[B-PT02_1AM2] Biocalcification and the Geochemistry of Proxies

[BPT02-12] カリブ海のサンゴ骨格に記録される北大西洋の窒素固定量の変動

*山崎 敦子¹、Hetzinger Steffen²、Reumont Jonas V.²、Manfrino Carrie³、角皆 潤⁴、渡邊 剛⁵ (1.東京大学大気海洋研究所、2.GEOMAR Helmholtz-Zentrum fur Ozeanforschung Kiel、3.Central Caribbean Marine Institute、4.名古屋大学大学院環境学研究科、5.北海道大学理学研究院)

キーワード：Coral skeletons, nitrogen isotopes, nitrogen fixation, Caribbean Sea, North Atlantic Ocean

Oceanic nitrogen fixation is important as new nitrogen in oligotrophic oceans and balances with denitrification in global nitrogen cycle controlling primary production. North Atlantic ocean is known to have higher nitrogen fixation rates, although the controlling factors have been debated by modern observations and sediment cores in geological time scales. Reef corals have been widely used as paleo-environmental proxy in oligotrophic oceans. Recent studies suggested that nitrogen isotopes of organic matter preserved in coral skeletons δ¹⁵N_coral have the potential to record coral nitrogen sources on decadal to millennia scale. In this study, we report recent 90-year records of nitrogen isotopes in Diploria sp. coral cores from Cayman Islands. δ¹⁵N_coral values were +1.9±2.6 (σ)‰ (n=139), which suggested that the variation of δ¹⁵N_coral was controlled by nitrogen fixation (~0‰) in ambient seawater. The trend line of δ¹⁵N_coral increased ~4‰ from 1920s to 2010s. This result suggests that nitrogen fixation rate in Caribbean Sea decreased during the past 90 years. Detrended δ¹⁵N_coral showed a negative correlation between Atlantic Multi-decadal Oscillation (AMO) index (R=-0.71, P‹‹0.001), which suggested that nitrogen fixation rate increased in higher SST condition leading an index for hurricane activity on multi-decadal scales. In this presentation, we discuss the relationship between nitrogen fixation and hurricane activity in global warming state.