Environmental sulfidation and Mo source jointly control Mo isotopic composition in sediments under methane seepage

Abstract The redox sensitivity of molybdenum (Mo) isotopes makes them valuable in tracing and reconstructing (paleo-)methane seepage. Yet, our understanding of the characteristics of Mo isotopic composition in the methane seepage environment is limited. In conjunction with prior research, this study further examines the Mo isotopic composition of sediments in various methane seepage environments and investigates its potential controlling mechanisms. The research highlights that the Mo isotopic composition in sediments is primarily influenced by the sulfidic state of the environment (strongly or weakly sulfidic) and the source of Mo: seawater or Fe-Mn (oxy)hydroxides. Based on these factors, four models of Mo isotopic composition in methane seepage environments are proposed: (1) Sediments in a strong methane seepage environment (with low Fe-Mn shuttling) exhibit heavy δ98Mo signatures and high MoEF. Furthermore, the δ98Mo values increase with increasing MoEF; (2) sediments in a strong methane seepage environment (with high Fe-Mn shuttling) display light δ98Mo signatures and high MoEF. In contrast, the δ98Mo values decrease with the increase of MoEF; (3) sediments from weak methane seepage environments (with low Fe-Mn shuttling) exhibit light δ98Mo signatures and low MoEF, similar to the upper continental crust values; and (4) sediments in weak methane seepage environments (with high Fe-Mn shuttling) display a light δ98Mo signature and a low MoEF. These models help in understanding the controls on Mo isotopic compositions in different methane seepage scenarios and can aid in the reconstruction of past methane seepage events and environmental conditions.