Distribution and isotopic composition of foraminifera at cold-seep Site 973-4 in the Dongsha area, northeastern South China Sea

Abstract

A 1375-cm-long gravity core (Site 973-4) was acquired from the Dongsha cold seep area of the northeastern South China Sea (SCS). We measured its stable isotopes of planktonic foraminifera and investigated benthic foraminiferal assemblage compositions. Accelerator mass spectrometry 14C analysis of planktonic foraminifera shows that some intervals are dated to Marine Isotope Stage (MIS) 3. Pulleniatina obliquiloculata has positive δ13C (0.25–1.25‰) except for a negative carbon isotopic excursion (up to δ13C = −1.15‰), but its light-carbon sources remain elusive. The benthic δ13C values (Uvigerina) of non-seep and cold-seep boreholes from the SCS have no systematic difference. Therefore, the benthic δ13C at Site 973-4 did not record the enhanced seepage activities (if occurred) during the last sea-level lowstand in the Dongsha area. In the pre-Holocene sediments, the foraminiferal assemblages have lower diversities, Shannon-Wiener indices, evenness indices, Simpson indices and higher calcareous proportions like typical cold-seep settings. The changes in oxygen levels (lower in the glacial period than those in the Holocene) indicated by foraminiferal indices reflect either bottom-water oxygen variations on the glacial-interglacial scale or enhanced seepage activities during the last glacial period.