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.
15 February 2024 in Journal of Geophysical Research: Solid Earth
Unraveling the short‐term behavior of the Earth's past geomagnetic field at regional scales is crucial for understanding its global behavior and, thus, the dynamics of the deep Earth. In this context, obtaining accurate full‐vector geomagnetic field records from regions where archeomagnetic data are absent becomes essential. Here, we present the first full‐vector archeomagnetic data from Central Asia, derived from the analysis of nine archeological kilns sampled in South Uzbekistan, dating back to the period between 200 BCE and 1429 CE. To obtain these new data, we conducted thermal and alternating field demagnetization procedures, along with Thellier‐Thellier paleointensity experiments, including partial thermoremanent magnetization checks, thermoremanent magnetization anisotropy and cooling rate corrections. The comparison between the new data, previous selected data from Central Asia, and available global models reveals important differences between approximately 400 BCE and 400 CE, especially concerning the geomagnetic field intensity element. In order to investigate this in detail, we have developed a regional update of the SHAWQ global models family by incorporating, for the first time, high‐quality data from Central Asia. The results suggest that this deviation is linked to non‐dipolar sources of the geomagnetic field in Central Asia reaching a maximum contribution around the first century BCE. According to the updated global paleoreconstruction, this non‐dipole feature, manifested at the Earth's surface as low intensities, is associated with the presence of a reversed flux patch at the core‐mantle boundary beneath this region.