Carbon isotopic records of nutrient-depleted surface water place constraints on the past fertility of the oceans and on past atmospheric pCO(2) levels. The best records of nutrientdepleted delta C-13 are obtained from planktonic foraminifera living in the thick mixed layers of the western equatorial and tropical Atlantic Ocean. We have produced a composite, stacked Globigerinoides sacculifer delta C-13 record from the equatorial Atlantic, which exhibits significant spectral power at the 100,000- and 41,000-year Milankovitch periods, but no power at the 23,000-year period. Similar to the record presented by Shackleton and Pisias [1985], surface-deep ocean Delta delta C-13 produced with the G. sacculifer record leads the delta O-18 ice volume record. However, the glacial-interglacial amplitudes of Delta delta C-13 differ between our record and Shackleton and Pisias [1985] record. Although large changes in Delta delta C-13 occur in the equatorial Atlantic during early stages of the last three glacial cycles, surface-deep Delta delta C-13 at glacial maxima (O-18 stage 2, late stage 6, and late stage 8) was only about 0.2 parts per thousand greater than during the subsequent interglacial. Our results imply that nutrient-driven pCO(2) changes account for about one third of the pCO(2) decrease observed in ice cores, and consequently, Delta delta C-13 should not be used as a proxy pCO(2) index. Enough variance in the ice core pCO(2) records remains to be explained that conclusions about pCO(2) and ice volume phase relationships should also be reexamined. As much as 40 ppm pCO(2) change still has not been accounted for by models of past physics and chemistry of the ocean.
