Dating sediments in coastal areas and potential applications for constraining the palaeo-environmental context of an archaeological site at Vohemar

We present an overview on different environmental zones within coastal areas and summarise the physical basis behind the three most important methods that are available to date Holocene coastal sediments. Besides radiocarbon and uranium series dating, Optically Stimulated Luminescence (Osl) has increasingly been applied for dating in coastal settings over the past decade. This is illustrated by a number of case studies showing that Osl can be applied to sediments from almost any kind of coastal environment, covering a potential dating range from some years up to several hundred thousand years. Osl dating may hence be the method of choice for deciphering natural environmental change along coasts as well as the presence and the impact of human occupation in such areas. In addition, we briefly show how and where these dating methods could be applied to constrain the palaeo-environmental context of an archaeological site at Vohemar in north-eastern Madagascar.

Characterization and age determination of Quaternary volcanism in the southern Ankaratra region (central Madagascar) through novel approaches in luminescence dating

This work introduces two novel approaches for the application of luminescence dating techniques to Quaternary volcanic eruptions: crystalline xenoliths from lava flows are demonstrated to be basically suitable for luminescence dating, and a set of phreatic explosion deposits from the Late Quaternary Vakinankaratra volcanic field in central Madagascar is successfully dated with infrared stimulated luminescence (IRSL). Using a numerical model approach and experimental verification, the potential for thermal resetting of luminescence signals of xenoliths in lava flows is demonstrated. As microdosimetry is an important aspect when using sample material extracted from crystalline whole rocks, autoradiography using image plates is introduced to the field of luminescence dating as a method for detection and assessment of spatially resolved radiation inhomogeneities. Determinations of fading rates of feldspar samples have been observed to result in aberrant g-values if the pause between preheat and measurement in the delayed measurements was kept short. A systematic investigation reveals that the phenomenon is caused by the presence of three signal components with differing individual fading behaviour. As this is restricted to short pauses, it is possible to determine a minimal required delay between preheating and measurement after which the aberrant behaviour disappears. This is applied in the measuring of 12 samples from phreatic explosion deposits from the Antsirabe – Betafo region in the Late Quaternary Vakinankaratra volcanic field. The samples were taken from stratigraphically correlatable sections and appear to represent at least three phreatic events, one of which created the Lac Andraikiba maar near Antsirabe. The obtained ages indicate that the eruptive activity in the region started in the Late Pleistocene between 113.9 and 99.6 ka. A second layer in the Betafo area is dated at approximately 73 ka and the Lac Andraikiba deposits give an age between 63.9 and 50.7 ka. The youngest phreatic layer is dated between 33.7 and 20.7 ka. These ages are the first recorded direct ages of such volcanic deposits, as well as the first and only direct ages for the Late Quaternary volcanism in the Vakinankaratra volcanic field. This illustrates the huge potential of this new method for volcanology and geochronology, as it enables direct numerical dating of a type of volcanic deposit which has not been successfully directly dated by any other method so far.