Age and nature of carbonatite emplacement in North Pakistan

The N Pakistan carbonatites of Loe Shilman, Silai Patti and those within the Ambela complex were formerly considered as comprising one alkaline igneous province associated with a Peshawar rift valley. New data show that these alkaline rocks occur in two distinct periods, Carboniferous and Tertiary, and are not related to any Tertiary rift faulting. K-Ar dates determined on biotites from the Loe Shilman and Silai Patti carbonatites reveal that the carbonatites were emplaced at 31±2 Ma, and along thrust planes, not rift faults. Subsequent movement of the thrusts reset the argon contents in some biotites to indicate a deformation age of 24±2 Ma for the carbonatites at both localities.The Koga carbonatite in the Ambela complex occurs as a plug associated with nepheline syenites and ijolites, and Rb-Sr isotope determinations on the silicate rocks give dates of approximately 297–315 Ma.The study implies that there were thrust movements associated with the Indian — Asian plate collision more than 31 Ma ago, which is much earlier than the 20 Ma date previously advocated for the initiation of thrusting.

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Zusammenfassung Die nordpakistanischen Karbonatite Loe Shilman, Silai Patti und jene aus dem Ambela-Komplex wurden bisher zusammen mit einer Alkaligesteins-Provinz betrachtet, deren Bildung mit dem Peshawar Riff zusammenhÄngt. Neue Daten zeigen auf, da\ die Alkaligesteine von Shilman und Silai Patti dem Karbon zuzuordnen sind und jene aus dem Ambela-Komplex dem TertiÄr. Beide Gesteinserien gehören keiner tertiÄren Riff-Faltung an. K-Ar Altersdaten, bestimmt an Biotiten aus den Karbonatiten von Loe Shilman und Silai Patti, ergeben ein Alter von 31±2 Ma für die Platznahme der Karbonatite entlang einer überschiebungszone. Nachfolgende Bewegungen in der überschiebungsbahn verÄnderten den Argongehalt in einigen Biotiten, so da\ sich ein Deformationsalter von 24±2 Ma ergibt. Die Koga Karbonatite aus dem Ambela Komplex kommen dagegen als Propfen vor. Sie sind assoziiert mit Nephelinsyeniten und Ijoliten. Rb-Sr Bestimmungen an diesen Silikatgesteinen ergeben ein Alter zwischen 297–315 Ma.Diese Untersuchung zeigt, da\ die Indisch-Asiatische Plattenkollision vor 31±2 Ma Jahren stattfand, da sie mit der überschiebungsbewegung assoziiert werden kann. Bisher wurde für diese Kollision ein Alter von 20 Ma angenommen.

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Résumé Les carbonatites du Pakistan septentrional, situées à Loe Shilman, Silai Patti et dans le complexe d'Ambela, ont été regardées jusqu'ici comme appartenant à une seule province magmatique alcaline associée au fossé de Peshawar. Des données nouvelles démontrent que ces roches alcalines appartiennent à deux périodes différentes, le Carbonifère et le Tertiaire, et qu'elles ne sont associées à aucun fossé tertiaire. Des datations K-Ar, effectuées sur les biotites des carbonatites de Loe Shilman et de Silai Patti, montrent que ces roches se sont mises en place il y a 31±2 Ma et ce, le long de failles de charriage et non de failles radiales. Des mouvements plus récents le long de ces charriages ont modifié la teneur en argon de certaines biotites, qui fournissent pour cette déformation un âge des 24±2 Ma dans les carbonatites des deux localités.La carbonatite de Koga dans le complexe d'Ambela est un culot associé à des syénites à néphéline et à des ijolites sur lesquelles les déterminations Rb-Sr donnent des aes approximatifs de 297 à 315 Ma.Cette étude montre que des mouvements de charriage, associés à la collision des plaques indienne et asiatique, ont eu lieu avant 31±2 Ma, c'est-à-dire beaucoup plus tÔt que les 20 Ma admis jusqu'ici.

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Loe Shilman, Silai Patti Ambela , , Peshawar. , Shilman Silai Patti , Ambela — . . Loe Shilman Silai Patti, K/, 31±2 . , 24 ± 2 . Koga Ambela «». - ; , Rb/Sr, 297–315 . , 31 ± 2 , . . , . , 20 .

     

Atmospheric turbidity at athens observatory

Summary In this study, values of the Ångström turbidity coefficient () determined from Solar radiation observations at the National Observatory of Athens over the period 1955–1972 are analysed. Mean daily turbidity lies between 0.020–0.100. Turbidity is higher in summer than in winter. The main factor determining the turbidity is the air mass type. The scavenging by rainfall probably has a considerable effect in determining this distribution. There is some evidence of a trend of increasing turbidity during the period.     

A combined central- and fissure-type phonolitic volcano in western Kenya

The Nyamaji volcano is a small eruptive complex of late Miocene age associated with the nearby Usaki ijolite and Sokolo carbonatite intrusion of Homa Bay in the Kavirondo Rift valley of Kenya. It is probably a satellite volcano to the major volcanic structure of Kisingiri - Rangwa which lies 25 km to the west. The Nyamaji volcanic complex is composed of agglomerates, breccias and tuffs erupted from a central vent, whilst at much the same time lavas were extruded from fissures which are now occupied by dykes. These two contemporaneous events gave rise to an interdigitated sequence of pyroclastic deposits and effusive lavas. The pyroclastic rocks of Vulcanian origin cover an area at least 30 km² in extent, are poorly bedded, and usually are about 25 m (80ft.) thick though they often thin to zero over topographic highs in the pre-existing landscape. At Nyamaji itself, the Strombolian style pyroclastic pile exceeds 330 m (1100 ft.) in thickness over an area of 1 km², and this marks the position of the original central vent. The fragmental material in the pyroclastic rocks includes ijolite, phonolite, nephelinite, trachyte, carbonatite, granite, and feldspathic and aegirine-bearing fenites; the matrix is sometimes calcareous, sometimes feldspathic. Nephelinitic lavas occur amongst the lowest lavas, but the lavas above are nearly all phonolitic. The oldest dykes are nephelinitic and are rare; the youngest dykes are phonolitic and are abundantly exposed. Both lavas and dykes contain xenoliths similar to those in the pyroclastic rocks. A series of volcanic plugs pierce the lavas. These plugs, mostly non-conduit type, average 200–500 m diameter, are mainly composed of glassy to very fine-grained phonolites, and show good flow structures. The plugs, especially those near the Ruri hills, tend to lie along N - S and E - W lines. The majority of the dykes also lie along these directions. The dominant structural directions within the nearby Usaki ijolite complex and the Wasaki carbonatite are also N - S and E - W, respectively. These directions are quite different from the axis of the Kavirondo rift valley which here is NE - SW, and from the strike of the Precambrian basement. The Nyamaji volcanic structure differs from nearly all the other East African volcanoes by its dominant phonolitic petrochemistry.     

Estimating Travel Time in Bank Filtration Systems from a Numerical Model Based on DTS Measurements

An approach is presented to determine the seasonal variations in travel time in a bank filtration system using a passive heat tracer test. The temperature in the aquifer varies seasonally because of temperature variations of the infiltrating surface water and at the soil surface. Temperature was measured with distributed temperature sensing along fiber optic cables that were inserted vertically into the aquifer with direct push equipment. The approach was applied to a bank filtration system consisting of a sequence of alternating, elongated recharge basins and rows of recovery wells. A SEAWAT model was developed to simulate coupled flow and heat transport. The model of a two‐dimensional vertical cross section is able to simulate the temperature of the water at the well and the measured vertical temperature profiles reasonably well. MODPATH was used to compute flowpaths and the travel time distribution. At the study site, temporal variation of the pumping discharge was the dominant factor influencing the travel time distribution. For an equivalent system with a constant pumping rate, variations in the travel time distribution are caused by variations in the temperature‐dependent viscosity. As a result, travel times increase in the winter, when a larger fraction of the water travels through the warmer, lower part of the aquifer, and decrease in the summer, when the upper part of the aquifer is warmer.     

Biogeomorphology in the marine landscape: Modelling the feedbacks between patches of the polychaete worm Lanice conchilega and tidal sand waves

Tidal sand waves are dynamic bedforms found in coastal shelf seas. Moreover, these areas are inhabited by numerous benthic species, of which the spatial distribution is linked to the morphological structure of sand waves. In particular, the tube-building worm Lanice conchilegais of interest as this organism forms small mounds on the seabed, which provide shelter to other organisms. We investigate how the interactions between small-scale mounds (height ∼dm) and large-scale sand waves (height ∼m) shape the bed of the marine environment. To this end, we present a two-way coupled process-based model of sand waves and tube-building worm patches in Delft3D. The population density evolves according to a general law of logistic growth, with the bed shear stress controlling the carrying capacity. Worm patches are randomly seeded and the tubes are mimicked by small cylinders that influence flow and turbulence, thereby altering sediment dynamics. Model results relate the patches with the highest worm densities to the sand wave troughs, which qualitatively agrees with field observations. Furthermore, the L. conchilegatubes trigger the formation of sandy mounds on the seabed. Because of the population density distribution, the mounds in the troughs can be several centimetres higher than on the crests. Regarding sand wave morphology, the combination of patches and mounds are found to shorten the time-to-equilibrium. Also, if the initial bed comprised small sinusoidal sand waves, the equilibrium wave height decreased with a few decimetres compared to the situation without worm patches. As the timescale of mound formation (years) is shorter than that of sand wave evolution (decades), the mounds induce (and accelerate) sand wave growth on a similar spatial scale to the mounds. Initially, this leads to shorter sand waves than they would be in an abiotic environment. However, near equilibrium the wavelengths tend towards their abiotic counterparts again. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

Control of the repeatability of high frequency multibeam echosounder backscatter by using natural reference areas

The increased use of backscatter measurements in time series for environmental monitoring necessitates the comparability of individual results. With the current lack of pre-calibrated multibeam echosounder systems for absolute backscatter measurement, a pragmatic solution is the use of natural reference areas for ensuring regular assessment of the backscatter measurement repeatability. This method mainly relies on the assumption of a sufficiently stable reference area regarding its backscatter signature. The aptitude of a natural area to provide a stable and uniform backscatter response must be carefully considered and demonstrated by a sufficiently long time-series of measurements. Furthermore, this approach requires a strict control of the acquisition and processing parameters. If all these conditions are met, stability check and relative calibration of a system are possible by comparison with the averaged backscatter values for the area. Based on a common multibeam echosounder and sampling campaign completed by available bathymetric and backscatter time series, the suitability as a backscatter reference area of three different candidates was evaluated. Two among them, Carré Renard and Kwinte, prove to be excellent choices, while the third one, Western Solent, lacks sufficient data over time, but remains a valuable candidate. The case studies and the available backscatter data on these areas prove the applicability of this method. The expansion of the number of commonly used reference areas and the growth of the number of multibeam echosounder controlled thereon could greatly contribute to the further development of quantitative applications based on multibeam echosounder backscatter measurements.     

The organic carbon pump in the Atlantic

The growth, sinking and remineralisation of phytoplankton constitute the soft-tissue carbon pump. Here we derive the expression for the strength of this carbon pump in terms of the nutrient utilisation and the carbon:nutrient ratio of plankton. Furthermore, we aim to determine how the physical environment affects this carbon pump. Thereto, we have used a plankton physiological model with a novel representation of the effect of temperature and light on plankton stoichiometry; it predicts that the effect of temperature on the carbon:nutrient ratio depends on the light conditions. Using this model combined with a mechanistic model for the mixed layer, we have studied the biological carbon pump at the BATS site and at various locations along a meridional transect through the Atlantic Ocean. Our results suggest that the soft-tissue carbon pump is stronger at mid-latitudes than in the (sub-)tropics and near the poles as a consequence of latitudinal variations in algal stoichiometry.     

Larval dispersion of the estuarine crab Neohelice granulata in coastal marine waters of the Southwest Atlantic

The estuarine brachyuran crab Neohelice granulata export their larvae from the parental intertidal population of the Mar Chiquita coastal lagoon, and probably other populations, to marine waters. The degree of larval dispersion or self-recruitment of populations is unknown. We evaluated the presence of all larval stages of N. granulata in coastal waters of Argentina between 37.9° and 35.8° S, at two different spatial scales: a broad scale of tens to hundreds of kilometers from the Río de la Plata estuary in the north, to Mar Chiquita lagoon in the south, and a small scale of hundreds of meters to some kilometers around the mouth of Mar Chiquita, during spring and summer. Additionally, we registered the larval composition and density at San Clemente creek population, in Samborombon Bay (Río de la Plata estuary), every 3 h along a 30-hour period. Evidence indicates that larval release of N. granulata is temporally synchronized with nocturnal ebb tides and all development from Zoea I to Zoea IV occur in areas close to the parental population, even with very different oceanographic characteristics. A possible mechanism based on salinity selection and wind-driven transport is proposed for such behavior, and some considerations related to the connectivity of present populations are made.