卡斯卡迪亚大陆边缘活动冷泉流场定量研究

A quantitative study on the active cold seeps flow field along the Cascadia margin

海底冷泉是一种重要的地质活动,当前对冷泉的研究逐渐从定性研究过渡到定量研究.本文利用遥控深潜器视频资料对美国西海岸卡斯卡迪亚大陆边缘活动冷泉进行研究.观察视频资料发现,研究区活动冷泉的释放主要有两种形式,一是甲烷气泡的缓慢逸散,二是含甲烷流体快速喷发形成羽状流.缓慢逸散的甲烷气泡多为透明椭球形,羽状流喷口处多为锥形,流体浓度高时呈白色不透明状,流体边缘处往往为灰色半透明.羽状流流动方向整体向上,内部往往存在复杂的湍流涡.针对研究区中两种释放形式,分别利用不同方法进行流场的定量研究,其中,冷泉羽流的流场定量分析是通过粒子图像测速技术来获得冷泉羽状流瞬时全场流动定量信息,在此基础上试算了羽流的卷吸系数.本文利用图像测速技术对冷泉流场进行定量研究,可为估算甲烷通量等研究提供新的数据来源,对研究冷泉地质过程及其对全球气候变化的影响都具有重要意义.

Cold seep is an important geological activity in the marine environment,which is an important source of methane and other greenhouse gases to the ocean and atmosphere.The current research on cold seeps tends to transform from a qualitative description to a quantitative research.In this study,we analyzed in-situ video image sequences of the active cold seeps at the Cascadian margin along the west coast of North America in 2018.According to the video image sequences,there are two main forms of methane seeping from the seafloor in the study area:one is the slow release of methane bubbles which are mostly transparent ellipsoid;the other is the rapid eruption of methane-containing fluid,forming a conical,white and opaque plume with gray and translucent edge.It is observed that the direction of the cold seep flow is generally upward,however,there are often complex turbulences inside.Furthermore,a quantitative investigation of the flow field induced by a large number of gas bubbles was carried out using particle image velocimetry(PIV)to obtain some quantitative,preliminary information about the flow.On this basis,the entrainment coefficient of the plume is calculated.The quantitative study of flow field of a plume by using particle image velocimetry can provide a new data source for the estimation of methane flux and related investigations,which is of great significance for the study of the geological process of cold seep and its impact on global climate change.