Modern analogue for the extinction and development of benthic foraminiferal fauna and its application to Neogene development

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This study was aimed to obtain basic information from the recent marine environment to understand the geohistric events. The recent processes are important to consider the prehistoric processes of extinction and development of foraminiferal species and assemblage, because we have not still enough evidence to explain the processes. I focused on the drastic changes of foraminiferal assemblage with man-made dikes and water gate in Lake Nakaumi for land reclamation project. Foraminiferal species have changed their assemblage composition and particularly they became extinct in the enclosed area over 30 years. The benthic disappearance in the enclosed area is explained by the following three processes : (1) stop of saline water into brackish area, (2) temporal development of halocline and fertilized bottom sediments, (3) desalinitization of bottom water. The desalinitization of bottom water was associated with corrosive waters for calcareous grains such as foraminiferal tests. The other study indicates natural changes are also critical for the assemblage changes. Sea-level increase occurred in the 1940s and early 1950s, which enhanced the development of coastal species into brackish area and brackish species also invaded into lower saline area. The dredging of bottom sediments, as one of the human activities, accelerated upward flow of high saline bottom water.I have participated Ocean Drilling Program Leg 199 at the end of 2001 and I have a chance to observe the Paleocene/Eocene benthic extinction event. The processes of the abrupt extinction of foraminifera would be explained in comparison with above mentioned modern processes in later studies.