Authigenic carbonates from seeps on the northern continental slope of the South China Sea: New insights into fluid sources and geochronology

Seep carbonates represent excellent archives of seepage activities near seafloors. In order to provide insights on sources of seeping fluids, stable carbon and oxygen isotopes and ⁸⁷Sr/⁸⁶Sr ratios were applied to seep carbonates from Shenhu, SW Dongsha, and NE Dongsha on the northern continental slope of the South China Sea. U/Th dating was selected to constrain the time involved in carbonate formation and the potential driving force. The δ¹³C values of the Shenhu and NE Dongsha carbonates vary from -52.3‰ to -32.6‰ (V-PDB), indicating that biogenic methane is the primary carbon source at both locations. The δ¹³C values of the SW Dongsha carbonates are much higher, ranging from -18.8‰ to -11.4‰ (V-PDB), suggesting that thermogenic methane is the predominant carbon source in this region. A relatively deep origin of thermogenic methane for the SW Dongsha seeps is also supported by somewhat lower ⁸⁷Sr/⁸⁶Sr ratios of the samples (from 0.709025 to 0.709097). The δ¹⁸O values of the Shenhu and NE Dongsha carbonates vary from 1.4‰ to 5.1‰ (V-PDB), while the δ¹⁸O values of the SW Dongsha carbonates range from 2.5‰ to 3.4‰ (V-PDB). Many samples from Shenhu and NE Dongsha demonstrate an ¹⁸O-enriched fluid source that most likely originated from waters released from the dissociation of locally abundant gas hydrate. U/Th ages of the Shenhu carbonates show a large range from 330 ka to 152 ka BP and the NE Dongsha carbonates have ages from 77 ka to 63 ka BP. Interestingly, U/Th ages of most samples are either within times of sea-level lowstands or correspond to periods when the sea-level was falling. Taken together, our data suggest that in the South China Sea gas hydrate dissociation during sea-level lowstands or falling stages resulted from reduced hydrastastic pressures, which in turn enhanced the seep activities and promoted the formation of authigenic carbonates close to the seafloor.