Following the 2004 Asian tsunami, worldwide tsunami scientists recognized the need for identifying stratigraphic signatures of paleo-tsunami events in the Indian Ocean and to develop a regional tsunami chronology. This project carried out detailed, fine resolution, sediment profile studies to recognize the signatures of paleo-coastal environmental change in southeastern and eastern Sri Lanka, so that they can be differentiated from the regional record of tsunami deposits.
Sediment cores (~100 m total sediment) were extracted along landward transects from nine lagoons, two swales, and one back barrier marsh along the prograding coastline in southeastern and eastern Sri Lanka. Multiple proxies, including grain size, magnetic susceptibility (MS), visible and near IR reflectance, chemical composition, mineralogy and texture of sediments, micro, and macro fossil contents were used for this study. Measurements of grain size, MS, reflectance, and x-ray florescence were taken at 1-2 cm resolution. AMS 14C dating was used for age determination.
Results indicate a sea level high-stand between 5200-4900 yrs BP followed by sea level stabilization. Gradual filling of the Kirinda paleo-estuary and the Okanda lagoon commenced around 4900 yrs BP as a result of this sea level stabilization. Proxy evidence and a high rate of erosion after 4500 yrs BP suggests a sea level regression after 4500 yrs BP. Beach ridge development started around 4900 yrs BP and intensified after 2500 yrs BP.
Climate proxies suggest a weakened monsoon system until ~3500 yrs BP, when monsoon activity gradually increased. Four drought phases around 5500, 4500-3500, 1500 and 300 yrs BP are recognized. Significant ~6yr, ~20-25 yr and ~128 yr cycles, in monsoon variability can be discerned in wavelet power spectra of climatic proxies. According to previous studies, these Indian monsoon cycles are controlled by ENSO and solar variability.
According to results, the most recent pre-2004 tsunami event likely occurred around 1000 yrs BP with the older events around 4200 yrs BP and 4900 yrs BP. The sedimentary record from these estuaries, lagoons, and beach-ridge plains shows clear localized evidence for several additional flood and storm events, and possibly two other tsunami events around 4500 yrs BP and 7000 yrs BP.