Geochemical Characteristics Of Site 4B Sediments From The Shenhu Area Of The South China Sea: Implications For Methane Seepage

Methane seepage plays an essential role in natural gas hydrate and climate change.In China,the South China Sea is an important reservoir of natural gas hydrate resources.Unveiling the history of methane seepage by studying the geochemical information of seafloor shallow surface sediments in the South China Sea is of great significance for the research of natural gas hydrate in China.Previous researchers have studied the lithology,structure and texture,?34S values of pyrites,TOC values of seafloor surface sediments of core Site 4B.They suggested that methane seeping was occurred in this area.In order to unveil the history of methane seeping,we analyzed the geochemical element characteristics of core Site 4B(water depth 970 m,total core length 300 cm)sediments,studied the variations of Mg/Ca and Sr/Ca ratios as a function of depths of the sediments,scrutinized the distribution character:istics of authigenic carbonate minerals,and investigated the grain size characteristics of the sediments and the correlation among elements.The major findings are:(1)Based on the analysis of the geochemical characteristics of rare earth elements of the Site 4B sediments,it is concluded that the sediments in the area are mainly derived from terrigenous debris materials,with a high degree of terrigenous debris mixing and a low content of mantle-derived volcanic materials and biogenic materials.And the core Site 4B sediments were originated from a stable source and have not changed dramatically in geological history;(2)Studies on redox-sensitive elements of the Site 4B sediments indicated that in the depth range of 0-70 cmbsf,the sedimentary environment in the study area was anoxic.The anaerobic methane oxidation causes a vulcanization environment in the depth of 186 cmbsf.Other horizons are in relatively oxidized sedimentary environments.(3)The study shows that the Mg/Ca and Sr/Ca ratios of the Site 4B sediments can be used to indicate the composition of high-Mg calcite and aragonite generated by methane seepage,so as to identify methane seepage events.(4)Under the condition of*the known Mg/Ca and Sr/Ca ratios as a function of depths of the Site 4B sediments,we calculated the relative contents of aragonite,high-Mg calcite,biogenic calcite,and detrital materials by using the Bayon's model.Combined with the lithology,structure and texture,?34S values of pyrites,TOC values,geochemical characteristics of rare earth elements,geochemical characteristics of trace elements,grain size characteristics and element correlation analysis of the Site 4B sediments,we postulated that the methane seepage was related to the mud volcano activity before 15.78 ka.The methane seepages had multi-stages variations.Strong methane seeping was observed at 24.15 ka B.P.,27.47 ka B.P.,and 31.05 ka B.P.,where the maximum seeping occurred at 31.05 ka B.P..In a methane seepage setting,seafloor surface sediments are more accessible than carbonate rocks.Analyzing the geochemical information of seafloor surface sediments to invert the history of methane seepages is of great significance to the exploration and research of global natural gas hydrate resources,the study of global climate change and the origin and evolution of life on earth,which has a broad application prospect.