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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., Financial support was given by the Spanish Ministry of Science and Innovation thorough the projects CERAC (HAR2016-75133-C3-1-P), CONCERAC (HAR2016-75133-C3-3-P), SPYKE (CGL2017-87015-P), SÚMATE (PID2020-113316GB-I00) and PULSES 5.K (PID2020-117105RB-I00). R. Bonilla-Alba thanks the FPI fellowship (PRE2018-085949) associated with the SPYKE project. This study has been also funded by the Ramón y Cajal program (contracts of M. Gómez-Paccard and V. Martínez-Ferreras, refs. RYC-2013-14405 and RYC-2014-15789). The authors wish to acknowledge the professional support of the CSIC Interdisciplinary Thematic Platform Open Heritage: Research and Society (PTI-PAIS). E. Beamud thanks the Geomodels Research Institute (UB) and the Grup de Geodinàmica i Anàlisi de Conques-2017SGR596 (Generalitat de Catalunya). A. Palencia-Ortas thanks the PTA contract of the Spanish Ministry of Science and Innovation.

International audience; In directional archaeomagnetic studies, it is rarely analysed whether the combustion structures display anisotropy of their thermoremanent magnetization (ATRM). It has been observed that, in specific cases like thin baked clays from the base of small hearths, archaeomagnetic directions can also be disturbed by the ATRM. We re-examine data from 56 combustion structures from Iberia and use them to analyse the ATRM effect on their archaeomagnetic directions. Flattening of inclinations up to 13 degrees has been found in highly anisotropic structures that can be adequately corrected by the ATRM tensors. We show how the lack of anisotropy corrections on directional values can dramatically deteriorate the ages obtained from archaeomagnetic dating, highlighting the importance of systematically analysing this property in future archaeomagnetic studies.

The chronology of the Late Bronze Age and the earliest stages of the Iron Age in the Mediterranean is an important topic of debate since the study of both local social dynamics and trade and colonial activity around the Mediterranean requires obviously well-established chronological frameworks. However, the exact chronology of the Early Iron Age in the Mediterranean region is still a problematic issue today since different, and in some cases unbalanced, sources of information (historical texts, material culture sequences, and radiocarbon dating results) are used. The NE Iberian Peninsula is not an exception and different time periods are proposed in the literature for the Early Iron Age. Here, and in order to provide a new and an independent input to feed this debate, we apply the archeomagnetic dating method to four archeological hearths from the Sant Jaume Complex, a set of several Early Iron Age archeological sites located in the north-eastern part of the Iberian Peninsula. The archeomagnetic dating results suggest that the abandonment of the studied structures (that can be linked to the abandonment of the archeological sites) most probably occurred before 650 BC, and therefore invalidate the age commonly ascribed to the Early Iron Age sites in this area. Our study provides, hence, new evidences that the traditional view of human settlement development in the NE Iberian Peninsula should be revisited., This project was supported by the CGL2015-63888-R (MINECO/FEDER) project and by the Ramón y Cajal program (MGP) of the Spanish Ministry of Economy and Competitiveness. Financial support was also given by the PICS International Program for Scientific Cooperation (CNRS-France and CSIC-Spain).

Since the publication of the European archaeomagnetic field model SCHA.DIF.3k in 2009, the number of paleomagnetic data derived from archaeological materials such as baked clays and volcanic rocks coming from Europe has increased by about 90% for directions and around 180% for intensities. Taking advantage of this increase, here we provide an updated regional archaeomagnetic model, called SCHA.DIF.4k, for the European continent and adjacent areas and now covering the last four millennia. To model the three geomagnetic elements, declination, inclination, and intensity, we use the regional R-SCHA2D technique in space and temporal basis of cubic splines. A critical selection of the archaeomagnetic and volcanic data available in a spherical cap of 30° centered at 40°N latitude and 10°E longitude has been considered. In addition, in order to better constrain the behavior of the archaeomagnetic field during the last centuries, we include the historical data of the HISTMAG compilation. The new regional model allows us to better define the paleomagnetic field over Europe as well as to generate new paleosecular variation curves for archaeomagnetic dating purposes. Using these curves, the dating precision has been estimated for the last 4 kyr. As expected, results show that it strongly depends on the data uncertainties, the temporal data distribution and the behavior of the geomagnetic field itself. In addition, the use of the full vector geomagnetic field, instead of the directional information exclusively, provides more precise archaeomagnetic dating results.

vEGU21: Gather Online | 19-30 April 2021, Recent archeomagnetic studies performed in different regions of the world have revealed unusual periods of sharp changes in intensity during the first millennium. Here we focus on the study of intensity variations between 600 BCE and 600 CE in central Asia, where an important intensity decrease seems to be present during the second half of the 1st millennium BCE. For this purpose, we present a new paleosecular variation (PSV) curve obtained from 51 new archeointensities and the selected previous data located within a radius of 1000 km around Termez (Uzbekistan). The new curve shows an intensity maximum around 400 BCE followed by a rapid decrease. When the virtual axial dipole moment (VADM) values are compared with the Dipole Moment estimations derived from different global geomagnetic models key differences are observed, suggesting an important non-dipolar effect for this feature. Finally, in order to constrain the spatial behaviour of this phenomenon and its global implications, we investigate the PSV intensity and VADM trends from twelve regions distributed among Central America, Europe and Asia. A VADM maximum is observed in Western Europe (Iberia and Germany) around 450 BCE, associated to rates of change of about 9 µT/century. This feature is also observed eastwards, in the Caucasus and the Levant, but associated to lower rates of changes. In Central Asia (Uzbekistan) our new study suggests that maximum values of about 14 µT/century, between 400-300 BCE, were achieved. In other regions, as Eastern Asia and Central America, rapid variations of the intensity are not observed during the targeted period.

Absolute past geomagnetic intensity values can mainly be recovered by fired archaeological materials and volcanic rocks. Here, we present 10 new archeointensities from the Mediterranean region that help to better constrain geomagnetic field intensity changes in Europe over the last two millennia. The new archeointensity results were obtained from the Thellier classical method including thermoremanent magnetization (pTRM) checks and both the TRM anisotropy and cooling rate corrections and were derived from at least three specimens. The new data presented, together with a selection of previous archeointensities satisfying a set of quality criteria, confirm the presence of several intensity maxima in Europe over the last 2000 years. In particular, the new archeointensities allow to better define the starting point of the double-oscillation feature that occurred in Europe during the second half of the first millennium CE, and reinforce the existence of a relative maximum at the end of the 14th century - beginning of the 15th century in Western Europe. From selected European archeointensities two new paleosecular variation curves are constructed for Western and Eastern Europe using temporal cubic b-splines in a bootstrap approach. The obtained curves suggest that the occurrence of the intensity maxima is characterized by a period of about 300 ± 50 years. In addition, our results suggest that the maxima do not occur simultaneously in Western and Eastern Europe, pointing out an intensity eastward drift with a mean lag-time of about 100 years.

The aqueduct of Les Ferreres is a major element of the Archaeological Ensemble of Tarraco. Although the ashlars of the aqueduct are stacked without mortar, lime was used in some parts and lime was certainly used in later repairs. Worthy of note is a coating mortar used in a well-documented restoration (1854–1856). In this study, a limekiln found near the Roman aqueduct has been archaeomagnetically dated to determine if it was used for the construction of the aqueduct or in later repairs. The mean values for the measured archaeomagnetic direction from the limekiln were compared with two different archaeomagnetic models (SCHA.DIF.3k and GUMF1), and both indicate that the limekiln is modern with ages only slightly older than the well-documented restoration. The extensive use of the coating mortar in that restoration is consistent with the need of onsite lime production. Additional archaeomagnetic intensity has not contributed to constrain further the obtained archaeomagnetic age but the intensity datum can be added to archaeomagnetic intensity datasets to enhance geomagnetic intensity field models. The paper illustrates how archaeomagnetic dating can be useful to characterize secondary structures of major cultural heritage monuments. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.

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 multicentennial 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., F.J.P.C. is funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 659901. F.J.P.C. and J.F.G.R. acknowledge the Marie Sklodowska-Curie project CLIMAGNET. M.G.P acknowledges the Ramón y Cajal Program. The authors are grateful to the Spanish research projects CGL2017-87015-P/CGL2015-63888-R of the Spanish Ministry of Economy and Competitiveness.

Coastal wetlands have been proposed as highly threatened by the ongoing and future climatic change, including projected sea-level changes as an additional forcing factor compared to more inland locations. The limited knowledge generated to date in this topic has been primarily focused on those areas attaining a high population density, and rarely deals with long-term (>50 years) dynamics. Here we present the first Holocene palaeoecological study carried in the Orinoco Delta, in NE Venezuela. The record presented here contains sediments from the last 6200 years and is located in a river-shore swamp dominated by the palm Mauritia flexuosa. Current human occupation is almost restricted to small settlements of the Warao indigenous culture, closely related to the use of M. flexuosa and other palm species present in the zone. The results show the occurrence of three well-distinguished palynological zones: (i) from 6200 to 5200 cal yr BP, characterised by mixed rainforest and other taxa related to salinity (coastal-like), low (negative) values of magnetic susceptibility and magnetic grain size, absence of transported clays, and the highest macrocharcoal particles abundance; (ii) from 5200 to 2950 cal yr BP, marked by a replacement of the mangrove-like vegetation by a more inland mixed-swamp forest community with low levels of charcoal, and (iii) from 2950 cal yr BP to present-day, characterised by the establishment of the current vegetation community, dominated by M. flexuosa, and an increasing trend in the charcoal curve since the last 700 years. A combination of regional (climatic changes) and local (sediment ontogeny) has been proposed as the key drivers influencing the vegetation succession recorded. The stabilisation of the sea-level that occurred during the mid-Holocene would have favoured the transgression of the coastal line, with the migration of the coastal-like vegetation seawards. Synchronous to this event, a trend towards drier conditions has been reported in the close Cariaco record, that could have also influenced the vegetation replacement. Between 3800 and 2800 years ago, the increased ENSO variability registered in Cariaco may have played a key role in the expansion of the Mauritia palm community. It is suggested that in our location, the potential inhabiting human populations were differently influenced by these environmental changes. First, the disappearance of the coastal resources could have favoured land abandonment, whereas the increase in the abundance of the palm might be influential for the arrival of other inland cultures that were previously used to manage Mauritia. This sequence shows the importance of the ecosystem services for the location inhabitants, highlighting the abandonment of the mid-Holocene culture coeval with the disappearance of its ecosystem. These results also provide information about the sensitivity and resilience in facing external stressors of both humans and vegetation, and will be valuable tools for managing the future of this ecosystem. © 2019 Elsevier Ltd, This study has been funded by BBVA Foundation (ref. BIOCON 08-031 ) to V. Rull, and the Spanish Ministry of Economy and Competitivity (Juan de la Cierva Incorporación contract, ref. IJCI-2015-24273 ) and the Catalan Agency for Universities and Research AGAUR and the EU programme Marie Curie COFUND (Beatriu de Pinós – Marie Curie COFUND fellowship, ref.: 2014 BP-B 00094 ) to E. Montoya.

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., This project was supported by the Ramón y Cajal program (RYC-2013-14405) and the CGL2015-63888-R (MINECO/FEDER) GEOMED project of the Spanish Ministry of Economy and Competitiveness. Financial support was also given by the PICS 2016 International Program for Scientific Cooperation (CNRS-France and CSIC-Spain).

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.

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 ⋅ 10 22 Am2 for the oldest materials (750 BC) and reaching 16 ⋅ 10 22 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.

In this work, we present 16 directional and 27 intensity high‐quality values from Iberia. Moreover, we have updated the Iberian archeomagnetic catalogue published more than 10 years ago with a considerable increase in the database. This has led to a notable improvement of both temporal and spatial data distribution. A full vector paleosecular variation curve from 1000 BC to 1900 AD has been developed using high‐quality data within a radius of 900 km from Madrid. A hierarchical bootstrap method has been followed for the computation of the curves. The most remarkable feature of the new curves is a notable intensity maximum of about 80 μT around 600 BC, which has not been previously reported for the Iberian Peninsula. We have also analyzed the evolution of the paleofield in Europe for the last three thousand years and conclude that the high maximum intensity values observed around 600 BC in the Iberian Peninsula could respond to the same feature as the Levantine Iron Age Anomaly, after travelling westward through Europe., Authors acknowledge the Spanish Ministry of Economy (CGL2014-54112-R and CGL2017-87015-P projects; BES-2015- 074575 and BES-2016-077257 grants), the Spanish Ministry of Education (FPU14/02422 grant), and the Autonomous Community of Madrid through the European Youth Employment Initiative (PEJ15/AMB/AI- 0203). M. G. P. and M. R. acknowledge the Ramón y Cajal program and the CGL2015-63888-R project of the Spanish Ministry of Economy and Competitiveness. Financial support was also given by the PICS International Program for Scientific Cooperation (CNRS-France and CSIC-Spain).

We present a source-to-sink environmental magnetic study of a sediment core from Lake Sanabria (north-west Iberian Peninsula) and rocks of its catchment. The results indicate the occurrence of magnetite, and probably also pyrrhotite, in sediments accumulated between ca. 26 and 13 cal ka BP in a proglacial lake environment. These minerals also appear to dominate the magnetic assemblage of Palaeozoic rocks from the lake catchment. This indicates that sedimentation was then driven by the erosion of glacial flour, which suffered minimal chemical transformation due to a rapid and short routing to the lake. A sharp change in magnetic properties observed in the lake sediments between 13 and 12.6 cal ka BP reflects the rapid retreat of glaciers from the lake catchment. Sediments from the upper half of the studied sequence, accumulated after 12.6 cal ka BP in a lacustrine environment with strong fluvial influence, contain magnetite and smaller amounts of maghemite and greigite. We suggest that greigite grew authigenically under anoxic conditions caused by enhanced accumulation of organic matter into the lake. The occurrence of maghemite in these sediments suggests pedogenic activity in the then deglaciated lake catchment before the erosion and transportation of detrital material into the lake.

The Balkan area provides an almost continuous record of the geomagnetic field variations during the last eight millennia but important data dispersion and chronological gaps are still observed. In order to improve this pattern, we oriented our research towards the study of Greek prehistoric ceramics and pottery collections. We present here new archaeointensity and mineralogical results from pottery and ceramics collected in two Bronze Age sites in Northern Greece, corresponding to the middle third and middle second millennium BC. Thermal demagnetization experiments, thermomagnetic curves, thermal demagnetization measurements of three axes composite isothermal magnetization and first-order reversal curves (FORC) diagrams were performed in order to define the main magnetic carriers and select the most promising samples for archaeointensity determination. The results suggest that the majority of the studied materials are dominated by a mixture of superparamagnetic and stable single domain grains, with very little magnetic interactions. In almost all samples the main magnetic carrier observed is a low coercivity mineral, most probably magnetite and/or Ti-magnetite. Classical Thellier experiments, including both the thermoremanent magnetization anisotropy and cooling rate corrections were performed on the selected samples. Two new archaeointensities were obtained and compared with previous data available for Greece and neighbouring countries and with global geomagnetic field models results. In order to investigate the causes of the high rate of failure observed in our archaeointensity experiments, we re-examined the results obtained from the study of four collections already published by better characterizing the magnetic and mineralogical properties of the studied fragments. Specific hysteresis curves performed at different layers of ceramic fragments reveal an important degree of inhomogeneity. Mineralogical analysis of selected samples from the totality of six Greek prehistoric sites studied, including new X-ray powder diffraction analysis and a detailed description of the different geological contexts, is provided. The results suggest that the sherds were all fired in medium-high temperatures (mostly between 500° and 800 °C). The nature of the baked clay paste in response to the local geology appears to account for a part of the variability in magnetic properties. We suggest that X-ray powder diffraction analyses and thorough examination of the geological environment in combination with a satisfactory knowledge of the corresponding excavations history might provide an additional tool to appropriately select pottery fragments before performing future archaeointensity experiments., The INSTAP Center has substantially contributed to the funding of this research during the years 2012–2014. MGP acknowledges the Ram´on y Cajal Program and the CGL2015–63888-R research project of the Spanish Ministry of Economy and Competitiveness.

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Financial support for this research was provided by a CSIC JAE-Doc postdoctoral research contract (MGP), the Spanish Ministry of Science and Innovation (projects CGL2008-02203/BTE, CGL2010-15767/BTE and CGL2011-24790) and Banco Santander – Universidad Complutense de Madrid (Grupo de Investigacion 910396).

New full-vector archaeomagnetic data for North Africa recovered from the study of six kilns, five from Tunisia and one from Morocco, are presented. Archaeological and historical considerations, along with three radiocarbon dates, indicate that the age of the kilns ranges between the 9th and 15th centuries AD. Rock magnetic analyses showed that the principal magnetic carriers are magnetite and low Ti titanomagnetite, along with variable contributions of thermally stable maghemite and a high coercivity phase with low unblocking temperatures. The magnetic mineralogy of the studied material is thermally stable and behaves ideally during archaeointensity experiments. Stepwise alternating field demagnetization isolated a single, stable, characteristic remanence component with very well defined directions at both specimen and structure levels. Mean archaeointensities have been obtained from successful classical Thellier experiments conducted on between five and eight independent samples per kiln. Thermoremanent magnetization (TRM) anisotropy and cooling rate effects upon TRM intensity have been investigated. The results showed that these effects are low for four of the six studied kilns, with differences between the uncorrected and corrected means of less than 3 per cent. For the other two structures differences between the uncorrected and corrected mean site intensities are 4.4 per cent and 5.8 per cent. These results highlight the necessity for TRM anisotropy and cooling rate corrections in archaeomagnetic studies if accurate archaeointensities are to be obtained. The new results suggest that high intensities occurred in Northwest Africa during the 9th century. Although more data are clearly needed to define this period of high intensity, the results are in agreement with the available European archaeointensity data. Acomparison between the newdata, other available archaeomagnetic determinations in nearby locations, and palaeosecular variation (PSV) curves derived from the regional SCHA.DIF.3k and global ARCH3K.1 geomagnetic field models shows good agreement between the new data and directional results derived from the models. However, some differences are observed between geomagnetic field models intensity results and available archaeointensity data for the studied regions. This highlights the need for new data for unexplored regions such as North Africa. The new data presented here better constrains the evolution of the geomagnetic field during historical times in this region. They represent a new step towards the construction of a reference PSV curve for Northwest Africa. Once established, this curve will represent a new dating method for this region.

A magnetostratigraphy-based chronological framework has been constructed in the Eocene sediments of the Montserrat alluvial fan/fan-delta complex (southeast Ebro Basin), in order to unravel forcing controls on their sequential arrangement and to revise the tectonosedimentary history of the region. The palaeomagnetic study is based on 403 sites distributed along an 1880-m-thick composite section, and provides improved temporal constraints based on an independent correlation to the geomagnetic polarity time scale. The new chronological framework together with sequence stratigraphy and geohistory analysis allow us to investigate the interplay between factors controlling the sequential arrangement of the Montserrat complex at the different temporal scales and to test for orbitally driven climate forcing. The results suggest that the internal stacking pattern in transgressive and regressive sequences sets within the more than 1000-m-thick Milany Composite Megasequence can be explained as the result of subsidence-driven accommodation changes under a general increase of sediment supply. Composite sequences (tens to hundreds of metres thick) likely reflect orbitally forced cyclicity related to the 400-kyr eccentricity cycle, possibly controlled by climatically induced sea-level fluctuations. This study also provides new insights on the deformational history of the area, and shows a correlation between (tectonic) subsidence and forelimb rotation measured on basin-margin deformed strata. Integration of subsidence curves from different sectors of the eastern Ebro Basin allows us to estimate the variable contribution of tectonic loads from the two active basin margins: the Catalan Coastal Ranges and the Pyrenees. The results support the presence of a double flexure from Late Lutetian to Late Bartonian, associated with the two tectonically active margins. From Late Bartonian to Early Priabonian the homogenization of subsidence values is interpreted as the result of the coupling of the two sources of tectonic load. © 2011 The Authors. Basin Research © 2011 Blackwell Publishing Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists., This manuscript is a contribution of the Institut de Recerca Geomodels and the Research Group of Geodynamics and Basin Analysis SGR 2009SGR1198, of the Age`ncia de Gestio´ d’Ajuts Universitaris i de Recerca (AGAUR) de la Generalitat de Catalunya. It has been developed in the framework of the MCI projects REMOSS 3D/4D (CGL2007-66431-C02-02) and COFORSED (CGL2010-17479).

The anisotropy of magnetic susceptibility (AMS) has provided important insights into early deformation conditions in compressive settings through characterization of tectonic fabrics in mudrocks that appear otherwise undeformed. The validity of these insights relies on the assumption that tectonic fabrics are rapidly locked shortly after sediment deposition. However, the time lag between deposition and the tectonic overprint has yet to be quantified to verify this assumption. Here we present AMS data from Late Holocene sediments recovered in two cores from Lake Issyk-Kul in the Kyrgyz Tien Shan fold-and-thrust belt. These sediments, in which magnetic fabrics reflect the preferred orientation of phyllosillicates, have typical tectonic magnetic fabrics with varying degrees of tectonic overprint likely controlled by small-scale active faults. The mean orientation of the susceptibility maxima parallels neighbouring active thrust faults and is perpendicular to the geodetically-derived local shortening direction. The age model for one of the studied cores, which is based on 7 calibrated accelerator mass spectrometer radiocarbon dates, indicates that sediments as young as 25 yr record this clear tectonic fabric, which is locked within sediments older than 1180 cal. yr BP. Our demonstration of rapid locking of tectonic fabrics in weakly deformed mudrocks shortly after deposition provides the required validation of their reliability for tectonic studies, and opens new opportunities for providing quantitative insights into the relationship between magnetic ellipsoids, shortening rates, and stress directions in compressive settings.

One of the main challenges of paleomagnetic research is to obtain high-resolution geomagnetic field intensity reconstructions. For the last millennia, these reconstructions are mostly based on archeomagnetic data. However, the quality of the intensity data available in the databases is very variable, and the high scatter observed in the records clearly suggests that some of them might not be reliable. In this work we investigate how the geomagnetic field intensity reconstructions and, hence, our present knowledge of the geomagnetic field in the past, are affected by the quality of the data selected for modeling the Earth's magnetic field. For this purpose we rank the European archeointensity data in four quality categories following widely accepted paleomagnetic criteria based on the methodology used during the laboratory treatment of the samples and on the number of specimens retained to calculate the mean intensities. Four geomagnetic field regional models have been implemented by applying the revised spherical cap harmonic analysis to these four groups of input data. Geomagnetic field models strongly depend on the used data set. The model built using all the available data (without any preselection) appears to be the less accurate, indicating some internal inconsistencies of the data set. In addition, some features of this model are clearly dominated by the less reliable archeointensity data, suggesting that such features might not reflect real variations of the past geomagnetic field. On the contrary, the regional model built on selected high-quality intensity data shows a very consistent intensity pattern at the European scale, confirming that the main intensity changes observed in Europe in the recent history of the geomagnetic field occurred at the continental scale. © 2014. American Geophysical Union. All Rights Reserved.

The Bardenas Reales Natural Park (western Ebro Basin, NE Spain) constitutes a fragile landscape where the prevailing semiarid climate and erodability of the bedrock have resulted in high sensitivity to erosion/sedimentation oscillations during the Holocene. In this paper, we present new chronologic, sedimentologic, paleomagnetic and environmental magnetic results from the oldest unit (Qah1) of a system of nested cut-and-fill alluvial sequences, which is 18. m thick and crops out extensively in the central part of the natural park at the Bardena Blanca Depression. Radiocarbon dating indicates that Early Holocene unit Qah1 is younger than 9.38-8.89. cal. kyr. BP and gives ages of 8.92-8.42 and 7.59-7.37. cal. kyr. BP in the middle and uppermost parts of the unit, respectively. Sedimentologic data indicate that distal alluvial sediments of unit Qah1 accumulated in sandy mud flats and ephemeral playa lakes under arid conditions. The coincidence of enhanced magnetite content with moderate to intense root bioturbation in the lowermost and likely also uppermost sediments of unit Qah1 suggests that magnetite formed authigenically in response to relatively wetter conditions at the beginning and end of this dry period. These results indicate that onset and cessation of alluvial aggradation mark the passing of a geomorphologic threshold, most likely plant cover in the watershed, during a progressive drying-wetting event. The radiocarbon-based age model of unit Qah1 indicates that alluvial aggradation under arid conditions began at 9.13. cal. kyr. BP and ended at 7.16. cal. kyr. BP. This period is strikingly centered at around the 8.2. kyr cold event but spans a longer time period, supporting the notion of a global climatic deterioration as the underlying cause for this event. Paleomagnetic directions do not clearly display the characteristic patterns recognized in paleosecular variation curves derived from other European lacustrine sediments and cannot be used to refine the radiocarbon-based age model for unit Qah1. Our results demonstrate a quick and strong response of alluvial systems to Early Holocene climate variability in fragile Mediterranean landscapes such as those at the semiarid Bardenas Reales Natural Park. © 2011 Elsevier B.V., This work was funded by projects CGL2006-08973/BTE and CGL2009-10455/BTE (Spanish Government and European Regional Development Fund) and by a CSIC JAE-Doc post-doctoral research contract (MGP).

[1] We present new paleomagnetic and rock magnetic data from mid and late Holocene sediments recovered in two gravity cores (C087 and C142a) from Lake Issyk-Kul (central Asia), for which independent radiocarbon-based age models are available. Our results indicate that sediments from core C087 are characterized by fine (pseudo single domain) magnetite grains and are reliable recorders of Holocene geomagnetic paleosecular variation (PSV) in central Asia, which is a region with poor data coverage. Similarity is found between the core C087 PSV record, which spans the last 5700 years, and the Lake Baikal PSV record, which is the only published Holocene central Asian PSV record of comparable length with an independent radiocarbon-based chronology. Our new results represent a step forward in constructing a reference PSV curve for central Asia that can be used to date sedimentary sequences. These results can also be used to improve the reliability and accuracy of global geomagnetic field models. We have not been able to disentangle past environmental changes in the Lake Issyk-Kul region based on rock magnetic properties, probably because the magnetite and hematite in the studied sediments have a mixed (fluvial and eolian) terrigenous origin.

Available archaeomagnetic data indicate that during the past 2500. yr there have been periods of rapid geomagnetic field intensity fluctuations interspersed with periods of almost constant field strength. Despite Europe being the most widely covered region in terms of archaeomagnetic data the occurrence and the behaviour of these rapid geomagnetic field intensity changes is under discussion and the challenge now is to precisely describe them. The aim of this study is to obtain an improved description of the sharp intensity change that took place in western Europe around 800 AD as well as to investigate if this peak is observed at the continental scale. For this purpose 13 precisely dated early medieval Spanish pottery fragments, four archaeological French kilns and three collections of bricks used for the construction of different French historical buildings with ages ranging between 335 and 1260 AD have been studied. Classical Thellier experiments performed on 164 specimens, and including anisotropy of thermoremanent magnetisation and cooling rate corrections, gave 119 reliable results. The 10 new high-quality mean archaeointensities obtained confirm the existence of an intensity maximum of ~85 οT (at the latitude of Paris) centred at ~800 AD and suggest that a previous abrupt intensity change occurred around 600 AD. Together with previously published data from western Europe that we deem to be the most reliable, the new data also suggest the existence of two other abrupt geomagnetic field intensity variations during the 12th century and around the second half of the 13th century AD. High-quality archaeointensities available from eastern Europe indicate that very similar geomagnetic field intensity changes occurred in this region. European data indicate that very rapid intensity changes (of at least 20 οT/century) took place in the recent history of the Earth's magnetic field. The results call for additional high-quality archaeointensities obtained from precisely dated samples and for a selection of the previously published data if a refined description of geomagnetic field intensity changes at regional scales is to be obtained. © 2012 Elsevier B.V., Financial support was given by a CSIC JAE-Doc contract (MGP), by the Nos. CGL2008-02203/BTE, CGL2011-24790, and CGL2010-15767/BTE projects from the Spanish ministry of Science and Innovation, the Complutense University, the Comunidad de Madrid Consolidación de grupos program (No. 910396), and by the Junta de Comunidades de Castilla-La-Mancha.

New Magnetostratigraphic data from the Montserrat alluvial fan and fan-delta system led to an unambiguous correlation to the GPTS and provided the basis for a quantitative estimate of its tectonosedimentary evolution. Local isostatic backstripping analysis has been performed. Tectonic pulses in the basin margin during the initial synsedimentary folding episode, expressed by unconformities and forelimb rotation rates, correlate to tectonic subsidence which shows a trend similar to the forelimb rotation rates. Three different frequency scales of sequential arrangement have been analysed on the fan-delta. The lowest frequency megasequence (3.2My) resulted from a combination of different factors (local subsidence-related architecture, long-term eustacy and regional basin configuration). The intermediate frequency Composite sequences (457±70ky) can be tied to astronomical cyclicity that controlled accommodation. Lack of periodicity (15 to 125ky) at higher-order sequential arrangement indicates that autocyclic processes were the main control. In tectonically active basin margins, sediment supply and subsidence are usually considered as major controls on sedimentary arrangement. In our study this assumption has been confirmed only for large-scale sequences. However, at intermediate scale sequences tectonics results in high accumulation rates favouring the development and preservation of astronomical-related cycles.

This paper presents new magnetostratigraphic results from a 1100‐m‐thick composite section across the marine to continental sediments of the central part of the SE margin of the Ebro basin (NE Spain). Integration with existing marine and continental biochronological data allows a robust correlation with the geomagnetic polarity time scale. The resulting absolute chronology ranges from 36.3 to 31.1 Ma (Priabonian to Rupelian), and yields an interpolated age of ∼36.0 Ma (within chron C16n.2n) for the youngest marine sediments of the eastern Ebro basin. This age is in concordance with a reinterpretation of earlier magnetostratigraphic data from the western South Pyrenean foreland basin, and indicates that continentalization of the basin occurred as a rapid and isochronous event. The basin continentalization, determined by the seaway closure that resulted from the uplift of the western Pyrenees, was probably coincident with a mid‐amplitude eustatic sea level low with a maximum at 36.2 Ma. The base level drop that followed the basin closure and desiccation does not appear associated to a significant sedimentary hiatus along the margins, suggesting a late Eocene shallow marine basin that rapidly refilled and raised its base level after the seaway closing. Rapid basin filling following continentalization predates the phase of rapid exhumation of the Central Pyrenean Axial Zone from 35.0 to 32.0 Ma, determined from the thermochronology data. It is possible then that sediment aggradation at the front of the fold‐and‐thrust belt could have contributed to a decrease in the taper angle, triggering growth of the inner orogenic wedge through break‐back thrusting and underplating. Contrasting sedimentation trends between the western and eastern sectors of the South Pyrenean foreland indicate that basin closing preferentially affected those areas subjected to sediment bypass towards the ocean domain. As a result, sediment ponding after basin closure is responsible for a two‐fold increase of sedimentation rates in the western sector, while changes of sedimentation rates are undetected in the more restricted scenario of the eastern Ebro basin.

International audience; A modern (1959) brick kiln from western Spain has been studied in order to conduct a quality control test of the archaeomagnetic method in a partially heated structure. The kiln exhibits two stable natural remanent magnetisation components: a low-temperature component(150­620 C) acquired during kiln use and a randomly oriented, high temperature component (500­680 C) acquired during the original firing of the bricks. A detailed rock magnetic study revealed a magnetic mineralogy dominated by non-stoichiometric magnetite and hematite, both of which contribute to the characteristic remanent magnetisation. Both the direction and intensity of the characteristic magnetisation have been compared with the known values for the geomagnetic field at the site location. Irrespective of the method used to determine the mean archaeomagnetic direction (principal component analysis, great circle analysis, with or without hierarchy) the results are statistically indistinguishable from each other and from the known field direction. In all cases the direction is within 5 ofthe expected value. Thellier-style archaeointensity determinations have been carried out on a smaller specimen set, with the mean result consistent with the known field value. The results demonstrate the reliability of the archaeomagnetic method in determining the featuresof the geomagnetic field in the past, even in cases with a complicated magnetic history. Furthermore, they highlight the importance of a uniform sample distribution in order to obtain truly representative values for the direction and intensity of the ancient geomagnetic field.

International audience; In this study six new intensity determinations are presented, obtained from five well dated archaeological sites, located in northern Greece and in Paros, Cyclades Islands. The fired structures consisted of ceramic and pottery kilns belonging to the Hellenistic, Roman and Byzantine periods. Between 8 and 21 samples of highly fired baked clays, tiles and bricks were taken, homogeneously distributed over the structures. The samples were analysed using the classical Thellier method, providing the past intensities and directions of the geomagnetic field recorded at each site. The intensity values have been corrected for anisotropy of thermal remanent magnetisation and cooling rate effects. Differences in the mean archaeointensities per site ranging from 1% to 11%, before and after TRM anisotropy and cooling rate corrections, were obtained. The new results indicate a decrease of 20% of the geomagnetic field strength in Greece, during the last four centuries BC. In order to compare our results with previously published data, a catalogue of archaeo- and palaeointensity results for the Aegean area has been established, covering the last 7 millennia. It consists of 336 data from Greece, western Turkey and Former Yugoslavia, collected from various authors. Weighting factors have been applied to these data, that then have been treated with a hierarchical Bayesian modelling, and a geomagnetic field intensity variation curve for Greece was constructed. A good agreement is observed when comparing the curve for Greece with the Bulgarian secular variation curve (SVC) for intensity. Satisfactory coincidence is also found with the archaeointensity data from Mesopotamia. Despite the presence of some time gaps, a more precise secular variation intensity curve has been constructed for Greece which, combined with a forthcoming directional SVC, will help for dating purposes.

International audience; Archeomagnetic studies on 14 kilns, a group of jar fragments, and a collection of baked bricks dated between 1000 and 1959 AD plus one Roman pottery kiln have been conducted in order to obtain high-quality archeointensity data to enhance the western European database. The Thellier method with corrections for anisotropy of thermoremanent magnetization (TRM) and for cooling rate dependence upon TRM acquisition was used. The effect of TRM anisotropy is only important for the bricks where the corrected and uncorrected mean intensities differ by more than 5%. Cooling rate correction factors determined per sample are up to 15% and per site up to 7.7%. Our 17 new data together with 62 previously published results were used to obtain, by Bayesian modeling, the geomagnetic field intensity over the past two millennia for western Europe. Our results indicate that geomagnetic intensity remained more or less constant between the 1st and 4th centuries and between the 14th and 16th centuries (mean values, at Paris, around 65 and 57 μT, respectively), whereas an important decrease occurs between 1600 and 1800 AD. The detailed evolution of geomagnetic field intensity during the High Middle Ages is not yet well established. Despite their differences, geomagnetic global models predict our results reasonably well. This work indicates the need to obtain a number of archeointensity data for each time interval in order to reliably record variations of the geomagnetic field and to test whether any relationship exists between field intensity and climate

[1] A first secular variation (SV) curve for the Iberian Peninsula was computed by hierarchical Bayesian method using a total of 134 archaeomagnetic directions with ages ranging from −775 to 1959 A.D. A general agreement is observed between the Iberian curve and the French and German SV curves, although some interesting differences were found, such as the occurrence of lower inclinations between the 11th and 14th centuries in the Iberian curve. The analysis of these three reference curves indicates that SV in western Europe is characterized by three major directional changes at −125, 200, and 1350 A.D. It is suggested that these cusps are regional features of the geomagnetic field. The Iberian curve has been compared with the predictions of the Jackson, CALSK7K.2, and Hongre global models. Despite large differences recognized between these models, even for the dipolar terms, they predict reasonably well the Iberian archaeomagnetic SV.

International audience; New archeointensity values have been determined from seven Spanish kilns sampled at the same archeological site and dated by archeological constraints as between 1100 and 1200 a.d. The directions of the characteristic remanent magnetization and paleointensities have been obtained from classical Thellier experiments conducted on 69 samples collected from the walls of the kilns. Thermoremanent magnetization (TRM) anisotropy and the cooling rate effect upon TRM intensity acquisition have been investigated in all the samples. Differences between mean archeointensities per site, before and after corrections for cooling rate are between 3 and 17%. The study of these seven “contemporaneous” kilns coming from the same archeological site and treated in the same way in the laboratory allows us to discuss the different factors that can cause the dispersion between sites generally observed in archeointensity studies. The results confirm, on one hand, the need for cooling rate corrections in this kind of study and, on the other hand, the need to obtain several archeointensity determinations per century in order to construct a reliable archeointensity curve. Finally, the new data provide a robust mean intensity (60.3 ± 3.3 μT at the latitude of Paris) for the middle of the XII century in Western Europe and constrain better the evolution of the geomagnetic field intensity during medieval times in this area.

International audience; A total of 58 new archaeomagnetic directions has been determined from archaeological structures in Spain. Together with five previous results they allow the compilation of the first archaeomagnetic catalogue for Spain, which includes 63 directions with ages ranging between the 2nd century BC and the 20th century AD. Characteristic remanence directions have been obtained from stepwise thermal and alternating field demagnetization. The hierarchical structure has been respected in the calculation of the mean site directions. Rock magnetic experiments reveal that the main magnetic carrier is magnetite or titanomagnetite with different titanium contents. The age estimate of the studied structures is generally well justified by archaeological constraints. For six structures the proposed date is also supported by physical methods. The data are in close agreement with the French secular variation (SV) curve. This catalogue represents the first step in the construction of a SV curve for the Iberian Peninsula, which will be of much use in archaeomagnetic dating and in modelling of the Earth's magnetic field in Western Europe.

Magnetization curves up to 9 T and AC susceptibility ( χ AC ) down to 4.2 K, of airborne suspended particulate matter collected in Madrid (Spain) are reported. The χ AC results present a broad maximum at 120 K reminiscent of the Verwey transition while magnetization curves reveal a paramagnetic signal superposed to ferromagnetism. These new data complement those obtained from isothermal remanent magnetization and indicate that magnetism in these particles is mostly due to highly oxidized magnetite or maghemite.