Estimation of advective methane flux in gas hydrate potential area offshore SW Taiwan and its tectonic implications

With the discoveries of Bottom Simulating Reflectors (BSRs), large and dense chemosynthetic communities and rapid sulfate reductions in pore space sediments, gas hydrates may exist in offshore southwestern Taiwan. Methane concentrations in pore space sediments have been measured to investigate if fluids and gases are derived from dissociation of gas hydrates. Very high methane concentrations and very shallow depths of sulfate methane interface (SMI) imply the high methane flux underneath the seafloor. Linear sulfate gradients, low total organic carbon (TOC) have been combined to describe the process of anaerobic methane oxidation (AMO) and calculate the iffusive methane flux in Chuang et al. (2010). However, the appearance of concave (or non-linear) profiles of sulfate in some cores might indicate advective fluid flows. Hence, the methane flux may be much greater under advective conditions. In this study, numerical transport-reaction models were applied to calculate the methane flux including diffusion and advection of dissolved sulfate and methane and the anaerobic methane oxidation of methane. According to the modeled results of three giant piston cores (MD05-2911, MD05-2912 and MD05-2913) collected during the r/v Marion Dufresne cruise in 2005, gas bubbling or bioirrigation may occur in these site. Values of the methane flux ranging from 1.91 to 5.17 mmol m-2yr-1 and upward fluid flow velocities around 0.05-0.13 cm yr-1 are related to different geologic structures in the active continental margin. Site MD05-2912 is located on the Tainan Ridge where anticlines and blind thrusts are the dominate structures. Site MD052911 is on the Yung-An Ridge characterized by emergent and imbricate thrusts

Methane Migration and Its Influence on Sulfate Reduction in the Good Weather Ridge Region, South China Sea Continental Margin Sediments

Bacteria sulfate reduction is a major pathway for organic carbon oxidation in marine sediments. Upward diffusion of methane from gas hydrate deep in the sedimentary strata might be another important source of carbon for sulfate reducing bacteria and subsequently induce higher rates of sulfate reduction in sediments. Since abundant gas may migrate upward to the surface as a result of tectonic activity occurring in the accretionary wedge, this study investigates the effect of methane migration on the sulfate reduction process in continental margin sediments offshore southwestern Taiwan. Piston and gravity core samples were taken in order to evaluate vertical and spatial variations of sulfate and methane. Pore water sulfate, sulfide, methane, sediment pyrite, and organic carbon were extracted and analyzed

Extremely High Methane Concentration in Bottom Water and Cored Sediments from Offshore Southwestern Taiwan

It has been found that Bottom Simulating Reflections (BSRs), which infer the existence of potential gas hydrates underneath seafloor sediments, are widely distributed in offshore southwestern Taiwan. Fluids and gases derived from dissociation of gas hydrates, which are typically methane enriched, affect the composition of seawater and sediments near venting areas. Hence, methane concentration of seawater and sediments become useful proxies for exploration of potential gas hydrates in a given area. We systematically collected bottom waters and sedimentary core samples for dissolved and pore-space gas analyses through five cruises: ORI-697, ORI-718, ORII-1207, ORII-1230, and ORI-732 from 2003 to 2005 in this study. Some sites with extremely high methane concentrations have been found in offshore southwestern Taiwan, e.g., sites G23 of ORI-697, N8 of ORI-718, and G96 of ORI-732. The methane concentrations of cored sediments display an increasing trend with depth. Furthermore, the down-core profiles of methane and sulfate reveal very shallow depths of sulfate methane interface (SMI) at some sites in this study. It implies sulfate reduction being mainly driven by the process of anaerobic methane oxidation (AMO) in sediments; thus indicating that there is a methane-enriched venting source, which may be the product of dissociation of gas hydrates in this area

An experimental study of the solubility and thermodynamic properties of nickel in the system NIO + HCL + H₂O

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