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[BCG07-01] Aragonitic spherulite in modern stromatolite has a core composed of amorphous calcium carbonate, calcite, and extracellular polymeric substances
Carbonate spherulite is a spherical grain with a diameter of several tens of micrometers to 1 mm, and is often included in microbial carbonates. The origin of carbonate spherulite is particularly attracting attention because large-scale oil reservoir rocks along the Atlantic coast contain abundant carbonate spherulite. In a previous study, spherical structures with a diameter of approximately 0.2 to 0.5 μm were observed in SEM from the center of carbonate spherulite, interpreted them as bacterial fossils, and proposed microbial origin of spherulites. However, the only basis for identifying these spherical structures as bacteria is their morphology, and since they are considerably smaller than typical prokaryotes, this interpretation remains questionable. To clarify the origin carbonate spherulite, this study observed and analyzed the interior of carbonate spherulite contained in a hot spring stromatolite.
The sample studied is carbonate deposits (travertine) developing at Nagayu hot spring in Oita Prefecture, where aragonite precipitates reflecting the high concentrations of Mg2+ and SO42− in the hot spring water. In this study, the sample was collected from area where thick cyanobacterial mats cover the travertine surface. Polarized and fluorescence light observations of a thin section showed that this deposit was stromatolite with fine laminations. This study selected a spherulite with a diameter of approximately 20 μm growing near the stromatolite surface where many filamentous cyanobacteria were distributed. A thin-foil section with a thickness of approximately 100 nm was made by focused ion beam (FIB) processing from this spherulite, and the interior was observed using a scanning transmission X-ray microscope (STXM) and a transmission electron microscope (TEM). The results showed that most part of the spherulite was aragonite, but sparse parts with a diameter of about 0.1 to 0.2 μm showing a curly string were observed near the center. Near-edge X-ray absorption fine structure (NEXAFS) analysis using STXM showed that amorphous calcium carbonate (ACC) and carboxy groups were present in these regions. In addition, some of them were surrounded by calcite with a diameter of approximately 2.0 to 2.5 μm. These results imply that ACC nanoparticles first accumulated around the negatively charged extracellular polymeric substances (EPS) of microorganisms, from which calcite crystal nuclei were partly formed due to the influence of the EPS; however, reflecting the surrounding water chemical composition, aragonite crystal nuclei were abundantly formed, and their crystal growth created the studied spherulite.
The sample studied is carbonate deposits (travertine) developing at Nagayu hot spring in Oita Prefecture, where aragonite precipitates reflecting the high concentrations of Mg2+ and SO42− in the hot spring water. In this study, the sample was collected from area where thick cyanobacterial mats cover the travertine surface. Polarized and fluorescence light observations of a thin section showed that this deposit was stromatolite with fine laminations. This study selected a spherulite with a diameter of approximately 20 μm growing near the stromatolite surface where many filamentous cyanobacteria were distributed. A thin-foil section with a thickness of approximately 100 nm was made by focused ion beam (FIB) processing from this spherulite, and the interior was observed using a scanning transmission X-ray microscope (STXM) and a transmission electron microscope (TEM). The results showed that most part of the spherulite was aragonite, but sparse parts with a diameter of about 0.1 to 0.2 μm showing a curly string were observed near the center. Near-edge X-ray absorption fine structure (NEXAFS) analysis using STXM showed that amorphous calcium carbonate (ACC) and carboxy groups were present in these regions. In addition, some of them were surrounded by calcite with a diameter of approximately 2.0 to 2.5 μm. These results imply that ACC nanoparticles first accumulated around the negatively charged extracellular polymeric substances (EPS) of microorganisms, from which calcite crystal nuclei were partly formed due to the influence of the EPS; however, reflecting the surrounding water chemical composition, aragonite crystal nuclei were abundantly formed, and their crystal growth created the studied spherulite.