Oxygen isotope mass-balance constraints on Pliocene sea level and East Antarctic ice sheet stability

The mid-Pliocene warm period (MPWP, 3.3-2.9 Ma), with reconstructed atmospheric pCO (sub 2) of 350-450 ppm, represents a potential analog for climate change in the near future. Current highly cited estimates place MPWP maximum global mean sea level (GMSL) at 21 + or - 10 m above modern, requiring total loss of the Greenland and marine West Antarctic Ice Sheets and a substantial loss of the East Antarctic Ice Sheet, with only a concurrent 2-3 degrees C rise in global temperature. Many estimates of Pliocene GMSL are based on the partitioning of oxygen isotope records from benthic foraminifera (delta (super 18) O (sub b) ) into changes in deep-sea temperatures and terrestrial ice sheets. These isotopic budgets are underpinned by the assumption that the delta (super 18) O of Antarctic ice (delta (super 18) O (sub i) ) was the same in the Pliocene as it is today, and while the sensitivity of delta (super 18) O (sub b) to changing meltwater delta (super 18) O has been previously considered, these analyses neglect conservation of (super 18) O/ (super 16) O in the ocean-ice system. Using well-calibrated delta (super 18) O-temperature relationships for Antarctic precipitation along with estimates of Pliocene Antarctic surface temperatures, we argue that the delta (super 18) O (sub i) of the Pliocene Antarctic ice sheet was at minimum 1 ppm-4 ppm higher than present. Assuming conservation of (super 18) O/ (super 16) O in the ocean-ice system, this requires lower Pliocene seawater delta (super 18) O without a corresponding change in ice sheet mass. This effect alone accounts for 5%-20% of the delta (super 18) O (sub b) difference between the MPWP interglacials and the modern. With this amended isotope budget, we present a new Pliocene GMSL estimate of 9-13.5 m above modern, which suggests that the East Antarctic Ice Sheet is less sensitive to radiative forcing than previously inferred from the geologic record.