Evidences of a Geomagnetic Field Intensity Decrease from 500 BCE to 50 CE in South Uzbekistan
Résumé
Recent studies indicate the existence of rapid fluctuations of the geomagnetic field intensity over the last 3000 years. These features have been observed during the first millennium BCE in the Mediterranean area (from Western and Central Europe to the Near East), and in other regions as the Canary and the Azores Islands, China and Korea. Nevertheless, due to the difficulty to obtain precise secular variation intensity curves, the spatial and temporal extension of these events, and the potential link between the different regional events observed are still under discussion. In this study, we focus on the rapid intensity decrease that seems to take place in central Asia during the second half of the 1st millennium BCE. We present 51 new archeointensity from South Uzbekistan, obtained by Thellier-Thellier method, also, rock magnetism experiments have been conducted in order to identify the principal magnetic minerals responsible for the magnetization. The results of those experiments show that the maghemite and/or magnetite, in some cases with low titanium content, are the main magnetic carriers. New data together with 73 previous archaeointensity data, allow us to better defined geomagnetic field intensity changes in South Uzbekistan between 600 BCE and 600 CE. The results indicate a rapid drop out in intensity between 500 BCE and 200 BCE with a minimum of 37.2 µT around 200 BCE – 50 CE. A comparison with different VADMs values available for different regions indicates that similar intensity trends are observed during this period at the continental scale. Finally, the results are compared with Axial Dipole Moment estimations derived from different global models, suggesting that this sharp continental decrease is controlled by non-dipolar sources.