Age and chemical composition of magmatic rocks from the Belkovsky Island, New Siberian Islands

The study was inspired by information on Paleozoic andesites, dacites, and diabases on the Belkovsky Island in the 1974 geological survey reports used to reconstruct tectonic evolution of the continental block comprising the New Siberian Islands and the bordering shelf. We did not find felsic volcanics or Middle Paleozoic intrusions in the studied area of the island. Igneous rocks are mafic subvolcanic intrusions including dikes, randomly shaped bodies, explosion breccias, and peperites. They belong to the tholeiitic series and are similar to Siberian traps in petrography and trace-element compositions, with high LREE and LILE and prominent Nb negative anomalies. The island arc affinity is due to continental crust contamination of mantle magma and its long evolution in chambers at different depths. K-Ar biotite age (252+/-5 Ma) of magmatism indicates that it was coeval to the main stage of trap magmatism in the Siberian craton at the Permian-Triassic boundary. The terrane including the New Siberian Islands occurred on the periphery of the Siberian trap province where magmatism acted in rifting environment. Magma intruded into semiliquid wet sediments at shallow depths shortly after their deposition. Therefore, the exposed Paleozoic section in Belkovsky Island may include Permian or possibly Lower Triassic sediments of younger ages than it was believed earlier.

La Colección Austral : 75 años de cultura en el bolsillo (1937-2012)

En 2012 la emblemática Colección Austral, pionera en el libro de bolsillo, cumplirá 75 años habiendo logrado pasar de colección a sello editorial dentro del Grupo Planeta, quien la adquirió al hacerse cargo de la editorial Espasa a comienzos de la década de los noventa. Es objeto de este artículo el análisis del significado, evolución y desarrollo de Austral desde su creación en 1937 hasta la actualidad, dentro del marco de la historia de la edición, considerando tanto el fondo como la forma; es decir, el contenido y el diseño. La metodología empleada ha sido el estudio de la bibliografía sobre el tema, y en especial de la propia colección en todos sus modelos. Se ha elaborado una base de datos con autores, títulos y fechas de publicación, para lo que ha sido necesario consultar ejemplares en diversas bibliotecas públicas y privadas con el fin de revisar todos los números. Se ha investigado asimismo sobre todas las variaciones de la colección, resultando seis modelos diferentes. Se concluye que la Colección Austral es paradigma del libro de bolsillo, agente difusor de la cultura en Iberoamérica y un proyecto difícilmente repetible

Composition and age of volcanic rocks from the Sinii Utes Depression, Southern Primoye

New data are reported on structure of sections, chemical composition, and age of volcano-sedimentary and volcanic rocks from the Sinii Utes Depression in the Southern Primorye region. The Sinii Utes Depression is filled with two sequences: the lower sequence composed of sedimentary-volcanogenic coaliferous rocks (the stratotype of the Sinii Utes Formation) and the upper sequence consisting of tephroid with overlying basalts. This work considers chemical composition and problems of K-Ar dating of basalts. The uppermost basaltic flow has K-Ar age 22.0±1.0 Ma. The dates obtained for the middle and upper parts of lava flows are underestimated. It is explained by their heating due to combustion of brown coals of the Sinii Utes Formation underlying the lava flow. Calculations show that argon could only partly have been removed from the basalts owing to conductive heat transfer and was lost largely due to infiltration of hot gases in heterogeneous fissured medium. Basaltic volcanism on continental margins of the southern Primorye region and the adjacent Korean and Chinese areas at the Oligocene-Miocene boundary preceded Early-Middle Miocene spreading and formation of the Sea of Japan basin. Undifferentiated moderately alkaline basalts of intraplate affinity developed in the Amba Depression and some other structures of the southern Primorye region and intraplate alkali basalts of the Phohang Graben in the Korean Peninsula serve as indicators of incipient spreading regime in the Sea of Japan. Potassic basalt-trachybasalt eruptions occurred locally in riftogenic depressions and shield volcanoes. In some structures this volcanism was terminated by eruptions of intermediate and acid lavas. Such evolution of volcanism is explained by selective contamination of basaltic melts during their interaction with crustal acid material and generation of acid anatectic melts.

Organic geochemistry of sediment samples from Prydz Bay

High molecular weight aliphatic hydrocarbons were extracted from sediments at two sites (741 and 742) drilled during Ocean Drilling Program Leg 119 in Prydz Bay, a major embayment on the continental shelf of East Antarctica. The distributions of n-alkanes and triterpenoid and steroid hydrocarbons suggest that the n-alkanes and steranes are mainly of terrestrial origin and that the hydrocarbons are immature to slightly mature in the Lower Cretaceous sediments and immature to mature in the Tertiary sediments. At Site 741, the Lower Cretaceous depositional sequence, which is generally characterized by immature hydrocarbons, is interrupted by sediment having more mature components, suggesting a change of source during part of Early Cretaceous time. At Site 742, the mature geochemical parameters of a Pliocene sample correlate with results reported elsewhere for Site 739. In all but one of the other Tertiary samples, the geochemical parameters indicate intermediate maturity. The Lower Cretaceous and Pliocene sediments average about 1.9% organic carbon, a value of interest from the point of view of potential sources of petroleum offshore from Antarctica.

Migration of C1 to C8 volatile organic compounds in sediments at DSDP Hole 75-530A

The distribution of C1 to C8 hydrocarbons in sediment samples from DSDP Leg 75, Hole 530A, indicates that significant amounts of methane and ethane have migrated from organic-rich to organic-lean shales in close proximity. Most compounds larger than ethane are not migrating out of black shales, where they occur in high concentrations. These results lead to a general model for assessing migration. In addition, three shale types are identified on the basis of organic carbon and pyrolysis products and patterns.

Hydrous and anhydrous pyrolysis of DSDP Leg 75 kerogens

The aliphatic hydrocarbon distributions obtained from the natural bitumens of three Leg 75 sediments were compared using computerised gas chromatography-mass spectrometry (C-GC-MS). The kerogens isolated from these sediments were heated in sealed tubes at 330°C using the techniques of hydrous (i.e. heating kerogen in the presence of water) and anhydrous pyrolysis (i.e. heating dry kerogen alone). These experiments were then repeated at a lower temperature (280°C). At 330°C, under anhydrous conditions, considerable destruction of biomarkers in the ancient kerogens (i.e. pre-Tertiary) occurred, whereas with water present significant amounts of hopanes were obtained. However, with more recent kerogens (which contain larger amounts of chemically bound water), both anhydrous and hydrous pyrolysis gave a similar suite of biological markers, in which long chain acyclic isoprenoids (C40) are significant components. Lowering the temperature of pyrolysis to 280°C yielded biological markers under both hydrous and anhydrous conditions for all kerogens. n-Alkenes were not detected in any of the pyrolysates; however, a single unknown triterpene was discovered in several of the hydrous and anhydrous pyrolysates. The results tentatively indicate that the chief value to petroleum research of kerogen hydrous pyrolysis lies in its ability to increase the yield of pyrolysate. High temperature hydrous pyrolysis (280-330°C), under high pressure (2000 psi), does not appear to mimic natural conditions of oil generation. However, this study does not take into account whole rock pyrolysis.

Osmium isotope composition of ODP Hole 121-758A

This study presents osmium (Os) isotope and elemental data for cleaned planktic foraminifera, authigenic Fe-Mn oxyhydroxides and pelagic carbonate host sediments from ODP site 758 in the southernmost reaches of the Bay of Bengal. The Os in the bulk sediments appears to be dominantly hydrogeneous (sourced by carbonate and Fe-Mn oxyhydroxide), but variations in this particular core are controlled by the presence of volcanic ash. Fe-Mn oxyhydroxide leachates (of the bulk sediments) from Holocene samples also yield an Os isotope composition close to that of seawater, but the record diverges from that of foraminifera at a depth corresponding to the oxic/post-oxic boundary, suggesting diagenetic mobilization of Os at depths below this. Holocene planktic foraminifera, cleaned using oxidative-reductive techniques, also give Os isotope compositions indistinguishable from modern seawater, but the record obtained for the past 150 kyr shows strong covaraitions of 187Os/188Os with both the local and global oxygen isotope record, with less radiogenic Os isotope compositions during glacial intervals. These results indicate that foraminifera provide a robust record of seawater Os isotope compositions, and comparison of the data obtained here with records from the other major oceans demonstrate global changes in 187Os/188Os over this time interval, while the covariation with oxygen isotopes suggest a process controlling the Os isotope composition that is in phase with global climate cycles. Global excursions to relatively unradiogenic 187Os/188Os during glacial intervals are consistent with decreased input of radiogenic continental material, reflecting cooler temperatures and reduced continental runoff. Modelling indicates that the shift to unradiogenic values during glacial intervals could be caused by an ~30% decrease in the global river flux, with an ~5% change in river composition. If the residence time of Os in the oceans is ~5 ka then the post-glacial recovery to present-day seawater values is consistent with a corresponding increase in the river flux of around 30%. However, if the residence time of Os is closer to 40 ka, as is suggested by the global river flux, then this demands either significant changes in both the riverine Os flux and composition of around 40% and 30%, respectively, that closely follow the oxygen isotope record, or else a short-lived post-glacial pulse of weathering some 75% greater than the steady-state flux. In either case, these results clearly indicate that climatic changes affect both the flux and composition of weathered material delivered to the oceans on glacial-interglacial timescales.