9:15 AM - 9:30 AM
[BCG05-02] Original organisms of Mesoproterozoic acritarchs reconsidered by morphological and chemotaxonomical methods
Keywords:acritarch, micro-FTIR, chemotaxonomy
Most species of Mesoproterozoic palynomorphs belong to acritarchs which have unknown origins. These acritarchs presumably originated from eukaryotic algae due to that modern eukaryotic shells/cell membranes composed of resistant macromolecules have similar morphological characters. Neoproterozoic and Paleozoic acritarchs have more complex morphologies including far different forms with modern palynomorphs. The morphological characters of some species of Neoproterozoic and Paleozoic acritarchs are similar to those of dinocyst while archeopyle, which is a really important structure to identify as dinocyst, is absent. The morphologic characters in more ancient acritarchs (e.g., Leiosphaeridia) observed from Paleo- and Mesoproterozoic sedimentary rocks are poor. Interestingly, their morphologies are rather similar to modern species of acritarchs observed from surface sediments. Some researchers applied macromolecular structures analysis (e.g., micro-FTIR analysis) as chemotaxonomic approaches to reveal the original organisms of acritarchs. However, it is difficult to interpret these data without a larger database about macromolecular structures of palynomorphs and an understanding of the diagenetic process of biomacromolecules in the sediments/sedimentary rocks. In this study, we reconsidered the morphological classification and developed the chemotaxonomic method of Mesoproterozoic palynomorphs including acritarchs in the sedimentary rocks.
The Mesoproterozoic dark-gray shales were collected from the Qaanaaq Formation in northwestern Greenland during the field survey in 2018 and 2019. Kerogen was separated from the sample by acid treatment and observed by a fluorescent microscope. Individual palynomorph was isolated by a manipulation system and dried on a gold mirror to measure the macromolecule using micro-ATR/FTIR. Leiosphaeridia, Simia, Tasmanites and Schizofusa are observed from the kerogen samples. These acritarchs have similar morphologies with modern eukaryotic algal palynomorphs such as prasinophycean phycoma, some species of dinocyst, zygospore of zygnematophyte algae and cell membrane of eustigmatophyte algae. Leiosphaeridia can be subdivided into some species by their diameters and wall thicknesses. In the Qaanaaq Formation, the relative abundances of thin-wall species are higher in the upper shale layers, which is possibly related to change in the depositional environments. On the other hand, aggregated acritarchs (Synsphaeridium and Squamosphaera) and some species of filamentous palynomorphs are also observed. Aggregated acritarchs and filamentous palynomorphs have similar morphologies with modern accumulated akinetes of freshwater N2-fixing cyanobacteria like Anabaena and filamentous cell walls of mat-forming cyanobacteria like Oscillatoria, respectively. The relative abundances of filamentous among these cyanobacteria-like palynomorphs are higher in the upper layers. The IR spectra from a part of these acritarchs are separated into four groups by using cluster analysis. Eukaryota-like Leiosphaeridia and prokaryota-like Synsphaeridium belong to different groups and spectra are similar to modern green algal remains and cyanobacterial cells, respectively. In addition, some palynodebris (fragments of larger fossils) have similar spectra with heated cellulose by a vacuum electric furnace using over 325 ℃, 30 hours. Therefore, from this data, the original macromolecular structures of Mesoproterozoic acritarchs were also maintained more easily due to smaller biodegradation by cellulase and other enzymes. In the future, as spectra of macromolecules are detected from much more species of modern and fossil palynomorphs, we will be able to understand the original organisms forming acritarchs.
The Mesoproterozoic dark-gray shales were collected from the Qaanaaq Formation in northwestern Greenland during the field survey in 2018 and 2019. Kerogen was separated from the sample by acid treatment and observed by a fluorescent microscope. Individual palynomorph was isolated by a manipulation system and dried on a gold mirror to measure the macromolecule using micro-ATR/FTIR. Leiosphaeridia, Simia, Tasmanites and Schizofusa are observed from the kerogen samples. These acritarchs have similar morphologies with modern eukaryotic algal palynomorphs such as prasinophycean phycoma, some species of dinocyst, zygospore of zygnematophyte algae and cell membrane of eustigmatophyte algae. Leiosphaeridia can be subdivided into some species by their diameters and wall thicknesses. In the Qaanaaq Formation, the relative abundances of thin-wall species are higher in the upper shale layers, which is possibly related to change in the depositional environments. On the other hand, aggregated acritarchs (Synsphaeridium and Squamosphaera) and some species of filamentous palynomorphs are also observed. Aggregated acritarchs and filamentous palynomorphs have similar morphologies with modern accumulated akinetes of freshwater N2-fixing cyanobacteria like Anabaena and filamentous cell walls of mat-forming cyanobacteria like Oscillatoria, respectively. The relative abundances of filamentous among these cyanobacteria-like palynomorphs are higher in the upper layers. The IR spectra from a part of these acritarchs are separated into four groups by using cluster analysis. Eukaryota-like Leiosphaeridia and prokaryota-like Synsphaeridium belong to different groups and spectra are similar to modern green algal remains and cyanobacterial cells, respectively. In addition, some palynodebris (fragments of larger fossils) have similar spectra with heated cellulose by a vacuum electric furnace using over 325 ℃, 30 hours. Therefore, from this data, the original macromolecular structures of Mesoproterozoic acritarchs were also maintained more easily due to smaller biodegradation by cellulase and other enzymes. In the future, as spectra of macromolecules are detected from much more species of modern and fossil palynomorphs, we will be able to understand the original organisms forming acritarchs.