Environmental Signals from Palynomorphs

While macrofossils of plants, found usually in distinct levels in geological sections, allow a fair understanding of the biodiversity and ecology of fossil vegetation at specific times, palynological assemblages can deliver a higher resolution picture of vegetation and climate change through time, especially due to their presence in drill cores. In addition, pollen and spores can be recovered from areas in which macrofossils are unknown (e.g. Prydz Bay). Palynological studies have been undertaken on various Palaeogene sections in West and East Antarctica, but complete recovery is rare, especially over the crucial time interval spanning the E/O boundary when climate dramatically deteriorated.

Cranwell (1959) carried out the earliest palynological studies in the Antarctic realm on a single sample of probable Palaeogene age from Seymour Island in the Antarctic Peninsula area. Subsequently, several early Tertiary stratigraphic sections on Seymour, Cockburn and King George Islands, and cores from the South Orkney Islands (ODP Leg 113, Site 696) and South Scotia Ridge (Bruce Bank, Eltanin Core IO 1578-59) have been subjected to detailed palynological analyses (Mohr, 1990; Askin et al., 1991. 1997; Grube, 2004; Grube and Mohr, 2004). Data used to reconstruct Tertiary vegetation and climate in East Antarctica have been derived from drilling campaigns in southern McMurdo Sound in the Ross Sea (CIROS-1, CRP-2/2A and CRP 3: Mildenhall, 1989; Askin and Raine, 2000; Raine and Askin, 2001; Prebble et al., 2006) and in Prydz Bay (MacPhail and Truswell, 2004). ODP Leg 189, in the Tasman Sea, cored the E/O boundary (Grube and Mohr, 2008). The glacial erratics of Eocene-aged sediments in southern McMurdo Sound have also yielded terrestrial palynomorphs.

Pollen assemblages in Antarctic Palaeogene strata contain many taxa comparable to those still found today in southern high latitudes, including areas of southern South America, Tasmania, Australia, New Zealand and New Caledonia. They vary quite substantially in the abundance of their major components, and consist of moss and fern spores, gymnosperm and angiosperm pollen. Ferns were species-rich until the Middle to early Late Eocene (Mohr, 2001) and include genera that live today under humid subtropical conditions, such as Cnemidaria (Mohr and Lazarus, 1994). At Bruce Bank (c. 46-44 Ma), fern spores dominated the assemblage, at intervals comprising more than 50% of the sporomorphs. During the Late Eocene, and even more so during the Oligocene, fern diversity and abundance dropped dramatically. During the Early Miocene, some of the taxa seem to return, but recycling of older Eocene spores cannot be completely ruled out, a problem prevalent in glaciogene sediments all around the Antarctic.

Gymnosperms were important components of the southern high latitude Palaeogene forests. Cycads seem to have been present until the Middle Eocene (Cycadopites), while Araucariaceae (Araucaria), Cupressaceae and especially Podocarpaceae pollen (Podocarpus, Phyllocladus, Lagarostrobus,

Dacrydium and Microcachrys) play a major role in the palyno-associations well into the Oligocene.

Angiosperms were relatively species-rich in the Palaeogene Antarctic pollen spectra with a dominance of various types of Nothofagidites (pollen comparable to that of extant Nothofagus, the southern beech), but diversity declined during the Late Eocene. Middle to Late Eocene assemblages from the Antarctic Peninsula area (ODP Leg 113, Site 696) show, in earliest sections, relatively large amounts of angiosperm pollen with a clear dominance of Nothofagidites, while in assemblages of latest Eocene and Early Oligocene age, moss spores become more common, and, except for Nothofagidites, almost no angiosperms are registered. The McMurdo Sound cores (CIROS-1, CRP-2/2A and CRP-3, Late Eocene to Oligocene) and glacial erratics (Middle to Late Eocene) are characterized by prominent Nothofagidites, particularly Nothofagidites lachlaniae and the N.fusca group. Various Podocarpus taxa are also abundant (Askin, 2000; Raine and Askin, 2001). In a relatively short sequence of CRP-3 (glaciomarine cycle 26) of Early Oligocene age, N. fusca-type, N. flemingii and N. lachlaniae contribute each about 23% of the total count (Prebble et al., 2006). In glaciomarine cycle 11 of Late Oligocene age, N. fusca-type pollen dominates with about 50%, followed by N. flemingii and N. lachlaniae. In Prydz Bay sections dated Late Eocene, Nothofagidites is clearly dominant at 41-57% of sporomorphs; the second largest group are conifer pollen that reach in a few samples up to 50% and more. Fern spores comprise 6-20% and cryptogams 3-6% (Macphail and Truswell, 2004) (Figs. 8.8 and 8.9).

During the warmer periods of the Palaeogene, the following families have been identified from pollen: Aquifoliaceae (includes holly), Casuarinaceae (she-oak), Cunoniaceae/Elaeocarpaceae, Epacridaceae (southern heath, now included within Ericaceae), Euphorbiaceae (spurge), Gunneraceae, Liliaceae, Myrtaceae, Nothofagaceae (including all four morphotype groups brassii-type, fusca-type, menziesii-type and ancestral-type; Dettmann et al., 1990) Olacaceae, Proteaceae (Gevuina/Hicksbeachia, Adenanthos, Carnarvonia, Telopea and Beauprea; Dettmann and Jarzen, 1991), Restionaceae (rush), Sapindaceae (soapberry) and Trimeniaceae (Prebble et al., 2006). In Prydz Bay sections (Late Eocene) and within the La Meseta Formation on Seymour Island (Eocene), Fischeripollis and Droseridites, that belong to Droseraceae (sundew), which are today restricted to moors or damp sites, are excellent ecological markers.

During the Late Eocene to Oligocene, Apiaceae, Asteraceae (daisy), possibly Campanulaceae (bellflower), Caryophyllaceae (carnation), Chenopodiaceae (now in the Amaranthaceae), Onagraceae (willowherb; Corsinipollenites) and Gramineae (grasses) seem to play a role as members of

20 30 40 50 60 70 80 0 10 20 30 40 0 10 20 30 40 50

20 30 40 50 60 70 80 0 10 20 30 40 0 10 20 30 40 50

Angiosperms Gymnosperms Cryptogams

Figure 8.8: Relative abundance (%) of major plant groups in ODP Leg 189, Site 1168. Asterisk marks the E/O boundary.

Angiosperms Gymnosperms Cryptogams

Figure 8.8: Relative abundance (%) of major plant groups in ODP Leg 189, Site 1168. Asterisk marks the E/O boundary.

the local vegetation (Mildenhall, 1989; Mohr, 1990; Askin, 1997; Askin and Raine, 2000; Raine and Askin, 2001; Prebble et al., 2006). During the Early Oligocene, a low shrub or closed Nothofagus-podocarp forest of small stature may have developed, occupying warmer sites on the Antarctic continent (Prebble et al., 2006). In colder phases, a tundra-like vegetation, evidenced by moss spores, few but relatively diverse herb pollen and a few Nothofagidites pollen, derived possibly from dwarfed southern beech, may have grown near the coast.

Palynological studies by Grube and Mohr (2008) of cores from ODP Leg 189, Site 1168 in the Tasman Sea show a clear response to E/O climate change. The abundance-time-chart (Figs. 8.8 and 8.9) for the Tasman Sea samples shows that during the latest Eocene, the pollen flora was dominated by the Nothofagaceae (especially the evergreen type Brassospora), with araucarian and podocarp conifers (gymnosperms) and typical fern families (cryptogams). Near the E/O boundary itself, there is a short peak in the occurrence of araucarian and some other gymnosperm pollen, as well as an increase in ferns, in response to a decline in Nothofagaceae. Surprisingly, however, there was no sustained change in terrestrial pollen after this that might reflect a major change in climatic regime. Vegetation typical of latest Eocene composition seems to have been restored during the earliest Oligocene, significant changes

Figure 8.9: Relative abundance (%) of sporomorphs in the Tasman Sea samples from ODP Leg 189, Site 1168. Asterisk marks the E/O boundary; Lines represent palynological ''events''.

Figure 8.9: Relative abundance (%) of sporomorphs in the Tasman Sea samples from ODP Leg 189, Site 1168. Asterisk marks the E/O boundary; Lines represent palynological ''events''.

being the decrease in Casuarinaceae angiosperms, and the gradual replacement of Osmundaceae ferns by Schizaeaceae and Gleicheniaceae. Nothofagus fusca- and menziesii-type pollen increase from about 33.8 Ma on, while other angiosperm pollen show a slight decline, which Grube and Mohr (2008) interpret as a gradual response to long-term cooling.

The pollen record in Figs. 8.8 and 8.9 also highlights a further short-lived episode of vegetational change at about 32.9 Ma. The pattern is similar to that at 33.7 Ma, with an increase in araucarian conifer pollen and in fern spores at the expense of the angiosperms, especially the Nothofagaceae - does this represent a later episode of cold climate? The pollen diagram in Fig. 8.9 also hints at cyclical changes, possibly at intervals of 0.8 m.y or even 0.4m.y.

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