The age and duration of the Matuyama-Brunhes transition from new K-Ar data from La Palma (Canary Islands) and revisited ~(40)Ar/~(39)Ar ages

摘要

Paleomagnetic investigations conducted on lava sequences from La Palma (Canary Islands) yielded several independent records of the Matuyama-Brunhes transition (MBT). Seven K-Ar ages of flows sampled across the MBT suggest a duration between 0 and 11 kyr, and provide a weighted mean age of 786 ± 3 ka (analytical uncertainty only) for the transition. This value is significantly older than the previously admitted age of 779 ± 2 ka, derived from volcanic or sedimentary sequences. When the 1% uncertainty on the calibration standard used in the present study is considered, our age for the transition recorded at La Palma becomes 786 ± 8 ka. We have recalculated the ~(40)Ar/~(39)Ar determinations from the previous studies using recent revisited ages for ~(40)Ar/~(39)Ar. We have recalculated the ~(40)Ar/~(39)Ar determinations from the previous studies using recent revisited ages for ~(40)Ar/~(39)Ar standards. This yields a global dataset of 23 K-Ar and ~(40)Ar/~(39)Ar determinations. Our best estimate for the MBT age derived from this compilation is 789 ± 2 ka (analytical error), or 789 ± 0 ka (total error). Within this 5 kyr uncertainty, which could be considered as a minimum estimate, this age remains compatible with recent determinations made by the astronomical time scale. However, the present study suggests that better control of the lock-in depth, as well as better quantification of uncertainties involved in the tuning of δ~(18)O records to orbital forcing models, are needed to improve further the astronomical polarity time scale derived from sedimentary records. In addition, absolute dating by K-Ar and ~(40)Ar/~(39)Ar will gain in accuracy when uncertainty determinations on standards will be significantly improved.