Supplementary information available at http://www.nature.com/srep/2014/140226/srep04187/full/srep04187.html#supplementary-information ; Characterised by long term cooling and abrupt ice sheet expansion on Antarctica ~14 Ma ago, the mid Miocene marked the beginning of the modern ice-house world, yet there is still little consensus on its causes, in part because carbon cycle dynamics are not well constrained. In particular, changes in carbonate ion concentration ([CO32−]) in the ocean, the largest carbon reservoir of the ocean-land-atmosphere system, are poorly resolved. We use benthic foraminiferal B/Ca ratios to reconstruct relative changes in [CO32−] from the South Atlantic, East Pacific, and Southern Oceans. Our results suggest an increase of perhaps ~40 μmol/kg may have occurred between ~15 and 14 Ma in intermediate to deep waters in each basin. This long-term increase suggests elevated alkalinity input, perhaps from the Himalaya, rather than other shorter-term mechanisms such as ocean circulation or ecological changes, and may account for some of the proposed atmospheric CO2 decline before ~14 Ma. ; This study is part of the Palaeoclimate and Palaeoenvironment core science programme at the British Geological Survey, funded by the Natural Environment Research Council (S.K., V.P.) and by grant ARC DP140101393 (J.Y.) ; Peer-reviewed ; Publisher Version