Icehouse climate

The remainder of the Eocene (48 to 34 million years ago; Fig. 1) was characterised by significant cooling of polar regions at high latitudes (Fig. 2)[1] This led to the contraction of (sub)polar biomes throughout the late Eocene and the growth of continental ice sheets on Antarctica at the Eocene-Oligocene boundary (34 million years ago). The sudden, widespread glaciation of Antarctica and the associated shift towards colder temperatures is the culmination of the most fundamental reorganisation of global climate known in the last 100 million years.

This event represents the ultimate transition from the early Cenozoic ‘greenhouse’ into the late Cenozoic ‘icehouse’ and, although it appears to be driven by CO₂ decline (Fig.3 and 4), the cause of the cooling from 48 to 34 Ma remains uncertain.  The existing pCO₂ records appear ambiguous and several other ideas have been proposed – e.g. closure of ocean gateways[2]. New pCO² records are clearly needed to resolve this issue.

Fig 1.a. shows the concentration of atmospheric carbon dioxide through the mid to late-Eocene (shown in blue) with values ranging from 750 to 2000 parts per million[3]. Fig 1.b. shows the temperature of the deep ocean during the same time period. This record suggests that water near the sea floor was much cooler than the early Eocene.

Figure 2.a και 2.b

Figure 2.a shows sea surface temperature records derived from organic biomarkers during the Eocene and Oligocene[4] . The white circles represent high-latitude temperatures while the red squares represent low-latitude temperatures. This illustrates pronounced cooling of high latitudes during the mid-to-late Eocene while the low-latitudes appear to remain relatively stable and warm.

Fig 2.b. represents the deep-water temperatures from benthic foram ¹⁸O during the Eocene and exhibit similarities with the polar regions (i.e. pronounced cooling)

Fig 3

Figure 3. Shows the two-step decline in atmospheric carbon dioxide across the Eocene-Oligocene boundary in Tanzania measured using boron isotopes derived from planktonic foraminifera shells[5]. The major increase in ¹⁸O seen in Figure 3a across the E-O reflects deep-ocean cooling and the growth of ice on Antarctica. It appears that pCO₂ decline is driving deep ocean cooling and ice sheet growth. This measurement comes from benthic foraminifera shells which record cooling and ice growth in their oxygen isotopic signature.

Fig 4

Figure 4. Reconstructed pco₂ records derived from two organic biomarkers at two low-latitude sites (blue circles and green diamonds)[6]. The grey line represents the deep ocean temperature record. The pCO₂ record shows a gradual decline in atmospheric carbon dioxide from the late Eocene into the early-mid Oligocene with a pronounced drop in pCO₂ across the Eocene-Oligocene boundary.

During the mid-to-late Eocene the temperature of the tropics remained warm and stable[7](Fig. 4).  This conflicts with the hypothesis that CO₂ is the driver of this Eocene cooling.  However, relatively few records exist of tropical temperatures during the Eocene. Therefore, the generation of new data sets for this latitude will be one of the aims of this project.