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OS1C-4：CHIMNEY-LIKE SEEP CARBONATES IN NORTHEASTERN DONGSHA AREA, SOUTH CHINA SEA: EVIDENCE OF GAS HYDRATE DESTRUCTION DURING LOW SEA-LEVEL STANDS
发布时间:2014-07-28
Xiqiu HAN 1,2*, Kehong YANG 1,2 , Yongyang HUANG 3
1. Key Laboratory of Submarine Geosciences, State Oceanic Administration, China; 2. Second Institute of Oceanography, State Oceanic Administration, China; 3. Guangzhou Marine Geological Survey, China
Seep carbonates precipitated around the cold seep sites documented the information of the past methane-rich fluid flow at continental margins. In this paper, chimney-like seep carbonate samples from the northeastern Dongsha area of the South China Sea were collected and studied in detail to understand the origin and nature of the venting fluids and their potential relationship with gas hydrate deposits underneath the seafloor. Based on the occurrence, morphology, petrology, mineralogy, C- and O- isotope compositions and age dating of the carbonates, combined with present and inferred past bottom water temperatures, the oxygen isotopic fractionation between calcite and water were used to estimate the equilibrium δ18O values of the precipitating fluids. The δ13C values ranging from −56.33‰ to−42.70‰ V-PDB and thus clearly show that the studied chimneys were mainly derived from biogenic methane oxidation. The calculated equilibrium δ18O values of the precipitating fluids ranged from 1.9‰ 0.3‰ to 0.6‰ 0.3‰ V-SMOW, with an average of 1.4‰ 0.3‰ V-SMOW which is heavier than those of seawater even at the last glacial maximum. It is considered that the formation of chimney-like carbonates was closely related to methane hydrate dissociation in the area during low sea-level stands. The methane hydrates contributed as much as 45.7% of water to the venting fluids. It is suggested that the climate and environmental changes (e.g.sea-level lowering, down-cutting canyons and mass wasting) are the major mechanisms maybe responsible for the destabilization of methane hydrates in the study area. The extensive occurrence of seep carbonates indicates that a large amount of the methane released from methane hydrate dissociation has been effectively captured and sequestered by microbial anaerobic oxidation of methane (AOM) before it escapes into the water column and atmosphere.
1. Key Laboratory of Submarine Geosciences, State Oceanic Administration, China; 2. Second Institute of Oceanography, State Oceanic Administration, China; 3. Guangzhou Marine Geological Survey, China
Seep carbonates precipitated around the cold seep sites documented the information of the past methane-rich fluid flow at continental margins. In this paper, chimney-like seep carbonate samples from the northeastern Dongsha area of the South China Sea were collected and studied in detail to understand the origin and nature of the venting fluids and their potential relationship with gas hydrate deposits underneath the seafloor. Based on the occurrence, morphology, petrology, mineralogy, C- and O- isotope compositions and age dating of the carbonates, combined with present and inferred past bottom water temperatures, the oxygen isotopic fractionation between calcite and water were used to estimate the equilibrium δ18O values of the precipitating fluids. The δ13C values ranging from −56.33‰ to−42.70‰ V-PDB and thus clearly show that the studied chimneys were mainly derived from biogenic methane oxidation. The calculated equilibrium δ18O values of the precipitating fluids ranged from 1.9‰ 0.3‰ to 0.6‰ 0.3‰ V-SMOW, with an average of 1.4‰ 0.3‰ V-SMOW which is heavier than those of seawater even at the last glacial maximum. It is considered that the formation of chimney-like carbonates was closely related to methane hydrate dissociation in the area during low sea-level stands. The methane hydrates contributed as much as 45.7% of water to the venting fluids. It is suggested that the climate and environmental changes (e.g.sea-level lowering, down-cutting canyons and mass wasting) are the major mechanisms maybe responsible for the destabilization of methane hydrates in the study area. The extensive occurrence of seep carbonates indicates that a large amount of the methane released from methane hydrate dissociation has been effectively captured and sequestered by microbial anaerobic oxidation of methane (AOM) before it escapes into the water column and atmosphere.
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