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

这项研究的目的是从最近的海洋环境中获取基本信息，以了解地球历史事件。最近的过程对于考虑史前有孔虫物种和组合的灭绝和发展过程是重要的，因为我们仍然没有足够的证据来解释这些过程。重点考察了纳考米湖围海造陆工程中人工堤防、水闸等有孔虫组合的剧烈变化。有孔虫物种已经改变了它们的组合组成，特别是在封闭地区30多年来它们已经灭绝。封闭区底栖生物的消失可通过以下三个过程来解释：(1)咸水进入微咸水区的停止；(2)盐跃层和肥沃的底层沉积物的时间发展；(3)底层水的淡化。底层水的脱盐与钙质颗粒的腐蚀性水有关，如有孔虫试验。另一项研究表明，自然变化对组合变化也是至关重要的。20世纪40年代和50年代初，海平面上升，促进了滨海物种向微咸水区的发展，微咸水物种也入侵到低盐度地区。作为人类活动之一的底泥疏浚，加速了高盐度底水的向上流动，我于2001年底参加了海洋钻探计划第199回合，并有机会观察到古新世/始新世底栖动物灭绝事件。有孔虫突然灭绝的过程将在以后的研究中与上述现代过程相比较来解释。

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.