Oxygen Isotope Mass-Balance Constraints on Pliocene Sea Level and East Antarctic Ice Sheet Stability

Abstract:

The mid-Pliocene Warm Period (MPWP, 3.3–2.9 Ma), with reconstructed atmospheric pCO₂ of 350–450 ppm, represents a potential analogue for climate change in the near future. Current highly cited estimates place MPWP maximum global mean sea level (GMSL) at 21 ± 10 m above modern, requiring total loss of the Greenland (GIS) and marine West Antarctic Ice Sheets (WAIS) and a substantial loss of the East Antarctic Ice Sheet (EAIS), with only a concurrent 2–3 ºC rise in global temperature. Many estimates of Pliocene GMSL are based on the partitioning of oxygen isotope records from benthic foraminifera (δ¹⁸O_b) into changes in deep-sea temperatures and terrestrial ice sheets. These isotopic budgets are underpinned by the assumption that the δ¹⁸O of Antarctic ice (δ¹⁸O_i) was the same in the Pliocene as it is today, and while the sensitivity of δ¹⁸O_b to changing meltwater δ¹⁸O has been previously considered, these analyses neglect conservation of ¹⁸O/¹⁶O in the ocean-ice system. Using well-calibrated δ¹⁸O-temperature relationships for Antarctic precipitation along with estimates of Pliocene Antarctic surface temperatures, we argue that the δ¹⁸O_i of the Pliocene Antarctic ice sheet was at minimum 1‰–4‰ higher than present. Assuming conservation of ¹⁸O/¹⁶O in the ocean-ice system, this requires lower Pliocene seawater δ¹⁸O (δ¹⁸O_sw) without a corresponding change in ice sheet mass. This effect alone accounts for 5%–20% of the δ¹⁸O_b difference between the MPWP interglacials and the modern. With this amended isotope budget, we suggest that Pliocene GMSL was likely 9–13.5 m and very likely 5-17 m above modern, which suggests the EAIS is less sensitive to radiative forcing than previously inferred from the geologic record.