JC11-K5 was dated by correlating ten macroscopically visible layers with counted annual layer sequences of adjacent cores and tephra shards related to the Askja eruption in 1875 CE. As a conservative estimate, we assigned a 2σ error of 10 years to the marker layers that we used for calculating the age-depth model in OxCal v. 4.2, a Bayesian age-depth modelling approach that provides posterior age uncertainties.