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

Abstract

ABSTRACT1 Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC 2 Institute of Oceanography, National Taiwan University, Taipei, Taiwan, ROC 3 Central Geological Survey, MOEA, Taipei, Taiwan, ROC * Corresponding author address: Prof. Tsanyao Frank Yang, Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC; E-mail: tyyang@ntu.edu.tw 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.

Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

“…Further evidence of gas vents came from seafloor photographs taken by a deep-tow camera system that show authigenic carbonates, carbonate mounds, chimney structures, and clam communities associated with bacterial mats on the sea floor where geochemical proxies for methane flux are high (S. Lin, personal communication). With all the geophysical data presented in this study, together with many geological observations (e.g., Jiang et al 2006;Huang et al 2006;Horng and Chen 2006) and geochemical analyses results (e.g., Lin et al 2006;Chuang et al 2006;Yang et al 2006), we suggest that enormous amounts of gas hydrate should exist beneath the seafloor off SW Taiwan.…”

Methane Venting in Gas Hydrate Potential Area Offshore of SW Taiwan: Evidence of Gas Analysis of Water Column Samples

“…This indicates that abundant gas hydrate deposits may exist in the sediments of both the passive and active continental margins offshore of SW Taiwan. In addition to the BSRs, geological and geochemical data also support the interpretation that gas hydrates may exist in southwestern Taiwan continental margin sediments (Chao and You 2006;Chuang et al 2006;Horng and Chen 2006;Huang et al 2006;Jiang et al 2006;Lin et al 2006;Oung et al 2006;Shyu et al 2006). Hence, the region has been considered a gas hydrate potential area.…”

“…We can observe higher radon concentrations wherever there is a higher flux of carrier gases at fault zones or hydrothermal areas. Initially we expected to observe high radon concentrations, especially at sites with high methane flux (e.g., Chuang et al 2006). However, in this survey, we did not measure any radon concentration in seawater higher than 50 Bq m -3 , which is the background radon concentration of ocean and tap water used.…”

Geological Study of Active Cold Seeps in the Syn-collision Accretionary Prism Kaoping Slope off SW Taiwan

“…13). Additional pieces of evidence in support of this region as active cold seep sites are the occurrences of distinct BSRs, shallow SMI and high concentration of dissolved methane in the bottom water (Chi et al 1998;Chuang et al 2006;Lin et al 2006;Liu et al 2006;Yang et al 2006).…”