Petrographic and geochemical characterization of seep carbonate from Alaminos Canyon,Gulf of Mexico

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 developed 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. Supported by the National Natural Science Foundation of China (Grant Nos. 40725011 and U0733003), the Hundred Talents Program of the Chinese Academy of Sciences (CAS) and the Foundation of Key Laboratory of Marginal Sea Geology of CAS (Grant No. MSGL07-21)