Foraminifers biodiversity and Holocene evolution of Phetchaburi coastal area (Thailand Gulf).

Abstract Twenty three surface sediments were collected in May 2002, in the Phetchaburi coastal area (Thailand Gulf) starting from the outer margin of the vegetated zone, to a depth of about 20 m, covering an area of about 800 km2. One core was taken in the north-eastern area of the Mae Khlong delta plain to investigate the recent paleoenvironmental evolution of this zone, thirteen core levels were collected for micropaleontological (foraminifers) investigation. This highly populated area is characterized by the inflow of the Mae Khlong, the second major river in Thailand, and the Khlong Ban Labun River. Empty tests pertaining to the thanathocoenosis are dominant and, as for the few living specimens, are most frequently represented by Ammonia spp. The collected species are typically of brackish-water settings. Foraminiferal diversity is rather low and increases from the northern part to the central-southern part of the studied area, where taxa typical of the Indo-Pacific realm (such as Miliolids and Schackoinella globosa) are more abundant than the cosmopolitan species such as Ammonia tepida and Gaudryina exilis. The geographic distribution of the four foraminiferal assemblages, defined on the basis of the cluster analysis, indicates that the northern sector is affected by severe environmental conditions. Since this area is affected by the Mae Khlong River input, the salinity fluctuation is probably a controlling factor on the foraminifers distribution. Ammonia tepida, A. inflata, Murrayinella murrayi and Miliolinella sp.1 are the most opportunistic species, able to tolerate wide environmental variability. As regards to the Holocene data, the study carried out on the core located on the Mae Khlong delta plain, indicates that the tidal flat paleoenvironment has been subject to different fresh-water influence, with a general prograding trend.