The effects of manganese nodules on concentration fields in adjacent pore waters and sediments are investigated theoretically with a three-dimensional (axisymmetric) diffusion-reaction model. The nodules are idealized as impermeable and poorly permeable disks and hemispheres. The first part of this paper examines the organic matter and coupled pore water oxygen fields in order to evaluate the local redox conditions under an isolated nodule. Oxygen is allowed to disappear on a free surface that is calculated as an output of the model. In sediments that become suboxic, like those at MANOP site C, the model predicts that pore waters neighboring a nodule are markedly more depleted in oxygen than in its absence. Although these reduced conditions may remobilize metals that would be available for nodule growth, these same conditions would appear to favor nodule dissolution. Typically, the presence of a nodule raises the local position of the zero-oxygen surface, but it is deeper than expected for large nodules on sediments containing only highly reactive organic matter. The redox changes calculated for an isolated nodule on a hypothetical, but representative, oxidized abyssal red clay appear inadequate to raise the redox boundaries enough to facilitate metal transport from reduced regions deeper in the sediment. In the later part of the paper the model is used to evaluate the influence of a nodule field on the outward flux of dissolved silica from a siliceous ooze. Although the flux attenuation is generally modest, some dense packings observed in the Pacific should produce substantial reductions (>35%), which should promote opal preservation. ¿ American Geophysical Union 1989