Pore-water geochemistry in methane-seep sediments of the Makran accretionary wedge off Pakistan: Possible link to subsurface methane hydrate

1. Introduction

Cold seeps associated with natural gas hydrate are common and unique geological features, which are widely observed on both active and passive continental margins (Kvenvolden, 1993; Milkov, 2004; Skarke et al., 2014; Mau et al., 2017) and characterized by strong spatial and temporal variations in fluid flux. In these systems, methane-rich fluids pass through the sediment-seawater interface and discharge into seawater; ultimately, a fraction of the methane potentially reaches the atmosphere (Boetius and Wenzhöfer, 2013; Feng, et al., 2018; Ceramicola et al., 2018). Although seeps are heterogeneous both in time and space, they are considered to be hotspots of element cycling on Earth, representing an area that is typified by various biogeochemical processes, and thus commonly sustaining unique oasis-type ecosystems at the seafloor (Dickens, 2003; Boetius and Wenzhöfer, 2013; Suess, 2018; Crémière et al., 2016).

Geochemical composition of pore water provides the most straightforward and important signals for seafloor cold seep fluid activities and the biogeochemical processes associated with sulfate consumption. When the upward methane driven by buoyancy and pressure gradient meets the downward diffusion of sulfate, methane is consumed via anaerobic oxidation of methane (AOM: ${\rm{CH}}_4 $+${\rm{SO}}_4^{2-} $→ $ {\rm{HCO}}_3^-$+ ${\rm{HS}}^- $+ ${\rm{H}}_2{\rm{O}}$) (Boetius et al., 2000; Reeburgh, 2007). Generally, AOM consumes up to nearly 90% of the total methane generated in the sediment, which reduces the release of methane to the hydrosphere and atmosphere (Reeburgh, 2007; Borowski et al., 1996). Besides that, organoclastic sulfate reduction (OSR: $2{\rm{CH}}_2{\rm{O}}$+${\rm{SO}}_4^{2-}$ →$2{\rm{HCO}}_3^-$+${\rm{H}}_2{\rm{S}}$) also profoundly contributes to dissolved sulfate consumption and has a pivotal effect on the early diagenesis processes of marine sediments (Berner, 1980). Both AOM and OSR increase the concentration of dissolved inorganic carbon (DIC) concentration and the alkalinity of pore waters (Boetius et al., 2000). However, the DIC derived from AOM usually has an anomaly depleted ¹³C feature, which is obviously different from OSR (Haese et al., 2003; Mazumdar et al., 2014; Solomon et al., 2014; Hu et al., 2015). Therefore, the changing concentrations of sulfate (${\rm{SO}}_4^{2-} $), DIC, as well as the δ¹³C of DIC are diagnostic indicators to distinguish the relative contributions of AOM and OSR (Masuzawa et al., 1992; Chen et al., 2010; Solomon et al., 2014; Feng et al., 2018). In addition, these reactions in cold seeps, including AOM, OSR, precipitation of authigenic carbonate, etc., changes in the downcore profiles of pore-water species, i.e., ${\rm{SO}}_4^{2-} $, CH₄, DIC, TA, Ca²⁺, etc. Thus, pore-water geochemistry is of great importance in understanding and identifying the complex biogeochemical processes acting in cold seep.

Methane seepages and gas emissions are pervasive and vigorous on the continental slope of the Makran accretionary wedge off Pakistan (von Rad et al., 1996, 2000; Delisle and Berner, 2002; Fischer et al., 2009, 2013). First evidences of anomaly gas concentrations in sediments were found during seismic investigations of bright spots (White, 1977). Then, high-resolution multichannel seismic investigation indicates that bottom simulating with reversed polarity (BSR) compared with the seafloor reflection marking the lower boundary of the gas hydrate stability zone were identified at a depth of approximately 500–800 m below sea floor throughout the continental slope (Minshull and White, 1989; von Rad et al., 2000). Furthermore, high methane concentration anomalies in water column, seep-associated authigenic carbonate, as well as chemosynthetic clams were discovered in the 1990s (von Rad et al., 1996, 2000). Additionally, some works on chemical compositions of pore water and the methane seepages activity in the cold seeps were conducted (Fischer et al., 2009, 2011). However, besides cold seepage on the continental slope (von Rad et al., 1996, 2000), a series of mud volcanoes were discovered on the shelf close to the coast (Delisle et al., 2002), on land (Ellouz-Zimmermann et al., 2008; Kassi et al., 2014), and seaward of the Makran accretionary wedge (Wiedicke et al., 2001). Therefore, it is ideal area for exploring the methane seepages activity and the associated biogeochemical processes.

Here, this study presented the pore water geochemistry characteristic of a piston core (S3) collected from the Makran accretionary wedge off Pakistan. Geochemical characteristics of the chemical constituents (Cl⁻, ${\rm{SO}}_4^{2-}$, ${\rm{NH}}_4^+$, I⁻, K⁺, Na⁺, Mg²⁺, Ca²⁺, Sr and DIC) in combination with δ¹³C_DIC were determined to characterize the methane seepages and methane-related biogeochemical processes in this area. Furthermore, an isotope mass balance model for DIC and δ¹³C_DIC was used to discuss the methane sources and its association with the local gas hydrate system.

2. Geological setting

The accretionary wedge of the Makran subduction zone was formed when thick sediments were scraped from the Arabian Plat oceanic crust that was subducted beneath the continental Eurasian Plate. Subduction and accretion have probably been initiated since approximately the Paleocene (Platt et al., 1988) and Eocene (Byrne et al., 1992), respectively. The modern Makran accretionary prism developed in the Late Miocene (Platt et al., 1985, 1988).

The accretionary wedge is characterized by an extremely low angle of subduction (<2°), moderate convergence rate (~4 cm/a), ultra-width (>500 km) of the accretionary complex, and a remarkably thick sediment (>7 km) as a result of high terrigenous sedimentation (~0.2 m/ka to <1 m/ka) (White, 1982). The extension of the total slightly arcuate Makran accretionary wedge is about 800 km from east to west (Pakistan-Iran). The submarine segment of the accretionary complex has formed the convergent Makran continental margin off Pakistan, which is characterized by a narrow shelf (generally <25 km wide) and a steep continental slope (~ 90 km wide), and leaded to the Oman abyssal plain. A series of frontally accreted imbricated thrust slices cut by erosive submarine canyons developed on the continental slope, and expressed as long, narrow, and steep accretionary ridges separated by ponded slope basins (Kukowski et al., 2001).

3. Sampling and methods

3.1 Sampling

A piston core (S3) was collected near the seep site Flare-3 reported by Römer et al. (2012) in the Makran accretionary area off Pakistan during the comprehensive environmental geologic survey of R/V Shiyan 3 in December 2017 (Fig. 1). The core length is 365 cm. The pore water extraction was conducted at 5−20 cm intervals immediately after sediment core retrieval onboard using Rhizon samplers (Seeberg-Elverfeldt et al., 2005) with the aperture size of 0.2 μm. Pore water subsamples for the concentrations and isotope analysis of DIC were poisoned with saturated HgCl₂. All the pore water subsamples were stored at 4°C until further analysis.

Figure  1.  Location of the sampling site at the Makran accretionary wedge off Pakistan (a), nests of living Bathymodiolus mussels at Flare 3 (Bohrmann, 2007) (b), and dead calyptogena clams scattered on the sea floor (Bohrmann, 2007) (c).

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3.2 Methods

The analyses of dissolved ion and stable isotopes were performed in the State Key Laboratory of Environmental Geochemistry, Third Institute of Oceanography (TIO), Ministry of Natural Resources (MNR, China). Before the analyses of the ion, the pore waters were diluted 1:100 with ultrapure water and filtered with 0.22 μm filter membrane. The dissolved ions concentrations of Cl⁻, ${\rm{SO}}_4^{2-}$, Br⁻, Mg²⁺, Ca²⁺, K⁺ and ${\rm{NH}}_4^+$ in pore water were measured by ICS-1100 ion chromatography (Thermo Fisher Scientific, USA). The precision of replicate measurements was ±2% (n=5). Dissolved trace elements (Ba, Sr and I) were analyzed using high resolution Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The analytical precisions were <5% for dissolved Ba and Sr, and <2% for I. Total alkalinity was measured by titration using Bruevich’s method (Ivanenkov and Lyakhin, 1978), with the precision <2%.

Analyses of stable oxygen and hydrogen isotope compositions of pore water were determined by a high-temperature pyrolysis system directly-coupled to an isotope ratio mass spectrometer (Gehre and Strauch, 2003). Stable oxygen and hydrogen isotope compositions were calculated in δ-notation in per mill relative to the V-SMOW standard. The analytical precisions were ±0.4‰ for δ¹⁸O and ±1.5‰ for δD.

The DIC concentrations and its stable carbon isotope were determined by continuous flow-isotope ratio mass spectrometer (CF-IRMS; Gasbench-Mat253, Thermo Fisher, USA). About 0.5 mL pore water sample was acidized with H₃PO₄. After 4 h reaction, the produced CO₂ was separated through a gas chromatographic column and transferred to a mass spectrometer for δ¹³C determination. Stable isotopic values were presented with the Vienna-Pee Dee Belemnite (V-PDB) standard. The analytical precision was ±0.1‰ for δ¹³C_DIC.

4. Results

4.1 Geochemical constituents of pore water

Depth profiles of Cl⁻,$ {\rm{SO}}_4^{2-} $, ${\rm{NH}}_4^+ $, I⁻, Ca²⁺, Mg²⁺, Sr, Ba, and total alkalinity are shown in Fig. 2. The concentration of the conservative element Cl⁻ generally showed constant seawater-like values in the upper layer, and obvious fluctuations varying between 354 mmol/L and 574 mmol/L in the underlying layer. Sulfate concentrations linearly decreased with depth from near seawater-like values to the minimum value of 2.8 mmol/L at the bottom (Fig. 2). The profiles of alkaline earth elements (Mg²⁺ and Ca²⁺) presented similar trend with sulfate, linearly decreased with the depth, and significant depletion of Ca²⁺ at the bottom of the core was noted with the minimum concentration of 1.4 mmol/L. In contrast, concentrations of ${\rm{NH}}_4^+$ and the alkalinity in sediment increased with depth, and reached maximum values at the bottom, with the range from 0.7 mmol/L to 1.4 mmol/L for ${\rm{NH}}_4^+$ and 4.3 mmol/L to 15.19 mmol/L for alkalinity.

Figure  2.  Concentrations of major components in pore water of the Makran accretionary wedge off Pakistan.

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4.2 Profiles of DIC concentration and δ¹³C

The profiles of DIC concentration and δ¹³C_DIC are presented in Fig. 3. The DIC concentration generally increased from 2.2 mmol/L at the uppermost part of the core to 7.59 mmol/L at 365 cm below sea floor. The δ¹³C_DIC decreased with the depth, from −10.5‰ to −31.0‰.

Figure  3.  Depth profiles of DIC concentration and δ¹³C_DIC in Core S3.

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4.3 Stable oxygen isotopes composition of pore water

The downcore profile of the δ¹⁸O is shown in Fig. 4. The δ¹⁸O showed a variation range between −4.5‰ and −0.4‰, with prominent increase in the interval of about 290-310 cm below sea floor, corresponding to the decrease of Cl⁻.

Figure  4.  Depth profiles of Cl⁻ concentration and δ¹⁸O in the pore water of Site S3.

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5. Discussion

5.1 Potential occurrence of gas hydrate

It is believed that the appearance of BSR can represent the base of the gas hydrate stability zone as a result of the acoustic impedance contrast between the overlying deposited gas hydrate and the underlying free gas layer (Shipley et al., 1979; Hyndman and Spence, 1992; Mazumdar et al., 2014). It can also occur due to the occurrence of small amounts of free gas below the gas hydrate stability zone with negligible presence of gas hydrate (Hyndman and Spence, 1992). The existence of high amplitude reflectors beneath the BSR reveals the presence of gas-charged sediments. According to previous seismic studies, the BSRs are widespread on the Makran accretionary wedge (Minshull and White, 1989; Minshull et al., 1992; von Rad et al., 2000; Sain et al., 2000; Shoar et al., 2014). In addition, the presence of gas hydrates and free-gas across the BSR in the Makran offshore are confirmed by the seismic characteristics, such as the blanking, instantaneous frequency and reflection strength (Ojha and Sain, 2009), and the velocity anomaly (Sain et al., 2000). In this study, the well-developed BSR with obviously reversed polarity was present in the seismic profile (Fig. 5), which may be related to the base of the gas hydrate stability zone and may define the phase boundary between gas hydrate-bearing sediments. In other words, there may be natural gas hydrate underlying/near our sampling site. In addition, several high angle fault rupture disrupting the overburden sediment can be clearly observed in the seismic reflection profiles (Fig. 5). Therefore, fluid associated with dissociation of gas hydrate may migrate through fracture to the shallow sediment. This assumption was supported by pore water anomalies as explained in the following.

Figure  5.  Multi-channel seismic data in the studied area just beneath the sampling site highlighting the presence of bottom simulating with reversed polarity (BSR) in the study area. Red lines represent the high angle faults.

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Low chloride fluids at Site S3 (Fig. 4) in comparison with seawater are observed at the lower part (between 290 m and 310 m below sea floor), corresponding with positive peak in and δ¹⁸O (Fig. 4). That can be attributed to the dissociation of gas hydrate and dehydration of clay minerals. However, the reaction clay mineral dehydration can be excluded since the feature of typical clay mineral transformation, i.e., Na⁺ enrichment and K⁺ depletion (Hesse, 2003), was not observed in Core S3 (Fig. 2). Cl⁻ concentrations and δ¹⁸O anomalies in pore water have been reported in a number of gas hydrate-bearing sediments, e.g., the Blake Ridge (Borowski, 2004), the Nankai Trough (Toki et al., 2004), the Cascadia margin (Egeberg and Dickens, 1999; Torres et al., 2004), the Barbados Ridge (Kastner et al., 1990), the Black Sea (Reitz et al., 2011) and the South China Sea (Luo et al., 2014). Due to the salt exclusion and preferentially retain the heavier isotope ¹⁸O in gas hydrates when they crystallize, the pore fluids could be additionally diluted by fresh water when gas hydrate dissociation. That result in the excursion of Cl⁻ concentration from baseline accompanied by the increase of pore water δ¹⁸O (Egeberg and Dickens, 1999; Hesse, 2003; Ussler and Paull, 1995; Wallmann et al., 2006; Torres et al., 2004; Wu et al., 2013). Previous studies found that gas hydrates are occurred widely in Makran accretionary wedge off Pakistan, even at the depth of a few centimeters at Site Flare 2 (Bohrmann, 2008) which was not far from our station. Therefore, this study attributes the pore water anomalies to gas hydrate dissociation, which mean that gas hydrate may present underlying or near the studied sedimentary section.

Additionally, this study found that the δ¹⁸O at Site S3 showed relatively negative values, which also has been found at ODP Site 1146 in the South China Sea and Nankai accretionary prism (Toki et al., 2014; Zhu et al., 2006). Besides that, Himmler et al. (2015) documented anomalously low δ¹⁸O in the seep carbonates in the oxygen-minimum zone of the Makran accretionary wedge off Pakistan, which were attributed to ¹⁸O-depleted fluids deriving the from formation of gas hydrate. Similarly, Site S3 is also situated in the oxygen-minimum zone with complicated deposition environment and vigorous seismicity (Fischer et al., 2013). Therefore, this study infers that the negative δ¹⁸O background value may have been affected by the formation of gas hydrate or a complex fluid, but this need to be further studied.

5.2 Sulfate reduction of Makran accretionary wedge

In marine sediments, downward diffusing sulfate is considered as the major acceptors for the anaeraobic oxidation of methane and organic matter through microbially mediated processes: OSR (Berner, 1980) and AOM (Reeburgh, 1976; Boetius et al., 2000). Both of the sulfate reduction processes result in an enhanced DIC accumulation in pore water. For Core S3, the sulfate concentrations rapidly decrease with depths, and the total alkalinity values increase steadily down depth and reach maximum at the bottom of the core (Fig. 2). This indicates the intensive microbial activities and actively sulfate reduction in the sediments.

${\rm{NH}}_4^+ $ in pore water is the available candidate to evaluate the decomposition of organic matter, as their concentrations depend on the amount of buried organic matter and its degradation rate (Muramatsu and Wedepohl, 1998; Graca et al., 2006; Tomaru et al., 2009). ${\rm{NH}}_4^+ $ in pore water is produced via decomposition of the nitrogen-bearing organic matter, which has been documented to be stoichiometrically related to sulfate reduction (Henrichs and Reeburgh, 1987; Berner et al., 1970). In this study, ${\rm{NH}}_4^+$ in sediment near-linearly increased with the depth, and ranged from 0.7 mmol/L to 1.4 mmol/L, pointing to relatively high rate degradations of organic matter. The cross-plots of the concentrations of ${\rm{SO}}_4^{2+}$ versus ${\rm{NH}}_4^+$ yield correlation coefficient of 0.53 for Site S3 (Fig. 6), similar to others sites of the cold seeps in the ocean, where sulfate reduction is associated with organic matter degradation (Ye et al., 2016; Yang et al., 2013; Reitz et al., 2011; Mazumdar et al., 2014; Xu et al., 2018). On the other hand, relatively higher ${\rm{NH}}_4^+$ than the upper part probably indicates that there may be a source from the organic matter degradation in the depth sediments rich in labile organic matter beneath the core (i.e., ex-situ origin; Mazumdar et al., 2014; Xu et al., 2018).

Figure  6.  Cross-plot of sulfate versus ammonium. A comparison was made among different locations of the other seas. The correlation coefficients were labeled closed to the fitting lines, together with corresponding annotations. Data for eastern Black Sea 11938 was taken from Reitz et al. (2011); Mahanadi Basin MD161-19 from Mazumdar et al. (2014); East China Sea from Xu et al. (2018).

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However, the quasi-linear shape sulfate profile and high alkalinity reflected a high AOM rate associated with high diffusive methane fluxes (Fig. 2; Borowski et al., 1996, 2000; Yang et al., 2013). Accordingly, the ratios of corrected produced DIC (ΔDIC+ΔCa²⁺+ΔMg²⁺) and reduced sulfate (Δ${\rm{SO}}_4^{2-}$) in pore water are usually used to evaluate the relative contributions of OSR and AOM (Masuzawa et al., 1992; Chen et al., 2010; Hu et al., 2015). Data obtained from Core S3 between slopes of 2:1 and 1:1, with slope of 1.23:1 (Fig. 7), suggested that sulfate reduction is contributed from a mixture of both OSR and AOM (Masuzawa et al., 1992). Also, AOM is the predominant biogeochemistry process for sulfate reduction with the contributions of ~77% in this area, whereas the contributions of OSR to sulfate reduction are ~23%. Additionally, the depleted ¹³C_DIC (−37.0‰V-PDB) (Fig. 3) at 320 cm supports this observation.

Figure  7.  Plot of sulfate concentration consumed versus DIC concentration produced and corrected for loss of Ca²⁺ concentration and Mg²⁺ concentration caused by authigenic carbonate precipitation. Diagonal lines indicate 1:1 and 2:1 ratios of DIC produced to sulfate consumed.

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5.3 Methane flux and sources

It was mentioned above that the linear ${\rm{SO}}_4^{2+}$ concentration gradient and (ΔDIC+ΔCa²⁺+ΔMg²⁺)/Δ${\rm{SO}}_4^{2-}$ ratio allow us to identify the contribution of AOM, which is driven by upward migration of methane. Generally, the depth of sulfate-methane interface (SMI) is a typically indicator to imply relative magnitude of methane flux, with shallower SMI depths is corresponding to higher methane fluxes (Haese et al., 2003; Luff and Wallmann, 2003). At Site S3, the steep and near-linear sulfate concentration profile with the shallower SMI depth of ~4 m below sea floor, pointed to relatively high AOM rate with large upward methane flux. However, methane flux is difficult to be quantified by pore water methane concentration due to degassing of methane during core recovery. Nevertheless, it can be inferred from the flux of sulfate diffusing from seawater due to the 1:1 stoichiometric of methane and sulfate for AOM (Wallmann et al., 2006; Chuang et al., 2013; Luo et al., 2013; Mazumdar et al., 2014). The sulfate diffusive flux is estimated according to Fick’s first law assuming steady state conditions (Berner, 1980):

where J refers to the diffusive flux (mol/(m^2·a)), φ refers to the porosity (this study assumed the porosity in our study to be 0.7) (Fischer et al., 2013), D_s refers to the sediment diffusion coefficient (m²/s), D₀ refers to the seawater diffusion coefficient, which can be calculated using equation presented by Boudreau (1997) with the bottom water temperature of 5°C. Sea-water sulfate D₀ at 5°C was 5.72×10⁻¹⁰ m²/s (Schulz, 2006). C and x indicate the sulfate concentration (mmol/L) and the sediment depth (m), respectively.

In this area, almost all of the upward-methane is consumed by AOM, which accounting for 77% of sulfate reduction; thus, the calculated methane flux is 77% of sulfate diffusive fluxes, which is equivalent to nearly 0.039 mol/(m²·a). However, compared with the flux from the Hydrate Site (Fischer et al., 2013) and the lower slope at the Makran accretionary wedge with a deeper SMI depth (ca. 6 m below sea floor) (unpublished data), it is higher than that. Additionally, compared with that from gas hydrate regions around the world, methane flux in the cold seep is notably higher than those in the northern South China Sea (0.014−0.035 mol/(m²·a) (Luo et al., 2013; Hu et al., 2015), and the Blake Ridge (0.02 mol/(m²·a), Borowski, 2004), whereas being lower than those from pockmarks and mud volcanos (Chen et al., 2010; Haese et al., 2003).

In general, methane is produced by two mechanisms in the marine sediments: microbial methane generated by CO₂-reduction or acetate fermentation, and thermogenic methane by thermal cracking of organic matter (Sackett, 1978; Whiticar, 1999). Thermogenic methane with the δ¹³C more positive than −50‰ (Whiticar, 1999) is generally, but not exclusively, enriched in ¹³C compared with bacterial methane (more negative than −60‰; Whiticar, 1999). Since δ¹³C values of methane in Site S3 are unavailable, pore water DIC concentration and its δ¹³C_DIC from the sulfate-methane transition zone (SMTZ) can be used to as an indicator for the source of methane (Kastner et al., 2008). Three possible sources of pore water DIC in marine sediments within the SMTZ are primarily included: (1) DIC diffusing from bottom seawater or seawater DIC being trapped within the sediment (SW); (2) generated via the OSR; (3) source from AOM. At SMTZ, methane is completely consumed, and carbon isotopic of DIC derived from AOM is the same as the methane oxidized. In order to distinguish sources of the pore water DIC in Site S3, this study use the mass balance model as follows (Borowski et al., 2000; Chen et al., 2010):

where X refers to the proportion of DIC contributed to the entire DIC pool, δ¹³C refers to the carbon isotopic constitution, and the subscripts AOM, SW and OM refer to DIC derived from the sources discussed above. This study estimated that the value of X_SW was 26.0% with δ¹³C_SW of 0‰V-PDB. The δ¹³C of sedimentary organic matter in Core S3 (−20.6‰V-PDB; unpublished data) was used for the δ¹³C_OM. The contributions estimated from DIC and sulfate stoichiometries were 57.0% for AOM, 17.0% for OM, and 26.0% for SW. In addition, the linear regression δ¹³C×DIC concentration versus DIC concentration has already been used to obtain DIC stable isotopic compositions added into the pore water DIC pool previously (e.g., Borowski et al., 2000; Ussler and Paull, 2008; Hu et al., 2018), thereby the calculated δ¹³C_add is −33.1‰V-PDB (Fig. 8). Accordingly, the estimated δ¹³C_methane in the Core S3 was −57.9‰V-PDB, indicating that the extraneous methane consumed by AOM has a mixed source including thermogenic and biogenic methane herein.

Figure  8.  The calculation of δ¹³C_add using the linear regression of δ¹³C×DIC versus DIC concentration at Site S3.

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5.4 Precipitation of authigenic carbonates

Precipitation of authigenic carbonate is one of the striking phenomenon in cold seep system with fluid discharge, which can be observed by the consumption of pore water Ca²⁺, Sr²⁺ and Mg²⁺ in shallow sediments (Gontharet et al., 2007; Cangemi et al., 2010; Haas et al., 2010; Hu et al., 2018; Luo et al., 2013; Nöthen and Kasten, 2011; Snyder et al., 2007). In Site S3, the concentrations of pore water Ca²⁺ and Mg²⁺ decreased significantly with the depth (Fig. 2), implying that authigenic carbonate may precipitate in the sediment. However, the concentration ratio of Mg/Ca increased from 6.6 in the near-surface sediment to approximately 40 at the bottom of the core (Fig. 9), which indicate that high Mg/Ca carbonate mineral phase are precipitating. As presented in Fig. 10, weight ratios plot of pore water Sr/Ca and Mg/Ca in Site S3 are close to or fall on the composition line of high Mg-calcite, indicating that high Mg-calcite in the sediment is the predominant precipitation of authigenic carbonate mineral phase. Generally, aragonite is often observed to form preferentially in the environments with high sulfate concentrations and high fluxes of methane-rich fluids near the seafloor, whereas high Mg-calcite and dolomite precipitation may dominantly occur in the sediment within or beneath the SMI (Mazzini et al., 2006; Bayon et al., 2007). In contrast, our study found that high Mg-calcite precipitation has taken place in the shallow sediment of Site S3 above the SMI. This scenario suggests the downward shift of SMI due to the decreased upward methane flux, and may point to comparatively low activity seepage environment (methane diffusive flux, nearly 0.039 mol/(m²·a)) relative to the advection-dominated transport of methane.

Figure  9.  Molar concentration ratios of Mg/Ca and Sr/Ca in Core S3. Both Mg/Ca and Sr/Ca ratios increase with depth obviously.

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Figure  10.  Plot of Sr/Ca versus Mg/Ca (weight ratios) in pore waters. The Sr/Ca and Mg/Ca ratios of the precipitating aragonite and high Mg-calcite as well as seawater were taken from Bayon et al. (2007) and Nöthen and Kasten (2011).

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6. Conclusions

Analysis of pore water geochemical characteristics of piston core (S3) sampled from shallow sediments at the Macron accretionary wedge off Pakistan revealed that the sulfate is consumed by both OSR and AOM. Proportion of sulfate consumed by the AOM is approximately 77%, which predominantly controls the sulfate concentration profiles. The near-linear sulfate profile, shallow SMI (~4 cm below sea floor) and methane flux toward the SMTZ (0.039 mol/(m²·a)) revealed the gas seepage activity in the study area. The calculated δ¹³C of the external methane in Site S3 was to be −57.9‰ based on the mass balance model of DIC, indicating that the methane consumed by AOM are most likely a mixed source, including thermogenic and biogenic methane. The weight ratios of pore water Sr/Ca and Mg/Ca in Site S3 suggested that the predominant precipitation of authigenic carbonate was high Mg-calcite, which may point that this area is a relatively low-activity seepage environment, which is in accordance with the estimated methane diffusive flux.

Acknowledgements

We thank the captains and crew of the R/V Shiyan 3 for their assistance in recovering the samples during the integrated environmental and geological expedition on the Macron accretionary wedge in December 2017.