Rare earth elements of seep carbonates: Indication for redox variations and microbiological processes at modern seep sites

摘要

At marine seeps, methane is microbially oxidized resulting in the precipitation of carbonates close to the seafloor. Methane oxidation leads to sulfate depletion in sediment pore water, which induces a change in redox conditions. Rare earth element (REE) patterns of authigenic carbonate phases collected from modem seeps of the Gulf of Mexico, the Black Sea, and the Congo Fan were analyzed. Different carbonate minerals including aragonite and calcite with different crystal habits have been selected for analysis. Total REE content (∑REE) of seep carbonates varies widely, from 0.1 ppm to 42.5 ppm, but a common trend is that the ∑REE in microcrystalline phases is higher than that of the associated later phases including micospar, sparite and blocky cement, suggesting that ∑REE may be a function of diagenesis. The shale-normalized REE patterns of the seep carbonates often show different Ce anomalies even in samples from a specific site, suggesting that the formation conditions of seep carbonates are variable and complex. Overall, our results show that apart from anoxic, oxic conditions are at least temporarily common in seep environments.