Rapid Intensity Decrease During the Second Half of the First Millennium BCE in Central Asia and Global Implications.

Recent paleomagnetic studies have shown that important short‐lived intensity fluctuations occurred during the first millennium BCE. However, the knowledge of the spatial and temporal extension of these features is still limited by the scarce availability of robust data. In this study we focus on the study of the intensity decrease that took place in Central Asia during the second half of the 1st millennium BCE after the high intensities that characterized the Levantine Iron Age Anomaly. Since previous archeointensities available for this period and region were obtained without accomplishing modern standards of quality, we present here new archeointensities that are derived from classical Thellier and Thellier experiments, including partial thermoremanent magnetization (pTRM) checks, thermoremanent magnetization (TRM) anisotropy and cooling rate corrections at the specimen level. The new 51 archeointensities, together with previous archeointensities, have been used to present a new local paleosecular variation curve for Central Asia. The results confirm the existence of an important geomagnetic field intensity decrease in South Uzbekistan from the 4th century BCE to the end of the 1st century BCE associated with rates of changes up to −15 μT/century. A critical analysis of the archeointensity global database indicates that this feature was present at continental scale, from Western Europe to Central Asia. However, this trend is not identified in other regions such as Japan or Mexico. Finally, the comparison with the dipole moment derived from recent global geomagnetic field reconstructions suggests a strong influence of non‐dipolar sources upon this continental intensity feature. Plain Language Summary: The Earth's magnetic field (also called the geomagnetic field) is continuously changing over time and space. Since the 19th century, the behavior of the geomagnetic field is known through direct observations performed in geomagnetic observatories and satellites. However, the only way to recover past geomagnetic field in ancient times is through paleomagnetic and archeomagnetic studies. In particular, archeomagnetism is the study of heated archeological materials, which include ferromagnetic particles acting as a compass and recording the geomagnetic field direction and strength at a certain time. When the archeological material cools down the ferromagnetic particles save the direction and intensity, like a photograph of the ancient geomagnetic field. Nowadays, there are very few archeomagnetic studies dealing with archeological materials from Central Asia. In this work, we present the variation of the geomagnetic field intensity between 600 BCE–600 CE in Uzbekistan deduced from a complete archeomagnetic investigation of 70 pottery fragments. The results confirm a rapid variation of geomagnetic field strength between 400 BCE and 100 BCE–1CE. To determine the spatial expression of this feature, we compared the obtained trend for Central Asia with the geomagnetic field intensity evolution in other regions of the world. Key Points: We present 51 new thermoremanent magnetization‐anisotropy corrected archeointensities for South UzbekistanAn important decrease in intensity is observed between 400 BCE and 100 BCE–1 CE, probably associated with non‐dipolar sourcesSimilar intensity V‐shaped trends are observed at continental scale [ABSTRACT FROM AUTHOR]