$^{40}$Ar/$^{39}$Ar, K–Ar and $^{230}$Th–$^{238}$U dating of the Laschamp excursion: a radioisotopic tie-point for ice core and climate chronologies
Résumé
A brief period of enhanced $^{10}$Be flux that straddles the interstadial warm period known as Dansgaard–Oeschger event 10 in Greenland and its counterpart in Antarctica, the Antarctic Isotope Maximum 10 is but one consequence of the weakening of Earth's magnetic field associated with the Laschamp excursion. This $^{10}$Be peak measured in the GRIP ice core is dated at 41,250 y b2k (= before year 2000 AD) in the most recent GICC05 age model obtained from the NorthGRIP core via multi-parameter counting of annual layers. Uncertainty in the age of the $^{10}$Be peak is, however, no better than ± 1630 y at the 95% confidence level, reflecting accumulated error in identifying annual layers. The age of the Laschamp excursion [Guillou, H., Singer, B.S., Laj, C., Kissel, C., Scaillet, S., Jicha, B., 2004. On the age of the Laschamp geomagnetic excursion. Earth Planet. Sci. Lett. 227, 331-343.] is revised on the basis of new $^{40}$Ar/$^{39}$Ar, unspiked K–Ar and $^{238}$U–$^{230}$Th data from three lava flows in the Massif Central, France, together with the $^{40}$Ar/$^{39}$Ar age of a transitionally magnetized lava flow at Auckland, New Zealand. Combined, these data yield an age of 40,700 ± 950 y b2k, where the uncertainty includes both analytical and systematic ($^{40}$K and $^{230}$Th decay constant) errors. Taking the radioisotopic age as a calibration tie point suggests that the layer-counting chronologies for the NorthGRIP and GISP2 ice cores are more accurate and precise than previously thought at depths corresponding to the Laschamp excursion