Enhanced trapping of molybdenum by sulfurized marine organic matter of marine origin in Mesozoic limestones and shales

Abstract

There have been many studies devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition. Among them, molybdenum (Mo) is frequently cited as an effective proxy for sediments and sedimentary rocks. Recently, Helz et al. (Helz, G.R., Miller, C.V., Charnock, J.M., Mosselmans, J.L.W., Pattrick, R.A.D., Garner, C.D., Vaughan, D.J., 1996. Mechanisms of molybdenum removal from the sea and its concentration in black shales: EXAFS evidences. Geochim. Cosmochim. Acta, 60, 3631–3642) and Adelson et al. (Adelson, J. M., Helz, G. R., Miller, C. V., 2001. Reconstructing the rise of recent coastal anoxia; molybdenum in Chesapeake Bay sediments. Geochim. Cosmochim. Acta, 65, 237–252.) suggested that Mo does not behave conservatively in the water column when H 2S reaches a threshold concentration. Above this concentration, a “switch” operates, and Mo is scavenged by forming bonds with metal-rich (notably iron) particles, sulfur-rich organic molecules and pyrite. In this paper, Mo-trapping by sulfur-rich organic matter (OM) in ancient marine deposits is emphasized. The following Mesozoic geological formations were selected for study because of their relatively high concentration of sulfurized OM: the Akkuyu Formation (Turkey), the Calcaires d'Orbagnoux (France) and Kimmeridge Clay (UK) and its timeequivalent in Boulonnais (France), the Kashpir oil shales (Russia), and the La Luna Formation (Venezuela). The sulfur-rich OM is identified by either measured organic-S abundance or kerogen microscope observation. Our results show that Mo is systematically more enriched relative to the other redox-sensitive/sulfide-forming elements studied (U, V, Ni, Cu, Zn, Cr), and Mo enrichment is positively correlated with the amount of sulfurized OM but not with pyrite abundance. These results illuminate the role played by sulfurized OM in geologic-scale Mo capture and retention, but they also underline the role played by reactive iron. Significant OM sulfurization is only possible when reactive iron is limited. Nevertheless, pyrite formation, though limited, could act as an initial Mo trap, prior to Mo uptake by OM that is sulfurized after the pyritization step. In future paleoenvironmental reconstructions, attention must be paid to this enhanced Mo enrichment in the presence of sulfurized organic matter. In such cases, the use of Mo could lead to overestimation of the reducing conditions of the depositional environment.