Trace elements geochemistry in OSR (organoclastic sulfate reduction)-dominated pyrite and SD-AOM (sulfate-driven anaerobic oxidation of methane)-dominated pyrite of western Andaman Sea (IODP Site U1447A) during the mid-late Quaternary

Some bioelements and redox-sensitive elements (e.g. Ni, Co, Se, Cu and Mo) can be more enriched in authigenic pyrite (FeS2) than in the host sediments or rocks. Organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM) provide most sulfide for pyrite formation. These two pathways of microbial activities uptake distinctive trace elements as nutrient or to produce required enzymes. Authigenic pyrite related to two different biogeochemical processes is anticipated to with differential signals in trace elemental compositions. In this study, we investigated a drill core from the mid-late Quaternary at western Andaman Sea (IODP 353 site U1447A) and hand-picked authigenic pyrite aggregates. In-situ and bulk sample analyses were respectively conducted on pyrite and host sediments for the trace elemental compositions. High-34S-pyrites (up to 40.80) that traditionally considered as products from SD-AOM are found in 0-140 mbsf (ca. 0-1.17 Ma) and are enriched in cobalt and nickel. This phenomenon might be resulted from large requirements for Co and Ni in reverse methanogenesis that happened in the SD-AOM. Pyrites in 140-250 mbsf (ca. 1.17-2.08 Ma) with 34S ranging from 37.63 to 46.09 are enriched in copper, zinc and molybdenum. We interpret the enrichment may be related to the remineralization of organic matter and subsequent incorporation of Cu, Zn, and Mo into pyrites. In contrast, the host sediments cannot suggest any difference in trace elemental compositions through the drill core. The results in this study demonstrate that SD-AOM-dominated and OSR-dominated pyrites are possibly be distinguishable on the basis of their trace elemental compositions. The trace elemental signatures of authigenic pyrite might help us to identify dominant formation processes of ancient pyrite record over the geological history.