Carbon and oxygen isotope characteristics of foraminiferan from northern South China Sea sediments and their significance to late Quaternary hydrate decomposition

摘要

Carbon and oxygen isotope and dating analyses of foraminiferan in sediment cores collected from three different areas of the northern slope of the South China Sea were conducted, in order to examine the records of the gas hydrate decomposition events since the late Quaternary under the conditions of methane seepage. The results show that: 1) the delta C-13 values of the benthic foraminiferan Uvigerina spp. (size range of 0.25-0.35 mm) are from -0.212% to -0.021% and the delta O-18 values of the planktonic foraminiferan Globigerinoides ruber (size range of 0.25-0.35 mm) are from -0.311% to -0.060%; 2) three cores (ZD2, ZD3 and ZS5) from the bottom of a hole are aged for 11 814, 26 616 and 64 090 a corresponding to the early oxygen isotope stage (MIS) I, III and IV final period, respectively; 3) a negative-skewed layer of carbon isotope corresponds to that of MIS II (cold period), whose degree of negative bias is -0.2%; and 4) the delta C-13 compositions of foraminiferans are similar to those of the Blake Ridge and the Gulf of Mexico sediments of the late Quaternary. According to the analysis, the reasons for these results are that the studied area is a typical area of methane seep environment in the area during MIS II due to the global sea-level fall and sea pressure decrease. Gas hydrate is decomposed and released, and a large number of light carbon isotopes of methane are released into the ocean, dissolved to inorganic carbon (DIC) pool and recorded in the foraminiferan shells. A pyrite layer developed in the negative bias layers of the foraminiferans confirms that the delta C-13 of foraminiferans is more affected by methane and less by the reduction of marine productivity and early diagenesis. The use of foraminiferan delta C-13 could accurately determine late Quaternary hydrate release events and provide evidence for both reconstructing the geological history of methane release events and exploring natural gas hydrate.