High-resolution stable isotope records as indicators of middle Eocene climate change

By Bridget Wade

High-resolution (3 kyr) stable isotope analyses have been conducted on late middle Eocene planktonic foraminifera from the western North Atlantic (Ocean Drilling Program, Leg 171B, Site 1052). The data indicate significant (>1 per mil) and abrupt changes in mixed layer planktonic foraminifera oxygen isotope results (d18O). These variations probably result from large oscillations in sea surface temperatures. The variability is greater than that seen in open ocean Pleistocene records and indicates that the middle Eocene climatic system was not consistently warm or stable. There were intervals when annual sea surface temperatures were up to 5 degrees C greater than modern mean values. These temperatures are similar to those recorded in the early Eocene, suggesting increased carbon dioxide or other greenhouse gases may have forced warm intervals.

New and existing planktonic foraminiferal biostratigraphic events of the late middle Eocene have been examined with a sampling resolution of 3 kyr. These have been calibrated to the astronomical time-scale to accurately define the timing of key biostratigraphic events, particularly the extinction of Morozovella spinulosa, which is a distinct biomarker for late middle Eocene sediments.

Fourier analysis reveals Milankovitch frequencies within the stable isotopic record. The long-period eccentricity signal (400 kyr) governs the large fluctuations in middle Eocene oxygen isotope results and produced significant changes in water column stability and thermal stratification. Large oscillations in sea surface temperatures occurred with surface water temperatures periodically reduced for 100 kyr. A possible explanation is that these abrupt shifts in planktonic foraminifera d18O represent orbitally forced variations in upwelling intensity, which greatly reduced sea surface temperatures. The direct effects of eccentricity on insolation are small. Therefore the prominent eccentricity variations were probably generated by nonlinear response to precessional forcing within the climatic system, rather than directly from eccentricity variations in solar insolation. Feedback effects within the oceanic - atmospheric system, possibly related to atmospheric transport, must have been important. The generally reduced surface to bottom temperature gradient in the Eocene may have facilitated the upwelling of deep water to the surface ocean.

It is concluded that the oscillations in the stable isotopic profiles in the western North Atlantic are due to climatic controls on the intensity of upwelling. The prominence of the eccentricity frequencies in middle Eocene climate records indicates that orbital modulation of solar insolation was an important parameter of climatic variability at this time.