Photometric monitoring of 2060 Chiron''s brightness at perihelion

The results of photometric and spectroscopic observations of comet/asteroid 2060 Chiron carried on at the Observatório do Pico-dos-Dias (Brazil), at the European Southern Observatory (Chile) and at the Mauna Kea Observatory (Hawaii) during 1996 are presented. The analysis of the photometric data shows that even at a minimum of brightness 2060 Chiron presents some activity. The absolute magnitude, H_v, varied from 6.79 in February to 6.22 in March. Therefore 2060 Chiron is still in a minimum of activity close to that of 1983–1985 and of 1994–1995.     

Integrated Radiolaria,benthic foraminifera and conodont biochronology of the pelagic Permian blocks/tectonic slices and geochemistry of associated volcanic rocks from the Mersin Mélange,southern Turkey: Implications for the Permian evolution of the northern Neotethys

Blocks and tectonic slices within the Mersin Mélange (southern Turkey), which are of Northern Neotethyan origin (Izmir–Ankara–Erzincan Ocean (IAE)), were studied in detail by using radiolarian, conodont, and foraminiferal assemblages on six different stratigraphic sections with well‐preserved Permian succesions. The basal part of the Permian sequence, composed of alternating chert and mudstone with basic volcanics, is assigned to the late Asselian (Early Permian) based on radiolarians. The next basaltic interval in the sequence is dated as Kungurian. The highly alkaline basic volcanics in the sequence are extremely enriched, similar to kimberlitic/lamprophyric magmas generated at continental intraplate settings. Trace element systematics suggest that these lavas were generated in a continental margin involving a metasomatized subcontinental lithospheric mantle source (SCLM). The middle part of the Permian sequences, dated by benthic foraminifera and conodont assemblages, includes detrital limestones with chert interlayers and neptunian dykes of middle Wordian to earliest Wuchiapingian age. Higher in the sequence, detrital limestones are overlain by alternating chert and mudstone with intermittent microbrecciated beds of early Wuchiapingian to middle Changhsingian (Late Permian) age based on the radiolarians. A large negative shift at the base of the Lopingian at the upper part of section is correlated to negative shifts at the Guadalupian/Lopingian boundary associated with the end‐Guadalupian mass extinction event. All these findings indicate that a continental rift system associated with a possible mantle plume existed during the late Early to Late Permian period. This event was responsible for the rupturing of the northern Gondwanan margin related to the opening of the IAE Ocean. When the deep basinal features of the Early Permian volcano‐sedimentary sequence are considered, the proto IAE oceanic crust formed possibly before the end of the Permian. This, in turn, suggests that the opening of the IAE Ocean dates back to as early as the Permian.     

Thermo-tectonic history of Ryoke Basement in Hohi volcanic zone, northeast Kyushu, Japan: Constraints from fission track thermochronology

Abstract Apatite and zircon fission track ages from Ryoke Belt basement in northeast Kyushu show late Cretaceous, middle to late Eocene, middle Miocene and Quaternary groupings. The basement cooled through 240 ± 25°C, the closure temperature for fission tracks in zircon, mainly during the interval 74-90 Ma as a result of uplift and denudation, the pattern being uniform across northeast Kyushu. In combination with published K-Ar ages and the Turonian-Santonian age of sedimentation in the Onogawa Basin, active suturing along the Median Tectonic Line from 100-80 Ma, at least, is inferred. Ryoke Belt rocks along the northern margin of Hohi volcanic zone (HVZ) cooled rapidly through ∼100°C to less than 50°C during the middle Eocene to Oligocene, associated with 2.5-3.5 km of denudation. The timing of this cooling follows peak heating in the Eocene-Oligocene part (Murotohanto subbelt) of the Shimanto Belt in Muroto Peninsula (Shikoku) inferred previously, and coincides with the 43 Ma change in convergence direction of the Pacific-Eurasian plate and the demise of the Kula-Pacific spreading centre. Ryoke Belt rocks along the southern margin of HVZ have weighted mean apatite fission track ages of 15.3 ± 3.1 Ma. These reset ages are attributed to an increase in geothermal gradient in the middle Miocene combined with rapid denudation and uplift of at least 1.4 km. These ages indicate that heating of the overriding plate associated with the middle Miocene start of subduction of hot Shikoku Basin lithosphere extended into the Ryoke Belt in northeast Kyushu. Pleistocene apatite fission track ages from Ryoke Belt granites at depth in the centre of HVZ are due to modern annealing in a geothermal environment.     

Dynamic evolution of the 1997–1999 El Niño–La Niña cycle in the southern California Current System

The development of the strongest El Niño event on record in the equatorial Pacific in 1997–1998 and the rapid transition to strong La Niña conditions in 1998–1999 had a large impact on the physical and biological environment of the West Coast. We investigate the evolution of the physical structure and circulation dynamics of the southern California Current System (CCS) during this period based on hydrographic data collected on 25 cruises over a 45-month period (February 1996–October 1999). The El Niño period was characterized by a significant increase in dynamic height, extreme water mass characteristics, a strengthening and broadening of the poleward nearshore flow, and a temporary reversal of net alongshore transport. By early 1999, conditions in the CCS had reversed. The data suggest that remotely driven forcing (propagating oceanic waves) contributed to the anomalies observed during the El Niño period, while the cool-water conditions of 1999 were most likely a result of anomalous local atmospheric forcing.     

Investigation of a group of rapidly rotating a stars having an emission excess at iras wavelengths

IRAS data on 13 normal and 4 shell A stars are analyzed to detect circumstellar matter around them. Most of the stars rotate rapidly; five of them have been studied before. By comparing the observed fluxes, IUE and ground-based, with Kurucz’s models in the 0.17-0.9 µm range, the stars’ parameters were determined ( $$T_{\hbox{\ \hbox{$\mid$}\kern -1em\lower .5em \hbox{$\leftarrow$}}} g_{\hbox{\ \hbox{$\mid$}\kern -1em\lower .5em \hbox{$\leftarrow$}}} $$ , and angular diameter), which were used to find the IR flux excesses over the photospheric emission. Infrared excesses were found in 15 of the stars. The excesses themselves are interpreted as thermal emission from dust particles with an effective temperature in the range from 70 to 200 K. The effective radius of the circumstellar disk is estimated to be from 7 to 400 R_*,     

Miocene shelf‐edge deltas and their impact on deepwater slope progradation and morphology,Northwest Shelf of Australia

Late‐middle Miocene to Pliocene siliciclastics in the Northern Carnarvon Basin, Northwest Shelf of Australia, are interpreted as having been deposited by deltas. Some delta lobes deposited sediments near and at the shelf break (shelf‐edge deltas), whereas other lobes did not reach the coeval shelf break before retreating landward or being abandoned. Shelf‐margin mapview morphology changes from linear to convex‐outward in the northern part of the study area where shelf‐edge deltas were focused. Location and character of shelf‐edge deltas also had significant impact on along‐strike variability of margin progradation and shelf‐edge trajectory. Total late‐middle and late Miocene margin progradation is ca. 13 km in the south, where there were no shelf‐edge deltas, vs. ca. 34 km in the north where shelf‐edge deltas were concentrated. In the central area, the deltas were arrested and accumulated a few kilometres landward of the shelf break, resulting in an aggradational shelf‐edge trajectory, in contrast to the more progradational trajectory farther north. This illustrates a potential limitation of shelf‐edge trajectory analysis: only where shelf‐edge deltas occur, there is sufficient sediment available for the shelf‐edge trajectory to record relative sea‐level fluctuations reliably. Small‐scale (ca. 400 m wide) incisions were already conspicuous on the coeval slope even before deltas reached the shelf break. However, slope gullies immediately downdip from active shelf‐edge deltas display greater erosion of underlying strata and are wider and deeper (>1 km wide, ca. 100 m deep) than coeval incisions that are laterally offset from the deltaic depocenter (ca. 0.7 km wide, ca. 25 m deep). We interpret this change in slope‐gully dimensions as the result of greater erosion by sediment gravity flows sourced from the immediately adjacent shelf‐edge deltas. Similarly, gullies also incised further (up to 6 km) into the outer shelf in the region of active shelf‐edge deltas.     

Rivers turned to rock: Late Quaternary alluvial induration influencing the behaviour and morphology of an anabranching river in the Australian monsoon tropics

Late Quaternary alluvial induration has greatly influenced contemporary channel morphology on the anabranching Gilbert River in the monsoon tropics of the Gulf of Carpentaria. The Gilbert, one of a number of rivers in this region, has contributed to an extensive system of coalescing low-gradient and partly indurated riverine plains. Extensive channel sands were deposited by enhanced flow conditions during marine oxygen isotope (OI) Stage 5. Subsequent flow declined, probably associated with increased aridity, however, enhanced runoff recurred again in OI Stages 4–3 (65–50 ka). Aridity then capped these plains with 4–7 m of mud. A widespread network of sandy distributary channels was incised into this muddy surface from sometime after the Last Glacial Maximum (LGM) to the mid Holocene during a fluvial episode more active than the present but less so than those of OI Stages 5 and 3. This network is still partly active but with channel avulsion and abandonment now occurring largely proximal to the main Gilbert flow path.A tropical climate and reactive catchment lithology have enhanced chemical weathering and lithification of alluvium along the river resulting in the formation of small rapids, waterfalls and inset gorges, features characteristic more of bedrock than alluvial systems. Thermoluminescence (TL) and comparative optically stimulated luminescence (OSL) ages of the sediments are presented along with U/Th ages of pedogenic calcrete and Fe/Mn oxyhydroxide/ oxide accumulations. They show that calcrete precipitated during the Late Quaternary at times similar to those that favoured ferricrete formation, possibly because of an alternating wet–dry climate. Intense chemical alteration of the alluvium leading to induration appears to have prevailed for much of the Late Quaternary but, probably due to exceptional dryness, not during the LGM. The result has been restricted channel migration and a reduced capacity for the channel to adjust and accommodate sudden changes in bedload. Consequent avulsions have caused local stream powers to increase by an order of magnitude, inducing knickpoint erosion, local incision and the sudden influx of additional bedload that has triggered further avulsions. The Gilbert River, while less energetic than its Pleistocene ancestors, is clearly an avulsive system, and emphasizes the importance in some tropical rivers of alluvial induration for reinforcing the banks, generating nickpoints, reworking sediment and thereby developing and maintaining an indurated and anabranching river style.     

Spatiotemporal drought variability in northwestern Africa over the last nine centuries

Changes in precipitation patterns and the frequency and duration of drought are likely to be the feature of anthropogenic climate change that will have the most direct and most immediate consequences for human populations. The latest generation of state-of-the-art climate models project future widespread drying in the subtropics. Here, we reconstruct spatially-complete gridded Palmer drought severity index values back to A.D. 1179 over Morocco, Algeria, and Tunisia. The reconstructions provide long-term context for northwest African hydroclimatology, revealing large-scale regional droughts prior to the sixteenth century, as well as more heterogeneous patterns in sixteenth, eighteenth, and twentieth century. Over the most recent decades a shift toward dry conditions over the region is observed, which is consistent with general circulation model projections of greenhouse gas forced enhanced regional subtropical drought.     

Eine Deutung der D-Schicht-Winter-Anomalie und des “Berliner Phänomens”

Zusammenfassung Die bisher ungeklärte Winter-Anomalie der D-Schicht sowie die explosionsartigen Erwärmungen der Hochstratosphäre im Winter können durch Staubpartikel interplanetaren Ursprungs (Meteorströme) hervorgerufen werden. Eine Anlagerung von Elektronen an die Staubteilchen in der Exosphäre bewirkt das Einfangen der kleinsten Partikel durch das erdmagnetische Feld und verhindert so ihr Verglühen in der Ionosphäre. Durch Sedimentation und turbulente Diffusion gelangt der Meteorstaub bis in die Ozonosphäre, wo er oxydiert wird und so eine plötzliche Wärmeentwicklung zur Folge hat. Die angelagerten Elektronen exosphärischen Ursprungs können bei zeitgerechter Photoablösung elektromagnetische Wellen dämpfen und so die Winter-Anomalie der D-Schicht hervorrufen.

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Summary The winter-anomaly of the ionospheric D-region for which no sufficient explanation existed up to now, as well as the explosive warming of the high stratosphere in wintertime are possibly caused by dust particles of inter-planetary origin (meteoric showers). The attachment of electrons on the dust particles in the exosphere causes the magnetic field of the earth to catch the smallest particles and so they do not evaporate in the ionosphere. By sedimentation and turbulent diffusion the meteoric dust reaches the ozonosphere where it is oxidized, which is followed by a sudden development of heat. After photo-detachment in due time the formerly attached electrons of exospheric origin are able to damp electro-magnetic waves and by this they cause the winter-anomaly of the D-region.

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Résumé L'anomalie de la couche D — inexplicable jusqu'ici — ainsi que les hausses prodigieusement rapides de la température de la haute stratosphère, deux phénomènes particuliers de l'hiver, peuvent être provoquées par des particules de poussière d'origine interplanétaire (courants de météorites). Des électrons adhérant, dans l'exosphère, aux grains de poussière permettent l'entraînement des plus petites particules par le champ magnétique terrestre et empèchent ainsi leur désintégration dans l'ionosphère. Par sédimentation et diffusion turbulente, la poussière cosmique parvient ensuite jusque dans l'ozonosphère où elle est oxydée. Il en résulte alors un dégagement subit de chaleur. Les électrons d'origine exosphérique adhérant à la poussière cosmique peuvent atténuer, s'ils s'en détachent en temps opportun sous l'effet du rayonnement solaire, les ondes électromagnétiques et provoquer ainsi l'anomalie d'hiver de la couche D.