ALBERT

Description: Characterization of geomagnetic field behaviour on timescales of centuries to millennia is necessary to understand the mechanisms that sustain the geodynamo and drive its evolution. As Holocene paleomagnetic and archeomagnetic data have become more abundant, strategies for regularized inversion of modern field data have been adapted to produce numerous time-varying global field models. We evaluate the effectiveness of several approaches to inversion and data handling, by assessing both global and regional properties of the resulting models. Global Holocene field models cannot resolve Southern hemisphere regional field variations without the use of sediments. A standard data set is used to construct multiple models using two different strategies for relative paleointensity calibration and declination orientation and a selection of starting models in the inversion procedure. When data uncertainties are considered, the results are similar overall regardless of whether we use iterative calibration and reorientation, or co-estimation of the calibration and orientation parameters as part of the inversion procedure. In each case the quality of the starting model used for initial relative paleointensity calibration and declination orientation is crucial and must be based on the best absolute information available. Without adequate initial calibration the morphology of dipole moment variations can be recovered but its absolute value will be correlated with the initial intensity calibrations, an effect that might be mitigated by ensuring an appropriate fit to enough high quality absolute intensity data with low uncertainties. The declination reorientation mainly impacts regional field structure and in the presence of non-zonal fields will result in a non-zero local average. The importance of declination orientation is highlighted by inconsistencies in the West Pacific and Australian sediment records in CALS10k.1b model. Great care must also be taken to assess uncertainties associated with both paleomagnetic and age data and to evaluate the effects of poor data distribution. New consistently allocated uncertainty estimates for sediment paleomagnetic records highlight the importance of adequate uncertainties in the inversion process, as they determine the relative weighting among the data and overall normalized misfit levels which in turn influence the complexity of the inferred field models. Residual distributions suggest that the most appropriate misfit measure is the L 1 norm (minimum absolute deviation) rather than L 2 (least squares), but this seems to have relatively minor impact on the overall results. For future Holocene field modelling we see a need for comprehensive methods to assess uncertainty in individual archeomagnetic data so that these data or models derived from them can be used for reliable initial relative paleointensity calibration and declination orientation in sediments. More work will be needed to assess whether co-estimation or an iterative approach to inversion is more efficient overall. This would be facilitated by realistic and globally consistent data and age uncertainties from the paleomagnetic community.

Description: A recent study of the Matuyama–Brunhes (M-B) geomagnetic field reversal recorded in exposed lacustrine sediments from the Sulmona Basin (Italy) provided a continuous, high-resolution record indicating that the reversal of the field direction at the terminus of the M-B boundary (MBB) occurred in less than a century, about 786 ka ago. In the sediment, thin (4–6 cm) remagnetized horizons were recognized above two distinct tephra layers—SUL2-19 and SUL2-20—that occur ~25 and ~35 cm below the MBB, respectively. Also, a faint, millimetre-thick tephra (SUL2-18) occurs 2–3 cm above the MBB. With the aim of improving the temporal resolution of the previous Sulmona MBB record and understanding the possible influence of cryptotephra on the M-B record in the Sulmona Basin, we performed more detailed sampling and analyses of overlapping standard and smaller samples from a 50 cm-long block that spans the MBB. The new data indicate that (i) the MBB is even sharper than previously reported and occurs ~2.5 cm below tephra SUL2-18, in agreement with the previous study; (ii) the MBB coincides with the rise of an intensity peak of the natural remanent magnetization (NRM) intensity, which extends across SUL2-18; (iii) except for a 2-cm-thick interval just above tephra SUL2-18, the rock magnetic parameters ( k , ARM, M r , M s , B c , B cr ) indicate exactly the same magnetic mineralogy throughout the sampled sequence. We conclude that either SUL2-18 resulted in the remagnetization of an interval of about 6 cm (i.e. during the NRM intensity peak spanning ~260 ± 110 yr, according to the estimated local sedimentation rate), and thus the detailed MBB record is lost because it is overprinted, or the MBB is well recorded, occurred abruptly about 2.5 cm below SUL2-18 and lasted less than 13 ± 6 yr. Both hypotheses challenge our understanding of the geomagnetic field behaviour during a polarity transition and/or of the NRM acquisition process in the Sulmona lacustrine sediment.

Description: Monthly means of the magnetic field measurements at ground observatories are a key data source for studying temporal changes of the core magnetic field. However, when they are calculated in the usual way, contributions of external (magnetospheric and ionospheric) origin may remain, which make them less favourable for studying the field generated by dynamo action in the core. We remove external field predictions, including a new way of characterizing the magnetospheric ring current, from the data and then calculate revised monthly means using robust methods. The geomagnetic secular variation (SV) is calculated as the first annual differences of these monthly means, which also removes the static crustal field. SV time-series based on revised monthly means are much less scattered than those calculated from ordinary monthly means, and their variances and correlations between components are smaller. On the annual to decadal timescale, the SV is generated primarily by advection in the fluid outer core. We demonstrate the utility of the revised monthly means by calculating models of the core surface advective flow between 1997 and 2013 directly from the SV data. One set of models assumes flow that is constant over three months; such models exhibit large and rapid temporal variations. For models of this type, less complex flows achieve the same fit to the SV derived from revised monthly means than those from ordinary monthly means. However, those obtained from ordinary monthly means are able to follow excursions in SV that are likely to be external field contamination rather than core signals. Having established that we can find models that fit the data adequately, we then assess how much temporal variability is required. Previous studies have suggested that the flow is consistent with torsional oscillations (TO), solid body-like oscillations of fluid on concentric cylinders with axes aligned along the Earth's rotation axis. TO have been proposed to explain decadal timescale changes in the length-of-day. We invert for flow models where the only temporal changes are consistent with TO, but such models have an unacceptably large data misfit. However, if we relax the TO constraint to allow a little more temporal variability, we can fit the data as well as with flows assumed constant over three months, demonstrating that rapid SV changes can be reproduced by rather small flow changes. Although the flow itself changes slowly, its time derivative can be locally (temporally and spatially) large, in particular when and where core surface secular acceleration peaks. Spherical harmonic expansion coefficients of the flows are not well resolved, and many of them are strongly correlated. Averaging functions, a measure of our ability to determine the flow at a given location from the data distribution available, are poor approximations to the ideal, even when centred on points of the core surface below areas of high observatory density. Both resolution and averaging functions are noticeably worse for the toroidal flow component, which dominates the flow, than the poloidal flow component, except around the magnetic equator where averaging functions for both components are poor.

Description: The Lower Danube Basin is one of the most important loess regions from Europe, which have provided excellent archives for long-term high-resolution palaeoclimate studies. The aim of this paper is to derive new information on the Middle–Late Pleistocene palaeoenvironment from a high resolution multiproxy assessment of the iron mineralogical composition at the Costinesti loess-palaeosol sequence located on the western Black Sea shore. It is the easternmost loess section in the Romanian loess region studied and its distinct pattern of the proxy records can be used to correlate the lower Danube loess to other key sites of the Moldavia and Ukraine loess regions. To investigate the climatic control on soft and hard ferromagnetic minerals we used several types of rock magnetic properties: magnetic susceptibility and its frequency dependence, anhysteretic remanent magnetization, isothermal remanent magnetization, hysteresis properties and FORC distributions, an unmixing model for isothermal remanent magnetization curves and high field (up to 8 T) isothermal remanence measurements. Our results show that the palaeosol horizons, formed during interglacials and climatically more favored periods of the Pleistocene, experienced pedogenic alteration, resulting in high amounts of superparamagnetic, single domain and pseudosingle domain magnetite/maghemite grains and hematite. The loess layers, formed during glacial periods, are mainly dominated by multidomain and/or pseudosingle domain oxidized magnetite and some hematite, all probably of aeolian origin. Goethite contribution is probably minor and constant both in loess and palaeosol horizons. We review the correlation of the loess sections from the lower Danube basin concluding that the new results can be interpreted as a support for the transition of a Mediterranean type climate to a steppe type climate in the last two interglacial periods in the western Black Sea. Because the pattern of magnetic susceptibility data from the lower Danube basin is changing relative fast with distance from the Black Sea shore, it probably reflects the local influence of the Black Sea on continental scale climatic oscillations during the last 600 ka. The values of background magnetic susceptibility of the Romanian loess-palaeosol sections indicate that the main source area of the dust changed during this climatic transition. Our analysis also shows that the age of the loess-palaeosol sections from the Eastern European low lands (Moldavia and Ukraine) must be revised to be in agreement with the chronostratigraphy of the sections from the Lower Danube Basin loess area.

Description: We present a new technique for modelling the global lithospheric magnetic field at Earth's surface based on the estimation of equivalent potential field sources. As a demonstration we show an application to magnetic field measurements made by the CHAMP satellite during the period 2009–2010 when it was at its lowest altitude and solar activity was quiet. All three components of the vector field data are utilized at all available latitudes. Estimates of core and large-scale magnetospheric sources are removed from the measurements using the CHAOS-4 model. Quiet-time and night-side data selection criteria are also employed to minimize the influence of the ionospheric field. The model for the remaining lithospheric magnetic field consists of magnetic equivalent potential field sources (monopoles) arranged in an icosahedron grid at a depth of 100 km below the surface. The corresponding model parameters are estimated using an iteratively reweighted least-squares algorithm that includes model regularization (either quadratic or maximum entropy) and Huber weighting. Data error covariance matrices are implemented, accounting for the dependence of data variances on quasi-dipole latitude. The resulting equivalent source lithospheric field models show a degree correlation to MF7 greater than 0.7 out to spherical harmonic degree 100. Compared to the quadratic regularization approach, the entropy regularized model possesses notably lower power above degree 70 and a lower number of degrees of freedom despite fitting the observations to a very similar level. Advantages of our equivalent source method include its local nature, the possibility for regional grid refinement and the production of local power spectra, the ability to implement constraints and regularization depending on geographical position, and the ease of transforming the equivalent source values into spherical harmonics.

Description: The rocks in the crust and the upper mantle of the Earth are believed to exhibit electrical anisotropy to some extent. It is beneficial to further understand and recognize the propagation of the electromagnetic waves in the Earth by investigating the magnetotelluric (which is one of the main geophysical techniques to probe the deep structures in the Earth) responses of the media with anisotropic conductivity structures. In this study, we examine the magnetotelluric fields over an idealized 2-D model consisting of two segments with axially anisotropic conductivity structures overlying a perfect conductor basement by a quasi-static analytic approach. The resulting analytic solution could not only contribute to the electromagnetic induction theory in the anisotropic Earth but also serve as at least an initial standard solution which could be used to validate the reliability and accuracy of the numerical algorithms developed for modelling the magnetotelluric responses of the 2-D media with much more general anisotropic conductivity.

Description: A series of important geological events occurred in the Tibetan Plateau area during the Jurassic, such as the collision of the Lhasa and Qiangtang Terranes, the closure of the Meso-Tethyan Ocean, the opening of the Neo-Tethyan Ocean and the cessation of the mega-monsoon. The ~3000 m thick Jurassic sedimentary sequence in the Qiangtang Basin on the central Tibetan Plateau, which is called the Yanshiping (YSP) Group, recorded these geological events. However, the chronology of the sequence is surprisingly poorly constrained. Here, we perform a detailed palaeomagnetic analysis on the ~1060 m thick middle and upper portions of the YSP Group (the Xiali and Suowa Formations) in the YSP section of the eastern Qiangtang Basin. Three bivalve zones at stratigraphic intervals of ~40–140, 640–800 and 940–1040 m are identified, which yield a Bathonian–Callovian age for the Lower Xiali Fm., a Callovian–Oxfordian age for the Lower Suowa Fm. and an Oxfordian–Kimmeridgian age for the Upper Suowa Fm. A total of 544 oriented palaeomagnetic samples were collected from the section. By combining thermal and alternating field demagnetizations, clear characteristic remanent magnetization (ChRM) directions are isolated for most of the samples. The robust ChRM directions pass fold and reversals tests, which support the primary nature of the ChRMs and yield a palaeopole at 76.8°N/297.2°E (dp = 2.2°, dm = 3.7°). A total of 27 normal and 26 reversed polarity zones were successfully recorded in the section. Combined with fossil age constraints, results suggest that the section is plausibly composed of a Callovian-Early Kimmeridgian age sedimentary sequence.

Description: Was the Yukon-Tanana Terrane (YTT), a California-sized part of south-central Yukon, an autochthonous or para-autochthonous part of northern British Columbia in the Early Cretaceous or was it part of a proposed allochthonous ‘Baja B.C.’ continent offshore of southern California? To answer this fundamental question, a paleomagnetic study has been completed on 347 specimens from 24 sites in the 114.7 ± 1.1 Ma Quiet Lake batholith. This 1300 km 2 pluton is composed mostly of massive medium-to-coarse grained biotite quartz monzonite that exhibits no evidence of either deformation or metamorphism, and that intrudes metamorphosed pre-Cretaceous basement rocks of the YTT in southern Yukon. The paleomagnetic analysis utilized thermal and alternating field step demagnetization, and saturation isothermal remanence methods. A well-defined characteristic remanent magnetization (ChRM) direction was isolated throughout the 500–585 °C temperature range at Decl. = 340.6°, Incl. = 77.4° ( N = 14 sites, k = 51.2, A 95 = 5.6°). The ChRM resides in magnetite with a low titanium content and is interpreted to be a primary thermoremanent magnetization. After correction for 490 km of geologically demonstrable dextral displacement on the inboard Tintina fault zone, the Quiet Lake batholith's paleopole is not significantly different at 95 per cent confidence from the co-eval 115 Ma reference paleopole for North America, giving non-significant translation and rotation estimates of 1.4° ± 5.1° (1) northwestwards and 10° ± 13° (1) clockwise, respectively. Thus, this is the first Early Cretaceous paleopole to show clearly that the YTT in Yukon is a para-autochthon that was part of North America's continental margin at that time. Further, after correction for Tintina fault displacement, the eight available Mesozoic YTT paleopoles agree closely with the North American apparent polar wander path (APWP). In contrast, the 22 paleopoles from the Intermontane Belt show the expected behaviour of an allochthonous thin-skin of terranes (i.e. ‘Baja B.C.’ behaviour) and record ~8° of northward translation and ~50° of clockwise rotation relative to both the YTT and North American APWPs during the Mesozoic and Paleogene.

Description: ‘Anomalous’ magnetic fabrics in dikes that appear to indicate flow into the wall confound many workers. Here, we present extensive magnetic data on five dikes from Tenerife, Canary Islands, and use these to interpret the causes of the anomalous fabrics. Comparison of the anisotropy of magnetic susceptibility (AMS) and anhysteretic magnetization (AARM) results show that, in some cases, the anomalous fabrics are caused by single-domain grains, which produce AMS fabrics perpendicular to the grain elongation, whereas AARM fabrics are parallel. To check this, hysteresis experiments were used to characterize the domain state. These show most are mixtures of pseudo-single-domain or single-domain plus multi-domain particles, but many have wasp-waisted hysteresis loops, likely indicating mixed populations of stable single-domain and superparamagnetic grains. First-order reversal curves were used to better characterize this and show mixtures of stable single-domain and superparamagnetic grains dominate the magnetic signal. Magnetic particles at the stable single-domain/superparamagnetic threshold are unstable at timespans relevant to the analytical techniques, so they produce complicated results. This suggests that anomalous AMS fabrics in dikes cannot simply be attributed to elongated stable single-domain particles and that mixtures of the different grain types can produce hybrid fabrics, in which the fabrics are neither perpendicular or parallel to the dike plane, that are difficult to interpret without extensive magnetic analysis.

Description: We present a detailed palaeomagnetic study from 35 sites on Holocene lava flows of the Tongariro Volcanic Centre, central North Island, New Zealand. Prior to the study the eruption ages of these flows were constrained to within a few thousand years by recently published high-precision 40 Ar/ 39 Ar geochronological data and tephrostratigraphic controls. Correlation of flow mean palaeomagnetic directions with a recently published continuous sediment record from Lake Mavora, Fiordland, allows us to reduce the age uncertainty to 300–500 yr in some cases. Our refined ages significantly improve the chronology of Holocene effusive eruptions of the volcanoes of the Tongariro Volcanic Centre. For instance, differences in the palaeomagnetic directions recorded by lavas from the voluminous Iwikau and Rangataua members suggest that individual effusive periods lasted up to thousands of years and that these bursts have been irregularly spaced over time. While over the last few millennia the effusive eruptive activity from Mt Ruapehu has been relatively quiet, the very young age (200–500 BP) of a Red Crater sourced flow suggests that effusive activity around Mt Tongariro lasted into the past few centuries. This adds an important hazard context to the historical record, which has otherwise comprised frequent relatively small, tephra producing, explosive eruptions without the production of lava flows.