东沙西南海域表层底栖有孔虫碳同位素对冷泉活动的指示

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

底栖有孔虫碳同位素负偏记录是冷泉活动的重要指标之一。对南海北部东沙西南海域4个含碳酸盐结核的表层沉积物样品中的6种底栖有孔虫进行了氧碳同位素分析,结果表明,表生属种Discanomalina semiungulata、Cibicides wullerstorfi、Cibicides pseudoungerianus、Cibicides lobatulus的碳同位素,相对同水深南海现代表生底栖有孔虫有明显的负偏记录,尤其是表生种D.semiungulata基本都偏负;内生种Lenticulina orbicularis也有部分负偏,而Uvigerina auberiana的碳同位素则明显偏正;几种底栖有孔虫的碳同位素波动幅度都明显较大,明显超出了南海最近两个冰期-间冰期旋回中碳同位素的变化幅度。其中生活在蠕虫管壁上的活体底栖有孔虫D.semi-ungulata和C.lobatulus的同位素都呈明显的负偏,认为该区底栖有孔虫碳同位素这种负偏和波动幅度大可能是由于冷泉活动造成的。活体底栖有孔虫碳同位素负偏表明该区可能存在有正在活动的冷泉,其中表生种D.semiungulata的大量出现可以作为南海冷泉活动的一个潜在指示标志。

The depleted carbon isotope （813 C） of benthic foraminifera was regarded as an important indica- tor of ancient cold methane seepage. In this study carbon isotope analyses were carried out for 6 benthic fo- raminiferal 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 Discanornalina semiungulata, Cibi- cides wullerstorfi , Cibicides pseudoungerianus and Cibicides lobatulus all show apparent negative excur- sions compared to the 813C values at the control sites in the SCS and, in particular, all values of D. semiun- gulata are depleted. The endobenthic Lenticulina orbicularis also has some negative carbon isotope excur- sions, while endobenthic Uvigerina auberiana reveals abnormal positive values compared with the control- ling sites. The amplitude of 813C variability of D. semiungulata （2.11%0）, L. orbicularis （1.49%0）, C. pseudoungerianus （2.08%0） and U. auberiana （1. 98%0） are apparent larger than that at the controlling sites （（0.4%o）, and also larger than the 81aC variability （1.1%0） 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 8laC. We consider that the depleted 81aC values with significantly larger variability observed in benthic foraminiferal species in the southwestern are- a off Dongsha Islands, SCS are possibly caused by cold seeping activity. And the negative excursion of car- bon 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.