Authigenic Carbonates from Cold Seeps of the South China Sea: New Insights into Fluid Sources, Formation Condition and Geochronology

Methane-rich fluid and gas expulsion often leads to formation of authigenic carbonates close to the seafloor along continental margins. This carbonates, consequently, represent excellent geochemical archives of methane emanation and possibly gas hydrate destabilization. Here, we report petrographic, mineralogical, geochemical, and geochronological characteristics of seep carbonates at Dongsha and Shenhu area from northern continental slope of the South China Sea. Samples from these areas provide environmental information on carbonate formation. Seafloor observation and samples acquired indicate that carbonates occur as concretions, nodules, chimneys, fragments, and massive blocks. The carbonates are primarily calcite, aragonite, and dolomite. The δ¹³C values of carbonates ranging from -52.3% to -32.6% (V-PDB) in Dongsha samples and from -49.8% to -23.8% in Shenhu samples suggest that biogenic methane and thermogenic methane are the primary carbon sources. A similarly large variability in δ¹⁸O values (1.4% to 4.2% V-PDB) demonstrates the geochemical complexity of the slope, with some samples pointing toward to ¹⁸O-enriched oxygen source that is possibly related to advection of ¹⁸O-enriched formation water and/or to the destabilization of locally abundant gas hydrate. Carbonates from both areas show low total rare earth elements (REE) concentrations that mostly less than 30 ppm. The shale-normalized REE of the carbonates are characterized by a slight light REE (LREE) and heavy REE (HREE) depletion. The shale-normalized REE patterns of the carbonates show no or positive Ce anomalies, suggesting that the redox conditions are likely to remain anaerobic at the time carbonates formed. Initial results of the U/Th dating indicate that carbonate precipitation started at least 116 ka BP. Remarkably, most of the carbonates revealed U/Th ages that point to formation during sea level lowstand. The results suggest that enhanced fluid flow during these time intervals closely related to sea level variations associated with glacial/interglacial cycles and possibly environmental change, e.g., variations in bottom-water temperatures that affected the stability of gas hydrate reservoirs. These correlations and their potential driving forces may be of interest for the assessment of global seep activities and their role in the past and future climates.