ALBERT

Beschreibung: The anaerobic degradation pathway of the saturated hydrocarbon n-hexane in a denitrifying strain (HxN1) was examined by gas chromatography-mass spectrometry of derivatized extracts from cultures grown with unlabeled and deuterated substrate; several authentic standard compounds were included for comparison. The study was focused on possible reaction steps that follow the initial formation of (1-methylpentyl)succinate from n-hexane and fumarate. 4-Methyloctanoic, 4-methyloct-2-enoic, 2-methylhexanoic, 2-methylhex-2-enoic and 3-hydroxy-2-methylhexanoic acids (in addition to a few other methyl-branched acids) were detected in n-hexane-grown but not in n-hexanoate-grown cultures. Labeling indicated preservation of the original carbon chain of n-hexane in these acids. Tracing of the deuterium label of 3-d1-(1-methylpentyl)succinate in tentative subsequent products indicated a deuterium/carboxyl carbon exchange in the succinate moiety. This suggests that the metabolism of (1-methylpentyl)succinate employs reactions analogous to those in the established conversion of succinyl-CoA via methylmalonyl-CoA to propionyl-CoA. Accordingly, a pathway is proposed in which (1-methylpentyl)succinate is converted to the CoA-thioester, rearranged to (2-methylhexyl)malonyl-CoA and decarboxylated (perhaps by a transcarboxylase) to 4-methyloctanoyl-CoA. The other identified fatty acids match with a further degradation of 4-methyloctanoyl-CoA via rounds of conventional #-oxidation. Such a pathway would also allow regeneration of fumarate (for n-hexane activation) from propionyl-CoA formed as intermediate and hence present a cyclic process.

Beschreibung: A temporary seismological network, which was located in the Antofagasta (northern Chile) region, recorded a large part of the aftershocks from a MW = 8.0 earthquake in 1995. Results from a tomographic inversion show that the aftershocks were concentrated in a plane, approximately 3 km thick spatial zone. Comparison of our results with earlier processed wide-angle reflection data, shows clearly that this zone is positioned at the top of the oceanic crust. Therefore, it represents the shearing boundary between the subducting oceanic and the continental plate, and gives an in-situ estimation of the thickness of the boundary. Outside of this zone, events were detected in the continental crust, as well as within the oceanic mantle.

Beschreibung: Seismic data from central Tibet have been combined to image the subsurface structure and understand the evolution of the collision of India and Eurasia. The 410- and 660-kilometer mantle discontinuities are sharply defined, implying a lack of a subducting slab beneath the plateau. The discontinuities appear slightly deeper beneath northern Tibet, implying that the average temperature of the mantle above the transition zone is about 300°C hotter in the north than in the south. There is a prominent south-dipping converter in the uppermost mantle beneath northern Tibet that might represent the top of the Eurasian mantle lithosphere underthrusting the northern margin of the plateau.

Beschreibung: Hot spots have been widely used as a fixed reference frame for plate motions; however, it should be expected that the underlying plumes are advected in a dynamic mantle. Here a simple numerical model of hot spot motion due to large-scale mantle flow is applied to the Easter, Hawaii and Louisville hot spots. Computations are performed for a range of different buoyant plume rising speeds, ages and locations, mantle density and viscosity structures, etc. It is shown that a westward motion of the Easter hot spot at several cm per year relative to Hawaii and Louisville hot spots is a robust result for a large range of model parameters. In order to obtain an appropriate boundary condition for mantle flow near the Easter hot spot and to find out on which plate to expect the Easter hot spot track for a given time, published isochrons were rotated to the past ridge locations in a reference frame that takes the computed motion of Hawaii and Louisville hot spots into account. Calculations that include flow in the mantle and therefore relative movement of hot spots yield a better explanation of the observed age distribution along the Sala y Gomez ridge than assuming hot spot fixity; at the same time they predict a hot spot track on the Nazca plate that is roughly similar to the shape of the Nazca and Sala y Gomez ridges. Furthermore, the Easter hot spot was in an intraplate location on the Nazca plate during the past 43 Ma, somewhat closer to the ridge between ∼43 and 26 Ma than between ∼26 Ma and now. In contrast, if hot spots are assumed fixed, a location close to the ridge and rather on the Pacific plate between ∼43 and 26 Ma is predicted. Based on morphology and gravity signature it is suggested that the Nazca ridge and the eastern Tuamotu Island Plateau were created by plume material that erupted at the East Pacific Rise while the Easter plume was located sufficiently close to enable such an interaction, whereas the Sala y Gomez ridge was formed above the Easter plume in an intraplate setting. This implies that, if hot spots have indeed moved as computed here, during formation of the Nazca ridge (between ∼43 and 26 Ma) plume material erupted at the ridge several 100 km away from the plume, whereas no volcanism occurred directly above the plume. It also implies that any seamounts on the Pacific plate east of the Tuamotu Plateau are unrelated to the Easter plume.

Beschreibung: Application of the Debye-Hückel theory for chemical reaction modeling of geothermal brines does not yield sufficiently accurate results. Thus, for the development of a new chemical reaction module for the numerical simulation model SHEMAT (Clauser and Villinger, 1990), the Pitzer formalism (Pitzer, 1991) is used to calculate aqueous speciation and mineral solubilities. It is based on an extended code of PHRQPITZ (Plummer et al., 1988). Using temperature dependent Pitzer coefficients, the system Na- K-Mg-Ca-Ba-Sr-Si-H-Cl-SO4-OH-(HCO3-CO3-CO2)-H2O can be modeled with sufficient accuracy for temperatures from 0° to 150°C. The incorporated carbonic acid system (set in parentheses in the list above) is valid for temperatures from 0 to 90°C, only. Flow, heat transfer, species transport, and geochemical reactions are mutually coupled for modeling reactive flow. Changes in porosity and permeability influence the flow and transport properties of the reservoir. These changes are taken into account by a relation derived from a fractal model of the pore space structure (Pape et al., 1999). A conceptual case study of the injection behavior of a geothermal installation focuses on the immediate vicinity of the well. The injection of cold water has a great influence firstly on the hydraulic conductivity of the aquifer indicated by continuous head pressure increase at the well and secondly on the equilibria between the minerals of the formation and the geothermal fluid. Reservoir changes are studied for the two cases of temperature dependence of solubility, prograde (i.e. barite) and retrograde (i.e. anhydrite). Dissolution of anhydrite induced by cooling down increases the permeability of the formation in a growing region around the borehole and precipitation at the temperature front decreases it. During the initial period of reinjection considered in this study, the negative effect on the injectivity by the colder water is partially compensated by the dissolution of anhydrite. Precipitation of barite around the borehole does not alter the permeability of the formation significantly because the volume of relocated mineral is too small.

Beschreibung: Paleomagnetic data are available for Mexico from various archives, covering irregularly the last 40,000 years. Data obtained from lake sediments are affected by ambiguous conversion of age to depth, and the paleomagnetic data themselves are not of high quality, as samples were only demagnetized at one field intensity. These lake sediment data are therefore considered as less reliable for construction of a secular variation curve, although they may give some qualitative information for the last 25,000 years. Archeological samples provide data for the period, 100-1200 A.D. Their 〈sup〉14〈/sup〉C ages are some times not well constrained, with unrecoverable analytical and statistical procedures, and often ages were only inferred from the stratigraphic position and the paleomagnetic record. The paleomagnetic record of these materials is of high quality with very low internal dispersion, although no magnetic cleaning techniques have been applied. Variations of inclination and declination with time are hampered by large age uncertainties and thus of limited value for the construction of secular variation curves. Paleomagnetic data from an U-Th dated stalagmite provide reasonable directions for most of the last 1200 years, being of lower quality in those parts with very low remanence intensity. The U-Th age determinations were interpreted in terms of a constant growth-rate, which might be only valid as a first-order approximation. Recent volcanic rocks from the Trans-Mexican volcanic belt provide 19 paleomagnetic directions of reasonable to high quality, and often also absolute paleointensities. Ages have been determined from 〈sup〉14〈/sup〉C analysis of charcoal or paleosols found below the lavas, and extend over the last 〈 similar-to 〉40,000 years. Therefore, volcanic rocks provide the most adequate archive for the construction of a secular variation curve, but much more volcanic structures need to be dated to obtain a semi-continuous secular variation curve. Comparison of the data from different archives is possible mainly for the last 2500 years and despite the aforementioned problems similar variations of declination and inclination are observed. This holds as well when comparing them to data from Holocene volcanic rocks from western North America

Beschreibung: The In-situ Laboratory Groß Schönebeck was installed in a former gasexploration well in the North German Basin. This down-hole laboratory will be used to develop hydraulic stimulation techniques to enhance permeability in a variety of sedimentary and volcanic rocks. These activities are part of a large multi- and inter-disciplinary project to develop geothermal technologies necessary for extracting hot fluids (〉 100°C) at sufficient rates (〉 50 m3/h) for geothermal electricity production. In 2000 the well was re-opened, and deepened to 4394 m. Drilling operations and related activities are summarized. Three cement plugs (separated by sections filled with old drilling mud) were drilled before the openhole section was reached. Operations were successful, except for an incident of differential sticking at a depth of 4167.8 m solved by employing a special spotting fluid. Soon after the end of the drilling, a first hydraulic production test and geophysical logging programm was performed.

Beschreibung: Azimuthal anisotropy in the upper mantle of many continental regions is documented by S-wave splitting measurements with SKS techniques. Here we present observations of splitting of the SKS seismic phase at seismograph stations in the Siberian platform, where few such data were known previously. The parameters of splitting are coherent: the fast direction everywhere is around 150°, and the delay of the slow split wave is close to 1.0 s. These observations provide no constraints on the distribution of anisotropy with depth. However, the Siberian platform is remarkable in that it is covered by a network of long-range profiles, where P waves from nuclear explosions are recorded at epicentral distances of 2000 km and more. Depending on epicentral distance these waves sample the upper mantle from the Moho to the transition zone. Two profiles run approximately parallel to the fast direction of the azimuthal anisotropy, whereas the directions of the two others are intermediate between the fast and slow. We examine the observed P traveltimes for their dependence on the azimuth and epicentral distance. With the available data on elastic anisotropy in mantle xenoliths, the values of P-wave anisotropy for horizontal propagation can be used to evaluate S-wave splitting for vertical propagation. It appears that the upper mantle between the Moho and 150 km depth is responsible for not more than about 30 per cent of the large-scale effect in the SKS phase. The major effect is accumulated in a broad low-velocity zone, the top of which is found at a depth of 150 km. Anisotropy within this zone can be caused by recent mantle flow. A similar distribution with depth might explain discrepancies between the estimates of azimuthal anisotropy from phase velocities of surface waves and SKS splitting in North America and South Africa.

Beschreibung: Within the first year following the activation of the GPS radio occultation experiment aboard the Challenging Minisatellite Payload (CHAMP) satellite more than 30,000 occultation events have been observed. Radio holographic analyses of 3783 occultation events observed between 14 May and 10 June 2001 reveal that in about 20–30% the received signal contains contributions from components reflected at Earth’s surface. This percentage number increases to about 70–80% at high latitudes. At low latitudes inhomogeneities in the refractivity field caused by the distribution of water vapor frequently prevent single-path propagation of the direct ray in the lower troposphere. On the basis of geometrical ray tracing and multiple phase screen simulations characteristic frequency shifts in the radio holograms’ power spectral densities are analyzed quantitatively. By correlating these frequency shifts with ground-level refractivity the events can be separated in three categories. First, the frequency shift depends almost linearly on reflection point elevation following about 0.22 km Hz−1. Second, within the subset of zero-elevation reflections an approximate correlation with specific humidity at ground-level is found. Thereby, specific humidities at the ground are derived on the basis of the GPS phase and amplitude observations, precise orbit data and ECMWF dry refractivities. The results are consistent with corresponding ECMWF values of specific humidity. Finally, in about 1.4% of the zero-elevation events the observed frequency shifts are inconsistent with the linear relationship between groundlevel refractivity and the observed frequency shift. Comparisons with results from multiple phase screen simulations suggest that in these cases signatures caused by diffraction effects of layered structures in the refractivity field are present.

Beschreibung: We report on observations of upper mantle anisotropy from the splitting of teleseimsmic shear waves (SKS, SKKS, and PKS) recorded by the ICEMELT broadband seismometer network in Iceland. In a ridge-centered hot spot locale, mantle anisotropy may be generated by flow-induced lattice-preferred orientation of olivine grains or the anisotropic distribution of magma. Splitting measurements of teleseismic shear waves may thus provide diagnostic information on upper mantle flow and/or the distribution of retained melt associated with the Iceland mantle plume. In eastern Iceland, fast polarization directions lie between N10°W and N45°W and average N24°W; delay time between the fast and slow shear waves are generally 0.7-1.35 s. In western Iceland, in contrast, the fast polarization directions, while less well constrained, yield an average value of N23°E and delay times are smaller (0.2-0.95 s). We propose that splitting in eastern Iceland is caused by a 100- to 200-km-thick anisotropic layer in the upper mantle. The observed fast directions in eastern Iceland, however, do not correspond either to the plate spreading direction or to a pattern of redial mantle flow from the center of the Iceland hot spot. We suggest that the relatively uniform direction and magnitude of splitting in eastern Iceland, situated on the Eurasian plate, may therefore reflect the large-scale flow field of the North Atlantic upper mantle. We hypothesize that the different pattern of anisotropy beneath western Iceland, part of the North American plate, is due to the different absolute motions of the two plates. By this view, splitting in eastern and western Iceland is the consequence of shear by North American and Eurasian plate motion relative to the background mantle flow. From absolute plate motion models, in which the Eurasian plate is approximately stationary and the North American plate is moving approximately westward, the splitting observations in both eastern and western Iceland can be satisfied by a background upper mantle flow in the direction N34°W and a velocity of 3 cm/yr in a hot spot reference frame. This inference can be used to test mantle flow models. In particular, it is inconsistent with kinematic flow models, which predict southward flow, or models where flow is dominated by subductions-related sources of mantle buoyancy, which predict westward flow. Our observations are more compatible with the flow field predicted from global seismic tomography models, which in particular include the influence of the large-scale lower mantle upwelling beneath southern Africa. While the hypothesized association between our observations and this upwelling is presently speculative, it makes a very specific and testable prediction about the flow field and hence anisotropy beneath the rest of the Atlantic basin.