Eo-Alpine Metamorphism in the Uppermost Unit of the Cretan Nappe System — Petrology and Geochronology

The uppermost unit of the Cretan nappe system contains a variegated series of high-grade metamorphic rocks. In the Léndas area, amphibolites are present characterized by the assemblage

$$\text{brown}\;\text{hornblende}\;+\;\text{diopside}\;+\;\text{plagioclase}\;\text{(An 50)}$$

while associated metapelitic gneisses consist of

$$\text{garnet}\;+\;\text{cordierite}\;+\;\text{biotite}\;+\;\text{sillimanite (andalusite)}\;\pm\;\text{K-feldspar}\;+\;\text{plagioclase (An 40-50)}\;+\;\text{quartz}.$$

Judging from relevant experimental data for the gneiss assemblage including the Fe/Mg distribution on coexisting garnet and cordierite, the P-T conditions of metamorphism are estimated at about 700° C and 5 kb water vapour pressure.K/Ar determinations on hornblendes from three amphibolites yielded cooling ages of 71.3, 71.2, and 71.1 (±1.7) m.y. respectively; biotites from three paragneisses gave 70.2 ± 1.4, 69.7 ± 1.2, and 67.9 ± 1.4 m.y. respectively. Assuming a sealing temperature against argon diffusion of 300° C, for biotite, and 500° C, for hornblende, a cooling rate of 100–200° C/m.y. is calculated. Thus a late Cretaceous (eo-Alpine) metamorphic event is established in the post-Cretaceous nappes of Crete.     

Grundwasser der Zentralsahara: Fossile Vorräte

Zusammenfassung Aufgrund von¹⁴C-Datierungen, die an mehr als 40 Grundwasserproben aus Südlibyen vorgenommen wurden, wird nachgewiesen, daß die Grundwasservorräte des Murzuk-Beckens weitgehend fossil sind. Sie werden unter den gegenwärtigen klimatischen Bedingungen kaum ergänzt, was sehr wahrscheinlich ebenso auf den Hauptteil der Grundwasservorräte anderer Becken der Sahara zutrifft. Für die frühere Vorstellung, das Grundwasser werde in regenreicheren Gebirgen etwa der südlichen Sahara ausreichend ergänzt und fließe im Unterbau großflächig nach Norden, gibt es keine Hinweise. In Abhängigkeit vom ursprünglichen Ablagerungsmilieu und vom Speichergestein haben die Grundwässer der verschiedenen hydrogeologischen Teilprovinzen des Murzuk-Beckens jeweils spezifische hydrochemische Eigenschaften, und sie sind unterschiedlich alt. Als Ursache hierfür ist anzusehen, daß die Regenerationsmöglichkeiten innerhalb der verschiedenen Einzugsgebiete nicht immer einheitlich waren, außerdem kam es stellenweise zur Vermischung älterer mit jüngeren Grundwässern. Von erklärbaren Ausnahmen abgesehen entstammen die untersuchten Wässer dem letzten Pluvial, einige gehören nachpluvialen Feuchtphasen an. Das¹⁴C-Alter der meisten Wasserproben liegt zwischen 20 000 und 40 000 Jahren.

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¹⁴C-dating of more than 40 samples of groundwater proves, that exploited ground-water in the Murzuk Basin area of southern Libya is mainly between 20 000 and 40 000 years old. This water, as probably most of the large groundwater reserves in other Saharian basins, has to be considered mainly fossil water without noteworthy recharge under present climatical conditions. The supposition, that Saharian groundwaters are recharged at mountainous areas in the south, as was postulated by several authors previously, has to be denied, at least as respects the central Sahara.Waters of all groundwater subprovinces investigated in southern Libya show reservoir specific chemical characteristics. Differences in age are partly depending on differing recharge possibilities at the recharge areas and partly they are the result of mixture between younger and older waters. The water mainly results from precipitations of the last pluvial period of the Sahara, some from rainfalls of later subpluvial periods.

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Résumé Des datations par la méthode¹⁴C, effectuées sur plus de quarante échantillons d'eau provenant des nappes souterraines du Sud de la Libye, prouvent que les réserves des nappes souterraines du Bassin de Mourzouk sont essentiellement fossiles. Dans les conditions climatiques actuelles, elles ne peuvent pratiquement pas être complétées. Cette constatation concerne aussi, très vraisemblablement, la partie principale des réserves des nappes souterraines des autres bassins du Sahara. Aucune indication ne semble capable d'étayer les concepts antérieurs traitant d'une alimentation, encore suffisante aujourd'hui, de la nappe souterraine, coulant vers le Nord sur de grandes surfaces dans le sous-sol du Sahara, par les montagnes riches en pluies à peu près au Sud du Sahara.Les nappes souterraines des différentes parties des provinces hydrogéologiques du Bassin de Mourzouk possèdent, respectivement, des propriétés hydrochimiques spécifiques, dépendant du milieu de dépôt primitif et de la roche réservoir. Leurs distances variables jusqu'à l'ancienne aire d'alimentation et aussi sûrement, par endroit, le mélange imaginable d'eaux plus jeunes et plus vieilles causent, en partie, leurs différences d'âge.Les eaux étudiées, sauf quelques exceptions insignifiantes et faciles à expliquer, proviennent, d'une part des dernières grandes pluies, et d'autre part, pour quelques unes, des phases humides (post-pluviales). Elles ont, en grande partie, 20 000 à 40 000 ans.

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¹⁴ , 40 , Murzuk . - . , . , , . , , Murzuk , . . , . - . , . 20 000 40 000 .

     

Late Quaternary barrier and fringing reef development of Bora Bora (Society Islands,south Pacific): First subsurface data from the Darwin‐type barrier‐reef system

The universally known subsidence theory of Darwin, based on Bora Bora as a model, was developed without information from the subsurface. To evaluate the influence of environmental factors on reef development, two traverses with three cores, each on the barrier and the fringing reefs of Bora Bora, were drilled and 34 uranium‐series dates obtained and subsequently analysed. Sea‐level rise and, to a lesser degree, subsidence were crucial for Holocene reef development in that they have created accommodation space and controlled reef architecture. Antecedent topography played a role as well, because the Holocene barrier reef is located on a Pleistocene barrier reef forming a topographic high. The pedestal of the fringing reef was Pleistocene soil and basalt. Barrier and fringing reefs developed contemporaneously during the Holocene. The occurrence of five coralgal assemblages indicates an upcore increase in wave energy. Age–depth plots suggest that barrier and fringing reefs have prograded during the Holocene. The Holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment. Fringing reef growth started 8780 ± 50 yr bp ; accretion rates average 5·65 m kyr^?1. The barrier reef consists of >30 m thick Holocene coralgal and microbial successions. Holocene barrier‐reef growth began 10 030 ± 50 yr bp and accretion rates average 6·15 m kyr^?1. The underlying Pleistocene reef formed 116 900 ± 1100 yr bp , i.e. during marine isotope stage 5e. Based on Pleistocene age, depth and coralgal palaeobathymetry, the subsidence rate of Bora Bora was estimated to be 0·05 to 0·14 m kyr^?1. In addition to subsidence, reef development on shorter timescales like in the late Pleistocene and Holocene has been driven by glacioeustatic sea‐level changes causing alternations of periods of flooding and subaerial exposure. Comparisons with other oceanic barrier‐reef systems in Tahiti and Mayotte exhibit more differences than similarities.     

Zur Entwicklungsgeschichte der Südschwarzwälder Hauptbewegungszone (Devon-Karbon-Zug Badenweiler -Lenzkirch) während des Variscikums

In the Southern Black Forest area of SW-Germany, pre-Permian paleozoic sequences seem to be confined to a narrow strip around Badenweiler-Schönau-Lenzkirch. It consists of uncertain (possibly M.- to U.-Devonian) shales and “Kulm”, i.e. L.-Carboniferous greywackes, conglomerates, sandstones and shales with volcanic intercalations. The rocks can be subdivided in a) Tournai-greywackes of marine origin. Volcanism reaches into the b) Visé characterized by marine limestones and dolomites. There seems to be a disconformity above followed by c) conglomerates and pyroclastic rocks. A late-hercynian (variscean) tectonic out-line of the occurrence was initiated by early-variscean basin-formation.     

Benthic storms in the Greenland sea

We present observations of strong, episodic, bottom-intensified currents from two current meter moorings, each of a year's duration, placed in the central Greenland Sea at 75°N, 8°W, in a water depth of circa 3340 m. The events, recorded by the current meters placed some 50 m above the sea floor, occur about 4 times a year and last about a week. They show currents of up to 43 cm/s, turning in direction, occasionally modulated by a signal of the frequency of the Coriolis parameter or the semi-diurnal tide. The temperature record at the current meter however remains constant to 0.01°C. The current direction measured at overlying meters correlates well with that of the deepest meter – the current speed does not. Independent, geological data also show evidence of strong bottom flows in the area.We discuss possible mechanisms for these `benthic storms', including the hypothesis of a sediment driven plume descending from the East Greenland continental slope. These high energy events have implications for sedimentation, shelf-basin exchange and boundary mixing processes. Normal mode theory is used to justify the dynamical response of the system to such a bottom-trapped impulse.     

Seasonal variability of Holocene climate: a palaeolimnological study on varved sediments in Lake Jues (Harz Mountains, Germany)

Studies combining sedimentological and biological evidence to reconstruct Holocene climate beyond the major changes, and especially seasonality, are rare in Europe, and are nearly completely absent in Germany. The present study tries to reconstruct changes of seasonality from evidence of annual algal successions within the framework of well-established pollen zonation and ¹⁴C-AMS dates from terrestrial plants. Laminated Holocene sediments in Lake Jues (10°20.7′ E, 51°39.3′ N, 241 m a.s.l.), located at the SW margin of the Harz Mountains, central Germany, were studied for sediment characteristics, pollen, diatoms and coccal green algae. An age model is based on 21 calibrated AMS radiocarbon dates from terrestrial plants. The sedimentary record covers the entire Holocene period. Trophic status and circulation/stagnation patterns of the lake were inferred from algal assemblages, the subannual structure of varves and the physico-chemical properties of the sediment. During the Holocene, mixing conditions alternated between di-, oligo- and meromictic depending on length and variability of spring and fall periods, and the stability of winter and summer weather. The trophic state was controlled by nutrient input, circulation patterns and the temperature-dependent rates of organic production and mineralization. Climate shifts, mainly in phase with those recorded from other European regions, are inferred from changing limnological conditions and terrestrial vegetation. Significant changes occurred at 11,600 cal. yr. BP (Preboreal warming), between 10,600 and 10,100 cal. yr. BP (Boreal cooling), and between 8,400 and 4,550 cal. yr. BP (warm and dry interval of the Atlantic). Since 4,550 cal. yr. BP the climate became gradually cooler, wetter and more oceanic. This trend was interrupted by warmer and dryer phases between 3,440 and 2,850 cal. yr. BP and, likely, between 2,500 and 2,250 cal. yr. BP.     

Geochemical controls on a calcite precipitating spring

A small spring fed stream was found to precipitate calcite by mainly inorganic processes and in a nonuniform manner. The spring water originated by rainwater falling in a 0.8 km² basin, infiltrating, and dissolving calcite and dolomite followed by dissolution of gypsum or anhydrite. The Ca²⁺/Mg²⁺ indicates that calcite is probably precipitated in the subsurface from a supersaturated solution. This water emerges from the spring still about 5 times supersaturated with respect to calcite and continues calcite precipitation. When 10 times supersaturation is reached, due to CO₂ degassing the precipitation is more rapid. The calcite accumulation from the stream with a flow of 5 l/s is calculated to be 12600 kg/yr with the highest rates in areas where CO₂ degassing is the greatest. The non-equilibrium, as shown by the high calcite supersaturation, is also reflected in a variable partitioning pattern for Sr²⁺ between the water and calcite.