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.Theδ^(13)C values of carbonates varied from-53.7‰to-39.3‰(average of-47.3‰based on Vienna Pee Dee Belemnite(V-PDB),n=9),and theδ^(18)O 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 infl uence of early diagenesis should be considered when using geological and geochemical proxies to reconstruct original methane seepage environments.

An insight into shallow gas hydrates in the Dongsha area,South China Sea

Previous studies of gas hydrate in the Dongsha area mainly focused on the deep-seated gas hydrates that have a high energy potential,but cared little about the shallow gas hydrates occurrences.Shallow gas hydrates have been confirmed by drill cores at three sites(GMGS208,GMGS209 and GMGS216)during the GMGS2 cruise,which occur as veins,blocky nodules or massive layers,at 8–30 m below the seafloor.Gas chimneys and faults observed on the seismic sections are the two main fluid migration pathways.The deep-seated gas hydrate and the shallow hydrate-bearing sediments are two main seals for the migrating gas.The occurrences of shallow gas hydrates are mainly controlled by the migration of fluid along shallow faults and the presence of deep-seated gas hydrates.Active gas leakage is taking place at a relatively high-flux state through the vent structures identified on the geophysical data at the seafloor,although without resulting in gas plumes easily detectable by acoustic methods.The presence of strong reflections on the high-resolution seismic profiles and dim or chaotic layers in the subbottom profiles are most likely good indicators of shallow gas hydrates in the Dongsha area.Active cold seeps,indicated by either gas plume or seepage vent,can also be used as indicators for neighboring shallow gas hydrates and the gas hydrate system that is highly dynamic in the Dongsha area.