Stable carbon and oxygen isotope record of planktonic foraminifera in ODP Site 138-851

From our knowledge of seasonal and interannual adjustments of the thermocline in this location, a deeper thermocline might be interpreted as either a decrease in the strength of the Equatorial Undercurrent (EUC) that results from lower mean wind strength or an increase in the Equatorial Countercurrent (ECC), which results from an increase in the strength of the southeasterly trade winds. A major shift from higher to lower carbon isotope values occurred at about 1.9 Ma, marking a transition to reduced planktonic-benthic d13C differences after 1.9 Ma.

The carbon isotopic data indicate that changes in the carbon isotopic composition of intermediate upwelling water occurs at higher frequencies than the glacial/interglacial changes in ice volume.