[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-03] Archeointensity variation for the past millennium from Reunion Island

*Koji Fukuma¹, Hisatoshi Ito² (1.Department of Environmental System Science, Faculty of Science and Engineering, Doshisha University, 2.Central Research Institute for Electric Power Industry)

Keywords:archeointensity, Thellier method, geomagnetic field model

Archeointensity variation for past one thousand years was obtained from volcanic rocks of Reunion Island in South Indian Ocean. Several geomagnetic field models such as gufm1 or cals-?k series have provided comprehensive global pictures of the geomagnetic field change for the past millennia. Although direct measurements for recent centuries form the basis of these models, for the more distant past archeomagnetic data are indispensable to construct the models. However, archeointensity data rely heavily on restricted regions and materials like archeological artifacts in Europe. This time we obtained reliable archeointensity data from lava flows of Piton de la Fournaise in Reunion Island. Hand samples of upper and lower clinkers were collected on twelve lava flows of archeomagnetic directional studies by Tanguy et al (2011). Emplacement ages are known based on historical records, 14C and archeomagnetic dating. An automated spinner magnetometer with thermal demagnetizer TSpin was utilized for Thellier measurements. Even strict selection criteria of TTA and curvature limit applied for the Thellier results still yielded paleointensity data for 64% samples and averaged paleointensities for 10/12 sites. Lava flows of recent centuries gave consistent archeointensity values with the presumed linear decay of gufm1. Reported lower g10 for 18th century seem restricted to Triaxe data on archeological artifacts. Older lava flows showed a monotonic decay of virtual dipole moment beginning at 1000 CE. Therefore the observed dipole moment decay after 1840 CE could have continued for the past millennium.