Timing of Seep Activities and Potential Driving Forces

Abstract

One of the foremost topics in seep research is the timing of seep activities and their potential driving forces. In the South China Sea, seep activities are primarily driven by gas hydrate dissociation–destabilization of gas hydrate leads to a release of methane. As decreases in pressure and increases in temperature promote gas hydrate dissociation, ocean warming and sea level lowstands are proposed to cause the dissociation of gas hydrate deposits and consequently induce methane seepage at the seafloor. Cross-slope investigations suggest that the bottom water pressure‒temperature conditions appear to have different impacts on seeps at different water depths. It is possible that seepage in the upper continental slope is more sensitive to sea level changes than that in the middle and lower continental slopes, which are more sensitive to bottom water temperature. Scientific drilling and the application of a range of geochemical and geophysical analytical approaches are proposed to advance our understanding of the temporal evolution of seep systems in the South China Sea.

12.1 Introduction

Understanding methane seepage dynamics in the past and present and the associated drivers and trigger mechanisms can be key for predicting methane emission scenarios in future oceans impacted by climate change (Boetius and Wenzhöfer 2013; Ruppel and Kessler 2017; Yao et al. 2020). Cold seeps emit materials into the oceans and leave characteristic footprints close to the seafloor, for example, extensive buildups of methane-derived authigenic carbonate minerals and a mass occurrence of seep-specific bivalves (Suess 2020). These footprints provide an excellent archive of past seepage and associated environmental parameters (Aharon et al. 1997; Bohrmann et al. 1998; Teichert et al. 2003; Bayon et al. 2009a, b, 2013, 2015; Feng et al. 2010; Chen et al. 2019; Himmler et al. 2019). Constraints on the parameters that control seepage dynamics can be obtained by dating seep carbonates and bivalve shells.

The South China Sea (SCS) is a natural laboratory for studying the development and evolution of methane seepage systems (Tong et al. 2013; Han et al. 2014; Feng and Chen 2015; Liang et al. 2017; Yang et al. 2018; Chen et al. 2019; Wei et al. 2020; Deng et al. 2021; Kunath et al. 2022; Wang et al. 2022), particularly the common trends across multiple geographic areas and the connection between gas hydrate dissociation and methane seepage dynamics (Feng et al. 2018). This chapter summarizes the materials and methods that have been used to constrain the timing of seepage in the SCS (Fig. 12.1) and explores how dating results can be exploited to study melt seepage activities under glacial-interglacial climate changes.

Fig. 12.1

Bathymetric image of the South China Sea showing locations of all hydrocarbon seeps that have been confirmed during the last two decades (yellow dots) (after Feng et al. 2018; Tseng et al. 2023). The locations of carbonate samples with dating results are illustrated (yellow dots with white circles). The white boxes are locations of gas hydrate drilling expeditions (GMGS 1−6) (after Wei et al. 2020)

12.2 Materials and Methods

Methane-derived authigenic carbonate represents an appealing archive of seepage dynamics since carbonate deposits are present throughout geological records (e.g., Hovland et al. 1987; Peckmann and Thiel 2004). Seep carbonates exhibit high fidelity for several other tracers, including the isotopic composition of carbon and oxygen, which can be used to constrain the source of methane-rich fluids and their formation environments and possible connections with gas hydrate dissociation (e.g., Bohrmann et al. 1998; Aloisi et al. 2000; Suess 2020). The systematics of carbonate carbon and oxygen isotope systems are discussed in Chap. 10.

Earlier studies mainly focused on seabed seep carbonates. However, recent studies have started to investigate seep carbonates from hydrate-bearing drill cores, and these can be used to determine past gas hydrate dynamics throughout the Late Quaternary (Fig. 12.2; Tong et al. 2013; Han et al. 2014; Chen et al. 2019; Wei et al. 2020; Deng et al. 2021). While seafloor samples are easy to collect and seep carbonates are dated from the seabed, sampling bias cannot be excluded. Authigenic seep carbonates from drill cores represent a continuous record of gas hydrate dynamics that can help address the sampling bias issue (Fig. 12.3).

Fig. 12.2

Typical morphologies of seep carbonate samples for dating. a A cut and polished surface of a carbonate sample from the seafloor at aite F. The sample was obtained during the ROPOS dive 2045 in 2018. The sample includes abundant cemented shell debris (arrows). The presence of shell fragments of chemosynthetic bivalves in the carbonates suggests that carbonate precipitation occurred close to the seafloor. b A cut and polished surface of a carbonate sample from the drilling site GMGS2-08F (at 61.8 m below the seafloor) in the dongsha area (from Chen et al. 2019). A microcrystalline matrix (yellow circle) usually represents initial seepage‐related precipitation, whereas cavity-filling cement (red circle) typically reflects a later stage (Feng et al. 2010)

Fig. 12.3

Lithology of cores that have been investigated for methane seepage timing (after Chen et al. 2019; Wei et al. 2020). The carbonate thickness is exaggerated for greater visibility

Uranium‐thorium (U‐Th) dating of carbonate offers a unique chronometer to assess the timing and duration of methane seepage on relatively long timescales up to approximately 500,000 years (Himmler et al. 2019; Wang et al. 2022). Most of the data accumulated in the SCS were obtained from applying U-Th techniques to seep carbonates through both solution-based multicollector inductively coupled-plasma mass spectrometry (MC-ICP-MS; Tong et al. 2013; Han et al. 2014; Yang et al. 2018; Chen et al. 2019; Wei et al. 2020; Deng et al. 2021; Wang et al. 2022) and the recently developed in situ laser ablation MC-ICP‒MS methods (Wang et al. 2022). The in situ U-series geochronological method has the potential to date seep carbonates efficiently and reliably. Researchers have directly determined the initial [²³⁰Th/²³²Th] ratios of seep carbonates and yielded a value of 0.7 ± 0.1 (2 SD, n = 12), which should be applied to future seep carbonate U-Th dating (Wang et al. 2022).

Microcrystalline matrix usually represents initial seepage‐related precipitation. However, precise dating of these matrix is difficult because of the high initial ²³⁰Th content due to variable amounts of detrital material and organic matter. In contrast, diagenetic cements in seep carbonates are often pure aragonites that have small uncertainties in their U-Th ages because they contain relatively less contamination. These diagenetic cements tend to reflect the last phase of carbonate precipitation (Fig. 12.2; Feng et al. 2010). In situ laser ablation has the potential to select appropriate regions to date within complex structures of seep carbonates and can obtain measurements efficiently (Table 12.1; Wang et al. 2022). Nevertheless, efforts directed toward applying U-Th dating to seep-specific bivalves proved to be infeasible because fossil mollusks often exhibit uranium contents that are many orders of magnitude greater than those in living shells, indicating uranium uptake after death and hence U-Th open-system behavior (Ayling et al. 2017). Due to the incorporation of methane‐derived fossil carbon upon carbonate precipitation (Paull et al. 1989; Aharon et al. 1997), the utility of radiocarbon dating is limited to shell materials that are cemented in carbonates (Fig. 12.2).

Table 12.1 Comparison of methods for U-Th dating of seep carbonates

Dating of authigenic carbonates preserved in sedimentary records can provide a chronology of past methane release events. This is usually achieved by the direct dating of seep carbonates using the uranium-series and by radiocarbon dating of shell materials cemented in seep carbonates. On the other hand, the timing of seeps can also be determined through dating of the host sediments in ancient sedimentary environments where authigenic carbonate is not present. However, one must assume that a shallow subsurface precipitation environment is present close to the sediment-water interface under extremely high methane flux conditions (e.g., Yao et al. 2020). The depth of the SMTZ may be independent from the stratigraphic chronosequence (Fig. 12.4). Dating the surrounding sediment is not a method for dating authigenic carbonates but a method to establish their maximum age (Yao et al. 2020; Guan et al. 2022). Such a method has been used to reveal methane seep dynamics since the last glacial maximum in the Shenhu area of the SCS (Zhang et al. 2022a, b).

Fig. 12.4

Image illustrating the relationship between the stratigraphic age and age of seep carbonates at high (left, a) and low methane fluxes (right, b). Under extremely high methane flux conditions, the age of precipitated seep carbonates (blue rectangle) is close to that of the stratigraphic chronosequence (yellow dot in the left image). Under extremely low methane flux conditions, the age of precipitated seep carbonates (red rectangle) is close to that of the stratigraphic chronosequence of the subsurface sediments (yellow dot in the right image) but much younger than that of the stratigraphic chronosequence of the neighboring sediments (green dot)

12.3 Methane Seepage and Hydrate Dissociation

From passive margins of the SCS, Tong et al. (2013), Han et al. (2014), and Yang et al. (2018) were the first to report chronology constraints on the timing of seepages across the continental slope of the northern SCS. These studies not only confirmed that the formation of seep carbonates in the northern SCS is inextricably linked to gas hydrates but also provided evidence for extensive seep activity during sea level lowstands, hence suggesting that hydrostatic pressure was the most important factor controlling gas hydrate stability. Recently, Wang et al. (2022) investigated the U-Th geochemistry and geochronology of 31 cold-seep carbonates recovered from the northern SCS by laser ablation in situ mass spectrometric analysis. The new age data suggest that cold seeps were likely continuously active since at least ~72 ka in the northern SCS, but different sites have contrasting durations (Fig. 12.5). The contrasting age spectra of seep carbonates from the upper continental slope of the northern SCS indicate that bottom water pressure‒temperature conditions might yield contrasting stability conditions for methane hydrates at different depths in the same geological setting. Accurate information on the bottom water pressure‒temperature history would thus be necessary to reliably quantify past marine cold seepage flux from the continental margins. While seep carbonate age data provide first-order constraints on past methane leakage activities, more geochemical proxy work is needed to reconstruct the magnitude of methane release in the geological past.

Fig. 12.5

a The seepage duration of cold seeps in the northern South China Sea includes four seafloor cold seep sites (Shenhu, Jiulong methane reef, Haima and Site F) and sediment core GMGS2-08 (Chen et al. 2019; Deng et al. 2021). The yellow circles represent the U-Th ages obtained by in situ U-Th dating by Wang et al. (2022); previously reported data with high [²³²Th/²³⁸U] or δ²³⁴U_ini values that deviate from the seawater value are not included (Wang et al. 2022). The red rectangles represent the U-Th age data of the seep cements from the Jiulong methane reef (Tong et al. 2013; Han et al. 2014; Wang et al. 2022) and GMGS2-08 cores (Chen et al. 2019; Deng et al. 2021). The green triangles represent ¹⁴C dating on shells cemented in the cold seep carbonate samples from Site F (Feng and Chen 2015; Wang et al. 2022), Haima (Liang et al. 2017; Wang et al. 2022) and GMGS2-08 cores (Deng et al. 2021). b Previously reported timing of seep carbonate formation worldwide versus global sea‐level changes (Chen et al. 2019 and references therein). Sea level curve from Rohling et al. (2009)

In a recent U-Th investigation of drilled seep carbonates from the Dongsha area, Chen et al. (2019) suggested that increased bottom-water temperature during Marine Isotope Stage (MIS) 5e caused methane hydrate destabilization at the SCS margin. This finding provided the first direct evidence that methane seepage possibly intensified during full sea level highstands. Additional investigation of deeply buried seep carbonates is required to assess whether this hypothesis holds true for earlier interglacial periods and to further test the relationships that link both methane hydrate stability and seepage intensity to Late Quaternary climate change. The assumption of intensified methane seepage was further confirmed by Deng et al. (2021) using the same sets of samples. Wei et al. (2020) analyzed three seep carbonate samples (3, 52.1 and 53.6 mbsf) recovered from site W08B in the Qiongdongnan Basin, China, and suggested that the formation of the carbonates was induced by the dissociation of gas hydrates. The dissociations took place at 12.2 ka BP and 131–136 ka BP.

12.4 Timescales of Methane Seepage

Knowledge of the timescales of gas hydrate dissociation and subsequent methane release is critical for understanding the impact of marine gas hydrates on the ocean-atmosphere system (Crémière et al. 2016; Ruppel and Kessler 2017; Ruppel and Waite 2020). However, assessments of the timescales over which gas hydrate systems respond to the processes that drive changes in pressure and temperature in the subsurface are limited. Site F and the Haima active methane seeps have been extensively investigated to study seep-impacted sediments, pore fluids, authigenic carbonates, and seep-dwelling faunas. The use of these materials or a combination thereof will ultimately allow us to constrain seepage intensities at different time scales (Feng et al. 2018). Each of these materials has its own validity in revealing the characteristics and mechanisms of seepage (Feng et al. 2018). For example, the geochemical data obtained from the solid fraction of sediments and from authigenic carbonates provide time-averaged information on biogeochemical processes on a timescale of years to centuries (Kiel et al. 2014; Oppo et al. 2020). Sediment pore waters and seep-dwelling fauna, on the other hand, provide information on much shorter timescales, spanning from days to months (cf., Valentine et al. 2005). A relevant case study was conducted by Luo et al. (2015) to estimate the time of pockmark formation in the SW Xisha Uplift of the SCS using reaction-transport modeling. They found that the pockmarks formed at least 39 kyr B.P. and that the termination of fluid seepage may be ascribed to gas hydrate stabilization or to complete depletion after the sea levels reached a relative highstand. The proposed method could be a significant tool for illuminating the temporal evolution of fluid seepage in pockmarks, especially if reliable age control is lacking, because most pockmarks on continental margins are presently dormant. Appropriate modeling tools are critical for establishing links between these multiproxy approaches to improve our understanding of seep systems.

12.5 Summary and Perspectives

This chapter summarizes findings from the first decade of seep geochronology in the SCS, which have revealed a number of global, regional, local, and geological processes that can render different seep dynamics with distinct timings. Since the seep activities of the SCS depend largely on the dissociation of locally abundant gas hydrates, age determination of seeps can be used to glean unique insights into the dynamics of gas hydrate systems, especially those located in the subsurface of marine sediments (Fig. 12.6). Despite recent progress, a number of key uncertainties remain in determining the timing of seeps, and these need to be resolved.

Fig. 12.6

Schematic diagram of the gas hydrate stability zone (GHSZ) extension in marine sediments of the South China Sea. The eight Guangzhou Marine Geological Survey (GMGS) gas hydrate drilling expeditions in the Dongsha, Shenhu, and Qiongdongnan areas are illustrated

Attaining a better understanding of the evolution of seepage through time and its links to gas hydrate reservoirs, seep ecosystems, and the amount of hydrocarbon escaping seabeds remains challenging (Skarke et al. 2014). Recent drilling campaigns and coring programs have contributed greatly to our understanding of gas hydrate dynamics in the SCS and in marine environments worldwide. The time is now ripe to start a dedicated program that targets these drilled cores and uses synergistic modeling–analytical approaches to shed new light on the timing of periods of enhanced seepage and its relationship to the dynamics affecting hydrate reservoirs.

The records of seepage indicators are usually incomplete; for example, many bivalves and carbonate nodules either on the seabed or in the drilling cores have not been dated or included in discussions (Fig. 12.4). In addition, sample-based case studies are mainly focus on longer timescales. For the short term, determining how much methane is being released to the oceans on regional to basin-wide scales remains a central challenge. Observing seafloor methane seeps in various environments is therefore critical.

Hydrocarbon seeps are uniquely prone to perturbation resulting from global change (e.g., Kennett et al. 2000; Ketzer et al. 2020; Kim and Zhang 2022; Weldeab et al. 2022). Focused paleoceanographic studies should also constrain bottom-water temperature changes on the continental slope of the SCS during glacial-interglacial cycles. In addition to continued sample analyses, more sophisticated numerical simulations are required to demonstrate the assumed gas hydrate dissociation effects before researchers hypothesize that seeps could have played a significant role as a climate feedback mechanism during the known large temperature and sea level changes of the Quaternary glacial-interglacial cycles (Li et al. 2023). The next decade of seep chronology is poised to offer many novel insights into the impact of seepage in response to typical tectonic, sedimentary, or climate triggers.