Foraminifera from gas hydrate-bearing sediments of the northeastern South China Sea: Proxy evaluation and application for methane release activity

Abstract

Anomalously low δ13C values of foraminifera in the stratigraphy are usually interpreted to reflect activities of paleo-methane seepage. The application of this proxy, however, requires to understand the cause of anomalously low δ13C signals in foraminifera. Here we present stable carbon and oxygen isotopes (δ13C and δ18O), Mg/Ca ratios and accelerator mass spectrometry (AMS) radiocarbon data from Cores GMGS2-08B (∼0–5.5 m below the seafloor [mbsf]) and 08C (2–23 mbsf) at ∼798 m water depth, recovered from the gas hydrate-bearing sediments in the northeastern South China Sea (SCS). The main objectives are to investigate the primary versus secondary signals of extremely negative δ13C values of foraminifera and further recognize the methane emission events (MEEs). The δ13C values of benthic (Uvigerina peregrina and Cibicidoides spp.) and planktonic (Globigerinoides ruber and Pulleniatina obliquiloculata) foraminifera in these two cores show distinct intervals with negative values (as low as −15.85‰) that indicate the shift of the sulfate methane transition zone (SMTZ) and the change in methane release flux in the past. The δ13C records of U. peregrina by a more exhaustive cleaning procedure designed to remove authigenic carbonate coatings show that most δ13C values are still lower relative to those in the normal marine environment. The Mg/Ca ratios of Uvigerina spp. by this exhaustive cleaning procedure display higher values and more variable for anomalous tests (as high as 13.40 mmol/mol) than normal ones. These observations indicate the 13C-depleted signals of foraminifera are mainly originated from diagenetic alteration of authigenic carbonate precipitates with extremely negative δ13C values. Furthermore, the mass-balance approach is adopted to assess the relative contribution of methane-derived authigenic carbonate in the low-δ13C signals in foraminifera and reveals the maximum contribution can be as high as ∼30% among the δ13C anomalies. Additionally, several major MEEs with distinct characteristics can be recognized according to remarkable δ13C excursions of foraminifera in these two cores.