Mid–Late Holocene Coastal Environmental Changes in Southeastern Sri Lanka: New Evidence for Sea Level Variations in Southern Bay of Bengal

The 2004 Indian Ocean tsunami highlighted the need to better understand the dynamics of the South Asian coastlines. Very little knowledge on paleo-coastal variability hinders the development of a paleo-tsunami chronology of the western Bay of Bengal. Also, limited sea level records for the southern Bay of Bengal are remarkably different from rest of the Indian Ocean due to alternating highstands and lowstands during mid–late Holocene. This study was undertaken to recognize how the coastal environment in tectonically stable eastern and southeastern coasts of Sri Lanka changed during mid–late Holocene and to derive evidence of sea level variability. Sediment cores were collected from Kirinda and Panama estuaries, Okanda lagoon and Vakarai beach ridge plain, situated on the eastern and southeastern coasts of Sri Lanka. Physical and chemical variations in sediments, caused by the coastal environmental changes occurred due to fluctuating eustatic sea level, were determined using X-ray fluorescence, visible reflectance, magnetic susceptibility and grain size. Chronology of events were developed using AMS 14C dates on bulk organic matter, wood, inorganic carbonate and mollusk shells. Results suggest that key periods of transition in the coastal environment in the studied area occurred between about 7300 and 3000 BP. Submergence of coastal environments by mid-Holocene transgression began around 7300 BP. Barrier development initiated at the end of the submergence phase around 4900 BP. Onset of beach ridge development in areas with high sediment supply took place around 4000 BP and continued later than 2400 BP. Marine influence ended around 3000 BP. These coastal environmental changes suggest that Holocene transgression, which started flooding southeastern coastal lowlands, either slowed or the sea level stabilized around 4900 BP. Sea level fell down to its present level at around 3000 BP by the subsequent slow regression. Some cores carry evidence for a possible pause in the transgression or a short lowstand that ended around 5200 BP.