On the age of the Laschamp geomagnetic excursion

The Laschamp geomagnetic excursion is a critical tie-point found directly in deep-sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux. Despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice, and to providing an independent calibration of the radiocarbon calendar, its timing remains poorly known. Previous K–Ar, 40Ar/ 39Ar, and U–Th isochron determinations from lava flows at the type locality in the Massif Central, France, vary widely, are imprecise, and suggest a mean age of about 46.2±2.5 ka (±2 σ). Results of 6 new unspiked K–Ar and 13 40Ar/ 39Ar incremental heating experiments on subsamples from three sites on the Laschamp and Olby flows are concordant and give a weighted mean age of 40.4±1.1 ka (2 σ uncertainty including analytical sources only) that is 10% younger than the previous estimates. Considering that the 40K→ 40Ar decay constant is not known to a precision better than ±2.4%, the most probable radioisotopic age for the Laschamp excursion is 40.4±2.0 ka (2 σ, analytical plus decay constant uncertainties). This new age for the Laschamp excursion agrees precisely with that deduced from the NAPIS-75 deep-sea sediment paleointensity stack when calibrated against the GISP2 ice core chronology using the O isotopes in ice and the magnetic properties of the marine cores.