ALBERT

Description: Paleomagnetic records from sediments, archeological artifacts, and lava flows provide the foundation for studying geomagnetic field changes over 0–100 ka. Late Quaternary time-varying spherical harmonic models for 0–100 ka produce a global view used to evaluate new data records, study the paleomagnetic secular variation on centennial to multimillennial timescales, and investigate extreme regional or global events such as the Laschamp geomagnetic excursion. Recent modeling results (GGF100k and LSMOD.2) are compared to previous studies based on regional or global stacks and averages of relative geomagnetic paleointensity variations. Time-averaged field structure is similar on Holocene, 100 ky, and million-year timescales. Paleosecular variation activity varies greatly over 0–100 ka, with large changes in field strength and significant morphological changes that are especially evident when field strength is low. GGF100k exhibits a factor of 4 variation in geomagnetic axial dipole moment, and higher-resolution models suggest that much larger changes are likely during global excursions. There is some suggestion of recurrent field states resembling the present-day South Atlantic Anomaly, but these are not linked to initiation or evolution of excursions. Several properties used to characterize numerical dynamo simulations as “Earth-like” are evaluated and, in future, improved models may yet reveal systematic changes linked to the onset of geomagnetic excursions. Modeling results are useful in applications ranging from ground truth and data assimilation in geodynamo simulations to providing geochronological constraints and modeling the influence of geomagnetic variations on cosmogenic isotope production rates.

Description: The production rate of cosmogenic nuclides such as 10 Be varies as a function of changes in primary cosmic ray flux, shielding by the Earth’s magnetic field and solar activity. Primary cosmic ray flux is generally assumed to be constant on short to million year time scales. The solar effect is generally smaller than the geomagnetic one on centennial and longer time scales. Higher frequencies solar modulation can be filtered out in order to minimize the solar influence. Another approach involves eigenanalysis techniques for separating the patterns of the geomagnetic field, the solar component and other inherent sources. The progress in global geomagnetic field reconstruction on long time scales provides estimates of dipole moment and regional intensity variations for any location on Earth. The new global, time-dependent, 100 ka geomagnetic field model (GGF100k) moreover offers the possibility to estimate the geomagnetic shielding against the incoming flux of cosmic rays in terms of the time evolution of the vertical cutoff rigidity, which is the minimum rigidity a vertically incoming particle must have in order to penetrate the geomagnetic field. We compiled more than 40 globally distributed records of 10 Be from sediments, loess and ice cores. As a first step, a global stack record of 10 Be production rates is built and compared with geomagnetic field intensity variations. The agreement is significantly better when only high resolution 10 Be records are included in the stack. Some sediment locations that provide both types of signals are first used to check for any delays in recordings of the signal in the two records. We particularly focus on investigating the Laschamp excursion ( ∼ 41 ka), when the lowest dipole moment is observed in the GGF100k model. We ultimately aim to utilize the 10 Be records to better constrain the minimum field intensity during the excursion.

Description: Reconstructions of the geomagnetic field on long timescales are important to understand the geodynamo processes in the Earth’s core and validate the field behavior observed in numerical simulations. A compilation of paleomag- netic sediment records and lava flow data was used to constrain the first global, time-dependent, geomagnetic field model spanning the past 100ka (GGF100k). We introduced smoothing kernels in the forward modeling to account for the different temporal resolution of the paleomagnetic sediment records and to fit higher resolution records bet- ter. The GGF100k model shows comparable spatial, but less temporal resolution compared to available Holocene geomagnetic field models. Using the GGF100k model we studied the dipole moment variations over the past 100 ka, the paleosecular variation activity, the time-averaged field, the evolving structure at the core-mantle boundary, and criteria for Earth-like behavior in geodynamo simulations. Axial dipole moment variations show a good agree- ment with the paleointensity stacks GLOPIS-75 and PISO-1500. The lowest dipole moment over the past 100 ka is observed during the Laschamp excursion at 40.9 ka BP, with a value of 2.44 × 10 22 Am 2 . The Laschamp field instability starts when reverse flux patches appear in low to mid latitudinal regions at the core-mantle boundary and then move towards the poles. The inverse flux patches enter the tangent cylinders and fluctuate inside for about 2 ka at different periods, first in the Northern hemisphere at 41.4 ka BP and then about 2.5 ka later in the Southern hemisphere. Apart from the Laschamp excursion that is recorded in most of the sediment records, other excursions appear only in limited regions and cannot be characterized as global events.

Description: The geomagnetic field variations on the continent of Africa are still largely undeciphered for the past two millennia. In spite of archaeological artefacts being reliable recorders of the ancient geomagnetic field strength, only few data have been reported for this continent so far. Here we use the Thellier-Coe and calibrated pseudo-Thellier methods to recover archaeointensity data from Burkina Faso and Ivory Coast (West Africa) from well-dated archaeological artefacts. By combining our 18 new data with previously published data from West Africa, we construct a reference curve for West Africa for the past 2000 years. To obtain a reliable curve of the archaeointensity variation, we evaluate a penalized smoothing spline fit and a stochastic modelling method, both combined with a bootstrap approach. Both intensity curves agree well, supporting the confidence in our proposed intensity variation during this time span, and small differences arise from the different methodologies of treating data and uncertainties. Two prominent peaks at around 740 AD and 1050 AD appear to be common in ours and several reference curves from other locations, indicating a general westward movement from China to Hawaii of a rather stable feature of the geomagnetic field. However, independent smaller peaks that do not correlate in different locations may hint to localized expressions of the geomagnetic field as a result of temporarily varying non-dipole sources.

Description: Global data compilations and the production of time-varying paleomagnetic field models over the past hundred thousand years provide insights into geomagnetic field evolution.

Description: There has been longstanding controversy about whether the influence of lateral variations in core-mantle boundary heat flow can be detected in paleomagnetic records of geomagnetic field behavior. Their signature is commonly sought in globally distributed records of virtual geomagnetic pole (VGP) paths that have been claimed to exhibit specific longitudinal preferences during polarity transitions and excursions. These preferences have often been linked to thermal effects from large low seismic velocity areas (LLVPs) in the lowermost mantle, but the results have been contested because of potential sensitivity to sparse temporal and spatial sampling. Recently developed time varying global paleofield models spanning various time intervals in 1–100 ka, three of which include excursions, allow us to complement assessments of spatial distributions of transitional VGP paths with distributions of minimum field intensity. Robustness of the results is evaluated using similar products from four distinct numerical dynamo simulations with and without variable thermal boundary conditions and including stable geomagnetic polarity, excursions and reversals. We determine that VGP distributions are less useful than minimum field intensity in linking the influences of thermal CMB structure to geographical variations in actual paleofield observables, because VGP correlations depend strongly on good spatial sampling of a sufficient number of relatively rare events. These results provide a basis for evaluating comparable observations from four paleofield models. The distribution of VGP locations provide unreliable results given the restricted time span and available data locations. Rough correlations of global distributions of minimum intensity with areas outside the LLVPs give some indications of mantle control during excursions, although the results for the eastern hemisphere are complex, perhaps highlighting uncertainties about the hemispheric balance between thermal and compositional variations in the lowermost mantle. However, access to other geomagnetic properties (such as intensity and radial field at the CMB) provides a strong argument for using extended and improved global paleofield models to resolve the question of mantle influence on the geodynamo from the observational side.