158-11 New insights on the molecular geochemistry of molybdenum and rhenium in sulfidic settings

Session: Redox-Driven Nutrient and Contaminant Dynamics in Terrestrial Systems

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Abstract:

Molybdenum (Mo) and rhenium (Re) have been used as a paleo-redox proxies for more than two decades. Within sulfidic waters, both elements undergo thiolation: Sulfur (S) replaces molecular oxygen (O) to form thiomolybdate (MoO_xS_(4−x)²⁻) and thioperrhenate (ReO_xS_(4−x)⁻) species. Thiomolybdates and thioperrhenates are known to easily concentrate in sulfidic sediments at orders of magnitude higher than average crustal backgrounds. Several burial pathways are probably involved and the quantitative importance of each remains an open debate.

A common misconception of MoO₄^2– thiolation is that it is accompanied by an immediate reduction of Mo(VI) to Mo(IV). Thiolation, however, describes only substitution, and not a reduction reaction. Several studies have showed that Mo speciation can be complex in sediments deposited in sulfidic settings. The reactions involved during Re fixation under anoxic conditions are highly contentious: some proposed a simple reduction step of Re(VII) to Re(IV), while others suggested thiolation to lead to Re sequestration by co-precipitation of an Fe-S-Re mineral. The potential for Re association with pyrite and magnetite has never been explored.

We determined Mo and Re speciation in synthetic and natural anoxic sedimentary deposits using µ-XANES and µ-EXAFS techniques. Combined with Mo and Re isotope measurements, our data, confirm that Mo and Re speciation is highly heterogenous even in persistent euxinic settings. These results show the complexity of Mo and Re geochemical behaviors in sulfidic environments and highlight that the interpretation of Mo and Re isotope compositions is not simple, even in the most stable sulfur-reducing environments.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9115

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