The Blake geomagnetic excursion recorded in a radiometrically dated speleothem

Abstract

One of the most important developments in geomagnetism has been the recognition of polarity excursions of the Earth's magnetic field. Accurate timing of the excursions is a key point for understanding the geodynamo process and for magnetostratigraphic correlation. One of the best-known excursions is the Blake geomagnetic episode, which occurred during marine isotope stage MIS 5, but its morphology and age remain controversial. Here we show, for the first time, the Blake excursion recorded in a stalagmite which was dated using the uranium-series disequilibrium techniques. The characteristic remanent magnetisation is carried by fine-grained magnetite. The event is documented by two reversed intervals (B1 and B2). The age of the event is estimated to be between 116.5±0.7 kyr BP and 112.0±1.9 kyr BP, slightly younger (∼3–4 kyr) than recent estimations from sedimentary records dated by astronomical tuning. Low values of relative palaeointensity during the Blake episode are estimated, but a relative maximum in the palaeofield intensity coeval with the complete reversal during the B2 interval was observed. Duration of the Blake geomagnetic excursion is 4.5 kyr, two times lower than single excursions and slightly higher than the estimated diffusion time for the inner core (∼3 kyr).