In this study, we analyzed stable calcium isotope results of authigenic carbonates from two cold seep areas of the Dongsha area and the Baiyun Sag in the northern South China Sea. The stable isotopes of carbon and oxygen as well as the mineral composition of authigenic carbonates were used to investigate control calcium isotope fractionation. The δ44/40Ca ratios of the southwestern Dongsha area samples ranged from 1.21‰ to 1.52‰ and the ratio of the Baiyun Sag sample was 1.55‰ of the SRM915a isotope standard. X-ray diffraction analysis showed that the carbonate samples consisted of dolomite, calcite and aragonite, with small amounts of high-Mg calcite and siderite. The δ13C values of the carbonates of the southwestern Dongsha area varied between δ49.21‰ and δ16.86‰ of the Vienna PeeDee Belemnite (VPDB) standard and the δ18O values ranged from 2.25‰ to 3.72‰ VPDB. The δ13C value of the Baiyun Sag sample was 2.36‰ VPDB and the δ18O value was 0.44‰ VPDB. The δ13C values of the carbonates of the southwestern Dongsha area revealed there is methane seeping into this area, with a variable contribution of methane-derived carbon. The sampled carbonates covered a range of δ13C values suggesting a dominant methane carbon source for the light samples and mixtures of δ13C values for the heavier samples, with possibly an organic or seawater carbon source. The δ18O values indicated that there is enrichment in 18O, which is related to the larger oxygen isotope fractionation in dolomite compared to calcite. The results of the Baiyun Sag sample exhibited normal seawater carbon and oxygen isotopic values, indicating that this sample is not related to methane seepage but instead to precipitation from seawater. The relatively high δ44/40Ca values indicated either precipitation at comparatively high rates in pore-water regimes with high alkalinity, or precipitation from an evolved heavy fluid with high degrees of Ca consumption (Raleigh type fractionation). The dolomite samples from the Dongsha area revea
WANG ShuHongYAN WenMAGALHAES H VitorCHEN ZhongPINHEIRO M LuisGUSSONE Nikolaus
Here we reported the fatty-acids and their δ13 C values in seep carbonates collected from Green Canyon lease block 185 (GC 185;Sample GC-F) at upper continental slope (water depth:～540 m),and Alaminos Canyon lease block 645 (GC 645;Sample AC-E) at lower continental slope (water depth:～2200 m) of the Gulf of Mexico.More than thirty kinds of fatty acids were detected in both samples.These fatty acids are maximized at C 16.There is a clear even-over-odd carbon number predominance in carbon number range.The fatty acids are mainly composed of n-fatty acids,iso-/anteiso-fatty acids and terminally branched odd-numbered fatty acids (iso/anteiso).The low δ13 C values (-39.99‰ to -32.36‰) of n-C 12:0,n-C 13:0,i-C 14:0 and n-C 14:0 suggest that they may relate to the chemosynthetic communities at seep sites.The unsaturated fatty acids n-C 18:2 and C 18:1 △9 have the same δ13 C values,they may originate from the Beggiatoa/Thioploca.Unlike other fatty acids,the terminally branched fatty acids (iso/anteiso) show lower δ13 C values (as low as -63.95‰) suggesting a possible relationship to sulfate reducing bacteria,which is common during anaerobic oxidation of methane at seep sites.
GUAN HongXiangFENG DongWU NengYouCHEN DuoFuROBERTS H HWU DaiDai
Seep carbonates were collected from the Alaminos Canyon lease area, Gulf of Mexico. The carbonates are present as slabs and blocks. Bivalve shell and foraminifer are the dominant bioclasts in carbonate. Pores are common and usually filled with acicular aragonite crystals. XRD investigation shows that aragonite is the dominate mineral (98%). Peloids, clotted microfabirc and botryoidal aragonite are de-veloped in carbonate and suggest a genesis linked with bacterial degradation of the hydrocarbons. The δ13C value of bioclasts in carbonate is from-4.9‰ to-0.6‰, indicating that the carbon source is mainly from sea water as well as the small portion incorporation of the seep hydrocarbon. The microcrystalline and sparite aragonite shows the δ13C value from-31.3‰ to-23.4‰, suggesting that their carbon is derived mainly from microbial degradation of crude oil. 14C analyses give the radiocarbon age of about 10 ka. Rare earth elements (REE) analyses of the 5% HNO3-treated solution of the carbonates show that the total REE content of the carbonates is low, that is from 0.752 to 12.725 μg·g-1. The shale-normalized REE patterns show significantly negative Ce anomalies. This suggests that cold seep carbonate is most likely formed in a relatively aerobic environment.
