Island, during three successive storms in October‐November 1989. The instruments were located approximately 200 m offshore on the seaward face of a shore parallel bar in a mean water depth of 2.2 m. Bed elevation measurements were made at 30‐min intervals with a range resolution of ±1 mm at horizontal spacings between 22 cm and 1.5 m, using an array of four acoustic sounders operating at 1, 2.25, and 5 MHz. It is shown that such an array can be used as (1) an interferometer to determine bedform properties and migration rates, (2) a leveling device to determine changes in local bottom slope, and (3) an erosion/deposition gauge. In the interferometric mode, time domain autocorrelation and cross‐correlation analyses of the bottom elevations are used to estimate the wavelengths, migration rates, and migration directions of sand ripples and megaripples. The ripple wavelengths (10–15 cm) and heights (3–5 cm) are comparable to measurements made by divers. Ripple migration rates were 1–6 cm h−1directed offshore in the direction of the mean shore normal near‐bottom current and increased with increasing mean current speed. Evidence of megaripple‐like features (2–5 cm height; 1.5–3 m length) is found, also migrating in the direction of the mean shore normal near‐bottom drift, at speeds of about 20 cm h−1. Bedload transport rates in the onshore‐offshore direction are estimated from the bedform migration rates. The ripple transport rate was small (an order of magnitude smaller than the transport of sediments in suspension by the mean offshore drift). In contrast, the large‐scale bedform transport appears to have been comparable to the mean current transport of suspended sediment. Interestingly, it appears that the total bedform transport was comparable to that predicted by Watanabe's (1981) net bedload transport formula, provided a coefficient appropriate to irregular wave conditions is used. During the third and highest‐energy storm event, the measurements indicate transition to flat bed, and generalized local erosion at a maximum rate of 8 cm h−1. This erosion event was associated with a general shoreward migration of the bar.
Sea level response to the 1986–1987 El Niño‐Southern Oscillation Event in the western Pacific in the vicinity of Papua New Guinea
Authors: Ridgway K. R. , Godfrey J. S. , Meyers G. , Bailey R.
Published: February 2008

Sea levels at the Papua New Guinea coast responded to the 1986–87 El Niño‐Southern Oscillation (ENSO) event much as would be expected for points on the inshore edge of a western boundary current driven by remote sea level disturbances. The first empirical orthogonal function of sea level in the region accounts for 60% of the variance, and shows a clear western boundary current signal. Sea level observations from tide gauges and estimates from adjacent expendable bathythermographs (XBTs) are in good agreement, so XBT data are used to supplement sea level results; specifically, we obtain an estimate of the western boundary current as a function of depth. The anomalous transport through Vitiaz Strait varied by about 15 Sv during the event, with possibly more following the eastern New Ireland coast. The western boundary current response appears to be located entirely equatorward of the inflow feeding it, as expected from linear theory. The boundary current followed the Trobriand Island ridge, rather than the main Papua New Guinea coastline, and appears to bifurcate along southern New Britain. A maximum response to the ENSO occurred south of New Ireland, where steric sea level appeared to vary by up to 45 cm due to the ENSO event; this may be an inertial feature.
Seasonal evolution of oxygen isotope‐salinity relationships in high‐latitude surface waters
Authors: Strain Peter M. , Tan Francis C.
Published: February 2008

The relationship between the isotopic composition of seawater and salinity and the variation of the isotopic composition of precipitation with latitude have frequently been used to identify freshwater sources and to tag water masses. Underlying these applications is the assumption that the isotope‐salinity relationship is affected only by the mixing of salt and fresh waters. Simple box models are used to show how δ18O‐salinity (δ‐S) relationships in the upper water column vary seasonally in areas where sea ice forms or melts. During melting, the δ‐S relationship takes a number of different shapes, but it returns to a straight line late in the melting season. During freezing, the δ‐S shape stays linear, but its slope varies. The models show that ice‐related processes can produce very large changes in the δ18O value of apparent freshwater components (δ18OS=0), the parameter usually used to label water masses. At the end of the melting season the δ18OS=0 value may be 10‰ more positive than at the beginning; at the end of the freezing season, δ18OS=0 may be more than 20‰ more negative than at the beginning. Thus the models provide explanations for both anomalously heavy δ18OS=0 values in the Labrador Current (−7.6‰) and anomalously light values in the Canadian arctic archipelago and the East Greenland Current (≈ −50‰). They can also explain the negative slope occasionally observed in δ‐S plots. Caution must be exercised in using δ18OS=0 to identify source waters derived from the Labrador Sea, which receives significant amounts of freshwater from Baffin Bay and Hudson Strait which, in many cases, have been altered by more than a single season of melting/freezing activity.
Wind‐forced variability of the deep eastern North Pacific: Observations of seafloor pressure and abyssal currents
Authors: Niiler P. P. , Filloux J. , Liu W. T. , Samelson R. M. , Paduan J. D. , Paulson C. A.
Published: February 2008

Data from an array of bottom pressure gauges and a string of current meters in the vicinity of 47°N, 139°W, are used to examine the deep‐ocean variability forced by ocean surface wind stress curl from August 1987 to June 1988. Bottom geostrophic currents are computed from the pressure gauge array, and these correspond well to the long‐period directly measured currents at 3000 m. The supratidal‐period bottom pressure variations are coherent at 95% confidence with the wind stress curl in period bands of 3–4 days and 15–60 days but removed in distances of 400 and 700 km to the northwest and the southeast, respectively. A linear, two‐layer hydrodynamic model is used to examine the theoretical forcing produced by random‐phased surface wind fields for the conditions of the eastern north Pacific and the 15‐ to 60‐day‐period observed response is reproduced credibly. To model 3‐ to 15‐day variations, more realistic models are required.
Directional attributes of the ocean surface wind stress vector
Authors: Geernaert G. L. , Hansen Finn , Courtney Michael , Herbers Tom
Published: February 2008

In a recent paper by Geernaert (1988) it was suggested that the stress and wind vectors are rarely aligned and that the deviation of the stress vector direction from the wind vector is related to the surface heat flux. It was argued that since cold air advection over the open ocean generally corresponds to upward heat flux, the consequent rotation of the winds to the right with height during cold air advection could be responsible for observations of the stress vector to be to the right of the wind flow for unstable stratifications. Conversely, observations of the stress vector to the left of the flow for stable stratifications were suggested to be associated with the turning of the wind vector to the left of the flow for warm air advection, or downward heat flux. In that study only 30% of the statistical variance of the angle between the wind and wind stress vectors could be explained by air‐sea heat flux, and it was suggested that the orbital velocity spectrum of surface waves (i.e., data unavailable in that study) could be important for explaining at least part of the remaining variance. In this study we report on measurements of the wind stress vector and wave directional spectrum collected during the Synthetic Aperture Radar X‐Band Ocean Nonlinearities experiment. Conducted on the Chesapeake Light Tower during 1988, the experiment yielded data reported herein that provide convincing evidence that the stress vector direction, during light winds and near‐zero air‐sea heat flux, is governed in large part by the direction of long waves on the ocean surface. Further, the measurements suggest that the stress vector has a direction which is a blend between the wind direction and the swell direction.
A comparison of gradient and observed over‐the‐water winds along a mountainous coast
Authors: Macklin S. Allen , Stabeno Phyllis J. , Schumacher James D.
Published: February 2008

Surface winds measured from April 11, 1987, through August 16, 1987, at five locations off the southeastern coast of the Alaska Peninsula indicate the complexity of coastal wind structure as a function of proximity to a mountainous coast. Proxy winds were computed for the same five locations from digitized sea level pressure fields using a simple geotriptic wind model. Shoreward of the Rossby deformation radius, the friction parameters of the geotriptic model vary from their open‐ocean values, slowing the winds and rotating them farther counterclockwise. These changing friction parameters account for the effect of the mountains on the surface wind distribution. Proxy winds and observed surface winds are coherent for periods longer than about 2 days and are better correlated farther seaward from the coast and for northeasterly alongshore winds and southeasterly onshore winds. Estimates of numbers of storms, regional wind stress, and regional vorticity determined from proxy winds are qualitatively accurate.
Dynamical properties of a buoyancy‐driven coastal current
Authors: Münchow Andreas , Garvine Richard W.
Published: February 2008

The outflow of buoyant waters from major estuaries affects the dynamics of inner continental shelves profoundly as lateral density gradients force an alongshore current. Often the Coriolis force causes the outflow to remain trapped near the coast. We observed one such current, the Delaware Coastal Current, on the inner shelf near the Delaware Estuary on the eastern seaboard of the United States. The spatial variability along the shelf, however, suggests at least two dynamically distinct regions that we term source and plume regions. In the source region we find fronts, a current whose width scales well with the internal deformation radius, and a ratio of relative to planetary vorticity that reaches unity, that is, the Rossby number is O(1). As nonlinear inertial forces in the across‐shelf momentum balance are weak, we suggest that such forces contribute to the along‐shelf momentum balance only. Farther downstream in the plume region, we find much reduced lateral density gradients, a current much wider than the deformation radius, and relative vorticities that are much smaller than the planetary vorticity. From our observations we compute nondimensional dynamical parameters, with which we discuss our observations. The Burger, Rossby, and Ekman numbers for the Delaware Coastal Current suggest that most models of buoyancy‐driven coastal currents do not apply to this coastal flow.
Variability of sources and sinks of CO2 in the western Indian and southern oceans during the year 1991
Authors: Poisson Alain , Metzl Nicolas , Brunet Christian , Schauer Bernard , Bres Bernard , Ruiz‐Pino Diana , Louanchi Ferial
Published: February 2008

For the period from January to September 1991 we describe spatial and temporal variations of sea surface carbon dioxide fugacity (fCO2) in the Antarctic, Subantarctic, subtropical, and tropical regions of the Indian Ocean (including the Red Sea). The measurements were made continuously with an infrared technique during seven cruises. We study the temporal variations of fCO2 at daily, monthly and seasonal scales in selected areas. High‐frequency variabilities of 20 μatm/d have been observed near polar frontal zone. Both spatial and temporal fCO2 variations are large near the subtropical and Subantartic fronts. In the subtropical domain, fCO2 decreases regularly from austral summer to winter. In January this region is a small CO2 sink with values near equilibrium with the atmosphere. In July, low fCO2 (300 μatm) leads to a CO2 flux of −4.5 mmol/m2/d into the ocean for the zonal band 23°S‐35°S. A quantitative study of monthly and seasonal fCO2 budgets is presented for the subtropical area. Considering first the observations at seasonal scale, it is shown that changes in fCO2 can be explained by temperature variations and air‐sea exchanges; the sum of biological and mixing processes, considered as the balance of the seasonal fCO2 budget, is close to zero. The monthly fCO2 budgets are then calculated. In that case, other processes must be taken into account to close the budget: the observations indicate that the effect of productivity exceeds the one of mixing in austral summer and the opposite in winter. We then describe the seasonal air‐sea fCO2 differences (ΔfCO2) for the whole western Indian Ocean and corresponding Antarctic sector (18,000 observations). In the equatorial and tropical regions the ocean is a CO2 source as was previously observed in the 1960s. In the subtropical area the CO2 sink dominates but varies strongly on a monthly scale. In the circumpolar front zones there is a large potential CO2 sink in summer. In the Antarctic waters, fCO2 spatial variability is very high at mesoscale, especially in the area of the Kerguelen plateau. Finally, it is shown that in some oceanic areas, well‐defined relations exist between fCO2 distribution and temperature and salinity. If we want to use them to constrain mappings of continuous fCO2 fields from sparse observations, such relations must be considered at regional and at least seasonal scales.
Measurement of the gas phase concentration of H2SO4 and methane sulfonic acid and estimates of H2SO4 production and loss in the atmosphere
Authors: Eisele F. L. , Tanner D. J.
Published: February 2008

Measurements of gas phase sulfuric and methane sulfonic acid (MSA) have been performed using a relatively new atmospheric pressure selected ion chemical ionization mass spectrometric technique at two field sites. Both gas phase acids are photooxidation products, and their concentrations are seen to qualitatively follow solar flux. While sulfuric acid concentrations typically decline in conjunction with declining solar radiation, they sometimes level off in the mid 105–106 molecules cm−3 range after dark, even in relatively clean air. The reason for this quasi‐stable nighttime sulfuric acid concentration is not well understood but may be a result of a steady state exchange of sulfuric acid between particles and the gas phase.

Measurements of OH, H2SO4, and SO2 concentrations in conjunction with aerosol number and size distribution also made possible the independent calculation of gas phase sulfuric acid production and loss rates. Calculated production and loss rates are seen to agree well in relatively clean air during the daylight hours. At night, however, the sulfuric acid concentrations and its calculated loss rate often have a nonzero value. In more polluted air masses, calculated gas phase sulfuric acid losses significantly exceed calculated production if H2SO4/aerosol reaction probabilities of 1.0 or 0.5 are assumed.
Atmospheric sulfur hexafluoride: Sources, sinks and greenhouse warming
Authors: Ko Malcolm K. W. , Sze Nien Dak , Wang Wei‐Chyung , Shia George , Goldman Aaron , Murcray Frank J. , Murcray David G. , Rinsland Curtis P.
Published: February 2008

Model calculations using estimated reaction rates of sulfur hexafluoride (SF6) with OH and O(1D) indicate that the atmospheric lifetime due to these processes may be very long (25,000 years). An upper limit for the UV cross section would suggest a photolysis lifetime much longer than 1000 years. The possibility of other removal mechanisms are discussed. The estimated lifetimes are consistent with other estimated values based on recent laboratory measurements. There appears to be no known natural source of SF6. An estimate of the current production rate of SF6 is about 5 kt/yr. Based on historical emission rates, we calculated a present‐day atmospheric concentrations for SF6 of about 2.5 parts per trillion by volume (pptv) and compared the results with available atmospheric measurements. It is difficult to estimate the atmospheric lifetime of SF6 based on mass balance of the emission rate and observed abundance. There are large uncertainties concerning what portion of the SF6 is released to the atmosphere. Even if the emission rate were precisely known, it would be difficult to distinguish among lifetimes longer than 100 years since the current abundance of SF6 is due to emission in the past three decades. More information on the measured trends over the past decade and observed vertical and latitudinal distributions of SF6 in the lower stratosphere will help to narrow the uncertainty in the lifetime. Based on laboratory‐measured IR absorption cross section for SF6, we showed that SF6 is about 3 times more effective as a greenhouse gas compared to CFC 11 on a per molecule basis. However, its effect on atmospheric warming will be minimal because of its very small concentration. We estimated the future concentration of SF6 at 2010 to be 8 and 10 pptv based on two projected emission scenarios. The corresponding equilibrium warming of 0.0035°C and 0.0043°C is to be compared with the estimated warming due to CO2 increase of about 0.8°C in the same period.