1.1 Introduction

Cold seeps are seafloor manifestations of methane-rich fluid migration from the sedimentary subsurface to the seabed and into the water column, and ultimately, some of the methane may even reach the atmosphere (Boetius and Wenzhöfer 2013). Marine hydrocarbon seeps are common features of continental margins worldwide (Suess 2020). Because of their relevance for oceanic emissions of the greenhouse gas methane, widespread chemosynthesis-based ecosystems, and their spatial overlap with areas containing gas hydrate, hydrocarbon seeps have been considered ideal natural laboratories for studying Earth’s mass and energy cycling from the geosphere to the exosphere (Boetius and Wenzhöfer 2013; Suess 2020). The term “冷泉” (the term “cold seep” in Chinese) was first introduced by Duofu Chen and his colleagues in 2002, as published in the first issue of the journal Acta Sedimentologica Sinica (Fig. 1.1). Since then, great progress has been made in regard to the hydrocarbon seeps in the South China Sea (SCS). To date, various types of samples, including authigenic minerals, seep-impacted sediments, and seep fauna, have been recovered from more than 40 seep sites, which cover a wide range of water depths in both the northern and southern continental margins of the SCS (Fig. 1.2).

Fig. 1.1
A book's front cover reads Acta Sedimentologica Sinica and a text in a foreign language. Below are two men inspecting the surface of the coast of a beach, among a pile of rocks.

Upper left: The Cover image of the first issue of the 2002 journal Acta Sedimentologica Sinica. Upper right: Front page of the manuscript on which the term “冷泉” (“cold seeps” in Chinese) was created by the lead editor and his colleagues of this book. Lower: Duofu Chen and Xianpei Chen (with hat; deceased). Photo: Qinxian Wang

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Fig. 1.2
A satellite map of the South China Sea highlights certain areas. An insert of an Asian continent, in which the area of the South China Sea is boxed.

Locations of known South China Sea seep sites (after Feng et al. (2018a); Wang et al. (2022a, b); and Tseng et al. (2023)). The active seep sites, Site F, Haima, and Yam are highlighted by red circles. The locations for gas hydrate drilling expeditions (GMGS 1–8) are marked by white boxes (after Yang et al. (2017) and Wei et al. (2020))

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1.2 South China Sea Hydrocarbon Seep Studies

Following their discovery in the 1980s on the Florida Escarpment in the Gulf of Mexico at a water depth of 3200 m (Paull et al. 1984), seeps have been widely found in the world’s oceans (Ceramicola et al. 2018; Suess 2020). Seafloor reflectivity or amplitude analysis using 2D/3D seismic datasets has been used for identifying and characterizing cold seeps since the late 1990s (Sun et al. 2011, 2012, 2013, 2017; Chen et al. 2015, 2018; Kunath et al. 2022). Studies on hydrocarbon seeps in the SCS have been driven by growing energy demands, specifically for gas hydrate resources, since the late 1990s. The presence of cold seeps in the SCS was first confirmed in June 2004 during the joint Chinese–German RV SONNE Cruise 177 (SO 177; Fig. 1.3; Suess et al. 2005). The cooperative research project between the Guangzhou Marine Geological Survey (GMGS) and Leibniz-Institute für Meereswissenschaften Kiel (IFM-GEOMAR) led to the discovery of a large area of methane-derived carbonate buildups, which are byproducts of hydrocarbon seepage. Carbonate buildups were found in three ridge crest segments (Site 1, Site 2, and Site 3) of the NE Dongsha area, which collectively cover approximately 400 km2 (Han et al. 2008). Moreover, additional methane-derived carbonates were dredged from the NE Dongsha area and SW Taiwan area of the SCS (Chen et al. 2005, 2006; Lu et al. 2005). Analyses of the carbonate samples, seep-impacted sediments (including pore waters), and animals have produced abundant data that have led to the accumulation of background knowledge on the SCS hydrocarbon seeps, e.g., the sources of seep fluids (Chen et al. 2005, 2006; Han et al. 2008; Yu et al. 2008; Feng and Chen 2015; Feng et al. 2015; Hu et al. 2017, 2018), hydrocarbon–mineral–element–microbe associations (Birgel et al. 2008; Ge et al. 2010, 2011, 2015; Guan et al. 2013, 2016, 2018; Han et al. 2013; Wang et al. 2014; Chen et al. 2016; Li et al. 2016; Lin et al. 2016a, b, c, d; 2017; Lu et al. 2017, 2018; Feng et al. 2018a; Gong et al. 2018, 2022; Wang et al. 2018, 2020; Yang et al. 2018), the timing of seepage, and driving forces (Tong et al. 2013; Han et al. 2014; Luo et al. 2014, 2015; Feng and Chen 2015; Wang et al. 2022a, b). Beyond documenting new seep systems, recent advances relate to seep footprints. Representative manifestations of seepage at the seafloor in the SCS are mud volcanoes, pockmarks, and carbonate deposits. The spatial distribution and morphology of these phenomena may provide information on the nature of the fluids, the conditions of their formation and evolution, and postformation secondary processes (e.g., Feng et al. 2018a).

Fig. 1.3
Two photographs of a boat in the ocean and 5 people assembling a bunch of rocks placed on a table inside a boat.

The German research vehicle SONNE at the South China Sea in 2004. “Rocks on deck” on the RV SONNE during SO177. Photos curtesy of Jun Tao

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The second major breakthrough on hydrocarbon seeps that involves detailed investigations using manned submersibles and remotely operated vehicles (ROV) started in the late 2000s (Lin et al. 2007; Machiyama et al. 2007). These underwater vehicles and robots have been routinely applied to examine seep systems since the early 2010s (Feng and Chen 2015; Feng et al. 2015, 2018a). At this stage, significant progress includes the detailed investigation of the known active seep Site F and the discovery of new active seeps: Haima and Yam (Table 1.1). The manned submersible Jiaolong was deployed in June–July 2013 to conduct a systematic survey and sampling of the active seep site, Site F, in the northern SCS. In 2015, another active cold seep, named the ‘Haima seep’, was discovered in the Qiongdongnan Basin of the SCS during the dives of the ROV Haima (Liang et al. 2017). These active seep sites were revisited annually from 2015 to 2022 by dives using remotely operated vehicles Faxian and Haima at Site F and the Haima seep, respectively (Liang et al. 2017; Zhang et al. 2017). The focus of these dives was to explore the interactions of fluids and chemosynthetic communities and to conduct in situ experiments on biogeochemical processes at hydrocarbon seeps (Fig. 1.4; Feng et al. 2018a, b; Wang et al. 2022a, b).

Table 1.1 Information o the three active seeps in the South China Sea
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Fig. 1.4
An illustration of the seep segment in the South China Sea. It consists of reduced sediment, a methane plume, mussels, tubeworms, and clams on the seafloor. Below the seafloor are gas hydrants and carbonates.

Schematic illustration of hydrocarbon seeps in the South China Sea. Highly concentrated hydrate deposits provide a supply of methane that stimulates the entire ecosystem at cold seeps

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By linking subsurface and seafloor biospheres, seeps have become natural laboratories to enhance our knowledge of the processes of hydrocarbon migration and associated hydrocarbon-based ecosystems in the marine environment. Extensive efforts on pore water studies from more than 250 sites have been made across the continental slope to detect hydrocarbon seeps in the SCS in the past two decades. The regional sulfate fluxes in the SCS highlight the importance of sulfate consumption fueled by deep-sourced methane, which is a factor that needs to be considered in any attempt to better quantify global fluxes of seawater sulfate in marine sediments (Hu et al. 2022, 2023; Chap. 8). In addition, drilling campaigns and coring programs have contributed greatly to our understanding of seep–hydrate dynamics on much larger timescales in the SCS (Fig. 1.2). These drilled cores merit further research to advance our understanding of the temporal evolution of seep systems (e.g., Chen et al. 2019; Wei et al. 2020). The macroecology of chemosynthesis-based ecosystems was first reviewed by Niu et al. (2017) and followed by Feng et al. (2018a) and Wang et al. (2022a, b). Documentation of which animals are present at the SCS seeps, how they interact with each other, and how the SCS seep animals are related to those from other seeps in a broader context have been important research areas during the last decade. Details about the macroecology of chemosynthesis-based ecosystems are presented in Chaps. 5 and 6.

Hydrocarbon seeps at the active margin of the SCS have also been extensively investigated in recent years (Fig. 1.5). For instance, elevated methane concentrations have been detected within seafloor sediments and bottom water (Chuang et al. 2010, 2013, 2019; Chen et al. 2017). Great progress was made in 2016, when Klaucke et al. (2016) investigated the Four-Way Closure Ridge and reported an active seep area, named the “Yam Seep”, on top of the northern ridge (Tseng et al. 2023). Seafloor observations showed active gas emissions and the presence of extensive authigenic carbonate slabs and chemosynthetic bivalves (Klaucke et al. 2016). Different from the seep activities on the passive margin of the SCS that are closely related to gas hydrate dissociation during either changing bottom water temperature or seabed pressure (Tong et al. 2013; Chen et al. 2019; Deng et al. 2021; Wang et al. 2022a, b), hydrocarbon seeps on the active margins of the SCS are most likely caused by tectonic activity (Tseng et al. 2023).

Fig. 1.5
A timeline of the seep research in the South China Sea. It starts with the year 2000 and ends in 2020. The year 2000 consists of the creation of the term cold seep and the discovery of Jiulong Methane Reef. It ends with 3 versions of R O V Haima dives to Haima seeps at the top, and their corresponding counterparts of 3 versions of R O V Faxian dives to Site F at the bottom.

Timeline of major milestones in South China Sea hydrocarbon seep research

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In contrast to the northern SCS, seeps in the southern SCS are less well investigated. Feng et al. (2018c) and Li et al. (2018) provided the first report of cold seep activity on the southern continental slope of the SCS. The temporal variations in methane seepage were reconstructed using geochemical analyses of pore waters and sediments. Recently, Huang et al. (2022) provided a detailed description of deep-routed fluid seepage and inferred its indication for hydrates in the Beikang Basin. Zhang et al (2023) analyzed either seep carbonate samples collected from the Beikang Basin to reveal the fluid sources and sedimentary environments as well. More future work is needed to better characterize the seeps in the southern SCS.

The past two decades have witnessed significant progress in understanding a variety of aspects of the SCS cold seep systems. Cold seeps offer a great opportunity to examine complex geological, chemical, and biological processes from an interdisciplinary angle, thus requiring a philosophy that guides cold seep research from the perspective of Earth system science. Due to the scientific background of the editors and space limit, many interesting aspects, e.g., microbial ecology of the SCS seeps, are not discussed. The distinct microbial communities of the SCS seeps that are unique from other seeps and from other seafloor ecosystems are presented elsewhere (Niu et al. 2022). In addition, as seep studies rely heavily on innovative sampling and observation apparatuses, advancements in deep-sea technology will play a crucial role in future seep studies. In particular, in-situ (or on-site) observations on the seafloor have become an indispensable part of seep studies, e.g., they can further enhance the validity of the simulation model. We look forward to the next decade of research on seep sites in the SCS and the advancement of knowledge that will result from it.