Luminescence dating of tsunami deposits from the algarve shelf, Portugal

Abstract

Offshore tsunami deposits have the potential to substantially improve our understanding of long-term tsunami hazard. This may be particularly true for the Portuguese Algarve coast, where onshore sediment records are restricted to the last 3000 years due to scarcity of sediment archives suitable for identifying tsunami deposits. This study investigates the potential of optically stimulated luminescence (OSL) for dating offshore tsunami layers from the Portuguese shelf and its background sedimentation cored during METEOR cruise M-152, by comparing luminescence ages with radiocarbon chronologies. Accurate luminescence ages for both offshore tsunami deposits and constraining shelf sediments can be achieved by using single grains of quartz in combination with the minimum age model. However, even single grain ages significantly over-estimate by several thousand years for some very poorly bleached tsunami deposits and underlying shelf sediments affected by erosion during the event. This demonstrates the necessity of using chronologies that are based on a large number of ages, as well as a combination of direct tsunami ages and limiting ages for background sedimentation. The combination of luminescence ages from this study with radiocarbon ages using a Bayesian approach yields an age of 3600–3800 years for a predecessor event of the 1755 CE Lisbon tsunami, formerly radiocarbon dated in the same offshore sediment cores to ca. 3600 cal years BP.