Geologically controlled intermittent gas eruption and its impact on bottom water temperature and chemosynthetic communities—A case study in the “HaiMa” cold seeps, South China Sea

Abstract

In 2018, the Institute of Deep Sea Science and Engineering investigated the “HaiMa” cold seeps in the northwestern part of the South China Sea using R/V “TanSuoYiHao” with manned submersible “ShenHaiYongShi.” Ten dives were conducted to make seafloor observations at six cold seeps (HM‐1 to HM‐6). By combining multibeam echosounder data and seismic profiles, a systematic study was made to understand fluid migration, accumulation, and eruption and their impact on the ecosystem. Location of gas flares observed in the multibeam echosounder data obtained in 2016 and 2018 is different, indicating a year‐level spatiotemporal variation of the cold seep activities. Seafloor observation shows that massive carbonates occur in HM‐1 and HM‐5, indicating two long‐term existed cold seeps (several thousands of years). Mussels covered large areas of HM‐2, HM‐3, and HM‐6, suggesting young active seeps. Elevated seabed with broken edges, mud cones, clay pools, and sudden increased water turbidity and temperature was encountered in multiple areas, which are inferred to be associated with shallow gas accumulation and eruption. Methane content of the headspace gas from shallow sediment at HM‐2 is higher than 99.5% with δ13C‐CH4 of −71.5‰ to 72.3‰, indicating a biogenic gas origin. It is inferred that the gas is sourced by reservoirs at ~1,280 mbsf, which are characterized by low frequency in the seismic profiles. Gas migrates from the gas reservoir to the top of the Basel uplift along the slope. High‐angle faults are widely developed between 800 and 240 mbsf, facilitating gas migration from the top of the Basel uplift to the shallow sediment. Most of the faults did not reach the seafloor; therefore, gas accumulated at shallow depth exhibits bright spots in the seismic profiles. When the pore pressure overcomes the overburden sediment, fractures will be created for gas entering the water column. Gas emission provides a pressure release mechanism; therefore, most fractures did not reach the seafloor as observed in the seismic profiles. However, it is speculated that gas emission will stop with growth of gas hydrates and authigenic carbonates, which continuously decrease the permeability of the original fractures. Pressure will be built up, and new fractures will be generated, leading to the formation of new seeps. This mechanism is inferred to result in the spatiotemporal variation of cold seep activity and affects the development of the cold seep ecosystem.

Physical Properties of Gas Hydrate-Bearing Pressure Core Sediments in the South China Sea

“…It has been suggested that the physical properties of gas hydrate-bearing sediments are closely related to the gas hydrate saturation and effective confining stress [24,26]. In addition, gas hydrate layers and veins have been discovered in marine sediments in marine sea areas, which can change the marine sediments from isotropic to anisotropic materials [47,48]. This inhomogeneity of the material properties increases the difficulty of predicting the stability of the seabed sediments during gas hydrate production [38].…”

Deeply Buried Authigenic Carbonates in the Qiongdongnan Basin, South China Sea: Implications for Ancient Cold Seep Activities

“…Therefore, unlike hydrothermal systems, which are mainly distributed in mid-ocean ridges [4,5], cold seeps are widely dispersed throughout the active and passive continental margins and sedimentary basins with thick sediments [3,[6][7][8]. Methane reacts with seawater sulfate via the sulfate-driven anaerobic oxidation of methane (SR-AOM), which produces hydrogen sulfide and bicarbonate [9], and provides nutrients for chemosynthetic communities [10][11][12]. Therefore, a special chemosynthetic ecosystem [13], widely distributed carbonate on the seafloor [14], and gas bubble emission from the seafloor [15,16] are important indicators for the identification of marine cold seeps.…”

“…Over the past two decades, several active cold seeps have been discovered on the northern slope of the South China Sea [31], including the Jiulong seeps [32], Haima cold seeps [10], and station F [33]. In recent years, a large number of seep carbonates have been collected from these cold seeps by multiple research expeditions [30][31][32]34].…”

“…Controlled by various factors, such as the gas supply, deep strata pressure, changes in the sea level, global climate change, and gas hydrate and carbonate formation, cold seeps are usually intermittently and periodically active [10,14,35]. Therefore, seep carbonates buried at different depths are valuable for reconstructions of the historical activities of ancient cold seeps [26].…”

The complete mitochondrial genome of a cold seep gastropod Phymorhynchus buccinoides (Neogastropoda: Conoidea: Raphitomidae)

“…Cold seeps is one of the extreme deep-sea environments, where fluid migrates upward from deep stratum to the seafloor under pressure that result from plate subduction or gravity compression [17][18][19]. It mostly occurs in geologically active and passive continental margins and trenches [19].…”