Carbon and Oxygen Isotope Characteristics of the Growth Profile of a Seep Carbonate Chimney from the Northeastern Slope of the South China Sea and Its Formation Model

Key words: 
• northeastern slope of the South China Sea /
• cold seep carbonate /
• carbon and oxygen isotopes /
• cold seep fluid sources, formation model /
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Abstract: This chimney(TVG14C11) was collected by SO177 cruise in 2004 from the northeastern Dongsha, South China Sea. It is composed of Mgcalcite.  Author did high resolution sampling on a cross section of the chimney to analyze their Cand Oisotope compositions. Based on their mineral composition, we used carbonatewater fractionation equation to calculate the δ18O of the precipitating fluid and discussed the fluid sources. We showed that the δ13C of the chimney varied from -50.14‰ to -43.92‰, and δ18O from 2.76‰ to 4.85‰. From the inner to the outer, the δ13C increased gradually while δ18O was decreaseing. The equilibrium δ18O of the precipitating fluid were in the range of 1.2‰-2.3‰VSMOW, which were significantly heavier than the modern seawater or glacial water. It is suggested that the water released from gas hydrate dissociation were responsible for the 18O enrichment. According to the two end-member mixing model, it was calculated that the precipitating fluid consisted of 53.6% gas hydrate water and 46.4% seawater during the precipitation stage of the inner layer. It got diluted gradually when it diffused into the sediments to form the outer layer, where the contribution of gas hydrate water decreased to 6.1%. Based on the growth structure and the variation of carbon and oxygen isotope composition, a growth model of the seep carbonate chimney was proposed.