bluuebunny/crossref_metadata_2025 huggingface dataset:
A model study of density intrusions into and circulation within a deep, silled estuary: Puget Sound
Authors: Lavelle J. W. , Cokelet E. D. , Cannon G. A.
Published: February 2008

A laterally averaged hydrodynamical model is used to investigate episodic intrusions of denser, more oceanic water across the shallow double‐silled entrance of Puget Sound and the resulting circulation within the interior deep basin. Tidal, freshwater discharge and wind effects are also encompassed in model results. Model simulations were made of the 4‐month period from December 1983 through March 1984. During that period, hourly measurements of currents and salinity were made at the estuary entrance and on four moorings in the interior basin. The first 60 days of salinity data were used with the model to choose rates of vertical turbulent mixing and the lag times between fresh water appearance at river gauges and its arrival in the main basin of the sound. Comparison of model results with the full 120 days of salinity and velocity data shows that the model incorporates the important circulation features of this estuary. The model was then used to study intrusions and mixing in the sill region. The model identifies these factors as controls on the occurrence and strength of intrusions: the trans‐sill salinity gradient, the intensity of tidal stirring and mixing over the sills, the supply of fresher surface water resulting from river runoff, and the winds. Time series of salinities in the interior basin show local maxima following local maxima in the trans‐sill salinity difference. Vertical transport in the sill region at both tidal and subtidal time scales is dominated by advection rather than diffusion. Though much of the vertical exchange occurs over the shallower sill, a depth depression between the two entrance sills appears to be the site of significant overturning. Downwelling of surface water occurs on the landward side of both sills. Substantial freshening of the interior basin occurs during the winter months because river discharges are large and are stirred downward in the sill region to be entrained into landward bottom flows. Extraordinarily large river discharges can suppress the effect of large trans‐sill salinity differences with the effect of weakening potential intrusions. Winds can indirectly influence intrusions by longitudinally positioning fresher, river‐influenced surface water in the sill region. Winds can directly influence intrusions via bottom compensation flows in Admiralty Inlet, with northward winds encouraging intrusions, and by changing salinities at the entrance to the sound, and thus the trans‐sill salinity gradient.
Application of a generalized scavenging model to thorium isotope and particle data at equatorial and high‐latitude sites in the Pacific Ocean
Authors: Clegg Simon L. , Bacon M. P. , Whitfield M.
Published: February 2008

A generalized trace metal scavenging model has been applied to particle cycle and 234Th data at three equatorial Pacific sites, and to dissolved and particulate 228Th, 230Th, and 234Th profiles at a single high‐latitude station. The one‐dimensional (and here steady state) model consists of two particle classes, small (suspended) and large (sinking). It is driven by particle primary production and parameterized from particle and tracer concentration and flux data. The model has been used to determine, as functions of depth, (1) rates of particle remineralization; (2) aggregation and disaggregation of the small and large‐particle classes, respectively, and (3) adsorption and desorption rate constants of thorium. The results are insensitive to die assumed large particle sinking rate of 150 m d−1. The profiles of all three tracers at Station P are described satisfactorily by a single set of model parameters. The adsorption rate constant, normalized for the effect of particle concentration, is enhanced in the upper 100 m of the water column by a factor of 2 or greater at all sites. Values of this and the other major model parameters are consistent with earlier results for a variety of oceanic environments.
Breaking waves affecting microwave backscatter: 2. Dependence on wind and wave conditions
Authors: Jessup A. T. , Melville W. K. , Keller W. C.
Published: February 2008

This paper is the second of a two‐part series on using microwave techniques for detection and characterization of wave breaking. The statistics of sea spikes detected using the method described in Part 1 are investigated as functions of friction velocity u* and of a Reynolds number Re* based on u* and the dominant surface wavelength. For vertical (VV) and horizontal (HH) polarization, the frequency of sea spikes and their contribution to the mean radar cross section show a roughly cubic dependence on u*. The percentage of wave crests producing sea spikes, Pss, is consistent with the Re* exponent of 1.5 reported by other investigators. At high friction velocities (u* between 40 and 50 cm s−1), sea spikes contribute between 10% and 15% to the mean radar cross section for VV polarization and between 15% and 25% for HH polarization. The data suggest that the average radar cross section of an individual sea spike does not depend on u*. Measurements of Doppler frequency and bandwidth are used to investigate the kinematics of the breaking process.
Iron, nitrate uptake by phytoplankton, and mermaids
Authors: Banse Karl
Published: February 2008

The critique by Martin et al. (this issue) of my recalculation of rates of nitrate uptake, and hence of algal division, from the Gulf of Alaska is shown to be incorrect. Neither can iron deficiency, if any, be shown to be connected with the demise of mermen and mermaids, although for different reasons.
A simple derivation of Hasselmann's nonlinear ocean‐synthetic aperture radar transform
Authors: Krogstad Harald E.
Published: February 2008

We present a simple derivation of Hasselmann's nonlinear spectral transform for the synthetic aperture radar imaging of ocean wave fields and discuss some of its implications.
Carbon monoxide concentrations and their relation to concentrations of total reactive oxidized nitrogen at two rural U.S. sites
Authors: Parrish D. D. , Trainer M. , Buhr M. P. , Watkins B. A. , Fehsenfeld F. C.
Published: February 2008

Concentrations of CO and NOy (total reactive oxidized nitrogen species) were measured simultaneously with many other atmospheric parameters, including SO2 concentration and wind direction, during three field studies (two rural and one suburban). The CO concentrations at the two rural sites can be described approximately by lognormal distributions, with the Scotia, Pennsylvania, site exhibiting higher levels (geometric mean, 214 parts per billion by volume (ppbv); standard deviation of the logarithms, 0.143) than Niwot Ridge, Colorado (geometric mean, 121 ppbv; standard deviation of the logarithms, 0.175). As expected, measurements made at Niwot Ridge when the wind was from the east, the direction of the principal nearby urban area, exhibited the highest CO levels. At Scotia, power‐plant‐dominated air parcels could be identified by larger [SO2] to [NOy] ratios and were characterized by smaller [CO] to [NOy] ratios. Simultaneous measurements of [CO] and [NOy] allowed the identification of air parcels that arrived at Niwot Ridge and Boulder (the suburban site) without significant recent anthropogenic input. The CO levels in these parcels were consistent with the tropospheric background concentration of CO at 40°N latitude: approximately 84 ppbv in late summer and 127 ppbv in early spring. The [CO] to [NOy] ratios observed in the Boulder suburban study provide a test for, and are consistent with, available emission inventories for these pollutants. At Niwot Ridge in east winds the observed ratios are consistent with the ratios expected from emission inventories, modified by photochemical transformation on time scales that are reasonable for transport from the location of emission to the measurement site. However, the rural data sets were not able to provide stringent tests of emission inventories.
Effects of nitrogen additions on annual nitrous oxide fluxes from temperate forest soils in the northeastern United States
Authors: Bowden Richard D. , Melillo Jerry M. , Steudler Paul A. , Aber John D.
Published: February 2008

Nitrous oxide (N2O) fluxes between aerobic soils and the atmosphere were measured each month for 2 years (except during the period of snow cover) in control and N‐fertilized plots in a red pine plantation and a mixed hardwood forest in the northeastern United States. Nitrogen was added as NH4NO3; application rates were 0 (control), 37 (low‐N), and 120 (high‐N) kg N/ha/yr in year 1. In year 2, application rates were 0 (control), 50 (low‐N), and 150 (high‐N) kg N/ha/yr. A total of 2520 individual flux measurements were made over the 2 years. Mean emission rates (μg N/m2/h) from the control plots were low (pine: year 1 = 0.17, year 2 = 1.10; hardwood: year 1 = 0.27, year 2 = 0.29). Fertilization resulted in only small increases in N2O effluxes even in the second year, with low rates in the low‐N plots (pine: year 1 = 2.19, year 2 = 3.87; hardwood: year 1 = 1.16, year 2 = 0.77) and also in the high‐N plots (pine: year 1 = 0.71, year 2 = 5.24; hardwood: year 1 = 1.12, year 2 = 0.90). The maximum N2O loss was 0.350 kg N/ha/yr from the red pine high‐N plot. Weak seasonal flux trends were noted in both stands, with highest efflux rates in spring and mid to late summer. No statistically significant relationships between fluxes and soil temperatures, soil moisture, or soil NO3− or NH4+ were detected. Low rates of net nitrification in both the control plots and the fertilized plots are thought to be responsible for the low N2O emissions. Small increases in net nitrification were measured in the forest floor of the the pine stand, with rates in the control, low‐N, and high‐N plots of 1.5, 4.5, and 8.5 kg N/ha/yr, respectively. Net nitrification rates in the hardwood plots were 0.1, 0.9, and 0.6 kg N/ha/yr in the control, low‐N, and high‐N plots, respectively. Low NH4+ availability in the forest floor and low NH4+ concentrations in soil solution below the rooting zone (0.5–0.7 m) suggest that NH4+ availability for nitrification is limited by competition with plant uptake and microbial immobilization, and that this competition is not alleviated by 2 years (50–150 kg N/ha/yr) of N addition.
Multiscale modeling of pollutant transport and chemistry
Authors: Odman Mehmet T. , Russell Armistead G.
Published: February 2008

A multiscale transport scheme is used in solving the coupled nonlinear system of atmospheric continuity equation. This upwind finite element scheme employs a local nonlinear filter to eliminate negative concentrations. Two different nesting techniques are considered to assess their potential of being used in photochemical models. The test problem is the advection of chemically reacting puffs in a rotational velocity field. The mechanism is a very simple description of atmospheric chemistry. The results are compared to the case where no advection takes place. The multiscale scheme performs well in matching the transport with nonlinear chemistry when constraint elements are used for nesting. The other technique, i.e., nesting with transition elements, that performed equally well in advection tests proved to be inappropriate for following the chemical kinetics. This result has a very important implication: transport schemes that will be used in air quality models should be tested with the chemistry. The efficiency of the transport algorithm is also discussed. Some comments are made about the general trends in pollutant transition from urban to regional scales.
Polar stratospheric cloud observations over the Antarctic continent at Dumont d'Urville
Authors: Stefanutti L. , Morandi M. , Guasta M. Del , Godin S. , Megie G. , Brechet J. , Piquard J.
Published: February 2008

The Istituto di Ricerca sulle Onde Electromagnetiche (IROE) two‐channel elastic backscattering lidar [Sacco et al., 1989], suitable for depolarization measurements, has been operated since January 8, 1989, at the French Antarctic base of Dumont d'Urville (66°40′S, 140°01′E). A continuous monitoring of the stratosphere was performed, which permitted measurement of the evolution of the background stratospheric aerosols and of polar stratospheric clouds (PSC) throughout the year. The data reported in this article correspond to the first year of measurements. Depolarization of the lidar signals was measured in order to obtain information on the type of clouds observed and on their particle size distribution. Both low (<10%) and high (>10%) depolarization ratios were detected, permitting discrimination between types Ia, Ib, and II PSC according to the classification given by Toon et al. (1990). Temporal continuity and high time resolution of the lidar measurements are evidence for altitude decreases in the PSC layers over periods of a few hours. These motions, if linked to sedimentation processes, led to values of velocity (≃10 cm s−1) compatible with large particles.
High‐resolution solar spectrum between 2000 and 3100 Å
Authors: Hall L. A. , Anderson G. P.
Published: February 2008

Solar spectra in the wavelength range 2000–3100 Å, measured near 40 km in the stratosphere in 1978 and 1983, have been combined and extrapolated to zero optical depth to provide a reference spectrum in 0.1‐Å resolution. The wavelength scale has been carefully compared with wavelength standards, and a cyclic instrumental effect has been removed, resulting in a wavelength accuracy to within 0.04 Å. The absolute intensities have been normalized to the previously reported 1983 spectrum in 1.0‐Å resolution.
A numerical study of the effects of air chemistry on fluxes of NO, NO2, and O3 near the surface
Authors: Gao W. , Wesely M. L. , Lee I. Y.
Published: February 2008

A numerical model was developed to investigate modifications of the vertical fluxes of NO, NO2, and O3 by rapid atmospheric chemical reactions in a horizontally homogeneous, neutrally stratified atmospheric surface layer. The one‐dimensional model coupled second‐order budget equations for fluxes with conservation equations for concentrations and incorporated simplified homogeneous gas kinetics describing the photolysis of NO2 and the recombination of NO and O3. The effects of hydrocarbons were ignored. Simulations were carried out for representative chemical concentrations at the top boundary, moderate atmospheric turbulent mixing, surface removal rates typical for vegetation in the daytime, and a range of NO emission rates from the surface. Results indicate that the effective eddy diffusivity for NO and NO2 can be significantly altered by chemical reactions and that the fluxes of NO and NO2 can vary strongly with height but that the impact of rapid atmospheric chemical reactions on O3 is small relative to its total flux. When the NO2‐to‐NO concentration ratio aloft is greater than the ratio for the photostationary state, photodecomposition of NO2 dominates the reaction cycle. In this case, NO flux increases with height even when the flux at the surface is zero, while the downward NO2 flux increases in magnitude with height and the magnitude of the downward O3 flux decreases. When the value of [NO2]/[NO] aloft is smaller than the photostationary value, these trends with height are reversed. Emission of NO at the surface strongly influences NO flux near the surface, while chemical reactions tend to determine the flux at heights above a few meters. A significant effect on NO2 occurs at the intermediate heights, but the relative change in O3 flux is minor.
Late Quaternary forcing of Indian Ocean summer‐monsoon winds: A comparison of Fourier model and general circulation model results
Authors: Clemens Steven C. , Prell Warren L.
Published: February 2008

Past fluctuations in the strength of summer monsoon winds in the Arabian Sea can be inferred from the grain size of lithogenic material blown in from the peripheral deserts. We partition a 400 kiloyear (kyr) time series of inferred wind strength into its dominant frequency components, and develop a Fourier model to evaluate the relative influence of potential monsoon forcing mechanisms operating over these dominant frequencies. Our results indicate that the primary external and internal forcing mechanisms determining the timing and strength of Arabian Sea monsoon winds over the late Quaternary are (1) solar insolation and (2) latent heat collected from the southern subtropical Indian Ocean and released over the Asian Plateau. Our results further suggest that the extent of glacial conditions plays only a minor role in determining the timing and strength of the Arabian Sea paleomonsoon. In contrast, sensitivity experiments using the National Center for Atmospheric Research community climate model (NCAR CCM) indicate that glacial boundary conditions play a dominant role in forcing the strength of Arabian Sea paleomonsoon winds. Such inconsistencies are illustrated by comparing CCM wind strengths to those inferred from geological data. CCM monsoon winds in the Arabian Sea are uniformly strong during interglacial oxygen isotope stages 1 and 5 and weak during glacial stage 2. The geological record, in contrast, indicates strong and weak monsoons during both glacial and interglacial stages. Examining CCM monsoon strength in the context of latent heat flux versus regional sea surface temperatures (SST) suggests that these discrepancies may be attributed to incorrect ocean‐atmosphere heat transfer in the southern hemisphere, possibly associated with the effects of prescribed model SST fields.
Trends in total ozone at Toronto between 1960 and 1991
Authors: Kerr J. B.
Published: February 2008

Ground‐based direct sun total ozone measurements made at Toronto between 1960 and 1991 are analyzed. During the 1980s the annual average of total ozone was observed to decrease by about 4.2%. This trend is attributed to a statistically significant decrease of ozone by about 7.0% during the late winter, early spring season. The observed trend is consistent with the mid‐latitude northern hemisphere results derived from the revised total ozone mapping spectrometer (TOMS) satellite data. Comparing the seasonal behavior of the 1980s trend with that of fluctuations between 1960 and 1980 indicates that the recent trend is distinct from the previous fluctuations which were presumably due to natural variability. Similar results are found when the analysis is applied to other stations.
Toward reconciliation of Late Ordovician (∼440 Ma) glaciation with very high CO2 levels
Authors: Crowley Thomas J. , Baum Steven K.
Published: February 2008

Although Phanerozoic glaciations usually coincided with times of estimated low atmospheric CO2, the Late Ordovician (440 Ma) glaciation is a significant exception. CO2 levels during that time may have been as much as 10 times greater than present. In an earlier paper we suggested that the unique geographic configuration of Gondwanaland may explain such a response, as the edge of the supercontinent was essentially tangent to the south pole, and the moderating effect of the nearby ocean may have suppressed the magnitude of summer warming on the landmass, thereby allowing glacial inception. One limitation to the earlier study was that it used a linear energy balance model (EBM). In this paper we further test the above hypothesis in a suite of experiments with a nonlinear EBM that allows for snow‐albedo feedback. We also utilize updated estimates for CO2 levels, decreases in solar luminosity, and variations in orbital forcing. Baseline experiments with no changes in luminosity or CO2 resulted in an ice‐covered area of 6.3 ×106 km2, 53% of the estimated area covered by the Ordovician ice sheet. Additional experiments for different combinations of orbital forcing, 7X/13X CO2, and −3.5%/−5.0% luminosity yielded 0–35% of the estimated ice area in the Late Ordovician. A crude estimate of possible topographic influences increased these numbers to7–47% of total estimated ice area. Additional factors related to ice sheet growth should increase these values somewhat. These results provide additional support for the high CO2/glaciation explanation, with the caveat that even the partial success occurs only when parameters are at the extreme end of their permissible range. The estimated duration of Ordovician glaciation is also consistent with the migration of Gondwanaland across the south pole, with a centrally located pole yielding ice‐free conditions in the summer. Thus identical levels of external forcing yield either glaciated or ice‐free conditions, with the solution dependent on location of the landmass. Although more work is required on this topic, our experiments suggest that there may be a relatively parsimonious explanation for this perplexing paleoclimate paradox.The results lend further support to the proposition that paleogeography significantly modifies the role of CO2 in the long‐term evolution of climate.
Electric field soundings through thunderstorms
Authors: Marshall Thomas C. , Rust W. David
Published: February 2008

We report on 12 balloon soundings of the electric field in thunderstorms. The maximum magnitude of E in the storms averaged 96±28 kV/m, with the largest being 146 kV/m. The maximum was usually observed between vertically adjacent regions of opposite charge. Using a one‐dimensional approximation to Gauss' law, we infer four to ten charge regions in the storms. The magnitude of the density in the charge regions varied between 0.2 and 13 nC/m3. The vertical extent of the charge regions ranged from 130 to 2100 m. None of our 12 storms had charge distributions that fit the long‐accepted model of Simpson and colleagues of a lower positive charge, a main negative charge, and an upper positive charge. In addition to regions similar to the Simpson model our storms had screening layers at the upper and lower cloud boundaries and extra charge regions, usually in the lower part of the cloud.
Measurements of dimethyl sulfide and H2S over the western North Atlantic and the tropical Atlantic
Authors: Andreae T. W. , Andreae M. O. , Bingemer H. G. , Leck C.
Published: February 2008

Airborne measurements of DMS and H2S were made off the east coast of the United States and over the tropical Atlantic off Brazil. Samples were collected through a fluorinated ethylene propylene Teflon inlet manifold. Dimethyl sulfide (DMS) was preconcentrated onto gold wool and analyzed by gas chromatography/flame photometric detection. H2S was collected on AgNO3‐impregnated filters and determined by fluorescence quenching. Use of a new scrubber material (cotton) to remove negative interference on DMS measurements was investigated. Comparison with a Na2CO3/Anakrom scrubber gave good overall agreement. Only under extreme conditions, e.g., on flight 9 (continental air mass, low humidity, high O3, and low DMS values) did Na2CO3 show noticeable loss of DMS compared to cotton. On most flights, especially in marine air masses with high humidity and relatively low O3, the results from both scrubbers agreed well with each other and with other instruments used during the intercalibration. Off the U.S. East Coast, DMS levels showed strong dependence on air mass origin with high values (up to 83 ppt) in marine tropical air masses and low values (10–20 ppt) in continental and polar air. Over the tropical Atlantic, DMS ranged over 20–100 ppt in the mixed layer. Nighttime values were a factor of 1.6–2.3 higher than daytime levels. DMS decreased with altitude to <1 ppt at 4000 m. H2S in the mixed layer off the U.S. East Coast ranged from 10 to 200 ppt. Significant influence from terrestrial and pollution sources was evident. H2S in air masses originating over the eastern seaboard was much higher than in continental polar air or over the remote tropical continents. In contrast, over the tropical Atlantic, concentrations were very low (5–10 ppt), typical of truly marine air. Night/day ratios were about 1.4. No significant geographical variability was seen in H2S levels over the tropical Atlantic. The correlation of atmospheric 222Rn and H2S was significant, with both being higher off the U.S. East Coast (H2S, 7–270 ppt; 222Rn, 2–20 pCi m−3) than over the tropical Atlantic (0.5‐10 ppt and 2–4 pCi m−3, respectively).
Spectral signature of oxidized pyroxenes: implications to remote sensing of terrestrial planets
Authors: Straub D'Arcy W. , Burns Roger G. , Pratt Stephen F.
Published: February 2008

The Fe2+ ions in pyroxenes occurring in basalts on surfaces of terrestrial planets are vulnerable to oxidation, particularly in magma in contact with the atmosphere; this produces Fe3+‐bearing assemblages that may mask spectral features of remnant Fe2+ in pyroxenes in telescopic reflectance spectra of oxidized planetary surfaces. To assess such interferences, reflectance spectra (0.3–2.6 urn) were obtained for a variety of pyroxenes that had been heated in air at elevated temperatures. Mossbauer spectral measurements enabled the relative proportions of Fe3+ and Fe2+ ions and Fe3+‐bearing phases to be determined in the reaction products. Nanophase hematite having particle diameters smaller than 10 nm obscures the pyroxene Fe2+/M2 site crystal field (CF) bands of oxidized enstatite and pigeonite. Intensification of Fe2+ → Fe3+ intervalence charge transfer transitions in calcic clinopyroxenes mask pyroxene Fe2+/M1 site CF bands, as well as the Fe2+/M2 site CF bands of augites and subcalcic hedenbergites. The ferric iron derived from oxidation of Fe2+ ions may remain as structural Fe3+ ions in the oxidized calcic clinopyroxenes or exist as clusters of nanophase hematite particles having diameters of a few unit cells.
Graben and extension in Northern Tharsis, Mars
Authors: Plescia J. B.
Published: February 2008

A morphometric analysis of north–south trending graben at 35°N in northern Tharsis is used to provide the first quantitative measurements of crustal extension in and strain for areas of the Martian crust that have undergone both extreme and mild deformation. These estimates also demonstrate the viability of the Viking data for making such quantitative assessment Graben in the region accommodate ∼8 km of post Early Amazonian east–west extension. This extension corresponds to a net regional strain of 0.45 – 0.61%; local strains vary from <1% to 5%. Extension is nonuniform and localized near 110° (south of Alba Patera and north of Ceraunius Fossae) because of reactivation of preexisting fractures in underlying basement and possibly due to superposition of stress fields from Tharsis and Alba Patera. A major mechanical discontinuity in the shallow crust, probably representing the base of or a strength contrast within the megaregolith, is observed. It occurs at a depth of 2 km on the profile flanks and is depressed to about 7.5 km near 110° due to loading of the crust by the volcano. A number of more shallow discontinuities occur within the construct and probably represent lithologic contacts.
Tektites in Cretaceous‐Tertiary boundary rocks on Haiti and their bearing on the Alvarez Impact Extinction Hypothesis
Authors: Izett G. A.
Published: February 2008

Relic tektites are associated with a Pt‐group metal abundance anomaly and shocked minerals in a thin marl bed that marks the K‐T boundary on Haiti. The presence of these three impact‐produced materials at the precise K‐T boundary enormously strengthens the Alvarez impact extinction hypothesis. The tektites occur in smectite spherules that have external shapes typical of tektites. Their chemical and physical properties are broadly similar to those of other tektite groups, except that the Haitian tektites have lower Si and higher Fe, Ca, and Na. On average, they contain more Sc, V, Cu, Zn, Ga, Sr, Sn, and Ba and less Cr, Ni, Co, B, Mn, and Hf than Other tektite groups. Amounts of rare earth elements (REE) in the tektites indicate that their progenitor materials were not melted mafic or ultramafic oceanic crust; rather they were sedimentary deposits having a bulk composition of andesite. Rare tektites contain unusually high amounts of CaO (∼20%) and S (0.4%), and these data suggest that some target materials consisted of CaSO4. Anhydrite beds occur in the subsurface at two candidate impact sites (Chicxulub and Manson). Sm‐Nd isotopic data for the tektites indicate that the melted precursor sediments were most likely deposited less than −400 m.y. between the Late Cretaceous and Silurian periods. Major chemical changes accompanied the diagenetic change of glass to smectite. The Haitian tektites are the first datable impact products in K‐T boundary rocks, and 40Ar‐39Ar ages of the glass show that the K‐T boundary and impact event are coeval at 64.5±0.1 Ma.
Variability of Upwelling Off NW Africa During the Latest Quaternary: Diatom Evidence
Authors: Abrantes F. F.
Published: February 2008

Diatom abundance and species composition were quantitatively studied in two latest Quaternary (≃130 ka to the Present) sequences from the continental margin of northwest Africa. Off this region, coastal upwelling is well developed under the influence of the NE trade winds. Variations in diatom abundance in these cores are inferred to represent changes caused by varying degrees of the upwelling fertility. Times of high productivity are marked by high relative frequencies of Chaetoceros, while low productivity is marked by the dominance of Aulacoseira granulata. Upwelling increased during glacial episodes (isotopic stages 2–4 and 6) relative to isotopic stages 1 and 5. During the late Holocene, primary productivity levels are similar to those for Stage 5, but in the early Holocene upwelling intensities seem to have been weaker than today. The paleoproductivity reconstruction based on the diatom record is supported by paleoproductivity estimations based on the organic carbon content of the sediments (Sarnthein et al., 1987a).
Equatorial Pacific Calcite Preservation Cycles: Production or Dissolution?
Authors: Archer D. E.
Published: February 2008

In the Equatorial Pacific Ocean, the depth and shape of the calcite lysocline appears to have changed significantly in response to climate forcing (Farrell and Prell, 1989). Taking the Farrell and Prell record at face value, the lysocline apparently became steeper during each of the last eight glacial stages. Also, the accumulation rate of calcite during the last glacial maximum was 2–4 times higher than it was during the previous interglacial (Arrhenius; 1952,1988; Broecker, 1971; Farrell, 1991). A numerical model for calcite dissolution in sediment is used to interpret these observations. The model was validated by comparison with observations from the present‐day ocean; these results are presented elsewhere. If the lysocline fluctuations in the equatorial Pacific are primarily a dissolution record, then changes in the glacial Pacific [CO3] of 20–40 µM can be inferred. Here I offer the alternative explanation that cycles in equatorial production are responsible for the observations. Higher rates of calcite and organic carbon rain to the sediment in the equatorial region would have depressed the calcite lysocline and increased the calcite accumulation rate, as observed. A twofold increase in glacial production appears to be adequate to explain the observations, but a precise determination is prevented by uncertainties in some of the model parameters.
Carbonate Variations in the Northeast Pacific during the Late Quaternary
Authors: Karlin Robert , Lyle Mitchell , Zahn Rainer
Published: February 2008

Quaternary glacial/interglacial changes in the carbonate compensation depth (CCD) have been determined from core suites in three regions of the NE Pacific. The cores have sedimentation rates ranging from 1.6 to 16 cm/kyr, based on oxygen isotope, radiocarbon, and/or carbonate stratigraphies. Carbonate values are higher in glacial than interglacial times and generally follow a pattern similar to that of the central equatorial Pacific. The sediment's carbonate profiles can be readily intercorrelated, allowing the construction of a regional carbonate stratigraphy. The extent of carbonate preservation is very sensitive to small changes in water depth but does not depend on geographic location. A depth transect of cores off northern California and Oregon provides a detailed record of glacial/interglacial shifts in the CCD through time. During the late Holocene, intensified carbonate dissolution at ∼7 ka is coincident with shifts in continental climatic indicators and may suggest the onset of the modern upwelling regime along the NW U.S. margin. During the last 45 ka the CCD has migrated more than 1800 m between glacial and interglacial times. Although carbonate patterns are similar to those in the equatorial Pacific, the amplitude of the carbonate fluctuations is much greater in the northeast Pacific. This suggests that regional mechanisms, such as glacial deepwater formation or enhanced dissolution due to interglacial noncarbonate productivity related to coastal upwelling, may modulate carbon cycling.
Fractal aggregates in geophysics
Authors: Meakin Paul
Published: February 2008

The combination of small particles to form larger structures is important in a broad range of natural processes, including the flocculation of organic and mineral particles in rivers, estuaries, and the open sea, the growth of smoke particles in the atmosphere, and accretion in the primordial solar nebula. Very often, these structures (cluster, flocs, and aggregates) have a low‐density random structure that can be described quite well in terms of the concepts of fractal geometry. The structure, growth kinetics, and properties of these “fractal aggregates” have been of considerable interest in statistical physics for the past decade. Other processes such as fluid‐fluid displacement in a porous medium, the dissolution of porous materials, dielectric breakdown, and random dendritic growth also lead to the growth of random fractals and can be understood in terms of aggregation models such as diffusion‐limited aggregation. The characterization of fractal aggregates produced by computer simulations, laboratory experiments, and natural processes is described, and some of the implications of the aggregates' fractal structure are discussed. The role played by fractal aggregates in geophysical processes such as sedimentation, air and water pollution, the evolution of river networks, and multiphase flow in porous materials is also discussed.
The K‐derived planetary indices: Description and availability
Authors: Menvielle M. , Berthelier A.
Published: February 2008

The 3‐hour K index was the first to provide an objective and quantitative monitoring of the irregular variations of the transient geomagnetic field observed in a given place. The use of K indices from a network of observatories to derive a planetary index of geomagnetic activity was suggested by Bartels when defining these indices. Then, Kp, am, Km, an, as, and aa were successively designed and accepted as International Association of Geomagnetism and Aeronomy indices. At present these K‐derived planetary indices are routinely computed and circulated. They make long homogeneous data sets and are widely used for long‐term and statistical studies in geomagnetism and solar‐terrestrial physics. After a short description of the main features of transient geomagnetic activity a definition of the K index as a measure of the irregular activity is given with a summary of its basic characteristics. The derivation of K‐derived planetary indices is described and discussed, and updated indications concerning their availability are presented. This short review provides users with the minimum required knowledge about these indices and may serve as an introduction to the Mayaud (1980) monograph.
Acceleration of the solar wind
Authors: Barnes Aaron
Published: February 2008

Stellar winds are thought to be the primary mass loss process for most stars and to play a particularly important role in mass and angular momentum transfer in newly forming stars. The physical processes that account for the acceleration of such winds are therefore of fundamental importance in astrophysics but are at present poorly understood, even for the case of the Sun. The question of just how the solar wind is accelerated to hypersonic speeds has been one of the most fundamental questions of space science since the beginning of the space age and is still far from a definitive solution. In this present review I give a general overview of some of the problems associated with trying to understand the physics of solar wind acceleration, how these problems have been approached, and the kinds of observational and theoretical efforts needed to facilitate closure between theory and observation.
Non‐uniform motion and extended media effects on the mutual coherence function: An analytic solution for spaced frequency, position, and time
Authors: Nickisch L. J.
Published: February 2008

An analytical expression for the two‐frequency, two‐position, two‐time mutual coherence function applicable to propagation through thick random media with nonuniform electron density and plasma velocity is derived using the phase‐screen/diffraction method (PDM). In this method the ionization is collapsed to a number of thin screens and diffraction is developed in the free space between. The resulting mutual coherence function converges rapidly to the continuum result as the number of screens representing the medium is increased. The effects of multiple scatter occurring over long distances and varying plasma velocity over the propagation path are shown to be important in HF propagation. Scattering functions (delay‐Doppler power spectra) obtained as Fourier transforms of the PDM mutual coherence function are compared to scattering functions measured by an HF channel probe. Nonuniform velocity profiles are shown to account for the variety of delay‐Doppler couplings observed.
Excitation of whistler mode signals via injection of polarized VLF waves with the Siple transmitter
Authors: Mielke T. A. , Elkins C. J. , Helliwell R. A. , Inan U. S.
Published: February 2008

Whistler mode waves of various polarizations were transmitted by the Siple Station, Antarctica, VLF transmitter and received near the geomagnetic conjugate point at Lake Mistissini, Quebec. Crossed 21‐km horizontal dipole antennas on top of the 2‐km‐thick ice sheet were used to transmit 2‐ to 4‐kHz waves alternately with right‐hand circular, left‐hand circular, and linear polarizations. Excitation of a multiplicity of magnetospheric propagation paths and the received signal strength were observed to depend on the transmitter antenna polarization. Where whistler mode growth and emission triggering occurred, saturated peak values of received signals were independent of antenna polarization and initial injected power levels, in agreement with previous findings. Propagation paths of ducted Siple signals observed at Lake Mistissini were identified with propagation paths deduced from natural whistlers, from which the L shell values and equatorial number densities for the paths were calculated. A combination of L shell data and models of antenna coupling into the whistler mode may aid in the location of ducts. Dynamics Explorer 1 satellite recordings of unducted Siple signals showed trends similar to the ground data on ducted signals. The observations are discussed in the context of a simplified model of the coupling from the Siple antenna into the ionosphere, which provides reasonable agreement with observations.
Radiation of whistler waves in magnetoactive plasma
Authors: Kondrat'ev I. G. , Kudrin A. V. , Zaboronkova T. M.
Published: February 2008

In this paper the radiation of ring electric and magnetic currents in magnetoactive plasma in whistler frequency range is investigated. Particular attention is concentrated on the problem of variation of the radiated power distribution over the spectrum of excited waves with variation of the radiator parameters. Attention is also given to the radiation pattern structure near the resonance cone and peculiar caustic directions corresponding to the conic refraction and the Storey angle. It is shown that for ring electric current the consideration of the inhomogeneity in its distribution along the ring can be of importance and can lead, in particular, to a large increase of the relative contribution of the quasi‐electrostatic whistler mode waves to the total radiation power. Detailed numerical calculations have been carried out for the conditions close to ionospheric ones.
Flexural flow folding and the paleomagnetic fold test: An example of strain reorientation of remanence in the Mauch Chunk Formation
Authors: Stamatakos J. , Kodama K. P.
Published: February 2008

The relationship between the remanent magnetization and the detailed strain geometry around a first‐order fold in the Appalachian Valley and Ridge Province was investigated to examine whether penetrative strains associated with folding can generate a apparent synfolding geometry from a prefolding magnetization. Paleomagnetic results from the Mississippian Mauch Chunk Formation on both limbs of the Frackville Anticline near Lavelle, Pennsylvania, yield two magnetic components, an intermediate unblocking temperature (300°C–600°C) Kiaman remagnetization and a two‐polarity high unblocking temperature (650°C–680°C) characteristic magnetization. When the magnetic directions are incrementally corrected for bedding tilt, the intermediate‐temperature component is most tightly clustered at 85% unfolding (D=176°, I=3°) and the high‐temperature component is most tightly clustered at 75% unfolding (D=184°, I=27°). Mesoscopic and microscopic structural fabric analyses suggest a strain history that includes a significant component of flexural slip/flow folding. In the coarser‐grained sandstone units, folding has largely been accommodated by slip on bedding, while in the finer‐grained beds, folding has been accommodated by grain‐scale deformation mechanisms such as pressure solution and low‐temperature plasticity. Finite strain measurements, determined from center‐to‐center distances between quartz grains, yield strain ellipsoids consistent with this folding model. Inclination of the characteristic component varies as a function of the magnitude of the finite strain. This variation suggests that the characteristic magnetization has been systematically reoriented with respect to bedding during folding. Remanence directions on the south dipping limb have been rotated to shallower inclinations, while those on the north dipping limb have been rotated to steeper directions causing the prefolding magnetization to appear synfolding. These rotations are in agreement with models of rigid particle rotation in deforming viscous media. Unlike the characteristic magnetization, the secondary component appears to be unaffected by the deformation, and its synfolding behavior is interpreted as the acquisition of a secondary magnetization during Alleghenian folding. These results show that it is important to consider penetrative strains when evaluating the significance of apparent synfolding magnetizations.
Early Middle Paleozoic Intraplate Orogeny in the Ogcheon Belt (South Korea): A new insight on the Paleozoic buildup of east Asia
Authors: Cluzel Dominique , Jolivet Laurent , Cadet Jean‐Paul
Published: February 2008

The polycyclic Ogcheon belt of South Korea represents the boundary between two Precambrian blocks. Investigation on the timing and kinematics of the block agglomeration is of considerable bearing on the understanding of the constitution of the Asian continent. We report here structural evidence showing that the earlier tectonic event dates back to early middle Paleozoic (the “Ogcheon tectonism” or “orogeny”). The ductile piling up of nappes of the Ogcheon belt corresponds to an intracontinental orogeny involving the opening and the subsequent closure of an aborted rift and not of a wide oceanic area. The nappes contain unequivocal evidence for ductile shearing with a northwest over southeast sense of movement. Field evidence includes the presence of S1 and S1–2 foliations bearing an extensive stretching lineation perpendicular to the belt, associated with a strongly noncoaxial deformation regime. The F1 isoclinal folds are commonly “A type” and sheath folds whose axes are parallel to the stretching lineation, whereas the southeast facing F2 recumbent folds have subhorizontal axes parallel to the belt. All lines of evidence (unconformity, superposed deformations, age of the metamorphism, etc…) imply that the D1–2 “Ogcheon tectonism” is pre‐middle Carboniferous (Late Silurian‐Early Devonian). The subsequent Indosinian (Middle Triassic) F3 upright synfolial folds that trend NNE‐SSW and later structures clearly overprint the middle Paleozoic ductile thrusts and isoclinal folds. The early Paleozoic Ogcheon Supergroup is formed of a thick volcanosedimentary pile deposited above an early platform sequence in a rift basin. On the basis of the geochemical characteristics of the metavolcanics, it appears that oceanization did not occur and Ogcheon rift aborted soon after it was formed. We suggest that middle Paleozoic ductile nappe structures have been formed by the structural inversion of the rift extensional features. During the ductile stacking episode, the early formed foliation and isoclinal folds have been folded in a continuum during the same tectonic event. Middle Carboniferous terrigenous formations have been deposited unconformably upon folded and thrust older rocks. A tentative correlation with early Paleozoic Imjingang, Qinling and Cathaysian belts within the Asian continent reveals that Ogcheon rift was emplaced within the South China plate and that Imjingang belt should represent the limit between North and South China blocks in the Korean peninsula.