Middle Miocene Southern Ocean cooling and Antarctic cryosphere expansion

Magnesium/calcium data from Southern Ocean planktonic foraminifera demonstrate that high-latitude (similar to55degreesS) southwest Pacific sea surface temperatures (SSTs) cooled 6degrees to 7degreesC during the middle Miocene climate transition (14.2 to 13.8 million years,ago). Stepwise surface cooling is paced by eccentricity forcing and precedes Antarctic cryosphere expansion by similar to60 thousand years, suggesting the involvement of additional feedbacks during this interval of inferred low-atmospheric partial pressure of CO2 (pCO(2)). Comparing SSTs and global carbon cycling proxies challenges the notion that episodic PCO2 drawdown drove this major Cenozoic climate transition. SST, salinity, and ice-volume trends suggest instead that orbitally paced ocean circulation changes altered meridional heat/vapor transport, triggering ice growth and global cooling.