Ocean Drilling Program, United States

Previous stable oxygen isotopic data from surface-dwelling foraminifera indicate that Eocene tropical sea surface temperatures (SSTs) were significantly lower than at present. Here we show that stable isotopic analyses (delta (super 18) O, delta (super 13) C) of the late mid-Eocene mixed-layer dweller Morozovella spinulosa are consistent with mid-Eocene mid-latitude SSTs close to, or slightly lower than modern temperatures at Blake Nose, western North Atlantic. In contrast, isotopic analyses of the benthic foraminifer, Nuttalides truempyi reveal a gradual fall in mean bottom-water temperatures from 8 to 7 degrees C over c. 500 ka years. These deep intermediate-water temperatures are significantly higher than modern ones and are similar to intermediate- and bottom-water temperatures recorded from earlier in Palaeogene and late Cretaceous time. Large shifts are seen in the delta (super 18) O and delta (super 13) C values of the planktonic foraminifers, of up to 1 per mil and 2.6 per mil, respectively, that probably reflect temperature and nutrient fluctuations controlled by regional changes in upwelling intensity and runoff. The surface to benthos delta (super 18) O gradient decreases from 3 per mil PDB to a minimum of c. 0.5 per mil PDB over 400 ka, which could relate to the intensity of upwelling. Spectral analysis reveals precessional forcing in the foraminiferal delta (super 18) O records, which shows the direct influence of low-latitude insolation on surface-water stratification. Monsoonal wind systems may have forced the upwelling cycles and/or freshwater input. The benthic foraminifer delta (super 18) O record also contains the obliquity cycle, in addition to the precessional cycles, indicating the inheritance of mid- and high-latitude forcing to subtropical deep waters.