[A-CG40] 沿岸海洋生態系─１．水循環と陸海相互作用

[ACG40-05] 固着性二枚貝イワガキ貝殻の炭素・酸素安定同位体比を用いた海底湧水環境の評価

*富永 修¹、竹内 優¹、杉本 亮¹、石田 健大¹、平井 タケル¹、中島 壽視¹、小路 淳²、本田 尚美⁴、谷口 真人³ (1.福井県立大学、2.東京大学大気海洋研究所、3.総合地球環境学研究所、4.福井県庁)

キーワード：海底湧水、炭素安定同位体比、酸素安定同位体比、イワガキ貝殻、ベイズミキシングモデル

Submarine Groundwater Discharge (SGD) results in various biogeochemical changes in the coastal environments, such as coastal primary production, eutrophication, and benthic production. The SGD condition of 6 sites along volcanic mountain coast in northern Japan was evaluated by using carbon and oxygen stable isotope ratio (δ¹³C and δ¹⁸O) in the shell of sessile bivalves Crassostrea nippona. In order to estimate the proportion of fresh water flow rate in SGD (FR) from the respective δ¹³C and δ¹⁸O of C. nippona shell, we used the three end member (sea water, riverine water and fresh underground water) bayesian stable isotope mixing model. The significant positive correlation was found between δ¹³C and δ¹⁸O of the shell and those of ambient water, respectively. FRs estimated from three source model were 3.1 -8.6% at low ²²²Rn sites and 21.9-28.5% at high ²²²Rn sites. At the site (Nishihama) of river mouth, the δ¹³C and δ¹⁸O of the shell collected were low (estimated FR 10.6-25.0%) and ²²²Rn concentration was also low. The contribution of DIC in SGD and riverine water for shell formation was estimated to be 19.8 % and 7.8%, respectively. In the present study, we demonstrated the effectiveness of δ¹³C and δ¹⁸O of C. nippona shell as the tool for estimating FR.