[B-PT02] バイオミネラリゼーションと古環境プロキシー

[BPT02-P06] Evolutionary change of crystallographic orientation and coccolith morphology: Case studies from Cenozoic coccolithophorids

*宇都宮 正志¹、小暮 敏博² (1.産業技術総合研究所　地質調査総合センター、2.東京大学大学院理学系研究科)

キーワード：ココリス、方解石、SEM-EBSD、結晶方位

Coccolithophores are unicellular phytoplanktonic algae which produce tiny platelets made of magnesium-free calcite, called coccoliths as their body scales. Coccoliths have a fossil record with continuous and abundant populations since the end of the Triassic to the present. The interlocking crystal units of calcite have two different crystal orientations: near-vertical c-axis (V-unit) and near-horizontal c-axis (R unit). Consistency between the taxonomy of coccolithophores and V/R unit composition suggests that the crystallographic orientation in coccoliths has a common phylogenetic origin. Despite the usefulness of the V/R model for higher-level taxonomy (such as families and genera), the influence of crystallographic orientation on morphological differences in closely related species remains unclear. Electron back-scattered diffraction (EBSD) operated in a scanning electron microscope (SEM) has the advantage of determining the relationship between the crystallographic orientation determined by EBSD and the three-dimensional coccolith morphology recognized by SEM. In this study, the crystallographic orientation and its relationship to the morphology of coccoliths were investigated for the Neogene-Quaternary calcareous nannoplankton Umbilicosphaera lineage. The c-axis of the calcite forming the distal shield elements was inclined upward at 57 to 66°from the coccolith plane. Accordingly, the outward dip angle of one of the {10-14} faces forming the surface of the distal shield of U. patera coccolith was shallower than those of U. sibogae and U. foliosa, explaining the nearly flat distal shield and the steep inner slope, formed by another equivalent {10-14} face, around the central opening of U. patera. Compared to the U. sibogae coccolith, the dip direction of the {10-14} of U. foliosa coccolith shifted counterclockwise largely from the radial direction of the nucleation site, which may be a reason for the bicyclic structure of the distal shield of this species. Our results showed that the evolution from U. rotula to U. patera during the Late Miocene was not accompanied by a change in crystallographic orientation. In contrast, the evolution from U. patera to U. sibogae and U. foliosa during the Pliocene was accompanied by a rotation of the orientation.