11.1 Introduction

Marine sediments are the largest methane reservoir on Earth, with greenhouse methane in the form of gas hydrates, free gas, and dissolved methane in pore-water (Wallmann et al. 2012; Joye 2020). At active seep sites, it is common that gas and fluids are transported from deep reservoirs into shallow surface sediments (Wu et al. 2011, 2022; Jin et al. 2022; Wang et al. 2022). However, most of the rising methane is consumed by methanotrophic microbial communities before it can reach the hydrosphere (Reeburgh 2007; Boetius and Wenzhöfer 2013). Methane is primarily consumed via anaerobic oxidation of methane (AOM) and aerobic oxidation of methane (AeOM) (Boetius and Wenzhöfer 2013). Sulfate-driven AOM in cold seep ecosystems is mainly performed by consortia of anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB; Boetius et al. 2000), whereas AeOM is performed by aerobic microbes and animal–microbe symbioses (Boetius and Wenzhöfer 2013).

To date, three types of microbial consortia have been demonstrated to mediate sulfate-driven AOM: ANME-1/Desulfosarcina/Desulfococcus (DSS), ANME-2/DSS, and ANME-3/Desulfobulbus spp. (DBB) (Hinrichs et al. 2000; Orphan et al. 2001; Knittel et al. 2003, 2005; Losekann et al. 2007). These archaeal assemblages are closely or loosely associated with their sulfate-reducing partners, varying with the methane and sulfate concentrations, temperature, and oxygen availability (Elvert et al. 2005; Knittel et al. 2005; Nauhaus et al. 2005). Lipid biomarkers and their compound-specific carbon stable isotope compositions are useful tools to monitor the distribution and functioning of microorganisms in seep environments (Birgel et al. 2006; Niemann and Elvert 2008; Stadnitskaia et al. 2008a, b; Blumenberg et al. 2015). This is because different ANME/SRB consortia biosynthesize different lipid biomarkers and produce distinct carbon differences from methane to biomarkers (Niemann and Elvert 2008). For example, ANME-2/DSS consortia are commonly characterized by large amounts of 2,6,11,15-tetramethylhexadecane (crocetane), high sn2-hydroxyarchaeol/archaeol ratios (>1.1), and low ai-C15:0/i-C15:0-fatty acid ratios, whereas high glycerol dibiphytanyl glycerol tetraethers (GDGTs) contents, low sn2-hydroxyarchaeol/archaeol ratios (<0.8), small amounts or absence of crocetane, and high ai-C15:0/i-C15:0-fatty acid ratios, are indicators of ANME-1/DSS (Niemann and Elvert 2008). Except for specific biomarkers, the maxima in rates of AOM and sulfate reduction are commonly correlated with the highest biomarker abundance and the lowest carbon isotopic signatures (Elvert et al. 2005).

The investigation of cold seeps in the SCS started in 2004 (Chen et al. 2005; Suess 2005). Since then, authigenic carbonates, sediments at active seep sites, and seep-dwelling bivalves have been collected from more than 40 seep sites, covering a wide range of water depths on both the northern and southern continental slopes of the SCS (Chen et al. 2005; Han et al. 2008; Liang et al. 2017). Lipid biomarkers of seep carbonates retrieved from the SCS were first reported in 2008 (Birgel et al. 2008; Yu et al. 2008). Based on the molecular fossils of seep carbonates, fluid sources, microbial consortia, and seepage dynamics were constrained (Guan et al. 2013, 2016a, b, 2018; Ge et al. 2015). At the Haima cold seeps, 13C-depleted isoprenoidal 3,7,11,15-tetramethylhexadecan-1,3-diol and two novel isoprenoids, namely, sn2-/sn3-O-hydroxyphytanyl glycerol monoethers, were found as hydrolysis products of isoprenoid diethers in authigenic carbonates (Xu et al. 2022a, b). Furthermore, lipid biomarker patterns combined with the compound-specific carbon stable isotopes of active seep sediments were used to uncover AOM, AeOM and oil degradation processes (Guan et al. 2022). In the seep ecosystems of the SCS, lipid biomarkers and compound-specific carbon stable isotope compositions have been studied in the Site F, northeast Dongsha, southwest Dongsha, Shenhu, and Haima cold seep ecosystems, revealing AOM, sulfate reduction, and oil biodegradation processes (Birgel and Peckmann 2008; Yu et al. 2008; Ge et al. 2010, 2011, 2015; Guan et al. 2013, 2014, 2016a, b, 2018, 2022; Lai et al. 2021; Li et al. 2021). In this chapter, we reviewed current lipid biomarker studies of seep ecosystems in the SCS. Fluid sources, consortia assemblages, and seepage dynamics during carbonate precipitation are summarized in detail.

11.2 Lipid Biomarker Inventories

11.2.1 Archaeal Biomarkers

The biomarker inventories of seep carbonates retrieved from NE Dongsha, Shenhu, Site F, and Haima revealed diagnostic lipids of specific groups of ANME/SRB (Fig. 11.1; Birgel et al. 2008; Yu et al. 2008; Ge et al. 2010, 2015; Guan et al. 2013, 2016a, b, 2018). The biomarker patterns and stable carbon isotope compositions were found to vary among individual study sites and geographic areas. Among these data, 13C-depleted isoprenoid hydrocarbons, crocetane, 2,6,10,15,19-pentamethyleicosane (PMI), and squalane, are common (Fig. 11.2; Birgel et al. 2008; Yu et al. 2008; Ge et al. 2011, 2015; Guan et al. 2013, 2016a, b, 2018). Crocetane is particularly abundant at ANME-2-dominated methane seeps, whereas it is only present in minor amounts or absent in ANME-1-dominated environments (Guan et al. 2013, 2016a, b; Ge et al. 2015). 13C-depleted PMI-acid specific to methanotrophic archaea was found in an authigenic carbonate from NE Dongsha (Guan et al. 2013). Monocyclic diphytanyl glycerol diethers (monocyclic MDGD), possibly derived from methanotrophic archaea, were observed in carbonate pipes in the Shenhu area (Ge et al. 2011). Other isoprenoids, including 13C-depleted phytanyl-glycerolmonoethers sn2-/sn3-O-phytanyl glycerol ethers, phytanol, and phytanoic acid, are common in authigenic carbonates and sediments (Guan et al. 2013, 2016a, b, 2018, 2022). The GDGT patterns are characterized by the dominance of GDGTs with 0–2 cyclopentane moieties (Guan et al. 2016a).

Fig. 11.1
3 graphs plot relative abundance versus time. All three graphs plot fluctuating trends, with graph (a) plotting C r slash p h as the highest, graph b plotting 18 is to 1 as the highest, and graph c plotting phytanol as the highest.

Gas chromatograms of the hydrocarbon fraction (a), carboxylic acid fraction (b), and alcohol fraction (c) from seep carbonates of the South China Sea. Istd: internal standard. a Pr: pristane; Cr/Ph: crocetane/phytane; PMI: 2,6,10,15,19-pentamethylicosane; Sq: squalane; gray dots: n-alkanes. Phy: phytanoic acid; gray triangles: n-fatty acids. DAGE: non-isoprenoidal dialkyl glycerol ether; ββ-32-ol: 17β(H), 21β(H)-C32-hopanol; sn2/sn3: sn2-/sn3-O-phytanyl glycerol monoethers; sn2'/sn3’: sn2-/sn3-O-hydroxyphytanyl glycerol monoethers; Ar: archaeol; sn2-/sn3-OH−Ar: sn2-/sn3-hydroxyarchaeol; gray diamonds: n-alcohols

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Fig. 11.2
18 bond-line chemical structures for crocetane, P M I, phytanol, phytanoic acid, archaeol, s n 2 hydroxyarchaeol, di-hydroxyarchaeol, phytanyl glycerol monoether, hydroxyphytanyl glycerol monoether, and macrocyclic glycerol diether among others.

Biomarkers of anaerobic methane-oxidizing archaea

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Biphytanic diacids, including acyclic biphytanic diacid, monocyclic biphytanic diacid, and bicyclic biphytanic diacid, were found in seep carbonates from NE Dongsha (the Jiulong methane reef; Birgel et al. 2008). Except for the most widespread isoprenoids above,  isoprenoidal 3,7,11,15-tetramethylhexadecan-1,3-diol and two novel sn2-/sn3-O-hydroxyphytanyl glycerol monoethers hydroxylated at C-3 of the phytanyl moieties were observed in seep carbonates from the Haima seep (Xu et al. 2022a, b). Most archaeal-specific isoprenoids are strongly depleted in 13C with δ13C values ranging from −140‰ to −84‰ (Fig. 11.3; Birgel et al. 2008; Yu et al. 2008; Ge et al. 2011, 2015; Guan et al. 2013, 2016a, b, 2018, 2022; Li et al. 2021).

Fig. 11.3
2 box and whisker plots of archaeal biomarkers and bacterial biomarkers. The former and latter plot values are for site F, Haima, N E Dongsha, and Shenhu, respectively. The former plots N E Dongsha as the highest while the latter plots Shenhu as the highest.

Comparison of the δ13C values of archaeal biomarkers (a) and sulfate-reducing bacterial biomarkers (DSS cluster) (b) in authigenic carbonates from various seep sites of the SCS. Isoprenoids include crocetane/phytane, PMI, phytanoic acid, phytanol, 3,7,11,15-tetramethylhexadecan-1,3-diol, sn2-/sn3-O-phytanyl-glycerolethers, sn2-/sn3-O-hydroxyphytanyl glycerol monoethers, archaeol, sn2-/sn3-hydroxyarchaeols, and di-hydroxyarchaeol. Sulfate-reducing bacterial biomarkers include i-/ai-C15:0, n-C16:1ω5, n-C17:1ω6, and cycC17:0ω5,6 fatty acids, the MAGEs were excluded. Data are taken from Birgel et al. (2008), Yu et al. (2008), Guan et al. (2013, 2014, 2016a, b, 2018), Ge et al. (2011, 2015), and Xu et al. (2022a)

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11.2.2 Sulfate-Reducing Bacterial Biomarkers

Seep carbonates and seep sediments from the SCS are known to contain various bacterial lipids (Fig. 11.4). The otherwise uncommon fatty acids C16:1ω5 and cycC17:0ω5,6 were found in high abundances and were assigned to the sulfate-reducing partners of ANME-2 (Elvert et al. 2003; Blumenberg et al. 2004, 2005). The presence of ai-/i-C15:0-fatty acids with ratios lower than 2 is an indicator of ANME-2/DSS-dominated seeps (Niemann and Elvert 2008; Guan et al. 2013). However, the ai-/i-C15:0-fatty acid ratio does not always discriminate the sulfate-reducing partners of ANME since their contents and δ13C values can be modified by SRBs involved in oil degradation (Guan et al. 2018, 2022; Krake et al. 2022). Non-isoprenoidal monoglycerol ethers (MAGEs), ascribed to DSS associated with ANME-2 and DBB associated with ANME-3 (Niemann and Elvert 2008), were only abundant in Haima seep carbonates (Xu et al. 2022a, b). SRB-specific fatty acids were less 13C-depleted than those originating from archaea because they can assimilate dissolved inorganic carbon (DIC) as a carbon source (Wegener et al. 2008). The SRB-specific biomarkers yielded δ13C values from −11133‰ to −111‰ in seep carbonates of the SCS (Fig. 11.3; Guan et al. 2013, 2016a, b, 2018, 2022). In addition, various DAGEs with extremely low δ13C values were found with high contents in authigenic carbonates from Site F, NE Dongsha, and Haima (Fig. 11.5; Guan et al. 2013, 2016a; Xu et al. 2022a, b).

Fig. 11.4
8 bond-line structures of different forms of fatty acids and 2 series of D A G Es.

Selected biomarkers of sulfate-reducing bacteria

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Fig. 11.5
2 box and whisker plots of archaeal biomarkers. The former and latter plot values are for site F, Haima, N E Dongsha, and Shenhu, respectively. Both plot N E Dongsha as the highest. A box with a question mark inside it is adjacent to the N E Dongsha plot.

δ13C values of DAGEs (a), and hopanoids (b) from seep carbonates of the SCS. Hopanoids include diplopterol, 17α(H),21β(H)-32-hopanol, 17β(H),21β(H)-32-hopanol, 17α(H),21β(H)-32-hopanoic acid, 17β(H),21β(H)-31-hopanoic acid, and 17β(H),21β(H)-32-hopanoic acid. Data are taken from Guan et al. (2013, 2014, 2016a, b, 2018); Ge et al. (2011, 2015

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11.2.3 Aerobic Methanotrophic Biomarkers

Another striking feature of the seep carbonates originating from the SCS is the significant amounts of hopanoids and steroids with moderate to significant 13C-depletions (Fig. 11.5; Guan et al. 2016a, b, 2018, 2022). These hopanoids generally include tetrahymanol, diploptene, diplopterol, hopanoic acids (e.g., 17α(H)-21β(H)-bishomo-hopanoic acid and 17β(H)-21β(H)-bishomo-hopanoic acid), and hopanols (e.g., 17α(H)-21β(H)-bishomo-hopanol and 17β(H)-21β(h)-bishomo-hopanol), reflecting a characteristic suitably agreeing with patterns found at other seep sites (Fig. 11.6; Birgel et al. 2008, 2011; Himmler et al. 2015; Natalicchio et al. 2015). However, bacteriohopanepolyols (BHPs) and their related geohopanoids are known to be produced by both aerobic and anaerobic bacteria (Talbot et al. 2001; Sinninghe Damste et al. 2004; Blumenberg et al. 2006; Rush et al. 2016). Therefore, diplopterol, diploptene, hopanols, hopanoic acids, and their possible diagenetic products generally cannot be used alone as reliable indicators of past aerobic methanotrophic bacteria at seeps (Birgel and Peckmann 2008). Generally, 13C-depleted 3β-methylated hopanoids, 4-methylated steroids, and BHPs aminotetrol/aminotriol and further related BHPs are diagnostic biomarkers of aerobic methanotrophic bacteria (Talbot et al. 2001; Birgel and Peckmann 2008; Birgel et al. 2011; Himmler et al. 2015; Natalicchio et al. 2015; Rush et al. 2016). In the SCS, hopanoids were observed in seep carbonates and sediments originating from NE Dongsha, Shenhu, Site F, and Haima, yielding δ13C values ranging from −31‰ to −101‰ (Fig. 11.5; Guan et al. 2013, 2014, 2016a, b, 2018, 2022). Regarding the Haima cold seeps, 4-methylated steroid 4α-methylcholesta-2,8(14),24-triene and two tetracyclic hopanoid ketones, C30-3β-methyl tetracyclic-ketone and C34-3β-methyl tetracyclic-ketone, were found in seep sediments (Guan et al. 2022). In this cold seep ecosystem, both 4-methylated steroid and tetracyclic hopanoid ketones were attributed to Type I and/or X methanotrophs, with precursors most likely including 4α-methylcholesta-8(14),24-dien-3β-ol and 3-methylated hopanoids, respectively (Guan et al. 2022). These hopanoids along with aerobic methanotrophic bacteria-specific biomarkers are indicators of aerobic methanotrophic bacteria at seeps.

Fig. 11.6
12 bond line structures of the diploptane, diploptene, tetrahymanol, diplopterol, lanosterol, and other 6 hopanoic acids.

Biomarkers of aerobic methanotrophic bacteria

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11.3 Authigenic Carbonate Formations Constrained by Lipid Biomarkers

11.3.1 Environmental Conditions Inferred from Biomarker Patterns

Microbial activity, diversity and related geochemical processes in cold seep ecosystems are of great concern because the upward ascending methane is mainly consumed by anaerobic methanotrophs in consortia with sulfate-reducing bacteria (Boetius et al. 2000). In the cold seep ecosystems of the SCS, microbial diversity has been reported at the NE Dongsha, Site F, Shenhu, and Haima cold seeps. Methanotrophic archaea (ANME-1, ANME-2, and ANME-3) and sulfate-reducing partners (Desulfosarcina/Desulfococcus) were observed, and their abundance varied with the depth and site (Niu et al. 2017; Cui et al. 2019; Zhuang et al. 2019). Processes of methanogenesis, AOM, and sulfate reduction could be archived by microbial activities at active seep sites (Boetius et al. 2000; Blumenberg et al. 2004). However, these advantages of microbial tools disappear during rock periods. Instead, lipid biomarkers are preferred because of their stable carbon skeleton and the carried information on their precursors (Aloisi et al. 2000; Peckmann et al. 2004, 2009; Birgel et al. 2008; Stadnitskaia et al. 2008a, b).

The lipid biomarker inventories of seep carbonates from NE Dongsha, Shenhu, Site F, and Haima revealed diagnostic lipids of specific groups of ANMEs and their sulfate-reducing partners (Fig. 11.1; Birgel et al. 2008; Yu et al. 2008; Guan et al. 2013, 2016a, b, 2018; Ge et al. 2015; Xu et al. 2022b). The biomarker patterns and carbon stable isotope compositions were found to vary among individual study sites and among different geographic areas (Figs. 11.3 and 11.5). Lipid biomarkers of ANMEs and SRBs originating from NE Dongsha and Site F carbonates commonly yielded lower δ13C values than those originating from Haima carbonates, reflecting differences in fluid compositions and methane types (Fig. 11.2; Guan et al. 2013, 2016a, 2018). NE Dongsha and Site F are located in the convergence zone of the active and passive margins, and seeps appear to be largely controlled by strike-slip faults (Liu et al. 2008, 2015; Chen et al. 2017; Feng et al. 2018). Methane at both sites indicates biogenic sources and is released from gas hydrate dissociation with δ13C values ranging from  −72.3‰ to  −69.4‰ (Zhuang et al. 2016). Regarding the Haima cold seeps, hydrate-bound methane is a mixture of biogenic and thermogenic gas, with most δ13C values ranging from −72.3‰ to −48.4‰ (Fang et al. 2019; Ye et al. 2019; Wei et al. 2019, 2021; Lai et al. 2022a, b). Potential methane types during carbonate precipitation could be inferred according to specific microbial consortia and δ13C values of archaea-specific isoprenoids because the δ13C differences from methane to archaeal-specific lipids reach approximately −30‰ and  −50‰ for ANME-1 and ANME-2, respectively (Niemann and Elvert 2008; Birgel et al. 2011; Himmler et al. 2015). The δ13C variability of lipid biomarkers originating from NE Dongsha, Site F, and Haima carbonates reflected the patterns of their parent methane and carbon fractionations from methane to lipids.

The application of molecular fossils in combination with their compound-specific isotope signatures is an efficient tool to reconstruct seepage intensities. At seep sites, the sulfate methane transition zone (SMTZ) is largely controlled by the seepage intensity (Borowski et al. 1996), which tends to occur shallower within sediments at sites with a high methane flux than that at sites with a relatively low methane flux (Luff and Wallmann 2003). The relatively high sulfate concentration and porosity on or near the seafloor surface favor the precipitation of aragonite over high-Mg-calcite (Fig. 11.7; Greinert et al. 2001; Haas et al. 2010). In regard to microbial consortia, ANME-2 is better adapted to higher methane concentrations and lower temperatures and is less sensitive to oxygen than ANME-1, whereas ANME-1 can tolerate moderate and lower methane concentrations (Elvert et al. 2005; Knittel et al. 2005; Nauhaus et al. 2005). In the SCS, including the Dongsha area, Site F, Shenhu, and Haima cold seeps, aragonite- and calcite-dominated carbonates are commonly formed at sites predominated by ANME-2 and ANME-1, respectively, indicating varying environmental conditions and seepage intensities (Guan et al. 2013, 2016a, b, 2018; Ge et al. 2015). Therefore, lipid biomarkers, combined with mineralogy, petrography, and carbon stable and oxygen isotope compositions, are effective indicators to reconstruct the formation conditions of seep-related carbonates.

Fig. 11.7
2 photos of artifacts at the top with two double-ended arrows labeled A N M E 1 slash D S S and A N M E 2 slash D S S on the right and left, and high M g calcite and aragonite on the right and left, respectively, on the bottom. Between the two are microbial communities, mineralogies, and environmental conditions.

Schematic model of relationship of microbial communities, mineralogies, and environmental conditions

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11.3.2 Sulfate Reductions Coupled to AOM and Organoclastic Oxidation

At marine seep sites, both AOM and oxidation of non-methane hydrocarbons can fuel sulfate reduction (Joye et al. 2004; Kallmeyer and Boetius 2004; Bowles et al. 2011; Kleindienst and Knittel 2020). Joye et al. (2004) found that sulfate-reducing rates generally exceeded AOM rates and concluded that the majority of sulfate reduction at Gulf of Mexico hydrocarbon seep sites was likely fueled by the oxidation of other organic matter. At oil-related cold seeps, hydrocarbons other than methane are the most likely reductants. Among sulfate reducers, DSS of Deltaproteobacteria are known as the primary non-methane hydrocarbon degraders at methane seeps (Kniemeyer et al. 2007; Kleindienst et al. 2012, 2014; Jaekel et al. 2013). DSS involved in sulfate-driven AOM lives in consortia with ANME, using CO2 as a carbon source and yielding extreme 13C-depletion of DSS-derived fatty acids (Wegener et al. 2008, 2015). In contrast, all known DSS capable of non-methane hydrocarbon degradation are free-living organisms, coupling substrate oxidization and sulfate reduction in a single-cell process (Kleindienst et al. 2012, 2014; Jaekel et al. 2013; Petro et al. 2019). These DSS were demonstrated to assimilate carbon from short-chain hydrocarbons in enrichment cultures (Jaekel et al. 2013; Kleindienst et al. 2014).

Although the δ13C values of methane and oil-related hydrocarbons overlap to a certain extent (−110‰ to −15‰ and higher than −35‰, respectively; Milkov and Etiope (2018)), methane in the SCS yielded notably lower carbon stable isotopes (lower than  −48‰) than oil-related hydrocarbons (Zhuang et al. 2016; Yang et al. 2018; Fang et al. 2019; Wei et al. 2019, 2021; Ye et al. 2019; Lai et al. 2022a, b; Liang et al. 2022). Consequently, the lipid biomarkers of DSS involved in AOM and oxidation of non-methane hydrocarbons are expected to be different. Guan et al. (2018) analyzed seep carbonates originating from the Haima cold seeps and found that i-/ai-C15:0-fatty acids were accompanied by less negative δ13C values, suggesting that the input of DSS was involved in the oxidation of non-methane hydrocarbons. Similar observations were also found for sediments obtained from the Haima cold seeps (Guan et al. 2022). These conclusions were supported by lipid biomarker comparisons between methane-seep carbonates and oil-seep carbonates (Krake et al. 2022). The authors found that oil-seep-related carbonates not only yielded a high abundance of DAGEs with more diverse alkyl chains but also heavier δ13C values on average than those of methane-seep carbonates (Krake et al. 2022).

Unresolved complex mixtures (UCMs) are an additional indicator of the oxidation of non-methane hydrocarbons. UCMs are commonly reported in seep-related carbonates or sediments, especially oil seepages (Sassen et al. 1993; Zhang et al. 2003; Naehr et al. 2009; Birgel et al. 2011; Feng et al. 2014, 2018; Li et al. 2021). UCMs contain naphthenes and other related compounds that cannot be completely resolved by conventional GC analysis (Zhang et al. 2003; Naehr et al. 2009; Birgel et al. 2011). At the Haima cold seeps, UCMs occurred in hydrocarbon fractions of both seep carbonates and sediments, with partially altered hydrocarbons (Guan et al. 2018, 2022). Since n-alkanes were not completely altered, these oils in the sediments were attributed to light to middle biodegradation (Peters and Moldowan 1993). Furthermore, the heavier δ13CTIC values of sediments retrieved from the Haima seeps were found to be correlated with a high total organic carbon content, which again agreed with non-methane hydrocarbons supplied to sulfate-reducing bacteria (DSS) and resulted in relatively heavier δ13C values of DSS-derived fatty acids (Guan et al. 2022).

11.3.3 Constraints on the Hydrolysis Process

In lipid biomarker analysis of authigenic carbonates originating from the Haima cold seep, 3,7,11,15-tetramethylhexadecan-1,3-diol and two novel sn2-/sn3-O-hydroxyphytanyl glycerol monoethers hydroxylated at C-3 of the phytanyl moieties were identified. Both methanogens and methanotrophic archaea were found to produce hydroxydiethers (Sprott et al. 1990, 1992, 1993; Rossel et al. 2008, 2011). Among methanotrophic archaea, ANME-1 contains abundant GDGTs and small amounts of sn2-/sn3-hydroxyarchaeols, whereas ANME-2 and ANME-3 are dominated by phosphate-based polar derivatives of di-hydroxyarchaeol, sn2-/sn3-hydroxyarchaeols and archaeol (Rossel et al. 2008, 2011). Regarding these Haima seep carbonates, the abundant crocetane and high ratios of sn2-hydroxyarchaeol relative to archaeol (>1.1) indicated the dominance of ANME-2 or ANME-3 (Xu et al. 2022b).

Generally, understanding the composition of molecular fossils, biological precursors and past diagenetic evolution process constitutes a key step in applying a fossil lipid indicator (Briggs and Summons 2014). The 13C-depleted short-chain isoprenoids in cold seep ecosystems are putative degradation products of archaeol and sn2-/sn3-hydroxyarchaeols, and can be attributed to ANMEs (Thiel et al. 2001a, 2018; Oba et al. 2006). According to Koga et al. (1993) and Oba et al. (2006), all hydroxyarchaeols were converted into monophytanyl glycerol ethers in the hydrolysis process with chloroform/methanol/concentrated HCl. Based on the sequential degradation pathway of hydroxyarchaeols, sn2-/sn3-O-hydroxyphytanyl glycerol monoethers are likely hydrolysis products of di-hydroxyarchaeol rather than sn2-/sn3-hydroxyarchaeols, whereas isoprenoidal 3,7,11,15-tetramethylhexadecan-1,3-diol is most likely a hydrolysis product of sn2-/sn3-O-hydroxyarchaeols, di-hydroxyarchaeol, and sn2-/sn3-O-hydroxyphytanyl glycerol monoethers (Fig. 11.8; Xu et al. 2022b). Except for these isoprenoid ethers, a similar relationship was found for MAGEs, DAGEs and corresponding non-isoprenoid short-chain alcohols in the Haima seep carbonates. The MAGEs, DAGEs, and non-isoprenoid short-chain alcohols yielded similar δ13C values, chain (side-chain) lengths and unsaturation patterns, indicating that non-isoprenoid alcohols might be either biosynthetic intermediates or degradation products of MAGEs and DAGEs (Xu et al. 2022b).

Fig. 11.8
A diagram demonstrates the hydrolysis pathway of isoprenoid glycerol ethers along with the byproducts. The bond-line structure of all the compounds is given.

Potential diagenetic pathways of isoprenoid glycerol ethers. Reprinted from Organic Geochemistry, 163, Xu et al. (2022b). Diagenetic fate of glycerol ethers revealed by two novel isoprenoid hydroxyphytanyl glycerol monoethers and non-isoprenoid alkyl glycerolethers, 104, 344, Copyright (2022), with permission from Elsevier

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11.4 Summary and Outlook

Authigenic carbonates sampled from the SCS contained varying contents of 13C-depleted archaeal and bacterial biomarkers, indicating their formation as a result of anaerobic oxidation of methane (AOM) via anaerobic methane-oxidizing archaea (ANMEs) and sulfate-reducing bacteria (SRBs). Based on various biomarker patterns, methane was predominantly oxidized by ANME-1/DSS and ANME-2/DSS, varying from site to site. Generally, ANME-2/DSS-specific crocetane, n-C16:1ω5, cyc-C17:0ω5,6-fatty acids, high sn2-hydroxyarchaeol/archaea ratios and low anteiso-C15:0/iso-C15:0-fatty acid (ai-/i-C15:0) ratios were found in aragonite-dominated carbonates, suggesting carbonates formed in seep ecosystems with high seepage intensities. Carbonates characterized by GDGTs, small amounts of crocetane, low sn2-hydroxyarchaeol/archaea ratios and high ai-C15:0/i-C15:0-fatty acid ratios were dominated by high-magnesium calcite, reflecting their formation at a greater depth with a relatively low seepage intensity. The oxidation of non-methane hydrocarbons fueled by sulfate reduction was demonstrated by abundant i-/ai-C15:0 fatty acids with relatively higher δ13C values. Distinct biomarker patterns and respective geochemical processes at cold seeps may be correlated with the microbial community succession sequence during seepage evolution because the different growth rates of aerobic methanotrophs, SRBs, and ANMEs could be recorded in an emerging cold seep ecosystem. Moreover, the ether hydrolysis products isoprenoidal 3,7,11,15-tetramethylhexadecan-1,3-diol and sn2-/sn3-O-hydroxyphytanyl glycerol monoethers were observed, reflecting a diagenetic process from isoprenoid dieters to isoprenoid monoethers and isoprenoid alcohols.

Although diagnostic biomarkers could be used to discriminate ANME-1/DSS and ANME-2/DSS consortia, it is unclear what factors control their ability to respond to changing seepage intensities and environments. Since cold seeps comprise a substantial carbon pool worldwide, it is critical to increase the efficiency of hydrocarbon biofilters. In addition to the well-advanced identification of sulfate-AOM, AeOM, and oxidation of non-methane hydrocarbons with biomarkers as outlined above, future studies combining culturing, molecular microbiology, and lipid biomarkers may provide further insight into (1) the modes and variability of hydrocarbon consumption possibly involving different electron acceptors and (2) the allochthonous and autochthonous contributions of seep carbonates to biomarker inventories.