Depth Of Sulfate-methane Transition Zone Research Articles

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.

Methane Index: Towards a quantitative archaeal lipid biomarker proxy for reconstructing marine sedimentary methane fluxes

In marine sediments, anaerobic methane oxidation (AOM) carried out by consortia of methanotrophic archaea (ANME groups) and sulfate reducing bacteria (SRB) represents a major sink for methane (40–290 Tg of C yr−1). The variability of seafloor methane seepage and AOM is crucial to sedimentary biogeochemistry and global carbon/sulfur cycling, yet poorly constrained for Earth’s history. Based on the rationale that a major group of ANME preferentially synthesizes certain tetraether lipids, Methane Index (MI) measured from sedimentary archaeal lipid biomarkers called GDGTs (glycerol dialkyl glycerol tetraethers) has been widely used as a qualitative indicator for methane-impact on sediments. However, it was unclear whether small amounts of diagenetic methane in porewater could cause high MI values or that high methane flux associated with, for example cold seeps, is required. Since not all ANME groups synthesize GDGTs, another contested issue is whether this index is broadly representative of AOM. Here, we use modern porewater and sedimentary lipid profiles from the world’s ocean to further explore MI’s potential to address these questions. The new data show that MI is quantitatively linked to sedimentary methane diffusive flux and the depth of sulfate-methane transition zone (SMTZ) which is the diagenetic front where AOM activity is mostly concentrated. In particular, high MI values (>0.3–0.5) are strongly associated with high methane fluxes (>∼0.1 mmol m−2 d−1) and shallow SMTZ depths (<1–3 m) commonly linked to gas hydrate dissociations. Sensitivity analyses suggest that when the variability of several key parameters (e.g., seawater sulfate concentration, geothermal gradient, and sediment porosity) is small, modern MI – methane flux and MI – SMTZ depth relationships can be applied to reconstruct methane history and diagenetic zonation of the geological past. Although only Group I of ANME archaea synthesizes GDGTs, the co-occurrence of AOM biomarkers (e.g., crocetane, PMI, and hydroxyarchaeol) that are attributed to ANME-2 or ANME-3 and high MI values suggest that MI is broadly representative of AOM. Lastly, we applied MI to probe the methane venting history by using new data from a Cascadia Margin site (Hydrate Ridge, IODP U1328) since the penultimate interglacial, and reinterpreted recently reported MI data from three Southern Ocean sites (ODP 1168, 1170, and IODP U1356) during the late Oligocene – early Miocene. Overall, these applications demonstrate that MI can serve as a sensitive and quantitative proxy to help broadly establish Cenozoic and Mesozoic history of marine methane biogeochemical cycling.

Vertical distribution of archaeal communities in cold seep sediments from the jiulong methane reef area in the south China sea

Jiulong methane reef, a large cold seep area on the continental slope of the South China Sea (SCS), was characterized by chemosynthetic fauna, bacterial mats, methane-derived carbonate structures, and a shallow depth of sulfate-methane transition zone (SMTZ) in gravity cores. To characterize the microbial diversity and variation in correlation to methane venting in the cold seep area, the archaeal community in sediments of the gravity Core CL11 was analyzed, using the Illumina MiSeq sequencing system. A total of 96,917 valid sequences were obtained and classified into 275 OTUs. Taxonomic analysis showed that all the archaeal sequences belonged to the phyla of Crenarchaeota (69.41%) and Eurarchaeota (25.26%). Vertical distribution of the archaeal communities showed that the Marine Benthic Group B (MBG-B) was the predominant group (58.92%) in the topmost sediment of the core, while percentages of the South African Gold Mine Euryarchaeotal Group (SAGMEG) sequences increased with depth. Our findings indicate that the archaeal community in the cold seep of Jiulong Methane Reef is unique and provides us new insights into the archaeal community composition of the cold seep area.

Enhanced methane flux event and sediment dispersal pattern in the Krishna–Godavari offshore basin: Evidences from rock magnetic techniques

Detailed rock magnetic measurements such as susceptibility (χ), susceptibility of anhysteric remanent magnetization (ΧARM), saturation isothermal remanent magnetization (SIRM), soft-isothermal remanent magnetization (soft-IRM) and hard-IRM (HIRM) were carried out on the topmost sediment samples of 73 gravity cores collected in the Krishna–Godavari (KG) offshore basin to understand the distribution of magnetic parameters and the associated diagenetic signatures related to upward methane flux. The variation in magnetic parameters in the KG offshore basin fluctuates around the average value (Χ = 105 × 10−8 m3 kg−1, ΧARM = 1.22 × 10−5 m3 kg−1, SIRM = 2040 × 10−5 Am2 kg−1, soft-IRM = 1807 and HIRM = 82) except at the location GC-07 (15.767°N, 81.814°E) where the magnetic parameters Χ, ΧARM and SIRM drop to ∼1/6th, ∼1/14th and ∼1/8th of the average near surface value indicating substantial loss in the concentration of primary magnetic iron oxide minerals. Anomalous low value of ΧARM/Χ and ΧARM/SIRM at GC-07 suggests that the magnetic remanence property is governed by coarser pseudo-single domain (PSD) or multi-domain (MD) grains. To understand the nature of the anomalous magnetic parameters at GC-07, complete downcore variation of magnetic parameters was also studied. The magnetic parameters were found to be even more depleted at all depths with susceptibility dropping (at 5.2 mbsf) to ∼1/200th of its average near surface value. Interestingly, this core shows a rapid increase in the concentration of dissolved methane from ∼0 nM at surface to ∼153 nM at around 5.2 mbsf where the sulfate concentration is also reduced to zero. The anaerobic oxidation of methane (AOM) and the sulfate methane transition zone (SMTZ) occur between ∼4.2 and 5.2 mbsf based on the present day sulfate concentration profile. The close association between the dissolution of magnetic iron oxide minerals and AOM at GC-07 suggests that the reduced magnetization is primarily due to the release of HS− by AOM. The magnetic iron oxide minerals are dissolved not only at SMTZ but also above the current depth of the SMTZ. It is likely that the methane flux was higher in the past leading to upward shift of the SMTZ, and the HS− released by AOM facilitated the dissolution of magnetic iron oxide minerals at shallower depths. Alternatively, the HS− diffusing upward from the current depth of SMTZ might have lead to dissolution at shallower depth. Therefore, additional geological/geochemical investigations are warranted at GC-07 to establish the dissolution mechanism of magnetic iron oxide minerals.The variability in different magnetic parameters within the slope basin and channels shows systematic decrease in the concentration of magnetic minerals and increase in the abundance of stable single domain (SSD) magnetic grains with water depth. The observed strong correlation between the silt fraction and the magnetic susceptibility for the slope basin and channels suggests that the variation in magnetic parameters is predominantly controlled by the sediment sorting/transport process as finer particles are deposited in deeper water.

Diagenesis of magnetic minerals in a gas hydrate/cold seep environment off the Krishna–Godavari basin, Bay of Bengal

We carried out detailed magnetic measurements of the core (MD161/8) located in the vicinity of Site NGHP-01-10, where ~128 m of hydrate is confirmed by drilling/coring, to understand the diagenesis of magnetic minerals in a gas hydrates/cold seep environment. The rock magnetic measurements along with SEM-EDS and XRD analyses show a zone of reduced magnetic susceptibility (zone 2) where most of the magnetic minerals are dissolved. The enhanced concentration of chromium reducible sulfur (CRS) in this zone suggests an intense pyritization process while isotopically depleted authigenic carbonates indicate sulfate reduction via anaerobic oxidation of methane (AOM). Therefore, the dissolution of magnetic minerals is attributed to the HS - released during AOM that has resulted in the reduction in the magnitude of magnetic parameters. Within zone 2, a zone of enhanced susceptibility (zone 2a) is observed between 17.68 and 23.6 mbsf, and is located beneath the present day sulfate-methane transition zone (SMTZ). The frequencydependent magnetic susceptibility and low temperature magnetic measurements suggest the abundance of fine grained superparamagnetic (SP) sized ferrimagnetic particles. The SEM-EDS and XRD analyses show the presence of greigite which occurs in interstices between the pyrite crystals. Such occurrence of greigite in sediments has important implications in the interpretation of paleomagnetic records. We evaluated the likely mechanism for the greigite formation in KG offshore basin and our data suggest that the formation of greigite may be related to either paleo-SMTZ or anaerobic oxidation of pyrite. It is unlikely that the formation of greigite can be explained by the downward diffusion of sulfide below the current depth of SMTZ. However, further investigations are required to ascertain the mechanism for the