Authigenic carbonates from methane seeps of the northern Congo fan: Microbial formation mechanism

摘要

Authigenic carbonates were collected from methane seeps at Hydrate Hole at 3113 m water depth and Diapir Field at 2417 m water depth on the northern Congo deep-sea fan during RV Meteor cruise M56. The carbonate samples analyzed here are nodules, mainly composed of aragonite and high-Mg calcite. Abundant putative microbial carbonate rods and associated pyrite framboids were recognized within the carbonate matrix. The δ~(13)C values of the Hydrate Hole carbonates range from -62.5‰ to -46.3‰ PDB, while the δ~(13)C values of the Diapir Field carbonate are somewhat higher, ranging from -40.7‰ to -30.7‰ PDB, indicating that methane is the predominant carbon source at both locations. Relative enrichment of ~(18)O (δ~(18)O values as high as 5.2‰ PDB) are probably related to localized destabilization of gas hydrate. The total content of rare earth elements (REE) of 5% HNO3-treated solutions derived from carbonate samples varies from 1.6 ppm to 42.5 ppm. The shale-normalized REE patterns all display positive Ce anomalies (Ce/Ce* > 1.3), revealing that the carbonates precipitated under anoxic conditions. A sample from Hydrate Hole shows a concentric lamination, corresponding to fluctuations in δ~(13)C values as well as trace elements contents. These fluctuations are presumed to reflect changes of seepage flux.