Early Miocene Nothofagus in Antarctica based on fossil leaves from the Transantarctic Mountains

Abstract:

Nothofagus (Southern Beech) is the most widely reported plant from Cenozoic Antarctic fossil assemblages. Most of the fossils are of pollen morphotypes and it is assumed that the plants the pollen represents were growing on the continent. However, because of the uncertainties with systematics, long-distance dispersal, and reworking, it has been difficult to interpret the assemblages in terms of paleoenvironments and paleoclimate. Here, we report an in situ assemblage of Nothofagus leaves and pollen from the Friis Hills (77⁰ 45’S, 161⁰ 28’E). The leaves are preserved as carbonaceous impressions in brown, fissile shales that represent the deposits of a lake occupying a glacial valley. The leaves most probably accumulated from deciduous shrubs. Based on the stratigraphic relationship to a tephra with a ⁴⁰ Ar/³⁹ Ar age of c. 20 Ma, the leaf assemblage is of early Miocene age. Nothofagus pollen from the shale suggests that at least 3 species were represented. A total of 227 leaves were examined and lengths, widths, and areas recorded. The preservation is generally good but only a few of the specimens represent complete leaves; the spectrum is 30-100%. Plots of the measurements of the leaves provide a summary of size variation but were otherwise not useful for separating out different taxa . The most useful characters for establishing differences between the leaves were the leaf margins, of which three or four types were distinguished: 1. margins entire or finely serrate between ribs; 2. margins with one or two convex lobes between the ribs; 3. margins with convex lobes over the ribs. The leaf study indicates that three or possibly four species were co-inhabiting the valley in the early Miocene and were part of a dynamic vegetation that colonized the valley with each deglaciation.  A possible analog today would be at low elevations in Tierra del Fuego where three species of Nothofagus coexist. The stem diameters of abundant wood preserved in adjacent facies indicates that the plants were shrubs and not trees, and that the landscape was tundra and not forest. Mean summer temperature was possibly 6-7⁰ C. By 14 Ma, immediately before the extinction of Nothofagus on the continent, diversity had declined to a single species. The research was supported by NSF grants 0739693 and 0948652.

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