南海F站位冷泉水体中的溶解甲烷

DISSOLVED METHANE IN THE SEEP SYSTEM AT SITE F IN THE SOUTH CHINA SEA

目前对南海北部陆坡的F站位冷泉的研究多从地质和生物角度开展,而冷泉羽流在水平和垂直方向上的扩散范围尚不清晰,其影响因素尚不明确。针对该问题,于2021年6月6~23日搭乘“科学”轮对该冷泉海域的溶解甲烷进行调查,使用搭载遥控无人潜水器(Remote Operated Vehicle,ROV)和温盐深系统(Conductivity,Temperature,Depth,CTD)的采水瓶采集了该冷泉及邻近水体中从近海底(距底2.5 m)至表层(5~10 m)多个层位的水样,并使用气相色谱仪在现场分析了水样中溶解甲烷的含量。结果表明,溶解甲烷在覆盖喷口的生物繁茂区水体中的含量为6 590~11 300 nmol/L,在生物稀少的毗邻区迅速降至85.0~354.8 nmol/L。水柱剖面的测量结果显示冷泉喷口区1 000 m以深的水体中溶解甲烷含量高于背景区,而1 000 m以浅的水体中溶解甲烷的含量与背景区相同(2.01~3.90 nmol/L);自喷口区顺流向下1 500 m处,甲烷羽流信号消失。综上结果,甲烷羽流的水平扩散距离≤1 500 m,上升高度≤125 m。溶解甲烷的含量在生物繁茂区的迅速降低是由于底栖生物对溶解甲烷的摄食所致,而其在繁茂区之外扩散过程中的逐渐降低是海水湍流混合导致的稀释效应所致。

Seep system titled “Site F” situates on the continental slope of the northern South China Sea. Its geological and biological characteristics have been sufficiently investigated. However, the diffusion range of the methane plume is still unknown and the influencing factors are not clear. To address this issue, we carried out a field investigation on June 6~23, 2021 onboard R/V Kexue(Science in Chinese). Discrete water samples of near-bottom water(2.5 m above seafloor)were collected with Niskin bottles mounted on a remotely operated vehicle(ROV), and samples of other depth ranging from near-bottom up to surface(5~10 m) were collected with Niskin bottles mounted on a CTD rosette. Immediately after sampling, dissolved methane was extracted using static headspace method and then quantified with a gas chromatograph.Concentration of dissolved methane([CH;]) in near-bottom water above the seeps ranged from 6 590 to 11 300 nmol/L and decreased rapidly to 85.0~354.8 nmol/L tens of meters away. Moreover, [CH;] within water column deeper than 1 000 m in the seep area was higher than that in the background area, whereas [CH;] within water column shallower than 1 000 m in the seep area was similar to that in the background area;the signal of methane plume disappeared at 1 500 m downstream from the seep area. So, the rising height of methane plume was ≤125 m and the horizontal diffusion distance along the bottom current was ≤1 500 m. The fast decrease of [CH;] in near-bottom water within seep area was probably due to rapid consumption by the chemosynthetic assemblages, whereas the gradual decrease of [CH;] beyond this area was probably due to dilution caused by turbulent mixing of seawater.