Intertidal and shoal benthic community metabolism in a temperate estuary: Studies of spatial and temporal scales of variability

Variability in benthic community metabolism, sediment chlorophyll and sediment organic carbon attributable to substrate type and season were examined. Substrate types included intertidalSpartina alterniflora marsh, mudflat, and sandflat and subtidal eelgrass (Zostera marina) and bare sand bottoms. Significant differences in parameter estimates were found among the different substrate types for each specific sampling data. When estimates were pooled over the entire study period, significant differences were found only for respiration rate and sediment pigment concentrations. The data indicate no obvious ranking in magnitude for any parameter that can be attributed to substrate type or season. Small-scale spatial variability and rapid temporal changes (turnover) are postulated as the principal reasons governing the overall results.     

Comparison of Methods for Sampling Dissolved Nitrogen in a Fractured Carbonate-Rock Aquifer

As part of an agricultural non-point-source study in the Conestoga River head waters area in Pennsylvania, different methods for collecting ground water samples from a fractured carbonate-rock aquifer were compared. Samples were collected from seven wells that had been cased to bedrock and drilled as open holes to the first significant water-bearing zone. All samples were analyzed for specific conductance, dissolved oxygen, and dissolved-nitrogen species. Water samples collected by a point sampler without pumping the well were compared to samples collected by a submersible pump and by a point sampler after pumping the well. Samples collected by using a point sampler, adjacent to major water-bearing zones in an open borehole without pumping the well, were not statistically different from samples collected from the pump discharge or from point samples collected adjacent to major water-bearing zones after pumping the well. Samples collected by using a point sampler without pumping the well at depths other than those adjacent to the water-bearing zones did not give the same results as the other methods, especially when the water samples were collected from within the well casings. It was concluded that, for the wells at this site, sampling adjacent to major water-bearing zones by using a point sampler without pumping the well provides samples that are as representative of aquifer conditions as samples collected from the pump discharge after reaching constant temperature and specific conductance, and by using a point sampler after pumping the well.     

Cumulate nodules as evidence for convective fractionation in a phonolite magma chamber

The physical mechanism by which chemical zonation develops in magma chambers has been controversial partly because unambiguous geological constraints have been lacking. The 11,000 years B.P. eruption of Laacher See Volcano produced a zoned tephra deposit and also ejected crystal-rich nodules which provide a snapshot of the materials crystallising at the magma chamber margins. New data on petrography and chemical compositions of nodules, their cumulate minerals and interstitial glasses are used to deduce the chemical evolution of the phonolite melt due to fractional crystallisation of the mineral assemblages. These data, together with those on the vertical zonation of the melt in the bulk of the chamber, are shown to be consistent with a model of stratification of the chamber by convective fractionation, in which a thin boundary layer of residual melt from fractional crystallisation ascends at the chamber side and accumulates at the roof. Crystallisation could have provided buoyancy to drive convection by enriching incompatible volatile components (mainly water) in the residual melt. Available fluid dynamic studies of single- and double-diffusive boundary layers are used to assess convection in the Laacher See chamber. The boundary layer is likely to have been: (1) laminar, which implies that the density gradient in the chamber steepened upwards; (2) in the counterflow regime, in which compositional and thermal layers flow in opposite directions; and (3) thin ( 10 cm), estimated from theory for a flat wall, suggesting that wall morphology could be important in determining boundary layer characteristics. Estimates of mass transfer rates due to this mechanism suggest that the chamber could have become stratified in a time of the order of 10³ years.     

Seltene Metallverbindungen in Sedimenten

Zusammenfassung An 2 Beispielen (Zinnober und Baryt) wird erörtert, wie es in Sedimenten zur mehr oder weniger frühzeitigen Ausscheidung von relativ seltenen Metallverbindungen kommt unter bemerkenswerter Konzentrationswanderung. In vielen, aber nicht in allen Fällen erscheint als Vorbedingung der Mineralbildung die Gegenwart von Hohlräumen im Sediment. Die Gegenwart organischer Materie scheint wenigstens in einem Teil der Fälle die Konzentrationswanderung zu begünstigen.     

Ein Versteinerter (Verkieselter) Wald im Chilenischen Tertiär

Zusammenfassung Ein neugefundenes Vorkommen eines verkieselten Waldes im Miozän des mittleren Chile gibt Anlaß zur Diskussion der Bildungsweise soldier Dokumente festländischer Vegetation der Vorzeit. Von dem Vorkommen bei Temuco wurden 23 Baumreste (meist Stubben) sowie der verkieselte Waldboden gesammelt und mikroskopiert. Boden und Stubben sind gekennzeichnet durch ungewöhnlich starke bakterielle Fäulnis, wobei nur die podocarpoiden Koniferen — überrepräsentiert — und Sporen als brauchbare paläobotanische Objekte übrig blieben. Es muß unter feucht-tropischem Klima ein Mischwald innerhalb eines Talsystems bestanden haben, der durch Grundwasser-Anstieg zum Absterben kam. Mit dem Grundwasser muß gelöste SiO₂ zugeführt worden sein.Weitgehende Analogie besteht zu einem verkieselten Waldboden des baltischen Miozäns, aber auch der Unterschied kälteren und trockeneren Klimas im letzteren Falle. Das gemeinsame geochemische Hauptproblem wird in den klimatischen Bedingungen gesehen, die die kieselreichen Verwitterungslösungen liefern konnten.

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The newly found appearance of a silicated forest in the miocene of the midland of Chile gives rise to a discussion of the way in which such documents of ancient continental vegetation were formed. The remains of 23 trees (mainly stumps) as well as the silicated forest soil found near Temuco were examined under the microscop. Soil and stumps are characterized by unusually strong bacterial decomposition so that only the podocarpoid conifers (excessively represented) and spores have been preserved as usable palaeobotanic objects. In a damp tropical climate a mixed forest must have existed in a valley system and this forest died down when the groundwater rose. In the groundwater dissolved SiO₂ must have been brought together with the forest soil.This is a close analogy with a silicated forest soil in the Baltic miocene, but there is also the difference of a colder and drier climate in the latter case.The climatic circonstances that provided the solucion of waetherd material particularly rich in SiO₂ is regarded as the main geochemical problem commun to both.

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Résumé Un nouveau gisement de forêt silicifiée dans le Miocène du Chili Central conduit l'auteur à discuter le mode de formation de ces témoins de la végétation continentale des temps anciens. 23 restes d'arbres (pour la plupart des souches), ainsi que des sols forestiers silicifiés provenant du gîte près de Temuco, ont été récoltés et ont fait l'objet d'un examen microscopique. Sols et souches sont caractérisés par un pourrissage bactériel particulièrement avancé, à la suite duquel les conifères podocarpoides — largement représentés — et des spores sont les seuls restes paléontologiques utilisables. Il a dû exister en climat tropical humide et dans un système de vallées une forêt mixte qui a fini par s'éteindre sous l'influence de la montée des eaux phréatiques. C'est avec ces dernières que de la silice en solution a dû être amenée.Une analogie très grande existe avec un sol forestier silicifié du Miocène de la Baltique, avec toutefois comme différence, dans ce dernier cas, l'existence d'un climat plus froid et plus sec. Le problème principal géochimique commun réside dans les conditions climatiques qui ont permis l'altération par voie de solutions riches en silice.

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Evolution of parental magmas of Miocene shield basalts of Gran Canaria (Canary Islands): constraints from crystal,melt and fluid inclusions in minerals

 Picritic units of the Miocene shield volcanics on Gran Canaria, Canary Islands, contain olivine and clinopyroxene phenocrysts with abundant primary melt, crystal and fluid inclusions. Composition and crystallization conditions of primary magmas in equilibrium with olivine Fo_90-92 were inferred from high-temperature microthermometric quench experiments, low-temperature microthermometry of fluid inclusions and simulation of the reverse path of olivine fractional crystallization based on major element composition of melt inclusions. Primary magmas parental for the Miocene shield basalts range from transitional to alkaline picrites (14.7–19.3 wt% MgO, 43.2–45.7 wt% SiO₂). Crystallization of these primary magmas is believed to have occurred over the temperature range 1490–1150° C at pressures ≈5 kbar producing olivine of Fo_80.6-90.2, high-Ti chrome spinel [Mg/ (Mg+Fe²⁺)=0.32–0.56, Cr/(Cr+Al)=0.50–0.78, 2.52–8.58 wt% TiO₂], and clinopyroxene [Mg/(Mg+Fe)=0.79–0.88, Wo_44.1-45.3, En_43.9-48.0, Fs_6.8-11.0] which appeared on the liquidus together with olivine≈Fo₈₆. Redox conditions evolved from intermediate between the QFM and WM buffers to late-stage conditions of NNO+1 to NNO+2. The primary magmas crystallized in the presence of an essentially pure CO₂ fluid. The primary magmas originated at pressures >30 kbar and temperatures of 1500–1600° C, assuming equilibrium with mantle peridotite. This implies melting of the mantle source at a depth of ≈100 km within the garnet stability field followed by migration of melts into magma reservoirs located at the boundary between the upper mantle and lower crust. The temperatures and pressures of primary magma generation suggest that the Canarian plume originated in the lower mantle at depth ≈900 km that supports the plume concept of origin of the Canary Islands. Received: 23 October 1995/Accepted: 21 February 1996     

Multi-stage magma ascent beneath the Canary Islands: evidence from fluid inclusions

Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from Gran Canaria, La Palma, El Hierro, Lanzarote and La Gomera (Canary Islands) contain abundant CO₂-dominated fluid inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to one or more of the following pressures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) between 0.2 and 0.4 GPa (the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal density distribution, the lower-density maximum comprising both texturally early and late inclusions. This is taken as evidence for an incomplete resetting of inclusion densities, and simultaneous formation of young inclusions, at well-defined magma stagnation levels. For Gran Canaria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts in the host basanites overlap at 0.9 to 1.0 GPa, indicating that such magma reservoir depths coincide with levels of xenolith entrainment into the magmas. Magma chamber pressures within the mantle, inferred to represent levels of mantle xenolith entrainment, are 0.65–0.95 GPa for El Hierro, 0.60–0.68 GPa for La Palma, and 0.55–0.75 GPa for Lanzarote. The highest-density fluid inclusions in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment. Inclusion data from all islands indicate ponding of basaltic magmas at Moho or lower crustal depths, and possibly at an additional higher level, strongly suggestive of two main crustal accumulation levels beneath each island. We emphasize that repeated magmatic underplating of primitive magmas, and therefore intrusive accretion, are important growth mechanisms for the Canary Islands, and by analogy, for other ocean islands. Comparable fluid inclusion data from primitive rocks in other tectonic settings, including Iceland, Etna and continental rift systems (Hungary, South Norway), indicate that magma accumulation close to Moho depths shortly before eruption is not, however, restricted to oceanic intraplate volcanoes. Lower crustal ponding and crystallization prior to eruption may be the rule rather than the exception, independent of the tectonic setting. Received: 30 May 1997 / Accepted: 6 February 1998     

Petrology, geochemistry, S, Cl and F abundances, and S oxidation state of sideromelane glass shards from Pleistocene ash layers north and south of Gran Canaria (ODP Leg 157)

Major elements, S, F, Cl concentrations and relative proportions of S⁶⁺ to total S were analyzed with electron microprobe in sideromelane glass shards from Pleistocene volcaniclastic sediments drilled during ODP Leg 157. Glasses are moderately to strongly evolved and represent a spectrum from alkali basalt, basanite and nephelinite through hawaiite, mugearite and tephrite to phonolitic tephrite. Measured S⁶⁺/ΣS (0.03–0.98) and calculated Fe²⁺/Fe³⁺ (2.5–5.8) ratios in the melt yield preeruptive redox conditions ranging from NNO−1.4 to NNO+2.1. The morphology of the glass shards, variations of S and Cl concentrations (0.010–0.127 wt% S, 0.018–0.129 wt% Cl), calculated preeruptive temperatures (1030–1200 °C) and oxygen fugacities suggest that glasses deposited even within the same ash layers have diverse origin and may have resulted from both submarine and subaerial eruptions. Most vesicle-free glasses are characterized by high concentrations of S and represent undegassed or slightly degassed submarine lavas, whereas vesiculated glasses with low concentrations of S and Cl are strongly degassed and can be ascribed to the eruptions in shallow water or on land. Sideromelane glass shards at Sites 953 are thought to have resulted from submarine eruptions northeast of Gran Canaria, glasses at Site 954 represent mostly volcaniclastic material of shallow water submarine and subaerial eruptions on Gran Canaria and Tenerife, and glasses deposited at Site 956 resulted from submarine or explosive eruptions on Tenerife. Received: 8 April 1997 / Accepted: 27 October 1997     

The lateral facies of ignimbrites at Laacher See volcano

Valley-fill ignimbrites of the Middle Laacher See Tephra (MLST) in the proximal depositional fan south of Laacher See volcano are laterally continuous with fine-grained overbank-facies deposits, 0.5–1.5 m thick, covering higher elevations and interfluves between the paleovalleys. The overbank deposits consist of up to 12 ash layers, each 4–10 cm thick, which show internal structures typical of ash-flow transport, such as poor sorting, reverse size-grading of pumice, local normal grading of coarse ash-sized lithics above a fine-grained basal layer, cross-stratification behind obstacles, and erosional unconformities. Thickness, median grain-size, and number of individual layers decrease systematically with distance from the vent. Overbank ash layers thicken at the valley slopes and form discrete valley-fill ignimbrite flow units in the paleochannels with median grain size increasing from 63 m to 350 m. Toward the center of paleochannels, however, the well-defined overbank facies is obscured by mutual erosion of individual flow units. Overlapping data fields in ternary grain-size variation diagrams indicate the overbank facies to have evolved from ash flows chiefly through depletion of lapilli and coarse ash. Overbank-facies ash layers do not represent dust layers resulting from elutriation clouds of ash flows (co-ignimbrite ash) or surge deposits developed on higher ground due to low concentration of solids. They are similar in some parameters to Taupo-type ignimbrite veneer deposits, but are interpreted differently. The thin, fine-grained, Laacher See ash layers are thought to have been deposited from diluted portions of the flow proceeding directly from the eruption column while the main pyroclastic flows were confined to the paleovalleys radiating away from the eruptive center. The wide distribution of the thin ash layers is attributed to the balance of deposition from various flow parts and turbulent entraining and heating of ambient air that sustained sufficient mobility of the diluted flows to spread across hills and level ground.     

15th International Symposium for the Advancement of Boundary Layer Remote Sensing (ISARS)

Announcement

15th International Symposium for the Advancement of Boundary Layer Remote Sensing (ISARS) 28-30 June 2010