The age of the Matuyama-Brunhes transition: New data from the Sulmona paleolake, Italy

The age of the Matuyama-Brunhes geomagnetic polarity transition (MBT) is a key datum for the Pleistocene time scale. Modern estimates of this age vary over a range approaching the 21 ka period of an orbital precession cycle. Singer (2014) placed the transition at 776 ±2 ka, in agreement with the estimate of 773.1 ±0.8 ka of Channell et al. (2010) based on an orbitally-tuned marine ice volume age model for North Atlantic sediment cores. The 40Ar/39Ar data of Singer (2014) are from basaltic lavas with transitional paleomagnetic directions and/or anomalously low paleointensities, representing episodic sampling of the geomagnetic field with limited stratigraphic context. New data from Sulmona paleolake in central Italy (Sagnotti et al., 2014) reveals directional and relative paleointensity records with unsurpassed stratigraphic resolution of the MBT. Here, biogenic carbonates contain numerous sanidine-bearing tuffs derived from nearby volcanoes of the alkalic Roman volcanic province. These tuffs, dated in the LSCE and BGC labs, punctuate the stratigraphic record of the MBT, documenting a smoothly varying sediment accumulation rate of 20-25 cm/ka during the interval between 720-810 ka. A minimum age of 781.3 ±2.3 ka (nominal calibrations of Nomade et al., 2005 and Steiger and Jäger, 1977) for the MBT is provided by a tuff 1 m above the MBT. Linear interpolation between bracketing tuffs yields an age of 786.1 ±1.5 ka for the MBT, and indicate a duration <100 years for the reversal. The Sulmona record thus clearly provides the best opportunity yet known to calibrate the age of the MBT. Applying any combination of reasonable modern choices of decay constants and standard calibrations to our data yields ages between 781 and 793 ka for the MBT, raising questions about the validity of the ice volume model of Channell et al. (2010) unless the transition was globally diachronous (Leonhardt and Fabian, 2007) over a time interval of several ka. Regardless of which calibrating parameters are used, our 40Ar/39Ar age for the MBT at Sulmona is 15 ±3 ka older than that of Singer (2014). These results underscore the need for high-precision dating applied to geomagnetic polarity transitions that are based on high-resolution magnetostratigraphy, in order to calibrate the GPTS and validate age models based on orbital tuning.