The structure and dynamics of the Bottom Boundary Layer in shallow sea areas without tidal influence: an experimental approach

This report describes extensive investigations of the near bottom layer of the Western Baltic (Mecklenburg Bight, Darss Sill and Arkona Basin) which were conducted over a 5 year period to determine the typical structure, vertical thickness, vertical turbulence structure, and spatial and temporal variability of this water mass with regard to the area's particular hydrographic conditions. Series of vertical profiles were obtained using the microstructure profiler MSS86, which is capable of measuring high resolution profiles of temperature, conductivity, current shear, light attenuation and pressure down to the seafloor. The near bottom current structure was simultaneously measured with conventional current metres at fixed depths. A typical vertical density structure of the near bottom layer was found. At all investigation sites the Bottom Boundary Layer was separated from the overlying water mass by a well pronounced thermohaline pycnocline. A homogeneous water layer was situated above the bottom with a mean thickness of 2.2 m and typical variation between 0.5 and 3.5 m. The thickness of both the homogeneous layer and of the near bottom layer vary considerably. It is suggested that horizontal advection is responsible for these fluctuations in thickness. The variation in thickness of the Homogeneous Layer is independent of the local mean current velocity, wind speed and energy dissipation rate. Over periods of about 2 days the thickness of the Homogeneous Layer is determined by the average wind speed. The Bottom Boundary Layer shows its own characteristic dynamic, which is largely decoupled from that of the remaining water body. A logarithmic layer was generally not resolved by the current measurements. From dissipation rate measurements, the wall layer was determined to be 0.9 m thick. There was no significant correlation between the dissipation rate and the local wind speed, or between the dissipation rate and local mean current u₁₀₀. This means that any simple parameterisation relating u₁₀₀ or friction velocity to the locally produced turbulence and consequently to the resuspension of sediment is probably not applicable to shallow sea areas with properties like the Western Baltic. The investigation of sediment concentration in the BBL illustrates the importance of local effects combined with advection. The sediment stratified layer covers only the bottom most 50 cm.     

Gabbroic xenoliths from La Palma, Tenerife and Lanzarote, Canary Islands: evidence for reactions between mafic alkaline Canary Islands melts and old oceanic crust

Gabbroic and hornblendite xenoliths from La Palma, Tenerife and Lanzarote fall into three main groups based on petrography and chemistry. One group (comprising all xenoliths from Lanzarote and some from La Palma) consists of highly deformed orthopyroxene-bearing gabbroic rocks that show a strong affinity to N-MORB and oceanic gabbro cumulates in terms of mineral chemistry and REE relations. However, they show mild enrichment in the most incompatible elements (particularly Rb+Ba±K) relative to intermediate and heavy REE, and their Sr–Nd isotope ratios fall within or close to the N-MORB field. The second group (60% of the xenoliths from La Palma) are gabbroic cumulates with zoned clinopyroxenes (Ti–Al-poor cores, Ti–Al-rich rims) and reaction rims of hornblende, biotite and clinopyroxene on other phases. Their trace-element and Sr–Nd isotope relations are in general transitional between N-MORB cumulates and Canary Islands alkali basalts, but they show strong enrichment in Rb, Ba and K relative to other strongly incompatible elements. The third group (comprising some xenoliths from La Palma and all those from Tenerife) are undeformed gabbroic and hornblendite rocks in which hornblende and biotite appear to belong to the primary assemblage. These rocks show strong affinities to Canary Islands alkali basaltic magmas with respect to mineral, trace-element, and Sr–Nd isotope chemistry. The first two groups are interpreted as fragments of old oceanic crust which have been mildly to strongly metasomatized through reactions with Canary Islands alkaline magmas. The reaction process is a combination of enrichment in elements compatible with biotite (and hornblende), and simple mixing between N-MORB cumulates and trapped alkaline magmas. The third group represents intrusions/cumulates formed from mafic alkaline Canary Islands magmas. Modeling indicates that locally up to 50% new material has been added to the old oceanic crust through reactions with ocean island basalts. Reactions and formation of cumulates do not represent simple underplating at the mantle/crust boundary, but have taken place within the pre-existing oceanic crust, and are likely to have significantly thickened the old oceanic crust.     

Petrology of basalts from the Mohns-Knipovich Ridge; the Norwegian-Greenland Sea

Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K₂O, TiO₂, and P₂O₅ contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe²⁺) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe²⁺) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C.Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30 N is also briefly considered.It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform.     

Estimating the amount of dewfall

Summary The equation for the turbulent diffusion of water vapour is developed to a form which makes it possible to estimate, in an approximative manner, the amount of dewfall on a short grass surface, or on a bare surface, using routine meteorological data. Introducing data typical for dew nights, the equation leads to estimates amounting to 0.10 to 0.20 mm/dew night. These figures are close to the dewfall amounts measured by the Duvdevani dew gauge at the same stations.

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Zusammenfassung Die Gleichung für die turbulente Diffusion von Wasserdampf wurde in einer Form entwickelt, die die angenäherte Schätzung des Taubetrages auf einer Rasenfläche oder auf einer vegetationslosen Oberfläche unter Benutzung üblicher meteorologischen Daten ermöglicht. Bei Anwendung kennzeichnender Werte für Nächte mit Taufall ergibt die Gleichung Schätzungen, die 0.10 to 0.20 mm pro Taunacht betragen. Diese Werte kommen den Taubeträgen, die mit dem Duvdevanischen Taugerät an denselben Stationen gemessen wurden, sehr nahe.

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Résumé L'équation pour la diffusion turbulente de la vapeur d'eau est développée en une forme qui donne la possibilité d'estimer, d'une manière approximative, la quantité de rosée sur un gazon ou une surface nue, en utilisant des donnes météorologiques de routine. En introduisant des données caractéristiques pour des nuits de rosée, l'équation conduit à des estimations qui arrivent à 0.10 to 0.20 mm par nuit de rosée. Ces valeurs sont proches des quantités mesurées par le jauge de rosée Duvdevani aux mêmes stations.

     

Latitudinal variation of tropospheric temperature lapse rate

Summary The latitudinal variation of tropospheric temperature lapse rate is examined on the basis of recently published data for the northern hemisphere. In January, the lapse rate systematically decreases from 6.4°C/km. for the equator to 3.6°C/km. for 70°N; in July, the lapse rate is nearly constant in respect of latitude between the equator and 50°N, but decreases from the latter poleward. The average January–July lapse rate decreases systematically from 6.3°C/km. for the equator to 4.7°C/km. for 70°N. This variation is closely described by the equation =6.25–2 sin⁴, where is the lapse rate, in °C/km, for latitude . For the north pole, the equation gives a lapse rate of 4.25°C/km.

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Zusammenfassung Die Änderung des vertikalen Gradienten der Troposphärentemperatur mit der geographischen Breite wird auf Grund neuen Zahlenmaterials für die Nordhemisphäre untersucht. Im Januar nimmt der Temperaturgradient allmählich von 6.4°C/km am Äquator bis zu 3.6°C/km in 70°N Breite ab; im Juli ist der Temperaturgradient in den Breiten zwischen dem Äquator und 50°N Breite fast gleich und nimmt von hier aus polwärts ab. Der mittlere Temperaturgradient für Januar–Juli nimmt allmählich von 6.3°C/km am Äquator bis auf 4.7°C/km in 70°N Breite ab. Die Breitenabhängigkeit läßt sich gut durch die Gleichung =6.25–2 sin⁴ darstellen, in der der Temperaturgradient in °C/km in der Breite ist. Für den Nordpol ergibt die Gleichung einen Temperaturgradienten von 4.25°C/km.

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Résumé La variation latitudinale du gradient thermique vertical de la température troposphérique est examinée en se basant sur des données publiées récemment d'une valeur de 6.4°C/km pour l'équateur à 3.6°C/km pour 70°N; en juillet le gradient est presque constant pour les latitudes situées entre l'équateur et 50°N, mais il diminue d'ici vers le pôle. Le gradient moyen janvier/juillet diminue systématiquement de 6.3°C/km pour l'équateur à 4.7°C/km pour 70°N. Cette variation peut être représentée par l'équation =6.25–2 sin⁴ où est le gradient en °C/km pour la latitude . Pour le pôle nord, l'équation donne un gradient de 4.25°C/km.

     

Crossings of ice-bound sea surfaces in history

Seven more-or-less well documentated cases of the use made of icebound sea areas in winter for the purposes of warfare are reviewed. The sea-ice crossings took place in 1495, 1577, 1581, 1658, 1809, 1940, and 1943, i.e. the first five occurred during the Little Ice Age. A book authored by a prominent Swedish personality (Archbishop Olaus Magnus) and published in 1555 says that warfare on frozen sea areas in winter by the Muscovites (Russians) and Swedes was as common as warfare by ships on the open seas in summer. There are indications of some crossings of ice-bound seas prior to 1495 and not necessarily for warlike activities. It seems that the Vikings too did some sea-ice crossings.The crossings of 1495, 1577, 1581, and 1940 involved the Gulf of Finland, that of 1809 the Gulf of Bothnia and the Aaland Islands area of the Baltic, that of 1658 the Danish Belts, and that of 1943 the Gulf of Taganrog in the Sea of Azov. In the first three cases the powers concerned were Muscovy (Russia) and Sweden which for centuries were fighting for supremacy in the Baltic and over the routes from the inner Baltic (Gulf of Finland and Bay of Riga) to western Europe. The case of 1809 involved, again, Russia and Sweden, though in the background of the conflict between the two were wider European issues of the Napoleonic wars. The 1658 crossing of the frozen-over Danish Belts was accomplished by the Swedes, forcing the Danes into submission: In the ensuing Peace Treaty Sweden for the first time in her history achieved her present territorial extent in the Scandinavian Peninsula. The case of 1940 was connected with the 1939–40 Winter War of Soviet Russia against Finland. The crossing of 1943 was made by German forces retreating from the Caucasus under the pressure of Soviet forces in World War II.The crossings of 1577, 1581, 1658, 1809, 1940, and 1943 took place between late in January and late in March; the case of 1495 appears to have taken place early in the winter season: probably late in November. Since in the period 1931–60 no part of the Gulf of Finland froze over before about the middle of December, the early date of the crossing of 1495 is possibly one of the many indications of cold winters during the Little Ice Age.     

The upper mantle under La Palma,Canary Islands: formation of Si−K−Na-rich melt and its importance as a metasomatic agent

 Mantle xenoliths hosted by the Historic Volcan de San Antonio, La Palma, Canary Islands, fall into two main group. Group I consists of spinel harzburgites, rare spinel lherzolites and spinel dunites, whereas group II comprises spinel wehrlites, amphibole wehrlites, and amphibole clinopyroxenites. We here present data on group I xenoliths, including veined harzburgites and dunites which provide an excellent basis for detailed studies of metasomatic processes. The spinel harzburgite and lherzolite xenoliths have modal ol−opx−cpx ratios and mineral and whole rock major element chemistry similar to those found in Lanzarote and Hierro, and are interpreted as highly refractory, old oceanic lithospheric mantle. Spinel dunites are interpreted as old oceanic peridotite which has been relatively enriched in olivine and clinopyroxene (and highly incompatible elements) through reactions with basaltic Canarian magmas, with relatively high melt/peridotite ratio. Group I xenoliths from La Palma differ from the Hierro and Lanzarote ones by a frequent presence of minor amounts of phlogopite (and amphibole). Metasomatic processes are also reflected in a marked enrichment of strongly incompatible relative to moderately incompatible trace elements, and in a tendency for Fe−Ti enrichment along grain boundaries in some samples. The veins in the veined xenoliths show a gradual change in phase assemblage and composition of each phase, from Fe−Ti-rich amphibole+augite+Fe−Ti-oxides+apatite+basaltic glass, to Ti-poor phlogopite+Cr-diopside±chromite+ Si−Na−K-rich glass+fluid. Complex reaction zones between veins and peridotite include formation of clinopyroxene±olivine+glass at the expense of orthopyroxene in harzburgite, and clinopyroxene+spinel±amphibole±glass at the expense of olivine in dunite. The dramatic change in glass composition from the broadest to the narrowest veins includes increasing SiO₂ from 44 to 67 wt%, decreasing TiO₂/Al₂O₃ ratio from >0.24 to about 0.02, and increasing K₂O and Na₂O from 1.8 to >7.0 wt% and 3.8 to 6.7 wt%, respectively. The petrographic observations supported by petrographic mixing calculations indicate that the most silicic melts in the veined xenoliths formed as the result of reaction between infiltrating basaltic melt and peridotite wall-rock. The highly silicic, alkaline melt may represent an important metasomatic agent. Pervasive metasomatism by highly silicic melts (and possibly fluids unmixed from these) may account for the enriched trace element patterns and frequent presence of phlogopite in the upper mantle under La Palma. Received: 15 January 1996 / Accepted 30 May 1996     

Climate of the Black Sea region around 0 C.E.

Indications of the climate of the Black Sea Region (the region up to about 500 km from the sea) are examined for a period of a few hundred years before and after 0 C.E. Much of the information is obtained from the work of Soviet scientists, some recent discovery regarding ice conditions on a high mountain of Turkey and from archeology of the region.Levels of the Black Sea, the Caspian and that of the large Lake Van were on the rise at the time. The most plausible cause for the level rise of the latter two exitless water bodies is increased precipitation and inflow from the drainage areas; in the Black Sea's case a contributory factor must have been the level rise of the world's oceans. Pollen investigations in the southern European Soviet Union, as well as the large quantities of wine and figs grown on the northern littoral of the Black Sea at the time, suggest that the climate was a little warmer than at present. The pollen investigations intimate a temperature level about 0.5 °C higher than the cold phase around the middle of the first millenium B.C. Support to the aforementioned inference is offered by the recently discovered ice conditions on Mt. Erciyas, Turkey, as they were 2000 years ago. It is also inferred that the precipitation level of the region was, generally, somewhat higher than nowadays.Finally, a brief review is made of glacier, tree-ring density and peatbog data for Europe and North America. They all show that the period around 0 C.E. was relatively warm.In 1986–90 visiting with the Department of Meteorology, University of Copenhagen, Copenhagen, Denmark.     

Ultramafic and mafic xenoliths from Hierro,Canary Islands: evidence for melt infiltration in the upper mantle

Three groups of ultramafix xenoliths were collected from alkali basalt in the island of Hierro, Canary Islands: (1) Cr-diopside series (spinel harzbugite, lherzolite, dunite); (2) Al-augite series xenoliths (spinel wherlite, olivine clinopyroxenite, dunite, olivine websterite); (3) gabbroic xenoliths. The main textures are granoblastic, porphyroclastic and granular, but poikilitic textures, and symplectitic intergrowths of clinopyroxene (cpx) + spinel (sp)±orthopyroxene (opx)±olivine (ol) (in rare cases cpx+opx), occur locally. Textural relations and large inter- and intra-sample mineral chemical variations testify to a complex history of evolution of the mantle source region, involving repeated heating, partial melting, and enrichment associated with infiltration by basaltic melts. The oldest assemblage in the ultramafic xenoliths (porphyroclasts of ol+opx±sp±cpx) represents depleted abyssal mantle formed within the stability field of spinel lherzolite. The neoblast assemblage [ol+cpx+ sp±opx±plagioclase (plag)±ilmenite (il)±phlogopite (phlog)] reflect enrichment in CaO+Al₂O₃+Na₂O+ FeO±TiO₂±K₂O±H₂O through crystal/liquid separation processes and metasomatism. The Al-augite-series xenoliths represent parts of the mantle where magma infiltration was much more extensive than in the source region of the Cr-diopside series rocks. Geothermometry indicates temperature fluctuations between about 900–1000 and 1200°C. Between each heating event the mantle appears to have readjusted to regional geothermal gradient passing 950°C at about 12 kbar. The gabbroic xenoliths represent low-pressure cumulates.