Tracing the Methane Events by Stable Carbon Isotopes of Benthic Foraminifera at Glacial Periods in the Andaman Sea

Stable isotopes of carbon and oxygen variations in foraminiferal shells have been widely used in paleo-environment studies. However, studies about the shells of benthic foraminifera in methane-hydrate-bearing sediments as reliable geochemical proxies to reconstruct the potential methane release events in the geologic past are rare. In this study, we present the stable carbon and oxygen isotopes of fossil benthic foraminifera including one epifaunal species ( Cibicidoides wuellerstorfi ) and two infaunal species ( Bulimina mexicana and Uvigerina peregrina ) from the Site U1447 of IODP 353 Expedition to trace methane events in the Andaman Sea, where one of the thickest and deepest gas hydrate stability zones was discovered. The δ 13 C values of benthic foraminifera show that there are eight distinct intervals with negative values in the last ~10 Myr, interpreted as a record of long-term fluctuations in methane emission. Six of these methane events occurred during the glacial sea-level lowstands in the last ∼1.1 Myr. We, therefore, infer that the trigger mechanism for these events might be the hydrate destabilization caused by sea level fall. The methane events that occurred at ~2.11 and ~5.93 Ma are more likely related to the sudden changes in sedimentation, either slide events or marked variations in sedimentation rate.