Connection of CO2 and Ice Sheet Inception at the EO Boundary Computer Modelling

While the onset of major, continental-scale glaciation in the earliest Oligocene has long been attributed to the opening of Southern Ocean gateways (Kennett and Shackleton, 1976; Kennett, 1977; Robert et al., 2001), recent modelling studies suggest declining atmospheric CO2 was the most important factor in Antarctic cooling and glaciation.

Climatic Eccentricity precession Obliquity (%) 6 p (deg) ° o ° 22 23 24 1 3 5

Climatic Eccentricity precession Obliquity (%) 6 p (deg) ° o ° 22 23 24 1 3 5

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-513C —Bulk sediment-<*-by physical property measurements Bulk sediment % CaC03

Figure 8.13: High-resolution isotope and % CaC03 records of the E/O boundary (Coxall et al., 2005). Age model is derived from orbital tuning.

-by mass-spectrometry

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-513C —Bulk sediment-<*-by physical property measurements Bulk sediment % CaC03

Figure 8.13: High-resolution isotope and % CaC03 records of the E/O boundary (Coxall et al., 2005). Age model is derived from orbital tuning.

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 CO2 concentrations) as the primary cause of Antarctic cooling and glaciation.

The recent development of coupled climate-ice sheet models capable of running long (>106 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 CO2 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), CO2 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 modelling studies exploring the role of orbital variability (DeConto and Pollard, 2003b), mountain uplift in the continental interior (DeConto and Pollard, 2003a), geothermal heat flux (Pollard et al., 2005), Antarctic vegetation dynamics (Thorn and DeConto, 2006) and Southern Ocean sea ice (DeConto et al., in press) in the E/O climatic transition.

The results of these studies can be summarized as follows. The timing of glaciation on East Antarctica was shown to be sensitive to orbital forcing, mountain uplift and continental vegetation, but only within a very narrow range of atmospheric CO2 concentrations around 2.8 times modern levels, close to the model's glaciation threshold. Once the glaciation threshold is approached, astronomical forcing can trigger sudden glaciation through non-linear height/mass balance and albedo feedbacks that result in the growth of a continental-scale ice sheet within 100kyrs (Fig. 8.14). The timing of glaciation appears to be insensitive to both expanding concentrations of seasonal sea ice and changes in geothermal heat flux under the continent; however, a doubling of the background geothermal heat flux (from 40 to 80mWm~2) does have a significant effect on the area under the ice sheet at the pressure-melt point (where liquid water is present), which may have had some influence on the distribution and development of sub-glacial lakes and subsequent ice-sheet behaviour.

While these modelling studies have certainly improved our understanding of the importance of atmospheric CO2 concentrations relative to other Cenozoic forcing factors, several important model-data inconsistencies remain unresolved. For example, long, time-continuous GCM-ice-sheet simulations of an increasing CO2 (warming) scenario show strong hysteresis once a continental ice sheet has formed (Pollard and DeConto, 2005). In these simulations, orbital forcing alone is not sufficient to produce the range of Palaeogene-Neogene ice-sheet variability (~ 50-120% modern Antarctic ice volumes) inferred by marine oxygen isotope records and sequence strati-graphic reconstructions of eustasy (Zachos et al., 2001; Pekar and DeConto, 2006; Pekar et al., 2006), pointing to the importance of additional feedbacks (possibly related to the marine carbon cycle and atmospheric CO2) in controlling Cenozoic ice-sheet variability. Furthermore, several recent isotopic analyses of deep-sea cores imply ice volumes during the peak Oligocene and Miocene glacial intervals that are too big to be accommodated by East Antarctica alone (Lear et al., 2004; Coxall et al., 2005; Holbourn et al., 2005). This suggests that either our interpretations of the proxy data are faulty, or episodic, bipolar glaciation occurred much earlier than currently accepted. These, among other unresolved controversies related to the climatic and glacial evolution of the high southern latitudes, will be the focus of future ACE modelling exercises and model-data comparisons.

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Figure 8.14: Ice volume (left) and corresponding ice-sheet geometries (right) simulated by a coupled GCM-ice sheet model in response to a slow decline in atmospheric C02 and idealized orbital cyclicity across the E/O boundary. The sudden, two-step jump in ice volume (left panel) corresponds to the Oi-1 event. The left panel shows simulated ice volume (red line), extrapolated to an equivalent change in sea level and the mean isotopic composition of the ocean (top). Arbitrary model years (left axis) and corresponding, prescribed atmospheric C02 (right axis) are also labelled. C02 is shown as the multiplicative of pre-industrial (280ppmv) levels. Ice-sheet geometries (right panels) show ice-sheet thickness in metres. Black arrows correlate the simulated geometric evolution of the ice sheet through the Oi-1 event (modified from DeConto and Pollard, 2003b). Reproduced with permission of The Geological Society Publishing House, Bath, UK.

A sea level 0 10 20 30 40 50 60

meters

4000 3500 3000 2500 2000 1500 1000 500

Figure 8.14: Ice volume (left) and corresponding ice-sheet geometries (right) simulated by a coupled GCM-ice sheet model in response to a slow decline in atmospheric C02 and idealized orbital cyclicity across the E/O boundary. The sudden, two-step jump in ice volume (left panel) corresponds to the Oi-1 event. The left panel shows simulated ice volume (red line), extrapolated to an equivalent change in sea level and the mean isotopic composition of the ocean (top). Arbitrary model years (left axis) and corresponding, prescribed atmospheric C02 (right axis) are also labelled. C02 is shown as the multiplicative of pre-industrial (280ppmv) levels. Ice-sheet geometries (right panels) show ice-sheet thickness in metres. Black arrows correlate the simulated geometric evolution of the ice sheet through the Oi-1 event (modified from DeConto and Pollard, 2003b). Reproduced with permission of The Geological Society Publishing House, Bath, UK.

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