Investigating the resetting of IRSL signals in beach cobbles and their potential for rock surface dating of marine terraces in Northern Chile

Abstract

Inactive shorelines represent valuable records for sea level change, shoreline variations and tectonics if we can constrain the timing of their formation. Where the associated beaches are cobble dominated, luminescence rock surface dating is a promising alternative to established dating approaches, since unlike other techniques it offers the potential to identify clasts unaffected by inherited ages. While luminescence rock surface dating has successfully been used on Holocene and Late Pleistocene beach ridges previously, in this study the potential of IRSL rock surface dating is evaluated for the magmatic cobbles of uplifted Pleistocene terraces along the tectonically active coast of northern Chile. Cobbles from an active beach were used to investigate the influence of cobble lithology on IRSL signal properties and the effectiveness of IRSL signal resetting in the rock. While alkaline and andesitic cobbles yield low IRSL intensities and limited signal resetting due to strong light attenuation, more favourable characteristics for dating were observed for some diorite and granite cobbles. Their IRSL signals were well reset in the uppermost few mm without any systematic difference between upper and lower surface. Some of them revealed bleaching plateaus with inherited ages close to zero after correction for laboratory residuals. For dating, cobbles from three Pleistocene marine terraces, for which new uranium‑thorium and ESR control ages on molluscs provide age control, were targeted. None of the associated IRSL rock surface burial ages agrees with the MIS 5 control ages of the terraces. Most of the selected cobbles are either too dark to allow for effective signal resetting or yield IRSL properties unsuitable for dating. Only one of the targeted cobbles shows both signs of signal resetting at its surface and sensitive IRSL signals, but its signal was already in field saturation due to dose rates >6 Gy/ka. In conclusion, our data indicate that beach cobbles with granitic to dioritic lithology combine appropriate IRSL properties and sufficient IRSL signal resetting for dating Holocene landforms. Last interglacial terraces may already be beyond the limit of IRSL dating for most cobbles of this lithology since they show large dose rates compared to IRSL sediment dating.