Chemical environment of cold seep carbonate formation on the northern continental slope of South China Sea: Evidence from trace and rare earth element geochemistry

Abstract

Trace element and rare earth element (REE) concentrations in cold seep carbonates from the northern continental slope of the South China Sea are used in this study to indicate source fluid characteristics and redox conditions. Carbonate samples from the two study areas (Shenhu and Dongsha) all show low total REE concentrations (mostly 10–20 ppm). In the Shenhu area, the shale-normalized REE patterns of the chimney carbonates display slight light REE enrichment, positive Ce anomaly, and a consistently positive Gd anomaly. These carbonate chimneys show a two-stage formation history, with a slight increase in the positive Ce anomaly from the rim to core. Trace and rare earth element data suggest that the core may have formed in a more anoxic condition than the rim. In the Dongsha area, seep carbonate samples show a slight heavy REE enrichment, with both negative and positive Ce anomalies, and more positive Gd anomalies, which are consistent with precipitation in an anoxic environment. Redox sensitive trace elements, such as Mo, U, Ni, V and Co, also indicate anoxic conditions for this cold seep carbonate precipitation.