The impact of the Laacher See Volcano (11 000 yr B.P.) on terrestrial vegetation and diatoms

Late-glacial lake sediments containing the Laacher See Tephra (LST, 11 000 yr B.P.) have been analyzed for their pollen and diatom content at three sites at varying distances from the volcano and on different bedrock geologies. The aim was to test the null hypothesis that this major volcanic eruption had no effect on terrestrial pollen or aquatic diatom assemblages. The pollen spectra at all sites show a short-lived increase in grass pollen following the LST. Partial redundancy analysis and associated Monte Carlo permutation tests suggest, however, that the LST had no statistically significant effect at two sites but it had a statistically significant impact on the pollen assemblages at the site nearest (60 km) to the volcano.The diatom assemblages at the three sites changed individualistically after the LST deposition, with increases inAchnanthes minutissima at one site, an expansion ofAulacoseira species at another, and an increase ofAsterionella formosa andFragilaria brevistriata at the third site. Partial redundancy analysis and associated permutation tests suggest a statistically significant change in diatoms in relation to the LST and associated changes in sediment lithology at the one site situated on acidic bedrock. No significant impacts were found at the sites on volcanic or calcareous rocks. Due to the interaction between tephra and sediment lithology, it is not possible to conclude if the statistically significant diatom changes were a direct result of the LST deposition or an indirect result of lithological changes following LST deposition.This is the first paper in a series of papers published in this issue on high-resolution paleolimnology. These papers were presented at the Sixth International Paeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. F. Lotter and Dr. M. Sturm served as guest editors for these papers.     

Magnetometer Array Study in North-Northeast Brazil: Conductivity Image Building and Functional Induction Modes

—Magnetovariational fields recorded by an array of magnetometers in the equatorial region of north-northeast Brazil are analyzed to infer the configuration of internal induced currents in and around the extensive intracratonic Parnaíba basin. Only nighttime magnetovariational fields were used because of the prevailing uniform source field conditions. For periods exceeding 40 min. the vertical fields at all inland sites are dominated by the effects of electric currents originating in the northeast, in the deep Atlantic Ocean. Below this period, although best developed in the 12–15 min. period range, the anomalous signatures are principally controlled by two distinct continental current paths. The first is associated with a N60°E trending graben-like structure in the southeastern part of the basin (named the Parnaíba Basin Conductivity Anomaly—PBCA) and the second appears as a subsurface sedimentary channel, from the NW corner of the array to the central part of the basin. This is named the LINK anomaly, as it connects the northwestern Marajó basin with the Parnaíba basin. While the PBCA is shown to highlight the importance of basement tectonics in the geological evolution of the Parnaíba basin, the LINK anomaly provides strong geophysical evidence of the direction of the sea intrusion into the region of the basin and possibly indicates the connectivity of the Parnaíba basin to the adjoining Amazon basin through the Marajó basin. Frequency and polarization dependence suggest that the induction response of individual structures is not determined by the local conductivity alone but also by their interconnectivity as well as by their linkage to the continental shelf and deep oceanic region.     

Review of Carboniferous-Permian volcanicity in Scotland

The main Carboniferous outcrop in Scotland is in the Midland Valley rift, though rocks of that age also occur farther south along the English Border. The cyclic sedimentary sequence comprises up to 3,000 m. of Dinantian (mainly Viséan) and 2,000 m. of Silesian beds which at one locality or another include lavas or tuffs almost at every level. The distribution of these volcanic rocks is known in exceptional detail because of mining and exploratory borings in the Viséan oil shale field and in the various Namurian and Westphalian coalfields.The lavas and tuffs form part of an alkaline (sodic) magma series. Their outpourings were greatest in volume during the Dinantian, with the formation of the extensive lava piles which now form the Clyde Plateau (maximum thickness c. 900 m.) in the west, the Garleton Hills (c. 600 m.) and Burntisland Anticline (c. 400 m.) in the east, and the Kelso Traps of the Tweed Basin (c. 120 m.), the Birrenswark Lavas of Dumfriesshire (c. 90 m.), the Kershopefoot Basalt of Liddesdale (c. 60 m.) and the Glencartholm Volcanic Beds (mostly tuffs) of Eskdale (c. 180 m.) to the south. Most of the lavas are varieties of olivine-basalt, but subordinate trachybasalts, trachytes and rhyolites are included in the upper parts of the Clyde Plateau and Garleton Hills successions. The areas covered by individual lava fields are difficult to assess because they varied from time to time and occasionally overlapped.By the end of the Dinantian this form of activity had ceased everywhere except in West Lothian, where it continued into early Namurian (E₁). Thereafter volcanicity continued periodically from scattered centres and gave rise to relatively short-lived ash-cones. They formed in Ayrshire (E₁ age) but were most abundant in Fife, occurring there almost throughout the Namurian succession. The highly explosive nature of this phase of volcanicity is apparent from the rarity of associated lavas (though minor basaltic intrusions occur in the necks) and from the presence within the surrounding sediments of thin layers of tuff representing fine ash carried for distances of up to 32 km from source. These layers, some of which are kaolinized and are a variety of tonstein, are of local use in correlation.The explosive phase is now known to have continued throughout Westphalian A and possibly into Westphalian B in Fife, but in Ayrshire volcanicity was more intermittent and is represented by two outpourings of basalt lavas, one late Namurian to early Westphalian in age and up to 150 m. thick, the other Stephanian in age and 90 to 237 m. thick. The Stephanian lavas and associated necks in Ayrshire, like some of the necks exposed on the classic Fife coastal section on the Firth of Forth, were until recently believed to be Permian, but there is now no positive evidence of Permian eruption in Scotland.Two groups of intrusions cut the Carboniferous rocks. The most extensive is of alkaline dolerites which are co-magmatic with the basaltic lavas and tuffs, and are of similar geographic distribution. They form sills up to 120 m. thick and are probably of various ages up to Stephanian. It is possible that some of them were high-level reservoirs from which diatremes issued. The other group is of quartz-dolerite or tholeiite which forms a series of east-west dykes and a sill-complex extending over an area of about 1500 km² and having a maximum thickness of 127 m. Their age is late Carboniferous.The distribution of the Carboniferous volcanic rocks is not apparently related to the rift-valley, but is clearly linked in places to the earlier Caledonoid structures which determined the pattern of sedimentation. The lavas and, to a lesser extent, the tuffs, influenced the sedimentary processes by partially enclosing basins, by causing local anomalies in cyclic sequences and by providing sources of atypical sediment.

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Zusammenfassung Im Midland Valley (Schottland) war der permokarbonische Alkali-(Na)-Vulkanismus besonders im Dinantium aktiv, wobei bis 900 m mächtige Laven (Olivinbasalte, untergeordnet Trachytandesite, Trachyte und Rhyolite) gefördert wurden. Gegen Ende des Dinantium hörte diese Art der Aktivität fast ganz auf, dafür setzte eine explosive, periodische Tätigkeit ein, die zu kurzlebigen Aschenkegeln führte. Ein Beweis für das früher angenommene permische Alter dieser Necks läßt sich nicht erbringen.Zwei Gruppen von Intrusiva sind in die karbonischen Sedimente eingedrungen. Die größere der beiden wird durch Alkali-Dolerite (Sills) gebildet, die co-magmatisch zu den Laven und Tuffen sind. Die andere besteht aus Quarz-Doleriten und Tholeyiten (vorwiegend Dykes), die spätkarbonisch bis frühpermischen Alters sind.Die Verbreitung der karbonischen Vulkanite ist nicht an das Midland Valley, sondern an frühere caledonische Strukturen gebunden und für den Sedimentationsprozeß von Bedeutung.

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Résumé Le principal affleurement carbonifère en Ecosse se trouve dans le graben du Midland Valley, quoiqu'on rencontre les roches de cet âge le long de la frontière anglaise. La succession du cycle sédimentaire comprend jusqu'à 3,000 m. de roches de l'âge Dinantien (principalement Viséen) et à 2,000 m. des couches Silésiennes qui dans une localité quelconque comprennent des laves ou des tufs à presque tous les niveaux. La distribution de ces roches volcaniques est bien connu dans tous les détails à cause des mines et des forages.Les laves et les tufs font partie d'une série alkaline (sodique) magma. Leurs éruptions atteignirent leur maximum en volume pendant Ie Dinantien, avec la formation du Plateau de Clyde (épaisseur maximum 900 m. env.) à l'ouest, les Collines de Garleton (600 m. env.) et l'anticlinal de Burntisland (400 m. env.) a l'est, et les Kelso Traps du bassin du Tweed (120 m. env.), les Birrenswark Lavas de Dumfriesshire (90 m. env.), le Kershopefoot Basait de Liddesdale (60 m. env.) et les Glencartholm Volcanic Beds (des tufs pour la plupart) d'Eskdale (180 m. env.) au sud. La plupart des laves sont des variétés de basalte-olivine, mais des quantié peu considérables de trachybasalte, trachytes et rhyolites sont comprises dans les parties supérieures dans les successions du Plateau de Clyde et des Collines de Garleton.A la fin du Dinantien, cette forme d'activité avait cessé partout excepté dans l'Ouest Lothian, où elle continuait jusqu'au commencement du Namurien. Après cela le volcanisme continua périodiquement des centres dispersés, et donna naissance à des cônes de cendre de relativement petite durée. Ceux-ci se formèrent en Ayrshire (age E₁), mais on les trouvait beaucoup plus souvent en Fife, et on les y rencontrait presque pendant toute la succession namurienne. La nature explosive de cette phase de volcanisme est apparente par la rareté des laves et par la présence dans le sédiment qui se trouve dans les couches étroites de tuf, résultat du cendre fin porté jusqu'à 32 km. de sa source. Ces couches-ci dont quelques-unes furent transformées en kaolin, sont une variété de tonstein qui s'emploie beaucoup en corrélation. La distribution des quelques cheminées volcaniques coïncide aux structures nord—est d'un âge précarbonifère.La phase explosive est maintenant connue d'avoir continué à travers toute la Westphalie «Äs» et possiblement d'avoir pénétré dans la Westphalie «B» en Fife, mais en Ayrshire le volcanisme était plus intermittent et est représenté par deux éruptions de lave basaltique, l'une pendant la fin du Namurien jusqu'au début du Westphalien et d'une épaisseur de 150 m., l'autre du Stéphanien et d'une épaisseur de 90 à 237 m. Les laves du Stéphanien et les cheminées associées étaient classées auparavant comme Permien, mais maintenant il n'y a aucune évidence positive d'éruption permianique en Ecosse.Deux groupes d'intrusions scindèrent les roches carbonifères. Le groupe le plus étendu est fait de dolérites alkalines qui sont co-magmatiques avec les laves basaltiques et les tufs, et sont de pareille distribution géographique. Elles forment des silles d'une épaisseur jusqu; à 120 m. et elles sont probablement d'âges variés jusqu'au Stéphanien. Il est possible que quelques-une d'entre elles furent des réservoirs de haut-niveau desquels des diatrèmes sortirent. L'autre groupe est fait de quartz-dolérite ou tholéiite qui forme une série de dikes est—ouest et une sille-complexe couvrant une étendue de 1,500 km² et ayant une épaisseur maximale de 127 km. Leur áge est de la fin du Carbonifère ou du début du Permien.

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Published by permission of the Director, Institute of Geological Sciences.     

Lithium isotope systematics of lavas from the Cook–Austral Islands: Constraints on the origin of HIMU mantle

Radiogenic isotope variations in lavas from the Cook–Austral volcanic chain have delineated three distinct mantle sources: a HIMU component, a depleted component (DM), and an enriched component (EM). To better constrain the mantle sources for South Pacific hot spot volcanism, we determined lithium isotopic compositions of lavas from Raivavae, Rapa, Mangaia and Tubuai of the volcanic chain. The study includes whole rock and mineral analyses. In general, δ⁷Li of most olivines resemble bulk rock composition whereas clinopyroxenes are variably lighter. This implies that clinopyroxene is more susceptible to diffusion-induced fractionation, in agreement with previous studies. Olivine δ⁷Li values span a narrower range than whole rock values, and do not extend to the very heavy compositions previously reported in HIMU bulk lavas. This discrepancy likely reflects alteration of bulk lavas, and suggests that Li-isotope analyses of bulk lavas should be interpreted with caution. Olivines from HIMU lavas have heavy δ⁷Li values (up to 6.2‰), and extend beyond the range reported for fresh MORB. Correlations between Li-isotopes and radiogenic isotopes suggest that the heavy δ⁷Li values in HIMU olivines are a source characteristic and not the result of post-magmatic alteration. Although the Li-isotope composition of recycled, dehydrated oceanic crust is currently under debate, our results suggest that HIMU lavas derive from a source containing recycled dehydrated oceanic crust, and that the “heavy” Li-isotope signature of altered oceanic crust is partially preserved during passage through the subduction factory.     

Large-Scale Structures in Comet Hale–Bopp

The plasma tails of comets clearly show the demarcation of the solar wind into distinct equatorial and polar regions (Brandt and Snow (2000), Icarus 148, 52–64).The boundary is determined by the maximum extent in latitude of the heliospheric current sheet (HCS). The observational record contains many well-observed equatorial comets, but observations of comets in the polar region are relatively rare. In addition to its size and brightness, comet Hale–Bopp had an orbital inclination of 89.4° and was well observed for months in the polar region. We document the comet's large-scale appearance throughout the apparition, including the polar region and its transition into the equatorial region. The bright dust tail hampered observations of the plasma tail, particularly near the head, but images taken with a CO⁺ filter show a very large disconnection event (DE) on May 7 and May 8, 1997. The time of disconnection is estimated at approximately May 4.0. This DE is associated with a crossing of the HCS. The model calculations of the HCS indicate that other crossings might have occurred in late April, but given the uncertainty in the calculation, the comet might have missed the HCS. Sparse observational coverage and the bright dust tail prevent further investigation of the potential earlier HCS crossings. The plasma tail shows anomalous orientations at the highest latitudes and possible explanations are discussed.