Origin and nature of cold seep in northeastern Dongsha area, South China Sea: Evidence from chimney-like seep carbonates

Abstract

The occurrence of seep carbonates is one of the characteristic features for cold seep sites at continental margins. The carbonates documented the venting history of methane-rich fluid. Compared to the chemoherm carbonates and carbonate pavements which formed on the sediment-water interface, chimney-like seep carbonates precipitated around fluid conduits below the sediment-water interface therefore better recording information of the past fluid flow and composition. Here the chimney-like seep carbonate samples from the northeastern Dongsha area of the South China Sea were studied 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 and C-and O-isotope compositions, combined with present and past bottom water temperatures and the timing of methane release events, the oxygen isotopic fractionation between calcite and water were used to estimate the equilibrium δ18 O values of the precipitating fluids. The δ 13 C 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 δ 18 O 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. 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.South China Sea, seep carbonate chimneys, carbon and oxygen isotopes, gas hydrate deposits, methane-rich fluids Citation:

A unique Fe-rich carbonate chimney associated with cold seeps in the Northern Okinawa Trough, East China Sea

“…Excluding the impact of the two subsamples of F5 and G15 situated in the latter filled branch channels, the chimney wall was obviously depleted in the carbon isotopic composition (δ 13 value ranging from À43.66‰ to À 28.09‰, average: 38.21‰, V-PDB, n¼27), which is slightly enriched than that in the South China Sea (Han et al, 2013;Tong et al, 2013). The lightest δ 13 C value was within the boundary between methane-produced either biogenically or thermogenically (Whiticar, 1999).…”

Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea

“…Geochemical proxies such as stable carbon isotope records of authigenic carbonates have been applied to evaluate the temporal variation of methane seep, owing to their very negative d 13 C (Peckmann and Thiel, 2004;Han et al, 2004Han et al, , 2008Han et al, , 2013Mazumdar et al, 2009Mazumdar et al, , 2011Bayon et al, 2009aBayon et al, , 2013Bayon et al, , 2015Cr emi ere et al, 2013;Tong et al, 2013;Tribovillard et al, 2013;Feng et al, 2014Feng et al, , 2015a. However, such carbonates are most intense at intermediate flow rates, and usually do not form in low and high flux conditions (Luff and Wallmann, 2003;Karaca et al, 2010).…”

Magnetic separation of calcite and aragonite for use in radioactive carbon analysis

“…1c). The chimney in this area do not contain visible conduits or channels, like other cold seep carbonate chimneys such as at the Kuroshima Knoll (Takeuchi et al, 2001); in northeastern Dongsha area, South China Sea (Han et al, 2013); or from Campos Basin offshore Brazil (Wirsig et al, 2012), that commonly contain conduits and channels. Nevertheless, the chimneys from Conical Seamount are still described as chimneys, primarily as the textures of our samples are analogous to the interior structures of the upper sections of other cold seep carbonate chimneys.…”