Cold seep status archived in authigenic carbonates: Mineralogical and isotopic evidence from Northern South China Sea

Abstract

Cold-seep carbonates are precipitated under high alkalinity conditions created by the anaerobic oxidation of methane in cold-seep sites. Multiple Ca–Mg-carbonate phases are identified, including aragonite, low-Mg calcite (LMC), high-Mg calcite (HMC), protodolomite, and dolomite. These phases result from different conditions that are related with cold-seep activities. Here, we report on the relationship between the Ca–Mg-carbonate phases and the cold-seep status. Authigenic carbonates were sampled from northern slope of South China Sea. Carbon isotopic compositions of samples from Shenhu area are lower than −40‰, indicating methane-derived carbon. The δ13C values of samples from Southwest (SW) Taiwan area range from ~−30‰ to ~−20‰, which is the result of the mixture of methane carbon and seawater carbon. Carbonate phases were identified according to the composition and structure results. Samples from Shenhu area are composed of protodolomite and HMC. Three zones were discovered from the center to the rim of the cross-section of the tube-like sample from SW Taiwan area. From the external to the internal zones, the carbonate phases are HMC; LMC and protodolomite; HMC, respectively. The intensity of superstructure reflections of the protodolomite from Shenhu area is stronger than that from SW Taiwan area, indicating higher MgCO3 content. Based on the formation conditions of Ca–Mg-carbonates from LMC to dolomite, those with higher MgCO3 content are formed in more active cold-seep environment. According to the distribution of carbonate phases in each sample, the cold seep flux was high in Shenhu area and was sustained for a long time. By contrast, the flux in SW Taiwan area was relatively low and not stable. It once became higher, but finally returned to low.