Palaeointensity of West Greenland Palaeocene basalts: asymmetric intensityaround the C27n-C26r transition

Palaeomagnetic sampling of the lower part of the Palaeocene flood basalts o f West Greenland, covering a composite succession of 1.6 km and close to 35 0 individual lava flows, revealed two polarity zones and a 200-m zone of in termediate directions. Ar-40/Ar-39 ages indicate that the transition is C27 n-C26r. Following palaeodirectional and rock magnetic analysis, 102 samples from 64 flows from the pre-transitional normal polarity zone, the zone of intermediate directions, and the post-transitional reverse polarity zone we re chosen for Thellier palaeointensity experiments. Altogether, 83 samples yielded reliable palaeointensity estimates. The mean intensity of the pre-t ransitional C27n field is 16.1 +/- 6.9 mu T (17 flows), Which is only twice the mean transitional field strength, 9.9 +/- 3.7 mu T (10 flows). The pos t-transitional C26r field intensity is generally high with a mean 43.3 +/- 15.0 mu T (21 flows). The asymmetric palaeointensity around the C27n-C26r t ransition with a high post-transitional field resembles the only other deta iled absolute palaeointensity studies of geomagnetic transitions recorded i n volcanic sequences in Hawaii (similar to 4 Ma) and Steens Mountain (simil ar to 15.5 Ma). These new data, therefore, suggest that a strong post-trans itional field might be a systematic feature of geomagnetic reversals, proba bly indicating a prolonged period of an unusual state of the geodynamo foll owing polarity transitions. (C) 2000 Elsevier Science B.V. All rights reser ved.