Trace element distribution in methane-seep carbonates: The role of mineralogy and dissolved sulfide

Abstract

Trace elements are widely used to constrain environmental conditions and biogeochemical processes through geologic time. Authigenic carbonates in particular are a reliable archive due to their ability to take up and store trace elements over long periods of time. This comparative study presents new data on the effect of ambient redox conditions on trace element distributions in authigenic seep carbonates forming in different geochemical and hydrographic environments. Carbonates from shallow and deep-sea hydrocarbon seeps of the Black Sea are compared to seepage sites from the northern Gulf of Mexico and the northern South China Sea, revealing that trace element inventories are to some extent site-specific and depend on carbonate mineralogy. Strongly euxinic conditions at Black Sea deep-water seeps favor the formation of low-magnesium calcite, whereas fluctuating redox conditions favors aragonite cement precipitation. The trends in mineralogy are accompanied by distinct enrichments and depletions of specific trace elements including uranium, iron, and manganese. Aragonite cements exhibit lower magnesium to strontium ratios and manganese contents than their low-magnesium calcite counterparts. Moreover, trace elements in Black Sea carbonates are enriched in barium, manganese, zinc, and nickel, and are depleted in iron. The new trace element data suggest that the presence of dissolved sulfide influences carbonate mineralogy, and, accordingly, trace element speciation and content. The results of this study have implications for trace element-based paleoenvironment reconstructions beyond seep environments, and may initiate a reassessment of authigenic carbonates as a trace element archive, in particular for carbonates from ancient and modern euxinic environments.