Products and Mechanism of the NO<Subscript>3</Subscript> Reaction with Thiophene

Products of the gas-phase reaction of the NO₃ radical with thiophene have been investigated using different experimental systems. On the one hand, experiments have been conducted in our laboratory using two different methods, a Teflon static reactor coupled to a gas chromatograph combined with mass-spectrometry (GC-MS) and a discharge flow tube with direct MS spectroscopic detection. A qualitative analysis in these cases indicates that possible products for the reaction of thiophene+NO₃ at room temperature include: sulphur dioxide, acetic and formic acids, a short-chain aldehyde, 2-nitrothiophene and 3-nitrothiophene. On the other hand, quantitative experiments have been performed in the European Photoreactor (EUPHORE) in Valencia, Spain. In this case, the major products were: HNO₃ (≈80%), nitrothiophenes (≈30%), SO₂ (≈20%), propanal (3%) and a fraction of particles (≈10%). The results obtained indicate that at least 70% of the reaction of NO₃ with thiophene proceeds by an H-abstraction process at room temperature. The mechanism of the reaction studied is proposed on the basis of experimental results.     

Paleoecological and climatic changes of the Upper Lerma Basin, Central Mexico during the Holocene

The record of Almoloya Lake in the Upper Lerma basin starts with the deposition of the late Pleistocene Upper Toluca Pumice layer. The data from this interval indicate a period of climatic instability that lasted until 8500 cal yr B.P., when temperature conditions stabilized, although moisture fluctuations continued until 8000 cal yr B.P. Between 8500 and 5000 cal yr B.P. a temperate climate is indicated by dominance of Pinus. From 5000 to 3000 cal yr B.P. Quercus forest expanded, suggesting a warm temperate climate: a first indication of drier environmental conditions is an increase in grassland between 4200 and 3500 cal yr B.P. During the Late Holocene (3300 to 500 cal yr B.P.) the increase of Pinus and grassland indicates temperate dry conditions, with a considerable increase of Pinus between 1100 and 950 cal yr B.P. At the end of this period, humidity increased. The main tendency during the Holocene was a change from humid to dry conditions. During the Early Holocene, Almoloya Lake was larger and deeper; the changing humidity regime resulted in a fragmented marshland, with the presence of aquatic and subaquatic vegetation types.     

Burial metamorphic episodes in the Andean geosyncline,Central Chile

The burial metamorphism of the Andean geosynclinal deposits of Central Chile is studied. The stratigraphic units under consideration, Lower Jurassic to Upper Cretaceous and/or Lower Tertiary in age, have an accumulated thickness of 15,000 to 28,000 m, cover an area of nearly 2,500 km², and consist predominantly of basic and acid lavas, ignimbrites and volcanic-clastic sediments, deposited in marine and continental environments. Unconformities break up the sequence into several stratigraphic-structural units.A specific burial metamorphic pattern, or series, characterizes each of the units. Each series consists of a succession of mineral assemblages, the metamorphic grade increasing downwards. The isograds are essentially parallel to the bedding planes. The grade range in each series covers different intervals between the zeolite and greenschist facies. Each unconformity corresponds to a mineralogie break in the alteration, higher grade assemblages generally overlying lower grade assemblages. The downward increase in metamorphic grade as well as the mineralogie breaks seem to be unrelated to the volcanic versus sedimentary character and to the continental versus marine depositional environment of the rocks.The repeated pattern of burial metamorphism between unconformities suggests a history consisting of several burial metamorphic episodes, each taking place prior to a folding period and leaving unaffected the underlying units, which were apparently sealed up by the previous episode. This mechanism provides an explaination for the presence of such low-grade burial metamorphic facies at the bottom of a stratigraphic column as thick as that of the Andean Geosyncline.

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Zusammenfassung Die Auswirkungen der Versenkungsmetamorphose auf die Ablagerungen der andinen Geosynklinale in Mittel-Chile wurden untersucht. Die betrachteten stratigraphischen Einheiten, unterer Jura bis obere Kreide und/oder unteres Tertiär, haben eine kumulative Mächtigkeit von 15 000 bis 28 000 m; sie bedecken ein Gebiet von fast 2500 km² und bestehen in der Hauptsache aus basischen und sauren Laven, Ignimbriten und vulkanoklastischen Sedimenten, die unter marinen und kontinentalen Bedingungen abgelagert wurden. Diskordanzen unterteilen die Abfolge in mehrere stratigraphisch-strukturelle Einheiten.Jede Einheit ist in einer besonderen, für die Versenkungsmetamorphose typischen Abfolge von Mineralgesellschaften geprägt. Der Grad der Metamorphose steigt mit zunehmender Tiefe. Die Isograden verlaufen im wesentlichen parallel zu den Schichtflächen. Die Bandbreite jeder der metamorphen Mineralgesellschaften bedeckt verschiedene Intervalle zwischen der Zeolith- und Grünschiefer-Fazies. Jeder Diskordanz entspricht auch ein Sprung in der mineralogischen Zusammensetzung, wobei die höhergradigen Mineralgesellschaften in der Regel die tiefergradigen überlagern. Der mit der Tiefe zunehmende Grad der Metamorphose und die Sprünge in der mineralogischen Zusammensetzung scheinen von dem vulkanischen bzw. sedimentären Charakter der Gesteine ebenso unabhängig zu sein wie von deren marinen bzw. kontinentalen Bildungsbedingungen.Die sich wiederholenden Muster der Versenkungsmetamorphose zwischen den Diskordanzen lassen vermuten, daß mehrere Metamorphosen stattfanden, wobei jede einzelne Umwandlung vor einer Faltungsperiode ablief und die unterlagernden Einheiten unverändert ließ, die anscheinend durch die vorangegangene Metamorphose versiegelt wurden. Dieser Mechanismus liefert eine Erklärung für die niedriggradigen Fazies der Versenkungsmetamorphose im Liegenden einer so mächtigen stratigraphischen Abfolge wie der der andinen Geosynklinale.

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Resumen En este trabajo se describe el metamorfismo de sepultamiento en los depósitos del Geosinclinal Andino. Las unidades estratigráficas estudiadas son de edad jurásica inferior a cretácica superior y/o terciaria inferior, tienen un espesor acumulado de 15.000 a 28.000 m, cubren un area de aproximadamente 2.500 km² y consisten predominantemente de lavas básicas y ácidas, ignimbritas y rocas sedimentario-volcánicas, depositadas en ambiente marino y continental.Cada una de las unidades estratigráficas está caracterizada por una serie metamórfica de sepultamiento específica. Cada serie consiste de una sucesión de asociaciones minerales con un grado de metamorfismo que aumenta hacia abajo. Las isogradas son esencialmente paralelas a los planos de estratificación. El rango de grado metamórfico de cada serie abarca diferentes intervalos entre las facies de zeolita y esquistos verdes. Cada discordancia corresponde a una discontinuidad mineralógica en la alteración en la que generalmente las asociaciones de mayor grado metamórfico cubren asociaciones de menor grado. El aumento de grado hacia abajo, lo mismo que las discontinuidades mineralógicas, parecen ser independientes tanto del carácter sedimentario o volcánico como del ambiente continental o marino de la depositación.La repetición de series metamórficas de sepultamiento entre discordancias, indica varios episodios metamórficos, cada uno de ellos anterior al plegamiento de la unidad afectada; al mismo tiempo, cada episodio metamórfico dejó sin afectar las unidades subyacentes, las que aparentemente habrían sido selladas por el episodio anterior.Este mecanismo explicaría la presencia de una facies metamórfica tan baja como la que existe en el fondo de una columna estratigráfica tan potente como la del Geosinclinal Andino.

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