The debated question on the possible relation between the Earth's magnetic field and climate has been usually focused on direct correlations between different time series representing both systems. ...However, the physical mechanism able to potentially explain this connection is still an open issue. Finding hints about how this connection could work would suppose an important advance in the search of an adequate physical mechanism. Here, we propose an innovative information-theoretic tool, i.e. the transfer entropy, as a good candidate for this scope because is able to determine, not simply the possible existence of a connection, but even the direction in which the link is produced. We have applied this new methodology to two real time series, the South Atlantic Anomaly (SAA) area extent at the Earth's surface (representing the geomagnetic field system) and the Global Sea Level (GSL) rise (for the climate system) for the last 300 years, to measure the possible information flow and sense between them. This connection was previously suggested considering only the long-term trend while now we study this possibility also in shorter scales. The new results seem to support this hypothesis, with more information transferred from the SAA to the GSL time series, with about 90% of confidence level. This result provides new clues on the existence of a link between the geomagnetic field and the Earth's climate in the past and on the physical mechanism involved because, thanks to the application of the transfer entropy, we have determined that the sense of the connection seems to go from the system that produces geomagnetic field to the climate system. Of course, the connection does not mean that the geomagnetic field is fully responsible for the climate changes, rather that it is an important driving component to the variations of the climate.
•New geomagnetic field reconstruction (SHAWQ2k) has been obtained for the last 2 ka.•A novelty weighting scheme based on quality-data is applied in modeling process.•New hints about the past ...dynamical behavior of the South Atlantic Anomaly are shown.•Reversed flux patch linked to the South Atlantic Anomaly is detected since 950 AD.
The South Atlantic Anomaly (SAA) is one of the most outstanding features of the present geomagnetic field. Thus, a good knowledge of the SAA is clue for a better understanding of the dynamical behavior of the geomagnetic field. To achieve this goal, paleomagnetic data are crucial since they provide the unique way to investigate past changes in the Earth's magnetic field. Here, we present a new global geomagnetic field reconstruction, the SHAWQ2k model, which is based on a critical revision of the global archeomagnetic and volcanic dataset. The new model provides an improved description of the geomagnetic field during the last 2 millennia, and yields surprising outcomes about the emergence and development of the SAA. It shows that the reversed flux patch observed at the core-mantle boundary and linked to the SAA, emerged in the Southern Hemisphere from at least 950 AD. This patch moved westward from the equator to southern latitudes, being clearly isolated after 1400 AD. In addition, since 1550 AD a second reversed flux patch moving northeastward is observed in the North Atlantic. The new data now available for the Southern Hemisphere coming from Africa and South America together with the use of an appropriated weighting scheme in the modeling process have improved our understanding of past geomagnetic field behavior and showed new evolutionary aspects of the SAA.
The identification of epsilon iron oxide (ɛ‐Fe2O3) as the low Curie temperature high coercivity stable phase (HCSLT) carrying the remanence in heated archeological samples has been achieved in ...samples from two archeological sites that exhibited the clearest evidence of the presence of the HCSLT. This uncommon iron oxide has been detected by Confocal Raman Spectroscopy (CRS) and characterized by rock magnetic measurements. Large numbers of ɛ‐Fe2O3 microaggregates (in CO) or isolated clusters (in HEL) could be recognized, distributed over the whole sample, and embedded within the ceramic matrix, along with hematite and pseudobrookite and with minor amounts of anatase, rutile, and maghemite. Curie temperature estimates of around 170°C for CO and 190°C for HEL are lower than for pure, synthetic ɛ‐Fe2O3 (227°C). This, together with structural differences between the Raman spectra of the archeologically derived and synthetic samples, is likely due to Ti substitution in the ɛ‐Fe2O3 crystal lattice. The γ‐Fe2O3‐ɛ‐Fe2O3‐α‐Fe2O3 transformation series has been recognized in heated archeological samples, which may have implications in terms of their thermal history and in the factors that govern the formation of ɛ‐Fe2O3.
Key Points
ɛ‐Fe2O3, a rare iron oxide polymorph, has been identified in archeological brick and baked clay
It has been found as part of the γ‐Fe2O‐ɛ‐Fe2O3‐α‐Fe2O3 transformation series
CRS and mineral magnetic measurements have proved a powerful combination for identifying complex magnetic mixtures in natural samples
•New archaeomagnetic data from the Early Iron Ages were obtained for Spain.•High geomagnetic fluctuation rates up to 8° and 16 μT/century were observed.•A new reconstruction model (SHAWQ-Iron Age) ...has been obtained.•New hints about the evolution of the Levantine Iron Age Anomaly (LIAA) are shown.•Normal flux patch (NFP) linked to the LIAA is observed at the core-mantle boundary.
Variations of geomagnetic field in the Iberian Peninsula prior to Late Iron Age times are poorly constrained. Here we report 14 directional and 10 palaeointensity results from an archaeomagnetic study carried out on 17 combustion structures recovered from six archaeological sites in eastern Spain. The studied materials have been dated by archaeological evidences and supported by radiocarbon dates (8th-5th centuries BC). Rock magnetic experiments indicate that the characteristic remanent magnetization is carried by a low coercivity magnetic phase with Curie temperatures of 500-575°C, most likely titanomagnetite/maghemite with low titanium content. Archaeointensity determinations were carried out by using the classical Thellier-Thellier experiment including pTRM-checks and magnetic anisotropy corrections. A new full vector Iberian Paleosecular Variation Curve for the Iron Age is presented. High fluctuation rates on both directions and intensities are observed during the Early Iron times that seems to be related with the Levantine Iron Age Anomaly (LIAA), the most prominent anomaly of the geomagnetic field of the last three millennia. Two intensity maxima were observed at Iberian coordinates, the oldest around 750 BC (associated with easterly declinations of around 23°) and the second 275 yrs later (475 BC) with northerly directions. The related virtual axial dipole moment was up to 14⋅1022 Am2 for the oldest materials (750 BC) and reaching 16⋅1022 Am2 for the materials corresponding to the end of the Early Iron Age.
In order to investigate the origin of the unusually high fluctuations of the palaeofield we have developed a new global geomagnetic field reconstruction, the SHAWQ-IronAge model, which is based on a critical revision of the global archeomagnetic and volcanic dataset. The new model provides an improved description of the evolution of the LIAA, which is related to a normal flux patch at the core-mantle boundary (CMB) below Arabian Peninsula clearly observed at around 950 BC. This flux patch expanded towards the north-west, while decreasing in intensity, reaching Iberia at around 750 BC. Around 600-500 BC, it underwent a revival below the European continent after that it seems to vanish in situ.
The main objective of this work is to compare directional (declination and inclination) volcanic and archaeomagnetic data for the last four centuries (~1600–1990) with the historical geomagnetic ...predictions given by the GUFM1 model which spans from 1590 to 1990. The results show statistical agreement between archaeomagnetic data and directions given by the geomagnetic field model. However, when comparing the volcanic data with the model predictions, marked inclination shallowing is observed. This systematically lower inclination has already been observed in local palaeomagnetic studies (Italy, Mexico and Hawaii) for the 20th century, by comparing recent lava flows with the International Reference Geomagnetic Field (IGRF) model. Here, we show how this inclination shallowing is statistically present at worldwide scale for the last 400 years with mean inclination deviation around 3° lower than the historical geomagnetic field model predictions.
The widespread occurrence of a novel, high coercivity magnetic phase in well‐heated archeological material is reported. Its properties are defined when it represents the dominant magnetic phase, ...although it is nearly always found as part of a mixture of magnetic phases. They are as follows: very high coercivity (remanence coercivity >600 mT), low unblocking temperatures (≤200°C) and high degree of thermal stability–this last property distinguishing it from goethite. The phase shows striking similarities to magnetic phases produced by thermal decomposition of nontronite (an Fe‐rich clay), where decomposition occurs after prolonged heating in air to high temperatures – conditions suffered by well‐heated archeomagnetic material. Preliminary results of Mössbauer and X‐Ray diffraction spectroscopy suggest that the phase is more likely to be a substituted hematite, rather than Fe‐cristobalite or a variant of ɛ‐Fe2O3.
Recent paleomagnetic studies have shown that important short‐lived intensity fluctuations occurred during the first millennium BCE. However, the knowledge of the spatial and temporal extension of ...these features is still limited by the scarce availability of robust data. In this study we focus on the study of the intensity decrease that took place in Central Asia during the second half of the 1st millennium BCE after the high intensities that characterized the Levantine Iron Age Anomaly. Since previous archeointensities available for this period and region were obtained without accomplishing modern standards of quality, we present here new archeointensities that are derived from classical Thellier and Thellier experiments, including partial thermoremanent magnetization (pTRM) checks, thermoremanent magnetization (TRM) anisotropy and cooling rate corrections at the specimen level. The new 51 archeointensities, together with previous archeointensities, have been used to present a new local paleosecular variation curve for Central Asia. The results confirm the existence of an important geomagnetic field intensity decrease in South Uzbekistan from the 4th century BCE to the end of the 1st century BCE associated with rates of changes up to −15 μT/century. A critical analysis of the archeointensity global database indicates that this feature was present at continental scale, from Western Europe to Central Asia. However, this trend is not identified in other regions such as Japan or Mexico. Finally, the comparison with the dipole moment derived from recent global geomagnetic field reconstructions suggests a strong influence of non‐dipolar sources upon this continental intensity feature.
Plain Language Summary
The Earth's magnetic field (also called the geomagnetic field) is continuously changing over time and space. Since the 19th century, the behavior of the geomagnetic field is known through direct observations performed in geomagnetic observatories and satellites. However, the only way to recover past geomagnetic field in ancient times is through paleomagnetic and archeomagnetic studies. In particular, archeomagnetism is the study of heated archeological materials, which include ferromagnetic particles acting as a compass and recording the geomagnetic field direction and strength at a certain time. When the archeological material cools down the ferromagnetic particles save the direction and intensity, like a photograph of the ancient geomagnetic field. Nowadays, there are very few archeomagnetic studies dealing with archeological materials from Central Asia. In this work, we present the variation of the geomagnetic field intensity between 600 BCE–600 CE in Uzbekistan deduced from a complete archeomagnetic investigation of 70 pottery fragments. The results confirm a rapid variation of geomagnetic field strength between 400 BCE and 100 BCE–1CE. To determine the spatial expression of this feature, we compared the obtained trend for Central Asia with the geomagnetic field intensity evolution in other regions of the world.
Key Points
We present 51 new thermoremanent magnetization‐anisotropy corrected archeointensities for South Uzbekistan
An important decrease in intensity is observed between 400 BCE and 100 BCE–1 CE, probably associated with non‐dipolar sources
Similar intensity V‐shaped trends are observed at continental scale
The production of cosmogenic isotopes offers a unique way to reconstruct solar activity during the Holocene. It is influenced by both the solar and Earth magnetic fields and thus their combined ...effect needs to be disentangled to infer past solar irradiance. Nowadays, it is assumed that the long-term variations of cosmogenic production are modulated by the geomagnetic field and that the solar field dominates over shorter wavelengths. In this process, the effects of the non-dipolar terms of the geomagnetic field are considered negligible. Here we analyse these assumptions and demonstrate that, for a constant solar modulation potential, the geomagnetic field exerts a strong modulation of multi-centennial to millennial wavelengths (periods of 800 and 2200 yr). Moreover, we demonstrate that the non-dipole terms derived from the harmonic degree 3 and above produce maximum differences of 7% in the global average radiocarbon production rate. The results are supported by the identification, for the first time, of a robust coherence between the production rates independently estimated from geomagnetic reconstructions and that inferred from natural archives. This implies the need to review past solar forcing reconstructions, with important implications both for the assessment of solar-climate relationships as well as for the present and future generation of paleoclimate models.
•New 37 archeointensities for the Central Mediterranean (1500 BCE–150 CE).•First evidence of a rapid intensity change between 1070 and 1040 BCE in Greece.•The Levantine Iron Age Anomaly (LIAA) is ...present in the Central Mediterranean.•A global analysis suggests that the LIAA vanishes to the west and to the east.
The magnitude and origin of the Levantine Iron Age geomagnetic Anomaly (LIAA), which spanned the first half of the first millennium before the common era, are not yet well understood. Recent archeomagnetic studies from the Levant and Western Europe suggest a western drift of this feature, stressing the importance of investigating the temporal and spatial behaviour of this event over the Central Mediterranean area. To analyse this issue, we here present 37 new archeointensity data obtained from the archeomagnetic study of 118 ceramics and brick fragments collected in 8 archeological sites in Greece and Italy with ages ranging between 1500 BCE and 150 CE. The samples were analysed using the classical Thellier and Thellier method for paleointensity determination, including the correction for the anisotropy effect of the thermoremanent magnetization (TRM) and for the cooling rate dependence upon TRM acquisition. The results reveal the first evidence of a high-intensity peak in Greece between 1070 and 1040 BCE associated to high virtual axial dipole moment (VADM) values of around 140 ZAm2. A global analysis of available paleointensities suggests that the origin of these high values is the same to the one which produced the maximum VADM of the LIAA in the Levantine region. Our results suggest that the source of the LIAA is located in the Levantine region vanishing to the north, to the west and to the east where lower VADMs are observed. In addition, another high intensity maximum, less pronounced than the one of the LIAA, seems to be present around 500 BCE all over Europe, from the Canary Islands to Turkey showing similar VADM values (around 150 ZAm2) in the different regions. Both events seem to span over a large region at the Earth's surface covering more than 60° of longitude, verifying an Earth's outer core origin for these intensity features.
•TRM anisotropy correction is an essential requirement in paleointensity studies.•Four new high-quality archeointensities are presented for NW Argentina.•The decrease of geomagnetic field intensity ...started in South America after 1600 CE.
A good characterization of the geomagnetic field strength at centennial to millennial time scales in the Southern Hemisphere is particularly crucial to disentangle the long-term evolution of the South Atlantic Anomaly (SAA), an intriguing geomagnetic feature currently observed at the Earth’s surface. Here we present 59 new archeointensities obtained from four well-dated groups of potteries with ages ranging between 1300 and 1500 CE and collected in Northwest Argentina. The new data were obtained in accordance with the Thellier paleointensity method including partial thermoremanent magnetization (pTRM) checks and TRM anisotropy and cooling rate corrections. We have also performed a comparative study of the efficiency of magnetic susceptibility, ARM and TRM anisotropy tensors to correct the TRM anisotropy effect upon intensity estimates. Our results suggest that the magnetic susceptibility tensor systematically underestimates the TRM anisotropy effect by 10–30%. Our new data, together with selected archeointensities already published, confirm that the decrease of the geomagnetic field intensity in South America started around 1600–1650 CE, due to the arrival of the SAA.
PDF
