[M-IS05] Global climate change driven by the Southern Ocean and the Antarctic Ice Sheet

[MIS05-P19] Seasonal flux and morphological changes in silicoflagellates in the Indian sector of the Southern Ocean

*Ryota Hayashi¹, Yusuke Okazaki², Koshiro Matsuo², Takahito Ikenoue³, Takuya Itaki⁴, Minoru Ikehara⁵ (1.Department of Earth and Planetary Sciences, School of Science, Kyushu University, 2.Department of Earth and Planetary Sciences, Graduates School of Science, Kyushu University, 3.Marine Ecology Research Institute, 4.Geological Survey of Japan, AIST, 5.Center for Advanced Marine Core Research, Kochi University)

Keywords:Southern Ocean, silicoflagellate, sediment trap

Seasonal silicoflagellate flux change was investigated in the Indian sector of the Southern Ocean. Time-series sediment trap was deployed at 3400m water depth of Station ECR-1 (55°S, 45°E, 4020m seafloor depth) near the Conrad Rise located in the southwest Indian sector of the Southern Ocean from February 2016 to February 2017. Stephanocha speculum was the dominant silicoflagellate species occupying greater than 95.4%. The silicoflagellate flux pattern corresponded to the biogenic opal mass flux exhibiting pronounced austral summer peaks. In the Southern Ocean, diatoms are the main contributor to the biogenic opal. However, onset of the silicoflagellate flux maximum in summer precede the diatoms. Detailed morphological investigation was performed on S. speculum skeletons. In austral summer, S. speculum var. bispicata with multiple apical pike was the most abundant morphotype. On the other hand, S. speculum speculum without apical pike was the abundant morphotype other seasons when flluxes of both silicoflagellates and biogenic opal were relatively low. This suggests that the number of the apical pikes have decreaced during diatom bloom. During the diatom bloom, diatoms consume most of dissolved silicic acid in the euphotic layer of the Southern Ocean. Earlier silicoflagellate summer peak may be due to silicic acid availability regulated by the subsequent diatom bloom, leading to the fine morphological changes in silicoflagellate skeletons.