bluuebunny/crossref_metadata_2025 huggingface dataset:
Rise and tilt of metamorphic rocks in the lower plate of a detachment fault in the Funeral Mountains, Death Valley, California
Authors: Hoisch Thomas D. , Simpson Carol
Published: February 2008

The Funeral Mountains in eastern California preserve a record of Early Cretaceous (?) metamorphism followed by ductile deformation, uplift, and low‐angle normal (detachment) faulting. 40Ar/39Ar age spectra indicate that cooling and uplift of the lower plate began in Cretaceous time. Uplift was accommodated by normal‐sense movement along a wide northwest dipping shear zone. Mylonitic fabrics, some of which have been dated as Late Cretaceous, deformed older high‐temperature metamorphic textures. Analyses of shear bands, mica fish, σ and δ porphyroclasts, grain shape fabrics, and folds indicate that the upper surfaces moved toward 299° ±12 (top to the northwest) relative to lower surfaces. Uplift continued until the near present, the youngest phase being accommodated by top‐to‐the‐northwest movement along the detachment fault, which formed subparallel to lower‐plate mylonitic fabrics. Fission track apatite data indicate that exposure of the lower plate to the surface occurred sometime after 6 Ma. Reconstruction along the movement vector places the Grapevine Mountains over the Funeral Mountains, having been displaced at least 40 km. Isograds and thermobarometry in pelitic schist from the lower plate indicate increasing pressures and temperatures of equilibration toward the northwest. The maximum temperature and pressure was determined on a sample from Monarch Canyon using thermobarometry, 700°C at a depth of 32 km. At Chloride Cliff, 5 km southeast of Monarch Canyon, 4 samples yielded 575°–600°C at depths of 19–27 km. At Indian Pass, 17 km southeast of Monarch Canyon, a temperature of 490°C was determined. In the southern Funeral Mountains, about 50 km southeast of Monarch Canyon, conodont color alteration indexes indicate temperatures of 325°–425°C. These data indicate that the lower plate is presently tilted strongly to the southeast from the orientation it maintained at the peak of metamorphism. Thermochronologic data (K‐Ar on muscovite, biotite, and hornblende, 40Ar/39Ar on hornblende, and fission track on apatite, titanite, and zircon) indicate that both tilting and the transition from ductile to brittle styles of quartz deformation are confined to the interval 21–6 Ma; during the latter part of this interval (11–6 Ma), rapid uplift and movement along the detachment fault are documented. The findings support current theories of detachment fault evolution in which a dipping fault surface undergoes rotation to a subhorizontal orientation while the lower plate undergoes a comparable tilt.
Posterosional volcanism in the Cretaceous part of the Hawaiian Hotspot Trail
Authors: Lonsdale Peter , Dieu Julie , Natland James
Published: February 2008

Multibeam bathymetry and seismic reflection profiling shows the Cretaceous products of the Hawaiian hotspot to be a ridge of coalesced guyots that prolong the N‐NW trend of the more isolated Paleocene Emperor Seamounts, a 10,000 km2 plateau (Detroit Plateau), where the trail changes strike to NW, and the Obruchev Rise hotspot ridge, which extends to the Kamchatka Trench. The northernmost guyots were submerged and tilted southeast by the load of new shield volcanoes added to the end of the chain, then secondary volcanism built small cones on their summit platforms and in a gap between two guyots. Dredged rock samples and the distribution of the cones indicates that on one guyot these submarine eruptions were in the alkalic postshield stage of Hawaiian volcanism, and at another were probably in the alkalic rejuvenated stage. Seamount‐building eruptions at Detroit Plateau produced lavas (nepheline melilitites) that belong geochemically to the alkalic rejuvenated stage, and are very similar to Pleistocene lavas in the Hawaiian Islands. However, these eruptions postdated passage off the hotspot plume by a much longer time than the 0.5–2.5 m.y. observed in the Hawaiian Islands, and were probably initiated by different tectonic processes.
Theory of alternating field demagnetization of multidomain grains and implications for the origin of pseudo‐single‐domain remanence
Authors: Xu Song , Dunlop David J.
Published: February 2008

Both analytical arid numerical models are developed to investigate the effects on alternating field (AF) stability of both microcoercivity associated with pinned walls and magnetic screening by soft walls in multi‐domain (MD) grains. It is shown that due to screening, AF stabilities of remanence in MD grains are usually significantly higher than their microcoercivities, the only exception being for two‐domain grains, in which there is no screening. One important implication is that hard “pseudo‐single‐domain” remanence observed in grains larger than single‐domain size but with AF stabilities higher than the expected maximum microcoercivity associated with domain wall pinning may be simply due to the screening by soft walls in individual grains. Moreover, we predict that MD grains containing a small number of defects do not necessarily show a low AF stability, for the reduction in the AF stability by a small microcoercivity in this case may be outweighed by a large screening due to a relatively large number of soft walls available in the grains. This prediction is supported by the experimental evidence that two types of magnetites (crushed and hydrothermally grown) with very different defect densities have similar values of median destructive field.
A model of multidomain thermoremanent magnetization incorporating temperature‐variable domain structure
Authors: Shcherbakov V. P. , McClelland E. , Shcherbakova V. V.
Published: February 2008

There are some fundamental experimental observations of properties of thermoremanent magnetization (TRM) and partial TRM (pTRM) in multidomain (MD) magnetite that cannot be explained by Néel's theories of TRM. We present experimental results that show (1) that pTRMs are additive at any temperature, (2) that a pTRM acquired in field H between temperatures T1 and T2 decreases on zero‐field cooling below T2 when normalized by Ms (T), (3) that thermal pre‐history has a strong effect on the intensity of a pTRM. These results strongly point to reorganization of domain structure during cooling being the dominant controlling factor in TRM acquisition in MD material. We further develop the approach of McClelland and Sugiura [1987] where TRM and pTRM are considered to be nonequilibrium states, and change in domain structure with changing temperature provides the driving force to allow a pTRM to shift toward the demagnetized state on zero‐field cooling, for example. A random element is essential in such a kinetically controlled system; in this paper we consider the physical mechanism providing this random element to be the variation of direction of the easy axis of magnetization throughout the grain due to local crystal defects, or stress effects due to the domains themselves, for example. Thermally driven domain structure changes then cause essentially random local changes of magnetization, which are governed by kinetic equations. Our model is developed by considering the magnetization of discrete cells within a cubic grain chosen to have reasonably uniform magnetic properties within the cell but probably different between cells, and the model satisfactorily explains our experimental observations. The strong effect of thermal prehistory is ascribed to the existence of a spectrum of local energy minima states, and the behavior of an MD grain is likened to that of a spin glass.
Receiver functions for the Tien Shan Analog Broadband Network: Contrasts in the evolution of structures across the Talasso‐Fergana Fault
Authors: Kosarev G. L. , Petersen N. V. , Vinnik L. P. , Roecker S. W.
Published: February 2008

We determined the structure of the crust beneath the central and western Tien Shan by analyzing broadband (1–20 s) analog seismograms of converted P‐SV phases generated by earthquakes at teleseismic distances and recorded by 14 seismograph stations. The one‐dimensional structures that best explain the waveforms reveal pronounced differences in crustal velocities east and west of the Talasso‐Fergana fault. Specifically, the transition zone between the crust and the mantle east of the fault is about 2 times broader than that west of the fault. Also, velocities at depths between 10 and 35 km are about 10% lower in the east, although the depth range of these lower velocities is not well resolved. The Talasso‐Fergana fault is an important boundary for other observables; in addition to the structural discontinuities observed at the surface, the areas east of the Talasso‐Fergana fault are associated with abnormally low mantle velocities, outcrops of basalt, low Q, and short‐wavelength variations in anisotropy. Integrating these observations, we interpret the broad mantle gradient as being due to vertical intrusions of mantle material into the lower crust. Likewise, the low velocities in the midcrust could be thermally induced or due to the introduction of magmatically derived fluids. The gross contrast in structure east and west of the Talasso‐Fergana fault could reflect a contrast in the dynamics of mountain building in these two regions. We postulate that while crustal shortening is the dominant mechanism controlling topography west of the Talasso‐Fergana fault, vertical uplift caused by a mass deficiency in the upper mantle may contribute significantly to the generation and maintenance of the high elevations east of the fault.
Fractal and length analysis of fractures during brittle to ductile changes
Authors: Velde B. , Moore D. , Badri A. , Ledesert B.
Published: February 2008

Linear Cantor's dust type fractal analysis of spacial distribution and fracture length analysis has been made on the fracture sets obtained in four deformation experiments performed at different confining pressures (0.5 to 5 kbar) using the Barre Granite. The fractures, mapped using petrographic microscope methods, were subsequently classed into those of shear, tension, and relaxation or decompression origin. Fracture lengths were found to have a distribution similar to a log normal frequency distribution for all sets. The difference between the mean lengths for the fractures of different orientations in a given experiment decreases with pressure, ie. the fracture lengths for all orientations (stress origins) tend to be more homogeneous as confining pressure increases. The fracture sets were also subjected to the linear Cantor's dust method of analysis. These analyses show that the fractal dimension for all of the fractures in a sample increases slightly as confining pressure increases. The difference in fractal dimension of individual fracture sets in a sample (shear, tension, decompression) tends to decrease with increased confining pressure. In the experiments with the lowest confining pressure, one finds the greatest contrasts in both fracture length and fractal dimension for fractures of different origin. These analyses show that one can describe the differences in fracture style in fracture fields using fractal measurements and length measurements. This follows the change from anisotropic stress fields to those more homogeneous through an increase of confining pressure. This shows that one can measure the change in fracture style in going from brittle rupture toward plastic deformation.
Deformation of fine‐grained aggregates of olivine plus melt at high temperatures and pressures
Authors: Beeman Mark L. , Kohlstedt D. L.
Published: February 2008

High‐temperature, high‐pressure constant displacement rate experiments have been performed to investigate the flow behavior of partially molten samples of fine‐grained olivine‐rich aggregates. Two‐phase samples with a grain size of ∼8 μm and dihedral angles between ∼45° and 60° were fabricated by hydrostatically hot‐pressing powders of San Carlos olivine plus ∼2 to 9 vol % of four different synthetic silicate melts that contained Al and either Ca or Na as well as Mg and Fe. Single‐phase olivine samples with a grain size of ∼2 μm were prepared from either San Carlos olivine powders or synthetic olivine powders. These samples were deformed at 1100° and/or 1200°C under a confining pressure of 300 MPa at strain rates between 10−6 and 10−4 s−1. At 1100°C, the two‐phase samples were either weaker than or comparable in strength to the single‐phase samples. At 1200deg;C, the two‐phase samples were consistently weaker than the single‐phase samples. Stress exponents of n ∼ 4, as well as comparison with published creep results, demonstrate that both the single‐phase samples and at least two of the two‐phase samples deformed predominantly by dislocation creep. In this case, the melt phase would be expected to have only a small effect on flow strength, and the reduced strength probably resulted from a water weakening of the olivine grains. The other two partially molten samples were a factor of 2 to 3 weaker than the single phase samples, suggestive of a change from dislocation to diffusion creep with the addition of wetting melt phases. A comparison of the rheology of single‐phase olivine aggregates deformed in Fe capsules with that of aggregates deformed in Ni capsules indicates that the strain rate increases with increasing oxygen fugacity,
, consistent with results published for olivine single crystals. On the basis of these experiments, it seems likely that the presence of a small amount of melt in a partially molten zone in the upper mantle will result in only a relatively minor localized reduction in rock strength.
Calorimetric study of MgSiO3 garnet and pyroxene: Heat capacities, transition enthalpies, and equilibrium phase relations in MgSiO3 at high pressures and temperatures
Authors: Yusa Hitoshi , Akaogi Masaki , Ito Eiji
Published: February 2008

Enthalpy of the MgSiO3 pyroxene‐tetragonal garnet transition was measured by high‐temperature solution calorimetry, giving ΔH°983 = 30.80 ± 3.11 kJ/mol at 983 K. Heat capacities of MgSiO3 pyroxene and tetragonal garnet were also measured by differential scanning calorimetry from temperatures of 150 to 700 K. Using these calorimetric data and published thermodynamic data, equilibrium phase boundaries of high‐pressure transitions in MgSiO3 were calculated at pressures up to 26 GPa and at temperatures up to 2600 K. In calculating the stability field of MgSiO3 tetragonal garnet, the effect of partial disorder of Mg and Si cations in the octahedral sites of garnet on the phase boundaries was examined. The calculated stability field of tetragonal garnet was generally consistent with the high‐pressure experimental data. The calculated boundaries for the pyroxene‐garnet and garnet‐perovskite transitions have negative and positive slopes, respectively.
The use of the precise observations of the celestial ephemeris pole in the analysis of geophysical excitation of Earth rotation
Authors: Brzeziński Aleksander , Capitaine Nicole
Published: February 2008

Perturbations of Earth rotation are currently observed as time variations of universal time UT1‐UTC and as changes in the terrestrial and the celestial orientation of the axis of the celestial ephemeris pole (CEP) as expressed by five Earth orientation parameters (EOP). On the other hand, dynamical theories of Earth rotation usually describe perturbations of the instantaneous rotation vector. A great improvement in the spatial and the temporal resolution of the EOP determination also raises the problem of the distinction between high‐frequency polar motion and nutation. Here we derive the time domain relations between polar motion of the rotation axis and the observed changes of the CEP as well as between the spatial and the terrestrial motion of the CEP. These relations were discussed by Brzeziński (1992a). Here they are presented in a much simpler and direct manner using only matrix transformations between the conventional Terrestrial Reference System and Celestial Reference System in the form proposed by Capitaine et al. (1986) and Capitaine (1990), based upon the concept of the nonrotating origin (Guinot, 1979). These relationships are first obtained in a strict form that enables us to estimate the order of magnitude of errors arising from various approximations. The final linear expressions are used in an algorithm that transforms arbitrary differential equations describing geophysical excitation of polar motion to the form using only the observed quantities as variables.
Magnetic and tectonic studies of the dueling propagating spreading centers at 20°40′S on the East Pacific Rise: Evidence for crustal rotations
Authors: Perram Laura Jean , Cormier Marie‐Helene , Macdonald Ken C.
Published: February 2008

We present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20°40′S on the East Pacific Rise. A majority of the data used were collected during a cruise aboard the R/V Moana Wave during which continuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly‐2‐aged crust. A three‐dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity. The results indicate consistent asymmetric spreading faster to the east, discontinuous high magnetizations in the discordant zone associated with the discontinuity, and southward migration of the feature at a rate of 90–100 mm/yr between Jaramillo and Brunhes time (0.95 to 0.73 Ma) with slowing during the Brunhes to less than 10 mm/yr. There is also an overlapping Jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the Nazca to the Pacific plate. In addition, areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector. Results from this second model support the presence of highly rotated pre‐Brunhes Nazca crust within Brunhes Pacific crust which has been deformed by bookshelf faulting. This indicates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the Nazca plate between the discontinuity and the Easter microplate to the south. The detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges.
A two‐dimensional micromagnetic model of magnetizations and fields in magnetite
Authors: Newell Andrew J. , Dunlop David J. , Williams Wyn
Published: February 2008

A two‐dimensional micromagnetic model is used to obtain magnetic domain structures for cubic particles of magnetite with sides 1 μm or less. We show that complex submicron structures evolve continuously with increasing particle size into recognizable classical domain structures. Two such sequences of equilibrium states are seen. The lowest energy state for particle sizes above 0.06 μm is the “vortex” state (already seen in three‐dimensional models). Another state is seen in larger particles which has angles between walls of 90°, 125°, and 145°, agreeing with domain observations of magnetite. These are usually called 71°, 109°, and 180° walls, based on the assumption that the domain moments are in the easy ([111]) directions, but we find that the domains are actually magnetized in the [100] and [110] directions. Because both states eventually become single domain in sufficiently small particles, there is no critical single domain size in a conventional sense, but there is a rapid decrease in normalized remanence over the size range 0.06 – 0.18 μm. Bloch‐like and Néel‐like walls are seen, but strong surface fields are only found over the Bloch‐like walls. Depending on the geometry of the particle observed, one may see only a partial expression of closure domains in domain observations.
Internal hydraulics, solitons and associated mixing in a stratified sound
Authors: Gan Jianping , Ingram R. Grant
Published: February 2008

Observations of tidally forced flow in a constricted and sill‐like region of a highly stratified sound are examined as a problem of two‐layer hydraulic exchange. Based on the current and salinity data, time and space dependent hydraulic conditions along the sound are discussed. It will be shown that the sound is subject to internal hydraulic controls at several locations, which account for the generation of strong internal hydraulic bores and drops in the upper layer. Internal solitary waves with 3–6 min period were found in the upstream propagating bore. Using results from different models of the solitary wave based on the Korteweg‐de Vries (KdV) equation, it is shown that the second‐order nonlinear term must be included in the two‐layer model. Results from a first‐order continuously stratified model, solved using the lowest mode eigenfunction, gave similar results as the second order two‐layer model. This implies that two‐layer models may ignore some properties of the real fluid and that internal solitons are also sensitive to the stratification characteristics of the water column. The vertical velocity shear, hydraulic characteristics of the flow and the presence of solitary waves were major contributors to vertical mixing in the sound.
Dynamic Reynolds stress modeling of turbidity currents
Authors: Brørs Bård , Eidsvik Karl J.
Published: February 2008

A turbidity current is idealized as a time‐varying, one‐dimensional boundary layer flow. A full Reynolds stress model appears to give significantly better predictions than one‐ and two‐equation turbulence closures. Model verification is obtained from stationary downslope experimental flows. Verification of the model is also supported by data from stratified boundary layers, represented by Monin‐Obukhov relations, and density interface flows.
Large‐scale thermal bending fracture of sea ice plates
Authors: Bažant Zdeněk P.
Published: February 2008

A hypothesis that large‐scale fracture of sea ice plates in the Arctic could be caused by the release of energy of thermal bending moments due to major temperature changes is advanced and examined. Bending propagation of a through‐the‐thickness crack along the floating plate, with negligible inertial forces, is analyzed, assuming the moment field in the plate near the traveling crack front and the fracture process zone to be in a steady state. The analysis uses the plate‐bending theory, and the second‐order geometric effects of the in‐plane normal forces are taken into account. Quasi‐elastic behavior is assumed, and creep is treated approximately according to the effective modulus method. The calculated temperature difference between the top and bottom of the plate required to produce this kind of fracture is found to be well within the range that actually occurs in the Arctic, but this cannot be regarded as a proof of the hypothesis because of the simplifying assumptions made as well as uncertainties about large‐scale fracture properties of sea ice. Further, it is shown that this type of fracture must exhibit a size effect, such that the critical temperature difference decreases in proportion to (plate thickness)−⅜. This might explain why large fractures often form in an intact thick plate rather than only in a thin plate and along lines of weakness. For the case that the in‐plane forces are significant, it is shown that beyond a certain critical crack length the thermally driven bending fracture (if it exists) must transit to a planar (nonflexural) fracture driven by the release of the energy of the in‐plane forces generated by wind and ocean currents. The effect of creep is to increase the required critical temperature difference, as well as the critical crack length for the aforementioned transition. For thermal bending fracture, the minimum possible spacing of parallel cracks increases with the plate thickness and is independent of the crack length, while after transition to planar fracture it increases in proportion to the crack length. The hypothesis of thermal bending fracture cannot be proven or disproven without new types of experiments and measurements in the Arctic, and their computer modeling.
Observations of the geostrophic current and water mass characteristics off Point Sur, California, from May 1988 through November 1989
Authors: Tisch Timothy D. , Ramp Steven R. , Collins Curtis A.
Published: February 2008

The Point Sur transect (POST) has been occupied 6–8 times per year since 1988 to resolve the flow in the California Current system at seasonal and interannual time scales. The POST extends offshore, normal to bottom topography, along 36°20′N, to 123°01.7′W where it doglegs southwest along the California Cooperative Fisheries Investigation (CalCOFI) line 67. Hydrographic observations from seven cruises over 2 years have been used to study variations of alongshore geostrophic velocities and water mass characteristics within these time scales. The California Undercurrent was a prominent feature in six of the seven sections analyzed and was very weak during a period of strong equatorward wind stress. The position of the undercurrent core varied from 12 to 42 km from shore while its strength varied from less than 5 cm s−1 to 35 cm s−1, with the maximum flow occurring in winter. The undercurrent (core) over the continental slope was found from 70 to 460 m depth throughout these seven cruises. The nature of the alongshore geostrophic velocities and the location and spatial extent of the California Undercurrent appear strongly related to specific wind events, both local and remote. Remote wind forcing from the south was believed to cause anomalous, strong poleward flow throughout the entire water column above 1000 dbar during a period of local equatorward wind stress.
The Point Arena eddy: A recurring summer anticyclone in the California Current
Authors: Lagerloef Gary S. E.
Published: February 2008

The propensity for an anticyclonic eddy (∼100‐ to 200‐km diameter) to develop west of Point Arena during the summer is documented with an analysis of satellite derived sea surface temperature (SST) data and a review of published historical hydrographic data. The satellite data span the period from 1981 through 1989 and the hydrographic data include CalCOFI surveys from 1949–1959 and a few more recent studies. A closed anticyclone was apparent in many of the historical dynamic height maps, while a meander pattern, lacking closed contours, characterized the coincident hydrographic 10‐m temperature maps. A conceptual model of an eddylike perturbation imbedded in a broader equatorward flow is adopted as a working hypothesis. In this simplistic model, the only distinction between an eddy (with closed contours) and a meander (without closed contours) is the relative strength of the eddylike perturbation. This led to a method to detect the eddies in the satellite SST data, which were dominated by meanderlike patterns: A linear trend was subtracted from each image to remove large‐scale SST gradients, leaving an expression of the residual eddylike perturbation. Composite images over select time periods were produced for analysis by computing spatial variance EOFs of the detrended data. With this approach, a warm‐core eddy was evident at Point Arena in summertime satellite SST data from 1981–1985 and 1987–1989 (formation of an eddy in 1986 remains unresolved because the readily available SST data were incomplete). The eddy was bounded on the inshore side by the jet and filament system studied during the 1987 and 1988 Coastal Transition Zone (CTZ) program. Recurrence of this eddy was apparently coupled with the recurring cold filament commonly observed at Point Arena during the summer. The coevolution of the eddy and meandering jet during 1988 was studied with sequential composite empirical orthogonal function (EOF) satellite SST images. These show that a deep anticyclonic meander was present near Cape Mendocino and directed the jet toward shore. In late July 1988 this meander pinched off a new eddy, and the subsequent jet flow was redirected southward and displaced farther offshore. These events account for the transition in the jet observed in the July CTZ hydrographic surveys. The length scale and position of the Point Arena eddy is consistent with a standing wave, phase locked to a prominent cape (in this case, Mendocino). The significance of the apparent splitting off of an eddy in 1988, as well as the apparent preponderance of anticyclones evident in these EOFs, is discussed relative to recent theoretical and numerical model developments.
Meridional fluxes of dissolved organic matter in the north Atlantic Ocean
Authors: Walsh John J. , Carder Kendall L. , Müller‐Karger Frank E.
Published: February 2008

Using bio‐optical estimates of gelbstoff and a few platinum measurements of dissolved organic carbon (DOCpt), a budget of the meridional flux of DOC and dissolved organic nitrogen (DON) across 36°25′N in the North Atlantic is constructed from previous inverse models of water and element transport. Distinct southward subsurface fluxes of dissolved organic matter (DOM) within subducted shelf water, cabelled slope water, and overturned basin water are inferred. Within two cases of a positive gradient of DOCpt between terrestrial / shelf and offshore stocks, the net equatorward exports of O2 and DOCpt from the northern North Atlantic yield molar ratios of 2.1 to 9.1, compared to the expected Redfield O2/C ratio of 1.3. In the first case, 63% of the apparent oxygen utilization demands of the water column may be met by DOC, instead of only 14% in the second scenario, preserving a role for falling particles in the sea. With a DOC/DON ratio of 10, the larger net southward export of DON across 36°25′N balances the postulated net northward input of 1.7 × 103 kg NO3 s−1 of unutilized nitrate within the Gulf Stream. Without an enhanced supply of DOM from the shelves, a zero seaward gradient of DOM in the third case suggests that none of the poleward nitrate flux is returned southward as DON, but instead a net poleward flux of DON prevails as well. Our present estimates are confounded, however, by the seasonal and multiyear variability of sinking processes in the North Atlantic. Future active and passive remote sensors, field programs, and simulation models must now discriminate between particulate and dissolved components of surface color signals to verify the importance of both continental margins and DOM in global biogeochemical cycles.
Spatial patterns of increases and decreases in the length of the sea ice season in the north polar region, 1979–1986
Authors: Parkinson Claire L.
Published: February 2008

Recently it was reported that sea ice extents in the northern hemisphere showed a very slight but statistically significant decrease over the 8.8‐year period of the Nimbus 7 scanning multichannel microwave radiometer (SMMR) data set. In this paper the same SMMR data are used to reveal spatial patterns in increasing and decreasing sea ice coverage. Specifically, the length of the ice season is mapped for each full year of the SMMR data set (1979–1986), and the trends over the 8 years in these ice season lengths are also mapped. These trends show considerable spatial coherence, with a shortening in the sea ice season apparent in much of the eastern hemisphere of the north polar ice cover, particularly in the Sea of Okhotsk, the Barents Sea, and the Kara Sea, and a lengthening of the sea ice season apparent in much of the western hemisphere of the north polar ice cover, particularly in Davis Strait, the Labrador Sea, and the Beaufort Sea.
Estimation of surface winds from upward looking acoustic Doppler current profilers
Authors: Brown J. , Barton E. D. , Trasviña A. , Velez H. S. , Kosro P. M. , Smith R. L.
Published: February 2008

Three upward looking acoustic Doppler current profilers (ADCP) were deployed beneath meteorological buoys in the Gulf of Tehuantepec, Mexico, during winter 1988–1989. Hourly averaged wind speed data from the buoys and from ship when in the vicinity were compared with surface acoustic backscatter intensity recorded at the ADCPs. The backscatter was found to be a significant predictor of wind speeds from both buoy and ship, the latter when within 50 km of the mooring site. There was no apparent saturation of the backscatter signal at the maximum wind speeds (<15 m s−1). The results cast doubt on the ability of near‐surface Doppler directional information to provide reliable estimates of wind direction.
Cycling of dissolved and particulate organic matter in the open ocean
Authors: Druffel Ellen R. M. , Williams Peter M. , Bauer James E. , Ertel John R.
Published: February 2008

Radiocarbon (Δ14C), δ13C, bulk carbon and organic constituent concentration measurements are presented for dissolved and particulate carbon pools from the North Central Pacific Ocean (NCP) and the Sargasso Sea (SS). We operationally define three overlapping pools of dissolved organic carbon (DOC): (1) DOC that is oxidizable by UV radiation (DOCuv); (2) “extra” DOC measured by Co/CoO flow‐through high‐temperature catalytic oxidation (DOCFt‐htc), which also has low Δ14C values like DOCuv (Bauer et al., 1992a); and (3) a potential residual DOC fraction that is the difference between DOC measured by discrete‐injection high‐temperature catalytic oxidation (DOChtc) and DOCFt‐htc, and which has unknown Δ14C signature. The distribution of a large fraction of DOC appears to be controlled by circulation of deep ocean waters between major oceans. The DOC in the SS is slightly younger than would be expected if circulation was the sole process controlling DOC cycling. We propose that there is more bomb 14C in the deep SS DOC to account for this difference. The Δ14C values of suspended, and to a lesser extent sinking particulate organic carbon (POC), decrease with depth, with the suspended POC displaying a much steeper gradient in the SS than in the NCP. These data reflect the incorporation of low‐activity organic matter into the POC pool, possibly through incorporation of DOC by physical adsorption and/or biological heterotrophy.
Sea ice melting and floe geometry in a simple ice‐ocean model
Authors: Steele Michael
Published: February 2008

A coupled sea ice‐ocean numerical model has been developed that addresses the role of floe geometry during summertime melting. The model contains a diagnostic equation for average floe diameter in addition to the usual prognostic equations for ice volume per unit area and ice concentration. The partition between melting on the top, bottom, and lateral (side) surfaces of floes is examined using time‐dependent simulations with differing initial average floe diameters. The different parameterizations for bottom and lateral melting in the literature are also compared and found to vary significantly. The results show that lateral melting is important only for floes with diameters less than O(30 m), given atmospheric thermal forcing typical of the central Arctic in summer. This means that the decrease in ice concentration over the summer is a strong function of floe diameter, in keeping with simple geometrical arguments. In all cases, about 80% of the net thermal energy that enters the ocean through leads goes toward melting ice, while the rest warms the ocean.
Heat and mass balances of the South Atlantic Ocean calculated from a numerical model
Authors: Matano Ricardo P. , Philander S. George H.
Published: February 2008

The general circulation model of Bryan (1969), modified by the introduction of open boundary conditions at the Drake Passage and between Africa and Antarctica, has been used to study the mass and heat budgets of the South Atlantic Ocean. The model was initialized with the climatological annual mean values of temperature and salinity of Levitus (1982) and forced at its surface with the climatological wind stress data of Hellerman and Rosenstein (1983). After 3 years of integration the model reached a quasi‐stationary state. A heat balance shows that the model transports 0.19 PW of heat toward the north across 30°S. While a large part of this heat is supplied by the atmosphere and involves the conversion of intermediate waters into surface waters, a comparison with climatological data of atmospheric heat fluxes suggests that an extra source of heat is necessary to maintain the northward heat flux.
Suppression of short sea waves in ship wakes: Measurements and observations
Authors: Milgram J. H. , Peltzer R. D. , Griffin O. M.
Published: February 2008

Measurements of short wave energy, surface tension distributions, and radar imagery of centerline ship wakes are presented. A direct association between reduced radar return and reduced scattering wave energy is demonstrated. The dominant influences of ship‐generated turbulence and surface film distributions upon reduced short wave energy in ship wakes are shown. The effect of turbulence is emphasized by the very slow regrowth of attenuated wave energy in the especially turbulent wake of a towed barge. The effect of surface film distributions is emphasized by the suppression of short wave growth during a wind puff in a wake that is about 1 hour old.
High‐latitude oceanic variability associated with the 18.6‐year nodal tide
Authors: Royer Thomas C.
Published: February 2008

Ocean temperatures in the upper 250 m in the northern North Pacific (60°N, 149°W) increased by more than 1°C from 1972 to 1986 but are now decreasing. Subsurface temperature anomalies are well correlated (∼0.58) with the air temperature anomalies at Sitka, Alaska; hence the coastal air temperatures can be used as a proxy data set to extend the ocean temperature time series back to 1828. Up to 30% of the low‐frequency variance can be accounted for with the 18.6‐year nodal signal. Additionally, spectral analysis of these air temperature variations indicates a significant low‐frequency peak in the range of the 18.6‐year signal. Similar low‐frequency signals have been reported for Hudson Bay air temperatures since 1700, for sea surface temperatures in the North Atlantic from 1876 to 1939, and for sea level in the high‐latitude southern hemisphere. The water column temperature variations presented here are the first evidence that the upper ocean is responding to this very long period tidal forcing. An enhanced high‐latitude response to the 18.6‐year forcing is predicted by equilibrium tide theory, and it should be most evident at latitudes poleward of about 50°. These low‐frequency ocean‐atmosphere variations must be considered in high‐latitude assessments of global climate change, since they are of the same magnitude as many of the predicted global changes.
Nitrogen uptake and new production in the Greenland Sea: The spring Phaeocystis bloom
Authors: Smith Walker O.
Published: February 2008

Nitrogen (nitrate and ammonium) uptake was measured in the Greenland Sea during April–May 1989 as part of the Coordinated Eastern Arctic Experiment (CEAREX). The region was dominated by a massive bloom of Phaeocystis pouchetii, and particulate nitrogen concentrations reached 450 mmol m−2 within the bloom. Particulate carbon:nitrogen ratios were similar to other polar assemblages, suggesting that the particulate matter composition was not dominated by carbon‐rich extracellular material. Vertically integrated f ratios ranged from 0.09 to 0.90 and generally decreased with time, as did nitrate concentrations. Nitrate uptake rates showed no temporal trend, but ammonium uptake rates were low initially and increased greatly by the end of the study. New production rates were very high, with carbon‐based productivities calculated from nitrate uptake rates and observed C:N ratios up to 4.9 g C m−2 d−1. Based on a simple mass balance of organic matter, much (∼60%) of the biogenic material produced in the Phaeocystis bloom apparently was removed from the surface layer. It is hypothesized that the formation of aggregates led to increased fluxes of particulate organic matter from the surface layer. As a result, Phaeocystis blooms may play important roles in the vertical transport of carbon and nitrogen in the Greenland Sea.
A numerical model study of parameters influencing tidal currents in the Irish Sea
Authors: Davies A. M. , Aldridge J. N.
Published: February 2008

The mathematical formulation of a mixed finite difference‐modal three‐dimensional tidal model of the Irish Sea using quadratic bottom friction is briefly presented. By using an expansion of modes in the vertical, a continuous current profile from sea surface to seabed can be computed, and accurate comparisons made with tidal currents measured at a range of positions throughout the water column. The sensitivity of current profiles to various eddy viscosity formulations, and coefficients of bottom friction are examined in detail. Also, the influence of open boundary conditions and water depths upon computed tidal elevations and currents is examined. In common with other earlier published calculations, initial results show that with previous formulations of eddy viscosity and friction, an accurate distribution of sea surface elevations is accompanied with computed currents which exceed observations by the order of 20 cm s−1. However, results from the sensitivity study presented here show that elevations and currents (above the bottom boundary) in good agreement with observations can be obtained with alternative formulations of viscosity and other friction coefficients, together with current input to the radiation condition. However, computed near‐bed currents are still larger than those observed, although a slight increase in water depth and bottom friction coefficients corrects this. Calculations clearly show that the computation of near‐bed currents, subject to the constraints of obtaining accurate elevation and current distributions is the most sensitive test of the model. Excellent agreement between computed and observed currents and elevations was obtained in a final series of calculations using appropriate parameterizations of eddy viscosity and bottom friction.
Intraseasonal variability of katabatic wind over east Antarctica and planetary flow regime in the southern hemisphere
Authors: Yasunari Tetsuzo , Kodama Satoshi
Published: February 2008

The Intraseasonal variation (30‐ to 50‐day period) of katabatic wind has been found at Mizuho plateau, East Antarctica, as a predominant mode of the subseasonal time scale variability. The atmospheric circulation in the southern middle and polar latitudes associated with this fluctuation of katabatic wind has been deduced by using the NMC gridded 500‐mbar zonal wind and geopotential height field. The axisymmetric circulation pattern to Antarctica is noticeable in the anomalous zonal wind and height field. That is, the strong (weak) katabatic wind phase corresponds to the shallower and broadened (deepened and narrowed) tropospheric circumpolar vortex with weaker (stronger) westerlies over the surrounding oceans. These results strongly suggest that the intraseasonal variation of katabatic wind regime over East Antarctica is part of the modulation of planetary flow regime and meridional circulation in the southern middle and high latitudes, i.e., part of the index cycle of the southern hemisphere.
Dry deposition of nitrogen dioxide and ozone to coniferous forests
Authors: Rondón A. , Johansson C. , Granat L.
Published: February 2008

The exchange of NO2, NO, and O3 between the atmosphere and coniferous forests has been studied by using a dynamic flow‐through chamber technique. The measurements were performed during summer at two coniferous forest sites in Sweden, Jädraås (Scots pine) and Simlångsdalen (Scots pine and Norway spruce). In Simlångsdalen, the flux of NO2 was found to be quantitatively determined by the stomatal openings. Generally, the flux of NO2 was towards the vegetation surface, and only in a few cases a small emission of NO2 was detected. The average deposition velocities per projected needle area were 1.5 ± 1.3 mm s−1 for Norway spruce and 1.4 ± 1.1 mm s −1 for Scots pine. The results from Jädraås are slightly different; the deposition velocity of NO2, at concentrations lower than 1 ppbv, was always lower than the stomatal conductance and net emissions of NO2 were observed at concentrations below 0.5–0.7 ppbv. The average deposition velocity per projected needle area observed in Jädraås was 0.8 ± 0.7 mm s−1. In only a few cases, both for Jädraås and Simlångsdalen, the uptake of NO2 was limited by mesophyllic resistance. The difference between Simlångsdalen and Jädraås may be attributed to physiological variations or nutrient supply. The exchange rate of NO between the air and vegetation per projected needle area was generally found to be less than the detection limit of the method (0.1 ng N m−2 s−1 ). The deposition velocity per projected needle area for O3 varied typically from 0 to 15.7 mm s−1 in Jädraås and between 0 and 9.0 mm s−1 in Simlångsdalen, with average values of 3.1 and 2.5 mm s−1, respectively. The ratio between the deposition velocity for O3 and stomatal conductance was in general larger than one, indicating deposition of O3 to the external surfaces of the vegetation. The residual deposition of O3 showed a marked diurnal variation with maximum values around noon. It seems to follow a complex mechanism in which several factors such as light intensity, temperature, humidity and chemical properties of the surface of the cuticle might be important. No significant differences between Norway spruce and Scots pine species were observed in terms of the dry deposition of NO2 and O3. The dry deposition of NO2 to the forest floor is of the same magnitude as the deposition to the canopy. The average deposition velocity of NO2 to the forest floor in Jädraås was equal to 4 mm s−1. The maximum contribution from the dry deposition of NO2 to the total deposition of oxidized nitrogen compounds to the forest was estimated to be about 5% and 30% (as nitrogen) for Jädraås and Simlångsdalen, respectively. The emission flux of NO from the forest soil is at least 4 times smaller than the deposition rate of NO2 to the floor.
Visible and near‐ultraviolet spectroscopy at McMurdo Station, Antarctica: 8. Observations of nighttime NO2 and NO3 from April to October 1991
Authors: Solomon S. , Smith J. P. , Sanders R. W. , Perliski L. , Miller H. L. , Mount G. H. , Keys J. G. , Schmeltekopf A. L.
Published: February 2008

Lunar absorption spectra have been used to determine the vertical column abundances of NO2 and NO3 above McMurdo Station, Antarctica (77.8°S), during the fall, winter, and spring seasons in 1991. The observed nighttime NO2 and NO3 amounts during the fall and spring were broadly consistent with model predictions and with daytime measurements. During winter, local photochemistry would imply a long polar night, with associated gradual removal of NO2 and buildup of NO3 and N2O5. The winter observations of both species differ from such simple predictions by factors greater than 10, with the measured NO2 being far larger than predicted while the measured NO3 is much smaller than predicted. The concurrent measurements of both NO2 and NO3 strongly constrain possible solutions for these discrepancies and suggest that air parcel excursions to sunlit, lower latitudes likely take place quite regularly throughout the winter (approximately every 3–7 days). Thus the concept of an extended period of polar night as often applied in modeling studies of polar photochemistry appears inconsistent with these observations.