Age models for ODP Leg 172 sites

Ten ODP sites drilled in a depth transect (2164-4775 m water depth) during Leg 172 recovered high-deposition rate (>20 cm/kyr) sedimentary sections from sediment drifts in the western North Atlantic. For each site an age model covering the past 0.8-0.9 Ma has been developed. The time scales have a resolution of 10-20 kyr and are derived by tuning variations of estimated carbonate content to the orbital parameters precession and obliquity. Based on the similarity in the signature of proxy records and the spectral character of the time series, the sites are divided into two groups: precession cycles are better developed in carbonate records from a group of shallow sites (2164-2975 m water depth, Sites 1055-1058) while the deeper sites (2995-4775 m water depth, Sites 1060-1063) are characterized by higher spectral density in the obliquity band. The resulting time scales show excellent coherence with other dated carbonate and isotope records from low latitudes. Besides the typical Milankovitch cyclicity significant variance of the resulting carbonate time series is concentrated at millennial-scale changes with periods of about 12, 6, 4, 2.5, and 1.5 kyr. Comparisons of carbonate records from the Blake Bahama Outer Ridge and the Bermuda Rise reveal a remarkable similarity in the time and frequency domain indicating a basin-wide uniform sedimentation pattern during the last 0.9 Ma.

Stable carbon and oxygen isotope ratios of planktonic foraminifera from the equatorial Atlantic

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 delta13C 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 delta13C 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 delta13C produced with the G. sacculifer record leads the delta18O ice volume record. However, the glacial-interglacial amplitudes of Delta delta13C differ between our record and Shackleton and Pisias [1985] record. Although large changes in Delta delta13C occur in the equatorial Atlantic during early stages of the last three glacial cycles, surface-deep Delta delta13C 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, Delta delta13C 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.