Ecosystem feedbacks in response to ocean acidification can amplify or diminish diel pH oscillations in productive coastal waters. Benthic microalgae generate such oscillations in sediment porewater and here we ask how CO₂ enrichment (acidification) of the overlying seawater alters these in the absence and presence of biogenic calcite. We placed a 1-mm layer of ground oyster shells, mimicking the arrival of dead calcifying biota (+Calcite), or sand (Control) onto intact silt sediment cores, and then gradually increased the pCO₂ in the seawater above half of +Calcite and Control cores from 472 to 1216 μatm (pH 8.0 to 7.6, CO₂:HCO₃⁻ from 4.8 to 9.6 × 10⁻⁴). Porewater [O₂] and [H⁺] microprofiles measured 16 d later showed that this enrichment had decreased the O₂ penetration depth (O₂-pd) in +Calcite and Control, indicating a metabolic response. In CO₂-enriched seawater: (1) sediment biogeochemical processes respectively added and removed more H+ to and from the sediment porewater in darkness and light, than in ambient seawater increasing the amplitude of the diel porewater [H⁺] oscillations, and (2) in darkness, calcite dissolution in +Calcite sediment decreased the porewater [H⁺] below that in overlying seawater, reversing the sediment–seawater H⁺ flux and decreasing the amplitude of diel [H⁺] oscillations. This dissolution did not, however, counter the negative effect of CO₂ enrichment on O₂-pd. We now hypothesise that feedback to CO₂ enrichment—an increase in the microbial reoxidation of reduced solutes with O₂—decreased the sediment O₂-pd and contributed to the enhanced porewater acidification.