Sedimentary record of late Holocene event beds in a mid-ocean atoll lagoon, Maldives, Indian Ocean: Potential for deposition by tsunamis

Abstract

Six Holocene sedimentary events (ranging in age from 420–890, 890–1560, 2040–2340, 2420–3380, 3890–4330, and 5480–5760 yrs BP) have been identified in the lagoon of Rasdhoo Atoll (Maldives; 4°N, 73°W), thereby underlining the importance of atoll lagoons as potential archives of environmental change. Holocene coastal sediments have been studied as archives for past tsunami and storm events but comparable sedimentological studies of mid-ocean atoll lagoons are rare. In ten vibracores covering the past 6.5 kyrs that are characterized by mudstone, wackestone, and floatstone background sedimentation, we found two types of event deposits: (1) several cm thick rudstone layers with redeposited corals like Acropora sp. and Seriatopora sp., which derive from the marginal and/or lagoonal reefs and have been transported into the lagoon and (2) thin (several mm) layers of wackestone, floatstone, and rudstone consisting of reef-derived components like coralline red algae, reef foraminifera (e.g., Amphistegina spp., Calcarina sp.), and redeposited coral fragments. Both types of event layers may be correlated among several cores, which we interpret as tsunami deposits. Five of the six events have temporal counterparts identified at the coasts of Thailand, Sumatra, and India. In the Maldives, close to the equator, no category 1–5 typhoons were recorded, but only tropical depressions and storms as potential triggers of event sedimentation have occurred rarely. Major earthquakes off western Indonesia and generated tsunamis, which potentially reach most parts of the Indian Ocean, are common.