Sulfate-methane Transition Zone Research Articles

Methane leakage through the sulfate–methane transition zone of the Baltic seabed

Methane leakage through the sulfate–methane transition zone of the Baltic seabed

Transformation of Mackinawite to Interlayered Greigite-Pyrrhotite and Pyrite in the Gaoping Submarine Canyon Sediments off Southwestern Taiwan

Iron monosulfides and neoformed pyrite below the sulfate‒methane transition zone (SMTZ) of rapidly accumulating turbiditic sediments from the Gaoping submarine canyon off southwestern Taiwan were examined by SEM-EDS-EBSD, HRTEM, and HAADF STEM to investigate their microstructural characteristics and processes of formation and transformation. Within a few meters below the SMTZ, mackinawite (Mkw) is largely replaced by interlayered greigite-pyrrhotite (Grg-Po) with {111}Grg//{001}Po and Grg//Po, followed by pyrite neoformation in clusters of disseminated matrix grains consisting of coalescing pyrite microcrystals, arrays of polycrystalline interlayer pyrite grains between the cleavage planes of layer silicates, with each grain’s core having inclusions of interlayered Grg-Po locally containing relict Mkw, and amassed pyrite microcrystals on the surface of porous interlayered Grg-Po micronodules. In the deeper sediments, neoformed pyrite is absent and Mkw is largely preserved, with partial replacement by interlayered Grg-Po having an overall topotactic relationship of ⟨110⟩Grg//⟨110 ⟩Po//⟨100⟩Mkw and {111}Grg//(001)Po//~{011}Mkw and a sharp reaction front without transitional profiles. The mineral grain boundaries and structural discontinuities with Mkw resulting from extensive interlayering between Grg {111} cubic close-packed segments and Po {001} hexagonal close-packed layers could serve as conduits for fluid flow and mass transport to drive the replacement reaction. The conversion of Mkw to metastable interlayered Grg-Po is inferred to occur through interface-coupled dissolution‒reprecipitation processes associated with partial oxidation while the partial replacement of interlayered Grg-Po ± minor relict Mkw by pyrite microcrystals with irregular grain boundaries and orientations probably occurred via a dissolution‒precipitation mechanism. Mkw could be initially formed by sulfate reduction driven by anaerobic oxidation of methane in reactive iron-rich sediments in paleo-SMTZs and subsequently transformed into interlayered Grg-Po followed by pyrite neoformation in the sulfidization front below the SMTZ or recent SMTZs in the Gaoping submarine canyon sediments.

Sulfate availability affect sulfate reduction pathways and methane consumption in freshwater wetland sediments

Methane (CH4) emissions from wetlands significantly contribute to global greenhouse gas fluxes, yet the mechanisms regulating CH4 production and oxidation in freshwater wetlands remain underexplored, particularly in the role of sulfate in the anaerobic oxidation of CH4. This study investigated the production, consumption, and release of CH4 in the sediments of the Qilihai wetland, focusing on the roles of hydrogenotrophic methanogenesis and sulfate dynamics across different seasons. CH4 concentrations ranged from 2.42 to 2290.52 μmol L⁻1, and δ13C–CH4 values became progressively more negative with depth, ranging from −87.37‰ to −57.18‰. Results indicate that hydrogenotrophic methanogenesis is the dominant pathway for CH4 production, particularly in sulfate-depleted environments, with CH4 concentrations in the sulfate-methane transition zone (SMTZ) strongly correlated with sulfate availability. Sulfate consumption through Sulfate Anaerobic Oxidation of Methane (SAOM) and Organoclastic Sulfate Reduction (OSR) demonstrated significant seasonal variation, with OSR accounting for up to 73% of sulfate consumption in the SMTZ. The SMTZ exhibited variations ranging from 2 to 40 cm in October, narrowing to 2–4 cm in July. These findings emphasize the complex interactions between sulfate availability, methanogenic pathways, and CH4 emissions in freshwater wetlands, highlighting the need for further research on sulfate dynamics and their implications for greenhouse gas emissions in the context of global climate change.

Effects of climate change and methane-rich fluid activity on sedimentary sulfur geochemical records in the northern South China Sea since mid-Pleistocene

Microbial sulfate reduction (MSR) and associated pyritic sulfide formation are important diagenetic processes in marine sediments. The sulfur isotopic composition of pyrite (δ34Spyr) is proven to be sensitive to changes in sedimentation rates and the content and reactivity of organic carbon, especially on the continental shelves and upper slopes (water depth < 350 m). However, the diagenetic responses of sulfur to variations in climatic and depositional conditions in the deep-sea sediments are still poorly understood. This study combines element contents and isotopes to characterize diagenetic interplays of sulfur, organic carbon, and methane in the continental slope sediments of the northern South China Sea since the mid-Pleistocene. Our data suggest that the total organic carbon (TOC) increased during glacial times, implying enhanced primary productivity due to increased nutrient supply by the East Asian Winter Monsoon, in addition to efficient transfer of organic carbon and better preservation of organic carbon due to reduced bottom water oxygen. Total sulfur and chromium reduction sulfur contents varied concomitantly with the TOC, suggesting an increased burial of organic carbon that enhanced the organoclastic sulfate reduction (OSR) and the formation of authigenic pyrite. The environmental changes did not induce a significant shift in δ34Spyr, due most likely to relatively low sedimentation rates and large fractionation in sulfur isotope through OSR during the glacial-interglacial cycles. Instead, it is hypothesized that the sulfate-driven anaerobic methane oxidation promoted the formation of a higher amount of authigenic pyrite. Consequently, it created a closed diagenetic system leading to positive excursions in δ34Spyr at the sulfate-methane transition zone. Our results suggest the vulnerability of pyrite formation and its sulfur isotopic composition to the changes in monsoon-driven primary productivity and the methane-rich fluid migrations in the continental margin sediments. This study complements the growing evidence for the local diagenetic controls on sedimentary sulfur geochemical records by highlighting the importance of early diagenesis in paleoenvironment reconstruction based on the content and sulfur isotopic composition of pyrite.

Occurrence characteristics of authigenic pyrite in the deep-sea environment and its paleoceanographic implications based on core sediments from IODP Expedition 342 Site U1406

Although authigenic pyrite (FeS2) in marine sediment is an important proxy for oxygen conditions and microbial activities at its formation site, very little is known about its morphological characteristics in the deep-sea (> 3500 m) environment and their paleoceanographic implications. Here, we report the occurrence characteristics of authigenic pyrite including texture, size distribution, and relative content in North Atlantic deep-sea sediments from Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406. The results of electron microscopy show octahedral pyrite microcrystals of 1.5–4.0 μm, the spherical texture of greigite (Fe3S4), and large pyrite framboids (average, 25 μm) relative to those of the continental margin. These features may demonstrate an unchanged supersaturation level of the pore water for octahedral pyrite after its initial formation, replacement of the precursor greigite by pyrite, and an expanded pyrite framboid formation zone with a low sulfate reduction rate in deep-sea sediments, respectively. Although the size distribution of pyrite framboids measured in this study falls into the range of pyrites formed in the sulfate–methane transition zone, based on onboard geochemical data and previous research, these pyrites most likely formed through organoclastic sulfate reduction during diagenesis. We assessed the implications of these findings for paleoceanographic changes, as the formation of diagenetic pyrite is controlled by the supply rate and reactivity of organic matter, which are closely related to ocean productivity and circulation. A comparison between the profiles of relative pyrite content and benthic foraminiferal oxygen isotope (δ18Obf) values indicated that these factors were anti-phased during the late Oligocene. Based on our results, we conclude that the formation of diagenetic pyrite was favored during interglacial periods due to increases in the amount and reactivity of organic matter driven by more energetic formation of proto-North Atlantic Deep Water. Our findings provide new insights into the formation of authigenic pyrite in deep-sea environments and its potential as a paleoceanographic proxy.

Cycling and persistence of iron-bound organic carbon in subseafloor sediments

Reactive iron (FeR) serves as an important sink of organic carbon (OC) in marine surface sediments, which preserves approximately 20% of total OC (TOC) as reactive iron-bound OC (FeR-OC). However, the fate of FeR-OC in subseafloor sediments and its availability to microorganisms, remain undetermined. Here, we reconstructed continuous FeR-OC records in two sediment cores of the northern South China Sea encompassing the suboxic to methanic biogeochemical zones and reaching a maximum age of ~100 kyr. The downcore FeR-OC contributes a relatively stable proportion of 13.3 ± 3.2% to TOC. However, distinctly lower values of less than 5% of TOC, accompanied by notable 13C depletion of FeR-OC, are observed in the sulfate-methane transition zone (SMTZ). FeR-OC is suggested to be remobilized by microbially mediated reductive dissolution of FeR and subsequently remineralized, the flux of which is 18–30% of the methane consumption in the SMTZ. The global reservoir of FeR-OC in microbially active Quaternary marine sediments could be 19-46 times the size of the atmospheric carbon pool. Thus, the FeR-OC pool may support subseafloor microorganisms and contribute to regulating Earth’s carbon cycle.

Ebullition dominates methane emissions in stratified coastal waters

Coastal areas are an important source of methane (CH4). However, the exact origins of CH4 in the surface waters of coastal regions, which in turn drive sea-air emissions, remain uncertain. To gain a comprehensive understanding of the current and future climate change feedbacks, it is crucial to identify these CH4 sources and processes that regulate its formation and oxidation. This study investigated coastal CH4 dynamics by comparing water column data from six stations located in the brackish Tvärminne Archipelago, Baltic Sea. The sediment biogeochemistry and microbiology were further investigated at two stations (i.e., nearshore and offshore). These stations differed in terms of stratification, bottom water redox conditions, and organic matter loading. At the nearshore station, CH4 diffusion from the sediment into the water column was negligible, because nearly all CH4 was oxidized within the upper sediment column before reaching the sediment surface. On the other hand, at the offshore station, there was significant benthic diffusion of CH4, albeit the majority underwent oxidation before reaching the sediment-water interface, due to shoaling of the sulfate methane transition zone (SMTZ). The potential contribution of CH4 production in the water column was evaluated and was found to be negligible. After examining the isotopic signatures of δ13C-CH4 across the sediment and water column, it became apparent that the surface water δ13C-CH4 values observed in areas with thermal stratification could not be explained by diffusion, advective fluxes, nor production in the water column. In fact, these values bore a remarkable resemblance to those detected below the SMTZ. This supports the hypothesis that the source of CH4 in surface waters is more likely to originate from ebullition than diffusion in stratified brackish coastal systems.

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.

Eutrophication and Deoxygenation Drive High Methane Emissions from a Brackish Coastal System.

Coastal environments are a major source of marine methane in the atmosphere. Eutrophication and deoxygenation have the potential to amplify the coastal methane emissions. Here, we investigate methane dynamics in the eutrophic Stockholm Archipelago. We cover a range of sites with contrasting water column redox conditions and rates of organic matter degradation, with the latter reflected by the depth of the sulfate-methane transition zone (SMTZ) in the sediment. We find the highest benthic release of methane (2.2-8.6 mmol m-2 d-1) at sites where the SMTZ is located close to the sediment-water interface (2-10 cm). A large proportion of methane is removed in the water column via aerobic or anaerobic microbial pathways. At many locations, water column methane is highly depleted in 13C, pointing toward substantial bubble dissolution. Calculated and measured rates of methane release to the atmosphere range from 0.03 to 0.4 mmol m-2 d-1 and from 0.1 to 1.7 mmol m-2 d-1, respectively, with the highest fluxes at locations with a shallow SMTZ and anoxic and sulfidic bottom waters. Taken together, our results show that sites suffering most from both eutrophication and deoxygenation are hotspots of coastal marine methane emissions.

Potential coupling of microbial methane, nitrogen, and sulphur cycling in the Okinawa Trough cold seep sediments.

The Okinawa Trough (OT) is a back-arc basin with a wide distribution of active cold seep systems. However, our understanding of the metabolic function of microbial communities in the cold seep sediments of the OT remains limited. In this study, we investigated the vertical profiles of functional genes involved in methane, nitrogen, and sulphur cycling in the cold seep sediments of the OT. Furthermore, we explored the possible coupling mechanisms between these biogeochemical cycles. The study revealed that the majority of genes associated with the nitrogen and sulphur cycles were most abundant in the surface sediment layers. However, only the key genes responsible for sulphur disproportionation (sor), nitrogen fixation (nifDKH), and methane metabolism (mcrABG) were more prevalent within sulfate-methane transition zone (SMTZ). Significant positive correlations (P < 0.05) were observed between functional genes involved in sulphur oxidation, thiosulphate disproportionation with denitrification, and dissimilatory nitrate reduction to ammonium (DNRA), as well as between AOM/methanogenesis and nitrogen fixation, and between sulphur disproportionation and AOM. A genome of Filomicrobium (class Alphaproteobacteria) has demonstrated potential in chemoautotrophic activities, particularly in coupling DNRA and denitrification with sulphur oxidation. Additionally, the characterized sulfate reducers such as Syntrophobacterales have been found to be capable of utilizing nitrate as an electron acceptor. The predominant methanogenic/methanotrophic groups in the OT sediments were identified as H2-dependent methylotrophic methanogens (Methanomassiliicoccales and Methanofastidiosales) and ANME-1a. This study offered a thorough understanding of microbial ecosystems in the OT cold seep sediments, emphasizing their contribution to nutrient cycling.IMPORTANCEThe Okinawa Trough (OT) is a back-arc basin formed by extension within the continental lithosphere behind the Ryukyu Trench arc system. Cold seeps are widespread in the OT. While some studies have explored microbial communities in OT cold seep sediments, their metabolic potential remains largely unknown. In this study, we used metagenomic analysis to enhance comprehension of the microbial community's role in nutrient cycling and proposed hypotheses on the coupling process and mechanisms involved in biogeochemical cycles. It was revealed that multiple metabolic pathways can be performed by a single organism or microbes that interact with each other to carry out various biogeochemical cycling. This data set provided a genomic road map on microbial nutrient cycling in OT sediment microbial communities.

Response of early diagenesis to methane leakage in the inner shelf of the East China Sea

Shelf seas are experiencing a rise in shallow gas leaks, primarily methane, raising concerns due to their environmental impact. However, the effect of the leaks on early diagenesis remains poorly understood. This study analyzes sediment lithology, organic geochemistry and porewater geochemistry of two short cores collected nearby the pockmarks in the muddy inner shelf of the East China Sea. Our findings clearly demonstrate the impact of methane leakage on early diagenesis, evidenced by the shallower position of the SMTZ (sulfate-methane transition zone), higher concentrations of uranium (U), vanadium (V), and manganese (Mn) in the porewater near and above the SMTZ, and downcore decrease in Mg2+, Ca2+, and Sr2+ concentrations versus increase in Mg2+/Ca2+ and Sr2+/Ca2+ ratios. Their profile variations and the difference between two cores are determined by the intensity of methane leakage. The estimated methane diffusive flux of 619 mmol m-2 yr-1 at YEC7–2 is roughly 8.5 times that at YEC7–1 (73 mmol m-2 yr-1), consistent with a shorter distance of YEC7–2 to the pockmark with active methane leakage. A schematic model is summarized to demonstrate the response of early diagenesis processes to the increasing methane leakages in response to changing sedimentation regimes from accretion to severe erosion. This study undoubtedly improves our understanding mutual promotion effect between seafloor erosion and gas leakage, and their impact on early diagenesis processes and resultant porewater geochemical changes and authigenic mineral records.

Geochemical and mineralogical features as indicators of sulfate-methane transition zones in shallow gas-bearing offshore sediments from Zhoushan

Subsurface shallow gas, primarily composed of biogenic methane (>95%), is highly-enriched in global offshore sediments. The redox conditions of methane-rich pore fluid play a crucial role in shaping the geochemical signatures associated with the anaerobic oxidation of methane (AOM) and vertical displacement of sulfate-methane transition zones (SMTZs) within deep sedimentary records. However, long-period, and continuous records of AOM and SMTZs in shallow gas-bearing offshore sediments are sparse. Here, we employed a comprehensive analyses of iron sulfide, elemental, and stable isotopies on a 63-m drilled sediment core (JC-1) from offshore Zhoushan. Four different layers U1–U4 were identified in 11.5–24 mbsf, 24−42 mbsf, 42−51 mbsf, and 51−63 mbsf, respectively, as revealed by the lithofacies and sedimentary structures. By comparing the pyrite contents, total sulfur to total organic carbon ratios, and δ34S-chromium-reducible sulfur values, the commonly seen organoclastic sulfate reduction occurred in U1 and U2, whereas significant AOM activities were found in U1 and U4. Further, the peaks of total sulfur to total organic carbon ratios, sulfur contents, and multiple positive δ34S-chromium-reducible sulfur values indicated the presence of AOM in SMTZs at 18.3, 52, and 59 mbsf. These intervals were featured by a sub-oxic or anoxic condition with strong methane seepages, evident from the scarcity of metals and the enrichment of Mo/Al and U/Al ratios. The presence of multiple SMTZs was consistent with the pore-water data of the adjacent YS4 and YS7 cores. In light of these findings, it is prudent to exercise caution when utilizing geochemical indicators, such as trace element proxies and the δ34S values of sulfide minerals, to comprehend the progression of fluctuating depositional settings in reaction to sea-level fluctuations and shallow gas reservoirs. This study provides a new perspective to better understand the evolution of unsteady depositional environments in response to sea-level changes and shallow gas reservoirs.

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.

Formation of non-evaporitic gypsum in gas hydrate-bearing sediments at Håkon Mosby mud volcano, SW Barents sea

Authigenic minerals such as carbonate and iron sulfide are known features at cold seeps worldwide and form as a result of the anaerobic oxidation of methane in the sulfate-methane transition zone (SMTZ). Recent findings of seep-related non-evaporitic gypsum raise questions regarding its distribution, formation mechanisms, and relationships with the other proxies for confident paleo-seep reconstructions. Here, we report new findings of gypsum in the seep-impacted sediments of the hydrate-bearing Håkon Mosby mud volcano, SW Barents Sea. We combined sediment core logging (XRF, X-Ray) with optical microscopy and scanning electron microscopy - electron dispersive energy to characterize the authigenic mineral fraction of the sediment. Three paleo-SMTZs marked by the co-occurrence of pyrite and gypsum accumulations were found at 7–10 cm, 13–18 cm and 19–21 cm, above the modern one (at ∼50 cm) and match with carbonate-poor and foraminifera-free sediment intervals. Our results indicate that gypsum formation required a descending SMTZ (decreasing methane flux), triggering the oxidation of previously-formed pyrite which supplied extra SO42− to the pore water. Acidic conditions caused by pyrite oxidation would have promoted the dissolution of carbonate particles, thus releasing Ca2+ that is available for gypsum formation. As seen in other studies, the presence of shallow gas hydrates seems to play an important role as a calcium source for gypsum precipitation, via ion exclusion during hydrate formation. HMMV represents an optimal natural laboratory to investigate the interplay between different processes in gypsum biogeochemistry and its potential use as a proxy for methane seeps in modern and ancient settings.

Biological and paleoceanographic controls on the postglacial sulfur isotope records in Arctic shelf sediments

The Arctic Ocean has experienced environmental fluctuations during the Pleistocene glacial-interglacial cycles. Since the last deglaciation, the reinundation of the Bering Strait and regional transgression have led to dramatic changes in the western Arctic Ocean, affecting coastal landscapes, depositional processes, marine ecosystem, and, inherently, biogeochemical element cycles. Here, we investigate the records of sulfur, carbon, and iron cycles in the Chukchi Sea of the western Arctic, with a primary focus on microbial sulfate reduction and subsequent pyrite formation. Variations in the pyrite abundance and sulfur isotopic composition, derived from a 10-m sediment core, demonstrate that dynamic changes in the factors governing pyrite formation – organic electron donor, reactive iron, and sulfate – are closely linked to the regional environmental conditions. In the early Holocene sediment, pyrite precipitation was impeded by the lack of organic matter, even in the presence of abundant sulfate and reactive iron. In the overlying sediment, pyrite contents increased due to vigorous microbial sulfate reduction fueled by enhanced availability of organic substrate and additional methane from the sulfate-methane transition zone. In general, the increased supply of organic matter to the Chukchi Shelf sediments can be attributed to the resumed inflow of nutrient-rich Pacific Water through the flooded Bering Strait. The increase in pyrite content, however, does not correlate exactly with either the increase in TOC or the opening of the Bering Strait, both of which precede the increased pyrite formation. These lags reflect the balance between marine primary production and terrestrial carbon sources, as well as the balance between the Pacific Inflow and the Beaufort Gyre. While a comparable increase in pyrite and TOC contents during the Holocene deglaciation has also been reported in the Black and Baltic Sea sediments, their distinct sulfur isotope records, contingent upon the availability of sulfate and reactive iron, highlight that pyrite can serve as a valuable proxy for tracking past climate and environmental change, especially in barren sedimentary records such as those found in the Arctic Ocean.

The relationships between vertical variations of shallow gas and pore water geochemical characteristics in boreholes from the inner shelf of the East China Sea

Shallow gas was widely recognized in the coastal region, especially in the estuarine delta areas with high organic matter (OM) burial flux. In this study, the vertical variations of shallow gas and the related geochemical indicators from two boreholes in the coastal region of the East China Sea (ECS) were investigated. Two gas-bearing layers were identified in the sediments from the Holocene and late Pleistocene series. Both boreholes exhibit a “typical” and an “inverse” Sulfate Methane Transition Zone (SMTZ). The “typical” SMTZs (SMTZ1 and SMTZ3) were in the upper part of the gas-bearing layers, where sulfate levels decrease and methane levels increase with depth. Conversely, the “inverse” SMTZs (SMTZ2 and SMTZ4) were in the lower part of the gas-bearing layers, exhibiting an increase in sulfate levels and a decrease in methane levels with depth, a phenomenon rarely documented in previous research. The downward variations of pore water geochemical characteristics indicates that these ions were related to Anaerobic Oxidation of Methane (AOM) processes. The increase in Ca2+ and Ba2+ concentrations and the gradual decrease in sulfate at the SMTZ reflect a series of biogeochemical processes resulting from the dissolution of carbonate and other minerals by AOM. The research indicates that sulfate in AOM may originate from multiple sources. Through analyzing the vertical distribution of shallow gas and the geochemical properties of pore water, this study elucidates the shallow gas formation mechanism and the features of the SMTZ, laying the groundwork for further investigations.

Characterization of the benthic biogeochemical dynamics after flood events in the Rhône River prodelta: a data–model approach

Abstract. At the land–sea interface, the benthic carbon cycle is strongly influenced by the export of terrigenous particulate material across the river–ocean continuum. Episodic flood events delivering massive sedimentary materials can occur, but their short-term impact on carbon cycling is poorly understood. In this paper, we use a coupled data–model approach to estimate the temporal variations in sediment–water fluxes, biogeochemical pathways and their reaction rates during these abrupt phenomena. We studied one episodic depositional event in the vicinity of the Rhône River mouth (NW Mediterranean Sea) during the fall–winter of 2021/22. The distributions of dissolved inorganic carbon (DIC), sulfate (SO42-) and methane (CH4) were measured in sediment porewaters collected every 2 weeks before and after the deposition of a 25 cm sediment layer during the main winter flood event. Significant changes in the distribution of DIC, SO42- and CH4 concentrations were observed in the sediment porewaters. The use of an early diagenetic model (FESDIA) to calculate biogeochemical reaction rates and fluxes revealed that this type of flood event can increase the total organic carbon mineralization rate in the sediment by 75 % a few days after deposition. In this period, sulfate reduction is the main process contributing to the increase in total mineralization relative to non-flood deposition. The model predicts a short-term decrease in the DIC flux out of the sediment from 100 to 55 mmolm-2d-1 after the deposition of the new sediment layer with a longer-term increase by 4 %, therefore implying an initial internal storage of DIC in the newly deposited layer and a slow release over relaxation of the system. Furthermore, examination of the stoichiometric ratios of DIC and SO42- as well as model output over this 5-month window shows a decoupling between the two modes of sulfate reduction following the deposition – organoclastic sulfate reduction (OSR) intensified in the newly deposited layer below the sediment surface, whereas anaerobic oxidation of methane (AOM) intensified at depth below the former buried surface. The bifurcation depth of sulfate reduction pathways, i.e., the sulfate–methane transition zone (SMTZ), is shifted deeper by 25 cm in the sediment column following the flood deposition. Our findings highlight the significance of short-term transient biogeochemical processes at the seafloor and provide new insights into the benthic carbon cycle in the coastal ocean.

Origin and geological significance of the first discovered seafloor oil seepage in the Beikang Basin, southern South China Sea

Seafloor oil seepage, resulting from fluids migration from depth, provides valuable insights into the characteristics of potential source rocks and hydrocarbons. For the first time, seafloor seepage oils were obtained by piston coring in the Beikang Basin, southern South China Sea (SSCS). Based on integrated analysis of the seismic data, core sediments, porewater, headspace gas and heat flow, the geochemical characteristics of seafloor seepage oils and their geological implications were elucidated. Results indicate that deep-sourced hydrocarbons intruded into subsurface sediments via paleo-uplift-related faults, fluid seepage pipes and mixed with in-situ modern marine sediments. Biomarkers suggest the presence of both marine and terrestrial organic matter, as well as a diverse range of thermal maturity levels from immature to mature. The seafloor seepage oil was most likely originated from coal-related source rocks in the Oligocene-Lower Miocene. Deep hydrocarbons were expelled through fractures induced by hydrocarbon-generating overpressure, as well as source rock-connecting faults and finally accumulated atop the paleo-uplift beneath the T3 unconformity. The gradually increased fluid pressure ultimately leads to the fracturing of overlying caprocks, resulting in the formation of fluid seepage pipe and the occurrence of oil seepage on the seafloor. Seafloor oil seepage and the accompanied high methane flux caused biogeochemical process, elements cycling and shoaled the depth of sulfate-methane transition zone (SMTZ), which provides valuable information in indicating gas hydrates occurrence. This study offers significant geological and geochemical information to reveal potential origin of source rocks and hydrocarbons, particularly in the case of limited drilling wells in the SSCS. In addition, this study also advances the understanding of gas-bearing fluids activity and the coupling relationship between deep and shallow strata under a focused fluid seepage background.

The influence of cold seepage on the grain size and geochemistry of sediments from the Laptev Sea shelf

The East Siberian Arctic shelf is renowned for its active methane-rich fluid seeps, confirmed by numerous gas flares in the water column and direct observations of methane release from the seafloor. These cold-seep sites serve as natural laboratories for studying the impact of methane seepage on early diagenesis, authigenic mineral formation, and the geochemical cycles of carbon, sulfur, and other elements. This study examines the grain size and chemical composition of two sediment types from the Laptev Sea shelf: cold-seep sediments influenced by sulfate-driven anaerobic methane oxidation, and unaffected “seep-free” sediments (i.e., control site). High methane concentrations (up to 649 μM/L) in the surface layer of sediments indicate that the sulfate–methane transition zone is found close to the sediment–water interface approximately 5–6 cm below the seabed. At the cold-seep site, thin layers containing up to 40% sand fraction were discovered among the fine-grained sediments. We infer that upward-migrating fluids may contribute to the remobilization of the fine fraction, resulting in concentration of coarse particles. Cold-seep sediments are characterized by molybdenum (Mo) enrichment compared to the lithological background, although one “seep-free” core exhibits evidence of authigenic Mo deposition. The Mo concentration (up to 16.8 ppm) indicates that enrichment occurs under sulfidic conditions restricted to pore water, which are caused by anaerobic methane oxidation. The high Mo concentration in one “seep-free” core likely implies past cold-seep activity. A general Mo–U covariation indicates that the molybdenum enrichment in cold-seep sediments from the Laptev Sea outer shelf is controlled by a weak particulate shuttle process.

Discerning the sulfur geochemical features of turbidites and methane-rich sediments from the South China sea

Upward diffusion of dissolved methane and associated anaerobic oxidation of methane (AOM) coupled with microbial sulfate reduction (MSR) are widespread in normal hemipelagic sediments of continental margins. The occurrence of authigenic sulfur enrichment in such sediments is usually regarded as an indicator of AOM; however, extensive MSR and authigenic iron sulfide generation also occur in turbidites. Therefore, research attention should be directed toward determining the geochemical features of authigenic sulfur mineralization in methane-rich background sediments and turbidites. Here we report the sedimentology and geochemical composition of sediments from four piston cores (CL30, CL44, CL47, and CL3A) containing turbidite layers from methane-rich areas of the South China Sea. The turbidite layers were deposited at 17–15 kyr BP or earlier during the last glacial period and were located above the present sulfate–methane transition zone (SMTZ). The median grain size, proportion of sand-sized grains, and Si, Ti, K, and Zr contents show marked elevation in the turbidite layers compared with the background sediments, indicating the input of large amounts of terrigenous detrital matter with coarser grains. High total sulfur (TS) contents and low δ34SAIS values (AIS: acid-insoluble sulfur) (−35.2‰ to −26.1‰) are observed within turbidite layers relative to the background hemipelagic sediments in core CL44. These results suggest that intensive sulfidization occurred as a result of enhanced organoclastic sulfate reduction rates in turbidite horizons, which were probably caused by the resultant decrease in downward oxygen flux and waning bioturbation after rapid deposition of turbidites. In comparison, TS contents are higher and δ34SAIS values are positive (0.2‰–46.8‰) within the SMTZ on account of the rapid consumption of sulfate and build-up of isotopically heavier hydrogen sulfide pools via intensive AOM. Moreover, authigenic barium (Ba) enrichments commonly occur in the depth interval immediately above the SMTZ rather than within turbidite layers; therefore, they serve as a useful indicator of the position of the SMTZ. This study presents distinct differences in the geochemical compositions of turbidity deposits and methane-rich background sediments and highlights that a multi-proxy approach including sedimentological and geochemical analyses should be used to constrain the types of MSR in deep-sea sediments, given the common occurrence of turbidites and subsurface methane release in marginal deep-sea areas. It is also suggested that if applied with caution, S–C–Ba geochemical patterns can be used to identify palaeo-SMTZs and palaeo methane-rich zones in turbidite-containing strata.