Marine Records of the Southern Ocean

The widespread presence of hiatuses in Southern Ocean sedimentary cores is one indicator of the reorganization of ocean circulation patterns that occurred during the MMCT, which is believed to have been associated with the expansion of the Antarctic Ice Sheet (e.g. Woodruff and Savin, 1989). Interbasinal offsets in benthic foraminiferal carbon isotopes suggest that the Early Miocene Southern Ocean received warm saline deep water sourced from low latitudes in the Tethyan region (e.g. Woodruff and Savin, 1989). The flow of this warm saline water mass likely ceased in the Middle Miocene, and the consequent reduction in meridional heat transport is a possible trigger for the ice-sheet expansion in the Middle Miocene. Furthermore, a number of Southern Ocean records reconstruct an increase in the strength of the ACC which may also have played an important role in the MMCT (e.g. Pagani et al., 2000).

During the Late Miocene, Southern Ocean records indicate further high-latitude cooling possibly associated with the establishment and/or growth of the WAIS and an intensification of the ACC (e.g. Kennett and Barker, 1990). By 6 Ma, the relative flux of NADW into the Southern Ocean reached modern proportions, and subsequently intensified yet further, supporting the speculation that the relative climatic warmth of the Early Pliocene was a result of enhanced thermohaline overturn (e.g. Kwiek and Ravelo, 1999). During the Pliocene, biological productivity increased suggesting a reduction in sea-ice extent/coverage at this time (e.g. Griitzner et al., 2005; Hillenbrand and Cortese, 2006). Furthermore, silicoflagellate assemblages have been used to pinpoint three intervals (3.7, 4.3-4.4 and 4.6-4.8 Ma) within the Pliocene when sea surface temperatures in the Southern Ocean were roughly 5° warmer than today (Whitehead and Bohaty, 2003).

10.8.7. Modelling Antarctic Middle Miocene to Pliocene Climates and Antarctic Ice Sheets

A number of palaeoclimate modelling studies have been conducted for the Middle Miocene to Pliocene period (e.g. Ramstein et al., 2005). However, only a few of them have focused on reconstructing the climate and environments of Antarctica (see Haywood et al., 2002b; Francis et al., 2007; Hill et al., 2007). A limited number of studies, using a prescribed EAIS within the climate model which is based on global sea-level estimates, suggest that during warm phases of the Pliocene (specifically the mid-Pliocene), climatic conditions of Antarctica could have ameliorated sufficiently to allow tundra forms of vegetation to exist in deglaciated regions. Idealized sensitivity experiments (e.g. Huybrechts, 1993), in which ice-sheet models are forced with different temperature and precipitation regimes, suggest that a surface temperature warming of between 17 and 20°C is needed to generate the ice-free corridor over the Pensacola and Wilkes subglacial basins hypothesized by Harwood (1983) and Harwood and Webb (1986) for the Pliocene. For temperature rises of less than 5°C, the models predict that the EAIS increased in size due to an increase in snowfall. More recent studies in which climatologies derived from globally evaluated climate modelling simulations for the Pliocene that are used offline to drive an ice-sheet model suggest that the mean state for the Pliocene Greenland Ice Sheet and EAIS were, to some extent, smaller than today. For Antarctica, the model results clearly show that the Wilkes and Aurora Basins are the areas of the EAIS that are most susceptible to increased temperatures.

Nevertheless, even in the most extreme scenario, the predicted EAIS still covers all but the northernmost reaches of the Wilkes subglacial basin, which is suggested to be a source of the diatoms found within the Sirius Group sediments (Hill et al., 2007).

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