Optical dating of cobble surfaces determines the chronology of Holocene beach ridges in Greenland

We constrain the age of a cobble beach ridge system in Greenland using Optically Stimulated Luminescence (OSL) dating of buried cobble surfaces. Luminescence signals from six cobbles are measured using infrared (IR) stimulation at 50 °C (IR₅₀) and 180 °C (pIRIR₁₈₀); these cobbles represent four progressively younger beach ridges lying between ~35 and ~6 m above present sea level. Luminescence–depth profiles show that the IR₅₀ signals in all the samples and pIRIR₁₈₀ signals in four out of six samples are well bleached to depths >1.2 mm into the cobble surfaces. Equivalent doses in well-bleached cobble surface slices (i.e. <1.2 mm) are measured and divided by the effective environmental dose rate to the cobble surfaces in order to derive burial ages. Both IR signals are corrected for anomalous fading using the sample-specific ratio of the field to the laboratory saturation levels. Our results are consistent with a depositional model predicting that the investigated portion of the beach ridge system built out continuously during relative sea level fall between ~5.3 and ~1.9 ka ago. The optical ages suggest that the rate of construction varied considerably, with the highest rates ~3.4 ka ago. Additionally, the lowest-elevation beach ridge investigated here provides the first evidence for a higher-than-present relative sea level, between 1.5 and 2 ka ago, in the Disko coastal region.