RECONCILING HIGH ATMOSPHERIC CO2 AND APPARENT HIGH ICE-VOLUME ESTIMATES DURING THE MIDDLE MIOCENE: NEW ISOTOPIC AND ELEMENTAL RATIO RECORDS FROM IODP EXPEDITION 371, SITE U1510

The Miocene Climatic Optimum (16.7-14.6 Ma) is the last time that atmospheric CO₂ levels were significantly higher than today, making this time interval an important analog for potential future warm climates. Reconstructions of deep-water temperatures based on benthic foraminifera Mg/Ca ratios and clumped isotopes, indicate bottom water temperatures 4 to 8˚C warmer than present. At face value, these results indicate ice-volume estimates were far larger than modern day and are incongruent with high atmospheric CO₂ levels as well as other studies that show ice volume during this time was smaller than the modern ice volume.

This study attempts to resolve the apparent mismatch between large ice-volume estimates concomitant with high bottom water temperatures and high atmospheric CO₂ for the span between 15 and 13 Ma. By taking into account higher isotopic values of seawater owing to increased evaporation history on δ¹⁸O_sw, and the larger size of the Antarctic continent during the Middle Miocene, the resulting ice-volume estimates are similar to estimates from backstripped stratigraphy from continental shelves, as well as from modeling studies that suggest a smaller ice sheet during the Miocene Climatic Optimum and an ice sheet volume near to slightly larger than the present-day ice sheet after the δ¹⁸O Mi-3 event (13.8 Ma).