Complicated Magnetic Mineral Assemblages in Marine Sediments Offshore of Southwestern Taiwan: Possible Influence of Methane Flux on the Early Diagenetic Process

Abstract

ABSTRACT1 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC * Corresponding author address: Dr. Chorng-Shern Horng, Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC; E-mail: cshorng@earth.sinica.edu.twIn this paper, we present the results of down core variations in magnetic susceptibility and magnetic mineral composition of fifteen piston cores retrieved from the active continental margin offshore of southwestern Taiwan, where a wide distribution of bottom simulating reflectors (BSRs) related to gas hydrate layers has been detected. X-ray analysis on magnetic extracts from the cores indicates that detrital magnetite and authigenic greigite, in various proportions, are the dominant magnetic minerals in sediments. Non-magnetic, authigenic pyrite is generally associated with greigite, but it may co-exist with magnetite or may solely occur in sediments. Consequently, the sediment sequences of the fifteen piston cores have complicated magnetic mineral assemblages that result in various magnetic susceptibility profiles. For core segments containing detrital magnetite as the only magnetic mineral, values of magnetic susceptibility are moderate with small variations (8 -15 × 10 -6 SI), which can be regarded as the susceptibility backgrounds for the initially deposited sediments. However, high magnetic susceptibilities relative to the backgrounds were found in core segments where magnetic mineral is enriched in greigite. Magnetic susceptibilities lower than the backgrounds were found in segments where neither magnetite nor greigite were detected. The complicated occurrence of magnetic minerals reveals that their host sediments at different levels have suffered various degrees of early diagenetic process ranging from oxic to anoxic conditions. Due to very low sedimentary organic matter content in the study area, detrital magnetite persisted in sediments that were subjected to Terr. Atmos. Ocean. Sci., Vol. 17, No. 4, December 2006 1010 initially oxic (or sub-oxic) conditions. However, the sedimentary organic matter is apparently insufficient for providing reducing environments for the formation or enrichment of authigenic iron sulfides. Instead, we propose that gaseous methane derived from gas hydrates in deeper sedimentary layers should be the most likely source of extra organic matter. The different flux intensities of this gas either through slow diffusion or rapid venting resulted in various anoxic conditions, which caused the dissolution or survival of magnetite and the formation or enrichment of greigite and pyrite.

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.…”

“…Geological and geochemical characteristics of the cored sediments have been investigated, they include: grain sizes, mineral components and physical properties of the sediments and clay mineral distribution (Jiang et al 2006), SO 4 2-, Cl -, organic carbon and sulfide distribution in the sediments , salinity and CH 4 concentration in the bottom water and CH 4 and carbon isotopic composition of the pore-space gases of cored sediments Yang et al 2006), B and Cl isotopes in the pore waters (Chao and You 2006), and magnetic properties of magnetic minerals in the sediments (Horng and Chen 2006). Methanederived authigenic carbonate with highly depleted carbon isotope values (-54 to -43%) have been found both on and offshore of 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.…”

Magnetic mineralogy and its implication of contemporary coastal sediments from South China

“…Magnetic minerals and their sources must be determined before one can relate magnetic parameters of marine sediments to the development history (Hilton 1987;Yu and Oldfield 1989). Some researches (Zhang et al 2001;Horng and Chen 2006;Garming 2006) discussed magnetic mineral assemblages and their influencing factors for intertidal and marine sediments. However, the mechanism between magnetic properties and minerals within coastal sediments remains unclear.…”