Neogene glacigenic debris flows on James Ross Island, northern Antarctic Peninsula, and their implications for regional climate history

Quaternary Science Reviews 28 (2009) 3138–3160

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Neogene glacigenic debris flows on James Ross Island, northern Antarctic Peninsula, and their implications for regional climate history Anna E. Nelson a, *, John L. Smellie a, Michael J. Hambrey b, Mark Williams c, d, Maryline Vautravers a, Ulrich Salzmann a, John M. McArthur e, Marcel Regelous f a

British Antarctic Survey, Geological Sciences Division, High Cross, Madingley Road, Cambridge CB3 0ET, UK Centre for Glaciology, Institute of Geography & Earth Sciences, Aberystwyth University, Aberystwyth, Ceredigion SY23 3DB, UK University of Leicester, Department of Geology, University Road, Leicester LE1 7RH, UK d British Geological Survey, Keyworth, Nottingham NG12 5GG, UK e University College London, Department of Earth Sciences, Gower Street, London WC1E 6BT, UK f Royal Holloway University of London, Department of Earth Sciences, Egham, Surrey TW20 0EX, UK b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 February 2009 Received in revised form 10 August 2009 Accepted 12 August 2009

Detailed sedimentological and microtextural analyses of newly-discovered late Neogene diamictites and other coarse-grained facies, mostly sandwiched between hyaloclastite of the James Ross Island Volcanic Group and Cretaceous sandstone and mudstone, indicate deposition mainly by glacigenic debris flows. The deposits on James Ross Island (northern Antarctic Peninsula) constrain the depositional setting, ice– bed dynamics and regional palaeoclimate. The sequences on James Ross Island vary in age but are mainly late Miocene and Pliocene. Unlike Neogene sedimentary sequences elsewhere in Antarctica, those on James Ross Island are unusually well-dated by a combination of 40Ar/39Ar and 87Sr/86Sr analyses on fresh interbedded lavas and pristine bivalve molluscs, respectively. The Sr isotopic ages of the debris flows cluster around 4.74, 4.89, 5.44, 5.78, and 6.31 Ma and probably date relatively warm periods in the northern Antarctic Peninsula region, when the bivalves lived under ice-poor or seasonally ice-free conditions. The bivalves are often remarkably well-preserved, lack adhering lithified sediment and, in at least two locations, are large, mainly unfragmented and sometimes articulated, suggesting that they were alive immediately prior to their incorporation in subaqueous debris flows at the margins of an advancing glacier. These fossiliferous glacigenic debris flows signify episodes of ice expansion during relatively warm periods, or ‘‘interglacials’’, of the late Miocene and Pliocene. The James Ross Island glacigenic sedimentary successions attain thicknesses of up to 150 m and extend over 4 km laterally. The high volume of glacigenic sediment delivery implicit in the James Ross Island successions indicates that a series of dynamic ice fronts crossed the region during the late Miocene and Pliocene epochs. Associated evidence, in the form of clast abrasion (including striations and faceting) and bedrock erosion, is indicative of basal sliding and subglacial sediment deformation active at the ice–bed interface and wetbased temperate or polythermal regimes, prior to remobilisation. The evidence further suggests two local ice caps on James Ross Island during the warm periods, as well as ice-overriding by the Antarctic Peninsula Ice Sheet from the west and northwest. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The preservation of debris flow deposits in glacigenic sequences in polar regions can provide key environmental clues of past climatic conditions and in particular, episodes of glacial expansion or inception. The delivery of glacial sediment to the margins of * Corresponding author. E-mail addresses: [email protected] (A.E. Nelson), [email protected]. uk (J.L. Smellie), [email protected] (M.J. Hambrey), [email protected] (M. Williams), [email protected] (M. Vautravers), [email protected] (U. Salzmann), [email protected]. uk (J.M. McArthur). 0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.08.016

terrestrial glaciers and to the grounding-line slopes of ice shelves and tidewater glaciers is governed by the growth and decay of ice masses. During glacier advances, sediment is transported to the glacier margins and deposited there mainly as dense, diamictic and conglomeratic debris flows. By contrast, ice ablation and decay results in ice-marginal processes such as increased glaciofluvial outwash and dead-ice sediment slumping. In marine environments, ice shelves decay and subglacial sediment transport processes predominate inland of the grounding-line. The aim of this paper is to document and interpret the terrestrial exposures of upper Neogene glacigenic debris flows on James Ross

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Island, northern Antarctic Peninsula. Our study describes the delivery of abundant locally and regionally-derived sediment to ice-proximal marine and terrestrial glacier settings. The paper also assesses the timing of sediment deposition based on the taphonomy of included fossils, and explores the configuration of Neogene ice masses in the northern Antarctic Peninsula. Although extensive Neogene sequences exist elsewhere on Antarctica (i.e. Pagodroma and Sirius groups; Haywood et al., 2009), the James Ross Island sequence is the best-dated Neogene glacigenic record on the continent. The importance of understanding Neogene climate and ice configuration is linked to the predictions of the Intergovernmental Panel on Climate Change (IPCC, 2007) regarding future temperature increases. The IPCC states that a doubling of atmospheric CO2 concentrations from pre-industrial values will result in a 3  C temperature increase by the end of the next century. The Pliocene was the last time the Earth experienced such conditions, making it an important palaeoanalogue (Haywood et al., 2000; Salzmann et al., 2009). Before the onset of widespread Northern Hemisphere glaciation c. 2.7 Ma ago and prior to the warm mid-Pliocene interval at c. 3.3 Ma, the Messinian stage (between 7.25 and 5.33 Ma) and the Zanclean stage (early Pliocene; between 5.33 and 3.6 Ma) were times of large-scale environmental modifications. (Lourens et al., 2004) These changes are recorded in the orography and seaway connections of the Arctic (Gladenkov et al., 2002), Panama (Marshall, 1988) and Indonesia (Li et al., 2006), and altered the overall Earth geography to resemble that of modern time. There are numerous signs of continental (cf. Fortelius et al., 2006), oceanic circulation (Li et al., 2006) and hydrological cycle changes (Cane and Molnar, 2001) that suggest global climatic modifications. The Antarctic Peninsula Ice Sheet is highly sensitive to climate change, as evidenced by the recent rates of glacier retreat and ice shelf decay (cf. Pritchard and Vaughan, 2007; Glasser and Scambos, 2008). In the Neogene, there is both onshore and offshore evidence for extensive ice cover on the peninsula even during warm periods in the late Miocene and Pliocene (i.e. Hillenbrand and Ehrmann 2002, 2005; Smellie et al., 2008; Johnson et al., in press). Seismic studies show that the Antarctic Peninsula Ice Sheet was more dynamic than both the West and East Antarctic Ice Sheets during the Neogene (Bart et al., 2005), while geochemical studies of ODP Leg 178 suggest repeated ice advances and retreats on the west side of the Antarctic Peninsula from 9.2 to 3.1 Ma (Hillenbrand and Ehrmann, 2005). Modelling of the Pliocene ice sheets suggest that the ice sheets, which were likely to have been smaller than that of today, fluctuated significantly, although the extent of these fluctuations is still unknown (Hill et al., 2008). As glacigenic debris flows signify ice expansion, the deposits investigated in this study will further constrain the limits of Neogene ice. 1.1. Glacigenic debris flows Glacigenic debris flow deposits are formed from gravity-driven processes acting on other glacigenic sediment found in icemarginal, supraglacial and sometimes subglacial areas. That sediment becomes unstable and will flow when: (i) the internal strength of the material is overcome by applied forces, such as porewater pressures and gravity; or (ii) the ‘driving moment is increased beyond a critical amount’ in response to adjacent processes (Drewry, 1986). Many different terms are used for these gravity-driven sediments, including flow till, sediment flow deposits, slumped till, and flowtill complex (Dreimanis, 1988). The term glacigenic debris flow is used throughout this paper. Debris flows occur in both subaerial and subaqueous settings. Subaerial debris flows commonly occur at steep glacier margins, where a wide range of gravitational processes predominate (e.g.

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falling, rolling and sliding) and deposition may ultimately be subaqueous. Four types of subaerial debris flows have been identified based on studies at the margin of Matanuska Glacier in Alaska (Lawson, 1979, 1982). Lawson’s flow types I–IV range from dense, stiff, slowly evolving flows (flow types I and II) with a minimal water content, weak fabric development, and coarse grain size distribution diagnostic of non-channellised flow; to low-density, dilated, rapidly evolving flows (flow types III and IV) with a higher water content, better developed fabric, and finer grain size distributions, diagnostic of flow in sediment fans or pools (Lawson, 1979). Subaqueous debris flows often have long run-out distances in glacigenic prograding wedges and trough-mouth fan sequences on continental margins of polar cross-shelf troughs (Hillenbrand et al., ´ Cofaigh et al., 2005; Dowdeswell et al., 2008). The run-out 2005; O distances are typically much longer compared with subaerial debris flows (Elverhøi et al., 2000). For example, the presence of voluminous debris flows (1800 km3) with run-out distances of 250 km on the Pacific margin of the Antarctic Peninsula has been attributed to major continental shelf margin collapse and expansion of the Antarctic Peninsula Ice Sheet in response to global cooling during the late Neogene (Diviacco et al., 2006). The late Neogene sediments described here were first described in the mid-1960s (Bibby, 1966), but were not interpreted as glacigenic until the 1980s (Sykes, 1989), and more than 80% of the exposures have been discovered since 2002 (Hambrey and Smellie, 2006; Smellie et al., 2006a; Hambrey et al., 2008). We have now visited and conducted detailed analyses of all known (over 80) localities during several field seasons between 2002 and 2008 on James Ross, Vega, and Cockburn islands, thus enabling the first comprehensive view of Neogene Antarctic Peninsula ice configuration and ice-bed dynamics. 1.2. Neogene global ice sheet configuration 1.2.1. Southern Hemisphere Southern Hemisphere proxy records elucidating the evolution of the Antarctic Ice Sheet dating back to the late Miocene and early Pliocene are scarce, especially considering the size of the continent. In most cases, sedimentary records from near Antarctica have a low time resolution, which is complicated by the lack of biogenic carbonates that hinder paleoceanographic investigations. More recently, however, Steig et al. (2008) have shown that the Antarctic continent as a whole is being affected by the last 50 years warming. High-resolution data on sediment cores obtained from multisensor studies suggest orbitally controlled size variations of the Antarctic Ice Sheet in the late Miocene (Grutzner et al., 2003). Now, 30 years of drilling efforts off Antarctica have culminated with ANDRILL and the successful drilling of a continuous record near the Ross Sea Ice Shelf confirming numerous sedimentary cycles that testify for changing environmental conditions in Antarctica (Naish et al., 2008). More recently, Krusic et al. (2009) found evidence of glacial advances in the Dry Valleys suggesting a wetter and therefore warmer early Pliocene. Further afield, other evidence includes glacial episodes found in Patagonia between 7 and 5 Ma (Rabassa et al., 2005), and changes in deep oceanic circulation recorded in sediments in the Argentina basin, thought to be related to episodic changes in Antarctic Bottom Water flow (HernandezMolina et al., 2006). 1.2.2. Northern Hemisphere The Northern Hemisphere ice configuration is relevant to the global understanding of the evolution of the cryosphere during the late Neogene. Long marine sequences recovered through ODP drilling in the North Atlantic Ocean have provided abundant data

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Fig. 1. Field location map, indicating all described localities visited during the 2005–2008 field seasons. CO ¼ Cape Obelisk; RP ¼ Rink Point; DD ¼ Dobson Dome; dD ¼ Davis Dome; DP ¼ Dreadnought Point; BH ¼ Berry Hill; LP ¼ Lookalike Peaks.

on the progression of the Neogene Northern Hemisphere glaciations (Larsen et al., 1994). Onshore data indicate that Iceland was not significantly glacierised before 4 Ma (Geirsdottir et al., 2007), whereas near Greenland there is evidence for multiple glaciations since the late Miocene, although the Pliocene Greenland Ice Sheet is thought to have been smaller than present-day (Larsen et al., 1994). In the North Pacific realm, the best-exposed outcrop repository of late Cenozoic glaciomarine rocks is the Yakataga Formation (Gulf of Alaska), which dates the earliest tidewater glacier marine incursions to no older than late Miocene, where enhanced precipitation may have also helped sustain extensive highland icefields on the Pacific margin of Alaska (Lagoe et al., 1993). 2. Regional setting 2.1. Geographical setting James Ross Island is an ice-capped volcanic island situated on the eastern side of the northern Antarctic Peninsula (Fig. 1). There are three principal reasons for working on James Ross Island: the glacigenic sediments contain pristine bivalves that can be dated by Sr isotope stratigraphy (Dingle et al., 1997; McArthur et al., 2006; Smellie et al., 2006a) and are interbedded with lavas for which an

extensive published 40Ar/39Ar chronology exists (Smellie et al., 2008); there are many ice-free areas that are readily accessible on foot or by helicopter; and the island is situated in a relatively northern low-lying location adjacent to the climatically-sensitive Antarctic Peninsula Ice Sheet, where fluctuations in ice sheet extent and thermal regime should be most clearly preserved. James Ross Island is dominated by a large ice cap 200–400 m thick that is centred on Mt Haddington, a polygenetic shield volcano, the ice over which rises to >1600 m a.s.l. The island also contains two smaller ice domes on the Ulu Peninsula, at Dobson Dome and Davis Dome (Fig. 1). Eastern and southern James Ross Island are characterised by several cliff-walled valleys containing retreating tidewater glaciers that radiate away from the Mt Haddington summit region, whereas ice-free low-lying areas are particularly extensive to the north and west (Figs. 2a,b). The latter has a stronger glacial erosional imprint, with distinctive volcanic mesa landforms, some ice-capped, in which soft Cretaceous strata are widely exposed in the low-lying ground between mesas. The Antarctic Peninsula Ice Sheet is particularly sensitive to climatic fluctuations reflecting very high precipitation and meltrates (e.g. van Lipzig et al., 2004), and its development over time has significantly influenced the geology of the James Ross basin and Prince Gustav Channel (Domack et al., 2005; Evans et al., 2005).

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volcanic rocks date back only as far as 6.2 Ma (Jonkers et al., 2002; Smellie et al., 2008). The volcanic rocks have also provided a uniquely detailed record of former ice thicknesses, which probably did not exceed w750–850 m during the eruptive period (Smellie et al., 2008, 2009). The glacigenic sediments are exposed in two stratigraphic positions: (1) unconformably resting on top of Cretaceous sediment at the base of the volcanic sequence; and (2) interbedded with the James Ross Island Volcanic Group. James Ross Island has the highest concentration of dateable Neogene glacigenic sediments in Antarctica, and they are much rarer elsewhere in the Antarctic Peninsula region (e.g. King George Island (c. 22 Ma): Troedson and Riding, 2002; Alexander Island (c. 7.5–5.4 Ma): Smellie et al., 1993; Brabant Island (<200 ka): Smellie et al., 2006b)). The abundance of localities is explained by the development of the very long-lived Mt Haddington stratovolcano, in which the products of more than 50 discrete eruptions have been identified, most of which have preserved glacigenic sediments at their base (Smellie et al., 2006a, 2008). 3. Methods 3.1. Sedimentology

Fig. 2. (a) Steep-sided, mainly volcanic, valley walls of southern and eastern James Ross Island. (b) Glacially-eroded landscape, including flat-top mesas, of northern and western James Ross Island.

Since 1950, the Antarctic Peninsula has experienced a warming of nearly 3  C, which has resulted in spectacular ice shelf collapses and a concomitant increase in the flow rates of many outlet glaciers (Pritchard and Vaughan, 2007; Glasser and Scambos, 2008). 2.2. Regional stratigraphy The James Ross basin formed as a back-arc basin east of a coeval volcanic arc situated in the Antarctic Peninsula. Long-lived subsidence within the basin created accommodation space for over 5 km of marine volcaniclastic sediments derived from the volcanic arc. The stratigraphic succession extends from James Ross Island eastward to Seymour Island and comprises Jurassic deep-marine mudstones with ash layers at its western margin overlain by early Cretaceous to late Eocene sediments. Abundant exceptionally preserved invertebrate faunas and fossil floras in the strata have yielded important climatic information and are the basis of a welldeveloped chronostratigraphy of the region (see papers within Francis et al., 2006). Resting unconformably on top of the Cretaceous strata is a basaltic volcanic field that crops out within an area of w6000 km2 (Nelson, 1975; Smellie, 1999). It is known as the James Ross Island Volcanic Group and is characterised by voluminous and laterally very extensive multiple lava-fed deltas, each formed of basalt lava ‘‘topset beds’’ overlying much thicker steep-dipping homoclinal hyaloclastite breccia ‘‘foreset beds’’ (cf. Skilling, 2002; Smellie, 2006) and less common tuff cones (Nelson, 1975). Although eruptions probably commenced no later than c. 10 myr ago, the in situ

The successions in this study were visited during the Antarctic summers of 2005–2008. Multiple sections were logged using the broad lithofacies classification of Hambrey and Glasser (2003). Three main lithofacies were distinguished: ‘diamictite’ (‘diamicton’ if not lithified) refers to lithified, massive, matrix-supported sediment comprising mainly subrounded and subangular clasts; ‘conglomerate’ (‘gravel’ if not lithified) refers to lithified, massive, clast-supported sediment, also containing mainly subrounded and subangular clasts; and ‘breccia’ refers to lithified, massive, matrix and clast-supported sediment comprising mainly angular clasts. This is a descriptive terminology, although generally the diamictites and conglomerates are glacigenic, whereas the breccias are volcanogenic. The volcanogenic breccias occur as large-scale homoclinally dipping foreset beds in lava-fed deltas. They have previously been described in detail (cf. Skilling, 2002; Smellie, 2006; Smellie et al., 2006a, 2008) and are largely undescribed here except where contact features are important for interpreting age relationships. The log descriptions include information about clast, fossil, and erratic content, bed structure and thicknesses, and contact relationships (Fig. 3). Clast analyses are essential for constraining transport paths and depositional environments (Table 1). Fifty clasts w35–125 mm in diameter were randomly collected for various analyses. Clast roundness and shape define whether the sediment was transported actively (e.g. subglacially) or passively (e.g. supraglacially or englacially) in a glacial system (Benn and Ballantyne, 1994). The Powers (1953) scale was used to determine clast roundness (Fig. 4), and the RA index (percentage of ‘angular’ and ‘very angular’ clasts in a sample) quantifies the roundness (Benn and Ballantyne, 1994). Clast shape was determined by measuring the length of the a (longest), b and c (shortest) axes of the pebbles and calculating their ‘C40 index’ (percentage of clasts with c:a ratios below 0.4; Benn and Ballantyne, 1993; Fig. 5). Clast surface features (e.g. striations and facets) were also recorded, as these have a bearing on transport mechanisms. Sediment provenance was determined from clast lithologies. There are five main clast lithologies in James Ross Island glacigenic sediments (cf. Pirrie et al., 1997; Smellie et al., 2006a; Hambrey et al., 2008): (1) Antarctic Peninsula-derived Mesozoic volcanic rocks (Antarctic Peninsula Volcanic Group); (2) plutonic rocks (Antarctic Peninsula plutonic group); (3) Permo-Triassic Trinity

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Fig. 3. Sedimentary logging key, including information on contacts, structures, and textures (adapted from Hambrey and Glasser, 2003).

Peninsula Group metasediments, phyllites and schists (Trinity Peninsula Group); (4) James Ross Island-derived Cretaceous sediments; and (5) late Neogene volcanic rocks (James Ross Island Volcanic Group). Clasts derived from the late Neogene alkaline James Ross Island Volcanic Group are easily distinguished from calc–alkaline volcanic rocks in the Antarctic Peninsula: they are sparsely olivine- and plagioclase-phyric basalts that are fresh and contain unaltered or partially smectite-altered glass and olivine in the groundmass. By contrast, Antarctic Peninsula Volcanic Group lavas are basalts, andesites and dacites that are typically multi-coloured due to hydrothermal alteration, are conspicuously porphyritic (typically both plagioclase and pyroxene) and lack groundmass olivine. Indurated Jurassic mudstones with thin ash beds (Nordenskjo¨ld Formation) also crop out on the Antarctic Peninsula and locally on western James Ross Island. Although lithologically distinctive, they were not observed in our samples. The sedimentary signatures of many glacigenic lithofacies (subglacial till, debris flows, ice-proximal iceberg rainout sediment) can be almost identical on a macroscale (i.e. they are typically massive diamictite or conglomerate). Clast macro-fabric strength and detailed micromorphology (see below) are used here to distinguish between any sedimentary facies of otherwise similar appearance. Fifty elongated clasts were selected from each logged

unit for clast fabric analysis, (i.e. measurement of a axis orientation). The three-dimensional measurements were plotted on contoured stereonets (Fig. 6) and eigenvalues (S1, S2, S3) calculated. Clast fabrics have also been used to infer palaeo-ice flow directions (e.g. Holmes, 1941), but more recently, glacial geologists have been using the method to distinguish the genetic history of the sediment and to determine the subglacial shear in the sediment during glacier flow (cf. Hart, 1994; Larsen and Piotrowski, 2003; Nelson et al., 2005). Soft-sediment micromorphology is a recent development used to examine glacigenic sediments in more detail. Many glacigenic sediments that appear macroscopically massive actually contain micro-scale deformation structures that contain key information about ice-bed coupling, subglacial sediment deformation, type of deformation (ductile vs. brittle) and degree of sediment shearing (e.g. van der Meer, 1993; Carr, 2001; Hiemstra, 2001). 3.2. Macropalaeontology The taphonomy of macrofossils in James Ross Island Volcanic Group glacigenic (and volcanogenic; Nelson et al., 2008) sediments has been assessed by field observation at several localities on James Ross Island, particularly at Blancmange Hill and Fjordo Belen (Fig. 1). Marine fossils are not always present in the

Table 1 Summary table of all quantitative clast data. Location description

Station #

Erratics (%)a total %

TPG

APVG

APpg

Striations

#b

RA Index

C40 index

S1

S2

S3

(1) (2) (3) (1) (2) (1) (2) (3) (4) (5) (1) (2) (3)

2 8 0 2 0 13 37 41 43 12 0 0 6

4 14 2 2 6 13 9 8 9 6 14 4 6

0.547 0.5314 0.505 0.5788 0.5608 0.5351 0.4928 0.5671 0.5399 0.5089 0.4839 0.4476 0.5579

0.3703 0.346 0.4133 0.2419 0.3504 0.3114 0.3986 0.3111 0.2733 0.368 0.3359 0.4207 0.2819

0.0826 0.1226 0.0817 0.1793 0.0889 0.1535 0.1086 0.1219 0.1868 0.1231 0.1802 0.1317 0.1602

(1) (1) (2) (1) (2)

0 2 14 10 0

0 0 10 4 7

0.436 0.442 0.431 0.424 0.499

0.389 0.351 0.38 0.33 0.311

0.175 0.206 0.188 0.246 0.19

Cret

Rockfall Valley

D5.14

10

75

0

0

25

many

Stoneley Point

D5.29

1 10

50 25

0 25

0 25

50 25

many many

Fjordo Belen

D5.4

5

0

0

0

100

Blancmange Hill

D5.7

0

0

0

0

0

many

Rhino Cliffs Jonkers Mesa Pecten Spur

D6.262 D6.269 D7.4

<1 0 1

100 0 50

0 0 0

0 0 50

0 0 0

many none many

Stickle Ridge

D7.1

10

50

25

5

20

many

a b

few

Fabric

TPG ¼ Trinity Peninsula Group; APVG ¼ Antarctic Peninsula Volcanic Group; APpg ¼ Antarctic Peninsula plutonic group; Cret ¼ Cretaceous. Sample number at each station number; analyses done per 50 pebbles.

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Fig. 4. Clast roundness diagrams, after Powers (1952). The final plot is a co-variance plot, comparing clast shape and roundness data of selected depositional environments around Storbreen (from Benn, 2004). Note the low RA and C40 indices of the subglacially transported material. WR ¼ well rounded; R ¼ rounded; SR ¼ subrounded; SA ¼ subangular; A ¼ angular; VA ¼ very angular. RA indices located in Table 1.

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Fig. 5. Clast shape ternary diagrams, plotting the ratios of clast axes lengths. As the first diagram suggests, those clasts positioned near the top apex are ‘blocky’ in shape, clasts near the lower right of the diagram are ‘elongate’ in shame, and those near the lower left of the diagram are ‘slab-like’. The horizontal line is positioned at 0.4, where the C40 index (percentage of clasts with a c:a axis ratio is 0.4) is calculated.

glacigenic sediments, but when preserved supply evidence about the provenance of debris flow material, about how far that material has been transported, and about the marine environment of the adjacent shelf. Macrofossils include marine bivalve molluscs and encrusting cheilostome bryozoans, together with calcified serpulid worm tubes and fragments of echinoderm. The encrusting bryozoans form large colonies (often several centimetres in diameter) on pebbles and cobbles of James Ross Island Volcanic Group rocks, with smaller colonies sometimes encrusting bivalves. Serpulid worm tubes also encrust pebbles or small cobbles. At some localities and horizons fossils are largely intact, including at Blancmange Hill exquisitely preserved bryozoans and articulated bivalves suggesting minimal transport. At other sites, including Fjordo Belen, fossils are fragmentary, with bivalves reduced to fragments of centimetre (or smaller) scale. In these cases the fossils may have undergone greater transport distances. Field observation of fossils has been supplemented by detailed assessment of bivalve shell preservation from the Blancmange Hill locality (Fig. 1) using cathodoluminescence, thin-section petrography and SEM micrography. With the exception of one specimen studied, the shell lamellae have no visible cement overgrowths or recrystallisation. The calcitic shell lamellae of the prismatic layer

are non-luminescent to weakly luminescent under cathodoluminescence, indicating there are no diagenetic cements present. One specimen has a diagenetic cement overgrowth on the external surface of the valve of bladed calcite crystals, which are strongly luminescent. Where the shells have been further analysed these overgrowths or sediments adhering to the surface of the shell have been removed.

3.3. Benthic foraminifera A total of 50 samples for micropalaeontological analyses were selected from nine sites on James Ross Island. Sample weights were circa 20 g in most cases. All samples were disaggregated in distilled water and shaken for at least 24 h on a rotary shaking table prior to wet sieving. In the few cases where the samples were too indurated these were gently crushed with a small hammer prior to treatment. All samples were wet sieved on a 63 micron mesh and dried for several hours at 45  C. Once dry they were dry sieved on 150 micron mesh. Any benthic foraminifera or rare ostracods were removed using a brush and placed into micropaleontological slides for identification and quantitative study.

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Fig. 6. Schmidt lower-hemisphere, equal-area, stereographic projections of 3-dimensional clast macro-fabric. Contour interval is 1% per 1% of area.

The taxonomy used for the study of these samples follows the one used by Jonkers et al. (2002). In general the fauna reported for these sediments is similar to those found since the Oligocene and the development of the cryosphere in Antarctica (Webb and Strong, 2006).

3.4. Palynology Neogene fossiliferous diamictites and three samples from the underlying unconsolidated Cretaceous sediment have been subsampled for palynological studies from Davis Dome, Fo¨rster Cliffs

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Table 2 Summary table of all Sr and Ar ages. Location description

Station no.

Latitude/longitude

40

Rockfall Valley Stoneley Point

D5.14 D5.29

63.87383S; 58.02960W 63.85777S; 58.10590W

5.36  0.05 5.36  0.05 5.64  0.25

Fjordo Belen Blancmange Hill

D5.4 D5.7

63.99072S; 57.52761W 63.99995 S; 57.63315W

0.66  0.22, 2.50  0.07 5.85  0.03

Rhino Cliffs

D6.262-3

64.2174S; 57.30313W

3.62  0.03

Jonkers Mesa Pecten Spur

D6.269 D7.4

64.189S; 57.22342W 64.1959S; 57.1259W

3.08  0.15 3.08  0.15

Stickle Ridge a b

D7.1

64.9664S; 57.9773W

Ar/39Ar ages (Ma)a

87 Sr/86Sr (mean in bold)

Mean Sr age (Ma)

Error (þ)

Error ()

(1) 0.708995 (2) 0.708983 (3) 0.708965 (4) 0.708987 (5) 0.708961 (6) 0.708969 (7) 0.708980 0.708977

6.31

0.32

0.14

4.89

0.35

0.53

5.77

0.23

0.30

5.44 4.74b

0.20 0.32

0.27 0.63

(1) 0.709037 (2) 0.709040 (3) 0.709032 (4) 0.709028 (5) 0.709060 (6) 0.709054 (7) 0.709029 (8) 0.709029 (9) 0.709042 0.709039 (1) 0.709020 (2) 0.708999 (3) 0.708997 (4) 0.709012 0.709007 (1) 0.709025 (2) 0.709019 (3) 0.709020 0.709022

6.16  0.08

Ar age represents oldest and closest lava-fed delta to sedimentary outcrop; all data from Smellie et al. (2008); all Ar ages have 2s uncertainty. This age at Pecten Spur is from previously published data from Smellie et al. (2006a).

and Ekelo¨f Point (Fig. 1). A total of 20 samples were processed following standard procedures of demineralisation and oxidation. After preparation of microscope slides, all palynomorphs including dinoflagellates, pollen, spores and acritarchs were counted using a normal light and UV-fluorescence microscope in order to identify differences in age and thermal maturation (van Gijzel, 1967). Most samples from Davis Dome and Fo¨rster Cliffs were either sterile or contained only few palynomorphs. In contrast, Neogene and Cretaceous samples from Pecten Spur showed exceptionally good preservation and density, allowing a total count of more than 120 pollen and spores per sample. Regardless of preservation, samples of all three regions showed very similar pollen assemblages.

accumulated mean of NIST 987 measured in the laboratory over many years. All Sr and Ar ages are presented in Table 2. 4. Glacigenic lithofacies descriptions Many Neogene clast-supported and matrix-supported, massive and stratified glacigenic sequences were identified around the periphery of James Ross Island. Here, the glacigenic lithofacies are described from the following four regions (Fig. 1, Table 3): Davis Dome (Rockfall Valley and Stoneley Point localities; Figs. 7 and 8), Fo¨rster Cliffs (Fjordo Belen and Blancmange Hill localities; Figs. 9 and 10), Ekelo¨f Point (Rhino Cliffs, Jonkers Mesa, and Pecten Spur localities; Figs. 11 and 12), and Stickle Ridge (Figs. 13 and 14).

3.5. Sr/Sr dating Strontium isotopic analysis was undertaken at the Radiogenic Isotope Laboratory of Royal Holloway University of London. Pectinid bivalves were fragmented, surface layers flaked off selected fragments under the microscope, and the core fragments washed briefly in dilute acid before being washed with ultra-pure water and dried in a clean environment. Analysed sub-samples were translucent flaky carbonate without visible surface stain that may denote contamination from Fe or Mn. Flakes were dissolved in nitric acid, evaporated to dryness, taken up in acid and Sr was extracted using Sr-Spec ion-exchange resin. Isotopic measurement was made with a VG 354 TIMS in dynamic mode using the peakjumping routine of Thirlwall (1991). At least three standards of NIST 987 were run in each turret of up to 13 samples, with repeat standard runs. Data were normalized to a turret mean for NIST 987 of 0.710248. Values of 87Sr/86Sr replicated within analytical uncertainty of 0.000015, which is the uncertainty of the

4.1. Massive, matrix-supported diamictite (Rockfall Valley, Pecten Spur) This lithofacies typically contains 20% clasts, mainly pebble and cobble-sized. Striae are present on some boulders, and the clasts are mainly James Ross Island Volcanic Group-derived (lava and hyaloclastite breccia), although <1% Antarctic Peninsula-derived pebbles are observed at both localities. Clast shape is mainly blocky with a low to medium fabric strength. The diamictite at Rockfall Valley also contains rip-ups of Cretaceous material, deformed into the diamictic matrix (Fig. 7d). At the upper contact with hyaloclastite breccia at Rockfall Valley, squeeze-up structures of diamictite penetrate the volcanigenic material, while basalt pillows from the hyaloclastite penetrate the diamictite (Fig. 7c). Micromorphological features include a strong clast long-axis preferred orientation, imbrication and curvilinear structures (Fig. 7e).

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Table 3 Summary table of lithofacies characteristics, interpretation and provenance, described by geographical location. Location description

Station # Lithofacies descriptions

% fossils % AP Microstructural features erratics

Rockfall Valley

D5.14

Massive, matrix-supported diamictite with Cretaceous rip-ups

0

10

Massive conglomerate

0

<1

Weakly-bedded conglomerate

0

0

Subaerial debris flow with shearing of underlying Cretaceous material during transport Microlaminae and multidirectional Dry-type subaerial debris flow laminar undulations in matrix; fine-grained intraclasts n/a Subaerial debris flow

Weakly-bedded fossiliferous conglomerate 5 units of weakly-bedded conglomerate 9 units of well-bedded fossiliferous conglomerate 1 thick unit of well-bedded conglomerate 2 thick units of weakly-bedded conglomerate 2 units of massive conglomerate Massive, fossiliferous conglomerate Massive, matrix-supported diamictite 3 thick units of weakly-bedded conglomerate

5

10

n/a

0

0

10–80

0

0

<1

0

0

0

0

20

0

1

1

0

0

Stoneley Point

Fjordo Belen Blancmange Hill Rhino Cliffs

D5.29

D5.4

D6.262

Jonkers Mesa D6.269 Pecten Spur

D7.4

Stickle Ridge D7.1

Depositional interpretation

Strong clast long-axis preferred orientation, clast imbrication, and curvilinear structures

Ice-proximal glaciomarine debris flows Parallel clast fabric, thin silt haloes, Subaerial debris flows clast imbrication, intraclasts Rotational feature, intraclasts – v Ice-proximal glaciomarine few structures debris flows n/a Ice-proximal, marine-emplaced debris flows n/a Ice-distal, marine-emplaced debris flows n/a 2 lobes of subaerial glacigenic debris flows n/a ice-proximal glaciomarine debris flow n/a Iceberg-rafted material; ice-distal n/a 3 distinct subaqueous debris flow events in an ice-proximal glaciomarine setting

Ice provenancea Haddington Ice Cap

Haddington Ice Cap

Haddington Ice Cap Antarctic Peninsula Ice Sheet Haddington Ice Cap Haddington Ice Cap Haddington Ice Cap Haddington Ice Cap Haddington Ice Cap Haddington Ice Cap Haddington Ice Cap Small ice cap to NW of Stickle Ridge

AP ¼ Antarctic Peninsula. a Ice mass responsible for the deposition of particular lithofacies.

4.2. Massive conglomerate (Rockfall Valley, Jonkers Mesa) This lithofacies comprises a high clast concentration (70–85%) of pebbles, cobbles, and boulders (Figs. 7b and 11c). Occasional Antarctic Peninsula-derived clasts are observed, and there is an absence of fossils. Clasts are mainly blocky and have a low fabric strength. Clasts within the conglomerate at Jonkers Mesa contain many fractured clasts, where the matrix is squeezed into the fractures. Microstructures include microlaminae, multidirectional laminar undulations in the matrix, and intraclasts. 4.3. Weakly- to well-bedded conglomerate (<5 m thick: Stoneley Point, Fjordo Belen) This lithofacies typically contains a high percentage of clasts (some of which are striated and facetted) that are derived entirely from the James Ross Island Volcanic Group. Clasts are blocky in shape and vary in fabric strength from weak to strong. The stacked conglomerates at Fjordo Belen (Fig. 9a) contain a high percentage of angular clasts. The base of each conglomeratic unit is erosive (Fig. 9b). Microstructural analyses reveal parallel clast fabrics, thin silt haloes (Fig. 9c), clast imbrication, and intraclasts. 4.4. Weakly- to well-bedded conglomerate (<5 m thick: Rhino Cliffs, Stickle Ridge) This lithofacies contains thick (10–40 m; Figs. 11a and 13) units of clast-rich (up to 80% clasts) sediment, some with steeply-dipping planar-stratified beds (14–24 ), in sequences 64 m (Rhino Cliffs) to 150 m (Stickle Ridge) thick. The Stickle Ridge outcrop can be traced for 4 km. Many clasts are striated, and there are a lack of fossils and Antarctic Peninsula-derived material within this lithofacies. Clasts are blocky in shape. Patches of muddy matrix and crudely-laminated fines are also observed within the conglomerate.

4.5. Weakly-bedded fossiliferous conglomerate (Blancmange Hill, Pecten Spur, Stoneley Point) This lithofacies contains a high percentage of clasts and fossil material (Figs. 7g, 9e–l and 11e). Clasts are blocky and have a low fabric strength. The fossiliferous conglomerates at Blancmange Hill and Pecten Spur lack Antarctic Peninsula-derived erratics, while those at Stoneley Point contain a high percentage of erratics (5–10%; Fig. 7f). This lithofacies, however, is unique due to the high percentage of macrofossils (mainly pectinid bivalves and encrusting cheilostome bryozoans, but also calcified worm tubes; Figs. 7g, 9e–g and 11e) and micro-fossils (foraminifers, ostracods, and echinidae spines; Figs. 9h–k) observed. Some bryozoans encrust the bivalve fragments. Two beds at Blancmange Hill contain 90–95% shelly material, where shell horizons are draped and deformed around pebbles and cobbles, and folded shelly lenses (sometimes recumbent) are also present (Fig. 9l). Seven out of 11 samples at Blancmange Hill contained microfauna, mostly benthic foraminifera but also ostracods and echinoid spines (two occurrences; Fig. 9k). For these seven samples the total abundance in benthic foraminifera was low, with just one to 57 foram tests present, and the number of foraminifera per gram of dry sample was never in excess of 3. However, two of the seven samples were dominated by 45% Ammoelphiediella sp (Fig. 9h), while three other samples were dominated by Cassidulina crassa (>50%; Fig. 9i). Secondary species are Cibicides lobatulus (Fig. 9j) and Astrononion antarcticum. At Pecten Spur, 28 taxa of benthic foraminifera were identified, mainly Cibicides lobatulus (24%), now-extinct Ammoelphidiella Antarctica (14%), Cribrononion sp. (12%), Nonionella bradii (11%), and Cassidulina crassa (10.5%; Jonkers et al., 2002). Palynomorph assemblages are dominated by pollen and spores of Nothofagidites spp., Podocarpidites spp., Proteacidites spp., Cyathidites spp. and Retitriletes sp. Along with other palynomorphs such as the dinoflagellate Isabelidinium spp. or

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Fig. 7. Field photographs of Davis Dome sections. The Rockfall Valley (D5.14) section contains: (a) granular mud overlying Cretaceous strata and (b) cobble conglomerate. (c) Contact between conglomerate and overlying hyaloclastite pillow breccias at Rockfall Valley section. Note pillow lava sunk into underlying conglomerate. Knife circled for scale. (d) Cretaceous rip-up clasts have been incorporated into the diamictite. (e) Photomicrograph of curvilinear structures within diamictite. Photographs (f, g) are of the Stoneley Point (D5.29) sections. (f) Fossiliferous, Antarctic Peninsula erratic-rich upper conglomerate. (g) Cheilostome bryozoan within the conglomerate.

Odontochitina spp. and the abundant acritarch Michrystridium sp., these assemblages are indicative of a pre-Neogene, most probably late Cretaceous age.

(þ0.32, 0.14) Ma was obtained on pristine bivalve material and may approximately represent the depositional age for the host sediment (see below).

5. Lithofacies interpretations and age, regional ice configuration and ice flow directions

5.1.2. Rockfall Valley The exposures at Rockfall Valley are likely to be subaerial icemarginal gravity flow deposits, deposited by a local ice cap or valley glacier, but having travelled a comparatively long-distance from its source, from the following evidence: the superimposition of diamictite and conglomerate suggests successive different debris flow depositing events, and the high concentration of boulders at the base of the conglomerate indicates sinking of coarse clasts through saturated sediment, typical of subaerial glacigenic debris flow deposition (Benn and Evans, 1998). The strong microfabric and clast imbrication observed in thin-section suggest a ‘wet-type’ debris flow (Lachniet et al., 1999). The sharp erosive lower contact with the Cretaceous, the brecciation of the in situ Cretaceous material, and the rip-up clasts within the lower unit both suggest shearing and subsequent incorporation of the underlying Cretaceous material as this wet-type debris flow was transported downslope. The low RA index (0, 2, and 8; indicating a high degree of abrasion of the clasts) suggests that the sediment has travelled far, and the very low proportion of Antarctic Peninsula-derived erratics suggests deposition from a local ice cap (i.e. on James Ross Island). The microstructural evidence (folding, faulting, mixing, poorly defined fabric, and the presence of intraclasts) in the overlying conglomerate unit indicate a ‘dry-type’ debris flow, in which intraclasts were not disaggregated because of low ambient water conditions (Lachniet et al., 1999). The two stacked debris flow deposits at Rockfall Valley were probably derived from the same valley glacier derived from the Haddington

In this section we collate all sedimentological, palaeontological, and chronological data from the four regions discussed in this study, to assess the Mio–Pliocene ice configuration (provenance and extent) in the northern Antarctic Peninsula region, ice flow directions, and the timing of ice advances that caused the deposition of glacigenic debris flows described. 5.1. Davis Dome (Rockfall Valley and Stoneley Point) 5.1.1. Age (Table 2) In the two glacigenic outcrops near Davis Dome, the strata are interpreted as glacigenic debris flows deposited in different settings and at different times. The contact relations with overlying hyaloclastite in Rockfall Valley indicate that the volcanic and glacigenic sediments were contemporaneous there. The hyaloclastite forms part of a lava-fed delta correlated with that at Rink Point and dated as 5.36  0.05 Ma (Smellie et al., 2008). Because the outcrop north of Stoneley Point occurs only in contact with Cretaceous strata, there is no field evidence for a Neogene age. However, its appearance is remarkably similar to other late Neogene sediments on the island, with which it is hereby correlated. The nearest lavafed deltas, at Stoneley Point, are Ar/Ar dated at 5.36  0.05 and 5.64  0.25 Ma (Smellie et al., 2008). However, the latter age is very poor and an older age is not precluded. A 87Sr/86Sr age of 6.31

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Fig. 8. Sedimentary logs of Rockfall Valley and Stoneley Point sections.

Ice Cap, but may have been deposited as a result of multiple glacier advances, or more likely, during one advance with two onlapping debris flow lobes containing differing amounts of ambient water in the sediment resulting from variable ice ablation conditions associated with each event.

5.1.3. Stoneley Point Both conglomerate beds near Stoneley Point were formed from debris flows and we suggest that they are related to two different ice masses originating from two directions, as follows: the lower conglomerate is notably devoid of fossils and erratics, which

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Fig. 9. Field photographs of Fo¨rster Cliffs sections. (a, b) Onlapping units of conglomerate at Fjordo Belen (D5.4). (c) Photomicrographs of microstructures within conglomerate. (d) Blancmange Hill (D5.7) section, containing a high percentage of macrofossil material including this articulated pectinid bivalve (e), cheilostome bryozoans Smittina (f), and calcified worm tubes (g). Foraminifers include (h) Amoelphidiellia antarctica, (i) Cassidulina crassa, and (j) Cibicides lobatulus. (k) Ostracods (Leptocythere sp.) and echinoderm spines are also present in the Blancmange Hill debris flows. (l) Folded pockets of shell fragments.

strongly suggests derivation from a local ice cap situated on James Ross Island. It is probably also a subaerial debris flow deposit. By contrast, the presence of marine fossils and abundant Antarctic Peninsula-derived erratics in the upper conglomerate indicate derivation of at least some debris from a marine setting, probably the Antarctic Peninsula Ice Sheet extending across the Prince Gustav Channel. The pectinid bivalves in the conglomerate are heavily fragmented, consistent with relatively long-distance transport. The occurrence of abundant James Ross Island Volcanic Group clasts indicates that the leading edges of far-travelled lavafed deltas formerly extended much further out into Prince Gustav Channel and have since been eroded back by successive advances of the ice sheet.

¨ rster Cliffs (Fjordo Belen and Blancmange Hill) 5.2. Fo 5.2.1. Age (Table 2) Neither of the Fo¨rster Cliffs glacigenic sediment outcrops described here are in contact with overlying volcanic material and it is unclear whether the sedimentary and volcanic deposits are contemporaneous. Multiple 87Sr/86Sr ages from pristine bivalves in the Blancmange Hill exposure cluster consistently around 4.89 (þ0.35, 0.53) Ma. From their abundance, lack of adhering lithified sediment and well-preserved condition (some are still articulated), the Blancmange Hill bivalves appear to have suffered neither erosional reworking from an older exposure, nor prolonged (kascale) exposure on the sea floor prior to their incorporation in the

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Fig. 10. Sedimentary logs of Fjordo Belen and Blancmange Hill sections. Note fossil density within Blancmange Hill units, indicated by the number of drawn shells to the right of the sedimentary log.

host conglomerates. Thus, they may have been extant and coeval with the ice advance responsible for the conglomerate deposition, and the 87Sr/86Sr ages also date the host deposit. The closest lavafed delta, which forms the summit of Blancmange Hill, is dated as 5.85  0.03 Ma; that forming Fo¨rster Cliffs 3 km to the east is 2.5  0.07 Ma; and the Terrapin Hill tuff cone nearby to the north is 0.66  0.22 Ma (Smellie et al., 2008). The Fjordo Belen debris flow

beds were clearly emplaced prior to the 2.5 Ma Fo¨rster Cliffs delta, but after both the Blancmange Hill lava-fed deltas and associated conglomerate exposure. 5.2.2. Fjordo Belen The debris flow beds at Fjordo Belen are similar to those at Rockfall Valley. Both are products of gravity-driven debris flows

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Fig. 11. Field photographs of Ekelo¨f Point sections. (a) Rhino Cliffs (D6.262) section, consisting of three stacked conglomerates. The lower conglomerate contains pockets of finegrained, contorted laminae which wrap around cobbles (b). (c) Jonkers Mesa (D6.269) section. (d) Pecten Spur (D7.4) section, containing 10 m of tuffaceous laminated sandstone, overlain by fossiliferous conglomerate (e) and diamictite.

derived from a local ice cap, with clast characteristics indicating relatively limited transport distances. The dominance of angular clasts may indicate passive supraglacial transport, but the presence of striations on clasts suggests active transport in basal ice at some stage. The microstructures indicate a wet-type debris flow (Lachniet et al., 1999, 2001). Although the presence of siltstone intraclasts suggests a lack of disaggregation in water, the intraclasts are indurated and Cretaceous in age, and much less liable to disaggregate, even in a well-saturated debris flow. 5.2.3. Blancmange Hill With their abundance of well-preserved macro- and microfossil debris, the Blancmange Hill beds were likely to have been emplaced in a marine setting and could not have travelled far, given the good state of fossil preservation (Figs. 9e–l). The microfaunal assemblage represents eurytopic species that are adapted to a wide range of environments. They bear a close resemblance to the eurytopic assemblage of Webb and Strong (2006) found within upper Oligocene–lower Miocene glaciomarine sequences in Victoria Land (CRP-2/2A). The onlapping, slightly-dipping, attenuated units of cobble-rich conglomerate with sharp erosive contacts and cobble-lag concentrations are key indicators of glacigenic debris flows. The presence of rotational micro-features also

supports debris flow transport and deposition. Transient turbulent flow conditions in some debris flows moving over bed irregularities triggers the development of ‘rotational cells’, which preferentially incorporate fine-grained matrix and creates fines-rich rotational domains in the resulting sediment (Phillips, 2006). This sediment was probably derived from a local ice cap/glacier advance, on account of the absence of Antarctic Peninsula erratics and that the sediment was transported a short distance before being deposited at Blancmange Hill, as the bivalves and bryozoans are wellpreserved and mainly intact. We suggest that northward-flowing ice from an expanded Mt Haddington Ice Cap advanced across a palaeo-basin to the east of present-day Croft Bay, scooping up marine sediments and associated abundant marine life and depositing them at Blancmange Hill. ¨ f Point (Rhino Cliffs, Jonkers Mesa, Pecten Spur) 5.3. Ekelo 5.3.1. Age (Table 2) Each of the three described clastic deposits in the Ekelo¨f Point region is overlain by hyaloclastite breccia of lava-fed deltas. At Rhino Cliffs, the contact is sharp, whereas squeeze-up structures composed of diamict intrude hyaloclastite breccias at both Pecten Spur and Jonkers Mesa. Thus only the glacigenic and overlying

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Fig. 12. Sedimentary logs of Rhino Cliffs, Jonkers Mesa, and Pecten Spur sections.

volcanic deposits at Pecten Spur and Jonkers Mesa were demonstrably coeval. At Rhino Cliffs, fossil pectinids in a large block of conglomerate fallen from younger beds have yielded a 87Sr/86Sr age of 5.77 (þ0.23, 0.3) Ma (D6.263.1). Of all analysed sub-samples, however, D6.263.1 did not comprise of white translucent calcite. The sub-samples were composed of clear, brown calcite and are suspected of being slightly altered. Alteration of biogenic carbonate from the Antarctic Peninsula typically decreases the 87Sr/86Sr value of a sample, so this age must be treated with circumspection; it is probably spuriously old. The overlying lava-fed delta has an 40 Ar/39Ar age of 3.62  0.03 Ma (Smellie et al., 2008). 87Sr/86Sr dating from pristine bivalves within the Pecten Spur deposit yielded ages of 5.44 (þ0.2, 0.27) and 4.74 (þ0.32, 0.63) Ma, whereas

the age of the overlying lava-fed delta is 3.08  0.15 Ma (Smellie et al., 2008). 5.3.2. Rhino Cliffs The thick exposure of conglomeratic units at Rhino Cliffs is interpreted as multiple lobes of gravity-driven debris flows with varying sediment/water ratios, probably marine-emplaced. Some of the beds apparently have sandy-gravelly matrices and might be deposits of concentrated or hyperconcentrated sediment density flows, but they generally lack diagnostic features, such as grading, internal stratification and laminated sandy (turbiditic) interbeds (cf. Mulder and Alexander, 2001). Conversely, most have minor muddy fine fractions that probably imparted some matrix strength

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Fig. 13. Field photographs of Stickle Ridge (D7.1) section. (a) Entire section, (b) onlapping units of conglomerate, dipping towards the SSE (c).

and buoyancy during transport. The lowermost units containing lenses of sorted fine material indicate the presence of running water or a slurry and the relatively steep angle of bedding suggests deposition as foresets on the distal slope of a grounding-line fan. The upper two units of weakly-bedded material indicate grounding-line retreat, resulting in ice-distal debris flows. The Rhino Cliffs conglomerates were delivered by a marine-terminating glacier originating from the Mt Haddington ice cap, that underwent an initial advance and emplacement of ice-proximal debris flows close to the grounding-line as represented by the steeply-dipping conglomerate, followed by subsequent retreat and emplacement of ice-distal debris flows as indicated by the weakly-bedded conglomerate. 5.3.3. Jonkers Mesa The onlapping geometry of the beds, clast-rich deposits, solely James Ross Island Volcanic Group-derived clasts and low RA index, suggest that the section at Jonkers Mesa resembles two lobes of subaerial glacigenic debris flows also deposited from a local ice cap or glacier containing relatively far-travelled clasts. 5.3.4. Pecten Spur The Pecten Spur section is dominated by weakly-bedded, fossiliferous James Ross Island Volcanic Group-derived conglomerate, interpreted as subaqueous debris flow deposits, laid down from successive sediment gravity flows close to the grounding-line of an ice cap centred over Mt Haddington. Jonkers et al. (2002) also interpreted these deposits as an accumulation derived from high concentration sediment gravity flows, but suggested that further work would uncover a more complicated depositional history. By contrast, the capping unit, composed of massive, matrix-supported diamictite, was likely a product of glaciomarine suspension sedimentation and associated iceberg rafting near a receding ice front. The Ekelo¨f Point/Cape Gage peninsula may have lain below sea level at the time, with marine-terminating glaciers or ice shelves originating from the Mt Haddington ice cap overriding and incorporating glaciomarine sediment en route to the shelf edge. The

foraminiferal species identified in the fossiliferous conglomerate are indicative of shallow water Antarctic shelf environments (Jonkers et al., 2002). The palynoflora at Pecten Spur closely resemble Campanian to Maastrichtian assemblages which have been previously described for other localities on James Ross Island (Pirrie et al., 1997). This indicates a contamination of the Neogene diamictites with reworked material from the underlying late Cretaceous sediment. No difference in colour or fluorescence signature of pollen grains, and thus in the degree of degradation, could be detected, suggesting a purely reworked Cretaceous origin of the palynoflora. The presence of an unambiguous in situ pollen flora would provide evidence for a substantial vegetation cover on James Ross Island during the warm Neogene, but was not identified. The two Sr ages of the pectens (5.4 and 4.74 Ma, both specimens well-preserved) also indicate reworking of at least some of the sediment, while the intact preservation indicate that some of the fossils in the deposit were probably alive at the time of incorporation. The upward lithofacies transition from conglomerate to diamictite at Pecten Spur represents: (1) a glacial advance delivering glaciomarine debris (fossiliferous conglomerate) to the area, followed by (2) glacial retreat and the consequent deposition of iceberg-rafted debris (massive, matrix-supported diamictite). Because of its position, shielded from the Antarctic Peninsula by the large Mt Haddington volcano, there would have been little opportunity for Antarctic Peninsula-derived material to be deposited there.

5.4. Stickle Ridge 5.4.1. Age (Table 2) The Stickle Ridge sequence is overlain by a lava-fed delta at the northern end of the ridge. Lapilli tuffs overlie the delta at that location, but they overstep it towards the south and come to rest directly on the Stickle Ridge sedimentary sequence. The age of the lava-fed delta is 6.16  0.08 Ma (the oldest in situ delta on James

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interpreted here. The lower two thick stratified conglomeratic units appear to have been deposited during successive discrete subaqueous debris flow events as foreset beds at the groundingline of a marine-terminating glacier during successive ice advances. Like the outcrop at Rhino Cliffs, some beds have sandy–fine gravelly matrices and might be deposits of hyperconcentrated or concentrated density flows, but with similar caveats. Contemporaneous minimum water depths of c. 100 m are implied by the thickness of the two stacked (southerly-prograding) units at Stickle Ridge. The low percentage of Antarctic Peninsula-derived clasts (<1%) and lack of marine fossils suggest that the ice was derived from a local, James Ross Island-based, ice mass. However, the two lower conglomerate units dip homoclinally to the SSE. There is no evidence for any tectonic deformation that may have tilted the section. As elsewhere on James Ross Island, the dips are believed to be primary and they suggest that an ice source was formerly situated approximately north of Stickle Ridge. Although the Antarctic Peninsula is situated north and west of James Ross Island, the nearabsence of Antarctic Peninsula-derived material within the debris flows makes it an unlikely source. Conversely, the dominance of local James Ross Island Volcanic Group clasts indicates that the ice overrode a volcanic massif, which is no longer obvious in the present-day landscape. An ice-capped volcanic centre situated to the northwest of Stickle Ridge is therefore envisaged, either near present-day Davis Dome or within present-day Prince Gustav Channel (as discussed below). All traces of the volcanic massif have been removed by erosion (see below). Conversely, the uppermost stratified deposit, which we were unable to examine as closely, forms a subhorizontal unit of mud-rich diamictite beds (80–90% mud) that drapes and truncates the underlying stratified, foresetlike conglomerates. The relationship resembles that seen at Rhino Cliffs and similarly might indicate final deposition during grounding-line retreat. 6. Discussion 6.1. Debris flow deposition

Fig. 14. Sedimentary log of Stickle Ridge section. Only the stratified conglomerates above c. 20 m. are described in this paper. The sediments below 20 m were probably deposited in an unrelated older period.

Ross Island), and correlation of the lapilli tuffs suggests that they are approximately 5.90 Ma (Smellie et al., 2008). A diamictite–mudstone–sandstone sequence underlies conglomerates at Stickle Ridge but it probably formed during an older glacio-volcanic event. Only the overlying conglomerates are

The debris flows described here probably accumulated mainly at the margins of expanding ice masses. They were deposited in both subaerial and subaqueous environments, the latter presumably close to the grounding-line of marine-terminating glaciers. That many of the debris flows were not transported far from their source is shown by the dominance of local (James Ross Island Volcanic Group) clast lithologies and the presence of fragile and sometimes intact, articulated bivalves and bryozoans. The mainly coarsely bedded units of clast-rich material are consistent with subaerial types I and II debris flow deposits (Lawson, 1979, 1982), whereas micromorphological studies suggest a combination of wet-type and dry-type subaerial flows (Lachniet et al., 1999, 2001). The two dominant debris flow lithofacies described in this study are diamictite and conglomerate. Most of the exposures described here comprise onlapping units of massive to weakly-bedded, coarse-grained conglomerate. The conglomerate contains 50–70% pebbles, cobbles, and boulders. Many of the clasts are striated and faceted, indicating previous transport by subglacial processes when the ice was coupled to the bed, before being incorporated into the debris flows (Boulton, 1978). Although striations are confined to the volumetrically abundant James Ross Island Volcanic Group clasts, it is possible that striations also affected Cretaceous sandstone and Antarctic Peninsula-derived clasts but they were not seen because of their scarcity as clasts (typically <1%), soft texture, and coarsegrain size (Kuhn et al., 1993; Hambrey and McKelvey, 2000). The preferred orientations of the fabrics are mainly moderate to high and the S1 eigenvalues are similar to those for dilatant

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deformation tills (e.g. Benn, 1994). This reflects similar clast mobility behaviour within a semi-viscous material. Clasts in dilatant tills and debris flows are able to move freely under transient strain (Benn, 1994). Fabric strength in glacigenic debris flow deposits is generally highly variable but is determined mainly by water content. Lawson (1979a) suggested that clast fabrics are stronger with elevated water content, whereas others argued that higher water content allows for more freedom of movement and therefore reduces the preferred orientation (Benn, 1994; Bennett et al., 2000). Other common characteristics of the debris flows described here and elsewhere are: sharp lower contacts between onlapping units, where the flow has eroded the underlying material during emplacement; and presence of intraclasts and rip-up clasts, indicating erosion and incorporation of underlying material during transport. 6.2. Timing of debris flow events Glacigenic sediments generally have a low preservation potential, but the lava flows that originated from Miocene/Pliocene volcanoes on James Ross Island overrode and preserved the sediments in pristine condition. The lavas provide a minimum age for sedimentation of the glacigenic deposits, as well as providing a well-dated stratigraphical framework (Smellie et al., 2008). Evidence for intimate field relations between the glacigenic beds and the volcanic rocks (e.g. mixing, mingling and/or intrusion of sediment in hyaloclastite) also sometimes indicate that the volcanic and sedimentary events were coeval and essentially contemporaneous. All of the debris flow deposits on James Ross Island, except for those at Jonkers Mesa and Rhino Cliffs, which are interbedded with volcanogenic units, rest unconformably on Cretaceous strata. The age of the sediments is further constrained by dating wellpreserved fossil bivalves using Sr isotope dating techniques. Table 2 outlines all new and published dates relevant to this study. Because there is some evidence that fossils of multiple ages have been reworked into the glacigenic deposits (Smellie et al., 2006a), between 1 and 4 shells were chosen from each unit to determine the range of ages present and to determine if the ages cluster significantly. Since the pectinid bivalves required ice-poor, even sea ice-free, conditions in which to live (see Berkman et al., 2004; Williams et al., 2009), they represent warmer conditions, or interglacials (cf. Smellie et al., 2006a). Sr ages for fossils in the debris flows cluster at 4.74, 4.89, 5.44, 5.78, and 6.31 Ma. Debris flow deposits at Pecten Spur contain shells with multiple ages, whereas multiple shells dated at the other three localities (Blancmange Hill, Rhino Cliffs and Stoneley Point) gave single ages within uncertainty. The latter result implies that: (1) there was probably no reworking of older material within those debris flows; and (2) the bivalves were probably living contemporaneously with debris flow deposition. As debris flows typically signal ice expansion, probably ice expansion also occurred during late Neogene interglacial periods. The Sr ages on bivalves in debris flow deposits presented in this study probably correspond to episodic advances in the dynamic Antarctic Peninsula Ice Sheet during the late Neogene. Offshore records of clay mineral variations in ODP Leg 178 suggest that multiple ice advances took place along western Antarctic Peninsula throughout the last 9 myr, and occurred even during relatively warm periods (Hillenbrand and Ehrmann, 2005). Although the opal deposition rate (a proxy for biological palaeoproductivity and linked to sea ice coverage) is particularly enhanced between 5.2 and 3.1 Ma, suggesting reduced sea ice cover (Hillenbrand and Fu¨tterer, 2002; Pudsey, 2002), there are also peaks in opal deposition dating back to 9.3 Ma, which spans the multiple warm periods identified by our Sr ages (Table 2).

Despite the combination of Ar and Sr isotopic ages obtained on the James Ross Island sequences, it is still challenging to establish precisely the timing of the debris flow events. However, from the broad distribution of the glacigenic debris flow deposits across the island and their wide range of ages, it is clear that northern Antarctic Peninsula glaciers were dynamic and delivered large volumes of sediment to ice margins during numerous late Miocene and Pliocene advances.

6.3. Late Neogene provenance and ice configuration Every debris flow described here is overwhelmingly dominated by local James Ross Island Volcanic Group material, suggesting that the main ice centres for the debris flows were situated on James Ross Island itself. The most important ice centre was probably a large ice cap located on Mt Haddington, which dominates the present-day geography (Fig. 1), as Mt Haddington was volcanically active (and therefore an important landscape element) at least as far back as latest Miocene time, and possibly back to 10 Ma (Jonkers et al., 2002). The Mt Haddington ice cap was responsible for delivering unfossiliferous sediment, as subaerial debris flows linked to terrestrial glaciers, eastwards to Ekelo¨f Point, northwards to Fo¨rster Cliffs, and northwest-wards to Rockfall Valley (Fig. 15). Conversely, late Miocene to early Pliocene advances by the Haddington ice cap towards Pecten Spur and Blancmange Hill resulted in fossiliferous glaciomarine material being ploughed along palaeobasins and dumped onshore as debris flows at the palaeo-shelf edge(s). The bivalves at both localities are well-preserved and less fragmented than at other sites, suggesting that the pectens were not transported far, and were essentially coeval with the ice in which they were incorporated. By contrast, a smaller ice centre, comprising a volcanic massif that was active in latest Miocene times, must have been situated on the northwest side of the Ulu Peninsula or perhaps even in presentday Prince Gustav Channel (Fig. 15). Support for a former volcanic massif is suggested by the SSE-dipping hyaloclastite breccia foreset beds that characterise both of the overlying lava-fed deltas at Stickle Ridge. They indicate delta progradation in a SSE direction, consistent with (and uniquely for James Ross Island) a northern vent area. The former presence of a northern upstanding volcanic massif would have also been capable of diverting any Antarctic Peninsula ice around, and away from, Stickle Ridge. This left only a local source of volcanic and possibly Cretaceous material to supply the conglomerates at Stickle Ridge. A small ice-capped satellite volcanic centre is therefore envisaged, situated to the northwest of Stickle Ridge, either near present-day Davis Dome or within present-day Prince Gustav Channel. All traces of the volcanic massif have been removed by erosion but it may have resembled the 1.69  1 Ma complex of three overlapping ice-covered satellite volcanoes seen today on Tabarin Peninsula (Smellie et al., 2006c). The exposure near Stoneley Point is the only debris flow deposit documented that contains significant amounts of Antarctic Peninsula-derived material (up to 10% of clasts). The debris flow responsible for the deposit, which also contains large amounts of highly fragmented fossiliferous material, was probably delivered by the Antarctic Peninsula Ice Sheet (Fig. 15). The ice sheet must have been grounded and the presence of incorporated shelly debris suggests that Prince Gustav Channel must have existed as a marine channel during late Miocene time (i.e. at w6.3 Ma, from Sr dating of the bivalves). Thus, it is clear from the sedimentological record that the regional and local morphodynamics of ice masses and the delivery of debris flows during the late Neogene Epoch in the northern Antarctic Peninsula region were determined by multiple ice

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Fig. 15. Conceptual model showing ice cap configurations during the late Neogene, based on debris flows discussed in this paper. Two ice caps are envisioned, one positioned over Mt Haddington and one positioned over present-day Ulu Peninsula. The large Antarctic Peninsula Ice Sheet was also present, delivering material to James Ross Island across the Prince Gustav Channel. The dashed outline represents the presentday coastline of James Ross and adjacent islands, while the black dots represent localities discussed in text.

sources: two local ice caps situated over present-day James Ross Island and a regional-scale ice sheet on the Antarctic Peninsula. 6.4. Climatic implications The presence of well-preserved marine fossils within three of the described glacigenic debris flow deposits (Stoneley Point, Pecten Spur and Blancmange Hill) has significant environmental implications. The marine life clearly could not have grown under full glacial conditions, i.e. with an ice sheet grounded far out on the present shelf, and the fauna must have required relatively ice-poor conditions, or even an ice-free sea (Williams et al., submitted). Berkman et al. (2004) have argued that Austrochlamys bivalves in particular, suggest dominantly ice-free seas. The diversity of the fauna, including pectinid bivalves, cheilostome bryozoans, mussels, echinoids, ostracods and foraminifera, also implies relatively favourable growing conditions (Figs. 10–12 of Jonkers et al., 2002). Therefore, the presence of shelly faunas in the James Ross Island Neogene sediments has come to be regarded as broadly indicating warm or ‘‘interglacial’’ conditions (Smellie et al., 2006a). Furthermore, despite their fragmentation, the shelly material is in a pristine compositional state, largely unaffected by diagenesis, suggesting that the animals were not deeply buried in sediment or lay exposed on the sea floor for a prolonged period prior to their glacial overriding and incorporation. Numerous unfragmented shells are also present in two of the debris flow deposits (at Pecten Spur and Blancmange Hill), suggesting that at times ice transport prior to deposition was minimal. This is also supported by the presence of shells still articulated. Thus, these animals of still articulated shells lived essentially coevally with the ice advance

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responsible for their reworking and deposition, and ice advances took place during relatively warm periods in the late Neogene. Sediment delivery to Antarctic continental margins during full glacial periods has been well-documented for the Quaternary (cf. Anderson, 1999; Dowdeswell et al., 2008) and the Cenozoic (Powell et al., 2000; Naish et al., 2007), but this is the first onshore record of Cenozoic ice advance during a warm period. Debris flows and other mass transport activity generally reflect periods of ice expansion (Forwick and Vorren, 2007) that are usually correlated with glacial conditions (Dowdeswell et al., 2002, 2008). However, the fossiliferous debris flows on James Ross Island were delivered to the shelf by glaciers advancing during relatively warm periods. The ‘snow-gun hypothesis’ of Prentice and Matthews (1991) suggests that the warming of low- and mid-latitude deep water, enhanced ocean evaporation and increased precipitation on continental Antarctica, as snow, triggered ice expansion during the Tertiary. Their suggestion has been supported by ice sheet models of East Antarctica during the Cenozoic (e.g. Huybrechts, 1993). Higher evaporation at high latitudes is also facilitated during warm periods by reduced sea ice coverage. Thus, during late Miocene and Pliocene warm intervals, a dynamic Antarctic Peninsula Ice Sheet delivered debris flows to the ice margins during ice sheet expansion in both warm and cold periods. Multiple expansions of the Antarctic Peninsula Ice Sheet during the period are also supported by offshore seismic studies (e.g. Bart et al., 2000; Barker et al., 2002). This conclusion is consistent with glacio-volcanic and modelling studies on James Ross Island that indicate even the interglacials were simply ice-poor rather than ice-free, thus enhancing the likelihood of significant ice advances occurring under relatively warm periods rather than only during full glacials (Smellie et al., 2008, 2009). 6.5. Ice-bed dynamics The sediment delivered to the front of glaciers provides clues about the basal thermal regime of the ice masses. Warm-based ice masses have a high sediment delivery rate as they are coupled with and erode the basal sediment and bedrock, whereas cold-based ice is frozen to its substrate, stiffer, moves more slowly and is not a strong landscape modifier (Gellatly et al., 1988; Atkins et al., 2002). Cold-based ice therefore does not incorporate much debris into its system (Goodfellow, 2007). The high sediment supply evident from the thick sequences that accumulated at Stickle Ridge and Rhino Cliffs (150 and 64 m, respectively) suggests that the thermal regime of the ice must have been warm-based, at least in those areas. Clast shapes and surface features (i.e. striations, clast abrasion) and sediment texture point to a warm-based glacier, which was able to couple to and erode bedrock, eventually transporting sediment to the ice margin. Similar features are present in the thinner sequences at Davis Dome, Fo¨rster Cliffs and Ekelo¨f Point, and similar conclusions probably apply. However, a polythermal regime, i.e. one in which warm-based patches of ice erode bedrock and underlying sediment, and adjacent patches of coldbased ice are locally frozen to the bed (Hambrey et al., 2008; Johnson et al., in press), is more likely as there is a lack of glaciofluvial material present. Field evidence for cold-based ice erosion has been demonstrated in some parts of Antarctica (e.g. Atkins et al., 2002), but the sediment/landform associations are quite different. Warm-based or polythermal ice advanced across James Ross Island by a combination of basal sliding and subglacial sediment deformation. All except one of the debris flows described here (Jonkers Mesa) contain few to many striated James Ross Island Volcanic Group-derived clasts. Striations do not form within debris flows as clasts are transported in a slurry rather than directly in

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contact with ice or the bed. The striated clasts observed in the James Ross Island debris flow deposits are reworked subglacial debris. Ice-bed coupling and subsequent subglacial sediment deformation were dominant mechanisms in the forward movement of the ice, and that the subglacially-eroded material was incorporated into subsequent debris flows during ice advance.

(Leicester) examined the bivalves for evidence of diagenesis. We also thank Colin Cunningham and Rob Wilson (Leicester University) for thin-section and SEM analyses of the fossils. Eugene Domack and an anonymous reviewer are thanked for their insightful, positive comments. References

7. Conclusions This paper provides the first descriptions and interpretations of multiple late Neogene glacigenic debris flow deposits on James Ross Island, northern Antarctic Peninsula. Glacigenic debris flows were common during late Neogene time on the island, and they are well-preserved through protection given by overlying volcanic units. Two of the sequences examined contain numerous relatively friable intact fossil pectinid bivalves and cheilostome bryozoans, suggesting that they have not travelled far within the debris flow and that they were living on the sea floor during the period of ice advance(s) that reworked them into the debris flows. Sr isotopic ages obtained on the fossil bivalves suggest that warm periods occurred in the northern Antarctic Peninsula at 4.89, 5.44, 5.77, and 6.31 Ma. Because the pectinid bivalves within the debris flows required relatively warm environmental conditions in which to live (i.e. not full glacials) and glacigenic debris flow deposition is indicative of episodes of ice expansion, we infer that episodes of ice expansion took place during relatively warm periods, or ‘‘interglacials’’ (sensu lato). The warm periods would have been characterised by reduced or even no sea ice, greater oceanic evaporation and greater continental precipitation, as snow, thus promoting ice sheet expansion ahead of full glacial conditions (cf. the ‘snow-gun hypothesis’ of Prentice and Mathews, 1991). The presence of a fossil-rich debris flow sequence, on the west coast of James Ross Island, that is also rich in Antarctic Peninsula-derived erratics, indicates that Prince Gustav Channel must have comprised open water even during latest Miocene times (w6.3 Ma). Some of the late Neogene deposits on James Ross Island are thick and laterally extensive, indicating a high sediment delivery to the ice margins and probably a glacier thermal regime that was polythermal. Clast features and textural fabrics of the deposits indicate that basal sliding and subglacial sediment deformation were both active processes that contributed to multiple ice expansions during the period. Three main ice centres were responsible for the regional ice morphodynamics. In addition to evidence for a regional-scale Antarctic Peninsula Ice Sheet, there were also two local ice caps, comprising a large ice cap on Mt Haddington and a smaller ice cap or dome situated in the northern Ulu Peninsula. The massif responsible for the Ulu Peninsula ice cap is now completely removed by erosion. Acknowledgements This work contributes to the British Antarctic Survey ISODYN project (Ice-House Earth: Stability or Dynamism?), and also to the SCAR ACE (Antarctic Climate Evolution) initiative. The authors particularly thank Captains Bob Tarrant and Nick Lambert and the officers and crews of HMS Endurance for their invaluable logistical support during the 2005–2008 Antarctic field seasons, when the majority of the fieldwork for this paper was carried out. Field assistants A. Clark, M. Gorin, M. Laidlaw, and T. Spreyer are gratefully acknowledged. We are also very grateful to Mike Tabecki for his production of numerous technically challenging diamictite and conglomerate thin-sections. Paul Taylor (NHM) gave advice on bryozoans and supplied the image used in Fig. 9f, whilst Ian Wilkinson (BGS) provided information on ostracods. Carys Bennett

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