The middle Miocene climate transition ∼ 14 Ma marks a fundamental step towards the current ice-house climate, with a ∼ 1 δ 18O increase and a ∼ 1transient δ 13C rise in the deep ocean, indicating rapid expansion of the East Antarctic Ice Sheet associated with a change in the operation of the global carbon cycle. The variation of atmo spheric CO2 across the carbon-cycle perturbation has been intensely debated as proxy records of pCO2 for this time in terval are sparse and partly contradictory. Using boron iso topes (δ 11B) in planktonic foraminifers from Ocean Drilling Program (ODP) Site 1092 in the South Atlantic, we show that long-Term pCO2 varied at 402 kyr periodicity between ∼ 14.3 and 13.2 Ma and follows the global δ 13C variation remarkably well. This suggests a close link to precessional insolation forcing modulated by eccentricity, which governs the monsoon and hence weathering intensity, with enhanced weathering and decreasing pCO2 at high eccentricity and vice versa. The ∼ 50 kyr lag of δ 13C and pCO2 behind ec centricity in our records may be related to the slow response of weathering to orbital forcing. A pCO2 drop of ∼ 200 μ atm before 13.9 Ma may have facilitated the inception of ice sheet expansion on Antarctica, which accentuated monsoon driven carbon cycle changes through a major sea-level fall, invigorated deep-water ventilation, and shelf-To-basin shift of carbonate burial. The temporary rise in pCO2 following Antarctic glaciation would have acted as a negative feedback on the progressing glaciation and helped to stabilize the cli mate system on its way to the late Cenozoic ice-house world.