Orbitally-paced climate evolution during the middle Miocene “Monterey” carbon-isotope excursion

One of the most enigmatic features of Cenozoic long-term climate evolution is the long-lasting positive carbon-isotope excursion or “Monterey Excursion”, which started during a period of global warmth after 16.9 Ma and ended at ∼ 13.5 Ma, approximately 400 kyr after major expansion of the Antarctic ice-sheet. We present high-resolution (1–9 kyr) astronomically-tuned climate proxy records in two complete sedimentary successions from the northwestern and southeastern Pacific (ODP Sites 1146 and 1237), which shed new light on the middle Miocene carbon-isotope excursion and associated climatic transition over the interval 17.1–12.7 Ma. We recognize three distinct climate phases with different imprints of orbital variations into the climatic signals (1146 and 1237 δ18O, δ13C; 1237 XRF Fe, fraction > 63 μm): (1) climate optimum prior to 14.7 Ma characterized by minimum ice volume and prominent 100 and 400 kyr variability, (2) long-term cooling from 14.7 to 13.9 Ma, principally driven by obliquity and culminating with rapid cryosphere expansion and global cooling at the onset of the last and most pronounced δ13C increase, (3) “Icehouse” mode after 13.9 Ma with distinct 100 kyr variability and improved ventilation of the deep Pacific. The “Monterey” carbon-isotope excursion (16.9–13.5 Ma) consists overall of nine 400 kyr cycles, which show high coherence with the long eccentricity period. Superposed on these low-frequency oscillations are high-frequency variations (100 kyr), which closely track the amplitude modulation of the short eccentricity period. In contrast to δ13C, the δ18O signal additionally shows significant power in the 41 kyr band, and the 1.2 Myr amplitude modulation of the obliquity cycle is clearly imprinted in the 1146 δ18O signal. Our results suggest that eccentricity was a prime pacemaker of middle Miocene climate evolution through the modulation of long-term carbon budgets and that obliquity-paced changes in high-latitude seasonality favored the transition into the “Icehouse” climate.