CARBON ISOTOPE OF BENTHIC FORAMINIFERA AND ITS IMPLICATIONS FOR COLD SEEPAGE IN THE SOUTHWESTERN AREA OFF DONGSHA ISLANDS, SOUTH CHINA SEA

The depleted carbon isotope (δ¹³C) of benthic foraminifera was regarded as an important indicator of ancient cold methane seepage. In this study carbon isotope analyses were carried out for 6 benthic foraminiferal species from 4 surface sediments with carbonate nodules in the southwestern area off Dongsha Islands, South China Sea. The carbon isotope of epibenthic species of Discanomalina semiungulata, Cibicides wullerstorfi, Cibicides pseudoungerianus and Cibicides lobatulus all show apparent negative excursions compared to the δ¹³C values at the control sites in the SCS and, in particular, all values of D.semiungulata are depleted. The endobenthic Lenticulina orbicularis also has some negative carbon isotope excursions, while endobenthic Uvigerina auberiana reveals abnormal positive values compared with the controlling sites. The amplitude of δ¹³C variability of D. semiungulata (2.11‰), L. orbicularis (1.49‰), C. pseudoungerianus (2.08‰) and U. auberiana (1.98‰) are apparent larger than that at the controlling sites ((0.4‰), and also larger than the δ¹³C variability (1.1‰) of benthic foraminifera during the last two glacial-interglacial cycles of the SCS. Living benthic foraminifera D. semiungulata and C. lobatulus, which attached on a tube worm, both show obvious depletion of δ¹³C. We consider that the depleted δ¹³C values with significantly larger variability observed in benthic foraminiferal species in the southwestern area off Dongsha Islands, SCS are possibly caused by cold seeping activity. And the negative excursion of carbon isotope in the living attached benthic foraminifera may indicate an active cold seepage existed in the studied area. We suggest that the epibenthic D. semiungulata be used as a potential indicator of cold seepage in the SCS.