Iron shuttle controls on molybdenum, arsenic, and antimony enrichment in Pliocene methane-seep carbonates from the southern Western Foothills, Southwestern Taiwan
Certain trace metals are anomalously enriched in sediments and pore fluids at cold hydrocarbon seeps. Recent studies ascribed such anomalies, particularly those of molybdenum (Mo), arsenic (As), and antimony (Sb), to so-called particulate shuttle processes by manganese and iron oxyhydroxides. However, there are still some uncertainties about the prevalence of this mechanism, and it is difficult to generally rule out an influence of deep ascending fluids on trace metal enrichments. The Pliocene Chiahsien seep carbonates from the southern Western Foothills of Southwestern Taiwan were precipitated through anaerobic oxidation of methane contained in fluids that yielded no evidence for an admixture of deep ascending fluids. These authigenic carbonates consequently represent a good example to explore the role of particulate shuttle processes on the enrichments of trace metals. The Chiahsien seep carbonates reveal strong enrichment of authigenic Mo relative to uranium (U), reflected in high Mo/U ratios that are much higher than those of seawater. Such a pattern of Mo–U covariation is best explained by a weak particulate shuttle process that operated during the formation of the Chiahsien seep carbonates. Furthermore, the seep carbonates show exceptional Mo, As, and Sb enrichments with no U enrichment and strong covariations between the former three trace metals and reactive iron. These findings indicate that iron oxyhydroxide particulate shuttles indeed played an intrinsic role in the enrichments of Mo, As, and Sb at the Pliocene seeps. Our findings confirm the potential importance of iron particulate shuttles on the biogeochemistry of marine cold seeps, but the factors governing the presence or absence of particulate shuttles are insufficiently understood and require more research on trace metal sequestration at seeps in the future.