Spatial pattern of localized disturbance along a Southeastern salt marsh tidal creek

Geomorphology may be an important predictor of vegetation pattern in systems where suceptibility to disturbance is unevenly distributed across the landscape. Salt marsh communities exhibit spatial pattern in vegetation at a variety of spatial scales. In coastal Georgia, the low marsh is a virtual monoculture ofSpartina alterniflora interspersed with patches of species that are more typical of the high marsh. These localized disturbances are most likely created by wrack mats, mats of dead vegetation which can compact and smother underlying vegetation creating bare patches for colonization by high marsh species. We investigated the spatial pattern of disturbed patches along a 2 km section of Dean Creek, a tidal creek at the southwestern end of Sapelo Island, Georgia, U.S. We used a discriminant model to explore the relationship between tidal creek morphology (e.g., the presence of drainage channels and creek bends) and the spatial distribution of disturbed patches. The model predicted vegetation pattern along the creek with relatively high accuracy (>70%). Areas where water movement is slowed or multidirectional (e.g., along creek bends and near drainage channels) were most susceptible to disturbance. Our findings suggest an important functional linkage between geomorphology and vegetation pattern in salt marsh communities.     

Geology of the volcanic-hosted Brockman rare-metals deposit,Halls Creek Mobile Zone,northwest Australia. l. Volcanic environment,geochronology and petrography of the Brockman volcanics

Summary Rare-metals mineralization of the Brockman deposit (Halls Creek Mobile Zone, NW Australia) is hosted in a fluorite-bearing, rhyolitic volcaniclastic unit informally termed the Niobium Tuff. The Tuff, more correctly described as a tuffaceous volcaniclastic deposit, is the lowermost unit of a sequence of trachyte-to-rhyolite lavas, trachyandesite subvolcanic rocks, and volcaniclastic units of the Brockman volcanics located within the Halls Creek Group, a thick, early Proterozoic volcano-sedimentary sequence. High precision SHRIMP ion-microprobe zircon dating of the Niobium Tuff gives an eruption age of 1870 ± 4 Ma. Regional geochronological constraints indicate the Niobium Tuff was deposited about 15 Ma before major orogenic activity affected the area. Despite folding, faulting and low-grade metamorphism, the Brockman volcanics show excellent preservation of primary volcanological features, including pillow-lavas and pillow-breccias, that suggest a dominantly subaqueous, below-wave-base emplacement environment. The style of eruption products and magma volume constraints suggest the trachyte-dominated volcanics were erupted from a small shield volcanic complex probably in a rift-related basin in a shallow-marine setting. The tectonic setting was intraplate but differs from most Tertiary to recent trachyte volcanic complexes which are largely subaerial, are built on relatively thick continental crust, and show no post-eruptive orogenic history. Brockman-style rare-metal deposits are characterized by preservation of subaqueous volcanics beneath a thick sedimentary sequence, eruption of early incompatible-element enriched products followed by less differentiated magmas, and fine-grained mineralogy influenced by alteration processes. Prospects exist for discovery of analogous deposits, particularly in early Proterozoic mobile belts and Tertiary intraplate shield volcanic provinces.

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Geologie der an Vulkanite gebundenen Seltene-Metalle-Lagerstätte Brockman, Halls Creek Mobile Zone, Nordwest-Australien. I. Vulkanologie, Geochronologie und Petrographie der Brockman-Vulkanite

Zusammenfassung Selten-Metall-Vererzung der Brockman-Lagerstätten (Halls Creek Mobile Zone, NW Australien) sitzt in einer Fluorit-führenden, rhyolitischen vulkanoklastischen Einheit auf, die informell als der Niob-Tuff bezeichnet wird. Es handelt sich hier um ein vulkanoklastisches Tuff-Sediment; dieses ist die unterste Einheit einer Abfolge vor Trachyt bis Rhyolit-Laven, trachyandesitischen Subvulkaniten and vulkanoklastischen Einheiten der Brockman Vulkanite innerhalb der Halls Creek Gruppe, einer mächtigen frühproterozoischen vulkanosedimentdren Abfolge. Prüzisionsdatierungen des Niob Tuffs mit der SHRIMP Ionen-Mikrosonde ergeben ein Eruptionsalter von 1870 +-4 Millionen Jahren. Regionale geochronologische Zusammenhänge zeigen, daß der Niob Tuff etwa 15 Millionen Jahre vor einer größeren Orogenese, die das Gebiet betroffen hat, abgelagert wurde. Trotz Faltung, Bruchtektonik and niedriggradiger Metamorphose zeigen die Brockman-Vulkanite einen hervorragenden Erhaltungszustand primärer vulkanologischer Erscheinungen. Diese urnfassen auch Kissenlaven und KissenBreckzien, die eine vorwiegend subaquatische Ablagerung in ruhigem Wasser erkennen lassen. Die Art der Eruptionsprodukte and das Magmavolumen zeigen, daß die Trachytdominierten Vulkanite von einem kleinen Schildvulkan stammer, wahrscheinlich in einem Becken in einer Rift-Situation im seichten marinen Milieu. Die tektonische Situation war intraplate, aber unterscheidet sich von den meisten tertiären bis rezenten trachytischen Vulkan-Komplexen, die hauptsächlich subaerisch sind, auf einer relativ mächtigen kontinentalen Kruste aufsitzen, and keine post-eruptive orogene Entwicklung zeigen. Seltene-Metalle-Lagerstatten des Brockman-Typs rind durch die Erhaltung subaquatischer-Vulkanite unterhalb einer machtigen sedimentdren Abfolge gekennzeichnet; welters durch frühe Eruptionsprodukte, die an inkompatiblen Elementen angereichert sind, auf die dann weniger differenzierte Magmen folgten, und schließlich durch einen feinkörnigen Mineralbestand, der vor Umwandlungsprozessen betroffen war. Es besteht die Möglichkeit der Entdeckung analoger Lagerstätten, besonders in frühproterozoischen mobilen Gürteln und in tertidren intraplate Schildvulkan Provinzen.

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The role of principles and methods in loess-paleosol investigations

In the process of loessification, the development of loess fabric, the role of zonal, regional and partly of local environmental factors is regarded decisive. In the loess sequences various genetic types of loess pockets and paleosols occur and reflect different climatic and paleogeographical conditions.As a consequence, even within a single profile the colour, grain size and mineral composition, CaCO₃ content, degree of weathering and fabric of loess may vary. Zonal, regional or even local variations in such loess properties may equally derive from syngenetic or postgenetic processes. For similar reasons, spatially and temporally different forms, types and subtypes of paleosol also developed.This way, loess and soil formation resulted in various spatial types in the same glacial, interstadial or interglacial phase in the various geographical zones and regions. Although there exist major loess regions where the interglacial soils (eg brown forest soils) are markedly distinct from interstadial steppe soils or from other types of humic soils, continental loess areas can also be observed where the paleosols developed under interglacial and interstadial conditions (eg steppe or forest steppe soils) cannot be referred into different genetic types. In such cases the changes in paleoenvironments are difficult and uncertain to reconstruct.The interregional correlation of paleosols is occasionally hindered by the various terminologies applied to denote them. In addition, there are uncertainties concerning the dating and identification of paleoenvironments of polygenetic soils and of soil complexes consisting of paleosol units: how many soil formation phases they represent and how long erosional gaps can be reckoned with between these phases.     

Glacial drainage towards the Mediterranean during the Middle and Late Pleistocene

To a varying degree the Middle and Late Pleistocene ice sheets in northern Eurasia redirected the drainage of major catchments in Europe and western Siberia from the North Sea and Arctic Ocean south to the Caspian, Black Sea, and ultimately the Mediterranean. During the Late Weichselian, glacial meltwater reached the Mediterranean through the Dniepr and Don catchments and to a minor extent through the Danube. During the Warthe Substage of the Saalian, meltwater from the Volga was most likely added. During the Drenthe Substagc of the Saalian the watershed shifted Par to the east, and meltwater reached the Mediterranean also from the Oh. Irtysh, Yenisei, and Tunguska catchments in Siberia. Depending on the extent of the ice sheets, the increase in freshwater supply during deglaciations resulted in reductions of Mediterranean overflow into the North Atlantic. Such overflow reductions may have reduced vapour transport to the ice sheets and thus accelerated wastage.     

Environmental impact of salmon net-pen culture on marine benthic communities in Maine: A case study

The environmental impacts of salmon net-pen aquaculture on the benthic environment were investigated at a commercial fish farm located in coastal Maine waters. This site has a sandy mud bottom and low current velocities, is subjected to episodic sediment resuspension, and way in production for 3 yr prior to this study: We examined both the increase in carbon flux to the benthos caused by the net-pen and the effects of the elevated flux on sediment biogeochemistry and the microbenthic communities. The experimental design involved the establishment of two study sites, an ambient site ca. 100 m from the net-pen and a treatment site around the pen. Sediment traps deployed 1 m above the sediment-water interface indicated that carbon flux to the benthos was increased 1-fold to 6-fold (to a maximum of 5 g m^?2d^?1) at the edge of the net-pen with little or no increase in carbon flux 10 m from the pen. Unlike carbon flux rates, sediment organic matter inventories showed a complex pattern of change over time. Mineral surface area, organic carbon and nitrogen, digestible protein, and sterol content were initially (April 1991) lower beneath the pen than in ambient sediments. During 1991 ambient sediment accumulated organic matter until July after which it decreased, to a low during November. In contrast, organic matter inventories of sediment beneath the pen remained low until July and then increased to a high during November. These latter gains were associated with the development of bacterial mats at the sediment-water interface. Beneath the pen, microbial and macrofaunal communities were shifted toward those commonly associated with organic enrichment but seasonal trends and storm-related resuspension events also significantly affected these sediment communities. When abundant, most epibenthic organisms were more numerous near the pen than in adjacent ambient areas. These results suggest that net-pen aquaculture can alter the benthic ecosystem in Maine Coastal waters but indicate that the effects are spatially limited.     

Genetic structure of bay anchovy (Anchoa mitchilli) populations in Chesapeake Bay

To determine the genetic structure of the bay anchovy (Anchoa mitchilli) within Chesapeake Bay, 16 isozyme systems encoding 21 loci for 20 population were examined using horizontal starch gel electrophoresis. Contingency Chisquare analysis revealed significant allelic frequency differences at nine loci (AAT-1, AAT-2, ALD-1, CPK-2, GAP-1, GLY-1, LDH-1, MDH-1, and MDH-2). Two loci, ALD-1 and MDH-1, were responsible for nine of 14 tests not conforming to Hardy-Weinberg expectations, with some of these deviations attributed to possible scoring and/or sampling error. Estimates for mean average heterozygosity were relatively high, ranging from 0.40 to 0.096, with 33–57% of the loci polymorphic. A low F_st value (0.041) along with high genetic identity estimates (I=0.997) indicated little substructuring of bay anchovy populations within Chesapeake Bay.     

On the causes of 18O-depletion and 18O/16O homogenization during regional metamorphism; the East Humboldt Range core complex,Nevada

 Previous stable isotope studies at Lizzies Basin revealed that metasedimentary rocks are ¹⁸O-depleted relative to protolith values, particularly in the lower parts of the section (Lower Zone) where the rocks are also isotopically homogeneous on a scale of hundreds of meters (quartz δ¹⁸O=+9.0 to +9.6 per mil). In contrast, metasedimentary rocks at higher levels at Lizzies Basin (Upper Zone) are less ¹⁸O-depleted and more heterogeneous in δ¹⁸O. In order to understand more fully the isotopic evolution of this terrane, a series of detailed, meter-scale traverses across various metamorphic and igneous lithologies were completed at Lizzies Basin, and at the structurally higher Angel Lake locality. Traverses in the Lizzies Basin Lower Zone and in the lower parts of Angel Lake (Angel Lake Lower Sequence) across various silicate lithologies, including abundant granitoids, reveal similar degrees of homogeneity, although the average δ¹⁸O values are higher at Angel Lake. In contrast, traverses which include substantial thicknesses of marble and calc-silicate gneiss and very little granitoid have more heterogeneous quartz δ¹⁸O values (+11.9 to +13.4 per mil), and also have a higher average δ¹⁸O (+12.9 per mil), than observed elsewhere. The scale of ¹⁸O/¹⁶O homogeneity in quartz observed at Lizzies Basin and Angel Lake (meters to hundreds of meters) requires fluid-mediated isotope exchange, which accompanied Tertiary metamorphism. There is a correlation between the degree of ¹⁸O-depletion in metasedimentary rocks, ¹⁸O/¹⁶O homogenization between lithologies, and the proportion of granitoids (leucogranites in particular) within any part of the section, and a corresponding anticorrelation with the proportion of marble. This points to a causal relationship, whereby the leucogranites (as well as the Tertiary hornblende diorite and biotite monzogranite) acted as both a relatively low-¹⁸O reservoir and a source of fluids to enhance exchange, while the marbles hindered isotope depletion and homogenization by acting as relatively high-¹⁸O reservoirs and impermeable layers. Material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments. Single-pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations. Fluid-mediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations. The generalized model for the isotopic evolution of the East Humboldt Range core complex provides an excellent opportunity to establish the main causes and controlling factors of ¹⁸O-depletion and ¹⁸O/¹⁶O homogenization during regional metamorphism. Received: 27 July 1993 / Accepted: 1 July 1994     

Impacts Of Management Practices On Groundwater Conditions In The Eastern Province,Saudi Arabia

Insufficient attention has been given to management issues of groundwater in desertic aquifers, especially pumping policies and the proper design of well fields, i.e., the spatial distribution among wells and among well fields. In Saudi Arabia, many wells have been drilled in small areas, and in some parts of the Kingdom, groundwater pumping has increased drastically during the last two decades. Heavy pumping from large numbers of clustered wells in closely spaced well fields has resulted in unacceptable impacts on water levels and water quality in these areas. Numerical techniques were used to assess the impacts of this practice in parts of the Eastern Province. If the existing water-management policies and trend of increased pumping continue, a considerable additional drawdown will occur. Therefore, proper groundwater-management and conservation schemes need to be adopted. The approaches include optimization of well-field designs and well operations, and the implementation of effective irrigation-scheduling techniques. The implementation of a comprehensive irrigation water-management system in a large irrigation project has resulted in significant recovery of groundwater in the project area in the Eastern Province. RÉSUMÉ: Trop peu d'attention a été accordée aux questions de gestion des eaux dans les aquifères désertiques, particulièrement en ce qui concerne la règlementation des pompages et le groupement judicieux des puits, i.e., la répartition en surface des puits et des champs de puits. En Arabie Saoudite, de nombreux puits ont été forés sur des surfaces réduites, et, dans certaines parties du Royaume, le pompage des eaux s'est accru de façon spectaculaire dans les deux dernières déciennes. Un pompage important à partir d'un grand nombre de puits concentrés dans des ensembles rapprochés les uns des autres a eu des effets inacceptables sur le niveau et la qualité des eaux. Des techniques d'analyse numériques ont été employées pour évaluer les conséquences de ces modes de gestion dans certaines parties de la Province de L'Est. Si l'actuelle gestion des eaux se poursuit et le pompage continue de croître, il en résultera un abaissement considérable du niveau d'eau. Il est donc nécessaire d'adopter dans ces régions des méthodes de gestion et de conservation appropriées. Ces méthodes consistent, entre autres, à optimiser l'agencement des groupements de puits et de l'opération des puits, ainsi qu'à adopter des techniques efficaces pour programmer les horaires d'irrigation. L'adoption d'un sytème intégré de gestion des eaux dans un grand programme d'irrigation a provoqué une remontée substantielle des eaux souterraines dans la partie de la Province de L'Est correspondant à la zone du projet. RESUMEN: Los problemas de gestión de las aguas subterráneas en acuíferos desérticos no han recibido suficiente atención, especialmente en lo que respecta a las políticas de captaciones y al diseño adecuado de los campos de pozos, es decir, la distribución espacial de los pozos y de los campos de pozos. En Arabia Saudita, muchos pozos se han excavado en áreas pequeñas, y en algunas partes del Reino, el bombeo de agua se ha incrementado drásticamente durante las dos últimas décadas. El fuerte bombeo en un gran número de pozos de bombeo agrupados, situados en campos de extracción próximos, ha dado lugar impactos inaceptables en los niveles y en la calidad del agua en estas áreas. Se usaron técnicas numéricas para evaluar los impactos de esta política de extracción en partes de la Provincia Oriental. Si la política de gestión del agua actual y el incremento en el bombeo se mantienen, darán lugar a un descenso de niveles adicional considerable. Por tanto, en estas áreas es necesario adoptar esquemas apropiados de gestón y conservación de las aguas subterráneas. Esto incluiría la optimización de los diseños y de los criterios de operación de los campos de pozos y la adopción de técnicas de riego efectivas. La puesta en marcha de un sistema de gestión de aguas de riego, dentro de un gran proyecto de irrigación, ha dado como resultado una recuperación significativa de los niveles de agua en el área de proyecto, situada en la Provincia Oriental.     

Strontium and oxygen isotope profiles across marble-silicate contacts,Lizzies Basin,East Humboldt Range,Nevada: constraints on metamorphic permeability contrasts and fluid flow

 Sr isotope profiles across marble-silicate rock contacts are used in conjunction with previously published oxygen isotope profiles to constrain fluid movement, porosity and permeability contrasts in migmatitic rocks from Lizzies Basin in the East Humboldt Range, Nevada. The ¹⁸O/¹⁶O systematics in the high-grade sequence have been interpreted to reflect infiltration of ∼2×10² m³/m² of a relatively low ¹⁸O hydrous fluid through the sequence, but with preservation of δ¹⁸O anomalies in thin marble bands due to a 30-fold lower porosity in the marble compared with silicate rocks (Wickham and Peters 1992). The Sr isotope profiles confirm that tracer exchange between marble and silicate rock was primarily by diffusion, and in one case, indicate that porosities differed by less than a factor of four in the ∼10 cm boundary layer which exhibits diffusive modification of ⁸⁷Sr/⁸⁶Sr ratios. This contrasts with modelling of the oxygen isotope profiles which imply porosity contrasts >10 for one marble band and >50 for a second marble band. Either strontium and oxygen isotope diffusion reflect different events (possible if fluid Sr contents varied with time) or porosity varied substantially with the silicate rocks. Oxygen isotope profiles in the deeper part of the metamorphic section in which δ¹⁸O values of silicate rocks have been homogenised and lowered, indicate similar diffusion distances (and thus porosity-time evolution) to oxygen isotopic profiles higher in the section. Comparison of strontium and oxygen isotope diffusion distances constrains fluid Sr contents to between ∼50 and ∼500 ppm deep in the section, but less than ∼10 ppm higher in the section. The difference is related to release of relatively saline, Sr-rich fluids, by the abundant leucogranites and associated skarns deep in the section (cf. Peters and Wickham 1995). Received: 9 December 1994/Accepted: 13 April 1995     

Occurrence of hypogenic caves in a karst region: Examples from central Italy

The caves of the Umbria and Marche regions in central Italy are made up of three-dimensional maze systems that display different general morphologies due to the various geological and structural contexts. At the same time, the internal morphologies of the passages, galleries, and shafts present some similarity, with solutional galleries characterized by cupolas and blind pits, anastamotic passages, roof pendants, and phreatic passages situated at different levels. Some of these caves are still active, as is the case for Frassassi Gorge, Parrano Gorge, and Acquasanta Terme, with galleries that reach the phreatic zone, where there is a rising of highly mineralized water, rich in hydrosulfydric acid, and with erosion of limestone walls and the formation of gypsum. Elsewhere there are fossil caves, such as Monte Cucco and Pozzi della Piana, where large speleothems of gypsum are present 500 m or more above the regional water table. In all of these important karst systems it is possible to recognize basal input points through fracture and intergranular porosity networks at the base of the oxidizing zone in the core of the anticline, where mineralized water rises up from the Triassic evaporitic layers in small hydrogeological circuits. Different underground morphologies can derive from the presence of a water table related to an external stream or from the confined setting of the carbonate rocks, underlying low permeable sedimentary cover, where artesian conditions can occur.