Deep ocean carbonate ion increase during mid Miocene CO2 decline

Characterised by long term cooling and abrupt ice sheet expansion on Antarctica similar to 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 similar to 40 mu mol/kg may have occurred between similar to 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 similar to 14 Ma.