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古環境指標としての浮遊性有孔虫

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概要 The Foraminiferida represents one order of single-celled protists that live either on the sea floor (benthic) or within the surface water column (planktonic) of marine environments. The soft tissue (p...rotoplasm) of foraminiferal cell is largely enclosed within a test mainly composed of secreted calcite minerals. This protozoan evolved a planktonic mode of life in the Middle Jurrasic in a neritic environment. The first morphologic prolification occurred in Albian time. The pattern of evolution in shell morphology can be viewed as the best example of iterative evolution. The three shell construction modes of Cretaceous, Paleogene and Neogene forms occurred during the time period from Cretaceous to Recent. The narrow adaptive temperature ranges of planktonic species have become useful tools in oceanographic studies. The nine living bioprovinces and three depth habitats are recognized worldwide. The living plankton foraminifers are divided into non-spinose and spinose categories, with subcategories based on other shell wall structures. These two groups differ from each other in such ecological features as depth habitats, presence of algal symbiont, trophic activity and reproduction cycle. More detailed sediment trap investigations of one or several year duration revealed a pattern of production and seasonal occurrence of each living plankton species. Many trap studies suggest a hypothesis that the subsurface-dwelling species become abundant when shoaling of thermocline occurs. The upwelling of nutrient rich, cool water available for primary production is related to shoaling of thermocline, which eventually controls the productivity in the overlying waters. For the past geologic interval, especially that of Quaternary, coiling ratios and species compositions of selected species have been used as a proxy of reconstructing paleotemperature. Furthermore, geochemical signals of oxygen-carbon isotopes and trace elements recorded in foraminiferal calcite shells provide an important tool for deducing such past oceanic conditions as nutrient, seawater temperature and discharge of river flux. The statistical analysis of faunal assemblages is, however, still a powerful method for the pre-Pleistocene time period because a potential for diagenetic alteration increases exponentially.続きを見る

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登録日 2009.04.22
更新日 2020.10.13