Orbitally forced climate change in late mid-Eocene time at Blake Nose (Leg 171B): evidence from stable isotopes in foraminifera
Bridget S. Wade, Dick Kroon, Richard D. Norris, 2001. "Orbitally forced climate change in late mid-Eocene time at Blake Nose (Leg 171B): evidence from stable isotopes in foraminifera", Western North Atlantic Palaeogene and Cretaceous Palaeoceanography, Dick Kroon, R. D. Norris, A. Klaus
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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 (δ18O, δ13C) 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 °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 δ18O and δ13C values of the planktonic foraminifers, of up to 1‰ and 2.6‰, respectively, that probably reflect temperature and nutrient fluctuations controlled by regional changes in upwelling intensity and runoff. The surface to benthos δ18O gradient decreases from 3‰ PDB to a minimum of c. 0.5‰ PDB over 400 ka, which could relate to the intensity of upwelling. Spectral analysis reveals precessional forcing in the foraminiferal δ18O 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 δ18O 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.