Mass-balance rates derived by mapping internal tephra layers in Mýrdalsjökull and Vatnajökull ice caps, Iceland

Internal tephra layers of known age have been detected by radio-echo soundings within the Mýrdalsjökull and Vatnajökull ice caps in Iceland. Assuming steady state, the estimated strain rates since these isochrones were deposited on the glacier surface have been used to calculate past average specific net balance rates in the accumulation zones along three flowlines on Mýrdalsjökull and one on Vatnajökull. For the period 1918–91 the specific mass-balance rate has been estimated to 4.5 and 3.5 m a−1 at 1350 m a.s.l. on the southern and northern slopes of Mýrdalsjökull, respectively. At 1800 m elevation on the Bárdarbunga ice dome in Vatnajökull, the specific net balance averaged over the last three centuries is estimated to be about 2.1 m a−1. Given this specific net balance, a revised age–depth timescale is presented for a 400 m deep ice core recovered in 1972 from Bárdarbunga. The ice at the bottom is estimated to be from AD 1750