Geochemical characteristics of cold-seep carbonates in Shenhu area, South China Sea

Cold seeps spread worldwide along the continental margins, which are closely related to the exploration of gas hydrates. Cold-seep carbonates have been reported to record the nature of seepage, including fluid source, sedimentary environment, and variation of seepage activity. We investigated the morphology, mineralogy, element compositions, and carbon and oxygen isotopes of 15 cold-seep carbonates collected from the Shenhu area, and compared them with 2 carbonates from the Haima cold seep, the South China Sea (SCS), to promote our knowledge of cold-seep system in SCS. Most of the Shenhu carbonates exhibit crust morphology, and some are in the form of chimneys and blocks. Their absolute (20%-65%) and relative carbonate mineral contents (mainly aragonite and calcite, with minor samples containing dolomite) vary significantly, indicating the multi-stage methane leakage in our study area. Some samples show a slight negative Ce anomaly, suggesting either the mixing of seawater or variation of the redox condition during the precipitation; the co-occurrence of strongly enriched U and Mo demonstrates anoxic condition during precipitation. The mixed genetic methane source was interpreted by delta C-13 of the Shenhu carbonates to range from -22.34 parts per thousand to -59.30 parts per thousand Vienna PeeDee Belemnite (VPDB), and the slight O-18-enrichment imprinted on the carbonates suggests the possible influence from hydrate dissociation. The Haima carbonates, with biogenic methane as the main gas source, were presumably formed in a stronger fluid flux by compared with our Shenhu samples.