CARBON STABLE ISOTOPES OF AUTHIGENIC CARBONATES AND BENTHIC FORAMINIFERA RECOVERED FROM SITES U1328 AND U1329 AS CO-INDICATORS OF EPISODIC METHANE SEEP EVENTS IN THE CASCADIA MARGIN

Methane seeps play a significant role in the evolution of pore water dissolved inorganic carbon (DIC) via anaerobic oxidation of methane (AOM), which could make authigenic carbonates precipitated and influence the benthic foraminifera living near seep environments. Two independent proxies involving the carbon isotopic composition of authigenic carbonates and benthic foraminifera (Uvigerina peregrina) were studied to verify the potential relationship between authigenic carbonates and foraminifera as co-indicators of episodic methane seeps during late Pleistocene and Holocene in the northern Cascadia margin's gas hydrate geo-system. Both authigenic carbonates and benthic foraminifera exhibit episodic negative carbon isotope excursions during the past 1.6 Ma at site U1328 and 8.5 Ma at site U1329. The carbon isotope excursions of benthic foraminifera coincide with those of authigenic carbonates at several methane seep stages, even though a profound carbon isotopic disequilibrium exists between the authigenic carbonates and benthic foraminifera. Methane seep-related AOM favors authigenic carbonate precipitation and also leaves imprints on the DIC that could be recorded by the calcification of benthic foraminifera. The carbon isotopic coincidence between authigenic carbonates and benthic foraminifera demonstrates that two proxies could record the same methane seep events. We combine the benthic foraminifera with authigenic minerals to reveal that the methane seep events could preclude the post-depositional alterations of the authigenic carbonates and delineate the specific history of episodic methane seep events.