17:15 〜 18:45
[BCG05-P08] Well-preserved radiolarian fossils within trace fossils from Miocene and Oligocene deep-sea sedimentary rocks in Japan
キーワード：珪質微化石、保存ポテンシャル、生痕化石、炭酸塩コンクリーション、タフォノミー、新生代
Introduction
Extraction of well-preserved microfossils from sedimentary rocks is an essential procedure in studies of micropaleontology and regional geology. Stratigraphy and distribution of intensely deformed sedimentary rocks can be reconstructed by correlation and dating strata based on data of occurrence of siliceous and calcareous microfossils such as radiolarians, diatoms, foraminifers, calcareous nannofossils and so on. In these analyses, preservation of diagnostic characteristics in the microfossil specimens is a key to obtain high-resolution data of the range for each taxon. In order to collect well-preserved microfossils, we have focused on siliceous microfossils within the infills of trace fossils, such as Tasselia ordamensis, and conducted taphonomic analysis of them (Kikukawa et al., 2024 in press). According to Kikukawa et al. (2024 in press), well-preserved siliceous microfossils such as radiolarians and diatoms are mainly preserved in the inner burrow fill sediments (central part of the trace fossil). However, more detailed comparisons of fine structures of the shells from the trace fossils and host rocks are required to understand the preservation potential of the trace fossils well. In this study, we evaluated the state of the preservation of the radiolarian fossils within the trace fossil Tasselia ordamensis from the deep-sea sedimentary rocks of the Miocene Amatsu Formation in Boso Peninsula and the Oligocene Nishinoomote Formation on Tanegashima Island. In addition, we compared the preservation of radiolarian fossils from the inside and outside of the trace fossils.
Material and Method
Samples were obtained from Tasselia ordamensis (trace fossil samples) and host rocks around the trace fossils (bulk rock samples) in the Amatsu and Nichinoomote formations to extract radiolarian fossils. Acid treatments were conducted to the samples. Radiolarian fossils were randomly picked from the residues, and 100 specimens for each sample were mounted and observed under SEM to identify species. In addition, samples for XRD analysis were taken from the trace fossil and bulk rock samples to determine and compare the mineral compositions.
Results
The XRD analysis suggested that the infill of the trace fossils were mainly composed of the carbonate minerals such as rhodochrosite (MnCO3), dolomite (CaMg(CO3)2) and calcite (CaCO3). However, these carbonate minerals were absent in the bulk rock samples. The exceptionally well-preserved spumellarian and nasellarian fossils were observable in the infill of both the Miocene and Oligocene trace fossils. Fine delicate structure of the shells, which were damaged in bulk rock samples, were remained intact in the specimen from the trace fossil samples. Furthermore, it was found that some unknown collodarian fossils were preserved only within the Miocene trace fossil samples.
Discussion
The difference in the state of preservation of radiolarian fossils between the trace fossil and bulk rock samples can be caused by protection from burial compaction and diagenesis by rapid growth of carbonate in the trace fossils. The growth rate of carbonate concretions that form in response to decaying organic matter are significantly fast relative to geological timescale (Yoshida et al., 2018). Tasselia ordamensis is interpreted to be mainly formed by excretion activities of trace maker, so that they may contain both bodily fluids of the animal (such as mucus) and organic waste matter (Kikukuawa et al., 2024 in press). These organic contents could be a potential source of biogenic CO32-which would promote the precipitation of carbonate minerals. In future works, well-preserved microfossils are required to be extracted from the trace fossils for further discussion of the radiolarian diversity throughout the Phanerozoic.
Reference
Kikukawa et al., 2024 in press. Revue de Micropaléontologie.
Yoshida et al., 2018. Scientific Reports 8, 6308.
Extraction of well-preserved microfossils from sedimentary rocks is an essential procedure in studies of micropaleontology and regional geology. Stratigraphy and distribution of intensely deformed sedimentary rocks can be reconstructed by correlation and dating strata based on data of occurrence of siliceous and calcareous microfossils such as radiolarians, diatoms, foraminifers, calcareous nannofossils and so on. In these analyses, preservation of diagnostic characteristics in the microfossil specimens is a key to obtain high-resolution data of the range for each taxon. In order to collect well-preserved microfossils, we have focused on siliceous microfossils within the infills of trace fossils, such as Tasselia ordamensis, and conducted taphonomic analysis of them (Kikukawa et al., 2024 in press). According to Kikukawa et al. (2024 in press), well-preserved siliceous microfossils such as radiolarians and diatoms are mainly preserved in the inner burrow fill sediments (central part of the trace fossil). However, more detailed comparisons of fine structures of the shells from the trace fossils and host rocks are required to understand the preservation potential of the trace fossils well. In this study, we evaluated the state of the preservation of the radiolarian fossils within the trace fossil Tasselia ordamensis from the deep-sea sedimentary rocks of the Miocene Amatsu Formation in Boso Peninsula and the Oligocene Nishinoomote Formation on Tanegashima Island. In addition, we compared the preservation of radiolarian fossils from the inside and outside of the trace fossils.
Material and Method
Samples were obtained from Tasselia ordamensis (trace fossil samples) and host rocks around the trace fossils (bulk rock samples) in the Amatsu and Nichinoomote formations to extract radiolarian fossils. Acid treatments were conducted to the samples. Radiolarian fossils were randomly picked from the residues, and 100 specimens for each sample were mounted and observed under SEM to identify species. In addition, samples for XRD analysis were taken from the trace fossil and bulk rock samples to determine and compare the mineral compositions.
Results
The XRD analysis suggested that the infill of the trace fossils were mainly composed of the carbonate minerals such as rhodochrosite (MnCO3), dolomite (CaMg(CO3)2) and calcite (CaCO3). However, these carbonate minerals were absent in the bulk rock samples. The exceptionally well-preserved spumellarian and nasellarian fossils were observable in the infill of both the Miocene and Oligocene trace fossils. Fine delicate structure of the shells, which were damaged in bulk rock samples, were remained intact in the specimen from the trace fossil samples. Furthermore, it was found that some unknown collodarian fossils were preserved only within the Miocene trace fossil samples.
Discussion
The difference in the state of preservation of radiolarian fossils between the trace fossil and bulk rock samples can be caused by protection from burial compaction and diagenesis by rapid growth of carbonate in the trace fossils. The growth rate of carbonate concretions that form in response to decaying organic matter are significantly fast relative to geological timescale (Yoshida et al., 2018). Tasselia ordamensis is interpreted to be mainly formed by excretion activities of trace maker, so that they may contain both bodily fluids of the animal (such as mucus) and organic waste matter (Kikukuawa et al., 2024 in press). These organic contents could be a potential source of biogenic CO32-which would promote the precipitation of carbonate minerals. In future works, well-preserved microfossils are required to be extracted from the trace fossils for further discussion of the radiolarian diversity throughout the Phanerozoic.
Reference
Kikukawa et al., 2024 in press. Revue de Micropaléontologie.
Yoshida et al., 2018. Scientific Reports 8, 6308.