Archeomagnetic Data Research Articles

Linking site formation processes to magnetic properties. Rock- and archeomagnetic analysis of the combustion levels at Riparo Gaban (Italy)

This study focuses on archeomagnetic directional data that are obtained from combustion levels in the Riparo Gaban rock shelter (northern Italy) in order to assess the degree of disturbance in the sediments and to provide new information to evaluate evolution of the geomagnetic field in the mid-Holocene. Combustion levels consist of ash, rubefied levels, or partially burned material and mixtures of these, or polygenic material. Rock magnetic investigations show that superparamagnetic to single-domain magnetite is the main carrier of the remanent magnetization, but small amounts of hematite and maghemite may also be present in the different materials. Rock magnetic parameters, e.g., the Koenigsberger ratio Q or the reversibility of thermomagnetic can be related to the degree of burning, and the dispersion parameter k to the degree of preservation. The grouping of archeomagnetic directions of a specific level is strongly dependent on the degree of physical disturbances. Directional means from thirteen levels can be compared with reference curves for paleosecular variation based on geomagnetic field models and other archeomagnetic data. The data from this study are in agreement with other archeomagnetic data from the Balkans in the time period 4900 BC to 4600 BC. • We infer geomagnetic field evolution during mid Holocene in North Italy. • We determine archaeomagnetic directions from well-stratified burnt levels. • We find links between physical disturbances and scatter of magnetic directions. • Formation temperatures based on magnetism and micromorphology agree well. • Our magnetic directions show similar trend to the available SV curves.

First archeointensity determinations on Maya incense burners from Palenque temples, Mexico: New data to constrain the Mesoamerica secular variation curve

We present archeointensity data carried out on pieces of incense burners from the ancient Maya city of Palenque, Chiapas, Mexico, covering much of the Mesoamerican Classic period, from A.D. 400 to A.D. 850. We worked on pieces from 24 incense burners encompassing the five Classic ceramic phases of Palenque: Motiepa (A.D. 400–500), Cascadas (A.D. 500–600), Otulum (A.D. 600–700), Murcielagos (A.D. 700–770), and Balunte ´ (A.D. 770–850). All the samples come from highly elaborate, flanged pedestal of incense burners that are undoubtedly assigned to a ceramic phase by means of their iconographic, morphological and stylistic analyses. Archeointensity measurements were performed with the Thellier–Thellier’s method on pre-selected samples by means of their magnetic properties. We obtained archeointensities of very good technical quality from 19 of 24 pieces, allowing the determination of a precise mean value for each ceramic phase, between 29:17 0:9 mT and 32:57 1:2 mT. The firing

WITHDRAWN: An Evaluation of Modern Pottery from Southern Africa as a Magnetic Recorder

AbstractStudies of the recent history of Earth's magnetic field have revealed a rich spatial and temporal structure, but face limitations by a lack of Southern Hemisphere archeomagnetic data. Studies of Iron Age (200‐1850 AD) peoples of southern Africa have revealed a potentially rich source of archeomagnetic information in the form of ceramics (specifically pottery). Additionally, contemporary pottery made with traditional techniques and materials can still be found. Reported here is the first step in addressing whether ancient pottery from southern Africa might faithfully record the geomagnetic field. We analyze contemporary pottery made with traditional techniques and methods. Rock magnetic measurements, including magnetic susceptibility as a function of temperature and magnetic hysteresis behavior, are discussed. Intensity results generated by three common paleointensity methods: Thellier‐ Coe double heating experiments, the multi‐specimen method of Dekkers and Böhnel, and Shaw's method (with and without the corrections of Kono) are compared to the known field at the time of firing. The Thellier‐Coe method reproduces the field (with an accuracy of 1.3 μT), the Shaw technique with the correction approach of Kono overestimates the field by 3.7%. The multispecimen method overestimates the field by 20%, however improvement upon this could be expected given recent improvements to the technique. These values bound the accuracies we can expect when applying the methods to ideal samples, representing a best‐case for dealing with archeological ceramics from southern Africa

High‐fidelity paleointensity determination from historic volcanoes in Japan

Providing high‐fidelity paleointensity estimates is pivotal to identify temporal fluctuation of ancient geomagnetic field. Despite abundance of available paleointensity data in Japan, only dozen of them satisfy modern standards with systematic alteration checks. High‐fidelity paleointensity estimates were obtained from historic andesitic lavas collected from Mt. Aso (Ojodake, ≈700 BC), Mt. Kirishima (Iwoyama, AD 1768; Ohachi, AD 1235), and Mt. Sakurajima (An‐ei, AD 1779; Nabeyama, AD 764–766). Variation of geomagnetic field intensity is distinctively different from the prediction of global models (ARCH3k.1, CALS3k.3, CALS3k.4, CALS7k.2, and CALS10k.1b) for the past 4000 years in Japan. The compilation of high‐fidelity Thellier data set in Japan showed two obvious high intensities at AD 590–765 and AD 1330–1435 and one low intensity at ≈700 BC. These time intervals with anomalous high/low intensities are nearly identical to three of the four potential archeomagnetic jerks recognized from the European archeomagnetic data, implying that the archeomagnetic jerks were global (or at least northern hemispheric) features. To improve the poor representation of regional geomagnetic field variation with respect to the global model prediction, more high‐fidelity paleointensity determinations are required in East Asia.

New archeomagnetic and 226Ra-230Th dating of recent lavas for the Geological map of Etna volcano

This work deals with the dating of Mount Etna lava flows and eruptive fissure deposits to the last four millennia following field investigations and stratigraphic data (BRANCA et alii, 2011a). We have studied 24 of these volcanic products, including 301 large samples, through high precision archeomagnetic dating checked by 226Ra-230Th radiochronology, thus providing additional material to the previous paper by TANGUY et alii (2007). In most cases our results allow attributing ages to the historical period, although two flows are shown to be prehistoric. For the historic lavas, archeoma - gnetic ages can be defined within decades, except for three of them that erupted during a time span (Greco-Roman epoch) when the geomagnetic field underwent little variation. Although 60% of these volcanics exhibit ages comprised between 700 AD and 1850, only one (1285) is mentioned by contemporary written accounts. We conclude that i) historical documents alone are insufficient to reconstruct a coherent sequence of eruptions, and ii) a multidisciplinary approach is necessary to obtain a comprehensive eruptive history of such a very active volcano, useful for both scientific and civil protection purposes, even for such a geologically recent period as that of the last 10 or 20 centuries. Thanks to these new archeomagnetic and 226Ra-230Th data coupled with stratigraphic data, a comprehensive volcanic history of the still-outcropping Mount Etna volcanics is now available for the last 2,400 years. KEY WORDS: Etna volcano, archeomagnetic dating, 226Ra- 230Th dating, stratigraphy, historical account.

Spline analysis of Holocene sediment magnetic records: Uncertainty estimates for field modeling

Sediment and archeomagnetic data spanning the Holocene enable us to reconstruct the evolution of the geomagnetic field on time scales of centuries to millennia. In global field modeling the reliability of data is taken into account by weighting according to uncertainty estimates. Uncertainties in sediment magnetic records arise from (1) imperfections in the paleomagnetic recording processes, (2) coring and (sub) sampling methods, (3) adopted averaging procedures, and (4) uncertainties in the age‐depth models. We take a step toward improved uncertainty estimates by performing a comprehensive statistical analysis of the available global database of Holocene magnetic records. Smoothing spline models that capture the robust aspects of individual records are derived. This involves a cross‐validation approach, based on an absolute deviation measure of misfit, to determine the smoothing parameter for each spline model, together with the use of a minimum smoothing time derived from the sedimentation rate and assumed lock‐in depth. Departures from the spline models provide information concerning the random variability in each record. Temporal resolution analysis reveals that 50% of the records have smoothing times between 80 and 250 years. We also perform comparisons among the sediment magnetic records and archeomagnetic data, as well as with predictions from the global historical and archeomagnetic field models. Combining these approaches, we arrive at individual uncertainty estimates for each sediment record. These range from 2.5° to 11.2° (median: 5.9°; interquartile range: 5.4° to 7.2°) for inclination, 4.1° to 46.9° (median: 13.4°; interquartile range: 11.4° to 18.9°) for relative declination, and 0.59 to 1.32 (median: 0.93; interquartile range: 0.86 to 1.01) for standardized relative paleointensity. These values suggest that uncertainties may have been underestimated in previous studies. No compelling evidence for systematic inclination shallowing is obtained from the analysis of the available database of Holocene sediment magnetic records. The analysis highlights the importance of collecting oriented cores, publishing and archiving unprocessed raw paleosecular variation determinations, and presenting a detailed chronology so that changes in the sedimentation rate can be assessed. With regard to future field models, workers should consider rejection of anomalous cores through comparisons to other sources and ensure that realistically large uncertainties are allocated to high‐latitude declination records.

An archeomagnetic analysis of burnt grain bin floors from ca. 1200 to 1250 AD Iron-Age South Africa

Abstract Recognition of the rapid decay of Earth’s magnetic field over the last 150 years, chronicled in magnetic observatory and satellite data, highlights the need for a higher resolution record of geomagnetic field behavior over the past millennium. Such a record would help us better understand the nature of the recent dramatic changes. A limitation of the existing database is undersampling of the Southern Hemisphere. Here we investigate the potential of obtaining archeomagnetic data from Iron-Age burnt grain bins from southern Africa. These structures preserve oriented material that can record both paleodirections and paleointensity information. Directional data collected from three sites (ca. 1200–1250 AD) fall 9–22° to the East of predictions. Thellier–Coe and Shaw paleointensity results differ from model values by ∼15%. The consistency of results between the three sites suggests that further investigations of these materials with different ages could markedly improve the current spatial distribution of the archeomagnetic database.

Archeomagnetism of Piton de la Fournaise: Bearing on volcanic activity at La Réunion Island and geomagnetic secular variation in Southern Indian Ocean

Historical and other recent lavas from Piton de la Fournaise (La Réunion Island) are studied using the large sample archeomagnetic method which provides here paleodirections of the Earth Magnetic Field with confidence cones between 0.9 and 2.5°, thus offering a precise record of the geomagnetic field and potential dating constraints. Ages of the lavas are known thanks to historical chronicles (from 1708) or 14C dating and their analysis give information about the directional secular variation (SV) in a region where observatory measurements are scarce and not available before the end of the 19th century. For the past 250 yr we find a high value of inclination (−50 to −55°) with respect to the geographic latitude (21°S), connected with a very restricted directional SV. Conversely, the older volcanic units present a larger SV with magnetic inclinations of between −31° and −46°, and declinations from 16°W to 10°E. These results, which are in reasonable agreement with instrumental measurements made on ships in the vicinity of La Réunion, allow us to infer that undated lavas of the northern part of the caldera emanated from eruptions during the second half of the 1700s. Other volcanic products (e.g. Mare Longue flow, Piton Chisny cone and flows and Pointe Langevin), despite their fresh morphology, are necessarily older than about 1500 AD. Further knowledge of the path of the SV in the more distant past, and therefore further archeomagnetic dating, is hampered by the lack of a precise chronology through radiometric or other similar methods. Available models of the geomagnetic field during the last millennium, which suffer from the scarcity of data in the Southern Hemisphere, are discussed in the light of our results.

Regional modeling of the geomagnetic field in Europe from 6000 to 1000 B.C.

We have developed a first low‐degree regional geomagnetic model for the European continent valid for the period 6000–1000 B.C. from a selected compilation of sedimentary and archeomagnetic data (the SCHA.DIF.8K model). This model provides information about both direction (declination and inclination) and intensity of the Earth's magnetic field. By connecting it with our previous model, SCHA.DIF.3K, valid from 1000 B.C. to 1900 A.D., and the IGRF, we furnish continuous geomagnetic field information for the last 8000 years in Europe. It has been developed using the Revised Spherical Cap Harmonic Analysis in 2 Dimensions technique (R‐SCHA2D) and using the norm of the Earth's magnetic field to constrain the inversion problem. The size of the cap is 22°, and the maximum degree of the expansion is 2. The linearization problem was solved by using the truncated Taylor's series applied to the expressions representing the relationship between the declination, inclination, and intensity data and the Cartesian components of the geomagnetic field. We used the geocentric axial dipole (GAD) field as our initial or reference field. For time, we used the classical sliding overlapping window method. The size of the window was set to 100 years shifted by 50 years. We compared the model's prediction with the input data, with the global CALS7K.2 model, and with new independent data. The regional model shows a better fit to the input and to the independent data than the global model, especially in terms of intensity, and agrees with the virtual axial dipole moment given by other studies. For the last 8000 years, the European geomagnetic field has recorded rapid changes or archeomagnetic jerks. The average field for the last 8000 years in Europe is indistinguishable from the GAD field.

A bootstrap algorithm for deriving the archeomagnetic field intensity variation curve in the Middle East over the past 4 millennia BC

Recent compilations of archeomagnetic intensity data have shown the existence of outliers. Acknowledging this problem, we propose a new way of generating and studying the uniqueness of regional archeomagnetic master curves bypassing the need to assume a normal data error distribution. Our approach lessens the weight of outliers by applying an iteratively re‐weighted least‐squares method combined with a bootstrap algorithm. Given a particular set of archeomagnetic data associated with experimental and dating uncertainties, we produce an ensemble of curves that are designed to sample the conditional probability distribution of the ‘true’ master curve. Using this technique, we propose an archeointensity master curve with its probability density for the Middle East region that covers the past four millennia BC.

Millennial Variations of the Geomagnetic Field: from Data Recovery to Field Reconstruction

Variations of the geomagnetic field over past millennia can be determined from archeomagnetic data and paleomagnetic sediment records. The resolution and validity of any field reconstruction depends on the reliability of such indirect measurements of past field values. Considerable effort is invested to ensure that the magnetic minerals carrying the ancient magnetization are good, if not ideal, recorders of the magnetic field. This is achieved by performing a wide array of rock magnetic and microscopy investigations, many of which are outlined here. In addition to data quality, the spatial and temporal distributions of archeomagnetic and sediment records play a significant role in the accuracy of past field reconstruction. Global field reconstructions enable studies of dynamic processes in Earth’s core. They rely on data compilations which ideally include information about the quality of a measurement and provide a useful archive for selecting data with the best characteristics. There is, however, a trade off between the total number of reliable data and the geographic or temporal coverage. In this review we describe the various types of paleomagnetic recorders, and the kind of measurements that are performed to gather reliable geomagnetic field information. We show which modeling strategies are most suitable, and the main features of the field that can be derived from the resulting models. Finally, we discuss prospects for progress in this kind of research.

Archeomagnetic dating in western North America: An updated reference curve based on paleomagnetic and archeomagnetic data sets

A robust database of paleomagnetic (PM) and archeomagnetic (AM) data for western North America covering the past 4 millennia is assembled from three data sets: (1) published PM data for dated lava flows, (2) recently published AM data generated by R. L. DuBois, and (3) published AM data generated by J. L. Eighmy. Chronologic information for the AM data sets has been reassessed, and the data points have been assigned to age ranges depending on their archeological contexts. We have selected the computer program ArcheoCourbe, available from M. Le Goff (http://www.ipgp.fr/∼legoff) or the auxiliary material (see also http://EarthRef.org), to construct AM reference curves and to quantitatively estimate AM dates for archeological and geological samples. The updated curve provides a more detailed record of prehistoric secular variation for western North America than was previously available and shows a short interval of rapid secular variation (SV) in the 10th or 11th centuries at near‐excursional rates. Similar and coeval features in the Hawaiian and, to a lesser extent, Arkansan SV data sets possibly indicate a rapid amplitudinal shift in the regional nondipole field at that time.

Archeomagnetic secular variation from Korea: Implication for the occurrence of global archeomagnetic jerks

Abstract An archeomagnetic paleosecular variation (PSV) was first defined in Korea using baked materials collected from 26 kilns or hearths with ages ranging from ∼ 1100 BC to AD 1790. Variations of geomagnetic declination and inclination from the Korean peninsula are distinctively different from the prediction of a global model (CALS3k.3 or CALS7K.2) for the past 3500 yr. In particular, a distinctive offset in magnitude and phase is noticeable between the observations and predictions at ∼ 745 BC, ∼ AD 300, and ∼ AD 1400–1700. A bi-plot of magnetic declination versus inclination displays three cusps at the corresponding time intervals. These time intervals are nearly identical to or at least overlap with three of the four potential archeomagnetic jerks suggested by Gallet et al. (2003) from the European archeomagnetic data. A comparison of the PSV curves for neighboring countries/regions revealed that European archeomagnetic jerks at ∼ 800 BC, ∼ AD 200, ∼ AD 800, and ∼ AD 1400 were all preserved in East Asia, suggesting that the archeomagnetic jerks were global (or at least northern hemispheric) features.

Estimate of the magnetic anisotropy effect on the archaeomagnetic inclination of ancient bricks

Abstract The magnetic fabric of 59 bricks coming from 5 ancient kilns has been studied by measuring the anisotropy of magnetic susceptibility (AMS) and the anisotropy of isothermal (AIRM), anhysteretic (AARM) and thermal (ATRM) remanent magnetization. The bricks are characterized by a well developed magnetic fabric that matches their flat shape. The shape of the anisotropy ellipsoids is in almost all cases oblate with the maximum and intermediate axes lying parallel to the large face of the brick and the minimum axis perpendicular to it. The directions of the principal axes are almost the same irrespectively of the type of anisotropy measured, whereas the degree of anisotropy of the AIRM, AARM and ATRM is much higher than the AMS. As the bricks lie horizontally within the kiln, the planar magnetic fabric results in an inclination shallowing of the archaeomagnetic direction with respect to that of the Earth's magnetic field at the time of their last cooling. Estimation of this effect on the grounds of ATRM measurements yields a shallowing that varies from 4° to 10° for individual samples. Such inclination difference may significantly bias archeomagnetic dating; for the case of the Canosa late-Roman kiln it leads to a dating error of more than two centuries.

Geomagnetic field for 0–3 ka: 1. New data sets for global modeling

Paleomagnetic and archeomagnetic records are used in both regional and global studies of Earth's magnetic field. We present a description and assessment of five newly compiled data sets, also used in the companion paper by Korte et al. (2009) to produce a series of time‐varying spherical harmonic models of the geomagnetic field for the last 3000 years. Data are drawn from our compilation of lake sediment records and from the online database, GEOMAGIA50v2. The five selections are available from the EarthRef Digital Archive at http://earthref.org/cgi‐bin/erda.cgi?n=944. Data are grouped according to the source of material, and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows (the ARCH3k_dat data set) and for sediments (SED3k_dat). The overall number of data is 55% greater than in previous compilations. Constrained data sets were selected using different criteria for each group. Winnowing of archeological data was based on uncertainties supplied by the original data providers. The lake sediment data assessment relied on preassigned age uncertainties and one or more of the following: comparisons with archeomagnetic data from the same region, regional consistency among several lakes, and consistency with global archeomagnetic models. We discuss relative merits of a larger unconstrained data set or a smaller (possibly) more reliable one. The constrained data sets eliminate a priori up to 35% of the available data in each case and rely on potentially subjective assessments of data quality. Given the limited data available our analyses indicate that iterative rejection of a small number (1–1.5%) of outlying data during global field modeling is a preferable approach. Specific regional comparisons among the models and data support the conclusion that Korte et al.'s outlier‐free CALS3k.3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0–3 ka field; the ARCH3k.1 model provides a better fit to the denser European archeomagnetic data and may be better in that region.

Updated archeomagnetic data set of the past 8 millennia from the Sofia laboratory, Bulgaria

This data brief reports the latest updates of archeomagnetic data obtained at the Sofia palaeomagnetic laboratory of the Geophysical Institute, Bulgarian Academy of Sciences. The current data set consists of measurements from 284 Bulgarian archeological sites covering the past 8000 years. There are also 54 sites from other European regions, namely, Serbia, Kossovo, Greece, Spain, Switzerland, Finland, and Russian Karelia, as well as five sites from Morocco in North Africa. The update of the archeomagnetic results consisted of a thorough revision of all geomagnetic field measurements as well as dating these measurements that were published in the original papers or in previous compilations. The updated results can be found in GEOMAGIA (http://geomagia.ucsd.edu) or as an Excel spreadsheet at the EarthRef.org Digital Archive (http://earthref.org/cgi‐bin/erda.cgi?n = 946).

A regional archeomagnetic model for Europe for the last 3000 years, SCHA.DIF.3K: Applications to archeomagnetic dating

The available European database of archeomagnetic field values and instrumental data has been used to produce a regional model for the geomagnetic field in Europe for the last 3000 years (from 1000 B.C. to 1900 A.D., connecting with the epoch covered by the IGRF models). This new model, SCHA.DIF.3K, constitutes an improvement with respect to the previous regional archeomagnetic model SCHA.DI.00‐F, which used relocated values and was only valid for the last 2000 years. The new model has been obtained by least sums of absolute deviation inversion of paleomagnetic data using spherical cap harmonics for the spatial representation of the field and sliding windows in time. An algorithm has been developed to jointly model the three archeomagnetic elements declination, inclination, and intensity. The resulting model provides the direction and intensity of the Earth's magnetic field over the European continent, northern Africa, and western Asia for the last 3000 years. The fit to the European archeomagnetic database is more accurate than that provided by global archeomagnetic models. In addition, this model represents a step forward in archeomagnetic dating studies (since the relocation error is avoided) and can also be used to study the rapid changes of the geomagnetic field (archeomagnetic jerks) that have been recently proposed.

Secular variation of the geomagnetic dipole during the past 2000 years

We have constructed a very simple model of a time‐varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka. The predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe. They were also compared with the Hongre et al. (1998) time‐varying spherical harmonic model and with the CALS7K‐2 model by Korte et al. (2005). We find that the misfits between the predictions of our simple dipole model and the data are equivalent to those of the spherical harmonic models for the European sites and not strikingly larger for the rest of the world. Many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records, which, along with the small number and poor geographical distribution of sites, leads us to conclude that the present state of the database does not allow the extraction of secular variations described by terms going beyond degree 2. It appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years. In a second step, we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment. The present decrease of the dipole did not begin prior to 1000 years ago, and the dipole was actually increasing from 0 until A.D. 500. The dipole moment of CALS7K is lower than the present estimate, probably due to large repartition of energy to higher harmonics to minimize the misfit between the inversion and the data. The tilt and strength of the dipole can predict the dipole field at any site and were used to derive the contribution of the nondipole field to values of paleointensity at Paris during the past 2 ka. The results show that the “archeomagnetic jerks” are associated with various configurations depending on the phase relationship between the nondipolar and dipolar parts of the field.

Eruptions of the last 2200 years at Vulcano and Vulcanello (Aeolian Islands, Italy) dated by high-accuracy archeomagnetism

The recent eruptive history of the Vulcano island (Southern Italy) was investigated through the high-accuracy “large sample” archeomagnetic method (Tanguy, J.C., Le Goff, M., Principe, C., Arrighi, S., Chillemi, V., Paiotti, A., La Delfa, S., Patanè, G., 2003. Archeomagnetic dating of Mediterranean volcanics of the last 2100 years: validity and limits. Earth Planet. Sci. Lett. 211, 111–124; Tanguy, J.C., Principe, C., Arrighi, S., 2005. Comment on “Historical measurements of the Earth's magnetic field compared with remanence directions from lava flows in Italy over the last four centuries” by R. Lanza, A. Meloni, and E. Tema. Phys. Earth Planet. Interiors 152, 116–120; Arrighi, S., 2004. The large sample archeomagnetic method applied to Neapolitan volcanoes and Aeolian Islands. PhD Thesis. University of Pisa, Italy, pp. 1–186). Age determination is based upon directional geomagnetic variation reconstructed from historically dated lavas in Southern Italy, and from archeological sites in Western Europe (Gallet, Y., Genevey, A., Le Goff, M., 2002. Three millennia of directional variation of the Earth's magnetic field in Western Europe as revealed by archeological artefacts. Phys. Earth Planet. Interiors 131, 81–89) relocated to Sicily. Results in the present paper were obtained on 12 sites including 185 samples weighing 0.5–1 kg, distributed over the Vulcanello platform lavas and pyroclastic cones, and on the lava flows from the Fossa cone. It is shown that the Vulcanello platform was built by nearly continuous activity between AD 1000 and 1250, which is more than a millennium younger than believed until now from questionable interpretation of imprecise historical accounts. Most of the lavas from the Fossa cone, whose ages were rather hypothetical or known with a large uncertainty, have erupted within the same period. However, the last “Pietre Cotte” obsidian flow is confirmed to date from 1720 ± 30, in agreement with historical data (1739).

Comment on “Recent eruptive history of Stromboli (Aeolian Islands, Italy) determined from high‐accuracy archeomagnetic dating” by S. Arrighi et al.

[1] Arrighi et al. [2004] (hereinafter referred to as AR2004) recently reported paleomagnetic directions gathered from Stromboli volcano by using the unconventional ‘‘large sample method’’ (LSM). These directions are partly different and have smaller confidence cones than those previously reported by Speranza et al. [2004] (hereinafter referred to as SP2004) for the same spatter lavas by using the traditional (core-drilling and systematic stepwise demagnetization) paleomagnetic technique. Hence AR2004 conclude that ‘‘traditional paleomagnetic sampling cannot yield sufficient precision’’, and is therefore unsuitable to study the paleosecular variation of the geomagnetic field recorded in volcanics, and to use it as a dating tool. Here we contend that the laboratory procedures and analysis methods employed by AR2004 yield a fictitious improvement in statistical uncertainty. We conclude that the traditional paleomagnetic techniques provide a more realistic estimate of archeomagnetic dating and related uncertainties.