Gas hydrate-bearing sediments are highly sensitive to redox fluctuations, primarily driven by methane fluxes associated with gas hydrate decomposition or cold seep activity. These sediments serve as valuable archives, preserving records of past redox states and methane emission events. Trace elements and rare earth elements (REE) have been widely applied as proxies to reconstruct such environmental conditions. Anoxic, methane-rich depositional environments characteristic of gas hydrate systems promote the enrichment of redox-sensitive major and trace elements (e.g., Fe, Mo, U, Ce) and REE in seep carbonates and surrounding sediments, suggesting that these settings act as important sinks for these elements. Methane also strongly shapes the geochemical signatures, with dynamic seepage events identified through indicators within the sulfate-methane transition zone (SMTZ), such as the barite front, molybdenum enrichment, and authigenic carbonate precipitation. However, relying solely on bulk sediment analysis often leads to ambiguous interpretations. Here we highlight that geochemical signals are fractionated differentially across these phases: carbonates capture fluid signatures, while pyrite hosts sulfur and chalcophile elements (e.g., Mo, As). This review synthesizes and evaluates previous findings, highlighting current knowledge, identifying uncertainties, and outlining directions for future research.
gas hydrate-bearing sedimentsmethane fluxrare earth elementsredox conditionssedimentary geochemistrytrace elementsThe author(s) declared that financial support was received for this work and/or its publication. This research is funded by DD20230400103 from China geological survey to HL, and National Natural Science Foundation of China (No. 42303075) to HY.section-at-acceptanceMarine BiogeochemistryIntroduction
Gas hydrate-bearing sediments are dynamic environments consistently characterized by methane activity, which significantly influences their geochemical characteristics. Understanding the redox conditions within these sediments and the past dynamics of methane seepage is crucial for deciphering past environmental changes and biogeochemical processes. Inorganic geochemical proxies have emerged as powerful tools for this purpose, offering insights into these complex systems.
Over the past decades, extensive research has been conducted on gas hydrate-bearing environments across various marine regions worldwide, including the South China Sea (Ge et al., 2010; Hu et al., 2015; Li et al., 2016; Sun et al., 2019; Liu et al., 2020), North Sea (Chen et al., 2005), Black Sea (Thiel et al., 2001), Sea of Marmara (Crémière et al., 2013; Yang et al., 2018), Sea of Japan (Snyder et al., 2007; Hiruta et al., 2009), Gulf of Mexico (Feng et al., 2009; Birgel et al., 2011; Hu et al., 2014; Crémière et al., 2016), Gulf of Cadiz (Wang et al., 2015) and some deep-sea fans and deltas (Feng et al., 2010; Rongemaille et al., 2011; Hu et al., 2014; Pierre et al., 2014). These studies have established a valuable foundation for interpreting the geochemical evolution of these unique sedimentary settings.
The behavior of trace elements and rare earth elements (REE) is particularly sensitive to the fluctuating pH and Eh conditions in methane-rich environments. Elements such as Mo, V, U, Ni, Co, Cu, Zn, and Ba are often enriched in cold seep sediments due to their tendency to remain stable under reducing conditions (Francois, 1988; Morford and Emerson, 1999; Russell and Morford, 2001; Tribovillard et al., 2006; Hu et al., 2014; Smrzka et al., 2019, 2020). Their geochemical cycles are intricately linked with authigenic minerals (e.g., carbonates, pyrite, barite) and the redox cycling of Fe and Mn (oxy)hydroxides (Tribovillard et al., 2006; McQuay et al., 2008; Griffith and Paytan, 2012; Large et al., 2017; Smrzka et al., 2020; Yang et al., 2023; Xue et al., 2025). In addition to variations in elemental contents, the distribution patterns and enrichment layers of these element ratios, such as U/Th, V/Cr, Ni/Co (Merinero Palomares et al., 2012; Sun et al., 2019; Algeo and Liu, 2020), and Sr/Ca, Mg/Ca (Bayon et al., 2007; Nöthen and Kasten, 2011), are also considered to be critical factors in the evolution of methane fluxes. Therefore, these trace elements are widely used as proxies for tracing variable redox changes caused by past methane seepage events (Kimura and Watanabe, 2001; Chun et al., 2010; Pälike et al., 2014). Furthermore, REE patterns, including Ce anomalies, are widely applied to reconstruct redox conditions and fluid characteristics (Feng et al., 2009; Ge et al., 2010; Himmler et al., 2010; Rongemaille et al., 2011; Tribovillard et al., 2013).
However, the dynamic nature of cold seep systems, with episodic and transient methane fluxes, leads to highly variable redox conditions. This complexity, coupled with microbial mediation of element migration and transformation (e.g., Fe, Co, Zn, Cu, Ni, B, P, V, W) by sulfate-reducing bacteria or methanogenic/methanotrophic archaea, presents substantial challenges for interpretation (Scherer et al., 1983; Barton et al., 2007; Glass and Orphan, 2012; Glass et al., 2014, 2017). Consequently, relying on a single geochemical proxy is often insufficient to fully unravel the intricate dynamics of gas hydrate-bearing environments.
While inorganic geochemical proxies are widely applied, exclusive reliance on bulk sediment analyses may lead to ambiguous interpretations because bulk sediments integrate terrigenous, biogenic, and authigenic components. Geochemical signals preserved in different sedimentary carriers—namely bulk sediments, authigenic carbonates, and authigenic pyrite—exhibit significant divergence in their formation pathways and sensitivities to methane flux (Tribovillard et al., 2006; Nöthen and Kasten, 2011; Smrzka et al., 2019; Liu et al., 2024). Bulk sediments reflect a mixed signal that is commonly modulated by broader environmental controls, such as sea-level fluctuations and primary productivity (Calvert and Pedersen, 1993; Algeo and Tribovillard, 2009; Chen et al., 2019; Li et al., 2024). In contrast, including carbonates and pyrite, more directly archive pore-fluid composition and seepage intensity (Peckmann et al., 1999, 2001; Chen et al., 2005; Campbell, 2006; Naehr et al., 2007; Smrzka et al., 2019). Cold-seep carbonates, conceptualized as a “Cold-Seep Carbonate Factory” (Dai et al., 2025), precipitate from methane-derived fluids, and their mineralogy (e.g., aragonite versus high-Mg calcite) and elemental ratios (e.g., Sr/Ca and Mg/Ca) are particularly sensitive to variations in fluid flux (Li et al., 2024). Furthermore, authigenic pyrite formed via sulfate-driven AOM (Liu et al., 2024) acts as an important sink for redox-sensitive trace metals (e.g., Mo, As, W) and preserves diagnostic isotopic signatures (e.g., light δ98Mo) associated with benthic Fe–Mn redox shuttling, which may be diluted or obscured in bulk sediment records (Miao et al., 2024b). Therefore, discriminating among these sedimentary carriers is critical for robust reconstruction of past methane seepage.
This review aims to synthesize current knowledge of inorganic geochemical proxies used in gas hydrate-bearing sediments. Specifically, we summarize the indicative value and practical application of proxies such as redox-sensitive trace elements (e.g., Co, Ni, Cu, Zn, V, Mo, U), Fe and Mn speciation, REE patterns (including Ce anomalies), and indicators for reconstructing seepage history (e.g., barite front, Mo enrichment, authigenic carbonate precipitation). Furthermore, we critically analyze and distinguish the specific geochemical signatures recorded in three distinct seepage archives: bulk sediments, authigenic carbonates, and authigenic pyrite. Our goal is to highlight their utility in determining past redox conditions and methane seepage history, and to emphasize the necessity of a multi-proxy approach for a more comprehensive understanding of these dynamic environments.
Methane biogeochemistry and geochemical zones in gas hydrate environments
Understanding the roles of redox conditions and biogeochemical processes is essential for sedimentary geochemical research. According to classical early diagenetic theory, organic matter is sequentially degraded using O2, NO3−, Mn(IV), Fe(III) and SO42− as different electron acceptors as these become thermodynamically available (Froelich et al., 1979). This process occurs before burial and creates distinct geochemical zones in sediment profiles: the oxygen reduction zone, nitrate reduction zone, manganese reduction zone, iron reduction zone, sulfate reduction zone, and methanogenesis zone. Methane seepage, often derived from hydrate decomposition, usually leads to the saturation of pore waters with hydrocarbon gases (Johnson et al., 2014). Moreover, the presence of methane enhances biochemical processes in gas hydrate environments. Methane-rich and H2S-rich fluids modify the distribution of redox zones in marine sediments, thereby creating a more acidic and reducing environment due to the accumulation of CO2 and H2S (Niewöhner et al., 1998; Aharon and Fu, 2000; Joye et al., 2010). As a result, the influence of the oxic zone weakens and is replaced by prominent reducing and anaerobic conditions (Chen et al., 2016). Strong methane leakage, such as at cold seeps, can generate large amounts of H2S, thereby forming a new sulfidic zone (Chen et al., 2016).
In marine sediments, biogeochemical processes such as sulfate reduction (SR), anaerobic oxidation of methane (AOM), and methanogenesis (MOG) collectively shape the geochemical profiles with depth (Goldhaber, 2003; Kong et al., 2021). Sulfate reduction (SR) is the most influential process and occurs mainly in two ways: organoclastic sulfate reduction (OSR) (Berner, 1980) and sulfate-driven anaerobic oxidation of methane (SD-AOM) (Murray et al., 1978; Devol and Ahmed, 1981; Boetius et al., 2000). The two net reactions are expressed stoichiometrically as:
2CH2O+SO42−→2HCO3−+H2SCH4+SO42−→2HCO3−+HS−+H2O
Most sulfate is consumed to form sulfide in deeper geochemical profiles. Within these zones, significant amounts of methane are also produced in marine sediments (Jørgensen and Kasten, 2006). Besides SD-AOM, methane can also be oxidized by many alternative electron acceptors, including O2, NO3−, Mn(IV) and Fe(III) oxides, perchlorate (ClO4−), and arsenate (As(V)). This occurs due to changes in redox conditions and varying types and rates of organic matter input (Caldwell et al., 2008; Wallenius et al., 2021). Aerobic methane-oxidizing bacteria (MOB), anaerobic methane-oxidizing archaea (ANME), and sulfate-reducing bacteria (SRB) can collectively oxidize methane, coupling AOM with the reduction of oxygen, sulfur, iron, manganese, nitrate, or nitrite (Wallenius et al., 2021) (Figure 1). Some marine methane-seep sediments use manganese oxides (e.g., birnessite) and iron oxides (e.g., ferrihydrite) to oxidize methane (Beal et al., 2009). To date, Fe-AOM and Mn-AOM have been highlighted in many research studies of methane-rich sediments (Sun et al., 2015; Rooze et al., 2016; Li et al., 2019; Liu et al., 2020; Yang et al., 2023; Xue et al., 2025).
Schematic illustration of geochemical zones and methane oxidation (MO) processes in gas hydrate-bearing sediments.
Geochemical zoning diagram showing methane flux in sediments. It illustrates the main pathways of methane oxidation: aerobic MO, N-AOM, SD-AOM, Fe-AOM, Mn-AOM, and methanogenesis. Each pathway is represented with chemical equations. Key zones are labeled as seawater and sediment, with additional labels such as SMI and SMTZ indicating specific transition zones. Paths for different chemical reactions are color-coded and linked to their respective zones.
As organic matter degrades during sedimentation, it reaches the sulfate-methane transition zone (SMTZ) (Reeburgh, 2007). This zone lies between the sulfate reduction and methanogenesis zones, where downward-diffusing sulfate and upward-diffusing methane meet, react, and are consumed (Barnes and Goldberg, 1976; Borowski et al., 1996, 1999). At this interface, AOM is the key and most active biogeochemical process, which also controls methane release into seawater (Hinrichs and Boetius, 2003; Reeburgh, 2007). AOM primarily occurs in the SMTZ (Knittel and Boetius, 2009) or can also occur in the gas hydrate zone (Cui et al., 2019). AOM is typically coupled to microbial sulfate reduction (MSR) through a reaction involving ANME and SRB (Raghoebarsing et al., 2006; Reeburgh, 2007; Knittel and Boetius, 2009).
The location and morphology of the SMTZ provide valuable information for understanding the occurrence and evolution of gas hydrates. Globally, SMTZ depth is closely related to sedimentation rate, organic matter input, and upward methane flux (Egger et al., 2018). In nearshore and continental shelf sediments, high sedimentation and organic matter burial rates lead to a shallow SMTZ, whereas in sediments farther offshore the SMTZ becomes deeper (Egger et al., 2018). Among these factors, upward methane flux is the most important factor affecting the position of the SMTZ and associated biogeochemical processes. Methane flux from gas hydrate decomposition or cold seeps affects sulfate consumption. Strong methane flux from decomposed hydrates or cold seeps accelerates sulfate consumption, thereby changing the SMTZ’s depth and thickness within sedimentary sequences (Borowski et al., 1996, 1999; Consolaro et al., 2015). Similarly, seepage intensity strongly affects AOM efficiency: vigorous seepage enhances AOM efficiency, while weak seepage results in less efficient methane oxidation (Joye et al., 2004; Boetius and Wenzhöfer, 2013). Therefore, AOM exerts a primary control on the distribution and thickness of the SMTZ in marine sediments (Borowski et al., 1999). By tracing the typical concentration profiles of methane, sulfate, and their products (sulfide and carbonate), the key characteristics of AOM can be identified (Borowski et al., 1996; Knittel and Boetius, 2009). Based on this understanding, SMTZ characteristics in gas hydrate-bearing sediments can be summarized in two ways (Paull et al., 2005; Castellini et al., 2006):
In low-flux methane seepage environments, the SMTZ is deeper, thicker, and more stable, and methane may be completely consumed by relatively less efficient AOM.
In high-flux methane seepage environments, AOM cannot fully balance the upward methane flux, allowing methane to escape directly into the bottom waters. The SMTZ is typically very shallow and varies depending on seepage intensity.
Element geochemical proxies of redox environmentsRedox-sensitive trace elements: principles and behavior
Trace elements differ in their sensitivity to redox conditions (Tribovillard et al., 2006), and many show distinctive behavior in methane-rich sediments (Hu et al., 2014; Smrzka et al., 2019, 2020). Among these, Mo, V, U, Ni, and Re are widely regarded as particularly robust proxies for reconstructing paleo-redox conditions, whereas elements such as Co, Cr, Cu, Zn, and Cd may provide complementary information depending on local geochemical and depositional settings (Calvert and Pedersen, 1993; Yarincik et al., 2000; Tribovillard et al., 2006; Algeo and Tribovillard, 2009). Their enrichment generally increases as the environment becomes more reducing, since their solubility is strongly controlled by the redox state, and they accumulate as environmental reducibility increases (Francois, 1988; Morford and Emerson, 1999; Russell and Morford, 2001; Tribovillard et al., 2006). To evaluate their enrichment, enrichment factors (EF) are commonly used (Tribovillard et al., 2006).
The geochemical behavior of these trace elements in methane-rich sediments follows specific patterns (Figure 2). High-valence species of Cr, U, and V are reduced and preferentially enriched under suboxic to anoxic denitrification conditions associated with nitrate reduction, whereas Ni, Cu, Co, Zn, Cd, and Mo are predominantly enriched under euxinic conditions, where free H2S is produced during sulfate reduction (Algeo and Maynard, 2004; Tribovillard et al., 2006; Smrzka et al., 2020). U, V, and Mo are highly sensitive to hypoxia: concurrent enrichments of these three elements typically indicate euxinic depositional condition, while weaker Mo enrichment may instead reflect an anoxic environment (Algeo and Maynard, 2004; Tribovillard et al., 2006). Ni, Cu, Zn, and Cd are released during organic matter degradation and often fix in sediment via pyrite formation (Huerta-Diaz and Morse, 1990; Morse and Luther, 1999; Tribovillard et al., 2006). Cr is primarily associated with terrigenous components, substituting for Al in clay mineral structures (Björlykke, 1974). Under strongly reducing yet non-sulfidic conditions, V can be immobilized through the formation of vanadium oxides or hydroxide phases, such as VO2+ and V(III)-bearing hydroxides [e.g., V(OH)3] (Algeo and Maynard, 2004; Tribovillard et al., 2006). Rhenium (Re) is sensitive to dissolved oxygen and serves as a proxy for oxygen penetration depth at seep sites (Smrzka et al., 2020).
Migration processes of trace elements in cold seep environments.
Diagram illustrating the geochemical processes in a marine sediment environment. Fluid flow is depicted from a cold seep, showing interactions with elements like manganese, iron, and copper. Layers include oxic, suboxic, anoxic, and sulfidic zones. Key interfaces are labeled: Sea-Water Interface (SWI), Sulfate-Methane Transition Interface (SMI), and Top of Gas Hydrate Stability Zone (TGHSZ). Various elements and compounds, including organic matter, methane, and barite, are shown undergoing transformations and interactions in different sediment layers.Trace element enrichment
The cycling of redox-sensitive elements is significantly affected by abundant H2S and CH4 from seepage (Figure 2; Tribovillard et al., 2013; Hu et al., 2015; Smrzka et al., 2020). Methane derived from gas hydrate decomposition reacts with pore water SO42−, producing H2S via AOM-MSR (Boetius et al., 2000; Knittel and Boetius, 2009). Such reactions commonly lead to anoxic or sulfidic conditions (Chen et al., 2016; Sun et al., 2019).
Case studies from different seep and hydrate systems illustrate the variability of trace element enrichment patterns. For example, Feng et al. (2010) found highly variable V, Mo, U, and Cd contents in seep authigenic carbonates from the northern Congo fan, which increased during periods of relatively high fluxes. Ge et al. (2010) reported that Mo, U, Ni, V, and Co recorded anoxic conditions during carbonate precipitation on the northern continental slope of the South China Sea. Deng et al. (2017) noted that gas hydrate-bearing sediments in the South China Sea were more enriched in Sr, Pb, Th, U, Zn, Cu, and Mo relative to the upper crust. Wang et al. (2019) applied tungsten (W) as a novel proxy in seep carbonate systems, noting its biological role in microbial AOM pathways (Barton et al., 2007; Glass and Orphan, 2012).
In contrast, Chen et al. (2016) observed that although Mo exhibited expected enrichment in sulfidic environments, Co, Cu, Ni, Zn, and V showed little enrichment during active methane seepage. They attributed this to weak OSR under low TOC conditions. Based on these results, Mo and U emerge as the most reliable indicators of methane seepage among redox-sensitive trace elements, with AOM-driven sulfidic conditions enhancing Mo accumulation (Chen et al., 2016). This interpretation is further confirmed by reports of extreme Mo enrichments (MoEF > 10) and seawater-like MoEF/UEF values in gas hydrate-bearing sediments worldwide (Hu et al., 2014, 2015; Chen et al., 2016; Deng et al., 2017; Liu et al., 2020).
The Mo-U pattern in sediments provides a widely applied proxy for depositional redox states (Figure 3). Mo typically occurs as soluble molybdate (MoO42−) under oxic conditions, but under reducing conditions it is reduced and removed from solution, commonly through transformation to lower-valence Mo species (Mo(VI) → Mo(IV)/Mo(V)) and subsequent fixation in sulfide- or organic-associated phases (Calvert and Pedersen, 1993). U is soluble as U(VI) under oxic conditions but is reduced and fixed in sediments as U(IV) under anoxia (Tribovillard et al., 2006). Thus, Mo and U enrichments jointly reflect the redox gradient: U is commonly removed at the Fe2+–Fe3+ transition zone (suboxic), whereas Mo requires sulfidic conditions (Morford and Emerson, 1999; Algeo and Tribovillard, 2009; Scott and Lyons, 2012; Tribovillard et al., 2012). Algeo and Tribovillard (2009) and Tribovillard et al. (2012) defined Mo and U enrichment factors of ~3–10 for suboxic conditions and > 10 for euxinic conditions. The MoEF/UEF ratio further refines this distinction: values of 0.1–0.3 indicate oxic–suboxic conditions, 0.3–1.0 indicates anoxic conditions, and > 1.0 indicates sulfidic conditions (Algeo and Tribovillard, 2009; Hu et al., 2014).
Schematic flow diagram illustrating the fractionation and enrichment mechanisms of Molybdenum (Mo) and Uranium (U) across marine sedimentary redox gradients. The diagram delineates the divergent behaviors of Mo and U under oxic, suboxic, and sulfidic (methane seep/euxinic) conditions. Mo enrichment (MoEF > 10) specifically requires free H2S for thiomolybdate formation, whereas U enrichment begins under suboxic conditions. (Modified after Algeo and Tribovillard, 2009; Scott and Lyons, 2012).
Flowchart illustrating Mo-U variations across redox settings from oxic to sulfidic conditions. Molybdenum is soluble as molybdate in oxic and suboxic settings, with enrichment in sediments in sulfidic conditions. Uranium is soluble as U(VI) in oxic conditions, reduced and fixed as U(IV) in suboxic environments, and continually enriched in anoxic and sulfidic conditions. Mo-U enrichment factors vary, being lower under oxic-suboxic conditions and higher in euxinic (sulfidic) settings.
Overall, Mo and U enrichments are generally more prominent and reliable indicators of redox conditions, but the degree of enrichment of other trace elements ultimately depends on specific depositional and diagenetic circumstances.
Trace element ratios as redox proxies
Element ratios are widely used as proxies to reconstruct paleo-redox environments. Ratios involving U, Th, V, Cr, Ni, and Co are widely applied to constrain redox states (Table 1). For example, Hatch and Leventhal (1992) proposed V/(V+Ni) as a proxy for evaluating paleoenvironmental and local redox conditions. Jones and Manning (1994) further evaluated five trace element ratios as potential redox proxies, including V/Cr, Ni/Co, Ni/V, (Cu+Mo)/Zn, and U/Th, though Ni/V and (Cu+Mo)/Zn were found to be less effective for ancient bottom water conditions. Building on this work, Algeo and Liu (2020) refined the application of trace-metal ratios in paleo-redox analysis. In general, ratios of U/Th > 1.25, V/Cr > 4.25, Ni/Co > 7.00, and V/(V+Ni) > 0.77 are considered indicative of suboxic and anoxic conditions. These proxies therefore provide important constraints on the redox state of the original sedimentary environment (Hatch and Leventhal, 1992; Jones and Manning, 1994; Pehlivanli et al., 2014; Chen et al., 2019).
Proxies of paleo-oxygenation.
Paleo-oxygenation facies
Oxygen concentration (ml/l)
U/Th
V/Cr
Ni/Co
V/(V+Ni)
FeHR/FeT
Oxic
8.0–2.0
< 0.75
< 2.00
< 5.00
< 0.60
< 0.22
Dysoxic
2.0–0.2
0.75–1.25
2.00–4.25
5.00–7.00
0.60–0.77
0.22–0.38
Suboxic and Anoxic
0.2–0.0 (sulfide present)
> 1.25
> 4.25
> 7.00
> 0.77
> 0.38
The limits for oxygen concentration are from Tyson and Pearson (1991). The limits for V/Cr, U/Th, and Ni/Co are from Jones and Manning (1994). The limits for V/(V+Ni) are from Hatch and Leventhal (1992). The limits for FeHR/FeT are from Poulton and Canfield (2011).
The V/Sc ratio is another commonly applied redox proxy in paleoenvironmental studies (Kimura and Watanabe, 2001; Li et al., 2018), and some studies have linked it to methane seepage (Li et al., 2016; Sun et al., 2019). However, its ability to resolve redox shifts directly related to methane leakage is not always straightforward. Similarly, the Pr/Ph ratio has been used to differentiate anoxic (< 1.0), suboxic (1.0–3.0), and oxic (> 3.0) conditions (Jia, 2012), but it has rarely been used in gas hydrate-related studies.
To better explain the application of these proxies in gas hydrate-bearing sediment, we summarize case studies from four seep regions (Figure 4).
Application of U/Th, V/Cr, Ni/Co, and V/(V+Ni) ratios in cold seep carbonates and gas hydrate-bearing sediments. (a) U/Th vs. Ni/Co; (b) U/Th vs. V/Cr; (c) U/Th vs. V/(V+Ni); (d) Ni/Co vs. V/(V+Ni). 1, SH-HS4DG: carbonate chimney samples from Shenhu (data from Ge et al., 2010); 2, DS-TVG: sediment samples from Dongsha (data from Ge et al., 2010); 3, QDN-C14: Qiongdongnan sediments (data from Sun et al., 2019); 4, CF: carbonate samples from Congo deep-sea fan seep (data from Hu et al., 2014); 5, GeoB: carbonate samples from Congo deep-sea fan seep (data from Feng et al., 2010); 6, Mexico AC645 and GB425: carbonate samples from the Gulf of Mexico (data from Hu et al., 2014); 7, carbonate chimney samples from Gulf of Cadiz (data from Merinero Palomares et al., 2012). The limits for V/Cr, U/Th, and Ni/Co are from Jones and Manning (1994). The limits for V/(V+Ni) are from Hatch and Leventhal (1992).
Four scatter plots (labeled a, b, c, and d) depict geochemical data differentiated by symbols and colors, including triangles, circles, squares, and stars. Axes are labeled U/Th, Ni/Co, V/Cr, and V/(V+Ni) to represent different chemical ratios. The plots divide regions into oxic, dysoxic, suboxic, and anoxic conditions. A legend identifies the data sources by different markers: SH-HS4DG, DS-TVG, QDN-C14, CF, GeoB, Mexico AC645, Mexico GB425, and Gulf of Cadiz. Data points are distributed across the plots, with varied concentrations and correlations.South China Sea
Shenhu samples contain a whole chimney and four chimney fragments, indicating more anoxic conditions during the core formation stage under variable seepage (Ge et al., 2010). Dongsha chimneys, either scattered on the seafloor or protruding from sediments, likely precipitated from fluids that were influenced by surrounding sediments (Ge et al., 2010). At Qiongdongnan Site C14, cores recovered from a pockmark field show a shift from anoxic to suboxic conditions, reflecting methane leakage during deposition (Sun et al., 2019). Most samples from Shenhu and Dongsha have high U/Th and Ni/Co ratios, indicating anoxic conditions strongly influenced by active methane seepage and AOM. In particular, U/Th ratios in SH-HS4DG samples have a wide range, suggesting greatly varying methane fluid intensities. In contrast, Qiongdongnan samples reveal a predominantly oxic depositional environment, which may represent a weaker and more stable seepage activity.
Congo deep-sea fan
Carbonate from the Congo deep-sea fan (CF carbonates, Hu et al., 2014; GeoB carbonates, Feng et al., 2010), collected from pockmarks at Hydrate Hole and Diapir Field, formed under sulfidic conditions influenced by strong AOM and episodic H2S release (Hu et al., 2014). Geochemical signatures indicate the pronounced effects of intense AOM and methane leakage: U/Th and Ni/Co ratios are consistent with oxygen-depleted conditions, whereas the wide range of V/Cr values suggests variability in methane leakage intensity.
Gulf of Mexico
Carbonates at site AC645 from the Gulf of Mexico show intermittent oxygenation during deposition, while carbonates at site GB425 from southern venting sites of a Gulf of Mexico mud volcano were affected by episodic methane leakage (Hu et al., 2014). AC645 samples exhibited polarized data, with some showing higher U/Th and Ni/Co values, indicating suboxic and anoxic conditions, pointing to intermittent methane emission. In contrast, GB425 carbonates show U/Th and Ni/Co values suggesting redox states fluctuating between oxic and dysoxic conditions, likely reflecting moderate methane seepage.
Gulf of Cadiz
Carbonate chimneys from the Gulf of Cadiz formed under oxygen-depleted conditions associated with methane seepage and AOM (Merinero Palomares et al., 2012). These samples show relatively low U/Th and Ni/Co ratios, but elevated V/(V+Ni) ratios, indicating a hypoxic depositional environment. V/Cr ratios span wide ranges, oxic, dysoxic, suboxic, and anoxic stages, reflecting staged shifts in redox conditions linked to variable seepage intensity.
Despite their utility, trace element ratios can yield inconsistent results in modern sediments (Guo et al., 2021). Interpretations are often one-dimensional if they fail to account for seepage intensity and basin-wise geochemical baselines. Variations in seepage intensity can non-linearly alter redox zonations and element precipitation. For instance, V/Cr ratios in Gulf of Cadiz carbonates span multiple redox categories solely due to fluctuating seepage flux, while trace element contents in Congo Fan carbonates correlate directly with fluid flux periods. Furthermore, fixed redox thresholds are often inapplicable across different regions due to basin-specific trace elemental baselines. As emphasized by Algeo and Liu (2020), local depositional contexts determine background signals, necessitating internal cross-calibration rather than universal standards. Moreover, high seepage flux can compress redox zones, decoupling element precipitation from theoretical predictions (Li et al., 2024). Consequently, these ratios are best suited for tracking relative shifts in redox states rather than defining precise conditions, underscoring the necessity of a multi-proxy framework.
Speciation of iron and manganese
In marine sediments, Fe speciation is widely used to distinguish oxic, ferruginous, and euxinic water column conditions. This approach has become central to reconstructing ocean redox chemistry (Table 1; Poulton and Canfield, 2011). Oxic conditions are typically defined by low ratios of highly reactive iron to total iron (FeHR/FeT< 0.22), while ferruginous conditions are indicated by FeHR/FeT > 0.38 and low ratios of pyrite iron to highly reactive iron (FePy/FeHR< 0.7–0.8). Euxinic conditions are characterized by FeHR/FeT > 0.38 and FePy/FeHR > 0.7–0.8, with values between 0.22 and 0.38 considered “equivocal”. Sequential extraction procedures further characterize reactive iron pools—including carbonate-associated Fe (FeCarb), oxide Fe (FeOx), and magnetite Fe (FeMag)—allowing for the evaluation of reactive iron cycling during methane seepage and diagenesis (Yang et al., 2018, 2023).
While manganese (Mn) geochemical behavior is often linked to iron, its role in methane cycling warrants distinct consideration due to its thermodynamic superiority (Slomp et al., 2013; Wu et al., 2020; Xue et al., 2025). Mn oxides serve as potent electron acceptors for anaerobic oxidation of methane (Mn-AOM), yielding significantly higher energy (approximately 20 times) than sulfate-driven AOM (Beal et al., 2009). This process proceeds via direct oxidation (MnR-AOM) or an indirect “cryptic sulfur cycle” (MnR-S-AOM), where Mn oxides recycle sulfide back to sulfate (Su et al., 2020; Cai et al., 2023; Xue et al., 2025). Geochemically, active Mn-AOM is signaled by elevated porewater Mn2+ concentrations below the SMTZ (Riedinger et al., 2014; Xiao et al., 2023). However, unlike Fe, Mn is extensively reduced to soluble Mn2+ in strongly reducing seep environments and tends to escape the sediment column, often leading to a net depletion of solid-phase Mn rather than authigenic accumulation (Nealson and Saffarini, 1994; Riedinger et al., 2014; Yang et al., 2018).
Iron (oxyhydr)oxides act as electron acceptors (Fe-AOM) following Mn depletion (Slomp et al., 2013; Egger et al., 2015). This process involves the direct coupling of methane oxidation with the reductive dissolution of crystalline Fe(III) oxides (Ettwig et al., 2016; Cai et al., 2018). In contrast to Mn, the liberated Fe2+ is more readily retained within the sediment through rapid reaction with HS− and HCO3−, precipitating as authigenic Fe(II) carbonates (e.g., siderite) or sulfides (Slomp et al., 2013; Roberts, 2015; Yang et al., 2018, 2023). A unique signature in hydrocarbon-rich, sulfide-limited settings is the formation of ferrimagnetic iron sulfides, such as greigite (Fe3S4) and pyrrhotite, which serve as specific magnetic proxies for assessing seepage intensity (Roberts, 2015; Yang et al., 2023).
Collectively, the internal cycling of Mn and Fe drives the “benthic redox shuttle,” facilitating the migration and enrichment of associated trace elements such as Mo, As, and V (Tribovillard et al., 2013; Hu et al., 2014; Smrzka et al., 2020). While both elements facilitate AOM, their distinct post-reduction fates—Mn export versus Fe retention—create diagnostic geochemical signatures crucial for reconstructing the history of fluid migration.
Elements related to organic matter
Redox states are closely linked to the distribution of organic carbon in sedimentary environments (Brumsack, 1989). Below the sulfate reduction zone, organic matter degradation drives methanogenesis (Johnson et al., 2014). Some studies have highlighted the relationship between total organic carbon (TOC) content and gas hydrate saturation (Waseda, 1998; Milkov, 2004; Johnson et al., 2014).
Authigenic barite minerals (e.g. BaSO4) are almost insoluble and unaffected by redox conditions. They can form when sulfate diffuses downward below the seafloor and reacts with upward-migrating barium-rich fluids (Greinert et al., 2002). For this reason, biogenic barium (Ba) concentrations are commonly used as a proxy for past primary productivity on geological timescales (Dymond et al., 1992; McManus et al., 1998). In deep-sea sediments, Ca mainly originates from biogenic carbonates. A positive correlation between Ca and Ba is frequently observed (Emerson and Hedges, 2008; Carter et al., 2020; House and Norris, 2020).
In gas hydrate-bearing sediments, strong positive correlations between Ba and TOC or between Ba and Ca suggest that organic matter input exerts a key influence on Ba distribution (Merinero Palomares et al., 2012; Sun et al., 2019; Carter et al., 2020). This highlights the importance of organic carbon in regulating both redox states and mineral authigenesis in seep environments.
Rare earth element proxiesThe total REE
REE accumulation is an important indicator of cold seep environments. REE concentrations in fresh water and seawater are generally very low (1–10 ng/L), with river water typically one order of magnitude higher than seawater (Yang and Li, 1999). Cold seep sediments, however, show REE enrichment compared to normal marine sediments (Figure 5a), largely due to early diagenesis (Freslon et al., 2014).
The distribution of REE in modern cold seep carbonates. (a) Average shale-normalized (PAAS; Taylor and McClennan, 1985) REE patterns for natural waters and modern cold seep carbonates from all over the world; (b) Plot of Ce/Ce* vs. Pr/Pr* after Bau and Dulski (1996) and Shields and Webb (2004). Ce/Ce* denotes 3CeN/(2LaN + NdN), and Pr/Pr* denotes 2PrN/(CeN + NdN), where N refers to normalization of concentration against the standard Post Archean Australian Shale (PAAS), Field I: no anomaly; Field IIa: positive La anomaly produces an apparent negative Ce anomaly; Field IIb: a negative La anomaly causes an apparent positive Ce anomaly; Field IIIa: real positive Ce anomaly; Field IIIb: real negative Ce anomaly; Field IV: positive La anomaly disguises a positive Ce anomaly. Data sources: 1. REE patterns of seawater, river water, and anoxic pore water are from Freslon et al. (2014); 2. the South China Sea (Wang et al., 2014); 3. Gulf of Cadiz (Wang et al., 2015); 4. Gulf of Mexico (Birgel et al., 2011); 5. Congo fan (Feng et al., 2010); 6. Nile deep-sea fan (Pierre et al., 2014); 7. North Sea (Crémière et al., 2016).
Two graphs are displayed. Graph (a) on the left shows various water sample types plotted against a range of lanthanides, with seawater represented by triangles, river water by circles, and anoxic pore water by diamonds, indicating their normalized concentrations to PAAS. Graph (b) on the right presents a scatter plot of Ce/Ce* against Pr/Pr* ratios for different regions, including the South China Sea, Gulf of Cadiz, Gulf of Mexico, Congo fan, Nile deep-sea fan, and North Sea, each denoted by different symbols. A legend is provided for reference.
Ancient hydrocarbon seep studies show that ∑REE concentrations vary widely (0.3–43.7 ppm; Feng et al., 2009). Large variations have also been pointed out between surface, shallow, and bottom sediments from gas hydrate areas (Budakoglu et al., 2015). In cold seep sediments, ∑REE concentrations often increase at shallow depths but decrease at greater depths due to Fe-oxide reduction (Elderfield and Sholkovitz, 1987; Haley et al., 2004; Himmler et al., 2010). Cold seep carbonates (e.g., aragonite) formed during AOM can also act as sinks for REE. Rapid carbon deposition may dilute ∑REE concentrations, making decarbonation necessary before geochemical analysis (Wu et al., 2017; Liu et al., 2020). Furthermore, to minimize the influence of detrital input, normalization to TiO2 (∑REE/TiO2) has been suggested (Tanaka et al., 2007).
REE patterns
Shale-normalized REE patterns provide insights into fluid sources and redox conditions. Modern seawater shows a characteristic pattern with a negative Ce anomaly, a positive La anomaly, and relative HREE enrichment (Feng et al., 2009; Ge et al., 2010; Himmler et al., 2010; Wang et al., 2018) (Figure 5a). In contrast, continental shelf sediments resemble river inputs, displaying flatter patterns with LREE enrichment, HREE depletion, and weak Ce and Eu anomalies (Piper, 1974; Goldstein and Jacobsen, 1988; Elderfield et al., 1990; Frimmel, 2009) (Figure 5a).
Cold seep carbonates worldwide generally show flat shale-normalized REE curves, slight Ce and Eu anomalies, MREE enrichment, and relative HREE depletion (Figure 5a). These patterns closely match the MREE-enriched signature of anoxic pore waters, suggesting that modern seep carbonates commonly precipitate under reducing conditions. Case studies from the South China Sea (Chen et al., 2005; Ge et al., 2010; Wang et al., 2014), Gulf of Cadiz (Wang et al., 2015), Gulf of Mexico (Feng et al., 2009; Birgel et al., 2011), Congo deep-sea fan (Feng et al., 2010), Niger delta (Rongemaille et al., 2011; Pierre et al., 2014), Makran accretionary prism (Himmler et al., 2010), North Sea (Crémière et al., 2016), and other seep sites confirm the use of REE patterns in tracking methane seepage, fluid sources, and precipitation conditions.
Cerium anomaly (Ce/Ce*)
Cerium (Ce) is the primary rare earth element that undergoes redox transformation in marine environment (Yang and Li, 1999). Ce(III) is readily oxidized to insoluble Ce(IV) and separates from other REE under oxidized and alkaline conditions, whereas in a reducing environment is reduced to soluble and is released from sediments to the pore water (Wright et al., 1987; Wilde et al., 1996). The Ce anomaly (Ce/Ce*) is thus a sensitive proxy for marine redox conditions. Historically, it was calculated as Ce/Ce* = 2CeN/(LaN + PrN) (Elderfield et al., 1990; Moffett, 1990). However, to avoid the influence of positive La anomalies common in seawater, the definition by Bau and Dulski (1996) is often preferred: Ce/Ce* = 3CeN/(2LaN + NdN), where N refers to normalization against PAAS or chondrite. Ce/Ce* ratios greater than 1.05 (a positive anomaly) indicate reducing conditions, while values below 0.95 (a negative anomaly) indicate oxic conditions (Taylor and McClennan, 1985). However, diagenetic modification is possible (Peckmann et al., 1999; Shields and Stille, 2001) and Ce anomalies are usually evaluated alongside Pr/Pr* (Bau and Dulski, 1996; Shields and Webb, 2004).
Cold seep carbonates display diverse Ce anomalies (Figure 5b). Positive Ce/Ce* anomalies in cold seep carbonates usually indicate the reducing conditions of gas hydrate-bearing anoxic environments (Feng and Chen, 2008; Feng et al., 2010; Birgel et al., 2011; Bian et al., 2012; Hu et al., 2014). For example, seep carbonates from the northern Congo fan display positive Ce anomalies (Ce/Ce* > 1.3), suggesting precipitation under anoxic conditions (Feng et al., 2010). Conversely, negative Ce anomalies have been observed in seep carbonates from the Gulf of Mexico, and the Gulf of Cadiz, implying oxic conditions (Figure 5b). In contrast, samples from the South China Sea exhibit variable Ce anomalies (ranging from negative to positive), reflecting high spatial heterogeneity in redox environments. These negative anomalies may result from high alkalinity during AOM (Hu et al., 2014; Wu et al., 2017; Liu et al., 2020), organic-rich pore fluids (Pourret et al., 2008), or temporarily oxic conditions (Birgel et al., 2011; Bian et al., 2012). The coexistence of both negative and positive Ce anomalies within individual deposits reflects the spatial and temporal variability of seep redox environments (Wang et al., 2014, 2015).
Sedimentary proxies for methane seepage dynamicsBarite front
In the marine environment, Ba exists in dissolved and particulate forms (Chow and Goldberg, 1960), and predominantly precipitates as barite mineral (Liguori et al., 2016). Seafloor methane seepage is a major source of barite precipitation, as fluids released from gas hydrate decomposition or seepage are enriched in Ba2+ (Hu et al., 2014). This leads to higher Ba concentrations in bottom waters, where barite precipitates in surface sediments (Torres et al., 1996; Dickens, 2001; Torres et al., 2002; Feng and Chen, 2007; Vanneste et al., 2013). Barite formation is sensitive to pore water SO42− concentrations. Barite cannot form below the SMTZ, where sulfate is fully exhausted (Dymond et al., 1992; Torres et al., 1996). With burial, barite deposited in sulfate-depleted zones dissolves and diffuses upwards as Ba2+ with migrating seep fluids. Upon encountering pore water sulfate above the SMTZ, barite re-precipitates, forming a distinct “barite front” (Figure 6; Torres et al., 1996; Dickens, 2001; Torres et al., 2003).
Schematic diagram of the formation of “barite front”, enrichment of Mo, and types of authigenic carbonate under weak and strong seepage (Modified after Chen et al., 2016).
Two side-by-side geochemical profiles comparing low and high methane flux environments. The diagrams showthe depth-dependent distribution of Molybdenum (Mo), Sulfate (SO42−), Barium (Ba), and Methane (CH4)concentrations. A prominent 'barite front' and authigenic carbonates (High-Mg calcite vs. Aragonite) are depictedshifting upwards and compressing as methane flux increases from left to right.
Global studies have shown that the barite front records long-term methane seepage activity. For example, Torres et al. (2003) systematically studied cold seep barite deposits across continental margins, while subsequent research in the Gulf of Mexico (Castellini et al., 2006; Feng and Roberts, 2011; Canet et al., 2014), San Clemente Basin (Torres et al., 2002), Storfjorden Trough (Yao et al., 2020), and the South China Sea (Liu et al., 2020), and ancient cold seeps (Gao et al., 2017) has revealed the geochemical coupling between Ba fluxes and CH4 cycling.
The barite front is not static, as barite remineralization occurs due to the dynamic nature of seepage systems. A persistent barite front generally indicates stable hydrocarbon leakage that may persist for over 104 years (Torres et al., 1996; Feng and Chen, 2007). Ancient barite fronts can mark past SMTZ depths, thereby recording variations in methane flux over time. However, evidence of older fronts may be erased if the SMTZ shoals upward (Feng and Chen, 2007). Additionally, smaller Ba peaks may exist in the sedimentary profiles, possibly related to past episodes of enhanced (paleo)productivity (Dickens, 2001).
Molybdenum enrichment
The enrichment of Mo in marine sediments reflects its conservative behavior as molybdate in oxic seawater and its enhanced particle reactivity under sulfidic conditions (Scott and Lyons, 2012). Two main pathways have been identified: (i) transient enrichment related to Mn cycling near the sediment-water interface, and (ii) permanent enrichment in sulfidic environments where Mo is converted to thiomolybdates (MoOnS4-n2−). The presence of free H2S is therefore a key factor. In euxinic conditions, Mo concentrations typically exceed 60 ppm and may reach 100 ppm, whereas in H2S-limited pore waters, values rarely exceed 25 ppm (Scott and Lyons, 2012).
Mo enrichments often highlight modern and paleo-SMTZs (Figure 6; Peketi et al., 2012; Pälike et al., 2014; Chen et al., 2016). Under strong cold seepage, the combination of AOM and SRB produces abundant H2S, creating sulfidic conditions favorable for Mo enrichment (Niewöhner et al., 1998; Aharon and Fu, 2000; Joye et al., 2010). Extremely high Mo concentrations in sediments are therefore often interpreted as evidence for intense AOM-driven H2S production (Peketi et al., 2012; Hu et al., 2014; Chen et al., 2016; Li et al., 2016; Liu et al., 2020).
Seepage strength influences the distribution of Mo. In weaker leakage systems, a deeper and wider SMTZ favors high-Mg calcite precipitation, with Mo enrichment spread across a broader sulfide zone (over vertical scales of meters or more) (Chen et al., 2016). Conversely, strong leakage leads to a shallower SMTZ (decimeter or centimeter depths) (Hu et al., 2015), compressing redox zones and favoring aragonite precipitation, bivalve survival, and Mo enrichment in surface sediments (Chen et al., 2016). Crucially, the extent of enrichment acts as a sensitive recorder of the sulfidization degree driven by seepage intensity (Scott and Lyons, 2012; Chen et al., 2016; Liu et al., 2020). In high-flux environments where dissolved sulfide concentrations exceed the threshold for quantitative thiomolybdate formation, Mo is efficiently scavenged and sequestered by iron sulfides (e.g., pyrite) or sulfurized organic matter, leading to extreme enrichments (Helz et al., 1996; Scott and Lyons, 2012; Chen et al., 2016; Li et al., 2018; Liu et al., 2020; Dai et al., 2025). Furthermore, the “benthic redox shuttle” significantly impacts Mo cycling by transporting adsorbed Mo—often enriched in light isotopes—from oxic waters to the sulfidic seep zone, where it is released upon reductive dissolution and subsequently fixed (Algeo and Tribovillard, 2009; Goldberg et al., 2009; Scott and Lyons, 2012; Hu et al., 2014; Jin et al., 2024; Miao et al., 2024b). Thus, combining concentration data with isotopic compositions provides a more robust constraint on the dynamic evolution of SMTZ and seepage history (Scott and Lyons, 2012; Chen et al., 2016; Miao et al., 2024b). U enrichment often parallels Mo, since high methane fluxes compress both suboxic and sulfidic zones, leading to U and Mo enrichment in shallow sediments.
Sr/Ca and Mg/Ca ratios
Within the SMTZ, AOM typically performed by a consortium of ANME and SRB, oxidizes large amounts of methane to HCO3− and reduces sulfate to HS−. This process increases environmental alkalinity, leading to the precipitation of authigenic carbonate minerals such as low- and high-Mg calcite, dolomite, and aragonite (Ritger et al., 1987; Boetius et al., 2000). The key reaction is (Berner, 1980):
Ca2++2HCO3−→CaCO3(s)+CO2+H2O
Two main types of cold seep authigenic carbonates are aragonite and high-Mg calcite. Aragonite preferentially precipitates from seawater supersaturated in Mg2+ and Sr2+ (Berner, 1975; Tesoriero and Pankow, 1996), often in layers with free gas or close to gas hydrate horizons (Bohrmann et al., 1998; Naehr et al., 2000). High-Mg calcite, although less stable, preferentially precipitates under anoxic and low SO42− conditions in methane seepage environments (Savard et al., 1996), especially within or below the sulfate reduction zone (Naehr et al., 2000; Greinert et al., 2001; Gieskes et al., 2005).
Because of their sensitivity to seepage intensity, Sr/Ca and Mg/Ca ratios are widely used to track fluid evolution (Bayon et al., 2007; Nöthen and Kasten, 2011; Yang et al., 2014; Chen et al., 2016; Sun et al., 2019; Liu et al., 2020). A four-endmember mixing model distinguishes between terrigenous input, biogenic calcite, aragonite (Sr-rich, reflecting strong seepage), and high-Mg calcite (linked to weak or diffusive seepage) (Bayon et al., 2007). For example, gas hydrate-bearing sediments from the South China Sea (Site GMGS2-W08) show a shift from high-Mg calcite to aragonite (Chen et al., 2016), reflecting a progressive increase in methane flux.
Distinct geochemical archives: differentiating signals in bulk sediments, authigenic carbonates, and pyrite
The core biogeochemical process in cold seep systems is SD-AOM, which generates HCO3− and H2S. These byproducts promote the precipitation of authigenic carbonates and pyrite, significantly altering sedimentary redox conditions and thereby influencing the geochemical cycling of trace elements (Ritger et al., 1987; Peckmann et al., 2001; Sassen et al., 2004; Crémière et al., 2016; Miao et al., 2024b). While bulk sediment analysis provides an integrated record of these processes, the geochemical signatures recorded in specific sediment carriers differ significantly in their formation mechanisms and sensitivity to methane flux (Peckmann et al., 2001; Nöthen and Kasten, 2011; Liu et al., 2024; Dai et al., 2025) (Table 2).
Summary of distinct geochemical signatures and controlling mechanisms in different sediment carriers of cold seep systems.
Geochemical proxy
Authigenic carbonates (MDAC)
Authigenic pyrite
Bulk sediments
Inorganic Carbon (δ13C) & Oxygen (δ18O) Isotopes
δ13C: Highly negative (< −25‰, typically −60‰ to −30‰) due to incorporation of AOM-derived DIC.δ18O: Positive anomalies (e.g., +0.4‰ to +7.87‰) reflecting 18O-rich fluids from gas hydrate dissociation.[Refs: Hesse and Harrison, 1981; Davidson et al., 1983; Ritger et al., 1987; Bohrmann et al., 1998; Aloisi et al., 2000; Peckmann and Thiel, 2004; Ussler and Paull, 2008; Wang et al., 2015; Crémière et al., 2016; Liu et al., 2020; Dai et al., 2025]
—
TIC: Elevated Total Inorganic Carbon (TIC) content with negative δ13CTIC values (< −5‰).Implication: Indicates the presence of dispersed authigenic carbonates within the sediment matrix.[Refs: Peketi et al., 2012; Li et al., 2024]
Sulfur Isotopes (δ34S) & Content
—
δ34S: Highly enriched values (up to +21‰).Mechanism: Closed-system Rayleigh distillation driven by high AOM rates enriches residual porewater in 34S.[Refs: Jørgensen et al., 2004; Borowski et al., 2013; Sun et al., 2019; Chen et al., 2021]
Content: High Total Sulfur (TS) and TS/TOC ratios (> 0.36).Mechanism: Decoupled from organic carbon burial due to excess H2S production from SD-AOM.[Refs: Miao et al., 2022; Liu et al., 2024]
Redox-Sensitive Elements
Enrichment: Mo, U, Ni, V, Cd, Co.Mechanism: Precipitation under anoxic/sulfidic porewater conditions.[Refs: Morford and Emerson, 1999; Algeo and Tribovillard, 2009; Hu et al., 2014; Wang et al., 2019; Smrzka et al., 2020]
Enrichment: Symbiotic Mo, As, and Fe.Mechanism: The “benthic Fe–Mn redox shuttle” transports Mo and As to the sulfidic zone, where they are captured by pyrite.[Refs: Tribovillard et al., 2013; Hu et al., 2014; Large et al., 2017; Miao et al., 2024b]
Mo/U: Co-enrichment (MoEF > 10) indicates compressed redox zones.W/Mn: Depletion (WEF< 2) due to high solubility of thiotungstates and Mn diffusion.[Refs: Erickson and Helz, 2000; Algeo and Tribovillard, 2009; Chen et al., 2016; Miao et al., 2024a]
Metal Isotopes
—
δ98Mo & δ56Fe: Isotopically light signatures (e.g., negative δ98Mo).Mechanism: Fractionation driven by the reductive dissolution of Fe–Mn oxides and subsequent capture by pyrite.[Refs: Goldberg et al., 2009; Fehr et al., 2010; Miao et al., 2024b]
Signal: Bulk measurements often reflect the light isotopic signatures of the authigenic pyrite fraction within the matrix.[Refs: Miao et al., 2024b]
Rare Earth Elements (REE)
Pattern: MREE enrichment (bulge).Ce Anomaly: Variable (positive/negative) depending on redox conditions and fluid source.[Refs: Haley et al., 2004; Himmler et al., 2010; Birgel et al., 2011; Wang et al., 2018; Smrzka et al., 2020]
—
Signal: Often diluted by terrigenous or biogenic components; authigenic REE signals may be less distinct than in pure carbonate phases.[Refs: Hu et al., 2014; Wang et al., 2014; Liu et al., 2020]
Mineralogy/Morphology
Aragonite: High flux (High Sr/Ca).High-Mg Calcite: Low/Diffusive flux (High Mg/Ca).[Refs: Bayon et al., 2007; Li et al., 2024; Dai et al., 2025]
Framboidal: Characteristic of rapid formation under high sulfide flux conditions.[Refs: Peckmann et al., 2001; Merinero Palomares et al., 2012; Yang et al., 2023]
—
Authigenic carbonates: direct records of methane-derived fluids
Unlike bulk sediments, which record a mixed signal of terrigenous input and authigenesis, authigenic carbonates, often referred to as methane-derived authigenic carbonates (MDAC), function as reliable archives of paleo-cold seep activity (Ritger et al., 1987; Bohrmann et al., 1998; Peckmann et al., 2001; Wilfert et al., 2015; Yao et al., 2020). The most diagnostic feature is the extremely negative authigenic carbonate δ13C values (ranging from −60‰ to −30‰), typically ranging from −45.8‰ to −9.7‰ (Wang et al., 2015), or as low as −62.5‰ (Feng et al., 2010; Crémière et al., 2016), which confirm that the carbon source is derived from the anaerobic oxidation of methane (biogenic or thermogenic) (Peckmann et al., 2001; Liu et al., 2020; Dai et al., 2025). Additionally, these carbonates typically exhibit positive δ18O anomalies (e.g., +0.4‰ to +7.87‰) (Liu et al., 2020), reflecting the incorporation of δ18O-rich fluids released during gas hydrate dissociation (Bohrmann et al., 1998; Aloisi et al., 2000; Dai et al., 2025).
The mineralogical composition and elemental ratios of authigenic carbonates are highly sensitive to methane flux intensity (Nöthen and Kasten, 2011). High flux rates favor the precipitation of aragonite, characterized by high Sr/Ca ratios, whereas low or diffusive flux rates favor high-Mg calcite, indicated by high Mg/Ca ratios (Bayon et al., 2007; Nöthen and Kasten, 2011; Chen et al., 2016; Dai et al., 2025).
Regarding REE, authigenic carbonates often display a characteristic MREE bulge (enrichment in middle rare earth elements) (Himmler et al., 2010; Hu et al., 2014; Wang et al., 2018; Zhu et al., 2019), attributed to the reductive dissolution of Fe–Mn oxides in anoxic pore waters (Haley et al., 2004; Rongemaille et al., 2011; Crémière et al., 2016; Smrzka et al., 2020). The Ce anomaly varies with redox conditions: positive anomalies (Ce/Ce* > 1) indicate strongly reducing environments (Feng et al., 2010; Wang et al., 2014; Zhu et al., 2019), while negative anomalies may reflect the influence of oxic seawater or specific diagenetic fluids (Algeo and Tribovillard, 2009; Feng et al., 2009; Birgel et al., 2011; Hu et al., 2014; Wang et al., 2019). Additionally, MDAC exhibit enrichment in redox-sensitive elements such as Mo, U, Ni, V, Cd, and Co (Hu et al., 2014; Wang et al., 2019; Smrzka et al., 2020), further demonstrating precipitation under anoxic conditions (Morford and Emerson, 1999; Algeo and Tribovillard, 2009). Therefore, the unique combination of isotopic, elemental, and mineralogical signatures makes MDAC a high-fidelity recorder, capable of documenting the intensity, duration, and fluid sources of past methane seepage.
Authigenic pyrite: redox and sulfur cycling proxies
Authigenic pyrite is formed when hydrogen sulfide (H2S or HS−) produced by SD-AOM reacts with Fe2+ (Berner, 1984; Ritger et al., 1987; Peckmann et al., 2001). The sulfur isotope (δ34S) composition of pyrite is a key indicator. High δ34S values (up to +21‰) (Peketi et al., 2012; Li et al., 2016) suggest that the sulfate consumption rate exceeded the sulfate diffusion rate in a relatively closed system, leading to the enrichment of 34S in the residual pore water pool, which is indicative of elevated methane flux (Jørgensen, 1979; Peketi et al., 2012; Borowski et al., 2013). Conversely, extremely negative δ34S values (down to −51.0‰) (Peketi et al., 2012; Sun et al., 2019) can occur in environments with weak methane leakage, resulting in large sulfur isotope fractionation similar to that observed in OSR (Li et al., 2016). An abnormal enrichment of authigenic pyrite and high pyrite content (e.g., 1.77 to 10.45 wt%) (Miao et al., 2024b) coupled with 34S enrichment is considered an effective indicator of methane seepage events (Peckmann et al., 2001; Jørgensen et al., 2004).
Pyrite also acts as a primary carrier for trace elements and other isotopes. Mo and As are often enriched and incorporated into the sediment concurrently with pyrite precipitation (Wang et al., 2019; Smrzka et al., 2020; Lin et al., 2021). This symbiotic enrichment is frequently linked to the “Fe–Mn shuttle process”, which releases Fe, Mo, and As from Fe–Mn oxyhydroxides undergoing reductive dissolution in the deep euxinic environment (Tribovillard et al., 2013; Hu et al., 2014; Lin et al., 2021; Miao et al., 2024b). This process can also influence isotopic signatures: authigenic pyrite may record a relatively light Fe isotopic composition (δ56Fe depletion) due to the reactivation of Fe (Miao et al., 2024b), and sediments may display a light δ98Mo signature due to the release of light δ98Mo from the reductive dissolution of Fe–Mn oxyhydroxides (Goldberg et al., 2009; Miao et al., 2024b).
Bulk sediments: integrated environmental signals
The geochemical signature of bulk sediment integrates the influence of SD-AOM on the sediment column and surrounding pore water (Liu et al., 2020). Methane seepage creates a strongly sulfidic environment, leading to the strong enrichment of Molybdenum (Mo) (MoEF > 10) (Algeo and Tribovillard, 2009; Chen et al., 2016; Miao et al., 2024b). A high Mo/W molar ratio (> 10) serves as a proxy for sulfidic environments characteristic of methane seepage, as Mo is sequestered as thiomolybdates (MoS42−) (Erickson and Helz, 2000) while Tungsten (W) forms thiotungstates (WOnS4-n2−) which are less readily buried, leading to W depletion (WEF< 2) (Miao et al., 2024a). Additionally, Uranium (U) shows moderate to strong co-enrichment with Mo (Algeo and Tribovillard, 2009; Hu et al., 2015; Liu et al., 2020), suggesting that high methane fluxes compressed the suboxic (U enrichment zone) and sulfidic (Mo enrichment zone) zones into a narrow region (Morford and Emerson, 1999; Algeo and Tribovillard, 2009; Chen et al., 2016). Conversely, Manganese (Mn) is typically depleted due to the reductive dissolution of Fe–Mn oxyhydroxides and subsequent upward diffusion (Calvert and Pedersen, 1993; Goldberg et al., 2009; Pälike et al., 2014; Miao et al., 2024b).
Regarding bulk content, methane seepage environments feature elevated total sulfur (TS) content and a high TS/TOC ratio (Li et al., 2018; Miao et al., 2024a), deviating from normal marine ratios (Berner, 1984) due to the additional H2S production from SD-AOM (Peketi et al., 2012; Miao et al., 2024b). Furthermore, elevated total inorganic carbon (TIC) content and negative δ13CTIC values (Liu et al., 2020; Miao et al., 2024a; Dai et al., 2025) indicate the presence of authigenic carbonates derived from AOM within the bulk sediment matrix (Li et al., 2024).
Summary
Gas hydrate-bearing and cold seep sediments exhibit distinctive geochemical characteristics, primarily driven by sulfate-methane interactions. These characteristics provide insights into both redox states and the temporal evolution of seepage activity.
Globally, seepage-related carbonates and sediments are enriched in trace and rare earth elements (REE), acting as elemental reservoirs. Hypoxia is a common feature, as reflected by the enrichment of redox-sensitive elements (e.g., Mo, U, V, Ni, Cu) and shale-normalized REE patterns resembling anoxic pore waters. Nonetheless, localized oxidation events also occur, reflecting the episodic and dynamic nature of methane fluxes. Thus, while Mo and U are generally more prominent and reliable indicators, the degree of enrichment of all trace elements depends on local circumstances. In some cases, proxies such as negative Ce anomalies or lower U/Th, V/Cr, Ni/Co, and V/(V+Ni) ratios may signal temporary oxygenation.
Reconstructing the history of seepage requires tracing SMTZ migration. Indicators include the barite front, Mo enrichment, and authigenic carbonate precipitation. Geochemical parameters (e.g., Sr/Ca and Mg/Ca ratios) can further provide additional constraints on seepage intensity. However, dynamic methane seepage often modifies earlier records, for example, through the dissolution of ancient barite fronts below a migrating SMTZ. Therefore, seepage reconstructions typically require integration with pore water ion data and other supporting evidence.
However, generalized interpretations of individual geochemical proxies are often insufficient. This review underscores the necessity of distinguishing among sedimentary carriers—bulk sediments, authigenic carbonates, and pyrite—to achieve reconstructions of methane seepage history. Geochemical signals are differentially partitioned among these phases: authigenic carbonates preserve high-fidelity records of fluid sources and methane flux intensity, whereas authigenic pyrite functions as a major sink for sulfur and chalcophile elements (e.g., Mo, As), recording sulfidization processes and benthic Fe–Mn redox shuttling. Decoupling these carrier-specific signals from the integrated bulk sediment record is essential to avoid misinterpretations caused by signal dilution or overprinting by background sedimentary inputs.
The effectiveness of inorganic geochemical proxies for characterizing methane-rich environments is well-established, but single-proxy approaches are insufficient. Instead, phase-specific multi-proxy frameworks are essential to capture the complex interplay among sediment sources, pore water chemistry, microbial processes, and methane fluxes. Finally, while this review has focused on elemental geochemistry, integrating isotopic approaches with conventional geochemical tools could provide complementary constraints on redox dynamics and methane cycling in future research.
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Anaerobic Methanotrophic Archaea
AOM
Anaerobic Oxidation of Methane
DIC
Dissolved Inorganic Carbon
EF
Enrichment Factor
Fe-AOM
Iron-Dependent Anaerobic Oxidation of Methane
HREE
Heavy Rare Earth Elements
LREE
Light Rare Earth Elements
MDAC
Methane-Derived Authigenic Carbonate
Mn-AOM
Manganese-Dependent Anaerobic Oxidation of Methane
MnR-AOM
Manganese Reduction-Coupled Anaerobic Oxidation of Methane
MnR-S-AOM
Manganese Reduction-Coupled Sulfur-Dependent Anaerobic Oxidation of Methane
MO
Methane Oxidation
MOB
Aerobic Methane-Oxidizing Bacteria
MOG
Methanogenesis
MREE
Middle Rare Earth Elements
MSR
Microbial Sulfate Reduction
OSR
Organoclastic Sulfate Reduction
PAAS
Post Archean Australian Shale
REE
Rare Earth Elements
SD-AOM
Sulfate-Driven Anaerobic Oxidation of Methane
SMI
Sulfate-Methane Transition Interface
SMTZ
Sulfate-Methane Transition Zone
SR
Sulfate Reduction
SRB
Sulfate-Reducing Bacteria
SWI
Sea-Water Interface
TGHSZ
The Top of Gas Hydrate Stability Zone
TIC
Total Inorganic Carbon
TOC
Total Organic Carbon
TS
Total Sulfur
Edited by: Khan M. G. Mostofa, Tianjin University, China
Reviewed by: Xiting Liu, Ocean University of China, China
Debbrota Mallick, University of Georgia, United States