Does modern seep aragonite record the molybdenum isotopic composition of seawater?

• Applicants: Zhiyong Lin, Ph.D.; Professor Dr. Jörn Peckmann; Professor Stefan Weyer, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 535578816

Changing redox conditions lead to different geochemical behavior of molybdenum (Mo), accompanied by characteristic fractionation and changes of its isotopic composition. Over the last decades, Mo isotope systematics were established as a powerful proxy for reconstructing paleo-redox conditions of the Earth’s atmosphere and oceans, using subrecent sapropels and ancient black shales deposited under sulfidic conditions as sedimentary archives. However, the level of dissolved sulfide is a key factor that controls both the removal of Mo from the ambient aqueous phase and the associated Mo isotope fractionation. Recent field and laboratory studies have indicated that also non-skeletal, authigenic marine carbonate has potential to record seawater Mo isotope composition. Still, the Mo isotopic signature recorded in pelagic carbonate sediments may be easily overprinted and modified due to the existence of accessory phases. Methane seepage occurs widely along continental margins and its deposits are common in the rock record. With seeps exhibiting various diagenetic environments, methane-seep limestones consist of multiple authigenic carbonate phases, mostly aragonite and high-Mg calcite but also low-Mg calcite and dolomite. Aragonite tends to precipitate near to the seafloor and occurs as a pure phase in many methane-seep limestones, commonly lacking any accessory phases. Compared to pelagic carbonate sediments, the purity of seep aragonite and its formation at shallow depth in pore waters with seawater-like δ98Mo signature represents an unrivaled opportunity to establish a new proxy for the Mo isotopic composition of seawater. However, the overall mechanisms and parameters controlling the Mo isotopic composition of seep aragonite have not yet been investigated and, thus, the relation between Mo isotope patterns of seep aragonite and seawater remains unconstrained. The project aims to (1) determine the relationship between the Mo isotopic composition of pure seep aragonite and seawater in modern marine settings and (2) evaluate if the reconstruction of the Mo isotope composition of ancient seawater will be possible with the new seep aragonite-based proxy. The outcome of this project is expected to enable the reconstruction of the Mo isotope composition of seawater with unrivalled accuracy for the Phanerozoic eon.

• DFG Programme: Research Grants