Geochemistry of samples from Valtournenche, Western Italian Alps

Slices of polycyclic metasediments (marbles and meta-cherts) are tectonically amalgamated with the polydeformed basement of the Dent Blanche tectonic system along a major Alpine shear zone in the Western Alps (Becca di Salé area, Valtournenche Valley). A combination of techniques (structural analysis at various scales, metamorphic petrology, geochronology and trace element geochemistry) was applied to determine the age and composition of accessory phases (titanite, allanite and zircon) and their relation to major minerals. The results are used to reconstruct the polyphase structural and metamorphic history, comprising both pre-Alpine and Alpine cycles. The pre-Alpine evolution is associated with low-pressure high-temperature metamorphism related to Permo-Triassic lithospheric thinning. In meta-cherts, microtextural relations indicate coeval growth of allanite and garnet during this stage, at ~ 300 Ma. Textures of zircon also indicate crystallization at HT conditions; ages scatter from 263-294 Ma, with a major cluster of data at ~ 276 Ma. In impure marble, U-Pb analyses of titanite domains (with variable Al and F contents) yield apparent 206Pb/238U dates range from Permian to Jurassic. Chemical and isotopic data suggest that titanite formed at Permian times and was then affected by (extension-related?) fluid circulation during the Triassic and Jurassic, which redistributed major elements (Al and F) and partially opened the U-Pb system. The Alpine cycle lead to early blueschist facies assemblages, which were partly overprinted under greenschist facies conditions. The strong Alpine compressional overprint disrupted the pre-Alpine structural imprint and/or reactivated earlier structures. The pre-Alpine metamorphic record, preserved in these slices of metasediments, reflects the onset of the Permo-Triassic lithospheric extension to Jurassic rifting.

Geochemistry and isotopes at DSDP Hole 79-546

At Site 546, below the Mazagan Escarpment at a water depth of 4 km, 36 m of salt rock was cored from the top of one of a field of salt domes. The core was studied by thin section and a variety of geochemical procedures. The salt rock contains 0.1 to 3% carnallite and lesser amounts of sylvite and polyhalite, which with the corresponding high level of bromide place it within the potash evaporite facies. The bromide profile is of a dominantly marine evaporite deposited in moderately shallow brine which, however, was not repeatedly desiccated. A mineralogical argument suggests that the brine surface was not below sea level. An average of about 5% elastics, with dispersed anhydrite, darken the salt rock to deep shades of red, brown, and gray green. Most of the included materials are in highly deformed boudins or dispersions in the salt rock that has also undergone cataclasis in a subsequent, probably tectonic, deformation. The salt rock is slightly deficient in anhydrite, and the usual separate beds and laminae of anhydrite are virtually absent. Stable isotope ratios of sulfur and oxygen in the sulfate are clearly derived from sea water of Permian to Scythian age, in contrast to the late Triassic or Early Jurassic age of evaporites onshore in Morocco and Portugal and the corresponding evaporites offshore Maritime Canada. In contrast to those evaporites off the axis of Atlantic rifting, the salt at Site 546 may have been deposited in a very early central rift fed by marine waters from Tethys through the Gibraltar or South Atlas fracture zones.

Stable isotopes and element concentrations of samples from the Wadi Bih, United Arab Emirates

Ocean acidification triggered by Siberian Trap volcanism was a possible kill mechanism for the Permo-Triassic Boundary mass extinction, but direct evidence for an acidification event is lacking. We present a high-resolution seawater pH record across this interval, using boron isotope data combined with a quantitative modeling approach. In the latest Permian, increased ocean alkalinity primed the Earth system with a low level of atmospheric CO2 and a high ocean buffering capacity. The first phase of extinction was coincident with a slow injection of carbon into the atmosphere, and ocean pH remained stable. During the second extinction pulse, however, a rapid and large injection of carbon caused an abrupt acidification event that drove the preferential loss of heavily calcified marine biota.

Tab. 2: Average geochemical composition of dolerites from Neuschwabenland

In western Neuschwabenland basic dikes occur in the Jurassic lavas and Permian sediments of Vestfjella as weil as in the Precambrian sedimentary-volcanogenic rock sequence of the Ahlmannryggen and in the Precambrian crystalline complexes of Heimefrontfjella and Mannefallknausane. The concentration of the dikes in Vestfjella is conspicuous. Two main directions of strike perpendicular to each other are recognizable, from which the NE-SW striking one is predominant. The direction of the dikes coincides with the Mesozoic and younger fracture tectonics. Age relationships by structural, petrographical and geochemical observations are confirmed by palaeomagnetic and radiometrie age determinations from PETERS et al. (1986). Considerations on the geochemistry of further dolerite occurrences from Antarctica and other regions of the Gondwana continent are pointed out. Finally comparisons with the analogous South African dike system show the geotectonic significance of the dolerite dikes for the break-up of Gondwana.

Geochemistry on Yangla copper deposit, China

The Yangla copper deposit, situated in the middle section of Jinshajiang tectonic belt between Zhongza-Zhongdian block and Changdu-Simao block, is a representative and giant copper deposit that has been discovered in Jinshajiang-Lancangjiang-Nujiang region in recent years. There are coupled relationship between Yangla granodiorite and copper mineralization in the Yangla copper deposit. Five molybdenite samples yielded a well-constrained 187Re-187Os isochron age of 233.3±3 Ma, the metallogenesis is therefore slightly younger than the crystallization age of the granodiorite. S, Pb isotopic compositions of the Yangla copper deposit indicate that the ore-forming materials were derived from the mixture of upper crust and mantle, also with the magmatic contributions. In the late Early Permian, the Jinshajiang Oceanic plate was subducted to the west, resulting in the formation of a series of gently dipping thrust faults in the Jinshajiang tectonic belt, meanwhile, accompanied magmatic activities. In the early Late Triassic, which was a time of transition from collision-related compression to extension in the Jinshajiang tectonic belt, the thrust faults were tensional; it would have been a favorable environment for forming ore fluids. The ascending magma provided a channel for the ore-forming fluid from the mantle wedge. After the magma arrived at the base of the early-stage Yangla granodiorite, the platy granodiorite at the base of the body would have shielded the late-stage magma from the fluid. The magma would have cooled slowly, and some of the ore-forming fluid in the magma would have entered the gently dipping thrust faults near the Yangla granodiorite, resulting in mineralization.

Euechinoidea and Cidaroidea respond differently to ocean acidification

The impact of the chemical changes in the ocean waters due to the increasing atmospheric CO2 depends on the ability of an organism to control extracellular pH. Among sea urchins, this seems specific to the Euechinoidea, sea urchins except Cidaroidea. However, Cidaroidea survived two ocean acidification periods: the Permian-Trias and the Cretaceous-Tertiary crises. We investigated the response of these two sea urchin groups to reduced seawater pH with the tropical cidaroid Eucidaris tribuloides, the sympatric euechinoid Tripneustes ventricosus and the temperate euechinoid Paracentrotus lividus. Both euechinoid showed a compensation of the coelomic fluid pH due to increased buffer capacity. This was linked to an increased concentration of DIC in the coelomic fluid and thus of bicarbonate ions (most probably originating from the surrounding seawater as isotopic signature of the carbon -delta 13C- was similar). On the other hand, the cidaroid showed no changes within the coelomic fluid. Moreover, the delta 13C of the coelomic fluid did not match that of the seawater and was not significantly different between the urchins from the different treatments. Feeding rate was not affected in any species. While euechinoids are able to regulate their extracellular acid-base balance, many questions are still unanswered on the costs of this capacity. On the contrary, cidaroids do not seem affected by a reduced seawater pH. Further investigations need to be undertaken to cover more species and physiological and metabolic parameters in order to determine if energy trade-offs occur and how this mechanism of compensation is distributed among sea urchins.

delta13Corg of Paleozoic land plants

Based on the evaluation of 1323 carbon isotope values for Silurian to Permian terrestrial organic carbon, measured on plant fossils, cuticules, humic coals and bulk terrestrial organic matter (TOM), we conclude that the temporal trend in d13CTOM records variations in the global carbon cycle, notably an increase in the fractional burial of light (terrestrial) organic matter in Late Palaeozoic sediments. d13CTOM values suggest that the Late Palaeozoic pO2 peak could have been restricted to a time frame of ca. 40 Ma. Carbon isotope data from four taxonomic groups reveal small differences that could be a consequence of habitat conditions. No significant differences in organic carbon isotopic composition in relation to variable climatic conditions are discernible. The carbon isotopic composition solely reflects C3 plant metabolism.

Compositions of paleosols from the Salarevo Formation, Sukhona River valley (Severnaya Dvina basin)

Paleosols crop out in the Sukhona River valley as several members up to 10 m thick embedded into the Salarevo Formation sediments. Principal characteristics of the paleosols include a dense network of root channels, indications of eluvial gley alteration, redistribution and formation of secondary carbonates represented by several generations, and formation of block-prismatic soil structure with specific clayey films at structural jointing faces. The paleosols are divided into a number of genetically interrelated horizons (from top to bottom): presumably organogenic accumulation (AElg), eluvial gley horizon (Elg), illuvial horizons (B1 and B2), illuvial gley horizon (Bg), and transitional horizons (ElBg and BElg). The paleosols formed under conditions of a semiarid climate with sharp seasonal or secular and multisecular oscillations of atmospheric precipitation. Such soils point to specific ecological environments existed in the northern semiarid belt of the Earth before the greatest (in Phanerozoic) biospheric crisis at the Permian-Triassic boundary.

Early Cretaceous dinoflagellate cycsts from ODP Hole 123-765C

Results of a preliminary study of Early Cretaceous dinocyst assemblages from Site 765 on the Argo Abyssal Plain, off northwestern Australia, are presented. The palynological sequence is interpreted in terms of Australian zones and is, in descending order, the late Aptian Diconodinium davidii Zone (Cores 123-765C-33R to -39R), the middle to early Aptian Odontochitina operculata Zone (Cores 123-765C-40R to -49R), the Barremian Muderongia australis Zone (Cores 123-765C-50R to -54R), and the Berriasian lower Batioladinium reticulatum Zone (Core 123-765C-59R). The dating of the sequence as late Aptian to Berriasian on the basis of dinocysts is supported, in part, by data concerning associated foraminiferal, radiolarian, and calcareous nannofossil suites.