The Fate of Vanadium-Bearing Iron Oxyhydroxides in Marine Sediments: Integrating Gel-Based In Situ Mineral Probes with Synchrotron X-ray Fluorescence Microspectroscopy

Vanadium (V) is a redox-sensitive trace metal often used as a paleoredox proxy in ancient marine sediments. However, our understanding of the early diagenesis of V is limited to laboratory-based simulations and bulk geochemical measurements of natural sediments. Microscale measurements are essential to exploring V geochemistry in organic-rich coastal sediments, where redox zonation changes over small spatial scales. Here, we describe an innovative in situ two-dimensional (2D) imaging approach to study redox-driven changes of V-bearing iron oxyhydroxides (lepidocrocite and ferrihydrite) in intertidal mudflat sediments of an Australian lagoon lake (Coombabah Lake, Queensland). Vanadium-bearing iron oxyhydroxides were suspended in a polyurethane-based hydrogel matrix, loaded on laser-cut acrylic probes, and exposed to mudflat sediments for up to 6 weeks. Changes in V speciation and Fe mineralogy were examined using synchrotron-based X-ray fluorescence (XRF) microspectroscopy for high-resolution chemical imaging of elemental (V, S, Fe) distributions combined with micro-X-ray absorption near-edge structure (μXANES) spectroscopy at the V and Fe K-edges for speciation analysis. Linear combination fitting of μXANES data revealed that solid-phase VV was reduced to VIV in the ferruginous zone of sediments and to a mixture of mainly VIV and some VIII (up to 10-20%) in the sulfidic zone, where the reduction correlated with the degree of sulfidation and conversion of the iron oxyhydroxides to FeS (up to 96%). The combination of gel-based mineral probes with synchrotron-based μXRF tools can unravel small-scale geochemical relationships and provide new insights into the early diagenesis of trace elements in marine sediments.