Cassidulinid foraminiferal provinces around Japan during the latest Cenozoic

Abstract

Little is known about the relationship between the wall microstructure of cassidulinid foraminifera and environments in which they live. To elucidate this problem, 76 species and subspecies belonging o the genera Islandiella, Cassidulinoides, Takayanagia, Cassidulina, Globocassidulina, Evolvocassidulina, Burseolina, Ehrenbergina, Paracassidulina, Hastilina, and Lernella, from upper Pliocene to Pleistocene sediments, are examined for their wall microstructure and paleobiogeographic distribution in the Japanese Islands. The wall microstructure (crystalline structure) of the family Cassidulinidae consists of four types, namely fibrous, bundle-shaped, intricate, and clumpy. The former two types correspond to optically radial texture and the latter two to optically granular texture. These crystalline structures are consistent within a given species. The paleobiogeographic distribution of cassidulinid foraminifera demonstrates the existence of three provinces: Japan Sea, Pacific Transitional, and Pacific Tropical to Subtropical. The Japan Sea Province is dominated by Islandiella species having fibrous and bundle-shaped wall structures, whereas the two Pacific Provinces are dominated by species possessing intricate structure. Cassidulina norvangi having clumpy crystalline structure is exclusively confined to the Japan Sea Province. Peculiarity of these wall microstructures to the respective biogeographical provinces appears to be largely controlled by the property of water masses, such as temperature; fibrous and bundle-shaped crystalline structures are characteristic of cold-water faunas, and the intricate crystalline structure of warm-water faunas.