The Greenhouse world: from Gondwana breakup to 34 million years

Some 200 million years ago (Ma) Antarctica was the centrepiece of the Gondwana super-continent, which began to break up around 180 Ma in the Jurassic Period of the Mesozoic Era. The Gondwanan fragments separated by sea-floor spreading largely between around 100 to 65 Ma, during the Cretaceous Period, although by this time Antarctica had already moved into a position over the South Pole (Figure 3.2). A rich record of plant and animal fossils from Late Cretaceous and early Tertiary times has been found on Seymour Island and Alexander Island in the Antarctic Peninsula. This demonstrates that, despite its polar position, Antarctica had a warm temperate climate that allowed the growth of lush forests (Figure 3.3).

These were inhabited by dinosaurs in the Cretaceous and mammals during the early Tertiary (Francis and Poole, 2002).

Figure 3.2 Apparent polar wander path for East Antarctica over the last 120 Ma (modified from DiVenere et al., 1994). The shaded area represents the modelled error envelope.

Analysis of changing biodiversity in the forests, and of the climate record preserved in the foliage, indicates that climate cooling signalled the decline of Antarctic warmth during the Middle Eocene, about 45 Ma, when heat-loving plants were lost from Antarctica and replaced by types such as species of Nothofagus trees that could tolerate cold climates (Francis et al., 2008a).

Over time, South America, Africa, India, Australia and New Zealand moved away from Antarctica, opening the South Atlantic, Indian and Southern Oceans (Figure 3.4). As the proto-Pacific Plate was subducted beneath the Antarctic Plate an active volcanic arc grew, the eroded remnants of which now form the Antarctica Peninsula. The western margin of Antarctica was thus characterised by volcanism and lateral movement of fragments of the formerly continuous Gondwana terrain (for example see McCarron and Larter, 1998). This western arc broke apart first at around 85 Ma south of New Zealand, but much later between South America and the Antarctic Peninsula to create the Drake Passage.

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Figure 3.3 Reconstruction of mid-Cretaceous forests of Alexander Island, Antarctica. Based on the work of J. Howe, J.E. Francis, and numerous British Antarctic Survey geologists.

Painted by Robert Nicholls (Francis et al., 2008a).

Over the last 40 million years, the Antarctic shelf experienced a series of tectonic, climatic and oceanographic events that lead to isolation from other oceans, establishment of colder conditions and complete replacement and impoverishment of the fish fauna. The deepening of the separations from other continental masses and the removal of the last barriers to circumpolar flow, allowed the establishment of the Antarctic Circumpolar Current (ACC) and, at its northern border, of the Antarctic Polar Front (APF), a roughly circular oceanic system running between 50°S and 60°S and extending to 2,000 m in depth.

Two key events allowed the establishment of the ACC: (1) the opening of the Tasman Seaway between Antarctica and Australia, which according to tectonics and marine geology occurred approximately 35.5-30 Ma (Kennett, 1977; Lawver and Gahagan, 2003; Stickley et al., 2004; Wei, 2004); (2) the opening of the Drake Passage between southern South America and the Antarctic Peninsula. The two openings allowed the development of the ACC. The timing of the opening of the Passage is controversial, with estimates ranging between 40 and 17 Ma (Barker and Burrell, 1977; Livermore et al., 2004, Scher and Martin, 2006). Such a relatively wide time window is due to contrasting views on the palaeo-elevation of key parts of the Scotia Arc and the Shackleton Fracture Zone (reviewed in Barker et al., 2007).

Recently, using neodymium isotopes to detect the presence of Pacific seawater in the Atlantic sector Scher and Martin (2006) proposed that the opening of the Drake passage occurred as early as 41 Ma, and was thus completed before the establishment of the Tasman Seaway.

The timing of the final onset of the ACC is also uncertain (Barker et al., 2007).

Estimates from sediment records range from late Eocene (40 Ma) to latest Miocene (8-10 Ma) (Wei and Wise, 1992; Scher and Martin, 2006). Concerns have been raised that some of these estimates could rather be climate-related and reflect a change in either temperature or productivity independent of ocean circulation (Barker et al., 2007). The older view that the ACC developed around the Eocene-Oligocene boundary leading to the first continental ice sheets on Antarctica by reducing heat transport into the region (Kennett, 1977) has been questioned by DeConto and Pollard (2003). Their modelling study shows the first ice sheets could have formed from a drop in atmospheric CO2 levels from 3 to 2 times pre-industrial levels and that the opening of Drake Passage could provide only 20% of the cooling. The

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formation of the circum-polar deep water current may have subsequently helped the intensification of glaciation (Francis et al., 2008b).

Despite the uncertainty on both the timing of final onset of the ACC and its importance as a driver of climatic change, palaeoclimatic reconstructions leave little doubt about the fact that oceanographic and climatic processes responsible for current day glacial conditions in the Antarctic started in the same period of time. In fact, switching of climate conditions from

“greenhouse” to “icehouse” in the Antarctic started in the late Eocene, 42 Ma, with an initial phase of strong but short-lived glaciations that match with the most recently published time estimates of formation of the Drake passage (Scher and Martin, 2006). A further drastic change occurred at the Eocene-Oligocene boundary ~34 Ma, (Tripati et al., 2005; Barker et al., 2007), leading to ice-sheet coverage similar in extent to the present ice sheet and contemporaneous with the opening of the Tasman Seaway.

Figure 3.4 Three maps showing the progressive separation of Antarctica and the other Southern Hemisphere continents, leading to the opening of “ocean gateways” that allowed the development of the Antarctic Circum-polar Current sometime after 34 Ma. Modified from Kennett (1978).

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The palaeontological record suggests these climatic events also influenced the Southern Ocean and its fish fauna. The best studied Antarctic fish fossils have been discovered in La Meseta Formation on Seymour Island (at the tip of the Antarctic Peninsula). The deposit from the late Eocene showed a fauna still “cool and temperate in character” that possibly lived in waters such as those found today around Tasmania, New Zealand and southern South America (Eastman, 2005). Accordingly, the earliest cold-climate marine Antarctic faunas are thought to date back to the latest Eocene-Oligocene (35 Ma).

With the ACC, the Southern Ocean became the cold isolated habitat that we know today. However, ice sheet coverage oscillated throughout the Cenozoic and probably produced repeated shifts of species distribution in Antarctic coastal waters, allowing allopatric speciation and diversification of Antarctic taxa (Rogers, 2007). In addition, the present APF is now suspected to be “leaky” and to allow transport of plankton north in mesoscale eddies with cold cores (Clarke et al., 2005).

Atmospheric CO2 levels ranged from ~3,000 ppm in the Early Cretaceous (at 130 Ma) to around 1,000 ppm in the Late Cretaceous (at 70 Ma) and Early Cenozoic (at 45 Ma) (e.g.

Pagani et al., 2005; Royer, 2006), leading to global temperatures at least 6 or 7ºC warmer than present at times. These high CO₂ levels were probably the consequence of volcanic outgassing. Temperatures peaked at ~85 Ma during the mid-Late Cretaceous when sub-tropical climates prevailed over the pole (Francis et al., 2008a). Even in this Cretaceous greenhouse world from oxygen isotope and backstripping data some researchers have inferred sea level changes in the order of tens of metres and intermittent polar ice sheets (Miller et al., 2008), although there is as yet no geological evidence (Thorn et al., 2007).

Temperatures peaked again at 50 Ma (Figure 3.5) and subsequently declined, as did atmospheric CO2 levels (Pearson and Palmer, 2000; Pagani et al., 2005; Royer, 2006).

Superimposed on the high CO2 world of the Early Cenozoic, deep-sea sediments provide evidence of the catastrophic release of more than 2,000 gigatonnes of carbon into the atmosphere from methane hydrates around 55 Ma ago, at the Paleocene-Eocene boundary, raising global temperatures by a further ~4-5°C, although they recovered after about 100,000 years (Figure 3.5) (Zachos et al., 2003; Zachos et al., 2005).