Hydrous aluminosilicate metasomatism in an intra-oceanic subduction zone: Implications from the Kurancali (Turkey) ultramafic-mafic cumulates within the Alpine Neotethys Ocean

The Kurancali ultramafic-mafic cumulate body, an allochthonous ophiolitic sliver in central Anatolia, is characterized by the presence of abundant hydrous phases (phlogopite, pargasite) besides augitic diopside, plagioclase, and accessory amounts of rutile, sphene, apatite, zircon, and calcite. Based on modes of the essential minerals, the olivine-orthopyroxene-free cumulates are grouped as clinopyroxenite, hydrous clinopyroxenite, phlogopitite, hornblendite, layered gabbro, and diorite. Petrographical, mineralogical and geochemical features of the rocks infer crystallization from a hydrous magma having high-K calc-alkaline affinity with slightly alkaline character, and point to metasomatised mantle as the magma source. Our evidence implies that the metasomatising component, which modified the composition of the mantle wedge source rock in an intra-oceanic subduction zone, was a H₂O, alkali and carbonate-rich aluminosilicate fluid and/or melt, probably derived from a subducted slab. We suggest that the metasomatic agents in the subarc mantle led to the generation of a hydrous magma, which produced the Kurancali cumulates in an island-arc basement in a supra-subduction-zone setting during the closure of the Izmir-Ankara-Erzincan branch of the Alpine Neotethys Ocean.     

Fully coupled modeling of seabed subsidence and reservoir compaction of North Sea oil fields

This paper focuses on the aspects of fully coupled continuum modeling of multiphase poroelasticity applied to the three-dimensional numerical simulations of the Ekofisk oil reservoir in the North Sea (56°29′–34′N, 03°10′–14′E). A systematic presentation is chosen to present the methodology behind fully coupled, continuum modeling. First, a historical review of the subsidence phenomena above an oil and gas reservoir is given. This will serve as a background against which the relevance of the present approach to compaction and subsidence modeling will be demonstrated. Following this, the governing equations for a multiphase poroelasticity model are briefly presented. Particular attention is paid to the analysis of the pore-compressibility term usually used in an uncoupled approach for characterising the host-rock deformation. A comparative numerical analysis is carried out to contrast and highlight the difference between coupled and uncoupled reservoir simulators. Finally, a finite-element numerical model of the Ekofisk field is presented and a significant result is a contour map of seabed subsidence which is in general agreement with the shape of the subsidence contours based on past bathymetric surveys. Analysis of the simulation reveals that, due to the downward movement of the overburden, oil migration occurs from the crest of the anticline in which the field is situated, towards the flank. The pore-pressure depletion in the reservoir is significantly delayed due to the replenishment of the reservoir energy via the formational compaction. Horizontal movement in the reservoir, which is neglected in traditional modeling, can be significant and comparable in magnitude to the vertical subsidence. Electronic Publication     

Holocene environmental change at Lake Njulla (999 m a.s.l.), northern Sweden: a comparison with four small nearby lakes alongan altitudinal gradient

We assess Holocene environmental change at alpine Lake Njulla(68°22N, 18°42E, 999 m a.s.l.) innorthernmost Sweden using sedimentary remains of chironomid head capsules anddiatoms. We apply regional calibration sets to quantitatively reconstruct meanJuly air temperature (using chironomids and diatoms) and lake-water pH(using diatoms). Both chironomids and diatoms infer highest temperatures(1.7–2.3°C above present-day estimates, includinga correction for glacio-isostatic land up-lift by0.6°C) during the early Holocene (c.9,500–8,500 cal. yrs BP). Diatoms suggest a decreasing lake-waterpH trend (c. 0.6 pH units) since the early Holocene. Usingdetrended canonical correspondence analysis (DCCA), we compare the Holocenedevelopment of diatom communities in Lake Njulla with four other nearby lakes(Lake 850, Lake Tibetanus, Vuoskkujávri, Vuolep Njakajaure) locatedalong an altitudinal gradient. All five lakes show similar initial DCCA scoresafter deglaciation, suggesting that similar environmental processes such ashigh erosion rates and low light availability associated with high summertemperature appear to have regulated the diatom community, favouring highabundances of Fragilaria species. Subsequently, the diatomassemblages develop in a directional manner, but timing and scale ofdevelopment differ substantially between lakes. This is attributed primarily todifferences in the local geology, which is controlling the lake-waterpH. Imposed on the basic geological setting, site-specific processessuch as vegetation development, climate, hydrological setting andin-lake processes appear to control lake development in northernSweden.     

Numerical calculation of weakly singular surface integrals

We consider semi-analytical and purely numerical integration methods for weakly singular integrals with point singularities on curved smooth surfaces. The methods can be applied to many practical computations in Geodesy, e.g. terrain corrections, Stokes' and Hotines' integral, surface potentials, and the solution of geodetic boundary value problems using integral equations. Current numerical integration techniques are reviewed. The most important semi-analytical and purely numerical techniques are described. Test calcualtions are done and the techniques are compared as regards accuracy and computational efficiency. Semi-analytical methods, which are based on some regularizing parameter transformations, are superior to purely numerical techniques. The best choice are modified polar coordinates defined in the parameter domain with the singularity as pole. Triangular coordinates show similar performance if carefully tuned. Extrapolation techniques and adaptive subdivision techniques behave poorly as regards accuracy and numerical efficiency. Standard integration techniques, which ignore the singularity, completely fail.     

Influence of artificial groundwater lakes on the abundance and activity of bacteria in adjacent subsurface systems

Bacterial abundances and activity, estimated by 4′,6-diamidino-2-phenylindole staining (DAPI) and the reduction of 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT), were investigated in two oligotrophic artificial groundwater lakes and the surrounding aquifers. To evaluate the effect of lake water on groundwater downstream, samples were taken from wells at different distances from the lakes, and the total number of bacteria and the number of active bacteria in these samples were compared with samples collected upstream. In addition, sterilized sandy sediments were exposed in groundwater wells to measure the number and activity of bacteria attached to particles. At one of the study sites, where the lake sediments were disturbed by dredging, total bacterial abundance and the number of respiring bacteria in the groundwater aquifer was clearly influenced by the lake water. The average bacterial abundances decreased from 2.6 ± 1.9 × 10⁵ cells ml⁻¹ in the well closest to the lake (S2) to 2.9 ± 3.8 × 10⁴ cells ml⁻¹ in the most distant one (S4), which was equivalent to cell numbers in the upstream well. The number of respiring bacteria showed a similar tendency with 1.3 ± 2.7 × 10⁴ active cells ml⁻¹ in S2 and 1.9 ± 1.5 × 10³ active cells ml⁻¹ in S4. At the second study site, which was not influenced by dredging, bacteria in the downstream wells seemed not to be affected by the lake water. The number and activity of bacteria, which colonized exposed sediments, were not significantly different in the upstream and downstream wells, indicating a minor influence of lake water on this habitat. Our results suggest that gravel-pit lakes may influence the free living bacterial assemblages in nearshore groundwater systems, but do not visibly affect numbers and activity of bacteria attached to the surface of aquifer sediments.     

Towards an integration of process measurements, archive analysis and modelling in geomorphology — the Kärkevagge experimental site, Abisko area, northern Sweden

The analysis of Holocene geomorphic process activity demands long–term data sets, which are available for the Kärkevagge catchment due to 50 years of intensive geomorphologic field studies. This data set is used in combination with additional field measurements, remote sensing and digital elevation model (DEM) analysis to provide input data for modelling Holocene valley development. On the basis of this information, geomorphic process units (GPUs) are defined by means of GIS modelling. These units represent areas of homogeneous process composition that transfer sediments. Since the data base enables the quantification of single processes, the interaction of processes within the units can also be quantified. Applying this concept permits calculation of recent sediment transfer rates and hence leads to a better understanding of actual geomorphic landscape development activity. To extrapolate these data in time and space the process–related sediments in the valley are analysed for depth and total volume, primarily using geophysical methods. In this fashion the validity of measured process rates is evaluated for the Holocene time scale. Results from this analysis are exemplified in a cross–profile showing some of the principal sediment units in the valley. For example, the measured modern rates on a slush torrent debris fan seem to represent the Holocene mean rate. This approach should also be suitable for revealing Holocene geomorphic landscape development in terms of climate change.     

Effects of collision-induced breakup on drop size distributions in steady state rainshafts

New equations and techniques for dealing with drop breakups are developed and applied to the modelling of the evolution of raindrop spectra in rainshafts. Breakup experiments byMcTaggart-Cowan andList (1975) served as data base.No matter what the original size distribution, the spectrum evolution will always lead to a Marshall-Palmer type equilibrium di tributionN=N ₀e^–D, with =constant andN ₀ proportional to the rainfall rateR. (D stands for raindrop diameter.) ForR29 mm h^–1 and an original Marshall-Palmer distribution, the required fall height to reach equilibrium is 2 km.The equilibrium distributions are characterized by linear relationships betweenR, the radar reflectivity factorZ, the liquid water content LWC and theN ₀ of the Marshall-Palmer distribution. Possible explanations for the discrepancy with observations are given.The fact that the all-water processes cannot produce drops withD2.5 mm (as confirmed by observations) leads to the conclusion that observed large raindrops withD5 mm represent melted hailstones and have not yet reached an equilibrium distribution. These latter conclusions were reached within the original assumption of videspread, steady state precipitation.     

Upper Precambrian and lowermost Plaeozoic correlatinos in West Africa and in the Western Part of Central Africa. Probable diachronism of the Late Precambrian tillite

The Late Precambrian or Eocambrian and the lowermost part of Palaeozoic rocks of West Africa and of the Western half of Central Africa include two lithostratigraphic sequences of sediments which we can identify as well on the craton as in the orogenic belts. The lower sequence, argillaceous, green colour, sometimes flyschoid, begins with a tillite or a mixtite. The upper sequence made of reddish, mainly continental sandstones, represents in some areas the molasse of the Pan-African Fold Belt.These two sequences, and notably at the base of the first one the tillite horizon, seem to be older in the South (Katangan and West-Congolian Chains) than in the North (Adrar of Mauritania). This diachronism is thought to be related to polar wandering, the South pole having shifted in that interval of time from South-Africa to West of the Senegal coast.

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Zusammenfassung Das obere Präkambrium oder Eokambrium und die Basis des Paläozoikums von Westafrika und im westlichen Teil von Zentralafrika sind aus zwei lithostratigraphischen Abfolgen gebildet, die gleichfalls auf den Kratonen und in den Gebirgsketten zu erkennen sind. Die untere feinklastische Abfolge, grün, manchmal flyschartig, beginnt mit einem Tillit oder einem Mixtit. Die obere rote, sandige Abfolge, kontinentalen Ursprungs, wird örtlich als eine von der pan-afrikanischen Orogenese gebildete Molasse angesehen.Diese beiden Abfolgen, besonders die Tillite, scheinen diachron und im Süden älter (Katangische und West-Kongolische Gebirgsketten) als im Norden (Adrar von Mauritanien) zu sein. Diese Verjüngung nach Norden kann mit einer Polwanderung in Verbindung gebracht werden; der Südpol zieht während dieser Zeit von Südafrika in das Gebiet westlich der senegalesischen Küste.

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Résumé Le Précambrian terminal ou Eocambrien et la base du Paléozoïque de l'Afrique de l'Ouest et de la partie occidentale de l'Afrique centrale comprennent deux ensembles lithostratigraphiques identifiables aussi bien dans les couvertures cratoniques que dans les chaînes. L'ensemble inférieur, argileux, vert, parfois flyschoïde, débute par une tillite ou une mixtite. L'ensemble supérieur, gréseux, rouge, à dominante continentale, est interprété dans certaines zones comme la molasse de la chaîne pan-africaine. ces deux ensembles, et en particulier la tillite á la base du premier, semblent plus anciens au Sud (chaînes katangienne et ouest-congolience) qu'au Nord (Adrar de Mauritanie). Cet âge plus jeune vers le Nord va de Pair avec le mouvement du pôle sud, qui, â époque, se déplace de l'Afrique du Sud au large des côtes du Sénegal.

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Limatic variables that control development of stripe rust disease on winter wheat

The frequency and severity of the stripe rust disease (caused byPuccinia striiformis) on winter wheat in the Pacific Northwest of the U.S.A. has increased since 1958 in association with climatic variation. From 1968–1979, rust intensities on ‘Gaines’ wheat were most highly correlated with accumulated negative degree days (NDD) between 1 December and 31 January and positive degree days (PDD) between 1 April and 30 June. NDD and PDD were calculated from a 7 °C base. Linear regression equations using NDD alone accounted for 76% of the variation in stripe rust. When NDD and PDD were combined, 88% of the variation in stripe rust was explained. When a growth index (GI), and NDD plus PDD were used as independent variables in a multiple regression analysis, 91% of the variation in disease was explained. Frequency of precipitation in June was correlated with stripe rust intensity, but when it was added to the multiple regression analysis, it explained less than an additional 1% of the variation. The relationships between NDD, PDD, and disease index help to explain why stripe rust was not severe from 1941 to 1957. Methods used in this research should be applicable to similar studies of the effect of climatic variation on other pests. This research was supported by a National Science Foundation Grant (ATM 76-21725), Climate Dynamics Program, Division of Atmospheric Sciences.     

Distribution of tetrahedral Al and Si in coexisting biotite and Ca-amphibole

The distribution of tetrahedrally coordinated ions between coexisting biotite and Ca-amphibole is found to follow a regular trend of approximately Al _Bi ^IV =1.95+0.35 Al _A ^IV . It is concluded that deviations permit the recognition of disequilibrium mineral associations. The potential influence of crystal-chemical factors is considered.