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

摘要

Anomalously low delta C-13 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 delta C-13 signals in foraminifera. Here we present stable carbon and oxygen isotopes (delta C-13 and delta O-18), Mg/Ca ratios and accelerator mass spectrometry (AMS) radiocarbon data from Cores GMGS2-08B (similar to 0-5.5 m below the seafloor [mbsf]) and 08C (2-23 mbsf) at similar to 798m 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 delta C-13 values of foraminifera and further recognize the methane emission events (MEEs). The delta C-13 values of benthic (Uvigerina peregrine 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 parts per thousand) that indicate the shift of the sulfate methane transition zone (SMTZ) and the change in methane release flux in the past. The delta C-13 records of U. peregrine by a more exhaustive cleaning procedure designed to remove authigenic carbonate coatings show that most delta C-13 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 C-13-depleted signals of foraminifera are mainly originated from diagenetic alteration of authigenic carbonate precipitates with extremely negative delta C-13 values. Furthermore, the mass -balance approach is adopted to assess the relative contribution of methane-derived authigenic carbonate in the low-delta C-13 signals in foraminifera and reveals the maximum contribution can be as high as similar to 30% among the delta C-13 anomalies. Additionally, several major MEEs with distinct characteristics can be recognized according to remarkable delta C-13 excursions of foraminifera in these two cores.