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, clo se to or slightly later in the redox sequence than U, and before reduction of Mo in more reducing, sulfidic sediments. This work investigates the beha vior of authigenic Re during oxidation and long-term burial in the sediment s of several turbidites from the Madeira Abyssal Plain. Exposure to bottom water O-2 clearly remobilizes authigenic Re, as found in previous work, but reimmobilization occurs in reducing sediments below the oxidation front ov er a depth range of 0.5-3 m. The large spread of the Re "burndown peak" mus t be kinetically controlled, perhaps influenced by the C-org content. Once reimmobilized, however, the authigenic Re signal remains immobile for at le ast 3.4 Myr. Authigenic U, in contrast, consistently forms peaks approximat e to 0.5 m broad, whereas authigenic Mo is lost on oxidation and does not a ppear to be refixed unless authigenic pyrite is formed. These results imply that any paleoenvironmental significance of authigenic Re will be compromi sed in low sedimentation rate environments (< <approximate to>10 cm/kyr) wh ere there is significant exposure of authigenic Re to bottom water O-2. How ever, authigenic Re records from rapidly accumulating continental margin se diments, with much less exposure to bottom water O-2, still offer considera ble potential as high-fidelity recorders of past reducing conditions. Copyr ight (C) 2000 Elsevier Science Ltd.