ADVANCING THE REDOX PROXIES: NEW INSIGHTS ON MOLYBDENUM AND RHENIUM GEOCHEMISTRY

Interpretations of the sedimentary distributions of molybdenum (Mo) and rhenium (Re) have enhanced our understanding of biospheric evolution over geologic time, yet their specific mechanics and pathways of burial remain to be elucidated. A basic understanding of their redox-dependent behaviors considers they are highly soluble under oxic conditions and become considerably less soluble under anoxic conditions leading to high enrichments in sediments. Unfortunately this approach is too simplistic since it neglects to study their speciation.

Mo has arisen as a powerful indicator of euxinic conditions, implying that sulfide was present in high concentrations in the water column. Although Re also accumulates under sulfidic conditions, Re removal under suboxic conditions has been observed. This contrasting characteristic would certainly make Re a valuable indicator of suboxia, ideally complementing Mo measurements. However, Mo and Re speciation in sedimentary records and natural waters under anoxic conditions remains largely unexplored and thus compromise their strengths as redox proxies.

The purpose of this talk is to present recent developments in refining Mo and Re speciation under oxygen-depleted conditions. Our new data, obtained by combining molecular scale characterization (XAFS) and chromatographic separation (IC-ICP-MS) of experimental and natural samples (modern and ancient), will be discussed to introduce new model to explain Mo and Re burial pathways, as well as future applications.