CARBON AND OXYGEN STABLE ISOTOPES OF BENTHIC FORAMINIFERA AS POSSIBLE INDICATORS OF EPISODIC METHANE SEEPS IN GAS HYDRATE GEO-SYSTEM——A STUDY FROM IODP EXPEDITION 311

The research of shallow infauna benthic foraminifera species Uvigerina peregrina and Bulimina mexicana collected from all five sites (U1325, U1326, U1327, U1328, and U1329) during the Integrated Ocean Drilling Program (IODP) Leg 311 at the Cascadia margin, northeast Pacific was carried out to reveal the response of carbon isotopic compositions of benthic foraminifera to the episodic methane seeps in gas hydrate geo-system. Both the two species exhibit dramatic negative carbon isotopic excursions and positive oxygen isotopic anomalies (-3.32‰~-0.22‰(PDB) and 3.4‰~4.91‰(PDB)) clearly indicative of gas hydrate dissociation influencing the dissolved inorganic carbon of the pore water via anaerobic oxidation of methane and further influencing the isotopic compositions of benthic foraminifera tests. The negative carbon isotopic excursions and positive oxygen isotopic values of benthic foraminifera tests from the five sites clearly indicate episodic methane seeps in the gas hydrate system. The episodic methane seep stages recognized at the five sites coupled well with the 100 kyr-scale eustatic sea-level fluctuations during the past 1.0 Ma, which demonstrates that glacioeustatic sea-level fluctuations play significant roles in the destabilization of gas hydrate geo-system.