Low geomagnetic field intensity in the Matuyama Chron: palaeomagnetic study of a lava sequence from Afar depression, East Africa

Palaeointensity variation is investigated for an inferred time period spanning from 2.34 to 1.96 Ma. Twenty-nine consecutive lava flows are sampled along cliffs 350m high generated by normal faulting on the Dobi section of Afar depression, Ethiopia. Magnetostratigraphy and K-Ar measurements indicate a lava sequence of R-N-R-N geomagnetic field polarities in ascending order; the lower normal polarity is identified as the Reunion Subchron. Reliability of palaeomagnetic data is ascertained through careful thermal demagnetization and by the reversal test. The Tsunakawa-Shaw method yielded 70 successful palaeointensity results from 24 lava flows and gave 11 acceptable mean palaeointensities. Reliability in palaeointensity data is ascertained by the similar values obtained by the IZZI-Thellier method and thus 11 reliable mean values are obtained from our combined results. After the older reverse polarity with the field intensity of 19.6 +/- 7.8 mu T, an extremely low palaeointensity period with an average of 6.4 mu T is shown to occur prior to the Reunion Subchron. During the Reunion Subchron, the dipole field strength is shown to have returned to an average of 19.5 mu T, followed by second extreme low of 3.6 mu T and rejuvenation with 17.1 +/- 5.3 mu T in the younger reverse polarity. This 'W-shape' palaeointensity variation is characterized by occurrences of two extremely weak fields lower than 8 mu T prior to and during the Reunion Subchron and a relatively weak time-averaged field of approximately 15 mu T. This feature is also found in sedimentary cores from the Ontong Java Plateau and the north Atlantic, indicative of a possibly global geomagnetic field phenomenon rather than a local effect on Ethiopia. Furthermore, we estimate a weak virtual axial dipole moment of 3.66 (+/- 1.85) x 10(22) Am-2 during early stage of the Matuyama Chron (inferred time period of 2.34-1.96 Ma).