Magnetotelluric investigation on Björkö impact structure,west of Stockholm,Sweden

This paper describes the application of magnetotelluric (MT) method to investigate Björkö impact structure located at west of Stockholm, Sweden. This structure has formed in crystalline rocks ca. 1.2 Ga ago and located relatively close to the district heating infrastructure of the Stockholm region, as the largest district heating system in Europe. Since impact structures mostly contain fractured rock volumes in the form of breccia formations, the occurred brecciation zones in this region are more favorable potential targets for geothermal investigations. The main objective is evaluating the capability of the study area to have potential for geothermal resources by mapping the subsurface structure. To image electrical characteristic of underground layers, 1D and 2D bimodal inversions of TE and TM modes of MT data are performed. The results are also compared with the outputs of the inversion of the determinant data (yielding a direction-independent average of the subsurface conductivity) along the same profiles, proving good accordance of the outputs. The processed resistivity sections at depth along with measuring various rock physical properties across two drilled boreholes at Björkö and Midsommar islands localized two conductors at depths of 1 km and from 2.5 to 4.5 km, which may be attributed to be a potential zone for geothermal energy retrieval.     

An experiment to study and control the Langmuir sheath around INTERBALL-2

The satellite INTERBALL-2 has an orbit with high inclination (62.8°), covering the altitude range between a few hundred and about 20000 km. The ambient plasma conditions along this orbit are highly variable, and the interactions of this plasma with the spacecraft body as well as the photo-electron sheath around it are considered to be interesting topics for detailed studies. The electric potential of the spacecraft with respect to the ambient plasma that develops as a result of the current equilibrium reacts sensitively to variations of the boundary conditions. The measurement and eventual control of this potential is a prerequisite for accurate measurements of the thermal plasma. We describe the purpose and technical implementation of an ion emitter instrument on-board INTERBALL-2 utilising ion beams at energies of several thousand electron volts in order to reduce and stabilise the positive spacecraft potential. First results of the active ion beam experiments, and other measures taken on INTERBALL-2 to reduce charging are presented. Furthermore, the approach and initial steps of modelling efforts of the sheath in the vicinity of the INTERBALL-2 spacecraft are described together with some estimates on the resulting spacecraft potential, and effects on thermal ion measurements. It is concluded that even moderate spacecraft potentials as are commonly observed on-board INTERBALL-2 can significantly distort the measurements of ion distribution functions, especially in the presence of strongly aniso-tropic distributions.     

Contrasting tonalite genesis in the Lyngen magmatic complex, north Norwegian Caledonides

The Lyngen gabbro (LG), defining the major part of the Lyngen magmatic complex, is characterised by layered gabbros of N-MORB affinity (western suite) and layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiite) to boninitic affinity (eastern suite). The boundary between the eastern and western suites is generally defined by a large-scale ductile shear zone of suboceanic origin, the Rypdalen shear zone (RSZ). Tonalites occur within the RSZ and in the eastern suite of the LG. Variations in field occurrence and chemical composition of the tonalites suggest that they represent two petrologically different groups. Tonalite intrusion (the Vakkas pluton) up to 5 km² large occur in the eastern suite of the LG, and are characterised by high Y contents (average 26 ppm) and high K₂O/Rb ratios (average 0.062) compared to tonalites on the RSZ. The Vakkas pluton has lightly concave REE (rare earth element) patterns with negative Eu-anomalies, and positive _ND-values (+3.7 to +3.9). Geochemical modelling based on the REE and field evidence suggests that these tonalites may have formed by fractional crystallization from a boninitic parental magma. Tonalites related to the RSZ form irregular veins and dikes that net vein the shear zone. They are characterised by low Y contents (average 6 ppm), low K₂O/Rb ratios (average 0.025), and highly variable contents of Na₂O, K₂O, Sr and Ba, compared to the Vakkas pluton. Tonalites related to the RSZ show substantial variation in the content of the LREEs. They possess low abundances of the HREEs, and absence of, or slightly positive Eu-anomalies. The tonalites have highly variable _ND-values (−0.6 to −9.4), probably resulting from enrichment of Nd from an external source. Geochemical modelling suggests that the LREE-rich tonalites formed by H₂O-rich partial melting of differentiated products from the eastern suite of the LG. The presence of B in the fluid phase is suggested by the presence of tourmaline-bearing tonalite pegmatites. Thus, the anatectic tonalites of this group could have been formed by water-excess melting of a variety of gabbroic cumulates of the LG. In the LG, LREE-depleted tonalites (_ND-values +5.1) also occur, and these are best explained in terms of partial melting of gabbroic cumulates from the transition zone between the eastern and the western suites of the LG.     

Rocket measurements of ULF electric fields and precipitating particles in the aurora

The electric component of ULF waves with a characteristic period of 1.5-3 s and an amplitude of tens of $\text{mV}\text{m}$ has been observed with a double probe on a rocket launched from Andöya in Norway into an aurora. The polarization of the waves was right and left handed nearly circular as well as nearly linear during different intervals of the flight. These measurements are compared with ELF and energetic electron measurements on the rocket and ULF measurements on the ground.Possible interpretations involving generation of the ULF waves by proton cyclotron resonance in the magnetotail or by streaming ions or electrons in the magnetosheath are outlined.     

Correction of Single Log-Derived Temperatures in the Danish Central Graben,North Sea

An equation for correcting log-derived temperatures measured at well depths between 1200 and 5400 m has been derived by comparing log-derived temperatures from wells in the Danish Central Graben (North Sea) with DST temperatures, from the same wells, that are believed to represent true formation temperatures. Equations developed previously using data over different depth ranges from Malaysia and Mexico yield fair results when applied to the North Sea. However, a better fit to the Danish data was obtained using new equations that are similar to those published in the earlier studies. The correction method here is based principally on time since end of circulation (TSC), but it also includes a small dependence on depth. In this study the true subsurface temperature (T_true) (°C) is given by where the correction factor f = 3.07·TSC^(–0.09)/(0.47·Z^(0.175)), T_surf is the seafloor temperature in °C, T_meas is the measured log temperature in °C, TSC is in hours, and Z is the depth below seafloor in meters. When TSC is not known, maximum probable, minimum probable, and most likely values can be estimated from the observed trend of TSC with depth.An estimate of the uncertainty in the corrected temperature can be obtained from the equation where is the standard deviation of the error in the correction factor f. This approach can be modified to include the additional uncertainty associated with unspecified TSC.     

Solution of the H-polarization induction problem by a wavenumber integral equation approach

Summary. The H -polarization induction problem is solved in terms of an integral equation, which in the horizontal direction is transformed into the wavenumber domain. By this transformation the usual complicated integral expressions for the Green's tensor elements are removed. By extracting asymptotic features from the system of linear equations, we reduce the number of equations considerably independent of whether the horizontal variation in the conductivity is continuous or discontinuous. Likewise we reformulate the problem so that arbitrary conductivity contrasts may be studied. The method is finally tested by comparing with analytic solutions, and good agreement is achieved. Furthermore the numerical results indicate that a small amount of wavenumbers is required.     

Export Production in the Subarctic North Pacific over the Last 800 kyrs: No Evidence for Iron Fertilization?

The subarctic North Pacific is a high nitrate-low chlorophyll (HNLC) region, where phytoplankton growth rates, especially those of diatoms, are enhanced when micronutrient Fe is added. Accordingly, it has been suggested that glacial Fe-laden dust might have increased primary production in this region. This paper reviews published palaeoceanographic records of export production over the last 800 kyrs from the open North Pacific (north of ∼35°N). We find different patterns of export production change over time in the various domains of the North Pacific (NW and NE subarctic gyres, the marginal seas and the transition zone). However, there is no compelling evidence for an overall increase in productivity during glacials in the subarctic region, challenging the paradigm that dust-born Fe fertilization of this region has contributed to the glacial draw down of atmospheric CO₂. Potential reasons for the lack of increased glacial export production include the possibility that Fe-fertilization rapidly drives the ecosystem towards limitation by another nutrient. This effect would have been exacerbated by an even more stable mixed layer compared to today. This revised version was published online in July 2006 with corrections to the Cover Date.     

Seep carbonate formation controlled by hydrothermal vent complexes: a case study from the Vøring Basin, the Norwegian Sea

Several hundred hydrothermal vent complexes were formed in the Vøring Basin as a consequence of magmatic sill emplacement in the late Palaeocene. The 6607/12-1 exploration well was drilled through a 220-m-thick sequence of Eocene–Miocene diatomites with carbonate nodules above the apex of one of these vent complexes. Analysed calcites and dolomites from this interval have isotopic signatures typical for methane seep carbonates, with low ¹³C signatures of –28 to –54 PDB. The data suggest that the vent complex acted as a fluid migration pathway for about 50×10⁶ years after its formation, leading to near-surface microbial activity and seep carbonate formation.