Insights into the long-term preservation of buried marine cadmium derived from micro-scale observations of Cd distribution and speciation in organic-rich shales

Cadmium (Cd) is a nutrient-like trace metal whose burial in marine sediments influences both the long-term (10 3 –10 4 yr) oceanic Cd mass balance and the interpretation of Cd-based paleoceanographic records. However, the sedimentary host phases that preserve Cd in the rock record remain poorly constrained. Here, we present the first direct micro-scale observations of Cd distribution and speciation in organic-rich marine shales from the Cape Verde basin (Hatteras Fm, ~94 Ma) and the North Sea basin (Sele Fm, ~56 Ma). Synchrotron micro-X-ray fluorescence (μXRF) maps show heterogeneous Cd distributions, ranging from diffuse matrix-bound Cd to discrete Cd-rich hotspots, but no consistent spatial association between Cd and Fe-rich phases, indicating that pyrite is not a major repository for Cd in these samples. Cadmium K-edge X-ray absorption near-edge structure (XANES) spectroscopy and linear combination fitting show that Cd is dominated by cadmium sulfide (CdS), which accounts for 33–100% of the Cd in analysed spots; Cd sulfate forms most of the remainder, with minor Cd carbonate in one non-sulfidic sample. We interpret Cd sulfate chiefly as a secondary oxidation product of originally-reduced Cd sulfide, and infer that CdS is the dominant long-term burial phase under euxinic to anoxic conditions. These results indicate that ancient Cd records may reflect diagenetic redistribution and stabilisation in sulfide phases rather than the full diversity of initial Cd host phases buried at the seafloor. Direct speciation constraints are therefore essential for linking sedimentary Cd enrichments and Cd isotope records to marine redox conditions, productivity, and the long-term oceanic Cd cycle. Synopsis The burial of cadmium in marine sediments is a primary control on the global cycle of this important trace metal. Using micro-imaging and X-ray speciation techniques, we show that ancient marine sedimentary deposits are likely dominated by cadmium sulfide.