Search Results (4)

The Total Organic Carbon (TOC) content plays a crucial role in gas hydrate exploration because a higher TOC content signifies a greater potential for buried gas hydrates. The regulatory mechanisms governing organic matter in sediment are intricate and influenced by various predominant factors unique to different regions. Notably, the Shenhu area in the South China Sea stands as a pioneering region for methane hydrate research. Despite its significance, limited research has focused on the burial patterns of TOC, resulting in an insufficient dataset to draw definitive conclusions. Consequently, a comprehensive understanding of the burial patterns and controlling factors of TOC within this area remains elusive. This study examines the pore-water characteristics, mineral composition, geochemistry, and sedimentary factors of four distinct sites within the Shenhu region of the South China Sea. The current depths of the Sulfate-Methane Interface (SMI) for sites CL54, CL56, CL57, and CL60 are identified as 28.6, 8.5, 31.9, and 8.1 m below the seafloor (mbsf), respectively. It’s noteworthy that these SMI depths align with locations known to harbor underlying gas hydrates. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analyses reveal that the primary sediment sources within this region encompass microbial shells (such as foraminifera and diatoms), clay, and terrestrial detritus. In addition, marine productivity exhibits a reverse correlation with TOC content, and both TOC content and Ce/Ce* ratios exhibit synchronous fluctuations with sedimentation rate. Drawing from the sedimentation rate, TOC content, as well as indicators of redox conditions (Mo_EF, Ce/Ce*, Mo/U) and productivity proxies (Ba/Al, P/Al) within the sampled sites, it becomes apparent that high sedimentation rate coupled with ‘anaerobic’ conditions foster favorable conditions for TOC accumulation. This comprehensive investigation not only provides valuable datasets but also offers insights into the intricate processes governing TOC accumulation. Full article

We compare sediment vertical methane flux off the Mahia Peninsula, on the Hikurangi Margin, east of New Zealand’s North Island, with a combination of geochemical, multichannel seismic and sub-bottom profiler data. Stable carbon isotope data provided an overview of methane contributions to shallow sediment carbon pools. Methane varied considerably in concentration and vertical flux across stations in close proximities. At two Mahia transects, methane profiles correlated well with integrated seismic and TOPAS data for predicting vertical methane migration rates from deep to shallow sediment. However, at our “control site”, where no seismic blanking or indications of vertical gas migration were observed, geochemical data were similar to the two Mahia transect lines. This apparent mismatch between seismic and geochemistry data suggests a potential to underestimate gas hydrate volumes based on standard seismic data interpretations. To accurately assess global gas hydrate deposits, multiple approaches for initial assessment, e.g., seismic data interpretation, heatflow profiling and controlled-source electromagnetics, should be compared to geochemical sediment and porewater profiles. A more thorough data matrix will provide better accuracy in gas hydrate volume for modeling climate change and potential available energy content. Full article

East Siberian Arctic Shelf, the widest and the shallowest shelf of the World Ocean, covering greater than two million square kilometers, has recently been shown to be a significant modern source of atmospheric methane (CH₄). The CH₄ emitted to the water column could result from modern methanogenesis processes and/or could originate from seabed deposits (pre-formed CH₄ preserved as free gas and/or gas hydrates). This paper focuses primarily on understanding the source and transformation of geofluid in the methane seepage areas using ions/trace elements and element ratios in the sediment pore-water. Six piston cores and totally 42 pore-water samples were collected in the East Siberian Sea and the Laptev Sea at water depths ranging from 22 to 68 m. In the active zones of methane release, concentrations of vanadium, thorium, phosphorus, aluminum are increased, while concentrations of cobalt, iron, manganese, uranium, molybdenum, copper are generally low. The behavior of these elements is determined by biogeochemical processes occurring in the pore-waters at the methane seeps sites (sulfate reduction, anaerobic oxidation of methane, secondary precipitation of carbonates and sulfides). These processes affect the geochemical environment and, consequently, the species of these elements within the pore-waters and the processes of their redistribution in the corresponding water–rock system. Full article

Widespread cold seeps along continental margins are significant sources of dissolved carbon to the ocean water. However, little is known about the methane turnovers and possible impact of seepage on the bottom seawater at the cold seeps in the South China Sea (SCS). We present seafloor observation and porewater data of six push cores, one piston core and three boreholes as well as fifteen bottom-water samples collected from four cold seep areas in the northwestern SCS. The depths of the sulfate–methane transition zone (SMTZ) are generally shallow, ranging from ~7 to <0.5 mbsf (meters below seafloor). Reaction-transport modelling results show that methane dynamics were highly variable due to the transport and dissolution of ascending gas. Dissolved methane is predominantly consumed by anaerobic oxidation of methane (AOM) at the SMTZ and trapped by gas hydrate formation below it, with depth-integrated AOM rates ranging from 59.0 and 591 mmol m⁻² yr⁻¹. The δ¹³C and Δ¹⁴C values of bottom-water dissolved inorganic carbon (DIC) suggest discharge of ¹³C- and ¹⁴C-depleted fossil carbon to the bottom water at the cold seep areas. Based on a two-endmember estimate, cold seeps fluids likely contribute 16–26% of the bottom seawater DIC and may have an impact on the long-term deep-sea carbon cycle. Our results reveal the methane-related carbon inventories are highly heterogeneous in the cold seep systems, which are probably dependent on the distances of the sampling sites to the seepage center. To our knowledge, this is the first quantitative study on the contribution of cold seep fluids to the bottom-water carbon reservoir of the SCS, and might help to understand the dynamics and the environmental impact of hydrocarbon seep in the SCS. Full article