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)

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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 during the accumulation of the Pleistocene section represented in the core. The diatom temperature curve thought to reflect the fluctuations of surface water temperature is derived by plotting Td values obtained at 86 core levels examined of the core. The fluctuation of the curve through the core sequence is interpreted in the light of diatom assemblages whose types were determined at a number of maxima and minima of Td values shown by the curve. In the upper two thirds of the core, from the core level D-66 (655-656cm) to the top, major changes in the Td values are marked by the changes in the type of assemblages, following a rule that can be related to the geographical distributions of these two parameters of diatom compositions in the modern sediments of the North Pacific; whereas, the same rule does not necessarily hold true in the lower one thirds of the core, from the bottom of the core up to the core level D-68 (675-676cm). It is concluded, therefore, that the diatom temperature curve derived illustrates the fluctuation of surface water temperatures in the upper two thirds more reliably than in the lower one third of the core V20-130. Three maxima of Td, ones at D-6 (60-61 cm), D-43 (425-426 cm) and D-62 (615-616 cm), are considered to reflect the temperature maxima of major warm stages during the Pleistocene; the major temperature minima of the cold stages are thought to be detected at D-3 (30-31 cm), D-8 (80-81 cm), D-31 (310-311 cm), D-47 (465-466 cm) and possibly at D-94 (935-936 cm). Based on preserved forms, the diatom flora of the North Pacific appears to have acquired species compositions very similar to that of the present-day flora in middle latitudes before the time of deposition of the lowermost core levels of the core V20-130. The section represented in the core can be divided into two biostratigraphic zones : the upper zone represents the upper two thirds of the core, and the lower zone represents the lower one third of the core. The lower zone corresponds approximately to the Teil zone (or local range zone) of Coscinodiscus wailesii of this part of the Pacific. The difference in species composition between the two zones is not great even though Coscinodiscus wailesii is lacking in the upper zone. It appears that the differences between the zones were caused by the changes that took place in certain other attributes of biocoenoses, other than species composition, which made different the structures of those biocoenoses on which the compositions of thanatocoenoses depended. Differences of this kind detected by comparing the types of diatom assemblages and Td values are interpreted as representing the important phases of the development of the North Pacific diatom flora during the Pleistocene in the middle latitudes of the western North Pacific.
1966-03-05

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)

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