Comparative behavior of authigenic Re, U, and Mo during reoxidation and subsequent long-term burial in marine sediments

The trace element Re is enriched in mildly reducing, suboxic sediments, close to or slightly later in the redox sequence than U, and before reduction of Mo in more reducing, sulfidic sediments. This work investigates the behavior of authigenic Re during oxidation and long-term burial in the sediments of several turbidites from the Madeira Abyssal Plain. Exposure to bottom water O2 clearly remobilizes authigenic Re, as found in previous work, but reimmobilization occurs in reducing sediments below the oxidation front over a depth range of 0.5–3 m. The large spread of the Re “burndown peak” must be kinetically controlled, perhaps influenced by the Corg content. Once reimmobilized, however, the authigenic Re signal remains immobile for at least 3.4 Myr. Authigenic U, in contrast, consistently forms peaks ≈ 0.5 m broad, whereas authigenic Mo is lost on oxidation and does not appear to be refixed unless authigenic pyrite is formed. These results imply that any paleoenvironmental significance of authigenic Re will be compromised in low sedimentation rate environments (≈10 cm/kyr) where there is significant exposure of authigenic Re to bottom water O2. However, authigenic Re records from rapidly accumulating continental margin sediments, with much less exposure to bottom water O2, still offer considerable potential as high-fidelity recorders of past reducing conditions.