Morphology of ash aggregates from wet pyroclastic surges of the 1982 eruption of El Chichón Volcano, Mexico

The detailed stratigraphic study of the pyroclastic surge units S1, IU, and S3 produced during the most violent phases of the 1982 eruption of El Chichón volcano, contains a complex succession of hydromagmatic events triggered by the interaction of different proportions of magma and external water. Component analyses of the horizons within single units reveal that almost all wet and cohesive horizons contain ash aggregates. Based on their morphology and internal structure four different types of aggregates were distinguished: (a) accretionary lapilli, (b) armored lapilli, (c) irregular aggregates, and (d) cylindrical aggregates. The first three types have been described in the volcanological literature (field and experimental studies); cylindrical forms are reported here for the first time. These hollow cylindrical aggregates consist of concentric layers of crystals and glass fragments set in a finer-grained matrix. They formed around millimeter-size foliage fragments that are locally preserved in the interior of the aggregates as scorched or completely carbonized vestiges. SEM analyses suggest different mechanisms of formation for the four types of aggregates. Irregular aggregates and armored lapilli formed nearly instantaneously, whereas accretionary lapilli and cylindrical aggregates resulted from progressive aggregation of ash in different regions of the eruptive cloud.All types of ash aggregates contain fractured particles. This common feature suggests that particles ruptured during fragmentation prior to the growth of the aggregates. Broken clasts with cracks filled by a fine-grained matrix only occur inside the cylindrical ash aggregates and to a lesser degree in some types of accretionary lapilli. This suggests that small thermal contrasts at the contact of warm particles with the colder fine-grained matrix of the aggregate cause existing small fractures to propagate and open as the already weakened clasts deform slightly. The occurrence of all four types of aggregates in some horizons indicates that several mechanisms of aggregation occurred nearly simultaneously. The pyroclastic clouds therefore were not only stratified in terms of density but the content of fluid phases also were not uniform. A dark-red, Fe-rich amorphous film (locally rich in P and S) envelops the particles and fosters their preservation in the deposits by forming a hard shell. The composition of this cement reflects the abundance of these elements in acid fluids of hydrothermal systems that were intersected by the conduit during the eruption. In distal areas, fallout aggregates were incorporated by dissipating pyroclastic surges.     

Nekton of new seagrass habitats colonizing a subsided salt marsh in Galveston Bay, Texas

Subsidence and erosion of intertidal salt marsh at Galveston Island State Park, Texas, created new areas of subtidal habitat that were colonized by seagrasses begining in 1999. We quantified and compared habitat characteristics and nekton densities in monospecific beds of stargrassHalophila engelmanni and shoalgrassHalodule wrightii as well as adjacent nonvegetated substrates. We collected 10 replicates per habitat type during April, July, October, and December 2001. Most habitat characteristics varied with season. Water temperature, salinity, and dissolved oxygen were similar among habitat types. Turbidity and depth were greatest inH. engelmanni beds and least inH. wrightii beds.H. engelmanni exhibited shorter leaves and higher shoot density and biomass core⁻¹ thanH. wrightii. Densities of almost all dominant species of nekton (fishes and decapods) were seasonally variable, all were higher in seagrass habitats than in nonvegetated habitats, and most were higher in one seagrass species than the other. Naked gobyGobiosoma bosc, code gobyGobiosoma robustum, bigclaw snapping shrimpAlpheus heterochaelis, and blue crabCallinectes sapidus, were most abundant inH. engelmanni. Brown shrimpFarfantepenaeus aztecus, brackish grass shrimpPalaemonetes intermedius, and daggerblade grass shrimpPalaemonetes pugio were most abundant inH. wrightii. PinfishLagodon rhomboides and pink shrimFarfantepenaeus duorarum were equally abundant in either seagrass. Most dominant nekton varied in size by month, but only two (L. rhomboides andC. sapidus) exhibited habitat-related differences in size. Nekton densities in these new seagrass habitats equaled or exceeded densities associated with historical and current intertidal smooth cordgrassSpartina alterniflora marsh. Continued seagrass expansion and persistence should ensure ecosystem productivity in spite of habitat change.     

Assessment of Offshore New Jersey Sources of Beach Replenishment Sand by Diversified Application of Geologic and Geophysical Methods

Beach replenishment serves the dual purpose of maintaining a source of tourism and recreation while protecting life and property. For New Jersey, sources for beach sand supply are increasingly found offshore. To meet present and future needs, geologic and geophysical techniques can be used to improve the identification, volume estimation, and determination of suitability, thereby making the mining and manag ing of this resource more effective. Current research has improved both data collection and interpretation of seismic surveys and vibracore analysis for projects investigating sand ridges offshore of New Jersey. The New Jersey Geological Survey in cooperation with Rutgers University is evaluating the capabilities of digital seismic data (in addition to analog data) to analyze sand ridges. The printing density of analog systems limits the dynamic range to about 24 dB. Digital acquisition systems with dynamic ranges above 100 dB can permit enhanced seismic profiles by trace static correction, deconvolution, automatic gain scaling, horizontal stacking and digital filtering. Problems common to analog data, such as wave-motion effects of surface sources, water-bottom reverberation, and bubble-pulse-width can be addressed by processing. More than 160 line miles of digital high-resolution continuous profiling seismic data have been collected at sand ridges off Avalon, Beach Haven, and Barnegat Inlet. Digital multichannel data collection has recently been employed to map sand resources within the Port of New York New Jersey expanded dredge-spoil site located 3 mi offshore of Sandy Hook, New Jersey. Multichannel data processing can reduce multiples, improve signal-to-noise calculations, enable source deconvolution, and generate sediment acoustic velocities and acoustic impedance analysis. Synthetic seismograms based on empirical relationships among grain size distribution, density, and velocity from vibracores are used to calculate proxy values for density and velocity. The seismograms are then correlated to the digital seismic profile to confirm reflected events. They are particularly useful where individual reflection events cannot be detected but a waveform generated by several thin lithologic units can be recognized. Progress in application of geologic and geophysical methods provides advan tages in detailed sediment analysis and volumetric estimation of offshore sand ridges. New techniques for current and ongoing beach replenishment projects not only expand our knowledge of the geologic processes involved in sand ridge origin and development, but also improve our assessment of these valuable resources. These reconnaissance studies provide extensive data to the engineer regarding the suitability and quantity of sand and can optimize placement and analysis of vibracore samples.     

Radio and EUV observations of a coronal hole

We present observations of a coronal hole made with the EUV spectroheliometer of the Harvard College aboard Skylab and with high resolution (2–4) radio telescopes at Culgoora and Fleurs Australia and Bonn, West Germany. We attempt to derive the density and temperature distributions in the transition region and inner corona from the combined observations. No one standard model can explain both sets of observations; characteristically, models based on EUV data yield higher radio brightnesses than are observed, while models based on radio data yield lower EUV line intensities than are observed. The discrepancy is essentially that the electron density inferred from the EUV data is about three times that inferred from the radio data.After examining several possible modifications of the standard models we suggest that the discrepancy would disappear if the abundances of the heavier elements were increased by about a factor of 10. Such increases could result from differential diffusion in the large temperature gradient of the transition region. We conclude therefore that models which incorporate thermal diffusion, as well as mass outflow and departures from ionization equilibrium, offer the greatest hope of reconciling the EUV and radio observations of coronal holes.     

Sedimentary basins of the atlantic margin of North America

Scismic exploration has identified eight distinct basin structures along the North American Atlantic continental margin forming a chain of elongate depocenters parallel to the continental slope and interrupted by transverse basement arches and impinging oceanic fracture zones. From south to north these are: South Florida—Bahamas Basin bounded on the north by Peninsular Arch and Bahama Escarpment fracture zone; Blake Plateau Basin with Cape Fear Arch and the impinging Great Abaco and Blake Spur fracture zones; Baltimore Canyon Trough bounded by the Long Island Platform and impinging Kelvin fracture zone; Georges Bank Basin with the bounding Yarmouth Arch; Scotian Shelf Basin with Scartarie and Canso Ridges and impinging Newfoundland Ridge fracture zone; Grand Banks Troughs and the intervening horst ridges; and the East Newfoundland Basin separated by Cartwright Arch and the impinging Gibbs fracture zone from the Labrador Shelf Basin.All the basins are characterized by great depths to basement filled with from 7 to 14 km of possible Triassic, Jurassic, Cretaceous and Tertiary sediments. Basement faulting controls the basins' boundaries and the faults have affected the overlying sediments. The major boundary faults of the basins undoubtedly formed during the initial rifting of the Atlantic margin in the Jurassic or perhaps Triassic. However, throughout the Mesozoic and Cenozoic these basement faults have moved in response to different orientations of stress and strain rates produced by continued spreading of the Atlantic Ocean. As a result, the basement faults of the Atlantic Margin were apparently influenced by at least three different local stress systems, spatially overlapping but temporally independent. These are the east—west extensional Atlantic Ocean stress system, the northwest—southeast extensional White Mountain stress system, and the north-south extensional Labrador Sea stress system.Some consequences of this basic tectonic setting were differential cross-strike tilts of the basin blocks with each basin moving somewhat independent of its neighbor. The resulting buildup of the basins' sedimentary geometries reflect these tectonic tilts and varying strain rates. Correlations are found between changes in orientation and rates of Atlantic sea-floor spreading with observed major sedimentary events such as progradations, planar bedding episodes, reef platform development, regressive hiatuses, and transgressions. An understanding of this marginal geosyncline could yield a model with predictability.     

Assessment of Offshore New Jersey Sources of Beach Replenishment Sand by Diversified Application of Geologic and Geophysical Methods

Beach replenishment serves the dual purpose of maintaining a source of tourism and recreation while protecting life and property. For New Jersey, sources for beach sand supply are increasingly found offshore. To meet present and future needs, geologic and geophysical techniques can be used to improve the identification, volume estimation, and determination of suitability, thereby making the mining and manag ing of this resource more effective. Current research has improved both data collection and interpretation of seismic surveys and vibracore analysis for projects investigating sand ridges offshore of New Jersey. The New Jersey Geological Survey in cooperation with Rutgers University is evaluating the capabilities of digital seismic data (in addition to analog data) to analyze sand ridges. The printing density of analog systems limits the dynamic range to about 24 dB. Digital acquisition systems with dynamic ranges above 100 dB can permit enhanced seismic profiles by trace static correction, deconvolution, automatic gain scaling, horizontal stacking and digital filtering. Problems common to analog data, such as wave-motion effects of surface sources, water-bottom reverberation, and bubble-pulse-width can be addressed by processing. More than 160 line miles of digital high-resolution continuous profiling seismic data have been collected at sand ridges off Avalon, Beach Haven, and Barnegat Inlet. Digital multichannel data collection has recently been employed to map sand resources within the Port of New York New Jersey expanded dredge-spoil site located 3 mi offshore of Sandy Hook, New Jersey. Multichannel data processing can reduce multiples, improve signal-to-noise calculations, enable source deconvolution, and generate sediment acoustic velocities and acoustic impedance analysis. Synthetic seismograms based on empirical relationships among grain size distribution, density, and velocity from vibracores are used to calculate proxy values for density and velocity. The seismograms are then correlated to the digital seismic profile to confirm reflected events. They are particularly useful where individual reflection events cannot be detected but a waveform generated by several thin lithologic units can be recognized. Progress in application of geologic and geophysical methods provides advan tages in detailed sediment analysis and volumetric estimation of offshore sand ridges. New techniques for current and ongoing beach replenishment projects not only expand our knowledge of the geologic processes involved in sand ridge origin and development, but also improve our assessment of these valuable resources. These reconnaissance studies provide extensive data to the engineer regarding the suitability and quantity of sand and can optimize placement and analysis of vibracore samples.     

Catchment‐scale contribution of forest roads to stream exports of sediment,phosphorus and nitrogen

The relative contribution of forest roads to total catchment exports of suspended sediment, phosphorus, and nitrogen was estimated for a 13 451 ha forested catchment in southeastern Australia. Instrumentation was installed for 1 year to quantify total in‐stream exports of suspended sediment, phosphorus, and nitrogen. In addition, a total of 101 road–stream crossings were mapped and characterized in detail within the catchment to identify the properties of the road section where the road network and the stream network intersect. Sediment and nutrient generation rates from different forest road types within the catchment were quantified using permanent instrumentation and rainfall simulation. Sediment and nutrient generation rates, mapped stream crossing information, traffic data and annual rainfall data were used to estimate annual loads of sediment, phosphorus, and nitrogen from each stream crossing in the catchment. The annual sum of these loads was compared with the measured total catchment exports to estimate the proportional contribution of loads from roads within the catchment. The results indicated that 3·15 ha of near‐stream unsealed road surface with an average slope of 8·4% delivered an estimated 50 t of the 1142 t of total suspended sediment exported from the catchment, or about 4·4% of the total sediment load from the forest. Stream discharge over this period was 69 573 Ml. The unsealed road network delivered an estimated maximum of 22 kg of the 1244 kg of total phosphorus from the catchment, or less than 1·8% of the total load from the forest. The average sediment and phosphorous load per crossing was estimated at 0·5 t (standard deviation 1·0 t) and 0·22 kg (standard deviation 0·30 kg) respectively. The lower proportional contribution of total phosphorus resulted from a low ratio of total phosphorus to total suspended sediment for the road‐derived sediment. The unsealed road network delivered approximately 33 kg of the 20 163 kg of total nitrogen, about 0·16% of the total load of nitrogen from the forest. The data indicate that, in this catchment, improvement of stream crossings would yield only small benefits in terms of net catchment exports of total suspended sediment and total phosphorus, and no benefit in terms of total nitrogen. These results are for a catchment with minimal road‐related mass movement, and extrapolation of these findings to the broader forested estate requires further research. Copyright © 2007 John Wiley & Sons, Ltd.