Lo tsunami del dicembre 2004: evidenze dell’impatto nell’area costiera di Khao Lak (mare di Andaman, Tailandia)

The study of the environmental evolution in coastal areas hit by December 2004 tsunami exhibits a great deal of interest in terms of both purely scientific research and environmental recovery and protection. The present project, started in 2006, aims to evaluate the variations occurred on coastal and shallow marine sediments and related populations of shelled benthic organisms; the study area comprises the intertidal and shallow infralittoral bottoms between Ban Khao Lak and Ban Chao Nam, along the Thai coast of the Andaman Sea. This area includes tracts heavily hit by the wave and other tracts where effects were moderate or locally absent. In this respect, an actuopaleontological approach - already applied to mollusks in several areas of the Gulf of Thailand - appears as a valid surrogate of the common yearly sampling of faunas. The comparison between recent thanatocoenoses, representing the benthic fauna settled before the tsunami event, and present populations will allow to define 1) the effect of the wave on soft-bottom communities, 2) the rates of recovery subsequently to the catastrophe and 3) the correspondence between pre-existing and newly settled communities. Moreover, the research aims to verify the possibility of building an evolution model of the pre-impact/impact/post-impact sequence, useful to better interpret layers recording very high energy conditions, that occur within sedimentary successions. In range of the project’s first phase, three scientific campaigns were carried out in 2006, in order to survey and sample the selected area. The analysis of samples is still in progress; nevertheless, some observations made contextually with the geomorphological coastal survey allowed to notice some evidences of the impact of the tsunami on intertidale and shallow infralittoral mollusks. Such evidences, besides supratidal shell and coral debris, mainly pertain to sessile species colonizing rocks, boulders and old coral blocks. Dense populations of oysters, frequently retaining both valves and/or traces of ligament, were noted on metric blocks deposited some hundreds of meters from the shoreline at the Khlong Kao (tidal channel) mouth; in the same area, several bivalved specimens of Isognomonidae along with single valves of Arcidae, Pitarinae, Gafrariinae, Cerithiidae and Strombidae were found on continental sediments. In the Khlong Rian bay, some blocks detached from seaward rocks were found in the intertidale belt, once again densely colonized by oysters; these colonies were partly alive and partly eroded (when in contact with the sandy shore). The crack-side of the blocks appears to have been very recently re-colonized by juvenile specimens. In the area of Laem Pakarang sand spit, bordered seaward by a wide fossil reef, several metric old coral blocks appear to have been detached from the reef’s edge and deposited landward nearly as far as the coastline. On the surface of the blocks, besides encrusting calcareous algae, a dense mollusk colonization is noted; different species of Ostreidae, Chama, Spondylus are present, sometimes with remnants of the ligament or dried soft parts, along with rock-boring bivalves (Arcidae, Pedum, Martesiinae) whose holes have been abruptly split by the blocks crashing one another. Clearly infralittoral shell debris are scattered all along the coast, some meters above high tide level (Khlong Rian bay, Ban Khuek Khak shores, Laem Pakarang south side).