Enrichment mechanism of trace elements in pyrite under methane seepage

Authigenic pyrite is an important recorder for methane seepage. During methane seepage, the sulfur and iron isotopic composition of pyrite will change, allowing it to be used as an indicator to identify methane seepage. However, the dissimilar behaviour of trace elements in authigenic pyrite during methane seepage remain unclear. To provide insights, we used pyrite samples from the 'Haima' seep locality to determine differences in trace element contents in pyrite obtained from the sulfate-methane transition zone (SMTZ) and that from normal (non-seepage) sedimentary environments. In the SMTZ, the content of cadmium (Cd) related to the organoclastic sulfate reduction in pyrite was low, while the molybdenum (Mo) content, which is highly sensitive to redox environments, was high. This discrepancy can be explained by the fact that sulfate ions (SO42-) in the SMTZ were preferentially consumed by sulfate driven anaerobic oxidation of methane (SD-AOM), which inhibited organodastic sulfate reduction and decreased the trace metals derived from organic matter in pore water. Simultaneously, intense SD-AOM produced more hydrogen sulfide (H2S), which was more conducive to Mo removal from pore water, and then more Mo adsorption onto pyrite. Further analysis shows that the Mo/Cd ratio of pyrite in the SMTZ (average value of 82.08) is significantly higher than that of the non-SMTZ (average value of 16.02). We believe that the Mo/Cd ratio has great potential to indicate methane seepage, and thus provides a new indicator for methane seepage research.