ALBERT

Beschreibung: We report a palaeomagnetic investigation of the last full geomagnetic field reversal, the Matuyama-Brunhes (M-B) transition, as preserved in a continuous sequence of exposed lacustrine sediments in the Apennines of Central Italy. The palaeomagnetic record provides the most direct evidence for the tempo of transitional field behaviour yet obtained for the M-B transition. 40 Ar/ 39 Ar dating of tephra layers bracketing the M-B transition provides high-accuracy age constraints and indicates a mean sediment accumulation rate of about 0.2 mm yr –1 during the transition. Two relative palaeointensity (RPI) minima are present in the M-B transition. During the terminus of the upper RPI minimum, a directional change of about 180 ° occurred at an extremely fast rate, estimated to be less than 2 ° per year, with no intermediate virtual geomagnetic poles (VGPs) documented during the transit from the southern to northern hemisphere. Thus, the entry into the Brunhes Normal Chron as represented by the palaeomagnetic directions and VGPs developed in a time interval comparable to the duration of an average human life, which is an order of magnitude more rapid than suggested by current models. The reported investigation therefore provides high-resolution integrated palaeomagnetic and radioisotopic data that document the fine details of the anatomy and tempo of the M-B transition in Central Italy that in turn are crucial for a better understanding of Earth's magnetic field, and for the development of more sophisticated models that are able to describe its global structure and behaviour.

Beschreibung: The anisotropy of magnetic susceptibility (AMS) of soft rocks was measured in order to distinguish between the effect of remote and local strain fields, determine the size of the related inelastic damage zone and resolve the fault-plane solutions of past earthquakes. The AMS fabrics were explored next to late Pleistocene syndepositional normal faults (total displacement up to ~3.5 m) that cross soft lacustrine rocks within the seismically active Dead Sea basin. ‘Deposition fabrics’ prevail meters away from the fault planes and are characterized by scattered maximum and intermediate principal AMS axes. ‘Deformation fabrics’ are detected up to tens of centimetres from the fault planes and are characterized by well-grouped AMS axes, in which one of the principal axes is parallel to the strike of the nearby fault. Variations in the AMS fabrics and magnetic lineations define the size of the inelastic damage zone around the faults. The results demonstrate that the deformation-driven magnetic fabrics and the associated inelastic damage zones are compatible with coseismic dynamic faulting and the effects of the local strain field during earthquakes. Most of the AMS fabrics show a conspicuous similarity to that of the fault-plane solutions, i.e., the principal AMS axes and instantaneous strain ellipsoids are coaxial. These results suggest a novel application of the AMS method for defining the shape and size of the damage zones surrounding dynamic faults and determining the full tensor of the local strain field.

Beschreibung: Correlated data errors (noise) are common in magnetotelluric (MT) data, but MT inversions typically neglect error correlations without investigating the impact of this simplification on inversion results. This paper examines effects of neglecting frequency- and spatially correlated noise on MT inversion, based on a nonlinear Bayesian formulation which quantifies the uncertainties of inversion results in terms of marginal posterior probability densities and credibility intervals. To do so, data with frequency- and spatially correlated noise of differing degrees are generated for several layered (1-D) synthetic cases. Bayesian MT inversions are carried out for these data sets with and without accounting for error correlation (i.e. applying full and diagonal covariance matrices, respectively, in the inversion), and the results are compared. For cases with noise that is strongly correlated over frequency or space, parameter uncertainties estimated using the diagonal-covariance simplification (neglecting error correlations) are found to often be significantly underestimated compared with results computed using the full covariance matrix.

Beschreibung: Presently, most loess/palaeosol magnetic susceptibility records are interpreted as following either the wind-vigour model or the pedogenic enhancement model. However redoxomorphic processes induced by waterlogging, often referred to gleying in the loess literature, are also known to alter loess deposits but their impact on loess/palaeosol magnetic susceptibility records has received little attention. The reported rock magnetic study aims to characterize the mineral magnetic response of loess to waterlogging-induced redoxomorphic processes, thus improving our understanding of mineral magnetic changes within loess deposits with respect to environmental and climate conditions. The Nussloch loess-palaeosol deposit (Rhine Valley, Germany) was targeted because it is one of the best-studied Pleniglacial deposits for Western Europe in which numerous tundra gley intervals have been identified. Moreover, a comprehensive high-resolution environmental magnetism study has never been undertaken for this site. Various rock magnetism experiments were conducted at both room and low temperatures to characterise the composition, concentration and relative magnetic grain size of the mineral magnetic assemblage. The relative changes in magnetic parameters within the investigated loess interval are primarily controlled by (1) varying concentrations of coarse-grained ferrimagnetic particles of detrital (aeolian) origin and (2) dissolution of fine-grained ferrimagnetic particles related to in situ post-depositional alteration promoted by waterlogging-induced redoxomorphic processes. Goethite is found to be ubiquitous throughout the studied interval and is argued to have both a primary (aeolian) and secondary ( in situ ) origin. We conclude, that redoxomorphic processes induced by waterlogging, if present, will hinder the interpretation of magnetic susceptibility variations within loess and palaeosol deposits following the expected relationships dictated by the wind-vigour and the pedogenic enhancement magnetism models.

Beschreibung: We investigate polarity reversals in the geodynamo using a rotating, convection-driven dynamo model. As the flow in rapidly rotating convection is dominated by columns aligned with the axis of rotation, the focus is on the dynamics of columnar vortices. By studying the growth of a seed magnetic field to a stable axial dipole field, we show that the magnetic field acts in ways that significantly enhance the relative helicity between cyclonic and anticyclonic vortices. This flow asymmetry is the hallmark of a dipolar dynamo. Strong buoyancy, on the other hand, offsets the effect of the magnetic field, establishing parity between positive and negative vortices. As the dipole field is deprived of the helicity required to support itself, the dynamo is pushed into a reversing state. This is a likely regime for polarity reversals in the Earth's core. The integral lengthscale at which buoyancy injects energy is not significantly different from the convective flow lengthscale, which implies that buoyancy does not feed vortices at the small scales where non-linear inertia is present. The lengthscale at which the Lorentz force acts in the reversing dynamo is small, which may allow the passive presence of non-linear inertia in the small scales.

Beschreibung: This paper presents new directional archeomagnetic data from nine Meso-/Neolithic fireplaces, sampled in a cave shelter, at Arconciel, in western Switzerland. Rock magnetic measurements indicate a homogenous magnetic mineralogy in all fireplaces, with magnetite as the main magnetic carrier. The remanent magnetization is stable and generally shows one characteristic directional component. Nine new directions, which were obtained from Arconciel, are combined with 356 other archeomagnetic data from a circular area with a radius of 700 km around this site, to obtain a penalized least square spline fit for the past 9000 yr. We found in general good agreement with other local compilations, such as the Balkan curve, the regional SCHA.DIF.8k model and with lake sediments from UK, Fennoscandia and Switzerland. Nevertheless, a time lag of several centuries is observed for a declination maximum between the archeomagnetic spline fit and the other European data records around 5900 BC. This time lag is also observed in the Swiss lake sediment record; therefore we interpret this shift as a local feature of the Earth's magnetic field.

Beschreibung: We demonstrate that first differences of polar orbiting satellite magnetic data in the along-track direction can be used to obtain high resolution models of the lithospheric field. Along-track differences approximate the north–south magnetic field gradients for non-polar latitudes. In a test case, using 2 yr of low altitude data from the CHAMP satellite, we show that use of along-track differences of vector field data results in an enhanced recovery of the small scale lithospheric field, compared to the use of the vector field data themselves. We show that the along-track technique performs especially well in the estimation of near zonal spherical harmonic coefficients. Moreover, lithospheric field models determined using along-track differences are found to be less sensitive to the presence of unmodelled external field contributions and problems associated with the polar gap are ameliorated. Experiments in modelling the Earth's lithospheric magnetic field with along-track differences are presented here as a proof of concept. We anticipate that use of such along-track differences in combination with east–west field differences, as are now provided by the Swarm satellite constellation, will be important in building the next generation of lithospheric field models.

Beschreibung: We investigate the influence of different thermal and velocity boundary conditions on numerical geodynamo models. We concentrate on the implications for magnetic field morphology, heat transport scaling laws, force balances and generation mechanisms. The field morphology most strongly depends on the local Rossby number, but there is some variation in the dipolarity of the field with boundary condition. Scaling laws also depend on the boundary conditions, but a diffusivity-free scaling is a good first order approximation for all our dipolar models. Our multipolar models, however, obey different scaling laws from dipolar models implying a different force balance in these models. We find that our dipolar models have a stronger degree of Lorentz–Coriolis balance compared to our multipolar models which have a stronger degree of Lorentz-inertial balance.The models with a stronger Lorentz–Coriolis dominance can be generated by either α, α 2 or α 2 mechanisms whereas the models with a stronger Lorentz-inertial balance are all α 2 dynamos. These results imply that some caution is necessary when extrapolating results from dynamo models to Earth-like parameters since the choice of boundary conditions can have important effects.

Beschreibung: Lake sediments are excellent sources of palaeoenvironmental and palaeoclimatic information because they provide continuous and high-resolution records. South America is of particular interest because it is the only landmass that stretches southward into the Pacific and Atlantic Oceans towards Antarctica. The aim of this study is to explore the relationship of magnetic parameters with elemental and palaeobiological data of Laguna Potrok Aike to develop a model of lake-level changes and related hydrological and climatic fluctuations. Magnetic measurements were performed on subsamples from 15 500 cal. BP to the present, and associated rock magnetic parameters were calculated to infer magnetic mineralogy, concentration and grain size. According to the model, parameters dependent on magnetic concentration and grain size are directly related to lake-level changes. During dry periods, the remanent coercivity displays high values, whereas the proportion of magnetite is relatively low. Low percentages of greigite are observed, indicating that the water of the lake was stratified at least four times during the studied period, at approximately 10 300, 8900, 8500 and 8300 cal. BP. The preservation of greigite by inhibiting its complete transformation into pyrite is associated with a rapid burial that occurs with high sedimentation rates. Thermal stratification could be caused by a slight cooling in the area triggered by a weakening of the Westerlies and/or low activity of the sun, sum to the effect of a flood of melt water in the North Atlantic.

Beschreibung: Motional induction in the ocean by tides has long been observed by both land and satellite measurements of magnetic fields. While these signals are weak (~10 nT) when compared to the main magnetic field, their persistent nature makes them important for consideration during geomagnetic field modelling. Previous studies have reported several discrepancies between observations and numerical predictions of the tidal magnetic signals and those studies were inconclusive of the source of the error. We address this issue by (1) analysing magnetometer data from ocean-bottom stations, where the low-noise and high-signal environment is most suitable for detecting the weak tidal magnetic signals, (2) by numerically predicting the magnetic field with a spatial resolution that is 16 times higher than the previous studies and (3) by using four different models of upper-mantle conductivity. We use vector magnetic data from six ocean-bottom electromagnetic (OBEM) stations located in the Northwestern Pacific Ocean. The OBEM tidal amplitudes were derived using an iteratively re-weighted least-squares (IRLS) method and by limiting the analysis of lunar semidiurnal (M2), lunar elliptic semidinurnal (N2) and diurnal (O1) tidal modes to the night-time. Using a 3-D electromagnetic induction solver and the TPX07.2 tidal model, we predict the tidal magnetic signal. We use earth models with non-uniform oceans and four 1-D mantle sections underneath taken from Kuvshinov and Olsen, Shimizu et al. and Baba et al. to compare the effect of upper-mantle conductivity. We find that in general, the predictions and observations match within 10–70 per cent across all the stations for each of the tidal modes. The median normalized percent difference (NPD) between observed and predicted amplitudes for the tidal modes M2, N2 and O1 were 15 per cent, 47 per cent and 98 per cent, respectively, for all the stations and models. At the majority of stations, and for each of the tidal modes, the higher resolution (0.25° 0.25°) modelling gave amplitudes consistently closer to the observations than the lower resolution (1° 1°) modelling. The difference in lithospheric resistance east and west of the Izu–Bonin trench system seems to be affecting the model response and observations in the O1 tidal mode. This response is not seen in the M2 and N2 modes, thereby indicating that the O1 mode is more sensitive to lithospheric resistance.