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JAMSTEC 2002 Annual Report Mutsu Research Group sediments on the order of to yr. In contrast, methods such as δ O are available for dating carbonate-containing sediments in this geological age range. We focused on amino acid chronology as an alternative dating method for siliceous sediments. We analyzed the enantiomeric ratio (D-isomer/L-isomer) of aspartic acid (Asp) in bulk diatom assemblages in siliceous sediment cores collected at Station (St.) (approx. lat ˚N, long ˚E) and St. (approx. lat ˚N, long ˚E) in the northwestern North Pacific. Radiocarbon and paleomagnetic ages were also obtained from both cores to use as reference ages. Asp ages estimated by the parabolic model were consistent with the reference ages (Figure ). However, although generally the D/L ratio of Asp increased with increasing depth in the core at St. , the ratio did not continue to increase below about m depth. The D/L ratio of Asp and the paleomagnetic age at that depth were . and kyr BP, respectively. Therefore, the Asp racemization reaction apparently does not continue to progress in diatom frustules older than this age. This finding implies that Asp chronology can be used to determine ages up to about kyr BP in sediments composed of diatom ooze. Although the Asp dating method using the parabolic model has a limitation of D/L ratio of Asp 0.4 0.3 0.2 0.1 D/L=a x square root (t) +C a=4.45x10 -4 , C=0.12, r 2 =0.94 kyr BP for siliceous sediments, it is available for the estimation of ages on the order of to yr BP, which is beyond the time range (up to kyr BP) datable by the C method (Harada et al., ). Personal Research 1. The study of ecology and genetics of living calcareous plankton Planktonic foraminifera and coccolithophores, a group of free-floating marine protista, secrete calcareous hard skeletons. They are very sensitive to the oceanic surface environmental changes and their shells settle quickly and deposit in the sediment after they died. Their calcareous tests provide considerable information about the sea water when they lived and are widely used for paleo-environmental reconstruction. In spite of their importance, their ecology was not well understood. In particular, there are few studies on the ecology of living cold water species. We then started () the collection of calcareous plankton from the surface seawater, and () their cultivation in the laboratory. Living specimens were collected monthly from the Tsugaru Strait (˚.'N, ˚.'E; ca. ~m below surface), northernmost part of Honshu, Japan. This area is located in the mixing zone of the Tsugaru warm current and the cold Oyashio component water, therefore it is suitable for the study of the ecology of cold water species, as well as the warm water species. The aims of this study are ) establishment of the cultivation protocols of calcareous plankton in the laboratory, ) elucidation of the ecology of the northern species, and ) clarification of the relationships between skeletal microstructure and gene. 0 100 200 300 400 500 600 700 Square root of 14 C and paleomagnetic ages Fig. 5 Relationship between the D/L ratio of Asp obtained by the parabolic model and the square root of the 14 C and paleomagnetic ages from the sediment core from St. 5. 188
Japan Marine Science and Technology Center Mutsu Research Group a (x200) b (x100) c (x100) d e f Fig. 6 Living calcareous plankton collected from the Tsugaru Strait. a~c: planktonic foraminifera a: Neogloboquadrina incompta (Cifelli) b: Globigerina bulloides d'Orbigny c: Globigerina quinqueloba (Natland) d~f: coccolithophores d: Emiliania huxleyi (Lohman) e: Gephyrocapsa oceanica Kamptner f: Coccolithus pelagicus (Wallich) 189
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