ENHANCED TRAPPING OF MOLYBDENUM BY SULFURIZED ORGANIC MATTER OF MARINE ORIGIN AS RECORDED BY VARIOUS PHANEROZOIC FORMATIONS

A large literature is devoted to trace metals and their value in assessing the paleoredox conditions of ancient marine deposition. Among them, molybdenum is frequently cited as a good diagnostic proxy for sediments and sedimentary rocks. Recently, Helz et al. (1996, Geochim. Cosmochim. Acta, 60, 3631-3642) and Adelson et al. (2001, Geochim. Cosmochim. Acta, 65, 237-252) suggested that Mo does not behave conservatively in the water column when H2S reaches a threshold concentration. Above this concentration, a sort of "switch" operates, and Mo is scavenged by forming bonds with metal-rich (Fe) particles or sulfur-rich organic molecules. This scheme is clearly consistent with the high Mo enrichment observed in conjunction with organic matter (OM) deposition in the presence of hydrogen sulfide in the water column and pore fluids in the present-day Cariaco Basin (Lyons et al., 2003, Chem. Geol., 195, 131-157, and this presentation) and Santa Barbara Basin (Zheng et al., 2000, Geochim. Cosmochim. Acta, 64, 4165-4178). In this contribution, Mo-trapping by sulfur-rich OM in ancient marine deposits is emphasized. The following geological formations were studied: the Frasnian-Famennian transition of France, the Kimmeridge Clay in the UK and the French lateral time-equivalent, the Kimmeridgian Akkuyu Formation of Turkey, the Calcaires d'Orbagnoux of France and the Kashpir Oil Shales (Russia). Our results show that for the samples containing sulfur-rich OM, Mo enrichment is systematically more pronounced than for other redox proxies such as V or U. Attention must be paid to this enhanced Mo enrichment in the presence of sulfurized organic matter, because in such cases, the use of Mo could lead to overestimation of the reducing conditions of the depositional environment.