Yves Gallet

Paleointensity estimates from archaeological pottery represent a key source of information on Holocene variations in geomagnetic field intensity. Yet, pottery rarely exhibits ideal single‐domain behavior that fully satisfies the theoretical assumptions underlying absolute paleointensity methods. Using two different experimental procedures, we empirically quantified the reliability, accuracy and precision of pottery archaeointensity determinations on 30 ceramic vessels produced from six geological clay types and fired in two traditional kilns. The ambient geomagnetic field inside the kilns was measured before and after the experiments, and temperatures were recorded throughout the process. Hysteresis, first‐order reversal curves and thermomagnetic susceptibility curves indicate that the experimental ceramics span a range of magnetic properties representative of typical archaeological pottery. A total of 317 specimens were analyzed using the Thellier‐IZZI method in three different laboratory fields, all corrected for anisotropy and cooling rate effects. The results confirmed that applying a laboratory field significantly higher than the ancient field can bias archaeointensity estimates. As non‐SD behavior can produce multiple near‐linear segments in the Arai plot, introducing large uncertainties in the estimates, we propose a new criterion to limit acceptable uncertainty levels. Nineteen samples, comprising 66 specimens, met the acceptance criteria currently applied with the Triaxe method, yielding indistinguishable results from those obtained with the Thellier‐IZZI method. Regardless of the method used, most but not all samples reproduced the expected field value within 5%. Thus, to ensure this accuracy at the 95% confidence level, we recommend averaging data from at least three pottery samples meeting strict criteria per context.

Paleointensity estimates from archaeological pottery represent a key source of information on Holocene variations in geomagnetic field intensity. Yet, pottery rarely exhibits ideal single-domain behavior that fully satisfies the theoretical assumptions underlying absolute paleointensity methods. Using two different experimental procedures, we empirically quantified the reliability, accuracy and precision of pottery archaeointensity determinations on thirty ceramic vessels produced from six geological clay types and fired in two traditional kilns. The ambient geomagnetic field inside the kilns was measured before and after the experiments, and temperatures were recorded throughout the process. Hysteresis, first-order reversal curves and thermomagnetic susceptibility curves indicate that the experimental ceramics span a range of magnetic properties representative of typical archaeological pottery. A total of 317 specimens were analyzed using the Thellier-IZZI method in three different laboratory fields, all corrected for anisotropy and cooling rate effects. The results confirmed that applying a laboratory field significantly higher than the ancient field can bias archaeointensity estimates. As non-SD behavior can produce multiple near-linear segments in the Arai plot, introducing large uncertainties in the estimates, we propose a new criterion to limit acceptable uncertainty levels. Nineteen samples, comprising 66 specimens, met the acceptance criteria currently applied with the Triaxe method, yielding indistinguishable results from those obtained with the Thellier-IZZI method. Regardless of the method used, most but not all samples reproduced the expected field value within 5%. Thus, to ensure this accuracy at the 95% confidence level, we recommend averaging data from at least three pottery samples meeting strict criteria per context.

We investigate the frequency of subduction‐related volcanic events over the past billion years. Our analysis reveals distinct peaks and troughs interpreted as significant fluctuations in global subduction flux. This approach has the advantage of being independent of paleogeographic reconstructions. However, it does not provide information on the spatial distribution of thermal heterogeneities at the core‐mantle boundary. This likely explains why the long‐term evolution of global subduction flux does not correlate in any simple way with the frequency of geomagnetic polarity reversals throughout the Phanerozoic. As an additional parameter, we suggest focusing on how variations in the Earth's inertia, due to changing subduction configurations over time, influence the thermal conditions at the core‐mantle boundary and, consequently, the magnetic reversal frequency.

The rich and extensively studied archaeological record of the Near East provides an opportunity to develop a comprehensive archaeomagnetic dataset for exploring the behavior of the geomagnetic field with high precision. The Levantine archaeomagnetic curve (LAC) project is an ongoing effort to develop a continuous high‐resolution geomagnetic intensity curve for the Levant and Mesopotamia. The first version of the LAC covered the period between 3000 and 550 BCE. Here, we report archaeointensity data from 169 samples compiled into 32 groups dating between the 7th and the 1st centuries BCE aiming at extending the LAC up to the end of the first millennium BCE. Twenty‐two groups are assembled from storage jar handles bearing different types of royal seal impressions, which were used in Judah as part of a taxation administrative system. These groups are combined with 10 other groups of pottery assemblages, three of which are from Hellenistic destruction layers dated using radiocarbon and coins. The new curve shows that the Levantine Iron Age Anomaly (LIAA) spanned 550 years (1100 ‐ 550 BCE) and that the rate of decline during the last spike around 600 BCE could have reached ∼0.6 μT/year. During the 6th century, the virtual axial dipole moment (VADM) dropped from 160 ZAm² to 125 ZAm² after which field intensity only slightly increased to 135 ZAm², until another considerable decline to ∼90 ZAm² during the 3rd to the 1st centuries BCE. We highlight the archaeomagnetic implication of the new curve in inferring the relative chronological relationship between different stamp types.

Our understanding of geomagnetic field intensity prior to the era of direct instrumental measurements relies on paleointensity analysis of rocks and archaeological materials that serve as magnetic recorders. Only in rare cases are absolute paleointensity data sets continuous over millennial timescales, in sub‐centennial resolution, and directly dated using radiocarbon. As a result, fundamental properties of the geomagnetic field, such as its maximum intensity and rate of change have remained a subject of lively discussion. Here, we place firm constraints on these two quantities using Bayesian modeling of well‐dated archaeomagnetic intensity data from the Levant and Upper Mesopotamia. We report new data from 23 groups of pottery collected from 18 consecutive radiocarbon‐dated archaeological strata from Tel Megiddo, Israel. In the Near East, the period of 1700–550 BCE is represented by 84 groups of archaeological artifacts, 55 of which were dated using radiocarbon or a direct link to clear historically dated events, providing unprecedented sub‐century resolution. Moreover, stratigraphic relationships between samples collected from multi‐layered sites enable further refinement of the data ages. The Bayesian curve shows four geomagnetic spikes between 1050 and 600 BCE, with virtual axial dipole moment (VADM) reaching values of 155–162 ZAm², much higher than any prediction from geomagnetic field models. Rates of change associated with the four spikes are ∼0.35–0.55 μT/year (∼0.7–1.1 ZAm²/year), at least twice the maximum rate inferred from direct observations spanning the past 180 years. The increase from 1750 to 1030 BCE (73–161 ZAm²) depicts the Holocene's largest change in field intensity.

Our understanding of geomagnetic field intensity prior to the era of direct instrumental measurements relies on paleointensity analysis of rocks and archaeological materials that serve as magnetic recorders. Only in rare cases absolute paleointensity datasets are continuous over millennial timescales, in sub-centennial resolution, and directly dated using radiocarbon. As a result, fundamental properties of the geomagnetic field, such as its maximal intensity and change rate have remained a subject of lively discussion. Here, we place firm constraints on these two quantities using Bayesian modeling of well-dated archaeomagnetic intensity data from the Levant and Upper Mesopotamia. We report new data from 23 groups of pottery collected from 18 consecutive radiocarbon-dated archaeological strata from Tel Megiddo, Israel. In the Near East, the period of 1700–550 BCE is represented by 84 groups of archaeological artifacts, 55 of which were dated using radiocarbon or a direct link to clear historically-dated events, providing unprecedented sub-century resolution. Moreover, stratigraphic relationships between samples collected from multi-layered sites enable further refinement of the data ages. The Bayesian curve shows four geomagnetic spikes between 1050 and 600 BCE, with virtual axial dipole moment (VADM) reaching values of 155–162 ZAm2 – much higher than any prediction from geomagnetic field models. Rates of change associated with the four spikes are ~0.35–0.55 μT/year (~0.7–1.1 ZAm2/year), at least twice the maximum rate inferred from direct observations spanning the past 190 years. The increase from 1750 BCE to 1030 BCE (73 to 161 ZAm2) depicts the Holocene’s largest change in field intensity.

Our understanding of geomagnetic field intensity prior to the era of direct instrumental measurements rely on paleointensity analysis of rocks and archaeological materials that serve as magnetic recorders. Only in rare cases absolute paleointensity datasets are continuous over millennial timescales, provide sub-centennial resolution, and are directly dated using radiocarbon. As a result, fundamental properties of the geomagnetic field, such as its maximum intensity and maximum rate of change have remained a subject of lively discussion. Here, we place firm constraints on these two quantities using Bayesian modelling of well-dated archaeomagnetic intensity data from the Levant and Upper Mesopotamia. We report new data from 23 groups of pottery collected from 18 consecutive radiocarbon-dated archaeological strata from Tel Megiddo, Israel. In the Near East, the period between 1700-550 BCE is now represented by 87 groups of archaeological artifacts, 57 of which dated using radiocarbon and/or direct association to clear historically-dated events, providing an unprecedented sub-century resolution. Moreover, stratigraphic relation between samples collected from multi-layered sited enable further refinement of the archaeomagnetic ages. The Bayesian curve shows four geomagnetic spikes between 1050 and 600 BCE, with virtual axial dipole moment (VADM) reaching values of 155-162 ZAm ² – much higher than any prediction from geomagnetic field models. Rates of change associated with the four spikes are ~0.35-0.55 μT/year (~0.7-1.1 ZAm ² /year) – at least twice the maximum rate inferred from direct observations spanning the past 190 years. Moreover, the increase from 1750 BCE to the first spike depicts the Holocene largest change in field intensity.

The use of secular variation in Earth’s magnetic field for dating purposes in archeology and volcanology began with the first developments in paleomagnetism. This paper traces the key contributions of Paul-Louis Mercanton, Pierre David, Bernard Brunhes, and Raymond Chevallier in the early 20th century, between the seminal works of Giuseppe Folgheraiter in the 1890s and Emile Thellier in the 1930s, all of whom expressed a strong interest in these applications. These researchers raised issues that are still at the forefront of present-day research, though archeomagnetists are now able to address them with modern tools and much larger sets of data. Surprisingly, in 1901, a first and long plea for the use of archeomagnetism as a dating tool came not from a paleomagnetist but from the limnologist François-Alphonse Forel.