From Greenhouse to Icehouse – The Eocene/Oligocene - UMass ...

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Climatic Eccentricity precession Obliquity (%) (deg) 22 23 24 1 3 5 δ 0.5 1 1.5 13C‰ VPDB MAR g/cm2/kyr 0 0.5 1 1.5 2 δ 18 O‰ VPDB 0.04 0 -0.04 0 0.5 1 1.5 2 31 32 33 Oi-1 C13r C13n C13n E. formosa C12r Mag. chrons LATE EOCENE EARLY OLIGOCENE Epoch Age (Ma) C12n Eoc. Oli. Inset Mio. Author's personal copy 210 3 From Greenhouse to Icehouse – The Eocene/Oligocene in Antarctica 345 4 220 Paleodepth (km) 5 230 0 10 20 30 40 50 Age (Ma) 240 34 C13n 245 ? 35 C13n.1 C13r 0 20 40 60 80 Bulk sediment % CaCO3 % CaCO3 by mass-spectrometry by physical property measurements MAR CaCO3 Bulk sediment δ18O δ13C Figure 8.13: High-resolution isotope and % CaCO3 records of the E/O boundary (Coxall et al., 2005). Age model is derived from orbital tuning.
346 J. E. Francis et al. Author's personal copy As the Drake Passage and Tasmanian Gateway widened and deepened during the late Palaeogene and early Neogene (Lawver and Gahagan, 1998), the ACC and Polar Frontal Zone (APFZ) presumably cooled the Southern Ocean by limiting the advection of warm subtropical surface waters into high latitudes (Kennett, 1977). While the opening of the Tasmanian Gateway does broadly coincide with the earliest Oligocene glaciation event (Oi-1) (Stickley et al., 2004), the tectonic history of the Scotia Sea remains equivocal. Estimates for the timing of the opening of Drake Passage range between 40 and 20 Ma (Barker and Burrell, 1977; Livermore et al., 2004; Scher and Martin, 2006), clouding the direct ‘‘cause and effect’’ relationship between the gateways and glaciation. A number of ocean modelling studies have shown that the opening of both the Drake Passage and Tasmanian gateways reduces poleward heat convergence in the Southern Ocean and cools sea surface temperatures by up to several degrees (Mikolajewicz et al., 1993; Nong et al., 2000; Toggweiler and Bjornsson, 2000). More recent, coupled atmosphere–ocean GCM simulations suggest a more modest effect, however. Huber et al. (2004) showed that the Tasmanian Gateway likely had a minimal effect on oceanic heat convergence and sea surface temperatures around the continent, because the warm East Australia Current does not travel any further south if the gateway is open or closed. The gateway’s effect on East Antarctic climate and snowfall was also shown to be minimal, pointing to some other forcing (perhaps decreasing atmospheric CO 2 concentrations) as the primary cause of Antarctic cooling and glaciation. The recent development of coupled climate–ice sheet models capable of running long (W10 6 years), time-continuous simulations of specific climate events and transitions (DeConto and Pollard, 2003a) has allowed simulations of the Oi-1 event that account for decreasing CO 2 concentrations, orbital variability and prescribed changes in ocean transport (DeConto and Pollard, 2003b; Pollard and DeConto, 2005). These simulations support the conclusions of Huber et al. (2004) as to the likely importance of CO2 by showing that, even if significant, tectonically forced changes in ocean circulation and heat transport had occurred around the E/O boundary, they would have had only a small effect on temperature and glacial mass balance in the Antarctic interior. Therefore, Southern Ocean gateways could only have triggered glaciation if the climate system was already close to a glaciation threshold. Considering the sensitivity of polar climate to the range of CO2 concentrations likely to have existed over the Palaeogene–Neogene (Pagani et al., 2005), CO 2 likely played a fundamental role in controlling Antarctica’s climatic and glacial sensitivity to a wide range of forcing mechanisms. This conclusion is supported by a number of additional
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