17:15 〜 18:45
[BCG06-P09] エディアカラ紀から初期カンブリア紀の微化石の顕微断層撮影と微量元素分析による古生動物及び植物の元素利用解明
キーワード：カンブリア爆発、エディアカラ紀、ICP-MS
The Ediacaran to Early Cambrian period is a very important age in biological evolution because metazoans came to be distributed all over the world during the period, which was the starting point of metazoan evolution in the following Phanerozoic era. The Weng'an biota is one of the most intensively studied Ediacaran biotas, and mainly comprises phosphatic microfossils in the ca. 580 Ma Doushantuo Formation, South China. The fossils retain many complicated structures derived from their soft tissues, replaced by phosphatic minerals during fossilization. These fossils include spherical microfossils with ornamental membranes, which are interpreted as animal embryos during cleavage. They also include irregular microfossils with reticulate microstructures, which are interpreted as algae. They are several hundred µm to one mm in diameter. Early Cambrian strata in the Kuanchuanpu region of South China also yield phosphatic spherical and irregular fossils, similar to those from Weng'an. The spherical and irregular microfossils are commonly interpreted as animal eggs/cleavage stage embryos and algae, respectively, based on their morphology; however, their species and ecology remain debatable.
The purpose of this study is to establish a new methodology to estimate the original kingdom and to apply the methodology to the Weng'an and Kuanchuanpu microfossils. The methodology comprises combination of in-situ chemical analysis of major and trace elements and organic matter contents using SEM-EDS, LA-ICP-MS, and LA-Raman spectroscopy with three-dimensional morphological analysis using Synchrotron X-ray Micro-CT analysis.
Synchrotron X-ray Micro-CT analysis allows us to select fossil samples that preserve biological structures and to estimate the original phyla of the fossils based on their morphology. Laser Raman spectroscopy and SEM-EDS measurements enable us to determine distribution of organic matter within the fossils and their host minerals. Some fossils are composed of phosphatic minerals with ubiquitous distribution of organic matter and carbonates, calcite and dolomite, with little amount of organic matter. Therefore, it is considered that the biological tissues of the original organisms were replaced by phosphate, whereas the voids were replaced by dolomite or calcite.
Their rare earth element patterns obtained using LA-ICP-MS show a negative Ce anomaly and positive Y anomaly for both embryo and algae fossils. The former indicates that the phosphate deposition occurred under the oxic condition, and the latter suggests that phosphate minerals were precipitated from the water column.
The animal embryo fossils are more enriched in Se, Co, and Zn, bioessential elements, than the algal fossils. Se, in particular, is used in antioxidant enzymes, possibly related to the Neoproterozoic oxygenation event. On the other hand, algal fossils have higher Cu, V, and Cr contents than the animal embryo fossils. Cupper is an essential element for plants used in photosynthesis. In addition, V and Cr have also been reported to contribute to plant growth. Therefore, it is considered that the ancient algae utilized these elements, similarly to living plants.
This method can be applied to fossils in which soft tissues are replaced by phosphatic minerals. The combination of chemical and morphological information may provide new insights into the Ediacaran and early Cambrian organisms.
The purpose of this study is to establish a new methodology to estimate the original kingdom and to apply the methodology to the Weng'an and Kuanchuanpu microfossils. The methodology comprises combination of in-situ chemical analysis of major and trace elements and organic matter contents using SEM-EDS, LA-ICP-MS, and LA-Raman spectroscopy with three-dimensional morphological analysis using Synchrotron X-ray Micro-CT analysis.
Synchrotron X-ray Micro-CT analysis allows us to select fossil samples that preserve biological structures and to estimate the original phyla of the fossils based on their morphology. Laser Raman spectroscopy and SEM-EDS measurements enable us to determine distribution of organic matter within the fossils and their host minerals. Some fossils are composed of phosphatic minerals with ubiquitous distribution of organic matter and carbonates, calcite and dolomite, with little amount of organic matter. Therefore, it is considered that the biological tissues of the original organisms were replaced by phosphate, whereas the voids were replaced by dolomite or calcite.
Their rare earth element patterns obtained using LA-ICP-MS show a negative Ce anomaly and positive Y anomaly for both embryo and algae fossils. The former indicates that the phosphate deposition occurred under the oxic condition, and the latter suggests that phosphate minerals were precipitated from the water column.
The animal embryo fossils are more enriched in Se, Co, and Zn, bioessential elements, than the algal fossils. Se, in particular, is used in antioxidant enzymes, possibly related to the Neoproterozoic oxygenation event. On the other hand, algal fossils have higher Cu, V, and Cr contents than the animal embryo fossils. Cupper is an essential element for plants used in photosynthesis. In addition, V and Cr have also been reported to contribute to plant growth. Therefore, it is considered that the ancient algae utilized these elements, similarly to living plants.
This method can be applied to fossils in which soft tissues are replaced by phosphatic minerals. The combination of chemical and morphological information may provide new insights into the Ediacaran and early Cambrian organisms.