[M-IS15] Paleoclimatology and Paleoceanography

[MIS15-P27] Seasonal changes in planktonic foraminiferal fluxes from sediment trap experiment in the subtropical Northwest Pacific.

*Yuri Hashimoto¹, Azumi Kuroyanagi², Atsushi Suzuki³, Kyoko Yamaoka³, Takeshi Fujii⁴, Shunsuke Kondou⁴, Jumpei Minatoya⁵ (1.Graduate School of Science, Tohoku University, 2.Tohoku University Museum, Tohoku University, 3.Geological Survey of Japan, AIST, 4.KANSO TECHNOS CO., LTD., 5.Japan Organization for Metals and Energy Security (JOGMEC))

Keywords:Planktonic foraminifers, sediment trap, subtropical Northwest Pacific

Planktonic foraminifers are zooplankton which have calcareous shells, and can record upper ocean environment in their assemblage and shells’ chemical composition. Thus, planktonic foraminifers are used for paleoenvironmental analysis. It is necessary to understand ecology of modern species and relation to ocean environment in order to reconstruct paleoenvironment more precisely. However, only a few studies have been reported, especially in the subtropical Northwest Pacific. In this study, we used time-series sediment trap samples from the subtropical Northwest Pacific with a two-week time resolution to examine in detail the seasonal changes in fluxes and assemblage of planktonic foraminifera. Foraminiferal fluxes showed distinct seasonal changes, with large peaks in December and July. Each species showed different seasonality, thus, it suggests that foraminiferal fluxes are regulated by different factors depending on the species. Similar patterns of variation were observed in the fluxes of some species, and the 11 main species were classified into four groups based on trends of variation and correlations. Group A is composed of subtropical symbiont-bearing species (Globigerinoides ruber, Trilobatus sacculifer, Globigerinoides conglobatus, Globigerinoides tenellus, Globoturbolotalita rubescens, Orbulina universa), which accounts for about 66% of total planktonic foraminifera under the oligotrophic environment of central subtropical gyre. Group B consists of three species of Globorotalia (Globorotalia truncatulinoides, Globorotalia scitula, Globorotalia scitula), which inhabit relatively deep water and show a peak once a year during summer. This peak is thought to reflect reproduction season. Group C is Globigerinita glutinata, which shows an irregular pattern. This pattern may be due to the wide habitat range of G. glutinata. Group D is Globigerinella siphonifera, which shows low flux throughout the year and showed no clear seasonal variation. Moreover, comparison with previous sediment trap experiments in the subtropical Northwest Pacific suggests that foraminiferal assemblage composition is influenced by subtropical water mass and magnitude of foraminiferal fluxes are controlled by the amount of organic matter.