Transient creep of the lithosphere and its role in geodynamics

Laboratory experiments with samples of rocks show that at small strains there is transient creep, at which the strain grows with time, and the strain rate decreases. Plate tectonics allows only small strains in the lithospheric plates, so that the lithosphere creep is transient. In geodynamics, the lithosphere is regarded as a cold boundary layer formed by mantle convection. If we assume that the lithosphere has a steady-state creep, which is described by power-law non-Newtonian rheological model, the low effective viscosity of the lower layers of the lithosphere, obtained by data on small-scale postglacial flows, is possible only at high strain rates in these layers. However, the high strain rates in the lithosphere induce large strains that contradict plate tectonics. Transient creep of the lithosphere leads to its mobility at small strains, removing the discrepancy between thermal convection in the mantle and plate tectonics, which holds in the case of power-law rheological model of the lithosphere.     

Zirconolite and xenotime U–Pb age constraints on the emplacement of the Golden Mile Dolerite sill and gold mineralization at the Mt Charlotte mine,Eastern Goldfields Province,Yilgarn Craton,Western Australia

In situ SHRIMP U–Pb dating of magmatic zirconolite (CaZrTi₂O₇) in the Golden Mile Dolerite from the Mt Charlotte gold deposit (Yilgarn Craton, Australia) has yielded the first robust emplacement age (2,680 ± 9 Ma) for the principle host-rock of gold mineralization in the Kalgoorlie district. In contrast, co-magmatic zircon gave ages from ~2.68 Ga to ~2.17 Ga, reflecting isotopic resetting of high-U and -Th crystals. In situ SHRIMP analysis of hydrothermal xenotime (YPO₄), which co-exists with gold in alteration pyrite, provided a Pb/Pb isochron age of 2,655 ± 13 Ma. This date indicates that the youngest deposit in the Kalgoorlie district (Mt Charlotte) formed at ~2.65 Ga, and provides a new minimum age for the structurally older Golden Mile deposit. Our results indicate that gold mineralization at Mt Charlotte is ~50 million years older than indicated by recent ⁴⁰Ar/³⁹Ar dating and places new constraints on the timing of late-stage regional faulting (D₄) in the province.     

The transition from pyroxene granulite facies to garnet clinopyroxene granulite facies. Experiments in the system CaO-MgO-Al2O3-SiO2

The pressure-temperature curve for the equilibrium anorthite+2enstatite=pyrope+diopside+quartz has been determined in the system CaO-MgO-Al₂O₂-SiO₂ to be between 13.4 and 14.0 kbars at 900° C. The slope up to 1,240° C is 8.5 bar/K. The entropy change at 1,200 K is 20 kJ. These data, combined with data from the literature, lead to a geobarometer equation which, when applied to rocks from the Agto area (West Greenland), gives pressure estimates of 6–10 kbars at 800° C. The results are consistent for rocks of differing Fe/Mg ratios and are consistent with independent pressure estimates.     

Making the EU Emissions Trading System: The European Commission as an entrepreneurial epistemic leader

The EU has developed the first and largest international emissions trading system in the world. This development is puzzling due to the EU's scepticism to international emissions trading in greenhouse gases (GHGs) in the run-up to the 1997 Kyoto Protocol. This article analyses how the EU ETS was initiated in the first place mainly from the perspectives of Liberal intergovernmentalism (LI) and multi-level governance (MLG). LI emphasises change in the positions of the EU member states as the key to understand what happened and why, whereas MLG opens up for change in the position of supranational entrepreneurial leaders as the key explanation. The main conclusion is that entrepreneurial epistemic leadership exercised by the European Commission was crucial for making the EU ETS. The principal means of leadership involved building up independent expertise on how an EU ETS could be designed, and mobilizing support from state and non-state actors at various levels of decision-making. This type of leadership may be needed more generally to deal with challenges characterized by high scientific uncertainty and social complexity in which learning is pertinent, such as climate change.     

The effect of a vertical temperature gradient in the lithosphere on seismic and tectonic waves

A linear analysis of the stability of the lithosphere considered as a viscoelastic layer with an equilibrium vertical gradient of temperature is carried out. The problem is solved with a complete system of linearized equations of a continuous medium represented in the dimensionless form and containing a set of dimensionless parameters that determine thermomechanical properties of the lithosphere. As a result of the stability analysis, decrements are found that give the time dependence of perturbations and correspond to high-frequency seismic waves and low-frequency tectonic waves. The frequency and velocity of seismic waves are determined by the elasticity and inertial properties of the lithosphere, and their attenuation, by viscous properties of the lithosphere. The temperature gradient existing in the lithosphere influences seismic waves very weakly. On the contrary, the pattern of tectonic waves is controlled by the temperature gradient and viscous properties, while the effect of elastic and inertial properties on these waves is negligibly small. The stability of a viscoelastic lithosphere is examined using such rheological models as the Maxwell, standard linear, and Andrade media (the frequency of tectonic waves is zero in the Maxwell medium).     

Barium, SiO2 (excess), and P2O5 as proxies of biological productivity in the Middle East during the Palaeocene and the latest Palaeocene benthic extinction event

The distribution of barium and other elements related to biological productivity has been studied in two Palaeocene sections from the Middle East. In the bathyal Ben Gurion section, Israel, Ba* (= Ba/Al₂O₃× 15%) concentrations are low, in the range 0.04% to 0.3% in the lower Palaeocene, and very high, 1% to 2%, throughout most of the upper Palaeocene. In the neritic Gebel Aweina section in Egypt Ba* values are low, < 0.1%, throughout the entire Palaeocene. The Ba* enrichments at Ben Gurion and their correlations with increases in P₂O₅ and opaline silica, and local and global δ¹³C maxima, indicate that upwelling and high productivity were important in this region during the late Palaeocene. The absence of Ba* enrichments in the shallower Gebel Aweina section probably reflects the strong depth dependence of biobarium deposition. In the uppermost Palaeocene, at the level where the global benthic extinction event is registered, Ba* concentrations in the Ben Gurion section increase to anomalous 6%. which suggests that upwelling and possibly wind strengths intensified during this event. The results speak against deep-water formation in this region since downwelling and not upwelling is required.     

Application of the Lorenz equations for studying thermal convection in the lithosphere

Stationary solutions including wave solutions with constant amplitudes are found for nonlinear equations of thermal convection in a layer with nonlinear rheology. The solution is based on the Fourier expansion of unknown velocities and temperatures with only the first and first two terms retained in the velocity and temperature series, respectively. This method, which can be regarded as the Lorenz method, yields the Lorenz equations that fairly well describe the thermal convection in a layer with Newtonian rheology if the Rayleigh number is not very large. The obtained generalization of the Lorenz equations to the case of an integral (having a memory) nonlinear rheology implies that only the first term is retained in the Fourier series for the stress components, i.e., the nonlinear rheological equation is harmonically linearized. However, in the Fourier series of temperature, it is essential to keep the second term: this term, which is independent of the horizontal coordinate, models the thermal boundary layer that characterizes the developed convection. We constructed the bifurcation curves that describe the stationary convection in the nonlinear integral medium simulating the rheology of the mantle, and analyzed the stability of stationary convective flows. The Lorenz method is applied to study small-scale thermal convection in the lithosphere of the Earth.     

Meteorological Observations in 1999 at the Arctic Station,Qeqertarsuaq (Godhavn), Central West Greenland/Evidence of a warmer climate around AD 600, Mittivakkat Glacier,South East Greenland

In october 1990 an automatic weather station was established at the Arctic Station (65° 15', 53°31'W), Qeqertarsuaq (Godhavn), Central West Greenland. The Station register parameters each 20 minutes, and the paramters have been described in an earlier paper in this journal by Nielsen et al.(1995). The present paper summarizes meteorological parameters during 1999.

During a field campaign in 1999, woody remains from Salix glauca were found on a nunatak, 515 m. a.s.l. on the Mittivakkat Glacier, South East Greenland. Radio carbon dating determined the age of a wood sample to AD 640. Together with analyses of macroscopic botanical remains and insect rests in peaty material found nearby, these results indicate, that a warmer climate prevailed near the glacier around AD 600. These findings are in accordance with temperature records based on studies of ice cores from the Greenland Ice sheet.