Research paperUsing iron speciation in authigenic carbonates from hydrocarbon seeps to trace variable redox conditions

- Mössbauer spectroscopy is used to constrain formation conditions of seep carbonate.
- Fe speciation revealed variable redox conditions at the Gulf of Mexico seep site.
- Black Sea stratified water column results in low Fe contents of seep carbonate.
- Fe speciation is useful to trace variable redox conditions in marine environments.

Seepage of hydrocarbon-rich fluids out of the marine sedimentary column is characterized by temporal changes of flow intensity and resultant spatially variable redox conditions. Authigenic carbonates at marine hydrocarbon seeps provide excellent geological and geochemical archives that serve to explore seepage dynamics over time. In this study, we investigated the potential of Mössbuaer spectroscopy and Fe contents of seep-related authigenic carbonates from the Congo Fan, the Gulf of Mexico, and the Black Sea for reconstructing past redox conditions and fluid seepage activity at cold seeps. The Fe speciation observed by Mössbauer spectroscopy and Fe contents suggest that (1) the Congo Fan carbonates precipitated in a sulfidic environment, (2) the formation conditions of seep carbonates were variable at the Gulf of Mexico seep site, ranging from oxic to suboxic and anoxic and even spanning into the methanogenic zone, and (3) the stratified water column of the Black Sea or suboxic condition resulted in low Fe contents of Black Sea carbonates. The study reveals that Fe speciation can provide constraints on the wide range of redox conditions that imprinted seep carbonates during the life span of seepage. Similarly, Mössbauer spectroscopy - particularly when used in combination with the analysis of redox-sensitive elements - is a promising tool to trace variable redox conditions in marine paleoenvironments other than seeps.