A preliminary study on the active cold seeps flow field in the Qiongdongnan Sea Area, the northern South China Sea

Abstract

Cold seep is an important geological process of methane fluid migration. Methane plumes at these seeps have a significant impact on the local carbon budget and may trigger massive precipitation of carbonates and cold-seep biome. As a powerful greenhouse gas, methane seeping/venting from a cold seep may be released into the atmosphere and have a huge effect on climate. Research on cold seep flow field is of great significance for understanding geological processes of cold seep and their impacts on global climate change. Several methods have been used to observe cold seeps, e.g., acoustical, geochemical, and optical methods. However, no flow field image of Haima cold seeps was shown by these methods so far. The particle image velocimetry (PIV) method is a commonly used approach in the study of fluid dynamics. It tracks the motion of tracer particles in a flow field over a small time interval, so that the instantaneous velocity field can be calculated using a spatial cross-correlation method. In this study, we applied the PIV method to analyze in-situ video image sequences of the newly discovered, active cold seep in the Qiongdongnan Basin of the northern South China Sea during 2019, and obtained some quantitative, preliminary information about the flow. According to the video image sequences, there are two ways of methane seeping from the seafloor in the study area. One is via methane bubbles, which are formed through gas hydrate decomposing, and then slowly escaping from the seafloor. The other is via plumes of methane-containing fluid, which erupt rapidly from the vent. Gas hydrate-coated methane bubbles may also exist over the cold seep. The velocity of the plumes is between 0.00756 and 10.984 px/frame, and the accelerated velocity is −233.077 to 169.617 px/frame2 (negative for velocity decreasing with time). The vertical velocity is −10.842 to 5.055 px/frame (positive direction for downward), and the horizontal velocity is −6.377 to 5.828 px/frame (positive direction to the right). The velocity of methane bubbles is about 0.238 m/s, consistent with previous studies’ findings; this agreement demonstrates the viability of the PIV method. These results show that the direction of the cold seep flow is generally upward. However, its internal flow field involves complex turbulence. The direction and velocity of the cold seep flow change with time, and its streamlines are sinuous. Although there are some velocity deviations, our results show that the PIV method is feasible for calculating the flow field image, and that it has some unique advantages, e.g., it is low-cost, non-invasive, and has high resolution. However, we stress that these results are preliminary, and research on cold seep flow field is just beginning. Compared to hydrothermal vents, the flow field of cold seeps is stilly poorly understood and more studies are needed, such as describing the cold seep flow field quantitatively and analyzing how and why it changes.