The role of climate feedbacks in the middle Miocene climate transition

A major reorganization of Earth's climate system in the middle Miocene is suggested by the abrupt globally-recognized ∼1‰ increase in benthic foraminifer delta18O at ∼14-Ma, thought to reflect some combination of Antarctic ice growth and global cooling. However the relative contribution of ice volume and temperature to this delta18O signal remain unconstrained. Consequently, the relative importance of feedbacks involved in the middle Miocene climate transition is unresolved.; Southern Ocean (∼55°S) paleoclimate records provide novel insight into the magnitude and phasing of Antarctic ice growth and global cooling as well as into the feedbacks involved in the middle Miocene climate transition. Seawater delta18O, calculated from benthic foraminifer delta 18O and Mg/Ca, reveals a stepwise, eccentricity-paced, expansion of Antarctic ice sheets between ∼15 and 13.8-Ma, which suggests ice growth began at the height of the Miocene Climatic Optimum (∼17--14-Ma) and preceded the major delta18O increase by ∼1-million years. During the early stages of middle Miocene Antarctic cryosphere expansion (15--14.2-Ma), Southern Ocean surface waters underwent a stepwise eccentricity-paced cooling (6--7°C) and the water column became well mixed ∼60-ky before the major delta18O increase. A ∼2°C cooling of regional bottom waters (14.2--13.8-Ma) reveals that ∼70% of the delta 18O increase relates to Antarctic ice growth. Planktonic foraminifer assemblages indicate that the biota responded to surface temperatures and not ice volume, providing further corroboration of paleotemperature and seawater delta 18O records.; The initiation of middle Miocene Antarctic ice growth coincided with an interval of relatively warm Southern Ocean waters, a well-stratified regional water column, inferred low atmospheric pCO2, and a shift in orbital dynamics. These relationships suggest that moisture availability was essential for cryosphere expansion in a system poised for ice growth. Invigorated oceanic/atmospheric circumpolar circulation (14.2--13.8-Ma), related to the developing cryosphere, may have destabilized the water column, further isolated Antarctica from low-latitude heat and moisture and thus stabilized the Antarctic cryosphere. Results suggest that the Antarctic cryosphere was especially sensitive to heat and moisture during low atmospheric pCO2 conditions, reinforcing the fundamental role of pCO2 in global climate change.