A newly discovered ancient cold seep in the northwestern part of the South China Sea: Evidence from the authigenic carbonate

The cold seep activity creates a unique environment on the deep sea floor, and gave birth to a unique authigenic mineral- seep carbonate. In 2018, a new paleo-seep was discovered in the vicinity of the Ganquan Plateau using the manned submersible "Shenhaiyongshi" and seep carbonate samples were recovered. Here, we performed geochemical and mineralogical analyses on the seep carbonates. The results show that the seep carbonates (SY069) are mainly low-magnesium calcites with negative excursion in 813C values (-43.46 to -33.83 VPDB) and positive excursion in 818O values (2.10 to 3.80 VPDB), indicating that the carbonate formation is mainly affected by methane from gas hydrate dissociation. Meanwhile, the rare earth elements (REE) in the seep carbonates are enriched in middle REEs and there is no Ce anomaly (0.93 to 1.08) after the Post Archean Australian Shale standardization, indicating that the seep carbonates were formed in an anoxic environment. In addition, the Y/ Ho ratio is low (29.39 to 42.03), indicating a low methane flux. Therefore, it is suggested that SY069 carbonate rocks were formed in anaerobic environment with low methane flux. The above results are helpful for us to better understand the formation process of seep carbonates. Moreover, the Ganquan Plateau also be one of the hydrate occurrence areas, which is of great significance for us to explore the hydrate.