40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey

Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects ⁴⁰Ar-³⁹Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; ⁴⁰Ar-³⁹Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger ⁴⁰Ar-³⁹Ar ages in the cores of coarser white micas, which in conjunction with ⁴⁰Ar-³⁹Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma⁻¹ and exhumation to the surface via thrusting. The ⁴⁰Ar-³⁹Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks. Received: 12 May 1998 / Accepted: 21 April 1999     

Effect of hydrocarbon volatility and adsorption on source-rock pyrolysis

Petroleum source-rock evaluations by pyrolysis are based on the concept that free hydrocarbons in rock samples are volatilized below 300°C while hydrocarbons cracked from kerogen come off at higher temperatures. The pyrolysis of pure hydrocarbons with different mineral matrices shows that free hydrocarbons containing 16 or more carbon atoms may not be evolved eblow 300°C, but at varying higher temperatures. The extent to which this occurs depends on the hydrocarbon volatility, the mineral matrix and the pyrolysis instrument design. Source-rock parameters which use the P₁ (S₁) peak may be not be reproducible between instruments if the rock contains appreciable amounts of high molecular weight hydrocarbons.     

Metamorphic evolution of the Sesia-Lanzo Zone,Western Alps: time constraints from multi-system geochronology

 The Sesia-Lanzo Zone is a polymetamorphic unit containing Hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the Alpine orogeny. Different parts of the unit record different stages on the P-T-deformation evolution, allowing multi-system isotopic studies to unravel the precise timing of the metamorphic history. New Rb–Sr white mica and U–Pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60–70 Ma. This substantially alters the common view of early- to mid-Cretaceous eclogite facies metamorphism in this unit. The new results are more consistent with the established geotectonic framework for the Alpine orogeny, since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension. It is also more concordant with recent studies of other units that demonstrate post-Cretaceous high-pressure metamorphism. Step-heated ⁴⁰Ar–³⁹Ar analysis of phengites yields good plateaux giving ages older than the corresponding Rb–Sr age. Such anomalously high ages indicate the presence of radiogenic argon-rich fluids in the grain boundary network under the fluid/pressure conditions acting during this high-pressure metamorphic event. The U–Pb sphene ages are variable in polymetamorphic rocks, and show inheritance of older Pb or sphene crystals into the high-pressure event. Two monometamorphic assemblages yield concordant ages at 66±1 Ma, reflecting crystallisation of the eclogite facies assemblage. The Gneiss Minuti Complex (GMC) lies structurally below the Eclogitic Micaschists, and its pervasive greenschist facies fabric yields tightly clustered Rb–Sr white mica ages at 38–39 Ma. This greenschist event did not affect the majority of the EMC. The ⁴⁰Ar–³⁹Ar ages of micas formed at this time were very disturbed, whereas micas surviving from an earlier higher pressure assemblage had their ⁴⁰Ar–³⁹Ar system reset. The greenschist event did not strongly affect U–Pb systematics in Hercynian age sphenes, suggesting that the GMC did not uniformly suffer an eclogite facies metamorphism during the Alpine cycle, but was juxtaposed against the EMC later in the orogeny. This model still requires that the locus of deformation and metamorphism (and possibly fluid flux) moved outboard with time, leaving the Sesia-Lanzo basement as a shear-bounded unreactive block within the orogenic wedge. Received: 12 October 1995/Accepted:25 June 1996     

Past and future climate variations in the Norwegian Arctic: overview and novel analyses

Sparse stations and serious measuring problems hamper analyses of climatic conditions in the Arctic. This paper presents a discussion of measuring problems in the Arctic and gives an overview of observed past and projected future climate variations in Svalbard and Jan Mayen. Novel analyses of temperature conditions during precipitation and trends in fractions of solid/liquid precipitation at the Arctic weather stations are also outlined. Analyses based on combined and homogenized series from the regular weather stations in the region indicate that the measured annual precipitation has increased by more than 2.5% per decade since the measurements started in the beginning of the 20th century. The annual temperature has increased in Svalbard and Jan Mayen during the latest decades, but the present level is still lower than in the 1930s. Downscaled scenarios for Svalbard Airport indicate a further increase in temperature and precipitation. Analyses based on observations of precipitation types at the regular weather stations demonstrate that the annual fraction of solid precipitation has decreased at all stations during the latest decades. The reduced fraction of solid precipitation implies that the undercatch of the precipitation gauges is reduced. Consequently, part of the observed increase in the annual precipitation is fictitious and is due to a larger part of the "true" precipitation being caught by the gauges. With continued warming in the region, this virtual increase will be measured in addition to an eventual real increase.     

Ecological consequences of land use change: Forest structure and regeneration across the forest-grassland ecotone in mountain pastures in Nepal

The ecotone, the spatial transition zone between two vegetation communities, is claimed to have more species than the adjoining communities. However, empirical studies do not always confirm higher richness at the ecotone. The ecotone position and structure are dynamic over time and space and it is driven by the changes in climate, land use or their interaction. In this context, we assessed the forest- grassland ecotone of temperate mountains in central Nepal by i） comparing species composition and richness across the ecotone, ii） analyzing if the forestgrassland ecotone is shifting towards the grassland center by colonizing them with trees, and iii） discussing the consequence of changed disturbance regime in the dynamics of this ecotone and the surrounding grasslands. We analyzed vegetation data sampled from belt transects laid across the forest- grassland ecotone in semi-natural grassland patches. Vegetation data consisting of species richness and composition, and size structure and regeneration of the two most dominant tree species, namely Rhododendron arboreum and Abies spectabilis, from the transects, were used to analyze the trend of the forest-grassland ecotone. Forest and grasslands were different in terms of floristic composition and diversity. Vascular plant speciesrichness linearly increased while moving from forest interior to grassland center. Spatial pattern of tree size structure and regeneration infers that forest boundary is advancing towards the grasslands at the expense of the grassland area, and tree establishment in the grasslands is part of a suceessional process. Temporally, tree establishment in grasslands started following the gradual decline in disturbance. We argue that local processes in terms of changed land use may best explain the phenomenon of ecotone shift and consequent forest expansion in these grasslands. We underpin the need for further research on the mechanism, rate and spatial extent of ecotone shift by using advaneed tools to understand the process indepth.     

The evolution of disk galaxies in clusters

We are carrying out a programme to measure the evolution of the stellar and dynamical masses and M/L ratios for a sizeable sample of morphologically-classified disk galaxies in rich galaxy clusters at 0.2 < z < 0.9. Using FORS2 at the VLT we are obtaining rotation curves for the cluster spirals so that their Tully-Fisher relation can be studied as a function of redshift and compared with that of field spirals. We already have rotation curves for ∼ 10 cluster spirals at z = 0.83, and 25 field spirals at lower redshifts and we plan to increase this sample by one order of magnitude. We present here the first results of our study, and discuss the implications of our data in the context of current ideas and models of galaxy formation and evolution. This revised version was published online in September 2006 with corrections to the Cover Date.     

Late Quaternary sediments and stratigraphy on the continental shelf off Troms and west Finnmark, northern Norway

Lithologic, paleontologic, and chronostratigraphic investigation of 13 gravity cores indicates the following environmental evolution: a high- (mid-) arctic period with a slight influx of ice-rafted debris occurred during the early middle Weichselian followed by a mid- (high-) arctic environment with a high influx of iceberg-rafted debris during the remainder of the middle Weichselian. The continental ice sheet probably did not extend beyond the inner shelf during middle Weichselian and a minimum relative sea level was ca. −120 m. A low-arctic environment occurred during (parts of) the late Weichselian with an initial winnowing of the sediments. The Norwegian Current entered the area during this substage. A high- (mid-) boreal environment occurred during the Holocene with high winnowing activity in the early Holocene. Winnowing is still very active on the shallower banks in contrast to the deeper banks where it has ceased. Relatively high percentages of carbonate in the form of biogenic skeletal remains occur in the Holocene sediments.     

A Younger Dryas Ash Bed in western Norway,and its possible correlations with tephra in cores from the Norwegian Sea and the North Atlantic

A bed of volcanic ash up to 23 cm thick is found in lacustrine and marine sediments in western Norway. It is formally mamed the Vedde Ash Bed, and its age is approximately 10,600 yr B.P., i.e., mid-Younger Dryas. The bed consits of pure glass having a bimodal basaltic and rhyolitic somposition. The geochemistry of the glass shards suggests an Icelandic source. By means of stratigraphic position and geochemistry, the ash is correlated with ash zones found in cores from the continental shelf, the Norwegian Sea, and the North Atlatic.