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Home , East Antarctic Ice Sheet, Meyer Hills, West Antarctica

Journal of the Geological Society, London, Vol. 164, 2007, pp. 1–6. Printed in Great Britain.

1 environments in the Neogene Sirius Group, : evidence from the

2 geological record and coupled atmosphere–vegetation models

1 2 3 4 3 J. E. FRANCIS ,A.M.HAYWOOD,A.C.ASHWORTH &P.J.VALDES 1 4 School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 2 5 Geological Sciences Division, British Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK

6 (e-mail: [email protected]) 3 7 Department of Geosciences, North Dakota State University, Fargo, ND 58105517, USA 4 8 School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, UK

9 Abstract: The Neogene Meyer Desert Formation, Sirius Group, at Oliver Bluffs in the Transantarctic 10 Mountains, contains a sequence of glacial deposits formed under a wet-based glacial regime. Within this 11 sequence fluvial deposits have yielded fossil plants that, along with evidence from fossil insects, invertebrates 12 and palaeosols, indicate the existence of tundra conditions at 858S during the Neogene. Mean annual 13 temperatures of c. À12 8C are estimated, with short summer seasons with temperatures up to +5 8C. The 14 current published date for this formation is Pliocene, although this is hotly debated. Reconstructions produced 15 by the TRIFFID and BIOME 4 vegetation models, utilizing a Pliocene climatology derived from the HadAM3 16 general circulation model (running with prescribed boundary conditions from the US Geological Survey 17 PRISM2 dataset), also predict tundra-type vegetation in Antarctica. The consistency of the model outputs with 18 geological evidence demonstrates that a Pliocene age for the Meyer Desert Formation is consistent with proxy 19 environmental reconstructions and numerical model reconstructions for the mid-Pliocene. If so, the East 20 Antarctic has behaved in a dynamic manner in the recent geological past, which implies that it may 21 not be as resistant to future global warming as originally believed.

22 There is currently great interest in the history of the East 1 circulation model (HadAM3 GCM) to drive a mechanistically 23 , in particular its stability and response to past 2 based biome model (BIOME 4). The study helped to identify 24 since its development in the Late Eocene (Barker 3 mid-Pliocene vegetation patterns in equilibrium with different 25 et al. 1999). Understanding the past behaviour of the ice sheet is 4 scenarios for Antarctic ice-sheet cover. However, because the 26 important because it provides critical information about how this 5 BIOME 4 model was effectively used offline to the climate 27 large store of fresh water, ice-equivalent to more than 50 m 6 model the interaction between climate and vegetation was not 28 global sea-level rise, may respond to future climate change. 7 fully resolved. 29 How the responded to a well- 8 A more realistic approach would allow vegetation to actually 30 documented phase of global warming during the mid-Pliocene, c. 9 grow and interact with climate. Therefore, to better understand 31 3Ma bp (e.g. Dowsett et al. 1999), is a topic of significant 10 the role of mid-Pliocene climate–vegetation feedbacks, the land- 32 debate. The ‘stabilist’ group argue that evidence of ancient 11 cover must be treated as an interactive element by incorporating 33 landscapes, mainly in the Dry Valleys in (Denton 12 a dynamic global vegetation model (DGVM). In this paper we 34 et al. 1993), suggests that a cold polar climate has been 13 examine whether the TRIFFID DGVM and BIOME 4 vegetation 35 maintained for at least the past 14 Ma and that the subsequent 14 models, which utilize a climatology produced by the HadAM3 36 mid-Pliocene warming event had little impact on Antarctic ice 15 GCM for the mid-Pliocene, can reproduce the Antarctic tundra 37 sheets and the local landscape. In contrast, the ‘dynamists’ 16 habitats interpreted from the geological evidence, in particular 38 propose that the ice sheet was unstable during the Neogene and 17 from fossil plants. The model, via the PRISM2 dataset, specifies 39 did respond to the warming phase during the mid-Pliocene by 18 ice extent, atmospheric CO2 and sea surface temperatures for the 40 partially deglaciating (Webb et al. 1984; Webb & Harwood 19 Pliocene, but the type of vegetation that could have existed in 41 1987). 20 Antarctica under these conditions is not prescribed and is 42 Much of this debate has centred on the Sirius Group strata, 21 unknown at present. Although this may not conclusively prove a 1 43 which crop out at numerous localities in the Transantarctic 22 Pliocene age for the Meyer Desert Formation it will demonstrate 44 Mountains. These strata, particularly the Meyer Desert Formation 23 whether tundra vegetation could actually exist in Antarctica 45 at Oliver Bluffs (Fig. 1), consist mostly of diamictites but also 24 under the Pliocene climate conditions prescribed within the 46 contain a unique and exceptional assemblage of fossil soils, 25 climate model. 47 animals and plants (discussed below) that signal the presence of 48 tundra-like habitats at 858S, an environment considerably warmer 26 Geological setting 49 than the present glacial conditions. 50 Attempts have been made to model Antarctic mid-Pliocene 27 Fossil plants have been discovered in the Meyer Desert Forma- 51 vegetation, to test whether the tundra habitat indicated by this 28 tion of the Sirius Group at Oliver Bluffs, in the Dominion Range, 52 fossil evidence can be reproduced. Haywood et al. (2002) used 29 (858079S, 1668359E), about 500 km 53 mid-Pliocene climatologies derived from an atmospheric general 30 from the (Francis & Hill 1996; Hill et al. 1996;

1

Article number = 05191 2 J. E. FRANCIS ET AL.

1 possible age must be younger than the onset of Antarctic 2 glaciation in the latest Eocene–earliest Oligocene (Barker et al. 3 1999). Whatever the age of the Meyer Desert Formation, the 4 sediments and the fossil assemblage, especially the plants, 5 indicate a period of warming and ice-sheet reduction so that 6 glacial margins were within 300 miles of the South Pole. We here 2 7 test the Pliocene age suggested for the Meyer Desert Formation 8 by comparing whether mid-Pliocene climates and vegetation 9 predicted for Antarctica by the TRIFFID–BIOME 4 vegetation 10 models and the HadAM3 GCM are consistent with the fossil 11 evidence.

12 Modelling

13 Models and experimental design

14 The global atmospheric model used for this mid-Pliocene study is the 15 HadAM3 component of the coupled Hadley Centre Model (HadCM3; 16 Pope et al. 2000). It has a horizontal grid resolution of 2.5833.758 in 17 latitude and longitude with 19 hybrid ó, p levels in the vertical, and a 3 18 timestep of 30 min. The model incorporates prognostic cloud, water and 19 ice, has a mass-flux convection scheme with stability closure and uses 20 mean orography. It was forced with continental configurations changed 21 by a 25 m sea-level rise, reduced ice-sheet size for Greenland (by 50%) 22 and Antarctica (by 33%), reconstructed sea surface temperatures (SSTs) 23 and estimated sea-ice extents all derived from the US Geological Survey Fig. 1. Location of Oliver Bluffs in the Transantarctic Mountains. 24 (USGS) PRISM2 digital dataset of mid-Pliocene boundary conditions 25 (Dowsett et al. 1999). PRISM2 is the latest in a line of datasets generated 26 by the USGS that reconstruct palaeoenvironmental conditions during the 27 1 Ashworth & Cantrill 2004). The Meyer Desert Formation mid-Pliocene (Dowsett et al. 1999, and references therein). Atmospheric 28 CO2 concentrations were set to a ‘Pliocene’ concentration (400 ppmv) 2 consists predominantly of glacial tillites, with some thin mud- 29 based on proxy data (e.g. Raymo & Rau 1992). The geographical extent 3 stones, siltstones and sandstones, deposited in temperate to sub- 30 of the Greenland and Antarctic Ice Sheets within the PRISM2 dataset 4 polar conditions during glacial advance and retreat (Webb et al. 31 was based on global sea-level estimates derived for the mid-Pliocene by 5 1996; Passchier 2001, 2004). The deposits were formed in and 32 Dowsett & Cronin (1990). The PRISM2 reconstruction uses model results 6 around ice-proximal fluvial and lacustrine environments, and 33 from M. Prentice (pers. comm., cited by Dowsett et al. 1999) to guide 7 represent periods of exposure and soil development, as well as 34 the areal and topographic distribution of Antarctic and Greenland ice. 8 colonization by plants and animals. 35 This study utilizes the TRIFFID dynamic vegetation model (Top-down 9 A Pliocene age has been assigned to the Meyer Desert 36 Representation of Interactive Flora and Foliage Including Dynamics) 10 Formation at Oliver Bluffs on the basis of marine diatoms 37 coupled to the HadAM3 GCM. The TRIFFID model, developed by the 38 Hadley Centre, defines the state of the terrestrial biosphere in terms of 11 recovered from the glacial sediments (Webb et al. 1996). They 39 soil carbon and the structure and coverage of five plant functional types 12 provide an age estimate of 3.8 Ma or younger. Similar diatom 40 (Broadleaf trees, Needleleaf trees, C3 grass, C4 grass and shrub) within 13 taxa were also recovered from the CIROS-2 core in the 41 each model grid box (Cox 2001). The areal coverage, leaf area index and 14 , interbedded with volcanic ash that provided a date of c. 42 canopy height of each type are updated based on a carbon balance 15 3 Ma (Barrett et al. 1992). Palaeosols in this formation have also 43 approach, in which vegetation change is driven by the net carbon fluxes 16 been assigned an age of 1.3–4.1 Ma (Retallack et al. 2001). The 44 calculated within the MOSES2 land surface scheme, which is part of the 17 age of the formation is, however, intensely debated because of 45 HadAM3 GCM (Cox 2001). 18 the questionable validity of the marine diatom evidence. For 46 The spin-up of the model was a multistage process. TRIFFID was 19 example, it is argued that the diatoms may have been wind- 47 initialized with 100% shrub coverage in all terrestrial model grid boxes 48 20 deposited (Burckle & Potter 1996; Barrett et al. 1997) or resulted not covered by the imposed PRISM2 ice sheets. Shrub was selected 49 because of its relatively neutral characteristics of surface albedo and 21 from fallout from a meteorite impact (Gersonde et al. 1997). 50 surface roughness. The first stage of the model spin-up involved an 22 Passchier (2004), however, on the basis of geochemical and 51 iterative coupling between the HadAM3 GCM and the TRIFFID model 23 provenance data, supports the hypothesis of Webb et al. (1996) 52 running in equilibrium mode. The HadAM3 GCM was integrated for 24 that the diatoms were included in clasts that were derived from 53 5 years after which the required fluxes of moisture and carbon were 25 an inland source in East Antarctica. Furthermore, the lack of 54 passed to the TRIFFID model. TRIFFID was then integrated for a total 26 indisputable evidence for Pliocene warm-based glaciation in 55 of 50 years, after which the updated vegetation and soil variables were 27 such as the Dry Valleys in Antarctica may imply that the 56 passed back to the HadAM3 GCM. This process continued for a total of 28 Sirius Group strata represent glaciation before the onset of deep 57 50 simulated HadAM3 years, and 500 simulated TRIFFID years. This 29 cooling in the Late Miocene (Marchant et al. 2002; Hicock et al. 58 approach, similar to a Newton–Raphson algorithm for approaching 59 equilibrium, is a proven and effective method in producing equilibrium 30 2003). 60 states for the slowest variables (e.g. soil carbon and forest cover). The 31 The Sirius Group as a whole was considered by Passchier 61 second stage involved using the HadAM3 and TRIFFID models in their 32 (2001, 2004) to be a product of multiple ice-sheet events, as 62 fully dynamic mode in which the vegetation cover and other character- 33 different exposures across the Transantarctic Mountains exhibit 63 istics were updated every 10 days. The model was integrated in this fully 34 different provenance and weathering histories. Thus, other depos- 64 dynamic mode for a total of 30 simulated years to allow the model to 35 its of Sirius Group sediments may not be the same age as the 65 adjust to short time-scale vegetation variability. 36 Meyer Desert Formation from Oliver Bluffs. The maximum 66 Vegetation models are relatively new tools and display considerable NEOGENE SIRIUS GROUP, ANTARCTICA 3

1 variation in their predictions for present-day vegetation and estimates of 1 TRIFFID model) is not predicted to occur anywhere in Antarc- 2 carbon storage. Therefore it is good practice to use two models so that an 2 tica. The closest areas to exhibit forest type vegetation are 3 estimate of uncertainty can be made for the Antarctic Pliocene vegetation 3 southern , South and southern . These 4 predictions. BIOME 4 represents the second vegetation model utilized in 4 results are consistent with palaeoenvironmental reconstructions 5 this study. This model translates climate model output from the Pliocene 5 derived from the geological record (see below). 6 experiment, specifically the seasonal cycle of temperature, precipitation 7 and solar radiation, into biome distributions (Kaplan 2001, and references

8 therein). BIOME 4 is the latest in a series of mechanistically based 6 Palaeoenvironmental reconstruction 9 models that are developed from physiological considerations that place 10 constraints on the growth and regeneration of different plant functional 7 Within the Meyer Desert Formation at Oliver Bluffs an excep- 11 types. These constraints are calculated through the use of limiting factors 8 tional sequence of fossil plants has now been described. They 12 for plant growth. These include the mean temperature of the coldest and 9 include fossil leaves assigned to Nothofagus beardmorensis 13 warmest months, the number of growing degree days (GDDs) above 0 10 (southern beech) (Hill et al. 1996); wood of Nothofagus type 14 and 5 8C, and the calculation of a coefficient (Priestley–Taylor coeffi- 11 (Francis & Hill 1996); and a low-diversity pollen assemblage 15 cient) for the extent to which soil moisture supply satisfies atmospheric 12 including podocarp conifers, Nothofagus-type angiosperms and 16 moisture demand. GDDs are calculated by linear interpolation between 13 bryophyte spores (Askin & Raine 2000). Cushion plants of 17 mid-months, and by a one-layer soil moisture balance model independent 14 vascular plants, mosses, seeds similar to modern Ranunculus, 18 of the HadAM3 GCM hydrology. 15 19 We ran BIOME 4 on a 2.5833.758 grid corresponding to the same and various other small flowers and seeds of a tundra plant 20 resolution as the HadAM3 GCM. Monthly mean surface temperature, 16 assemblage have been described (Ashworth & Cantrill 2004). 21 precipitation and cloud cover were compiled from 10 simulated years of 17 Some aspects of the plant fossils provide more specific 22 our Pliocene experiment to provide the climatic information necessary 18 information about the growing environment. The small pieces of 23 for the BIOME 4 model. 19 fossil wood contain many very narrow growth rings, showing that 20 the small ‘twigs’, less than 10 mm in diameter, are in fact 21 extremely slow-growing mature stems of great age (Francis & 24 Model results (Pliocene Antarctic climate) 22 Hill 1996). The slow growth and the asymmetrical character of 25 The specification of the PRISM2 boundary condition dataset 23 the growth rings are characteristic of dwarf shrubs with a 26 within the HadAM3 GCM results in a dramatic warming over 24 prostrate habit that grow in nutrient-poor soils in cold conditions, 27 Antarctica relative to the present day. Surface temperatures 25 commonly seen in shrubs such as willow that survive in 28 remain below 0 8C in all areas of Antarctica for 9 out of 26 the high Arctic. In addition, abrasion scars in the fossil twigs 29 12 months of the year. In contrast, surface temperatures over 27 indicate that the shrubs were often subject to surface abrasion by 30 Antarctica for the present day remain at 0 8C or below for the 28 wind and outwash flood sediments. Francis & Hill (1996) 31 entire year. During December, January and February (particularly 29 concluded that the wood at Oliver Bluffs came from dwarf 32 December and January) surface temperatures in , 30 prostrate shrubs that hugged the tundra surface and grew very 33 near coastal localities, in deglaciated parts of East Antarctica, 31 slowly in the cold tundra conditions. A mean annual temperature 34 and in the Transantarctic Mountains region climbed above 32 of c. À12 8C was proposed, by comparison with similar vegeta- 35 freezing to a maximum of +8 8C (mean +4 8C; Fig. 2a and b). 33 tion that survives today in Arctic tundra environments. 36 The predicted warming over Antarctica in the mid-Pliocene 34 The Nothofagus leaves are preserved as leaf mats, formed as 37 experiment is a result of the prescribed warmer 35 the result of autumnal leaf fall, probably preserved within small 38 SSTs, reduced terrestrial ice cover and lower surface albedo 36 sheltered hollows on the tundra surface. The presence of 39 values. Furthermore, the reduced ice volume lessens the strength 37 deciduous Nothofagus led Hill et al. (1996) to propose mean 40 of surface winds over much of the (caused by reduced 38 annual temperatures of À15 8C for Meyer Desert Formation 41 elevation connected to a smaller ice cap) and acts as a further 39 times, with maximum lower temperatures of À22 8C (the lower 42 positive feedback mechanism on surface temperatures. 40 tolerance limits of Nothofagus today) but with a short summer 43 Although little annual variation in the amount of precipitation 41 season during which temperatures rose above freezing to allow 44 between the Pliocene and present day is predicted, significant 42 plant growth. 45 seasonal differences between winter and summer are observed 43 Tundra environments are also indicated by fossil insects and 46 (Fig. 2c and d). More specifically, precipitation rates increase for 44 invertebrates. Fossil peat and calcareous lake deposits at Oliver À1 47 the Pliocene experiment (by 0.2–2 mm day ) over widespread 45 Bluffs have yielded freshwater gastropods and clams. Remark- 48 areas of the Southern Ocean as well as over deglaciated regions 46 ably, parts of listeroderine weevils have been found in the peats, 49 of the Antarctic continent itself. This precipitation increase is 47 similar to weevils living today in Patagonian tundra moorlands, 50 caused by the warmer prescribed SSTs and surface temperatures 48 plus a puparium of a fly (Ashworth & Kuschel 2003; Ashworth 51 over the Southern Ocean for the Pliocene control experiment, 49 & Preece 2003; Ashworth & Thompson 2003). Palaeosols are 52 causing enhanced evaporation from the ocean surface and hence 50 also present within this sequence and have characteristics of 53 precipitation. 51 modern tundra soils. They formed in an active permafrost layer 52 under a climate with mean annual temperatures of À11 to À3 8C À1 53 and precipitation of 300–1100 mm a (Retallack et al. 2001). 54 Model results (Pliocene Antarctic vegetation cover) 54 Thus, a wealth of geological evidence clearly indicates that 55 The predicted BIOME 4 vegetation distribution over deglaciated 55 during deposition of the Meyer Desert Formation at Oliver Bluffs 56 areas of Antarctica is dominated by cushion forb lichen moss 56 tundra conditions prevailed at times. Although the main sedimen- 57 tundra, prostrate shrub tundra, dwarf shrub tundra or shrub 57 tary sequence is composed of diamictites indicative of lodgement 58 tundra (Fig. 3c). The fractional coverage (%) of plant functional 58 and supraglacial tills that formed during the presence of warm- 59 types predicted to occur in Antarctica by the TRIFFID model is 59 based glaciers at this site, at times the glaciers retreated to 60 dominated by C3 grasses and shrubs, which, at high latitudes, is 60 expose a tundra landscape with morainal banks and outwash 61 diagnostic of a tundra environment (Fig. 3a and b). Forest type 61 plains. An ice-marginal environment at the end of a glacier near 62 vegetation (represented by broadleaf or needle leaf trees in the 62 the head of a fjord was proposed by Ashworth & Kuschel 4 J. E. FRANCIS ET AL. (a) (c)

(b) (d)

-4.0 -2.0 -1.0 -0.5 -0.2 0.2 0.5 1.0 2.0 4.0 -15 -10 -5 -3 -1 -0.5 0.5 1 3 5 10 15

Fig. 2. Predicted difference in (a) total precipitation rate (mm dayÀ1) averaged over the December–January–February (DJF) season, (b) total precipitation rate (mm dayÀ1) averaged over the June–July–August (JJA) season, (c) surface temperature (8C) during the DJF season and (d) surface temperature (8C) during the JJA season between the mid-Pliocene and present day using the HadAM3 GCM running with the TRIFFID model in a fully dynamic mode.

1 (2003). Poorly developed soils formed during periods of expo- 1 Formation and indicate that a significant deglaciation occurred in 2 sure (Retallack et al. 2001) and plants colonized drained ridges 2 Antarctica and that a tundra environment existed during the 3 (Ashworth & Cantrill 2004). During times of glacial retreat the 3 period during which the Meyer Desert Formation was deposited. 4 climate was still cold, with mean annual temperatures in the 4 Mean annual temperatures of À12 8C, deduced from fossil 5 region of À12 8C and many months of freezing but with a couple 5 evidence, indicate that a much colder climate prevailed than has 6 of months in summer when temperatures rose above freezing. 6 been suggested previously (e.g. Mercer 1972; Webb & Harwood 7 1993; Webb et al. 1996). Even though the short summer months 8 were warm enough to permit the growth and reproduction of 7 Significance and implications 9 plants and the temperatures (c.168C warmer than present in the 8 Results of the HadAM3 GCM using the PRISM2 dataset and 10 Beardmore region) would have promoted shrinkage of the ice 9 TRIFFID–BIOME 4 vegetation models are consistent with 11 sheet, as shown in our models, the climate would have been cold 10 geological proxy environmental data from the Meyer Desert 12 enough to prevent extensive melting in inland basins. NEOGENE SIRIUS GROUP, ANTARCTICA 5 TRIFFID Model - Fractional Coverage of Plant Functional Types (%) on Antarctica a) C3 Grasses b) Shrub

02 5 10 15 20 25 30 35 40 45 50 60 70 80 90

c) BIOME4 Model - Prediction of Antarctic Biome Distributions

Barren/Ice Cushion forb lichen moss tundra Prostrate shrub tundra Dwarf shrub tundra Shrub tundra Steppe tundra

Fig. 3. Predictions of mid-Pliocene Antarctic vegetation cover derived from the TRIFFID and BIOME 4 vegetation models. (Note the exclusive occurrence of C3 grasses and shrubs in the predictions generated by the TRIFFID model (a, b) and tundra plant functional types produced by the BIOME 4 model (c).) Modern coastlines for Antarctica, Australia, South Africa and are shown. White regions inside the modern coastline of Antarctica represent ocean. Approximate location of Oliver Bluffs is marked by filled circle.

1 The consistency of our model results with geological evidence 1 least consistent with the proxy environmental reconstructions and 2 does not prove that the Meyer Desert Formation is Pliocene in 2 numerical model reconstructions for the mid-Pliocene presented 3 age, and it does not exclude the possibility that it formed in 3 in this paper. If the formation is Pliocene in age then it implies a 4 earlier periods of global warmth, but it does, however, demon- 4 number of things: (1) Nothofagus existed in Antarctica until 5 strate that a Pliocene age for the Meyer Desert Formation is at 5 relatively recently, expanding its geographical extent during 6 J. E. FRANCIS ET AL.

1 climatic ameliorations and retreating to coastal refugia during 1 million years ago. Nature, 359, 816–818. Barrett, P.J. Bleakley, N.I. Dickinson, W.W. Hannah, M.J. Harper, 2 cold periods; (2) the East Antarctic Ice Sheet has behaved in a 2 , , , & 3 M.A. 1997. Distribution of siliceous microfossils on Mount Feather Antarc- 3 dynamic manner in the recent geological past; (3) the East 4 tica and the age of the Sirius Group. In: Ricci-Carlo, A. (ed.) The Antarctic 4 Antarctic Ice Sheet may not be as resistant to future climate 5 Region: Geological Evolution and Processes. Terra Antarctica, 4, 763–770. 5 change as originally believed. 6 Burckle, L.H. & Potter, N. Jr 1996. Pliocene– diatoms in Paleozoic 7 and Mesozoic sedimentary and igneous rocks from Antarctica: a Sirius 8 problem solved. Geology, 24, 235–238. 6 Conclusions 9 Cox, P. 2001. Description on the TRIFFID Dynamic Global Vegetation Model. 10 Hadley Centre, Meteorological Office, Technical Report, 24. 7 The ‘Pliocene’ Meyer Desert Formation (Sirius Group) located 11 Denton, G.H., Sugden, D.E., Marchant, D.R., Hall, B.L. & Wilch, T.I. 1993. 8 in the Transantarctic Mountains contains a sequence of glacial 12 East Antarctic ice sheet sensitivity to Pliocene climate change from a Dry 13 Valleys perspective. Geografiska Annaler Stockholm, 75a, 155–204. 9 deposits formed under a wet-based glacial regime. Within this 14 Dowsett, H.J. & Cronin, T.M. 1990. High eustatic sea level during the Middle 10 sequence fluvial deposits have yielded fossil plants that, along 15 Pliocene: evidence from the southeastern U.S. Atlantic Coastal Plain. 11 with evidence from fossil insects, invertebrates and palaeosols, 16 Geology, 18, 435–438. 12 indicate the existence of tundra conditions at 858S during the 17 Dowsett, H.J., Barron, J.A., Poore, R.Z., Thompson, R.S., Cronin, T.M., 18 Ishman, S.E. & Willard, D.A. 1999. Middle Pliocene paleoenvironmental 13 Neogene. Mean annual temperatures of c. À12 8C are estimated, 19 reconstruction: PRISM2. US Geological Survey Open-File Report, 99-535. 14 with short summer seasons with temperatures up to +5 8C, 20 Francis, J.E. & Hill, R.S. 1996. Fossil plants from the Pliocene Sirius group, 15 climate conditions much colder than previously considered. 21 Transantarctic Mountains: evidence for climate from growth rings and fossil 16 This palaeoenvironmental reconstruction is compared with 22 leaves. Palaios, 11, 389–396. Gersonde, R. Kyte, F.T. Bleil, V. et al. 17 outputs derived from the TRIFFID and BIOME 4 vegetation 23 , & 1997. Geological record and 24 reconstruction of late Pliocene impact of the Eltanin asteroid in the Southern 18 models that utilized a Pliocene climatology derived from the 25 Ocean. Nature, 390, 357–363. 19 HadAM3 GCM (running with prescribed boundary conditions 26 Haywood, A.M., Valdes, P.J., Sellwood, B.W. & Kaplan, J.O. 2002. Antarctic 20 from the USGS PRISM2 dataset). The models also predict 27 climate during the middle Pliocene: model sensitivity to ice sheet variation. 21 tundra-type vegetation in Antarctica for the mid-Pliocene. 28 Palaeogeography, Palaeoclimatology, Palaeoecology, 182, 93–115. 29 Hicock, S.R., Barrett, P.J. & Holme, P.J. 2003. Fragment of an ancient outlet 22 The agreement between geological data and model outputs is 30 glacier system near the top of the Transantarcic Mountains. Geology, 31, 23 consistent with a Pliocene date for the Meyer Desert Formation. 31 821–824. 24 If so, the East Antarctic Ice Sheet has behaved in a dynamic 32 Hill, R.S., Harwood, D.M. & Webb, P.N. 1996. Nothofagus beardmorensis 25 manner in the recent geological past and this implies that it may 33 (Nothofagaceae), a new species based on leaves from the Pliocene Sirius 34 Group, Transantarctic Mountains, Antarctica. Review of Palynology and 26 not be as resistant to future global warming as originally 35 Palaeobotany, 94, 11–24. 2728 believed. 36 Kaplan, J.O. 2001. Geophysical applications of vegetation modeling. PhD thesis, 2930 37 Lund University, Lund. 31 J.E.F. thanks D. Harwood et al. for supplying wood fossil from Oliver 38 Marchant, D.R., Lewis, A.R. & Phillips, W.M. et al. 2002. Formation of 32 Bluffs and the University of Leeds Special Payment Fund, Trans- 39 patterned ground and sublimation till over Miocene glacier ice in Beacon 33 Antarctic Association and the Royal Society for support. A.M.H. and 40 Valley, southern Victoria land, Antarctica. Geological Society of America 34 P.J.V. thank the Natural Environment Research Council for supporting 41 Bulletin, 114, 718–730. 42 Mercer, J.H. 1972. Some observations on the glacial geology of the Beardmore 35 this work through the provision of High Performance Computing, and 43 Glacier area. In: Adie, R.J. (ed.) Antarctic Geology and Geophysics. 36 acknowledge the USGS PRISM Group for use of their materials. A.C.A. 44 Universitetsforlaget, Oslo, 427–433. 37 and J.E.F. acknowledge support from NSF grant OPP0230696 for support 45 Passchier, S. 2001. Provenance of the Sirius Group and related upper Cenozoic 38 of field studies on the Meyer Desert Formation of the Beardmore Glacier. 46 glacial deposits from the Transantarctic Mountains, Antarctica: relation to 39 This paper is published as part of the British Antarctic Survey’s GEACEP 47 landscape evolution and ice sheet drainage. Sedimentary Geology, 144, 48 263–290. 40 programme that investigates climate change over geological time scales. 49 Passchier, S.P. 2004. Variability in geochemical provenance and weathering 41 The work also contributes to the SCAR ACE initiative (Antarctic Climate 50 history of Sirius Group strata, Transantarctic Mountains: implications for 42 Evolution). 51 Antarctica glacial history. Journal of Sedimentary Research, 74, 607–619. 52 Pope, V.D., Gallani, M.L., Rowntree, P.R. & Stratton, R.A. 2000. The 53 impact of new physical parametrizations in the Hadley Centre climate 54 model—HadAM3. Climate Dynamics, 16, 123–146. 43 References 55 Raymo, M.E. & Rau, G.H. 1992. Plio-Pleistocene atmospheric CO2 levels inferred 13 44 Ashworth, A.C. & Cantrill, D.J. 2004. Neogene vegetation of the Meyer Desert 56 from POM ä C at DSDP Site 607. EOS Transactions, American Geophysical 45 Formation (Sirius Group), Transantarctic Mountains, Antarctica. Palaeogeo- 57 Union, Spring Meeting, 73, 95. 46 graphy, Palaeoclimatology, Palaeoecology, 213, 65–82. 58 Retallack, G.J., Krull, E.S. & Bockheim, G. 2001. New grounds for reassessing 47 Ashworth, A.C. & Kuschel, G. 2003. Fossil weevils (Coleoptera: Curculionidae) 59 the palaeoclimate of the Sirius Group, Antarctica. Journal of the Geological 48 from latitude 85 degrees S Antarctica. Palaeogeography, Palaeoclimatology, 60 Society, London, 158, 925–935. 49 Palaeoecology, 191, 191–202. 61 Webb, P.N. & Harwood, D.M. 1987. The Sirius Formation of the Beardmore 50 Ashworth, A.C. & Preece, R.C. 2003. The first freshwater molluscs from 62 Glacier region. Antarctic Journal of the United States, 22, 8–12. 51 Antarctica. Journal of Molluscan Studies, 69, 89–92. 63 Webb, P.N. & Harwood, D.M. 1993. Pliocene Nothofagus (southern beech) from 52 Ashworth, A.C. & Thompson, F.C. 2003. Palaeontology: a fly in the biogeo- 64 Antarctica: phytogeography, dispersal strategies, and survival in high latitude 53 graphic ointment. Nature, 423, 135–136. 65 glacial deglacial environments. In: Alden, J., Mastrantonio, J.C. & Odum, 54 Askin, R.A. & Raine, J.I. 2000. Oligocene and Early Miocene terrestrial 66 S. (eds) Forest Development in Cold Climates. Plenum, New York, 135–165. 55 palynology of the Cape Roberts Drillhole CRP-2/2A, Victoria Land Basin, 67 Webb, P.N., Harwood, D.M., McKelvey, B.C., Mercer, J.H. & Stott, L.D. 4 56 Antarctica. Terra Antarctica, 7, 493–501. 68 1984. Cenozoic marine sedimentation and ice volume variation on the East 57 Barker, P.F., Barrett, P.J., Cooper, A.K. & Huybrechts, P. 1999. Antarctic 69 Antarctic craton. Geology, 12, 287–291. 58 glacial history from numerical models and continental margin sediments. 70 Webb, P.N., Harwood, D.M., Mabin, M.C.G. & McKelvey, B.C. 1996. A marine 59 Palaeogeography, Palaeoclimatology, Palaeoecology, 150, 247–267. 71 and terrestrial Sirius Group succession, middle Beardmore Glacier–Queen 60 Barrett, P.J., Adams, C.J., McIntosh, W.C., Swischer, C.C. & Wilson, G.S. 72 Alexander range, Transantarctic Mountains, Antarctica. Marine Micro- 61 1992. Geochronological evidence supports Antarctic deglaciation three 73 palaeontology, 27, 273–297.

74 Received 9 January 2006; revised typescript accepted 13 July 2006. 76 75 Scientific editing by Jan Zalasiewicz

1 Please confirm this is a suitable point for reference to Fig. 1. 2 300 miles - please give metric equivalent. 77 3 Is some explanation needed for ‘sigma’ and ‘rho’? 4 Askin & Raine; Barrett et al. 1997 - should spelling be ‘Terra Antartica’ here?