Interpretation of Diatom Thanatocoenoses from the North Pacific Applied to a Study of Core V20-130 (Studies of a Deep-sea Core V20-130. Part IV)

• Main Authors: Kanaya Taro, Koizumi Itaru
• Format: Report
• Language: English
• Published: Tohoku University 1966
• Subjects: Pacific; Subarctic
• Online Access: http://hdl.handle.net/10097/28788; https://tohoku.repo.nii.ac.jp/?action=repository_uri&item_id=11985; https://tohoku.repo.nii.ac.jp/?action=repository_action_common_download&item_id=11985&item_no=1&attribute_id=18&file_no=1

The geographical distribution of diatom species in modern sediments has been analysed on the basis of samples representing the uppermost layers of deep-sea cores collected at widely distributed stations in the Pacific ranging in latitude from 55°N to 55°S. More than 220 taxa have been identified from 118 samples subjected to the analysis. Analyses made on the distributions of each taxon and of recurrent groups formed by combination of diatom taxa resulted in a broad classification of modern diatom thanatocoenoses in the surface layer of the Pacific deep-sea sediments. Seven assemblages are recognized in this proposed classification the of modern diatom thanatocoenoses in the North Pacific. They are : Subarctic assemblage, Northwest Marginal assemblage, Subarctic-Central Mixed assemblage, Northeast Transitional assemblage (North), Northeast Transitional assemblage (South), Central assemblage and Equatorial assemblage. The geographical distribution, as well as the species compositions of these assemblages, reflects the present-day surface water circulation and distribution of upper watermasses in the North Pacific. The identification of assemblages in ancient thanatocoenoses, therefore, provides criteria useful in the interpretation of ancient oceanic conditions that controlled the diatom record in deeper layers of North Pacific cores. Cold and warm water forms can also be discriminated through the analyses, and a variable Td can be defined that reflects the present-day latitudinal distribution of surface water isotherms in the North Pacific. The variable is defined as Td=(Xw/Xc+Xw)×100, where Xc and Xw are frequencies of cold and warm water forms, respectively, in a random count of 200 diatom specimens made for a sediment sample. Td values also provide criteria to be useful for the interpretation of the past records preserved in the North Pacific cores. Diatoms in the core V20-130 (36°59′N, 152°36′E; depth, 6547m; length, 1039cm) were studied in an attempt to infer the fluctuation of surface water temperatures ...