Travel time controls the magnitude of nitrate discharge in groundwater bypassing the riparian zone to a stream on Virginia's coastal plain

Groundwater that bypasses the riparian zone by travelling along deep flow paths may deliver high concentrations of fertilizer‐derived NO₃^? to streams, or it may be impacted by the NO₃^? removal process of denitrification in streambed sediments. In a study of a small agricultural catchment on the Atlantic coastal plain of Virginia's eastern shore, we used seepage meters deployed in the streambed to measure specific discharge of groundwater and its solute concentrations for various locations and dates. We used values of Cl^? concentration to discriminate between bypass water recharged distal to the stream and that contained high NO₃^? but low Cl^? concentrations and riparian‐influenced water recharged proximal to the stream that contained low NO₃^? and high Cl^? concentrations. The travel time required for bypass water to transit the 30‐cm‐thick, microbially active denitrifying zone in the streambed determined the extent of NO₃^? removal, and hydraulic conductivity determined travel time through the streambed sediments. At all travel times greater than 2 days, NO₃^? removal was virtually complete. Comparison of the timescales for reaction and transport through the streambed sediments in this system confirmed that the predominant control on nitrate flux was travel time rather than denitrification rate coefficients. We conclude that extensive denitrification can occur in groundwater that bypasses the riparian zone, but a residence time in biologically active streambed sediments sufficient to remove a large fraction of the NO₃^? is only achieved in relatively low‐conductivity porous media. Instead of viewing them as separate, the streambed and riparian zone should be considered an integrated NO₃^? removal unit. Copyright © 2011 John Wiley & Sons, Ltd.     

On the transfer of stratospheric ozone into the troposphere near the north pole

A series of nearly daily ozone vertical profiles obtained at station T-3 on Fletcher's Ice Island (85°N, 90°W) during the period January-March 1971 shows several significant ozone intrusions into the troposphere. These intrusions are not only associated with enhanced ozone amounts in the stratosphere but also require tropopause folding events to transport ozone into the troposphere. These folds in the Arctic tropopause appear to be capable of contributing significantly to the ozone budget of the Arctic troposphere during the late winter and spring seasons. The importance of tropopause folding for bringing ozone into the troposphere seen in the daily ozone profiles confirms the results found in the Arctic Gas and Aerosol Sampling Program aircraft flights.     

Long-term effects of crude oil on microbial processes in subarctic marine sediments: Studies on sediments amended with organic nutrients

Subarctic marine sediments amended with various organic compounds were exposed to fresh Cook Inlet crude oil at a concentration of 50 ppt for either 6 or 8 months. After the sediments were initially treated, they were returned to the approximate location where they were collected and left to be exposed to natural environmental conditions until they were retrieved for analysis. As a result of crude oil treatment, the activities of the enzymes that hydrolyse structural polysaccharides were reduced and the activities of the enzymes that hydrolyse storage polysaccharides were stimulated. In addition to these changes, we observed changes in phosphatase activity, nitrogen fixation rates, potential denitrification rates, methane concentrations, CO₂ production rates, and the glucose uptake and mineralization rates. The effect of the crude oil perturbation was different depending on the organic compound used in the amended sediments. Many of these differences could be explained by the effect of crude oil on the hydrolases which were responsible for degrading the compound in question. The results of this study suggest the effect of crude oil on microbial processes may be affected by the type of organic material present in the impacted marine sediment. This study also illustrates the value of measuring hydrolase activity in studies designed to determine the effects of this or any other pollutant on microbial processes in marine sediments.     

Carbon,hydrogen and nitrogen isotopes in solvent-extractable organic matter from carbonaceous chondrites

Solvent extractions were done on the carbonaceous chondrites Murray, Murchison, Orgueil and Renazzo, using CCl₄ and CH₃OH. Between 2 and 10% of the total carbon in these meteorites is extractable by ordinary techniques, most of it in CH₃OH. After demineralization with HF, perhaps as much as 30% of the total carbon in Murray may be extractable with CH₃OH. The extracts from Renazzo have isotopic ratios which suggest that they are mainly terrestrial organic matter, with lesser contributions from indigenous organics. The CH₃OH-soluble organic matter from Murchison and both untreated and HF-treated Murray has δ¹³C values of about +5 to + 10%. and δ¹⁵N values of about +90 to +100%., both of which are significantly higher than the bulk meteorite values. The Orgueil CH₃OH-extract also has a δ¹⁵N value well above the value in residual organic matter. Values for δD of +300 to +500%. are found for the CH₃OH-soluble organic matter. The combined data for C, H and N isotopes makes it highly unlikely that the CH₃OH-soluble components are derivable from, or simply related to, the insoluble organic polymer found in the same meteorites. A relationship is suggested between the event that formed hydrous minerals in CI1 and CM2 meteorites and the introduction of water-soluble (methanol-soluble) organic compounds. Organic matter soluble in CCl₄ has essentially no nitrogen, and δ³C and δD values are lower than for CH₃OH-soluble phases. Either there are large isotopic fractionations for carbon and hydrogen between different soluble organic phases, or the less polar components are partially of terrestrial origin.     

Processes controlling precipitation of carbonate cement and dissolution of silica in reef and near-reef settings

Carbonate cementation in the surface layer of reefs and beachrock eliminates porosity and partially replaces detrital quartz grains. The uptake and release of CO₂ by photosynthesis and respiration in reef communities cause a shift in the carbonate buffer system of seawater. Field studies and experimentation simulating the natural settings show minimum values of CO₂ (1.9 mmoles 1^?1) and HCO₃^? (2.4 meq 1^?1) in association with maximum values of pH (9.8) and O₂ (> 100% oxygen saturation) in waters covering corals and algae prior to sunset. The converse is true for these variables prior to sunrise, when minimum values of pH (7.6) and O₂ (<66% oxygen saturation) occur with maximum values of CO₂ (2.7 mmoles 1^?1) and HCO₃^? (2.7 meq 1^?1). Experimental tanks containing plain seawater showed almost no diurnal variability in pH (a constant 7.5–7.6) or O₂ (80–90% oxygen saturation) measurements. Seawater adjacent to reef biomass, with elevated pH and supersaturated with calcium carbonate, is periodically pumped into the underlying reef and beach sediments due to pressure-buildups between the reef framework or algal zones and open waters. Carbonate precipitation and quartz dissolution follow. As the system equilibrates, pH values progressively decrease.     

The Collision Manifold of the Collinear One-Bumper Two-Body Problem

We present a special model which is a caricature of the collinear three-body problem. Near triple-collision behavior for the model is governed by the collision manifold. We study the bifurcations of the dynamics on this manifold as we vary the ratio of masses. Both analytic and numerical results are presented.This revised version was published online in October 2005 with corrections to the Cover Date.     

Tectonics and volcanism along the Pacific Marginal Zone of Central America

The relation of volcanism to tectonics in the Central American region has been established by a review of the literature as well as by field and photogeological work of the authors. The association of a deep trench off the Pacific coast, a parallel seismic belt, and a similarly oriented chain of volcanic vents farther inland, has been recognized by numerous earlier workers. These features form a tectonic unit, and are here termed the Pacific marginal zone.Seismic foci on the northeastern edge of the southern part of the Middle American Trench define a fault zone which dips under the continent and suggests movement of the oceanic plate underneath the Central American continent.An impressive chain of volcanic cones and associated shallow seismic foci is aligned along a prominent graben, best developed in Nicaragua where it is known as the Nicaraguan Depression. This feature probably originated in late Tertiary time. The southeastern end of the graben terminates at the northern ranges of the Talamanca Cordillera in Costa Rica. At the Gulf of Fonseca the trend of the graben changes from NW to E-W; the graben continues as the Central Depression of El Salvador at a higher elevation. Only the fault of the seaward border of the graben is defined in Guatemala, where it is represented by a NW trending volcanic chain.In Nicaragua and El Salvador the oldest cones are situated on the north-eastern boundary fault zone of the graben. These include remnants of the largest volcanic structures of the region. All the active volcanoes are on the southwestern boundary fault belt. Cross fracturing of this fault system controlled later northerly trending cone alignments, often with the youngest cones on the south end.The main graben and associated faults are considered results of tensional stresses on the crest of geanticlinal arching on the landward side of the Middle American Trench. This arching is believed to be due to regional compression originated by the movement of the oceanic plate against the mainland.In northwestern Central America the youngest stresses produced N-S normal faults that are marked by well-defined scarps. These stresses may be the result of right-lateral motion along the underthrust fault zone. The complex Comayagua graben north of the Gulf of Fonseca, the Ipala graben and associated faults in southeastern Guatemala, and the Guatemala City graben are all north-south features illustrating the extent and youth of this structural trend.The distribution of volcanic vents along the Nicaraguan Depression and the N-S trends underscores the close tectonic control of volcanism.

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Zusammenfassung Der Zusammenhang zwischen Tektonik und Vulkanismus im mittelamerikanischen Raum wird aufgezeigt aufgrund einer Literaturdurchsicht und feldgeologischer Arbeiten der Verfasser. Die Beziehungen zwischen einem Tiefseegraben vor der pazifischen Küste, einem dem Festland parallelen seismischen Gürtel und einer ebenso orientierten Vulkankette weiter im Inland sind von früheren Bearbeitern bereits erkannt worden. Diese drei Einheiten werden hier zusammengefaßt und als pazifische Randzone bezeichnet.Erdbebenherde am Nordostrand des mittelamerikanischen Tiefseegrabens weisen auf eine Störungszone hin, die unter den Kontinent einfällt. Der Bewegungssinn spricht für ein Abtauchen der ozeanischen Platte unter das mittelamerikanische Festland.Eine eindrucksvolle Vulkankette und die mit ihr verbundenen Erdbebenherde sind längs eines bedeutenden Grabens aufgereiht, der in Nicaragua am deutlichsten ausgebildet ist und dort Nicaragua-Senke genannt wird. Diese tektonische Einheit entstand wahrscheinlich im Jung-Tertiär. Der Graben beginnt im Südosten an den nördlichen Ausläufern der Talamanca-Kordillere in Costa Rica.In Nicaragua verläuft er NW. Am Golf von Fonseca schwenkt er in eine E-W-Richtung und setzt sich in El Salvador in einem höheren Niveau als Zentral-Depression fort. In Guatemala ist nur der Südrand des Grabens erkennbar. Er ist gekennzeichnet durch eine in NW-Richtung aufgereihte Vulkankette. In Nicaragua und El Salvador liegen die ältesten Vulkane auf dem nordöstlichen Grenzbruch des Grabens. Zu dieser Gruppe zählen die größten Rumpfvulkane. Die meisten Kegel und alle tätigen Vulkane liegen hingegen auf dem südwestlichen Grenzbruchstreifen. Auf Querbrüchen haben sich jüngere, nördlich ausgerichtete Kegelreihen gebildet. Die jüngsten Kegel liegen meistens am südlichen Ende der Querbrüche.Es wird angenommen, daß der Hauptgraben und seine Störungen im Scheitel einer zum mittelamerikanischen Tiefseegraben parallel verlaufenden Aufwölbung entstanden sind. Die Aufwölbung kann auf Kompression der ozeanischen Platte gegen das Festland zurückgeführt werden.Im nordwestlichen Zentralamerika bewirkten jüngere Beanspruchungen die Bildung von N-S-Brüchen, die heute morphologisch als Steilkanten hervortreten. Diese Beanspruchungen können durch rechtslaterale Bewegungen entlang der pazifischen Störungszone ausgelöst worden sein. Beispiele dieser N-S-Struktur sind: Die Comayagua-Grabenzone, nördlich des Golfs von Fonseca, der Ipala-Graben und sein Störungssystem in Südost-Guatemala und der Guatemala-Stadt-Graben. Diese N-S-Störungen kennzeichnen das Ausmaß und das geringe Alter der Grabenbildung.Die Anordnung der Vulkane entlang der Nicaragua-Senke und der N-S-Brüche stellen die engen Beziehungen zwischen Tektonik und Vulkanismus in der pazifischen Randzone Mittelamerikas heraus.

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Resumen Mediante información tomada de la literatura, trabajos de campo e interpretation fotogeológica, se establece la relación entre el volcanismo y la tectónica en la región de América Central. Se usa la denominación de zona marginal del Pacífico para identificar, como una unidad, la asociación ya reconocida por otros autores de la profunda Fosa Mesoamericana frente a la costa del Pacífico de America Central y las franjas paralelas de actividad sísmica y de cadenas volcánicas.Los focos sísmicos en el borde nororiental de la Fosa Mesoamericana definen una gran zona de afallamiento inclinada hacia el continente, cuyos movimientos indican que un bloque de la corteza oceánica se sumerge bajo la porción continental de America Central.Una imponente serie de conos volcánicos, asociada a focos sísmicos poco profundos, está situada a lo largo de un gran graben conocido como Depresión de Nicaragua, por ser en ese país donde aparece más claramente definido. Este graben, que posiblemente se originó durante el Terciario tardío, termina en su extremo suroriental contra las estribaciones de la parte norte de la Cordillera de Talamanca, en Costa Rica. En Nicaragua el graben está orientado hacia el NW hasta el Golfo de Fonseca donde toma un rumbo E-W a través de El Salvador, donde se encuentra a mayor elevación. En Guatemala se conoce únicamente una línea de fallas, la más próxima al mar, definida por una cadena de volcanes que tiene nuevamente dirección NW.En Nicaragua y El Salvador los conos volcánicos más antiguos están situados a lo largo de las fallas que limitan al graben en su lado nororiental. Allí se encuentran las ruinas de los edificos volcánicos más grandes de la región. La mayoría de los conos que incluye todos los actualmente activos, se localizan a lo largo de las fallas del borde suroccidental del graben. Fracturas transversales controlan la localization de varios grupos de conos alineados de N-S. Los volcanes más jóvenes se encuentran frecuentemente en el extremo sur.Se postula que el graben principal y otras fallas del mismo sistema han sido formados como resultado de esfuerzos de tensión, en la cresta de un geanticlinal que resultó del arqueamiento del lado terrestre de la zona marginal del Pacífico. Dicho arqueamiento se cree que resulta de la compresión regional debida al movimiento del bloque oceánico contra el continente.En la parte noroccidental de América Central los esfuerzos más recientes produjeron fallas orientadas N-S, fácilmente identificables por sus bien marcadas escarpas. Estos esfuerzos pueden haber resultado del desplazamiento dextromóvil a lo largo de las zonas de afallamiento paralelas a la Fosa Mesoamericana.El complejo graben de Comayagua, al norte del Golfo de Fonseca, y los graben de Ipala y de la Ciudad de Guatemala, orientados también de norte a sur, ilustran la extension y la joven edad de las fracturas.La distribución de los focos volcánicos a lo largo de la Depresión de Nicaragua y de las fracturas N-S muestran de manera evidente, la estrecha relación que existe entre la tectónica y la localización del volcanismo en la zona marginal del Pacifico de America Central.

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