The δ13C of equatorial Atlantic surface waters: Implications for Ice Age pCO2 levels

Carbon isotopic records of nutrient-depleted surface water place constraints on the past fertility of the oceans and on past atmospheric pCO2 levels. The best records of nutrient-depleted δ13C 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 δ13C 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 Δδ13C produced with the G. sacculifer record leads the δ18O ice volume record. However, the glacial-interglacial amplitudes of Δδ13C differ between our record and Shackleton and Pisias [1985] record. Although large changes in Δδ13C occur in the equatorial Atlantic during early stages of the last three glacial cycles, surface-deep Δδ13C at glacial maxima (18O stage 2, late stage 6, and late stage 8) was only about 0.2‰ greater than during the subsequent interglacial. Our results imply that nutrient-driven pCO2 changes account for about one third of the pCO2 decrease observed in ice cores, and consequently, Δδ13C should not be used as a proxy pCO2 index. Enough variance in the ice core pCO2 records remains to be explained that conclusions about pCO2 and ice volume phase relationships should also be reexamined. As much as 40 ppm pCO2 change still has not been accounted for by models of past physics and chemistry of the ocean.

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