Abstract In the last decades, palaeomagnetic research has provided us with a picture of the temporal and spatial behaviour of the Earth's magnetic field (EMF) from its origin up to the present day. Well-dated palaeomagnetic data offer important sources of information about the past variation of the geomagnetic field and have shown that it is characterized by temporal fluctuations such as reversals, excursions and spikes. Despite the advances in our understanding of EMF behaviour, the current dataset is biased towards high and northern latitudes and, therefore, several questions remain open to debate, such as the origin and evolution of the EMF and the frequency and spatial distribution of its variations. This Special Publication focuses on the study of the temporal and spatial evolution of the EMF in the past through new data from palaeomagnetic and rock magnetic studies of archaeological materials, sediments and lavas from Europe, Africa, Australia, New Zealand, India and Baltic Sea, and their applications in archaeology, stratigraphy and climate. This paper summarizes our current knowledge on geomagnetic field variations in the past, open questions and future challenges and gives an overview of the volume's context, which aims to disclose fundamental properties of the Earth's magnetic field evolution.

Abstract Archaeointensity and rock-magnetic studies were undertaken on 49 baked clay artefacts from four archaeological sites (Ter, Junnar, Nalasopara and Kanheri) in Maharashtra, India. Rock-magnetic properties, including bulk magnetic susceptibility, magnetic remanence and thermomagnetic analysis, indicate the presence of a low-coercivity magnetite in fine (superparamagnetic, single domain) grain-sizes. The ratio of anhysteretic remanent magnetization to saturation isothermal remanent magnetization, the reversible high-temperature susceptibility curves and the 3-axes isothermal remanent magnetization tests also indicate that the artefacts dominantly possess fine-grained magnetic particles, carrying a stable thermoremanent magnetization (TRM). Archaeointensity was estimated using Coe's modified Thellier method corresponding to the linear behaviour of natural remanent magnetization loss and TRM gained plots, which were evaluated with ThellierTool4.0 software. Cooling rate and anisotropy of the TRM corrections were applied and the corrected intensities were used to calculate a mean archaeointensity value for each one of the four sites. The new archaeointensity values were plotted along the existing Indian archaeointensity values derived only from archaeological artefacts, and were compared with the SHA.DIF.14k and ARCH10k.1 global models’ predictions. The present study aims to improve the overall understanding of Indian geomagnetic field variation in the past by providing new high-quality archaeointensity results. However, still more archaeointensity values are required to develop a reliable secular variation curve for India.

Abstract Hangi stones, used to retain heat in traditional Maori earth ovens (hangi), may carry records of Earth's magnetic field when they were last used. Sixteen archaeological features, including 12 hangi, from eight sites were sampled and their palaeomagnetic data used to construct the first archaeointensity record for New Zealand, covering the past 700 years. A combination of radiocarbon dating of associated charcoal and archaeomagnetic dating of palaeomagnetic directions was used to obtain a ‘preferred’ date of each palaeointensity. A plot of virtual axial dipole moment (VADM) for the SW Pacific region outlines steady VADM values of about 8 × 10 22 A m 2 from 1000 to 1300 AD and 9.5 × 10 22 A m 2 from 1500 AD to the present, with a sharp peak in the early fifteenth century when the VADM reached about 13 × 10 22 A m 2 . This peak bears many similarities to archaeomagnetic ‘jerks’ and ‘spikes’ in Northern Hemisphere records from the first millennia BC and AD. However, it is the first such feature to be found in the Southern Hemisphere at this date, suggesting, in accordance with recent modelling, that it may be a feature of the non-dipole field, associated with rapid growth and decay of an intense flux patch on the core–mantle boundary.

Abstract New archaeointensity results were obtained from 14 groups of baked-brick fragments collected in and around Pisa (Tuscany, Italy). The fragments were assembled from civil and religious buildings whose dating of construction or renovation, over the past millennium, was constrained by documentary sources. This collection, analysed using the Triaxe protocol, was found particularly suitable for intensity experiments, with a success rate of c. 84% corresponding to 276 fruitful specimens associated with 125 independent brick fragments. The Tuscan data clearly show a peak in intensity at the transition between the sixteenth and seventeenth centuries. They are also in very good agreement with, and complementary to, a dense dataset previously obtained in France. Considering the results available within a 700 km radius of Beaune (between Paris and Pisa), all satisfying a set of quality criteria, a mean geomagnetic field intensity variation curve was constructed for the past millennium using a newly developed transdimensional Bayesian technique. This curve, which thus incorporates the new Tuscan results, allows a better recognition of three intensity peaks (during the twelfth century, the fourteenth century and around AD 1600) in western Europe. The detail of this curve is a clear illustration of the centennial-scale resolution that can be achieved using accurate archaeointensity data.

Abstract Several archaeological baked clay structures from Bulgarian Neolithic sites were archaeomagnetically studied. According to the ancient firing conditions, the collected materials are dwelling remains and ovens with different heating histories. The variability of magnetic properties is greater in the burnt dwelling remains compared to the ovens. The prevailing magnetic minerals are magnetite/titanomagnetite and epsilon iron oxide. Less often, the presence of hematite is suggested. For one of the ovens, a well-pronounced evolution in magnetic properties was observed between its successive levels. The collected materials possess stable magnetic mineralogy and the success rate of archaeointensity determination experiment is 84%. The experiment completely failed only for the structure where epsilon iron oxide is not identified and where the lowest median destructive fields are determined. The obtained archaeomagnetic results are summarized as 10 new reference points. The structures subjected to the same firing event are combined as one feature. Their dating is based on well-established relative Neolithic chronology and stratigraphic constraints. The new data coincide fairly well with the Bulgarian dataset and with the data available for the neighbouring countries, confirming an intensity maximum around 5550 BC and a minimum around 5400 BC. No indications of geomagnetic ‘spikes’ or ‘jerks’ are detected.

Abstract The importance of the full understanding of the Holocene geomagnetic field spans from human history and archaeology, to palaeoclimatic changes and engineering, to geomagnetic field modelling and unravelling of the geomagnetic field characteristics and anomalies. Unfortunately, the dearth of data from large under-covered areas such as oceans, the African and South American continents, and the southern hemisphere (only 4–6% of the global datasets) dramatically limits our knowledge of the temporal and spatial evolution of the geomagnetic field and its application. Here, a review of all data from the African continent is presented in order to encourage and motivate a new generation of palaeomagnetic and archaeomagnetic studies. New data will sharpen the palaeomagnetism as a dating tool, improve our knowledge of local/global geomagnetic features, and will help to finally answer some of the fundamental questions in palaeomagnetism, like the temporal and spatial distribution of the palaeointensity peaks described, and the origin and evolution of the South Atlantic Anomaly.

Abstract A relative palaeointensity determination was obtained using the pseudo-Thellier technique on sediment from Ther, Tirna Basin, Latu-Osmanabad District, Maharashtra, India. The stability of the natural remanent magnetization was investigated by alternating field (AF) demagnetization. Rock magnetic studies suggest that the main carriers of magnetization are ferrimagnetic minerals, predominantly pseudo-single-domain magnetite. To varying degrees, the smoothed palaeoinclination and palaeodeclination patterns of the Tirna Basin are similar to other Asian palaeosecular variation records CALS3k.4, CALS10k.1 and SED3k.1, with an age offset. Measurements of intensity of the natural remanent magnetization left after AF demagnetization v. intensity of anhysteric remanent magnetization gained at the same peak were carried out on a set of samples. A jackknife re-sampling scheme provides error estimates for the palaeointensity. A good agreement pattern can be observed between the Tirna Basin relative palaeointensity proxy and other global curves with an age shift. Although some temporal offsets of palaeointensity features between different records have been recognized, their similar shape suggests that the palaeointensity can give a globally coherent signal and may be used as a relative dating technique. For the first time, relative palaeointensity data for the past 2 kyr from India is presented here, which complement the existing archaeological records but with an additional input.

Abstract A marine sediment core from the western Mediterranean provides a new high-resolution 4500 year record of palaeomagnetic secular variation and relative palaeointensity. In 2013, the 7.1 m C5 core was recovered from the Tyrrhenian Sea as part of the NextData climate data project. The coring site, 15 km offshore from the Volturno river mouth, is well located to record combined marine and terrestrial palaeoclimatic influences, and the fine-grained, rapidly deposited sediments are effective palaeomagnetic recorders. We investigate the palaeomagnetic field direction and strength recorded in the core, which provide a valuable high-resolution record of Holocene geomagnetic variation in the area. Using rock magnetic techniques, we constrain the magnetic mineralogy of the studied sediments and confirm their suitability for palaeomagnetic analysis. Palaeomagnetic declination and inclination records were determined by stepwise alternating-field demagnetization, and relative palaeointensity estimates were obtained based on normalization to anhysterestic and isothermal remanent magnetization and to magnetic susceptibility. The age of the core is well constrained with a tephra and biostratigraphic age model, and its magnetic records are compared with relevant core and model data for the region, demonstrating that our record is compatible with previous results from the area. An automated curve matching approach is applied to assess the compatibility of our data with the existing secular variation path for the Mediterranean area.

Abstract We have studied the rock and palaeomagnetic properties and 14 C dating of a c. 205 m core from Site M0060 (Anholt Loch, BSB1 at Kattegat), recovering clays, (silty) sands and sandy clays. We took 297 8 cc samples at c. 50 cm intervals down-core. χ was measured along with AF demagnetization of the NRM up to 80 mT. ChRM was isolated between 0 and 25 mT. A weak VRM was removed at 5 mT. The intensity shows a positive relationship with χ . At Site M0060 the upper lithologic units (i.e. 0–100 mcd) show inclinations that vary within 10° on either side of the GAD prediction (i.e. +72°). Curie points indicate minerals with temperatures of 360–400, 520, 575 and 610°C. We obtained calibrated 14 C determinations for 15 levels, with the oldest age from 78.87 mbsf to c. 17 940 cal BP. The J , inclination, χ , ARM, SIRM, SIRM/ χ and ARM/ χ palaeomagnetic (i.e. inclination) wave forms results from the top c. 100 mcd correlate well to the deglacial inclination wave forms master curve for Fennoscandia. The best correlation to this curve shows four oscillations of the inclination record of Site M0060 from 11 to 14 ka BP. Shallow negative inclinations are characteristic of the deeper coarse-grained sediments deposited during the rapid wasting of the Fennoscandian ice-sheet.

Abstract The existing controversy regarding the climatic response to given changes in the geomagnetic field intensity (e.g. climate cooling or warming in periods of geomagnetic field strengthening) raises scepticism about the existence of any relationship between both variables. The main shortcomings of the recent understanding of the geomagnetic field–climate relationship are the expectations for: (i) its universal character over the globe; and (ii) the zonal homogeneity of this relationship. This paper offers a new conceptual framework aimed at resolving the existing controversy and explaining the mechanism for the appearance of geomagnetic footprints on climatic records. We show that particles trapped in the Earth's radiation belts are subject to geomagnetic lensing in the parts of their trajectories closest to the surface, being focused or defocused at different regions over the world. The irregularly distributed ionization, created in such a way near the tropopause, activates various ion–molecular reactionships which affect the atmospheric composition there. Thus, the regional changes in the ozone and water vapour are furthermore projected onto the surface temperature and pressure. Consequently, the geomagnetic field–climate relationship is not a rigid but, rather, a flexible one because it is mediated by the near-tropopause ionization, ozone and humidity, with each of them being subject to the other's influence.