Obliquity and long eccentricity pacing of the Middle Miocene climate transition

The Middle Miocene East Antarctic ice sheet expansion (EAIE), which is indicated by an abrupt similar to 1 parts per thousand increase in global benthic foraminiferal O-18 at similar to 13.8Ma, marks the Middle Miocene climate transition (MMCT) and has been related to astronomically modulated changes in the global carbon cycle. Here, we present high resolution (3-4kyr) benthic foraminiferal O-18 and C-13 records from IODP Site U1337 in the central equatorial Pacific, which spans the period 12.2-15.8Ma. The isotopic records clearly demonstrate significant imprints from periodic variations in the Earth's orbital parameters, particularly the obliquity (40kyr) and the long eccentricity (400kyr) cycles. While the benthic O-18 and C-13 exhibit nearly identical amplitudes for glacial-interglacial cycles from 15.8 to 12.2Ma, the long-term trends in the benthic O-18 and C-13 had started to reverse after the beginning of the EAIE. Within the 400-kyr band, the benthic -O-18 and C-13 displays a constant phase relationship between 15.8 and 12.2Ma. At the 41-kyr band, however, a phase reversal reaching >180 degrees between -O-18 and C-13 occurs from 13.8Ma to 14.0Ma during the period of the EAIE. A similar phase relationship of benthic foraminiferal -O-18 and C-13 at the 400-kyr band and the 41-kyr band is also observed at ODP Site 1146 from the northern South China Sea. This phase jump occurs when the long-term trends in O-18 and C-13 split, suggesting a decoupling of the global ice volume and ocean carbon reservoir changes during the Middle Miocene.