Two thousand years of archeointensity from West Africa

This study presents 17 archeointensity estimates from Senegal and Mali, two neighboring countries in West Africa, for the period 1000 BCE to 1000 CE. The archeological artifacts used in this study were collected during the course of two separate projects, together spanning 22 years and across 8 separate excavations. A primary objective of this study was to get accurate dates, hence, only samples with independent age constraints from pottery style, detailed stratigraphy and C-14 dates were used. A total of 236 specimens from 63 samples were subjected to a double heating paleointensity experiment (IZZI method) from which 95 specimens were selected using a set of very strict selection criteria. The paleointensity results were corrected for differential cooling rate effects and remanence anisotropy. Additionally, we demonstrate the equivalence of using tensors derived from anhysteretic and thermal remanences for correcting remanent anisotropy of the specimens and use the former for the anisotropy correction. Our data show good agreement with the most recent paleosecular variation model but are lower than the pre-existing data, which are mostly from Egypt and Morocco. The presence of substantial non-axial-dipolar contributions in the region is evident when virtual axial dipole moments (VADMs) from the published literature are calculated for 20 latitudinal bands and compared with our data the average VADM values show a distinct latitudinal gradient. A prominent feature of this dataset is an intensity high observed prior to 700 CE in both Senegal and Mali. Comparing this peak with existing records from regions further to the north suggests a small but significant temporal offset and is interpreted to be additional evidence for a geomagnetic field with a significant and rapidly changing non-axial-dipolar contribution. (C) 2013 Elsevier B.V. All rights reserved.