Ocean Circulation Tectonic Isolation

The progressive opening of oceanic gateways (Fig. 9.1) and progressive tectonic isolation during the Cenozoic stages of Gondwana breakup have been indicated as critical threshold events in the climatic deterioration and inception of ice sheets since the first deep-sea oxygen isotopic records were

Eccentricity p CO2 (ppmv)

Eccentricity p CO2 (ppmv)

Figure 9.8: Potential controls on climate and ice sheet behaviour across the Oligocene-Miocene boundary and origin of the Mi-1 d18O event. Obliquity and eccentricity orbital target data from Laskar et al. (2004). 818O data are from Zachos et al. (2001b) and pCO2 data are from Pagani et al. (2005). All data are plotted against the astronomical time scale presented by Billups et al. (2004). See text for discussion.

Figure 9.8: Potential controls on climate and ice sheet behaviour across the Oligocene-Miocene boundary and origin of the Mi-1 d18O event. Obliquity and eccentricity orbital target data from Laskar et al. (2004). 818O data are from Zachos et al. (2001b) and pCO2 data are from Pagani et al. (2005). All data are plotted against the astronomical time scale presented by Billups et al. (2004). See text for discussion.

recovered (Shackleton and Kennett, 1975; Kennett, 1977). Testing of this hypothesis has proven particularly difficult due to uncertainties in the timing of gateway opening and inception of deep-water circulation. While estimates for the timing of the opening and deepening of the Tasmanian Gateway between Australia and Antarctica are reasonably well constrained to the Eocene-Oligocene boundary (Stickley et al., 2004), estimates for the opening of a deep Drake Passage range from the middle Eocene (~50Ma) to the late Miocene (~6Ma) (Barker and Burrell, 1977; Barker et al., 2007; Livermore et al., 2007). However, some estimates suggest that deep-water circulation through the Drake Passage may well have been coincident with the Oligocene-Miocene boundary (Barker and Burrell, 1977, 1982). Pekar et al. (2006) and Pekar and Christie-Blick (2008) suggested, however, that Southern Ocean water masses were still relatively poorly mixed in the late Oligocene-early Miocene and that individual records used to compile the composite Cenozoic oxygen isotope curve by Zachos et al. (2001a) were drawn from water masses with different temperature and salinity histories. Hence, introducing an artifact from the splice of different isotopic records (Pekar and Christie-Blick, 2008) previously interpreted to represent significant oceanic warming in the latest Oligocene (Zachos et al., 2001a).

A number of numerical ocean modelling studies (Mikolajewicz et al., 1993; Nong et al., 2000; Toggweiler and Bjornsson, 2000; Sijp and England, 2004) have shown that the opening of a deep circum-Antarctic passage can cool the Southern Ocean by 1-3°C. While the amount of cooling in these studies is somewhat dependent on modelling details associated with the treatment of the atmosphere (Huber and Nof, 2006), the effects of this range of cooling on continental climate and ice-sheet mass balance have been shown to be small relative to the effects of the falling Cenozoic CO2 concentrations (DeConto and Pollard, 2003a; Huber et al., 2004). For example, recent model simulations testing the importance of sea ice feedback on Antarctic Ice Sheets show that the continental interior is relatively insensitive to changes in Southern Ocean sea surface temperatures, and the effect of even large changes in ocean heat transport and sea ice is generally limited to the continental margins (DeConto et al., 2007). Conversely, the expansion of the EAIS, as presumed to have occurred at Mi1, has a dramatic effect on simulated Southern Ocean sea surface temperatures and sea ice distributions via the ice sheet's influence on regional temperatures and low-level winds (DeConto et al., 2007; Fig. 9.9). As these simulations clearly show, a growing Mi1 ice sheet would have cooled Southern Ocean sea surface temperatures by several degrees, pushing the 0 °C isotherm equatorwards and increasing the area, thickness, and fractional cover of seasonal and perennial sea ice (DeConto et al., 2007). Furthermore, as the katabatic wind field increased in

Figure 9.9: South polar seasonal temperatures, sea ice and winds in response to a growing ice sheet as simulated by a GCM (DeConto et al., 2007). (a-c) Climatic conditions in a pre-Mi1 world with isolated ice caps in the continental interior (Top). (d-f) Climatic conditions with a fully glaciated east antarctica as presumed to have existed at the time of Mi1 (bottom). With the exception of ice sheet geometry, boundary conditions are identical in both simulations including the same late Palaeogene palaeogeography, 2 x pre-industrial CO2 (560 ppmv), and a relatively cold austral summer orbit conducive to Antarctic Ice Sheet growth. Ice sheet geometries are taken from prior GCM-ice sheet simulations of Antarctic glaciation (DeConto and Pollard, 2003a). Austral Summer (December, January, February) and winter (June, July, August) seasonal climatologies are shown on the top and bottom of (a-f), respectively. (a, d) Seasonal surface (2 m) air temperature, (b, e) seasonal sea ice extent and thickness in metres and (c, f) lowest level (sigma level — 0.189) GCM winds with vector scale length equivalent to 2°C per mT

of wind velocity.

d e intensity, the enhanced polar easterlies and westerlies would have increased ocean frontal divergence and upwelling, with possible implications for the marine carbon cycle and CO2 drawdown (DeConto et al., 2007). Such mechanisms have been implicated as important contributors to the dynamics of Quaternary glacial cycles (Stephens and Keeling, 2000; Archer et al., 2003), but they have yet to be considered in a Miocene context.

While discussions of tectonic influences on Antarctic climate evolution usually focus on Southern Ocean gateways, Miocene ice sheets could have also been sensitive to changes in tropical climate associated with Tethyan tectonics. As the Southern Ocean gateways were widening and deepening, the eastern Tethys was closing. Ocean modelling studies have shown that the progressive closure of the Tethys affected the location of deep-water formation and the thermohaline component of the meridional overturning circulation, ocean heat transport, and both tropical and high-latitude sea surface temperatures (Hotinski and Toggweiler, 2003; von der Heydt and Dijkstra, 2006). While the Antarctic interior appears to have been relatively insensitive to changes in the Southern Ocean, the modern Antarctic interior receives much of its moisture from the low mid-latitudes and significant changes in the tropics and associated teleconnections to polar latitudes could be important. Considering the timing of Tethyan closure relative to Antarctic Ice Sheet expansion in the Miocene, the sensitivity of ice-sheet evolution to low-latitude versus circum-Antarctic sea surface temperatures should be tested in future modelling studies.

The perspective provided by numerical climate modelling suggests falling greenhouse gas concentrations around the time of the Oligocene-Miocene boundary (Pagani et al., 2005) had a greater impact on Antarctic climate than the direct, physical effects of ocean gateways. However, the indirect effects of the gateways, including their influence on the marine carbon cycle and atmospheric CO2 should also be considered. These indirect effects may be found to be more important to Cenozoic climate events like Mil than the direct influence of the gateways on ocean circulation and heat transport (Mikolajewicz et al., 1993; DeConto and Pollard, 2003a; Huber et al., 2004).

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