K-Ar dating,X-Ray diffractometry,optical and scanning electron microscopy of glauconies from the early Cretaceous Kurnub Group of Jordan

The glaucony of the early Cretaceous Kurnub Group in Jordan has been isotopically dated using the K–Ar method. The glaucony occurs in an arenaceous dolomite unit, referred to here to as glaucony marker unit (GMU), located in the upper part of the Kurnub Group, that persists throughout Jordan. The glaucoliths of the heavy fraction are dark green in colour, ovoidal or mammilated, with a mainly cracked smooth surface, whereas the light fraction glaucoliths are light green in colour, irregular in shape and have a rough or porous surface. Both fractions exhibit boxwork and rosette microstructure, whereas lamellar microstructure is restricted to the heavy glaucoliths. X-ray diffraction and chemical analysis placed the glauconies of the GMU of the Kurnub Group in Odin and Matter's evolved to highly evolved class corresponding to glauconitic mica and suggested that they should be well-closed chronometers. On the other hand, petrographic investigation proved these glauconies to be unaltered, whereas the other altered ones are discarded from the age determination. The unweathered, highly evolved, heavy glauconies that are neither tectonized nor deeply buried best fulfil recommendations regarding appropriate samples for K–Ar dating. The apparent age constrained, within the analytical uncertainty limits, from the most evolved glaucony is 96·1±1·1 Ma and suggests that the GMU is of Albian age. The other less evolved glauconies, which are still within the evolved to highly evolved class of Odin and Matter, yield a mean apparent age of 93·6±1·0 Ma, which is probably slightly younger than the true depositional age by 2–3 Ma due to genetic and historical uncertainties, as indicated by the petrographic and sedimentological data. Thus the upper part of the Kurnub Group, where the GMU is located, is of Albian age. © 1998 John Wiley & Sons, Ltd.     

Strong lensing optical depths in a ΛCDM universe

We investigate strong gravitational lensing in the concordance ΛCDM cosmology by carrying out ray tracing along past light cones through the Millennium Simulation, the largest simulation of cosmic structure formation ever carried out. We extend previous ray-tracing methods in order to take full advantage of the large volume and the excellent spatial and mass resolution of the simulation. As a function of source redshift we evaluate the probability that an image will be highly magnified, will be highly elongated or will be one of a set of multiple images. We show that such strong lensing events can almost always be traced to a single dominant lensing object and we study the mass and redshift distribution of these primary lenses. We fit analytic models to the simulated dark haloes in order to study how our optical depth measurements are affected by the limited resolution of the simulation and of the lensing planes that we construct from it. We conclude that such effects lead us to underestimate total strong lensing cross-sections by about 15 per cent. This is smaller than the effects expected from our neglect of the baryonic components of galaxies. Finally we investigate whether strong lensing is enhanced by material in front of or behind the primary lens. Although strong lensing lines of sight are indeed biased towards higher than average mean densities, this additional matter typically contributes only a few per cent of the total surface density.     

A nitrogen fixation estimate for the Baltic Sea based on continuous pCO2 measurements on a cargo ship and total nitrogen data

Organic matter production and nitrogen fixation in the central Baltic Sea were studied on the basis of high-resolution CO₂ partial pressure data that were obtained from an automated measurement system deployed on a cargo ship. The net organic carbon (OC) production was calculated from a surface water CO₂ mass balance and used to estimate the nitrogen uptake by organic matter during the period March to August 2005. It was shown that the net OC production continued despite the exhaustion of dissolved inorganic nitrogen (DIN) after the spring bloom in April. The nitrogen demand for this production was calculated on the basis of the C/N ratio of organic matter. It was of the same order of magnitude than the winter DIN concentration that fuelled the spring bloom. Since the atmospheric DIN deposition was negligible and no indications of alternative DIN sources were found, it was assumed that N₂ fixation had taken place despite the low temperatures (4–8 °C) in April/May. This “cold fixation” amounted to 74 mmol m^?2 whereas a value of 99 mmol m^?2 was obtained for the summer N₂ fixation during June/July. Due to the contribution of the April/May N₂ fixation, a total annual rate (173±35 mmol m^?2) was obtained for 2005 which is considerably higher than presently accepted estimates. These findings were confirmed by a nitrogen budget based on long-term data (1993–2006) for total nitrogen and total phosphorus concentrations. Furthermore, these data revealed a 30% increase in N₂ fixation during the years 1994–2006.     

Aluminium grain boundary diffusion in polycrystalline mullite ceramics

Aluminium tracer diffusivities were measured in polycrystalline mullite. The artificial aluminium isotope ²⁶Al was used as tracer isotope. An advanced preparation technique for the ²⁶Al₂O₃ tracer source allowed to apply secondary ion mass spectrometry (SIMS) in order to analyse ²⁶Al depth distributions in the polycrystalline material. Pre-exponential factors and activation enthalpies were determined for compositions of 78 wt.% Al₂O₃, 22 wt.% SiO₂ (high-alumina material) and of 72 wt.% Al₂O₃, 28 wt.% SiO₂ (low-alumina material), respectively. A strong dependence of the ²⁶Al grain boundary diffusivity on the composition is observed. The results are discussed in comparison to previous data on grain boundary diffusivities of oxygen in mullite samples from the same batch. Dedicated to Prof. Hermann Schmalzried on the occasion of his 75th birthday.     

The Kansas University meteorit

Abstract— A new 2.8 kg meteorite find from the United States, Kansas University, is classified as an L6 (S3) chondrite based on optical microscopy and electron microprobe analysis of mafic minerals. Natural thermo-luminescence (TL) and ¹⁴C measurements suggest that the Kansas University meteorite has a short terrestrial age compared to other meteorite finds from this area and is not paired with the other local meteorite finds, Densmore (1879), Lawrence, Kansas and Long Island, Kansas.     

Timing of Uralian orogenic gold mineralization at Kochkar in the evolution of the East Uralian granite-gneiss terrane

Gold mineralization at Kochkar (Urals, Russia) is hosted mainly by quartz lodes, which developed at lithological contacts between mafic dikes and granitoids of the Plast massif during late Carboniferous to early Permian, regional E–W compression in the East Uralian Zone (EUZ). The alteration mineralogy in mafic dikes comprises biotite, actinolite, albite, K-feldspar, quartz, epidote, tourmaline, sericite, pyrite, arsenopyrite, chalcopyrite, sphalerite, fahlores, galena, bismuthinite, and gold, and in Plast granitoids quartz, sericite, calcite, epidote, and ore minerals. Geochemically, an enrichment of Si, K, Rb, Ba, S, base metals, W, and Au can be observed. The ore fluid had δ¹⁸O values between 8.2‰ and 9.5‰ typical for metamorphic or deep magmatic fluids. The tectonometamorphic evolution of the EUZ is marked by peak metamorphic conditions at 635±40°C and 5–6 kbar through 500±20°C during gold mineralization, and 300–350°C and 2–3 kbar. The last event was dated on a late, barren quartz vein formed during greenschist facies metamorphism at 265±3 Ma by the Rb–Sr method. Fluids related to this overprint had a δ¹⁸O value of 5.2‰ and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70685 indicating that they are largely equilibrated with metamorphic lithologies of the EUZ. The Plast granitoids and the adjacent Borisov granite, which was dated at 358±23 Ma (U–Pb zircon age), have an adakitic character. This, together with the arc-signature of the mafic dikes, supports the setting of the EUZ within the Valerianovsky continental arc. Eastward subduction of the Uralian Ocean below this arc began during the late Devonian to early Carboniferous. Between 320 and 265 Ma, the oblique closure of the ocean resulted in doming of granitoid massifs in a sinistral transpressional regime, subsequent retrograde gold mineralization during E–W compression and a later greenschist facies overprint. This long-lasting retrograde evolution of the EUZ was caused by the lack of postcollisional collapse. Heat for a “deep-later" type of metamorphism and triggering the auriferous fluid system was supplied by radiogenic heating of an overthickened crust. The greenschist facies overprint at Kochkar and coeval crustal melting in the EUZ was additionally initiated by local external heating of the terrane. This could have been caused by syn- to postcollisional slab rollback or delamination resulting in magmatic underplating of the EUZ, which postdates orogenic gold mineralization at Kochkar. The tectonic interpretation of the EUZ indicates that gold mineralization at Kochkar formed in a mid-crustal environment of a continental magmatic arc at the cessation of active subduction predating post orogenic plutonism.     

Apport des isotopes stables dans l'estimation des altitudes de recharge de sources thermales du MarocRecharge altitude estimation of thermal springs using stable isotopes in Morocco

The recharge altitude estimation of thermal springs from northern and eastern Morocco using ¹⁸O and ²H contents requires the definition of regional isotopic altitudinal gradients (?0.25‰ for 100 m for the Rif and ?0.30‰ for the East) and the calculation of residence time using ¹⁴C. The altitudes of emergence vary widely between 170 and 1040 m under the altitude of the recharge areas. The ¹⁸O and ²H compositions of palaeowaters ($\text{>10}\text{000}$ yr BP) indicate two effects, altitude and palaeoclimate. To cite this article: A. Winckel et al., C. R. Geoscience 334 (2002) 469–474.     

Adaptation: Sensitivity to Natural Variability, Agent Assumptions and Dynamic Climate Changes

The role of adaptation in impact assessment and integrated assessment of climate policy is briefly reviewed. Agriculture in the US is taken as exemplary of this issue. Historic studies in which no adaptation is assumed (so-called "dumb farmer") versus farmer-agents blessed with perfect foresight (so-called "clairvoyant farmer") are contrasted, and considered limiting cases as compared to "realistic farmers." What kinds of decision rules such realistic farmer-agents would adopt to deal with climate change involves a range of issues. These include degrees of belief the climate is actually changing, knowledge about how it will change, foresight on how technology is changing, estimation of what will happen in competitive granaries and assumptions about what governmental policies will be in various regions and over time. Clearly, a transparent specification of such agent-based decision rules is essential to model adaptation explicitly in any impact assessment. Moreover, open recognition of the limited set of assumptions contained in any one study of adaptation demands that authors clearly note that each individual study can represent only a fraction of plausible outcomes. A set of calculations using the Erosion Productivity Impact Calculator (EPIC) crop model is offered here as an example of explicit decision rules on adaptive behavior on climate impacts. The model is driven by a 2xCO₂ regional climate model scenario (from which a "mock" transient scenario was devised) to calculate yield changes for farmer-agents that practice no adaptation, perfect adaptation and 20-year-lagged adaptation, the latter designed to mimic the masking effects of natural variability on farmers' capacity to see how climate is changing. The results reinforce the expectation that the likely effects of natural variability, which would mask a farmer's capacity to detect climate change, is to place the calculated impacts of climate changes in two regions of the US in between that of perfect and no adaptation. Finally, the use of so-called "hedonic" methods (in which land prices in different regions with different current average climates are used to derive implicitly farmers' adaptive responses to hypothesized future climate changes) is briefly reviewed. It is noted that this procedure in which space and time are substituted, amounts to "ergodic economics." Such cross-sectional analyses are static, and thus neglect the dynamics of both climate and societal evolution. Furthermore, such static methods usually consider only a single measure of change (local mean annual temperature), rather than higher moments like climatic variability, diurnal temperature range, etc. These implicit assumptions in ergodic economics make use of such cross-sectional studies limited for applications to integrated assessments of the actual dynamics of adaptive capacity. While all such methods are appropriate for sensitivity analyses and help to define a plausible range of outcomes, none is by itself likely to define the range of plausible adaptive capacities that might emerge in response to climate change scenarios.