MagNetZ With Yoseph Dres (2025/06/11) - Archeomagnetic study of Iron Age burnt floors from Botswana

Archeomagnetic study of Iron Age burnt floors from Botswana: Implications for the South Atlantic Anomaly The video is also available for download at: https://earthref.org/ERDA/2764/ and can be referenced with the doi: 10.7288/V4/ERDA/2764 Title: Archeomagnetic study of Iron Age burnt floors from Botswana: Implications for the South Atlantic Anomaly Author: Yoseph Dres from the University of Rochester, USA Prior archeomagnetic studies of Iron Age sites in southern Africa have defined geomagnetic directional and intensity variations suggesting a recurrent South Atlantic Anomaly (Tarduno et al., 2015; Hare et al., 2018). Moreover, this recurrence has been hypothesized to reflect flux expulsion promoted by interaction of the fluid iron core and the core-mantle boundary beneath the African Large Low Shear Velocity Province (Tarduno et al., 2015). Here we discuss new sampling in Botswana to further define the archeomagnetic record. We focused our sampling in the Magagarape Hills, an area that may have seen occupations from ca. 500 to 1000 CE. We recovered fully oriented samples from burnt grain bin floors and a burnt hut. Rock magnetic data, including magnetic susceptibility versus temperature (in air), magnetic hysteresis, and first-order reversal curves, were collected in the University of Rochester Paleomagnetism Laboratories to identify the magnetization carrier minerals. Alternative field (AF) and thermal (TH) demagnetization measurements were conducted to determine magnetic directions. The magnetic hysteresis and first-order reversal curves indicate the presence of pseudo-single-domain grains with varying coercivities. Some magnetic hysteresis loops exhibit a wasp-waisted behavior, indicating mixed coercivities. Additionally, magnetic susceptibility results show Curie temperatures ranging from 580-625°C; near magnetite compositions are dominant with minor hematite relative contributions. AF demagnetization to 100 mT removed 70-80% of the natural remanent magnetization (NRM), while thermal demagnetization to 580 °C in most cases removed 90% of the NRM, with any remaining remanence removed by treatment to 600-650 °C. Single component stable magnetic directions are observed after demagnetization at approximately 400 °C. Our preliminary data show a rapid change of the magnetic field during the Iron Age of southern Africa. ----------------------------------------------------------- 00:00 - Intro 00:15 - Presentation 22:17 - Questions 31:32 - Announcements