Hydrochemical Characteristics and Evolution Mode of Cold Seeps in the Qiongdongnan Basin, South China Sea

Abstract

Submarine cold seeps have recently attracted significant attention and are among the most effective indicators of gas hydrate in the oceans. In this study, remotely operated vehicle (ROV) observations, seismic profiles, core sediments, bottom seawater, and fluid vented from cold seeps in the deep-water Qiongdongnan Basin were used to investigate the origin and evolution of cold seeps and their relationships with gas hydrate. At stations A, B, and C, inactive cold seeps with dead clams, cold seep leakage with live clams, and active cold seeps with a rich mussel presence, respectively, were observed. The salinity and Na+ and Cl- concentrations of the cold seeps were different from those of typical seawater owing to gas hydrate formation and decomposition and fluid originating from various depths. The main ion concentrations of the bottom seawater at stations B and C were higher than those at station A, indicating the substantial effects of low-salinity cold seep fluids from gas hydrate decomposition. The Na+-Cl-, K+-Cl-, Mg2+-Cl-, and Ca2+-Cl- diagrams and rare earth element distribution curves of the water samples were strongly affected by seawater. The concentrations of trace elements and their ratios to Cl- in the bottom seawater were high at the stations with cold seeps, suggesting the mixing of other fluids rich in those elements. Biochemical reactions may also have caused the chemical anomalies. Samples of HM-ROV-1 indicated a greater effect of upward cold seep fluids with higher B/Cl-, Sr/Cl-, and Ba/Cl- values. Moreover, the Re/Cl- value varied between fluid vents, possibly due to differences in Re precipitation strength. Differences in cold seep intensity are also believed to occur between areas. The cold seep fluxes changed from large to small before finally disappearing, showing a close connection with gas hydrate formation and decomposition, and influenced the local topography and ecosystems.

Gas Hydrate Accumulation and Occurrence Associated with Cold Seep Systems in the Northern South China Sea: An Overview

“…Based on the comprehensive interpretation and analysis of 2D/3D seismic data, subbottom and multibeam data, and manned deep submersible (HOV) and/or unmanned deep submersible (ROV) observations and sampling, scholars are able to investigate submarine cold seep systems in situ, and our understanding of the cold seep system in the northern SCS has been greatly improved [92][93][94][95][96][97][98][99][100][101][102]. Through submergence observations and sampling, several regions of a cold seep system and its associated gas hydrate accumulations have been discovered (Figure 1(a)).…”

“…Through Haima ROV observations, seabed microbial communities were found in two of the main investigation sites. Using the sampling manipulator arm, sampling grab, and shallow drilling rig of the Haima ROV, bivalve organisms were collected, and carbonate rock samples were obtained via push coring [102]. In the same year, the Haima ROV of the GMGS and the Deep Sea Warrior HOV of the Sanya Deep Sea Research Institute conducted a joint deep-diving investigation in the Haima Cold Seep and its eastern area, which provided abundant precious information and data for further research of the cold seep and for evaluation of its gas hydrate resource potential [84] (Figure 2(f)).…”

Study on the Mechanism of Natural Gas Hydrate Decomposition and Seabed Seepage Triggered by Mass Transport Deposits

“…Previous studies have investigated submarine microlandforms and fluid seepage in this area, uncovering various types of micro-landforms, such as pockmarks, spiny seamounts, and domed seamounts. Marine geological investigations over several years have established a close correlation between submarine micro-landform formation, gas hydrate formation, and decomposition, in addition to the widespread development of the mass transport deposit (MTD) systems [20]. However, current research inadequately examines the causal mechanisms of submarine seepage involving hydrates and MTDs.…”

Geochemical characteristics of sediment and pore water affected by cold seeps in southern South China Sea

“…Cold seep commonly occurs along the continental margin, which is generated by the concentrated upward migration of dissolved hydrocarbons and/or gaseous hydrocarbons (mainly methane) (e.g. Suess, 2014;Wan et al, 2020), and functions effectively in the element circulation (Tribovillard et al, 2013;Feng D. et al, 2018;Smrzka et al, 2020). Additionally, cold seep often nurtures a unique ecosystem with a series of typical biochemical reactions (Levin, 2005;Levin and Sibuet, 2012).…”