The Terrestrial Fossil Record of East Antarctic Climate

The onset of Antarctic glaciation during the Oligocene dramatically altered terrestrial environments ultimately resulting in widespread extinction of plant and animal taxa (Cantrill, 2001). Tundra environments, based on pollen from the Cape Roberts borehole, are known to have existed in the Ross Sea region through the Oligocene until at least the early Miocene (Askin and Raine, 2000; Raine and Askin, 2001). The final extinction of plants and animals with more complex life histories is assumed to be associated with the shift to the polar desert climate which prevails today. Whether or not this immediately followed the mid-Miocene warm interval or was as late as the mid-Pliocene is the most outstanding problem in Neogene climatic history in need of resolution.

Pre-Pleistocene glacigenic sequences (Sirius and Pagodroma Groups) are patchily exposed throughout the Transantarctic and Prince Charles mountains. They are dominated by glaciomarine fjordal sediments, lodgement tills and outwash deposits (see previous section for more details). At the Oliver Bluffs on the upper Beardmore Glacier, fossiliferous sequences occur within the Meyer Desert Formation. The discovery of wood (Carlquist, 1987) and later Nothofagus leaves (Hill and Truswell, 1993; Webb and Harwood, 1993; Francis and Hill, 1996; Hill et al., 1996) from a location about 500 km from the South Pole makes this arguably the most important fossil site in Antarctica. The fossils provide an unusual window into Neogene environments and biogeography but until questions surrounding their age can be better resolved, their value as palaeoclimatic proxies is lessened.

The deposits have been assigned a Pliocene age using a biostratigraphic argument based on reworked marine diatoms (Harwood, 1986; Webb et al., 1996). Recently, however, cosmogenic dates obtained on boulders from moraines that post-date the Meyer Desert Formation indicate ages of greater than 5 Ma (Ackert and Kurz, 2004). The authors propose various age models based on different rates of weathering and discuss different amounts of uplift, but based on their discussion, a Miocene age for the Meyer Desert Formation is preferred over a Pliocene age (Ackert and Kurz, 2004). If the older age for the Meyer Desert Formation prevails, then it is tempting to speculate that the degree of melting of the ice sheet that resulted in fjords opening up into the Transantarctic Mountains was associated with the mid-Miocene climatic optimum. That interpretation would be more acceptable to researchers in the Dry Valleys who present evidence that the shift from wet-to cold-based glaciation occurred during the Middle Miocene (Denton et al.. 1993; Marchant et al., 1993a; Sugden, 1996).

The fossil Nothofagus leaves from the Meyer Desert Formation were described as a new species, N. beardmorensis, but with morphological similarities to extant species in South America and Tasmania (Hill et al., 1996). Originally, a krummholz growth form (scrubby, stunted growth form) was assigned to the Nothofagus (Webb and Harwood, 1993) but later studies of the growth ring anatomy (Francis and Hill, 1996) suggested a low prostrate and spreading habitus, more similar to ground-hugging shrubs growing today along exposed parts of treeline on Isla Navarino at the southern tip of South America (Ashworth and Kuschell, 2003, p. 193). The prostrate growth forms implied colder growing conditions than estimated using the krummholz interpretation, and considerably lower than the comparison to the cold-temperate rain forest of southern South America made on the basis of a pollen study (Askin and Markgraf, 1986; Mercer, 1986). Nearest living relative comparisons, along with physiological experiments into cold tolerance of Nothofagus, yielded theoretical estimates of mean summer month temperatures of at least 5°C for 3 months and low winter month temperatures of —15 to —22°C. Mean annual temperature (MAT) was estimated to have been in the range of —8 to — 12°C (Francis and Hill, 1996; Ashworth and Kuschel, 2003) compared to an interpolated sea-level MAT at latitude 85°S today of —26°C (Ashworth and Kuschel, 2003).

Although much of the research to date has focused on Nothofagus, the vegetation was more complex consisting of cushion plants, grasses, ranunculids (buttercups) and mosses (Ashworth and Cantrill, 2004).

It should be emphasized that to date, only one site has been reported from the Beardmore Glacier and leaves and wood of Nothofagus have not been reported from other Sirius Group sequences although they have been discovered recently in pre-Pleistocene glacial sequences of the western Dry Valleys (Ashworth et al., unpublished).

The Meyer Desert Formation also contains fossils of beetles, including two weevil species, a higher fly, single species each of a freshwater gastropod and bivalve, an ostracod and a fish (Ashworth and Kuschel, 2003; Ashworth and Preece, 2003; Ashworth and Thompson, 2003; Ashworth et al., 1997). Using overlapping autecological requirements for Nothofagus, listroderine weevils and freshwater molluscs, mean summer temperatures are estimated to have been 4-5°C for at least two summer months and the MAT was estimated to be — 8°C, similar to the temperatures estimated from the physiological requirements of the Nothofagus fossils (Ashworth and Kuschel, 2003; Ashworth and Preece, 2003). The palaeosols, which are stratigraphically higher than the fossiliferous beds, indicate polar conditions (Retallack et al.. 2001), and their development may signal the end of warmer and wetter climates in Antarctica.

Regardless of age, there is a question regarding the likelihood of a tundra biome re-establishing itself in the interior of Antarctica during a warm phase if it had been extirpated during an earlier cold phase. The question is important because it sets limits on how cold the climate of Antarctica could have been before the Meyer Desert Formation biota colonized the interior. Did the species disperse from refugia around the margins of Antarctica, in which case the climate had to have been continually warmer around the margins of the continent through at least the Early Miocene? Or, if they became extinct prior to the mid-Miocene climatic optimum or mid-Pliocene warm interval (~3 Ma), could they have reinvaded the continent?

For Nothofagus, seed germination is considered to be critical to answering this question. Studies suggest that Nothofagus seeds cannot survive prolonged periods of immersion in salt water (Hill et al., 1996) and consequently cannot be easily dispersed across oceanic barriers. The implications of this are that components of the vegetation survived on Antarctica even during earlier glacial phases. There is a scenario, however, based on phylogenetic and palynological studies, that allows for the possibility that Nothofagus may have dispersed from Australia to New Zealand after the Tasman Sea formed (McGlone et al., 1996). Even if trans-Tasman dispersal occurred, it would have been from land areas with similar climatic conditions that would favour successful colonization. For organisms to disperse from South America or New Zealand to Antarctica would require not only much greater distances of dispersal but for organisms to re-establish themselves in complex communities in a different climatic regime. This makes the long-distance dispersal scenario for recolonization of Antarctica even less probable. We note that since deglaciation at the end of the Pleistocene, climatic conditions on many of the sub-Antarctic islands have been favourable for colonization by Nothofagus, listroderine weevils and the types of freshwater molluscs found in the Meyer Desert Formation, but it has not occurred. This strongly implies that refugia were present on the margins of Antarctica, from which plants, insects and freshwater molluscs could migrate to the continental interior during a time or times of significant climatic warming (cf. Convey et al., in press).

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