Sulfate-driven Anaerobic Oxidation Of Methane Research Articles

Hydroxylated GDGTs-0 in marine methane seep environments: A putative indicator for archaeal methanogenesis

Marine methane seeps are environments with a high microbial diversity, but are known for one biogeochemical key process, the sulfate-driven anaerobic oxidation of methane (SD-AOM) performed by anaerobic methane-oxidizing archaea (ANME) and sulfate-reducing bacteria (SRB). SD-AOM is also the dominant process at methane seeps in the South China Sea based on the lipid biomarker inventory of authigenic seep carbonates characterized by crocetane, a high sn2-hydroxyarchaeol over archaeol ratio, low contents of glycerol dibiphytanyl glycerol tetraethers (GDGTs), and δ13Clipid-methane offsets of −52 ± 4 ‰. Combined with the dominance of aragonite over other carbonate minerals, such pattern suggests high seepage intensity with a predominance of ANME-2/SRB consortia. Interestingly, the studied seep carbonates also yielded some uncommon biomarkers, which may derive from methanogenic archaea. Methylotrophic methanogenesis has been shown to be the dominant mode of methanogenesis in seep environments where non-competitive substrates like methanol or trimethylamine are abundant. The presence of methylotrophic methanogens is possibly indicated by high contents (more than 50 % of all GDGTs) of hydroxylated GDGTs-0 (OH-GDGT-0 and 2OH-GDGT-0) with extreme 13C-depletion (−128 ‰ to −116 ‰); this unique pattern is recognized in only some of the studied seep carbonates, while other carbonates are dominated by typical distributions of ANME-2 lipids, also comprising GDGTs with 0 to 4 rings, but lacking high contents of OH-GDGTs. The overall lack of crenarchaeol, the specific biomarker of planktonic Thaumarchaeota, agrees with the tentative assignment of the highly abundant OH-GDGTs-0 to methanogenic archaea. Such interpretation is necessarily circumstantial considering that the compound and carbon isotope composition of the membranes of ANMEs and methanogenic archaea is similar. Although production of OH-GDGTs has been previously reported for planktonic Thaumarchaeota and ANMEs, OH-GDGT-0 as sole and highly abundant OH-GDGT has only been recognized in one culture of methanogenic archaea. Therefore, the high abundance of 13C-depleted hydroxylated GDGTs-0 compared to only minor contents of regular, ANME-derived GDGTs 1–4 with similar 13C-depletions can possibly be used as an indicator for methylotrophic methanogenesis in seep environments. Future experimental work is needed and should test if 13C-depleted hydroxylated GDGTs-0 are indeed biomarkers of methylotrophic methanogenesis at seeps.

Isotopically light Mo in sediments of methane seepage controlled by the benthic Fe–Mn redox shuttle process

Methane seepage has been extensively observed in various continental margin settings. It has profound effects on the marine redox environment and the molybdenum (Mo) cycles in marine sediments. Therefore, there has been much recent attention on the redox-sensitive behavior of Mo in methane seepage environments. However, the characteristics of the Mo isotope composition in the cold-seep system remain poorly understood. In this study, we performed geochemical analyses, including Mo content and isotope composition, on sediment samples (core QDN-MS6) from the “Haima” cold-seep deposit area in the South China Sea. The analysis reveals a significant concentration of authigenic pyrite in the mid-section of QDN-MS6 core (373–403 cm). Moreover, the δ34S value in this interval is notably elevated with high total sulfur/total organic carbon ratio. Additionally, the sediments in the mid-section exhibits substantial enrichment in Mo (enrichment factors of Mo ranging from 5.29 to 39.32). This implies that the sediments in the mid-section are influenced by sulfate-driven anaerobic oxidation of methane. Most notably, the sediments in the mid-section displayed distinct low δ98Mo values (with an average of −0.7‰). After careful consideration, we ruled out the influence of organic matter, an oxic environment, a weakly sulfidic environment, and incomplete removal of thiopolybdate as contributing factors. Based on δ56Fe-Fe/Al ratios, (Mo–U) enrichment factors, and As enrichment factors, we propose that the “benthic Fe-Mn redox shuttle process” is the primary cause of the observed light δ98Mo signatures in sediments. This newly identified mechanism sheds light on Mo isotope cycling in methane seepage environments and enhances our understanding of the Mo isotope cycling process.

Enrichment pattern of tungsten in sediments under methane seepage environments: Applicability as a proxy for tracing and reconstructing (paleo-)methane seepage

The release of hydrogen sulfide by sulfate-driven anaerobic oxidation of methane (SD-AOM) during methane seepage activities has a strong impact on the evolution of marine redox conditions and the cycles of redox-sensitive elements (e.g. Mo and U). Consequently, the sedimentary enrichment or depletion of these elements is often used to trace and reconstruct (paleo-)methane seepage activity. In recent years, tungsten (W) has shown promise as an indicator of marine redox changes, but its applicability to trace methane seepage activity is unknown. This study identified significant differences in the W content of sediments in methane seepage environments compared to suboxic environments not influenced by methane seepage and oxic environments in the South China Sea. The sediments subjected to methane seepage were characterized by low W contents, similar to those of euxinic Black Sea sapropels. It is believed that the sulfidic environment resulting from SD-AOM is the primary cause of W depletion in sediments subjected to methane seepage. By examining W's interaction with its twin element, Mo, we present WMo enrichment patterns in various environments. In methane seepage environment (sulfidic environments), sediments had high MoEF (> 10) and low WEF (< 2) with a high Mo/W molar ratio (> 10); in suboxic environments not influenced by methane seepage, sediments exhibited a WMo enrichment pattern similar to the UCC (the MoEF and WEF values are predominantly below 1 and 2, respectively, with a low Mo/W molar ratio, <1.5); in oxic environments (enrichment of FeMn (oxy)hydroxides), sediments exhibited high MoEF (2 < MoEF < 10) and high WEF (> 2), with a medium Mo/W molar ratio (1.5–10). This further indicates that the Mo/W molar ratio has great potential in tracing and reconstructing (paleo-)methane seepage activities.

Origins of authigenic gypsums and carbonate minerals in sediments at a cold seep site in the Sea of Marmara

In the Western High of the Sea of Marmara, where pervasive gas hydrate and hydrocarbon gas seepage occurs, late Pleistocene-Holocene sediments are composed of a lower lacustrine and an upper marine unit. The sedimentary evolution, gypsum formation and carbonate anomaly, under the complex sedimentary environment at cold seep sites on the Western High, have not been systematically well explained. The transition from the lacustrine to marine unit occurs at around 7.5 m below the sea floor (mbsf), with a sulfidization front present in the lacustrine sediments just below it, and the marine unit includes a sapropel layer between 5.20 and 7.20 mbsf. The sedimentary sequence is characterized by authigenic minerals including pyrite, carbonates, and gypsum in the upper 0.50 mbsf, corresponding to the present-day sulfate-methane transition zone (SMTZ) near the seafloor. Mineralogical, chemical and isotopic compositions (13C, 18O, 34S, 26Mg) of the sediments in the core studied show their close relationship with intensive sulfate-driven anaerobic oxidation of methane (AOM) and other hydrocarbons. The dissolution of carbonate, related to the gas and oil migration, and organic matter enrichment might be the main cause of carbonate deficiency in the sapropel unit. Heavy carbon isotopic composition (δ13CCarb = 6.98‰) of carbonate at 4.30 mbsf implies that the carbon might be the residual by-product of subsurface biodegradation of seeping petroleum. The formation of secondary gypsum and partial dissolution of carbonate in the Sea of Marmara is commonly considered of being associated with the aerobic oxidation of pyrite. However, the euhedral authigenic gypsum studied might be with different origins, possibly including anaerobic oxidation of Fe-sulfides, carbonate deficiency in the sapropel unit and AOM. Gypsums below SMTZ are with the size of hundreds of microns. Linear pits and grooves are developed on these gypsum surfaces, probably due to corrosion by the methane-rich fluid of latest hydrocarbon seepage. Platy hexagonal gypsum in SMTZ, without any corrosion on crystal surface, might be associated with the changing flux of the acidic composition (e.g. CO2) in the seepage. Depleted-26Mg by about 1‰ in the SMTZ might be caused by the upward seepage of hydrocarbons. Magnesium isotope of authigenic carbonate could be a valid proxy for the recognition of SMTZ in a hydrate geo-system.

Application of pyrite trace-metal and S and Ni isotope signatures to distinguish sulfate- versus iron-driven anaerobic oxidation of methane

The formation of authigenic pyrite in marine sediments involves multiple reactions between ferrous iron (Fe2+) and hydrogen sulfide (H2S). Ferrous iron is commonly provided through the reductive dissolution of Fe-(oxyhydr)oxides by organic matter (i.e., dissimilatory Fe reduction), dissolved sulfide (i.e., abiotic Fe reduction) or methane (i.e., Fe-AOM), whereas sulfide is supplied by organoclastic sulfate reduction (OSR) or sulfate-driven anaerobic oxidation of methane (SD-AOM). Since Rayleigh-type distillation operates widely in sediments of gas-hydrate-bearing zones, sulfur and nickel isotope compositions (i.e., δ34S and δ60Ni) cannot readily distinguish OSR- from SD-AOM-associated pyrite. However, these microbial pathways may yield different patterns of trace-element enrichment in pyrite. To better understand the linkage of trace-element patterns to specific microbial pathways (i.e., Fe reduction, Fe-AOM, OSR and SD-AOM), and to evaluate the use of S and Ni isotopic signatures as tracers for pyrite formation pathways in methane-rich sediments, we report pyrite-associated trace element and δ34S and δ60Ni isotope analyses of sediments from a gas hydrate borehole (Site GMGS4-SC-03) from the Shenhu area, Pearl River Mouth Basin, South China Sea. Pyrite formed in conjunction with Fe- and/or SD-AOM exhibits abundant framboidal overgrowths and extremely high δ34S (up to +142.8‰) and δ60Ni (up to +2.72‰), representing the highest stable S and Ni isotopic compositions of pyrite reported to date. These pyrite morphologies are enriched in Co and Ni, which may be a diagnostic signature of an SD-AOM pathway. By contrast, OSR-associated pyrite is enriched in Cu and Zn due to OSR-induced release of trace elements from decaying organic matter. In addition, the relationship of As to Cu and/or Zn can distinguish microbial Fe/Mn reduction from Fe/Mn-AOM, because microbial Fe/Mn reduction releases trace elements from both Fe/Mn-(oxyhydr)oxides (i.e., As) and organic matter (i.e., Cu and Zn), whereas Fe/Mn-AOM only releases trace elements from Fe/Mn-(oxyhydr)oxides. Furthermore, an observed covariation between As and either Co or Ni in most pyrite with high δ34S, indicates that this pyrite captured both As released during Fe/Mn-AOM and Co and Ni from SD-AOM. Thus, the high nickel isotope values measured in this study likely dominantly reflect release of isotopically heavy Ni from Fe- and Mn-(oxyhydr)oxides. Our results demonstrate that the trace-element composition of pyrite in gas-hydrate-bearing sediments can record the geochemical signature of the dominant microbial processes.

Authigenic carbonate and native sulfur formation in Messinian (upper Miocene) marine sediments: Sedimentological, petrographical and geochemical constraints

Carbonate concretions accompanied by elemental sulfur were found in an early Messinian (Late Miocene) marine succession of NW Italy. The rocks were studied with an integrated approach including sedimentological, petrographical, stable isotope (carbon, oxygen, and multiple sulfur isotopes), and lipid biomarker analyses. Unlike other examples from Messinian strata of the Mediterranean area, the studied carbonate and sulfur concretions did not derive from the diagenetic replacement of sulfate minerals. Three lithofacies were distinguished: a) laminated lithofacies representing aphotic carbonate stromatolites enclosing fossils of filamentous sulfide-oxidizing bacteria; b) brecciated lithofacies deriving from the brecciation of carbonate stromatolites by mud injections; c) sulfur-bearing lithofacies deriving from the precipitation of thin laminae of elemental sulfur at or close to the sediment-water interface. The carbon and oxygen stable isotope composition of authigenic carbonate minerals and lipid biomarkers indicate that the initial formation of the laminated lithofacies was favored by organoclastic sulfate reduction in the shallow subsurface close to the sediment-water interface, producing sulfide that sustained dense microbial mats of sulfide-oxidizing bacteria at the seafloor. Calcification of the mats and consequent formation of stromatolites were possibly favored by nitrate-driven sulfide oxidation at the seafloor. The subsequent brecciation of the stromatolites was apparently the consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM) in an underlying sulfate-methane transition zone (SMTZ). Focused fluid flow from below, possibly resulting from destabilization of gas hydrates, was not only responsible for the brecciation of the stromatolites, but also for the delivery of bicarbonate ions and the consequent precipitation of additional, 13C-depleted calcite (δ13C values as low as −52‰). Along with bicarbonate, also hydrogen sulfide was produced by SD-AOM at the SMTZ and was transported upwards. The oxidation of hydrogen sulfide at or close to the seafloor promoted the formation of elemental sulfur characterized by δ34S values and Δ33S values close to coeval seawater sulfate.

Origin of carbonate minerals and impacts on reservoir quality of the Wufeng and Longmaxi Shale, Sichuan Basin

The Ordovician-Silurian Wufeng and Longmaxi Shale in the Sichuan Basin were studied to understand the genesis and diagenetic evolution of carbonate minerals and their effects on reservoir quality. The results of geochemical and petrological analyses show that calcite grains have a negative Ce anomaly indicating they formed in the oxidizing environment of seawater. The high carbonate mineral contents in the margin of basin indicate that calcite grains and cores of dolomite grains appear largely to be of detrital origin. The rhombic rims of dolomite grains and dolomite concretions with the δ13C of –15.46‰ and the enrichment of middle rare earth elements were formed during the sulfate-driven anaerobic oxidation of methane. The calcite in radiolarian were related to the microbial sulfate reduction for the abundant anhedral pyrites and δ13C value of –11.34‰. Calcite veins precipitated in the deep burial stage with homogenization temperature of the inclusions ranging from 146.70 °C to 182.90 °C. The pores in shale are mainly organic matter pores with pore size mainly in the range of 1–20 nm in diameter. Carbonate minerals influence the development of pores through offering storage space for organic matter. When calcite contents ranging from 10% to 20%, calcite grains and cement as rigid framework can preserve primary pores. Subsequently, the thermal cracking of liquid petroleum in primary pores will form organic matter pores. The radiolarian were mostly partially filled with calcite, which combining with microcrystalline quartz preserved a high storage capacity.

The Characteristics and Origin of Barite in the Giant Mehdiabad Zn-Pb-Ba Deposit, Iran

Abstract Mehdiabad is the world’s largest Mississippi Valley-type (MVT) Zn-Pb deposit (394 million tonnes [Mt] of metal ore at 4.2% Zn, 1.6% Pb) and contains significant barite resources (&amp;gt;40 Mt). Such large accumulations of barite are not common in carbonate-hosted Zn-Pb deposits. Therefore, the origin of the barite and its association with the Zn-Pb mineralization is of significant interest for further investigation. Field work and petrographic studies indicate that the Zn-Pb-Ba orebodies in the Mehdiabad deposit are hosted by Lower Cretaceous carbonate units of the Taft and Abkuh Formations. Fine- to coarse-grained barite with lesser siderite formed in three stages (S1, S2, and S4), along with a quartz-sulfide stage (S3) with minor quartz, sphalerite, galena, chalcopyrite, and pyrite, and the main Zn-Pb sulfide stage (S5) with massive sphalerite and galena. The barites have δ34S values from 17.7 to 20.6‰, δ18O values from 13.2 to 16.8‰, Δ33SV-CDT values from –0.001 to 0.036‰, and initial 87Sr/86Sr ratios from 0.707327 ± 0.000008 to 0.708593 ± 0.000008 (V-CDT = Vienna-Canyon Diablo Troilite). The siderites have δ13CV-PDB values from –3.8 to –2.7‰, and δ18OV-SMOW values from 18.2 to 20.9‰ (V-PDB = Vienna-Pee Dee Belemnite, V-SMOW = Vienna-standard mean ocean water). These geochemical data, and the barite morphology, point to a diagenetic origin for all stages of barite. We suggest that S1 and S2 barite precipitated from pore fluids at the sulfate-methane transition zone in a methane-diffusion-limited environment with increasing methane content. S4 barite precipitated when the methane- and barium-bearing cold-seep fluid migrated to the shallow carbonate sediments and formed a methane-in-excess setting. For the three stages, the SO42- in barite came from the residual SO42- in pore fluids undergoing sulfate-driven anaerobic oxidation of methane, and the Ba2+ came from dissolved biogenic barite and terrigenous materials in the Taft and Sangestan Formations. Primary fluid inclusions trapped in S3 quartz have salinities of 5.6 to 8.1 wt % NaCl equiv and homogenization temperatures of 143.8° to 166.1°C. The quartz has δ18OV-SMOW values ranging from 9.8 to 22.5‰ and δ30Si values from –1.3 to –0.9‰. These data indicate hydrothermal fluid flow occurred between the diagenetic S2 and S4 events. Secondary fluid inclusions with salinities of 17.70 to 19.13 wt % NaCl equiv and homogenization temperatures of 123.0° to 134.0°C are found in the S3 quartz, too. They might represent the hydrothermal event formed by basinal brines in S5. According to the ore textures and the comparison of the sulfur isotopes between S5 Zn-Pb sulfides and the digenetic barites, the barite provided a host and a sulfur source for the later Zn-Pb mineralization. The relationship between barite and the Zn-Pb mineralization indicates that significant accumulations of sulfates may be a critical exploration target for this kind of giant deposit.

Sulfate-driven anaerobic oxidation of methane inferred from trace-element chemistry and nickel isotopes of pyrite

In marine sediments, formation of pyrite (FeS2) is promoted by both organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM), and these two microbial pathways might yield differing patterns of sulfur and nickel isotopic fractionation and trace-element enrichment. To better understand these pathways, we analyzed the geochemistry of authigenic pyrite aggregates from the Upper Quaternary of the western Andaman Sea (International Ocean Discovery Program, Expedition 353, Site U1447A). 34S-enriched pyrites (to ∼+41‰) in Unit Ib are interpreted as products of SD-AOM, and their enrichments in Co and Ni and low δ60Ni values (−0.63‰ to −0.09‰) may represent diagnostic signatures of a reverse-methanogenesis microbial pathway. In contrast, 34S-depleted pyrites (∼–46‰ to –38‰) in both Units I and II are consistent with sulfide formation through early diagenetic remineralization of organic matter, and their relative enrichments in Cu and V and heavier δ60Ni values (to +0.41‰) may be diagnostic of this alternative microbial pathway. This study reports for the first time a fractionation of Ni isotopes linked to preferential uptake of isotopically light Ni by the enzyme Mcr (M reductase enzyme) in reverse methanogenesis and demonstrates that OSR- and SD-AOM-associated pyrites are potentially distinguishable on the basis of their δ60Ni compositions. Such geochemical signatures may prove useful in determining processes of pyrite formation in paleo-depositional systems and in identifying ancient methane emission events.

Quantification of the sources of sedimentary organic carbon at methane seeps: A case study from the South China Sea

Large amounts of methane are stored along continental margins in the form of methane hydrate. Methane hydrate is sensitive to environmental change, resulting in the release of substantial quantities of methane upon its destabilization. In the marine subsurface, microorganisms consume most of the methane in the sulfate-methane transition zone and convert a certain amount of methane into organic matter. Burial of such organic matter deriving from methane oxidation represents a long-term carbon sink, therefore mitigating the effect of a prominent greenhouse gas on global warming. However, the controls on the formation and consumption of sedimentary organic matter at marine seeps remain poorly constrained, impeding accurate quantification of carbon burial at seeps and its role in the marine carbon cycle. To gain new insight on the effect of seeps on carbon burial, sediments from two seep sites of the South China Sea (Site F, Haiyang 4) and a nearby reference site (Jiulong canyon) were analyzed for total organic carbon contents (TOC), δ13CTOC and Δ14C values, total inorganic carbon contents (TIC) and δ13CTIC values, as well as organic nitrogen (N) contents. Depth distributions reveal that the TOC at seeps (Site F, 0.55% ± 0.08%, Haiyang 4, 0.67% ± 0.11%) is higher than in sediments not affected by seepage (Jiulong canyon, 0.50% ± 0.10%). The enrichment of total sulfur, high TIC, and negative δ13CTIC values in sediments from Site F and Haiyang 4 agrees with locally prominent sulfate-driven anaerobic oxidation of methane (AOM) and resultant precipitation of authigenic carbonate. At the two seep sites, the sediments are characterized by more negative δ13CTOC and Δ14C values and a lower TOC/N ratio, indicating a contribution of the microbial fixation of 13C-depleted methane to the local pool of sedimentary organic matter. A linear increase in 14C ages of organic carbon with sediment depth at the reference site indicates steady depositional conditions, whereas the 14C age of organic carbon at seeps is typified by contributions of methanotrophy-derived carbon. A Δ14C mass balance approach shows that carbon derived from fossil methane accounts for at least 10 to 20% of the organic carbon preserved in the sediments. By comparing seeps, hydrate-bearing sediments, modern coastal sediments, and Archaean rocks, it becomes apparent that the carbon stable isotope composition of organic carbon in seep sediments is mainly controlled by the rate of methane oxidation and marine primary productivity.

Geochemical record of methane seepage in carbon cycling and possible correlation with climate events in the Qiongdongnan basin, South China Sea

At cold seeps, sulfate-driven anaerobic oxidation of methane (SD-AOM) is the dominant biogeochemical process, which can significantly affect the marine carbon-sulfur cycle. In this paper, multiple sedimentary indicators in core QS-1 were reported for methane seepage from Haima seep of the South China Sea (SCS). By combining total carbon (TC), total organic carbon (TOC), total sulfur (TS), total inorganic carbon (TIC), as well as the carbon and oxygen isotope compositions of benthic foraminifera Uvigerina spp. In sediments, our aim was to explore the geochemical evidence on methane seepage activities in this area. We identified three methane release events (MREs) with different intensities and timescales during MIS 1–2, characterized by particular anomalies of negative δ13CTIC (as low as −41.42‰) and δ13CU.spp (as low as −5.50‰), high contents of TS and high S/C ratios, simultaneously. High S/C ratios and negative values of δ13CTIC effectively indicate the intense SD-AOM and SD-AOM-related process in core QS-1. Three MREs, occurred at high or low sea level, may due to the climate-driven increase of hydrostatic pressure and seafloor temperature. The insights for sediment carbon-sulfur trace element, carbon isotopes of TIC contents and benthic foraminifera have obvious implications for tracing past methane seepages and its possible link with climate changes.

Extremely variable sulfur isotopic compositions of pyrites in carbonate pipes from the northern South China Sea: Implications for a non-steady microenvironment

To further constrain the microenvironment during the growth of carbonate pipes at cold seeps, we present the whole-rock carbon and oxygen, and in situ pyrite sulfur isotopic compositions along the cross-sections in pyrite-bearing carbonate pipes from the northern South China Sea. The whole-rock geochemical data suggest that these carbonate pipes were formed in an anoxic and methane-rich environment. Furthermore, in situ δ34S values in pyrites across the carbonate pipe display an extremely wide range on a centimetre scale. The majority of pyrites have δ34S values ranging from 19.4‰ to 45.8‰, reflecting the dominance of sulfate-driven anaerobic oxidation of methane (AOM) at and below the sulfate methane transition zone (SMTZ). Two abnormal areas show heavier or lighter δ34S values than the majority of pyrites, respectively. Pyrites in the abnormal area I have δ34S values ranging from 62.2‰ to 130.3‰, suggesting an extremely 34S-enriched microenvironment produced via Rayleigh fractionation of limited sulfate in a closed system. The model calculation indicates that the maximum positive δ34S value could be produced when ∼93% sulfate is consumed. Pyrites in the abnormal area II have δ34S values ranging from −32.7‰ to −18.0‰, and are interpreted as a result of early organiclastic sulfate reduction with a low proportion of later sulfate-driven AOM above the SMTZ. Collectively, the extreme variations in δ34S values point to locally migrating SMTZ in a non-steady microenvironment during the growth of carbonate pipes. This study reveals that the in situ pyrite δ34S value is a diagnostic tool for tracing the change in the local microenvironment, and could be a step towards a more comprehensive picture of biogeochemical cycles at continental margins.

Porewater geochemistry indicates methane seepage in the Okinawa Trough and its implications for the carbon cycle of the subtropical West Pacific

The Kuroshio Current, originating from the West Pacific Warm Pool and flowing through the Okinawa Trough, connects the equator and the subtropical West Pacific and plays an important role in the carbon cycle in the West Pacific Ocean. Previous studies have focused on the effects of surface export productivity and bottom redox conditions on the carbon cycle in the late Quaternary. Recent studies have demonstrated that methane seepages are very common in the Okinawa Trough, however, their significance for the carbon cycle remains not well understood. Here, methane seepages and sulfate-driven anaerobic oxidation of methane (SD-AOM) are constrained by the geochemical characteristics of porewater at site CSHC-4 (Length: 1555 cm) derived from the water depth of 934 m on the western continental slope of the Okinawa Trough. Using the △(DIC+ Ca2++ Mg2+)/△SO42− ratio, it is suggested that the sulfate reduction in the upper part of site CSHC-4 is dominated by organoclastic sulfate reduction (OSR), while that between 655 and 1055 cm is gradually dominated by SD-AOM with deepening depth. However, with the depletion of sulfate, the methanogenesis process predominates at the bottom of site CSHC-4. The depth of the sulfate-methane interface (SMI) and the methane diffusive flux of site CSHC-4 is calculated to be ∼1156 cm and ∼ 63 mmol m−2 yr−1, respectively. Near the SMI, the changes in concentrations of Ca2+, Mg2+, Sr2+ and ratios of Mg/Ca and Sr/Ca in the porewater indicate the possibility of the precipitation of authigenic high-Mg calcite in the sediments. In contrast, the increase in the concentration of Ba2+ in porewater below the SMI may imply the dissolution of barite (BaSO4). In addition, redox-sensitive elements in porewater exhibit various geochemical behaviours due to changes in redox conditions caused by SD-AOM. The above SD-AOM processes will affect the reconstruction of surface productivity and redox conditions of bottom water in the Okinawa Trough since the late Quaternary based on biogenic elements (e.g., Ca and Ba) and redox-sensitive elements (e.g., U and Cr), which requires careful evaluation. Therefore, our new findings illustrate the modern methane seepage in the Okinawa Trough and provide a reference for calculating the ancient carbon cycle in the subtropical West Pacific.

Multiple sulfur isotope systematics of pyrite for tracing sulfate-driven anaerobic oxidation of methane

Sulfate-driven anaerobic oxidation of methane (SD-AOM) is ubiquitous in marine sedimentary environments, governing the global methane budget and the redox evolution of Earth's surface. Tracing SD-AOM and organoclastic sulfate reduction (OSR) from the pyrite archive is key to the reconstruction of SD-AOM activity and paleoenvironmental interpretation. However, discriminating the origins of pyrite – basically SD-AOM and OSR – is commonly challenging due to frequent overlap of δ34Spy values. Multiple sulfur isotopes of pyrite are expected to be an effective tool to distinguish between OSR and SD-AOM, yet variable uncertainties and unknowns remain. Here we investigated the δ34S and Δ33S values of pore-water sulfate and authigenic pyrite from a piston core taken on the continental slope of the South China Sea. A positive Δ33S - δ34S correlation of pore-water sulfate is observed in the upper OSR-dominated zone, resulting in Δ33S and δ34S values of sulfate diffusing into the sulfate-methane transition zone (SMTZ) >0.1‰ and >30‰ larger than the corresponding values of seawater sulfate. A negative Δ33S - δ34S trajectory of pore-water sulfate and pyrite is observed for the SMTZ, agreeing with low sulfur isotope fractionation characteristic for SD-AOM and a diagnostic large Δ33S - δ34S field of SD-AOM-derived pyrite. These findings elucidate that the multiple sulfur isotope systematics of pyrite in methane-bearing sediment depends on (1) mass transport effects of dissolved sulfate and sulfide, (2) the relative contribution of OSR and SD-AOM to the pore-water sulfide and pyrite pools, and (3) the sulfur isotope fractionation during microbial sulfate reduction. Our study highlights the importance of mass transport dynamics on the isotopic composition of pyrite, a factor that needs to be considered in any attempt to reconstruct the origin of early diagenetic pyrite and the paleoenvironmental setting with multiple sulfur isotopes.

Iron oxides impact sulfate-driven anaerobic oxidation of methane in diffusion-dominated marine sediments

Microbial iron (Fe) reduction by naturally abundant iron minerals has been observed in many anoxic aquatic sediments in the sulfidic and methanic zones, deeper than it is expected based on its energetic yield. However, the potential consequence of this “deep” iron reduction on microbial elemental cycles is still unclear in sediments where diffusion is the dominant transport process. In this contribution, we experimentally quantify the impact of iron oxides on sulfate-driven anaerobic oxidation of methane (S-AOM) within the sulfate methane transition zone (SMTZ) of marine diffusive controlled sediments. Sediments were collected from the oligotrophic Southeastern (SE) Mediterranean continental shelf and were incubated with 13C-labeled methane. We followed the conversion of 13C-labeled methane as a proxy of S-AOM and monitored the sediment response to hematite addition. Our study shows microbial hematite reduction as a significant process in the SMTZ, which appears to be co-occurring with S-AOM. Based on combined evidence from sulfur and carbon isotopes and functional gene analysis, the reduction of hematite seems to slow down S-AOM. This contrasts with methane seep environments, where iron oxides appear to stimulate S-AOM and hence attenuate the release of the greenhouse gas methane from the sediments. In the deep methanic zone, the addition of iron oxides inhibits the methanogenesis process and hence methane gas production. The inhibition effect deeper in the sediment is not related to Fe-AOM as a competing process on the methane substrate, since Fe-AOM was not observed throughout the methanic sediments with several iron oxides additions.

Effects of sulfate reduction processes on the trace element geochemistry of sedimentary pyrite in modern seep environments

Although it has been suggested that the trace element patterns of sedimentary pyrite can be affected by early diagenesis, detailed studies on modern marine environments addressing this hypothesis are scarce. Organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM) are the dominant sulfate reduction processes in organic-rich and methane-rich marine sediments, and commonly result in the formation of authigenic pyrite. To better understand how OSR and SD-AOM affect the trace element geochemistry of pyrite, we applied laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) to analyze trace element contents of pyrite from two methane-seep sites (DH-CL11 and HS148) of the South China Sea. The trace element data are complemented by the measurement of iron isotope compositions of authigenic pyrite. This new data set, supplemented by published sulfur isotope data, improves our understanding of the diagenetic processes during early diagenetic pyrite formation. High δ56Fe and δ34S values of pyrite reflect the dominance of SD-AOM within sulfate-methane transition zones (SMTZs) at both sites (i.e., from 705 to 767 cm below seafloor at site DH-CL11 and from 490 to 690 cm below seafloor at site HS148), whereas pyrite from shallow depth characterized by low δ56Fe values points to early OSR processes. Within the SMTZs, pyrite exhibits highly variable but overall high manganese (Mn) contents (12 to 732 mg/kg at site DH-CL11; 12 to 1145 mg/kg at site HS148), compared to OSR-derived pyrite from shallow depth. Similar to this pattern, pyrite formed at the SMTZ shows higher lead (Pb), zinc (Zn), copper (Cu), and vanadium (V) contents than pyrite formed at shallower depths above the SMTZ. Enrichments of these metals in pyrite forming at the SMTZ are best explained by enhanced reductive dissolution of iron and manganese (oxyhydr)oxides under sulfidic conditions induced by SD-AOM, and subsequent trace element incorporation into pyrite. Arsenic (As), molybdenum (Mo), and antimony (Sb) distributions in pyrite vary between the two study sites. Pyrite from shallow depth of site HS148 has high contents of all three elements, indicating effective immobilization of dissolved As, Mo, and Sb during pronounced OSR. In contrast, apparently less intense OSR at shallow depth of site DH-CL11 allowed for more dissolved As, Mo, and Sb to diffuse downward to greater depth, resulting in the enrichments of these elements in pyrite formed at or close to the SMTZ. Positive correlations between As and cobalt (Co), and between As and nickel (Ni) typify pyrite derived from both OSR and SD-AOM for both study sites, and As-rich pyrite from site DH-CL11 is characterized by a distinctly high Co/Ni ratio. Both patterns suggest that the presence of As affects the uptake of Co and Ni into pyrite. Our study documents that pyrite element contents in methane-rich environments are significantly influenced by different sulfate reduction processes during early diagenesis, allowing the differentiation of OSR-derived pyrite from SD-AOM-derived pyrite by means of trace element geochemistry.

Sulfur and Oxygen Isotope Records of Sulfate-Driven Anaerobic Oxidation of Methane in Diffusion-Dominated Marine Sediments

Organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM) are the two major microbial pathways for sulfate consumption in marine sulfur cycle. The relative changes of sulfur and oxygen isotope ratios in pore water sulfate are affected by the mode of microbial sulfate reduction and have been applied as an indicator for assessing methane excess environments. However, so far, this isotope proxy fails to distinguish sulfate reduction processes fueled by the oxidation of organic matter or by diffusing methane. To better understand the mechanism of sulfur and oxygen isotope partitioning during OSR and SD-AOM, coupled sulfur and oxygen isotopic compositions of pore water sulfate (δ34SSO4 and δ18OSO4) were investigated from four methane diffusing sites (CL56, CL57, CL59, and CL60) of the South China Sea, supplemented by carbon isotopic composition of dissolved inorganic carbon (DIC) and sulfur isotopic composition of pyrite in bulk sediments. Pore water sulfate and DIC concentrations, as well as calculated net sulfate reduction rates suggest that the sulfate reduction at site CL57 was mainly dominated by OSR, whereas sites CL56, CL59, and CL60 were likely impacted by both OSR and SD-AOM. Furthermore, the trend of cross-plotting δ18OSO4 versus δ34SSO4 values from site CL57 was distinguishable from sites CL56, CL59, and CL60, although all study sites show similar patterns to those derived from methane limited environments. This further indicates the trajectory of sulfur and oxygen isotope partitioning was affected by the mode of sulfate reduction (i.e., OSR vs. SD-AOM). At site CL57, the low net sulfate reduction rate would lead to enhanced oxidation of intermediate sulfur species during OSR, thus leading to a higher slope in the δ18OSO4 vs. δ34SSO4 cross-plot (1.26). In contrast, the higher net sulfate reduction rates at sites CL56, CL59, and CL60 due to the impact from SD-AOM would lead to lower slopes in the δ18OSO4 vs. δ34SSO4 cross-plots (0.78 ± 0.11). This study provides new insights into the sulfur and oxygen isotope systematics during microbial sulfate reduction processes in methane diffusing environments.

Sulfate concentrations affect sulfate reduction pathways and methane consumption in coastal wetlands

Coastal wetlands are an important source of methane emissions, and understanding the mechanisms that control methane emissions from coastal wetlands is of great significance to global warming. Anaerobic oxidation of methane driven by sulfate is an important process to prevent methane emissions from coastal wetlands. The effects of environmental changes on this process and the function of the sulfate-methane transition zone (SMTZ) are poorly understood. In this study, spatiotemporal variations in pore-water geochemistry (concentrations of SO42-, CH4 and DIC as well as δ13C-DIC and δ13C-CH4) in the Beidagang wetland, Tianjin, China, were investigated to unravel factors controlling the role of anaerobic oxidation of methane in coastal wetlands. Results show that the geochemical profile of pore-water is characterized by significant spatial and temporal variability, which may be related to changes in sulfate concentration, temperature and dissolved oxygen. The carbon isotope fractionation factors (εC) during methane oxidation range from 8.9‰ to 12.5‰, indicating that the sulfate-driven anaerobic oxidation of methane (S-AOM) dominates the methane oxidation in the Beidagang coastal wetland in both winter and summer, in both high and low salinity wetlands, and in both open water and littoral areas. However, sulfate concentration has a strong influence on the sulfate reduction pathways and methane consumption. The consumption of methane and sulfate by S-AOM is more significant in coastal wetlands with high sulfate concentrations, with S-AOM consuming nearly all of the upward-diffusing methane (96%) and downward-diffusing sulfate (96%). In addition, the dissolved inorganic carbon (DIC) produced in the pore-water mainly comes from methanogenesis, accounting for more than 80% of the total DIC pool, but in the areas with high sulfate concentrations in water column, the contribution of S-AOM to the DIC pool is greater, although only a small fraction of the total DIC pool (9%). The depth and width of the SMTZ show a clear spatial and temporal pattern, with active methanogenesis activity and upward high methane flux shoaling the SMTZ and increasing the risk of high methane emissions from coastal wetlands with low sulfate concentrations. Our findings highlight the importance of sulfate-driven anaerobic oxidation of methane in coastal wetlands and the effect of sulfate concentration on it. It contributes to our understanding of the mechanism of methane production and emissions from the coastal wetland system, particularly in light of the increased demand for coastal wetland restoration under global warming.

Elemental and Isotopic Signatures of Bulk Sedimentary Organic Matter in Shenhu Area, Northern South China Sea

Hydrate-bearing sediments provide excellent materials for studying the primary sources and diagenetic alterations of organic matter. In this study, the elemental and isotopic signatures of total organic carbon (TOC), total inorganic carbon (TIC), total nitrogen (TN), and total sulfur (TS) are systematically investigated in three hydrate-bearing sediment cores (∼240 m) retrieved from the Shenhu area, South China Sea. All sediment layers from three sites are with low TOC content (average 0.35%) with marine and terrestrial mixed sources (-23.6‰ &amp;lt; δ13Corg &amp;lt; -21.4‰). However, the generally low δ15N (2.49–5.31‰) and C/N ratios (4.35–8.2) and their variation with depth cannot be explained by the terrestrial sources (Pearl River) and marine sources, binary end-member mixing processes. Contribution from lateral allochthonous organic matter from the mountainous river is considered after excluding other possible factors and ingeniously elucidating the organic matter origins. Furthermore, specific layers in W01B and W02B exhibit elevated S/C ratios (up to 2.39), positive bias of δ34S-TS (up to +29.7‰), and negative excursion of δ13C-TIC (up to -8.29‰), which are the characteristics of sustained occurrence of sulfate-driven anaerobic oxidation of methane. The occurrence of coupled carbon–sulfur anomaly may be accompanied by deep hydrocarbon leakage and the formation of hydrate with high saturation.

Magnesium Isotopes in Pore Water of Active Methane Seeps of the South China Sea

The magnesium (Mg) isotopic composition of marine authigenic carbonates is considered as promising archive of ancient seawater geochemistry and paleoenvironments. Previous experimental and theoretical work has shown that Mg isotope fractionation during carbonate mineral formation is a function of mineralogy and precipitation rate. However, information on Mg isotope fractionation is limited for well-defined precipitation rates in natural settings. Here, we investigate pore waters from sediments of an area of active methane seepage in the South China Sea. Low δ13C values (&amp;lt; −48.3‰ VPDB) of dissolved inorganic carbon (DIC) near the sulfate-methane transition zone (SMTZ) indicate that sulfate-driven anaerobic oxidation of methane (SD-AOM) is the predominant biogeochemical process. Pore water composition of dissolved Mg, calcium (Ca), and strontium (Sr) agrees with aragonite as the dominant carbonate mineral at the site ROV1, and high Mg-calcite at sites ROV2 and ROV4. Calculated carbonate precipitation rates are 0.92 μmol cm−2 yr−1 for site ROV2 and 1.24 μmol cm−2 yr−1 for site ROV4; these estimates are similar to previous calculations for seeps from other areas. The pore water δ26Mg values (−0.88‰ to −0.71‰) obtained for the three study sites are similar to those of seawater, in accord with a minor effect of Rayleigh fractionation due to abundant supply of Mg from seawater and insignificant consumption of Mg during carbonate precipitation. The modeled Mg isotope fractionation (ϵ = −2.0‰ to −1.0‰ for core ROV2; ϵ = −1.3‰ to −0.3‰ for core ROV4) can be explained by kinetic isotope fractionation during carbonate precipitation. The calculated carbonate precipitation rates and the degree of fractionation of Mg isotopes support the notion that fractionation is small at high precipitation rates. However, the carbonate precipitation rates calculated for the studied seep environments are much smaller than those in laboratory experiments, documenting a discrepancy of isotopic fractionation between carbonate authigenesis in laboratory experiments and natural environments. These results, including the modeled precipitation rates, provide new constraints for Mg isotope fractionation in natural settings.