87Sr86Sr in Late Proterozoic carbonates: evidence for a “mantle” event at ∼900 Ma ago

Strontium isotopic measurements were made on Late Proterozoic carbonates from West African Craton. Comparison of samples with acceptable trace element patterns with coeval data from southern Africa and with the published Australian results suggests that the ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratio of the Late Proterozoic sea water evolved in the following manner about 0.7075 at 1000 ± 50 Ma, 0.7056 to 0.7074 at 900 ± 50 Ma, 0.7068 to 0.7091(0.7106) at 800 ± 50 Ma, 0.7074 to 0.7077 at 700 ± 50 Ma, and 0.7076 to 0.7089(0.7096) at 600 ± 50 Ma ago. The variations are comparable in magnitude and frequency to those described previously for the Phanerozoic. Strontium isotopic values in the radiogenic part of this range suggest that the continental river flux of Sr into Late Proterozoic oceans was of comparable isotopic composition to its present day counterpart (～0.711). Consequently, the non-radiogenic ${}^{87}\text{Sr}{}^{86}\text{Sr}$ value at ～900 ± 50 Ma ago signifies a large flux of “mantle” strontium into the ocean at this time. Because the present time resolution is only about 75 ± 25Ma, additional sampling as well as better stratigraphie resolution and more definite selection criteria are required for construction of a more detailed Late Proterozoic sea water curve.     

Polyphase metamorphic sedimentary xenoliths from Mt. Amiata volcanics (Central Italy); evidence for a partially disrupted contact aureole

Metasedimentary inclusions in the crustal anatectic rhyodacites of Mt. Amiata volcanic complex can be subdivided into a silica-poor group, rich in aluminous minerals, and a small silica-rich group containing abundant quartz. Textural observations, supported by mineral chemistry provide evidence for three metamorphic events. The regional metamorphic M₀ is often associated with deformation, and two subsequent progressive thermometamorphic events, M₁ and M₂, are caused by the magmatic heat. Mineral assemblages of M₁ are indicative for the pyroxene-hornfels facies, and assemblages of M₂, in combination with evidence for partial melting and sanidinization, suggest sanidinite facies conditions. The inclusions are interpreted mainly as xenolithic fragments of a contact aureole, which were remetamorphosed after incorporation by the magma. The aureole formed in pre-mesozoic formations during accumulation of the melt in a magmachamber. Constraints on pressure conditions for M₁ indicate the possibility of a fairly large depth of the heat-source giving rise to Mt. Amiata geothermal field.

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Zusammenfassung Metasedimentäre Einschlüsse in den krustal anatektischen Rhyodaziten des vulkanischen Komplexes Monte Amiata lassen sich untergliedern in eine Silica-arme Gruppe, reich an Aluminium-Mineralien, und eine kleine Silica-reiche Gruppe mit Quarz im Übermaß. Die beobachteten Texturen, unterstützt von der Mineralchemie weisen auf drei metamorphe Ereignisse. Die Regionalmetamorphose M₀ ist häufig verbunden mit Deformation, während zwei nachfolgende progressive Thermometamorphosen M₁ und M₂ verursacht worden sind von der magmatischen Wärme. Die Mineralgesellschaften des M₁ sind representativ für die Pyroxen-Hornfels Fazies und die Gesellschaften des M₂, in Kombination mit Anweisungen für partielle Aufschmelzung und Sanidinisation, suggerieren Umstände der Sanidinit-Fazies. Die Einschlüsse werden interpretiert als xenolithische Fragmente einer Kontaktaureole, die nochmals metamorphosiert wurden nach der Aufnahme ins Magma. Die Aureole entstand in pre-mesozoischen Formationen während der Akkumulation des Schmelzes in einer Magmakammer. Die geschätzten Druckbedingungen für M₁ sind im Einklang mit der Möglichkeit einer ziemlich tiefen Position der Wärmequelle gehörend zum Geothermalfeld des Monte Amiata.

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Résumé Les enclaves métasédimentaires des rhyodacites anatectiques crustales du complexe volcanique du Monte Amiata se laissent subdiviser en deux groupes: l'un pauvre en silice avec des minéraux alumineux abondants, et l'autre, moins important, contenant un excès de quartz. Les textures, supportées par la minéralogie chimique témoignent de trois événements métamorphiques. Le métamorphism régional M₀ est souvent associé avec une déformation tandis que deux événements successifs de métamorphisme progressif, M₁ et M₂, ont été causés par la chaleur magmatique. Les assemblages des minéraux de M₁ représentent le faciès des cornéennes à pyroxene, et les assemblages de M₂, en combinaison avec indications de fusion partielle et sanidisation, suggèrent les conditions du faciès des sanidinites. Les enclaves sont interprétées comme des fragments xénolithiques d'une auréole de contact, qui ont été remétamorphosés après incorporation par le magma. L'auréole s'est formée dans les formations pré-mésozoiques pendant l'accumulation du magma dans un réservoir. Les conditions P-T de M₁ permettent de supposer que la source thermique donnant lieu au champ géothermique du Monte Amiata, occupe une position assez profonde.

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: , , . , , . ₀ , — M₁ M₂ — . M₁ - , ₂ , . , , . - , . M₁, , , .

     

Tektonische Untersuchungen an Molasse-Geröllen

Zusammenfassung An einer orientiert entnommenen Kollektion von Geröllen aus der Unteren Süßwasser-Molasse bei Immenstadt (Allgäu, Bayern) wird ein neues Verfahren zur Messung von Geröllen angewandt. Die Gerölle werden in Gerölldiagrammen dargestellt, die es ermöglichen, Form, Größe und Orientierung und alle Gefügemerkmale der Gerölloberfläche exakt abzubilden und zu vergleichen. Striemen und Lösungsgruben auf den Geröllen werden geometrisch erfaßt und statistisch ausgewertet. Danach ergeben sich Rückschlüsse auf die felsmechanische Beanspruchung des Geröllverbandes durch die Alpen.

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A collection of orientated pebbles from the subalpine, lower freshwater Molasse near Immenstadt (Allgäu, Bavaria) was studied by a novel measuring method. The pebbles are presented in diagrams that allow to portray and to compare exactly shape, roundness, size, orientation and all structures on the surface. Striation and excavations by solution on the pebbles are recorded geometrically and interpretated statistically. Hence follow conclusions about the alpine strain of the frame.

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Résumé Une nouvelle méthode de mesure des galets est appliquée à une collection de galets orientés de la molasse d'eau douce inférieure d'Immenstadt (Allgäu, Bavière). Des diagrammes sont faits, qui permettent de figurer et de comparer exactement la forme, la taille et l'orientation des galets, ainsi que toutes les caractéristiques structurelles de leur surface. Les stries et les trous de dissolution sur les galets sont représentés avec leur géométrie et sont exprimés statistiquement. On en tire des conclusions sur la sollicitation tectonique de cette formation de galets par les Alpes.

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Immenstadt (Allgäu, Bayern) . , , , . . . , . , .

     

Probleme der atmosphärischen Evolution im Präkambrium

Zusammenfassung Infolge des Sauerstoff-Unterschusses im Material von Erdmantel und Erdkruste besitzen deren Entgasungsprodukte generell reduzierenden Charakter und kommen somit als Lieferant von freiem Sauerstoff nicht in Frage. Die Quelle für den Luftsauerstoff muß deshalb in nicht-geologischen Prozessen zu suchen sein, bei denen der Sauerstoff nachträglich aus oxidischen Verbindungen (H₂O, CO₂) freigesetzt wurde. Da als Energiequelle für diese Reaktionen praktisch nur das Sonnenlicht zur Verfügung steht, muß die Anwesenheit von freiem Sauerstoff in der Atmosphäre letztlich auf einem photochemischen Effekt beruhen.Hierbei ist zu unterscheiden zwischen 1. einem anorganischen photochemischen Effekt, der mit dem kurzwelligen (energiereichen) Teil des Sonnenspektrums arbeitet (insbesondere 1500–2100 Å) und 2. einem organischen photochemischen Effekt, der mit dem Spektralbereich des sichtbaren Lichtes arbeitet, aber auf die Mitwirkung eines Katalysators (Chlorophyll) angewiesen ist. Der Anteil des ersteren Prozesses (Photodissoziation von Wasserdampf) am Aufbau der sauerstoffreichen Atmosphäre dürfte bei weitem geringer gewesen sein als der des zweiten (biologische Photosynthese von Kohlehydraten aus CO₂ und H₂O unter Freisetzung von Sauerstoff als Stoffwechselprodukt), so daß der heutige Luftsauerstoff im wesentlichen als biogen gedeutet werden kann. Die geochemische Evolution der Atmosphäre und die Evolution des Lebens sind somit auf das engste miteinander verknüpft. Mit dem Übergang von der primär reduzierenden zur oxidierenden Atmosphäre konnte beim Überschreiten des sog. Pasteur-Punktes (O₂-Partialdruck 10^–2 des heutigen Wertes) der Umschlag vom fermentativen zum oxidativen Energiestoffwechsel erfolgen. Es kann mit guten Gründen angenommen werden, daß die explosive Differenzierung des Lebens an der Wende Präkambrium-Kambrium eine Folge des höheren Energiegewinns bei der Atmung war. Sowohl die Aufrechterhaltung des Lebens als thermodynamisch metastabiler Zustand (mit einer Entropie weit unterhalb des Maximums) als auch seine Differenzierung im Laufe der Evolution bedürfen der ständigen Zufuhr von Energie, sind also nach der bekanntenBoltzmannschen Beziehung im Grunde genommen äußerst unwahrscheinliche Vorgänge. Freier Sauerstoff, der eine vollständige Verbrennung der Kohlehydrate zu Kohlendioxid und Wasser ermöglichte (und damit den Wirkungsgrad des Energiestoffwechsels im Vergleich zur anaeroben Gärung auf etwa das Vierzehnfache erhöhte), spielte bei der Evolution der Metazoen offensichtlich die Rolle Maxwellschen Dämons, der für die Aufrechterhaltung des thermodynamischen Ungleichgewichts (bzw. der Negentropie) in größeren Zellverbänden verantwortlich war.

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Because of the oxygen deficiency within the material of the earth's crust and mantle, degassing of the lithosphere does not furnish free oxygen, but gases possessing a strongly reducing character. Therefore, other than geological sources must have provided the free oxygen forming a major constituent of the present atmosphere. It is reasonable to conclude that this oxygen has been liberated from gaseous oxides such as water vapour and carbon dioxide. Since a large-scale dissociation of these compounds can only be brought about by solar radiation, photochemical activity must be responsible for the presence of molecular oxygen in the earth's atmosphere.In this context, we have to distinguish between 1. an inorganic photochemical process working with the energy-rich short wave-lengths of the solar spectrum (2 between 1500 and 2100 Å) and 2. an organic or biologic photochemical process which utilizes the spectral range of the visible light. In addition, the biologic process requires a porphyrin body (chlorophyll) to be present as a catalyst. The oxygen-producing capacity of the inorganic process, i. e., photodissociation of water vapour, is considerably smaller (about 10^–3) than that of the organic process (photosynthesis of carbohydrates from CO₂ and H₂O, releasing oxygen as a metabolic by-product). Accordingly, the bulk of the oxygen now present in the atmosphere may safely be regarded as being biologic in origin.Thus, the geochemical evolution of the atmosphere and the evolution of life are closely related and interdependent. The rise of the oxygen pressure of the ancient atmosphere to about 10^–2 P.A.L. (Present Atmospheric Level) enabled life to pass thePasteur point and change from fermentative (anaerobic) to respiratory (aerobic) metabolism. There is reason to believe that the appearance of the first metazoan faunas in the Late Precambrian, about 0.7×10⁹ years ago (Nama-Ediacara fauna), was a direct result of the enormous rise in metabolic energies derived from the more efficient aerobic metabolism.Life as a nonequilibrium steady state, being characterized by an entropy far below the level of its surroundings, requires a constant supply with free energy for its maintenance. As may be concluded from the impact of respiration on the diversification of life at the dawn of the Phanerozoic, this diversification was also dependent on an increased supply in free energy. Evolution thus being— just as life itself — an energy-absorbing (endergonic) process, it must be regarded as highly improbable in terms of theBoltzmann theory of thermodynamics. The mechanism of respiration, using oxygen as a final hydrogen acceptor in energyreleasing biologic redox processes (thereby increasing metabolic energies by the factor of fourteen), obviously played the role of the Maxwellian demon which caused the process of evolution to go into the direction of more order (or diversification) and less entropy.

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Résumé Le présent travail donne un aperçu de l'état actuel de nos connaissances concernant l'évolution de l'atmosphère terrestre depuis le Précambrien ancien. La transformation de l'atmosphère primitive réductrice en l'atmosphère actuelle oxydante paraît, pour la plus grande part, être due à des facteurs biologiques. En raison du parallélisme frappant entre l'évolution organique et l'évolution atmosphérique, la Paléontologie revêt une importance particulière pour l'histoire de l'atmosphère terrestre.

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. , (₂, ₂). . . , . : 1) , ( ) ( 1500–2100 ), 2) , (). . . ( ) , ( O_{2 2}O , ), , . . . . ( 10^–2 ) , — . , ( ), , , .

     

Geochemical trends across an arc-continent collision zone: magma sources and slab-wedge transfer processes below the Pantar Strait volcanoes, Indonesia

Four volcanoes in the Pantar Strait, the westernmost part of the extinct sector of the east Sunda arc, show remarkable across-arc variation in elemental abundances (K₂O: 1.2 to 4.3%), trace element ratios (Pb/Ce: 0.4 to 0.18; Ce/Yb: 20 to 55) and isotope ratios (¹⁴³Nd/¹⁴⁴Nd: 0.51263 to 0.51245; ⁸⁷Sr/⁸⁶Sr: 0.7053 to 0.7068; ²⁰⁶Pb/²⁰⁴Pb: 19.29 to 19.15). Pb isotopes are decoupled from Sr and Nd isotopes, with the frontal volcanoes showing the higher Nd and Pb and lower Sr isotopic ratios. The isotopic and trace element ratios of the volcanic samples are best explained by modification of a MORB-type source (with Indian Ocean island basalt-type Pb isotopic characteristics) by a fluid and a partial melt of subducted continental material (SCM). The frontal volcano contains the highest proportion of the fluid component, with a small contribution of partial melt. The source of the rear-arc volcano is strongly influenced by a partial melt of SCM that had undergone a previous dehydration event, by which it lost most of its fluid-mobile elements such as Pb. The SCM partial melt was in equilibrium with both rutile and garnet, whereas mantle melting took place in the presence of residual mica. The relatively large across-arc increase in incompatible elements can be explained by a combination of increasing addition of SCM partial melt, changing mantle wedge fertility and smaller degrees of partial melting toward the rear of the arc. Comparison with a more westerly across-arc transect shows that the relatively low ¹⁴³Nd/¹⁴⁴Nd ratios of the frontal volcano, and the decoupling of Pb from Sr and Nd isotopes are unique to the Pantar Strait volcanoes. This is likely to reflect magma generation in a collisional environment, where the leading edge of the Australian continent, rather than subducted sediment, contributes to the magma source.     

Surface-rupturing history of the Bree fault scarp, Roer Valley graben: Evidence for six events since the late Pleistocene

Since 1996 paleoseismological investigations have been used to develop the surface- rupturing history of the Bree fault scarp, the morphologically best-defined segment of the southwestern border fault of the Roer Valley graben in northeastern Belgium. The first studies determined that the escarpment is associated with a surface fault, and they exposed evidence for three surface displacements since about 40 ka BP. The most recent eventprobably occurred between 1000 and 1350 yr cal BP. Geophysical and trenching studies at a new site near the southeastern end of the fault scarp reconfirmed the coincidence of the frontal escarpment with a shallow normal fault, which displaces the Middle Pleistocene `Main Terrace' of the Maas River, as well as overlying coversands of Saalian to late Weichselian age. Different amounts of displacement shown by the two youngest coversand units indicate two discrete faulting events, but primary evidence for the coseismic nature of these events is sparse. Radiocarbon and optically stimulated luminescence dating constrainthe age of these events to the Holocene and between 14.0 ± 2.3 ka BP and 15.8 ± 2.9 ka BP, respectively. In addition, four older surface-rupturing events are inferred from the presence of four wedge-shaped units of reworked Main Terrace deposits that are interbedded with coversand units in the hanging wall of the trench and in shallow boreholes. These wedges are interpreted as colluvial wedges, produced by accelerated slope processes in response torejuvenation of the fault scarp, most probably in a periglacial environment. Luminescence dating indicates that five out of a total of six identified faulting events are younger than 136.6 ± 17.6 ka. The antepenultimate event was the largest faulting event, associated with a total fault displacement in excess of 1 m. Thus, the newly investigated trench site represents the longest and most complete record of surface rupturing recovered so far along the Bree fault scarp. This study also demonstrates the viability of the paleoseismological approach to identify past large earthquakes in areas of present-day moderate to low seismic activity.     

Geochemical characterisation of organic matter in Keg River Formation (Elk point group, Middle Devonian), La Crete Basin, Western Canada

Carbonates from the Keg River Formation, La Crete Basin, Alberta, western Canada were examined in order to define: (a) oil source rock potential; (b) bulk maceral composition; (c) extract yield; and finally (d) facies variations using PY-GC-MS. Thirty samples from 6 different wells were examined from the lower Keg River member and 4 from the upper Keg River member using conventional geochemical methods. As maturity differences are absent within the sample set, variations in TOC, T_max, hydrogen index, organic petrography and extract yields are caused by variability in organic matter input, which is revealed by molecular characterisation using PY-GC-MS. Lower Keg River member bituminous wackestones are excellent potential source rocks containing Types I–II and Type II organic matter. Types I–II organic matter contains large well preserved (up to 200μm in diameter) thick-walled Tasmanites (10–15% of sample) and akinete algal cells indicative of algal blooms within an amorphous bituminite. Type II organic matter contains a higher proportion of degraded alginites/bituminite relative to well-preserved alginites. Extract yields (mg/g TOC) were seen to increase from Types I–II to Type II organic matter. PY-GC-MS revealed that 1,2,3,4-tetramethylbenzene was a major peak in most samples. This is a pyrolysis product arising from β-cleavage of C₄₀ diaromatic carotenoids incorporated within the kerogen during diagenesis. The source of this compound is thought to be from an unknown diaromatic compound with a 2,3,6-/3,4,5-trimethyl substitution pattern and isorenieratene, which is specific to photosynthetic green sulphur bacteria (Chlorobiaceae) suggesting that the photic zone was at least partially anoxic during deposition of these samples. The relative abundance of this compound/n-C11-alkene and organic sulphur (calculated from the thiophene ratio) both increase from Types I–II to Type II organic matter. This trend was grossly similar to the trend seen in the variability of extract yield with hydrogen index. A similar trend for HI and T_max indicates samples containing a higher proportion of degraded alginites/bituminite relative to well-preserved alginite are more labile than Type I–II organic matter. Upper Keg River member marls contain Type II organic matter, which is characterised by heavily degraded algal material within a bituminous groundmass. Pyrolysates of two of the marl samples contain only low amounts of 1,2,3,4-tetramethylbenzene, in contrast to the bituminous wackestones, indicating that the depositional environment/source input was different during deposition of the marl samples. Although both marls contain similar organic matter, their pyrolysates were significantly different. One marl (1141.9 m) was highly paraffinic containing dominantly short-chain alkene/alkane doublets, while the other marl (1137.6 m) contained a bimodal n-alkane/alkene distribution and high amounts of alkylphenols, which may be derived from preservation of resistant algal polyphenolic molecules or suggest a terrestrial input.     

Distribution and Migrations of two Cerithid Snails on a Sand Flat in Bermuda

Abstract. Two species of cerithid prosobranchs live on a sand flat in Bermuda. At low tide during day light Batillaria minima is found on the surface whereas Cerithium lutosum remains buried in the sediment. This correlates with much higher lethal and coma temperatures, as well as much greater resistance to desiccation, of the former species. However, at other diurnal or tidal stages the two species co-occur in the same layer of sand. Despite greater resistance to physical factors the population of B. minima seems to be declining since in 1980 juveniles of this species were rare on Tuckers Town Flat whereas the population of C. lutosum was thriving.