古環境指標としての腕足動物殻の炭素・酸素同位体組成の有用性

概要

論文の詳細を見る
Carbon and oxygen isotopic compositions of brachiopod shells have been used for the reconstruction of Paleozoic global carbon cycles and seawater temperatures. Brachiopods occur throughout the Phanerozoic and their secondary shell layer consists of low-magnesium calcite. It is assumed that the shells were precipitated in isotopic equilibrium with ambient seawater. Recent studies, however, showed that isotopic composition of secondary shell layer varies due to "vital effects" and that the criteria proposed in previous studies to distinguish the diagenetic alteration of fossil brachiopod shells are not always perfect. In order to increase the reliability of the isotopic composition of brachiopod shells as a proxy for ancient seawater conditions, it is necessary to clarify variations in isotopic composition in a single shell and within and between species by studying modern brachiopods and to compare the variations to the measured environmental conditions such as temperature, salinity, and isotopic composition of seawater. This will allow selection of species and shell portions that best reflect the isotopic composition of ambient seawater. The information about shell ultrastructure, isotopic and trace elemental compositions of modern specimens are also needed to quantify the degree of modification of original isotopic composition resulting from diagenetic alteration by examining fossil forms.
日本地球化学会の論文
2006-12-26