Evolution of Ocean Temperatures and Global Ice Volume During the Eocene to Oligocene from the Ocean Isotope Record

The evolution of climate during the Eocene and Oligocene can be determined from the deep-sea isotope and trace element records of ocean temperatures and ice volume. Earlier isotope work suggests that the primary transition from greenhouse to icehouse world took place during the Late Eocene and Early Oligocene, with large, permanent ice sheets appearing on Antarctica at 34 Ma (Zachos et al., 1992, 1996, 2001; Miller et al., 1998; Coxall et al., 2005). This transition was preceded by a period of long-term cooling which initiated near the Early-Middle Eocene boundary, roughly 50 Ma, following a sustained period of Early Eocene warmth. The Eocene cooling trend was not monotonic, but followed a somewhat step-like pattern with several reversals, the most substantial of which was the Middle Eocene climatic optimum (MECO) (Bohaty and Zachos, 2003; Jovane et al., 2007). By the Late Eocene, the climate on Antarctica appears to have cooled sufficiently to allow for the formation of small, ephemeral ice sheets, a state that persisted until ~34Ma, when most of East Antarctica became glaciated by a large ice sheet (Fig. 8.12). From that time forward, the ice sheet was a permanent feature of Antarctica. For the remainder of the Oligocene, this ice sheet waxed and waned, most likely in response to orbital forcing (Naish et al., 2001).

The long-term cooling trend that facilitated the formation of continental ice sheets has been attributed to either changes in palaeogeo-graphy or the concentration of greenhouse gases. Geographical isolation of

818O

818O

Ice-free Temperature (°C)

Figure 8.12: Global compilation of oxygen isotope records for the Cenozoic (after Zachos et al., 2001). Solid bars span intervals of ice-sheet activity in the Antarctic and Northern Hemisphere. Reproduced with permission of the Geological Society Publishing House, Bath, UK (Zachos et al., 2001; Billups and Schrag, 2003; Bohaty and Zachos, 2003; Lear et al., 2004).

Ice-free Temperature (°C)

Figure 8.12: Global compilation of oxygen isotope records for the Cenozoic (after Zachos et al., 2001). Solid bars span intervals of ice-sheet activity in the Antarctic and Northern Hemisphere. Reproduced with permission of the Geological Society Publishing House, Bath, UK (Zachos et al., 2001; Billups and Schrag, 2003; Bohaty and Zachos, 2003; Lear et al., 2004).

the Antarctic continent with the tectonic widening of ocean gateways is often cited as one means of driving long-term cooling (see above). A second possible mechanism is a decline in atmospheric CO2. Recent modelling studies (see below) suggest that the former would have had little impact on heat fluxes and mean annual temperatures on Antarctica, while the latter would be a more effective way of cooling the continent (Huber and Sloan, 2001). However, the record of Eocene CO2 has until recently (Pagani et al., 2005) lacked sufficient resolution to fully test this possibility.

Recent investigations of marine cores have largely focused on improving two aspects of the Eocene and Oligocene climate reconstructions: (1) the resolution of proxy records of ocean temperature and ice volume, which has promoted the development of high-resolution, orbitally tuned records, and (2) quantifying changes in ice volume, which has spurred the development and application of palaeotemperature proxies. These studies have benefited in part from efforts of the ODP to recover highly expanded, stratigraphically intact sediment sequences spanning the E/O. The latest high-resolution records show that ice volume and ocean temperatures varied in a periodic fashion with power concentrated in the long eccentricity and obliquity bands (Fig. 8.13; Coxall et al., 2005). The latter is consistent with the presence of a large polar ice sheet. Prior to the Late Eocene, however, power in the obliquity band is relatively weak (Palike et al., 2001), suggesting little to no ice volume on Antarctica. These records have also revealed distinct climatic variability coherent with the lower frequency components of obliquity with periods of 1.25 m.y.

The recent development of seawater temperature proxies, Mg/Ca and TEX86 (Schouten et al., 2003), has improved estimates of ice volume from oxygen isotope records. In particular, the first low-resolution benthic Mg/Ca records suggest that much of the 818O increase just after the E-O boundary (33.4 Ma) was the result of a substantial increase in ice volume as deep-sea temperature was fairly constant (Lear et al., 2000; Billups and Schrag, 2002). Most recently, Lear et al. (2008) have interpreted a marine cooling of ~2.5°C associated with this ice growth from exceptionally well preserved foraminifera well above the calcite compensation depth. Though still controversial, the magnitude of ice-volume increase would have exceeded that of the present-day Antarctic Ice Sheet.

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