Contrasting molybdenum cycling and isotopic properties in euxinic versus non-euxinic sediments and sedimentary rocks: Refining the paleoproxies

The redox-sensitive transition element molybdenum is present in the Earth's crust at trace concentrations but is abundant in seawater, marine sediments, and ancient sedimentary rocks. These occurrences have led to the development of a suite of paleoredox proxies based on the bulk concentration of Mo, its isotopic composition, and the covariation of Mo with total organic carbon (TOC) in ancient black shales. However, these proxies have almost exclusively targeted the identification and interpretation of euxinic environments, where bottom waters are both anoxic and sulfidic. Here we present a discussion of the Mo geochemistry of non-euxinic sediments, where hydrogen sulfide is sometimes present yet always restricted to pore waters. We propose a new paleoredox application that uses Mo concentrations to help distinguish between environments where sulfide was present in the bottom waters and environments where sulfide was restricted to pore waters. Under ideal conditions, it is also possible to infer the presence of Mn-oxides recycling in non-euxinic paleoenvironments based on the Mo isotope composition of ancient black shales, providing a critical constraint on bottom water oxygenation.

► In non-euxinic environments sulfide is restricted to the sediments.
► In euxinic environments sulfide is present in the bottom waters.
► In non-euxinic environments Mo enrichments remain below 25 ppm.
► In euxinic environments Mo enrichments exceed 25 ppm.
► Mo isotopes can be used to identify presence of Mn-oxide cycling.