Paired benthic foraminiferal trace metal and stable isotope records have been constructed from equatorial Pacific Ocean Drilling Program Site 1218. The records include the two largest abrupt (<1 Myr) increases in the Cenozoic benthic oxygen isotope record: Oi-1 in the earliest Oligocene (~34 Ma) and Mi-1 in the earliest Miocene (~23 Ma). The paired Mg/Ca and oxygen isotope records are used to calculate seawater δ18O (δw). Calculated δw suggests that a large Antarctic ice sheet formed during Oi-1 and subsequently fluctuated throughout the Oligocene on both short (<0.5 Myr) and long (2--3 Myr) timescales, between about 50 and 100% of its maximum earliest Oligocene size. The magnitudes of these fluctuations are consistent with estimates of sea level derived from sequence stratigraphy. The transient expansion of the Antarctic ice sheet at Mi-1 is marked in the benthic δ18O record by two positive excursions between 23.7 and 22.9 Ma, each with a duration of 200--300 kyr. Bottom water temperatures decreased by ~2¿C over the 150 kyr immediately prior to both rapid δ18O excursions. However, the onset of each of these phases of ice growth is synchronous, within the resolution of the records, with the onset of a 2¿C warming over ~150 kyr. We suggest that the warming during these glacial expansions reflect increased greenhouse forcing prompted by a sudden decrease in global chemical weathering rates as Antarctic basement silicate rocks became blanketed by an ice sheet. This represents a negative feedback process that might have operated during major abrupt growth phases of the Antarctic ice sheet.