Nano-sized graphitic carbon in authigenic tube pyrites from offshore southwest Taiwan, South China Sea, and its implication for tracing gas hydrate

Abstract

Gas hydrates are a significant energy resource and are usually detected by bottom simulating reflection and submarine geochemical anomalies. Authigenic minerals are related to gas hydrates, with carbonates, sulfates and sulfides being important tracing minerals. Authigenic tubular pyrites were collected from offshore southwest Taiwan in the South China Sea, and were investigated by scanning electron microscopy(SEM) and high-resolution transmission electron microscopy (HRTEM). Authigenic tubular pyrite was composed of framboidal pyrite, within which nanosized graphitic carbon of low crystallinity was discovered. The graphitic carbon coexisted with pyrite and had a texture similar to carbon nanotubes and nanocones, indicating that they likely precipitated from carbon supersaturated C-H-O fluid. Pyrite may act as a catalyst for the conversion of CH 4 to C. The discovery of nanosized graphitic carbon in pyrite indicated it was deposited in sediments that were supersaturated with methane fluid. Thus, nanosized graphitic carbon may be another tracing species for submarine gas hydrates. The discovery of nanosized graphitic carbon deposited in a low temperature environment will enlighten our understanding of the laboratory synthesis and industrial production of graphitic carbon.nano-sized graphitic carbon, authigenic tube pyrite, gas hydrate, South China Sea Citation:Zhang M, Sun X M, Xu L, et al. Nano-sized graphitic carbon in authigenic tube pyrites from offshore southwest Taiwan, South China Sea, and its implication for tracing gas hydrate. Chinese Sci Bull, 2011Bull, , 56: 2037Bull, −2043Bull, , doi: 10.1007 Gas hydrates are an ice-like crystalline material consisting predominantly of methane and water molecules, and are formed under high pressure and low temperature on continental slopes/rises [1]. As a newly discovered clean energy source during the last two decades, gas hydrates are gaining worldwide attention on account of their importance in energy exploration, and through increasing awareness of seafloor structure geo-hazards (stability) and global climate change. Gas hydrates predominantly occur at depths of 300-3000 m around the edges of continents and in marginal marine basins. Most information on the distribution, concentration, composition and origin of gas hydrates in deep marine sediments comes from geophysical evidence such as bottom simulating reflection and strong seafloor acoustic reflectivity. These techniques are commonly used to infer the distribution of gas hydrates on continental margins, in addition to gas hydrate samples collected during the deep sea drilling project and ocean drilling program. Pyrite is a common authigenic mineral within the marine sediments of the continental margin. It is the product of

Calcium isotope fractionation and its controlling factors over authigenic carbonates in the cold seeps of the northern South China Sea

“…In recent years, many gas hydrate cold seeps have been found in the northern South China Sea. Occurrences of gas hydrates by the observation of BSRs and cold seeps have also been reported in southwestern Taiwan [43][44][45]. Chen et al [46] reported for the first time the occurrence of carbonates originating from cold seeps in the South China Sea and showed that those carbonates probably formed at presently active gas seepage sites.…”

The Morphological Characteristics of Authigenic Pyrite Formed in Marine Sediments

“…Except for the individual crystal, pyrite microcrystals are mostly found assembled as framboids [37]. [38][39][40][41][42], octahedral form [8,38,[43][44][45], pyritohedral form [12,38,43,[45][46][47], and intermediate [13,33,44,48,49] [38][39][40][41][42], octahedral form [8,38,[43][44][45], pyritohedral form [12,38,43,[45][46][47], and intermediate [13,33,44,48,49] under different marine environments, among which (O6,C5,C6,I1,I5,P5,P6) are BSE images, (O1-O5, C1-C4,I2-I4,I6,P1-P4) are SE images.…”

“…Previous studies revealed a sequence of morphology evolution of pyrite crystals [7] and pyrite aggregates [8], the morphological characteristics and size distribution of pyrite in normal marine sediment [9][10][11] and in sulfate-methane transition zone (SMTZ) [12][13][14], the various growth conditions during pyrite formation [15]. The morphology of pyrite provides important scientific significance for studying marine sedimentary successions, tracking microbial activities, and diagenetic processes [9,[16][17][18].…”

“…Figure 2. The scanning electron micrographs of euhedral pyrite in cubic form[38][39][40][41][42], octahedral form[8,38,[43][44][45], pyritohedral form[12,38,43,[45][46][47], and intermediate[13,33,44,48,49] under…”

“…Figure 2. The scanning electron micrographs of euhedral pyrite in cubic form[38][39][40][41][42], octahedral form[8,38,[43][44][45], pyritohedral form[12,38,43,[45][46][47], and intermediate[13,33,44,48,49] under different marine environments, among which (O6,C5,C6,I1,I5,P5,P6) are BSE images, (O1-O5, C1-C4,I2-I4,I6,P1-P4) are SE images.…”