11:30 〜 11:45
[BCG05-04] Seeing is believing - in situ visualization of radiolarians within their enveloping rock matrix
キーワード：Synchrotron MCT, Radiolaria, Paleozoic, 3d imaging
Over the past 500 years since the first microscopes, continuous refinement and development of technology have been at the forefront of advances in knowledge of biology. Obviously, the study of any microfossil group is empowered by the technology available at the time of any given study. Haeckel’s series of monographs clearly illustrate the power of optical microscopy available in the 1800s.
New techniques to extract the siliceous skeletons of radiolarian microfossils from their host rocks, advances in scanning electron microscopy (SEM) and the development of an understanding of the importance of plate tectonics combined in a late 20th-century golden era during which radiolarians were widely investigated. Japan was one of the leaders with numerous studies of radiolarians from open plate stratigraphy (OPS) successions that characterise its widespread subduction complexes. Techniques developed and refined in Japan have been exported globally.
However, while useful for radiolarians with distinctive external morphologies that could be extracted from rock strata, SEM observation proved less useful for complex forms with multiple shells that are dense and not easily resolved. This is particularly the case with Early Paleozoic spherical forms and has impeded understanding of the early evolution of this group.
Over the past two decades developments in x-ray computed tomography (CT) to micro and nano-scales have led to the application of new technology to resolve internal structure of optically dense multi-shelled radiolarians that characterize Early Paleozoic faunas. Using high-resolution x-ray microCT we have now been able to obtain precise 3D tomographic imagery of most early radiolarian genera and species from Upper Cambrian to Ordovician rocks. Data can be processed using 3D imaging software to construct models for detailed study. These can even be processed using 3D printers to produce large replica fossils. Unfortunately obtaining such scans and processing terabytes of data is extremely time-consuming.
New technology that addresses these issues is now available. Using the new microcomputed tomography beamline (MCT) on the Australian Synchrotron we obtained exceptional imagery of matrix-free radiolarian specimens revealing hitherto unresolvable internal detail. Numerous specimens were scanned in a fraction of the time normally taken for traditional microCT work.
In other cases, it appeared there was no solution. Differences in taphonomy sometimes result in fossil material that can be observed in two dimensions using microscope thin sections but is otherwise not extractable from its rock host. This makes it extremely difficult to examine the three-dimensional structure of such radiolarians. Radiolarians appear to be common amongst the fossil material present in the so-called tight oil shales that characterise unconventional hydrocarbon resources. In samples from the Uppermost Ordovician Wufeng and Longmaxi formations in the Sichuan Basin of China the skeletons of radiolarians appear to be well preserved but are replaced by a mixture of silica and calcite and locally coated with bituminous material. It is impossible to extract such material using known techniques and while serial sectioning could be used to generate a series of two-dimensional images that could be reconstructed digitally this is a destructive process leaving nothing curatable. Fortunately, however the materials that replace and/or coat the original skeletal test of the radiolarians and the surrounding rock matrix exhibit significant differences in their optical densities. Using the Australian Synchrotron MCT beamline, we have for the first time been able to see through mini-cores and image radiolarians inside their enveloping rock host. Initial results using 3D imaging software have generated astonishingly high-quality models of these otherwise invisible fossils. This technology is new but clearly has an incredible future.
New techniques to extract the siliceous skeletons of radiolarian microfossils from their host rocks, advances in scanning electron microscopy (SEM) and the development of an understanding of the importance of plate tectonics combined in a late 20th-century golden era during which radiolarians were widely investigated. Japan was one of the leaders with numerous studies of radiolarians from open plate stratigraphy (OPS) successions that characterise its widespread subduction complexes. Techniques developed and refined in Japan have been exported globally.
However, while useful for radiolarians with distinctive external morphologies that could be extracted from rock strata, SEM observation proved less useful for complex forms with multiple shells that are dense and not easily resolved. This is particularly the case with Early Paleozoic spherical forms and has impeded understanding of the early evolution of this group.
Over the past two decades developments in x-ray computed tomography (CT) to micro and nano-scales have led to the application of new technology to resolve internal structure of optically dense multi-shelled radiolarians that characterize Early Paleozoic faunas. Using high-resolution x-ray microCT we have now been able to obtain precise 3D tomographic imagery of most early radiolarian genera and species from Upper Cambrian to Ordovician rocks. Data can be processed using 3D imaging software to construct models for detailed study. These can even be processed using 3D printers to produce large replica fossils. Unfortunately obtaining such scans and processing terabytes of data is extremely time-consuming.
New technology that addresses these issues is now available. Using the new microcomputed tomography beamline (MCT) on the Australian Synchrotron we obtained exceptional imagery of matrix-free radiolarian specimens revealing hitherto unresolvable internal detail. Numerous specimens were scanned in a fraction of the time normally taken for traditional microCT work.
In other cases, it appeared there was no solution. Differences in taphonomy sometimes result in fossil material that can be observed in two dimensions using microscope thin sections but is otherwise not extractable from its rock host. This makes it extremely difficult to examine the three-dimensional structure of such radiolarians. Radiolarians appear to be common amongst the fossil material present in the so-called tight oil shales that characterise unconventional hydrocarbon resources. In samples from the Uppermost Ordovician Wufeng and Longmaxi formations in the Sichuan Basin of China the skeletons of radiolarians appear to be well preserved but are replaced by a mixture of silica and calcite and locally coated with bituminous material. It is impossible to extract such material using known techniques and while serial sectioning could be used to generate a series of two-dimensional images that could be reconstructed digitally this is a destructive process leaving nothing curatable. Fortunately, however the materials that replace and/or coat the original skeletal test of the radiolarians and the surrounding rock matrix exhibit significant differences in their optical densities. Using the Australian Synchrotron MCT beamline, we have for the first time been able to see through mini-cores and image radiolarians inside their enveloping rock host. Initial results using 3D imaging software have generated astonishingly high-quality models of these otherwise invisible fossils. This technology is new but clearly has an incredible future.