Eccentricity-paced atmospheric carbon-dioxide variations across themiddle Miocene climate transition

Abstract. The middle Miocene climate transition ~ 14 Ma marks a fundamental step towards the current “icehouse” climate, with a ~ 1 ‰ δ18O increase and a ~ 1 ‰ transient δ13C 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 atmospheric CO2 across the carbon-cycle perturbation has been intensely debated as proxy records of pCO2 for this time interval are sparse and partly contradictory. Using boron isotopes (δ11B) in planktonic foraminifers from drill site ODP 1092 in the South Atlantic, we show that long-term pCO2 variations between ~ 14.3 and 13.2 Ma were paced by 400 k.y. eccentricity cycles, with decreasing pCO2 at high eccentricity and vice versa. Our data support results from a carbon-cycle model study, according to which increased monsoon intensity at high eccentricity enhanced weathering and river fluxes in the tropics, resulting in increasing carbonate and organic carbon burial and hence decreasing atmospheric CO2. In this scenario, a combination of the eccentricity-driven climatic cycle and enhanced meridional deep-ocean circulation during Antarctic ice-sheet expansion may have both contributed to the pCO2 rise following Antarctic glaciation, acting as a negative feedback on the progressing glaciation and helping to stabilize the climate system on its way to the late Cenozoic “icehouse” world.