HOLOCENE SEA-LEVEL RISE, TSUNAMIS, AND ISLAND INSTABILITY IN THE MALDIVES (INDIAN OCEAN): AN EXAMPLE OF GEOLOGICAL HAZARDS IN LOW-LYING ISLAND NATIONS

The Maldives are among the lowest and flattest countries in the world. The archipelago consists of 22 major atolls with 1.200 sand and rubble islands that rise no more than 5 m above mean sea level. The Maldives are inhabited by almost 400.000 people that live on some 300 islands. The large majority of inhabitants (>100.000) live in the capital Male, an island that measures only 5 km2. Drilling on a central Maldivian atoll margin nearby has shown that reefs grew with rates of >15 m/kyrs in the early Holocene and kept pace with very high rates of sea-level rise. Sea-level rise decreased to ca. 2 m/kyr around 7-6 kyrs BP, and has been rising by <1 m/kyr since 6 kyrs BP. Lagoonal reefs did not keep pace with sea-level rise but only caught up some 2.000 yrs BP. The question as to whether late Holocene sea level exceeded modern levels is still debated. The presently estimated sea-level rise for the 21th century of 20-60 cm (rates of 2-6 m/kyr) should not be a problem in the light of the early Holocene history. However, much higher rates of sea-level rise have recently been predicted for the 21st century (75-190 cm/century). Also, global change including temperature rise and bleaching, coral desease, ocean acidification, and anthropogenic island modification have decreased the ability of reefs and reef islands to keep pace with rising sea level. Our drilling on marginal reefs has furthermore shown that only the upper 1-2 m are cemented whereas most of the Holocene reefs are largely uncemented. As a consequence, parts of the upper slope of Male have collapsed, thereby underlining the danger of extensive building on these islands. Tsunamis, like on 26 December 2004, inundated and modified many reef islands. Higher sea levels will make the islands of the Maldives more vulnerable to such events. Therefore, vibracoring in atoll lagoons in the Maldives aims at identifying coarse-grained event beds in the Holocene lagoonal muds in order to put together a tsunami archive for the past ca. 10.000 years and to estimate tsunami frequency. Potential tsunami beds may not be confused with storm layers as the Maldives are to a large part located outside the cyclone belt.