Petrology and geochemistry of cold seep carbonates from the northern Okinawa Trough, East China Sea: implications to early diagenesis.

Carbonate samples were collected from the northern Okinawa Trough in the East China Sea in 2013. The petrology, mineralogy, carbon and oxygen isotopes, and rare earth elements (REEs) of these samples were analyzed. Aragonite, high-Mg calcite, and dolomite were the main carbonate minerals, the contents of which varied greatly among the carbonate samples. Petrological observations revealed the common occurrence of framboidal pyrites. The δ13C values of carbonates varied from −53.7‰ to −39.39‰ (average of −47.3‰ based on Vienna Pee Dee Belemnite (V-PDB), n=9), and the δ18O values ranged from 0.6‰ to 3.4‰ (average of 1.9‰; V-PDB, n=9). The carbon and oxygen isotope characteristics indicated that the carbonates precipitated during the anaerobic oxidation of methane. The carbon source was a mixture of thermogenic methane and biogenic methane, possibly with a greater contribution from the former. The oxygen isotope data showed that gas hydrate dissociation occurred during carbonate precipitation. The Ce anomalies suggested that the carbonates precipitated in an anoxic environment. A slight enrichment of middle REEs (MREEs) could be attributable to the early diagenesis. The structures, minerals, oxygen isotopes, and MREEs all indicated that the carbonates experienced some degree of early diagenesis. Therefore, the influence of early diagenesis should be considered when using geological and geochemical proxies to reconstruct original methane seepage environments. [ABSTRACT FROM AUTHOR]