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Collaborative Research: Potential direct geologic constraints on ice sheet thickness in the central Transantarctic Mountains during the Pliocene warm period We propose to map and date glacial deposits in the central Transantarctic Mountains to

help constrain the configuration of the East Antarctic Ice Sheet during past periods of

warmer-than-present climate, specifically the Pliocene warm period 5-3 Ma. Along their length, the Transantarctic Mountains (TAM) are abutted by the East Antarctic Ice Sheet (EAIS) and form a topographic barrier to ice draining into the Ross Sea Embayment. Ice thickness in the upper TAM thus is controlled fundamentally by the thickness of the EAIS, while glacial deposits in these areas record past periods when the ice-sheet was at least as extensive as it is today. We propose to exploit this depositional record of glaciation in order to constrain when the EAIS was as thick as or thicker than present on long (Pliocene-Miocene) timescales. Such constraint is important because of evidence for large-scale fluctuations in sea level during past warm periods and recent hypotheses attributing much of this variability to Antarctica. Specifically, this proposal aims to demonstrate:

x Glacial geologic and surface-exposure investigations into sites in the upper TAM can provide constraints on past EAIS fluctuations and configuration for potentially long periods;

x That previous work at our proposed sites indicates a high likelihood of a viable geologic record being preserved there;

x How we have the means to collect the record and overcome potential issues associated with geochronology on these timescales and in this environment.

Our proposed research would tap a rare opportunity to provide geologic resolution of ice-sheet behavior from Antarctica itself in order to help address outstanding questions of EAIS history and, more specifically, ice-sheet dynamics under climate conditions warmer than present. Together with the documented existence of pre-Pleistocene glacial deposits in the upper TAM, the demonstrated efficacy of cosmogenic nuclides for dating polar landforms renders the investigation feasible and within the capabilities of the proposed field and analytical approach. With the growing focus on Antarctica’s role in past – and future – sea level, developing further the available record of glaciation in East Antarctica is a logical next step towards understanding long-term ice-sheet stability. Former configuration of the EAIS: Evidence from the Antarctic continent East Antarctica today is dominated by a cold polar climate that precludes significant surface melting over the ice sheet. Together with its largely terrestrial nature and thermal isolation, such conditions have supported a traditional view of long-term EAIS stability (Shackleton & Kennett, 1975; Kennett, 1977; Huybrechts, 1993; Sugden et al., 1993; Marchant et al., 1996; Barrett, 1999). Just how long the EAIS has maintained a configuration similar to present, however, has not been resolved conclusively and remains an important question in our understanding of Antarctica’s evolution. Detailed assessment of the TAM glacial geologic record has revealed distinct morphological differences among deposits indicating that

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glaciologic, and thus climatic, conditions have varied considerably in the past. For example, pioneering studies by J. Mercer identified two principal types of deposits, the first comprising lodgement tills associated with wet-based glaciation, the second characterized by loose, generally thin ablation tills (Mercer, 1968, 1972). The former (collectively termed the Sirius Group) have since been observed throughout the higher TAM (Barrett & Powell, 1982; McKelvey et al., 1991; Webb et al., 1987; Wilson et al., 1998), commonly overlying molded, striated bedrock, and have been interpreted as representing overriding of the mountains by an enlarged EAIS (Mayewski, 1975; Denton et al., 1984, 1989; Webb et al., 1984; Marchant et al., 1993). However, despite the widespread occurrence and distinct composition of these wet-based deposits, we still do not know when this period of ice-sheet expansion occurred. Certain minimum-limiting age constraint for the Sirius Group is afforded by overlying deposits at several sites (see below), though existing estimates for their emplacement range from >5-10 Ma (e.g., Bruno et al., 1997; Schäfer et al., 1999; Ackert & Kurz, 2004) to the early-mid Miocene (Marchant et al., 1996).

Fig. 1. Locations of sites discussed in text:

RG - Reedy Gl.; SG - Scott Gl.; ShG -

Shackleton Gl.; BG - Beardmore Gl.; HG -

Hatherton Gl; SVL - Southern Victoria

Land; McM - McMurdo Sound; LG -

Lambert Gl.; PB - Prydz bay.

The physical characteristics of deposits overlying the Sirius Group are suggestive of cold-based ice (Mercer, 1972; Bockheim et al., 1986; Prentice et al., 1986). Furthermore, the geometry and conformity of these deposits to the landscape have been interpreted as indicating an ice configuration similar

to today: a large ice sheet in East Antarctica draining via TAM outlet glaciers into the Ross Sea Embayment (Bromley et al., 2010). Coupled with chronologic constraint for cold-based deposits (see below), these geologic observations have been used to argue for enduring cold polar conditions in East Antarctica potentially since the mid Miocene. Implicit is the general stability of the EAIS since that time (Denton et al., 1993; Sugden et al., 1995; Marchant & Denton, 1996). A key limitation of this argument, however, is the sparse coverage of the existing chronologic dataset, both spatially and temporally. To date, surface-exposure chronologies extending beyond the last glaciation exist only at a few sites, including southern Victoria Land (Brown et al., 1991; Brook & Kurz, 1993; Brook et al., 1993, 1995; Ivy-Ochs et al., 1995; Bruno et al., 1997; Schäefer et al., 1999; Strasky et al., 2009), Beardmore Glacier (Fig. 1; Ackert, 2000; Ackert & Kurz, 2004), and Reedy Glacier (Fig. 1; Bromley et al., 2010), while 40Ar/39Ar has been applied in the Dry Valleys (Hall et al., 1993; Marchant et al., 1993, 1996). Moreover, these datasets targeted a small number of moraines, leaving much of the geologic record of EAIS glaciation in the TAM unresolved. We note, also, that not all terrestrial records are in agreement regarding past ice-sheet configuration. For example, sedimentologic investigations at Lambert Glacier (Fig. 1) invoke a dynamic fjord environment there until as recent as ~1 Ma (Hambrey & McKelvey, 2000), a scenario

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supported by marine evidence from neighboring Prydz Bay (Fig. 1; Mahood & Barron, 1996; Quilty et al., 2000; Whitehead et al., 2005). Considering the extensive catchment of Lambert Glacier (~7% of the EAIS; Higham et al., 1997), such marginal fluctuations likely would reflect changes throughout much of the EAIS.

Offshore, Antarctic marine records display a broader range of potential EAIS configuration during the Pliocene warm period. For example, whereas recent sedimentologic data from McMurdo Sound (Fig. 1) indicate persistent cold-based conditions along the ice-sheet margins (Barrett & Hambrey, 1992; McKay et al., 2009), despite generally warmer sea-surface temperatures (Scherer et al., 2007; Naish et al., 2009), provenance studies of ice-rafted debris suggest a dynamically unstable EAIS and widespread collapse during warmer periods (Williams et al., 2010; Pierce et al., 2011). In contrast, interpretations of sea-floor morphology on the continental shelf have invoked expansion of the ice sheet during the Pliocene due to enhanced accumulation (Bart & Anderson, 2000; Bart, 2001; Cowan, 2002), a scenario supported by some model simulations (Hill et al., 2007) and evidence for accumulation-induced thickening of EAIS outlet glaciers during Pleistocene interglacials (Higgins et al., 2000; Todd et al., 2010).

In summary, the existing Antarctic dataset presents a tantalizing, yet equivocal, view of ice-sheet behavior during past warm periods. Geologic evidence suggests the EAIS maintained a configuration similar to present during at least parts of the Pliocene, while the marine record leaves open the possibility of larger-scale variability. As a fundamental component of this dataset, the TAM glacial record has demonstrated potential to expand our knowledge of former ice-sheet behavior, yet thus far has been exploited only partially, leaving our understanding of the EAIS during the Pliocene warm period incomplete. To help address this shortcoming, we propose to constrain the chronology of pre-Pleistocene moraines deposited by the EAIS in the central TAM, thereby expanding the geologic record of the ice sheet during this crucial time.

Distal evidence for a dynamic EAIS during the Pliocene Numerous studies have suggested that the Pliocene represents a closer analog for our future than does any time period since (e.g., Zubakov & Borzenkova, 1988; Dowsett & Cronin, 1990; Bart, 2001; Huybrechts, 2009): average temperatures were elevated relative to today (e.g., Budyko, 1982) and sea level generally was higher (Raymo et al., 2006). Exactly how much sea level will rise over the coming centuries is one of the foremost problems in our field and depends ultimately on the capacity of the world’s ice sheets for change. As the largest remaining ice sheet, the stability of the EAIS is of critical importance, yet its behavior and configuration during former warm periods such as the Pliocene is poorly constrained.

Distal reconstructions of Pliocene sea level range from 15 m (Krantz, 1991) to 60 m above modern levels (Haq et al., 1987), a degree of variability which Raymo et al. (2011) argued likely reflects dynamic topography effects: most sea-level reconstructions are based on elevations of paleo-shorelines and relict marine deposits and do not account for the influence of mantle flow and glacio-isostatic adjustment. Such variability in the observational record complicates reconstruct����ȱ�����ȱ��ȱ������ȱΈ18O data, for although these high-resolution records reveal generally lighter values during the Pliocene than subsequently (Prentice & Matthews, 1988; Lieseki & Raymo, 2005), the relative contributions of temperature and ocean volume are difficult to establish without calibration against independent sea-level data (Naish & Wilson, 2009). Despite these uncertainties, most conceptual models and computer-based simulations of Pliocene conditions, including

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global ice-sheet configuration, assume a sea-level value of at least +25 m and must therefore invoke considerable reductions in the cryosphere, including the EAIS (e.g., Dowsett & Cronin, 1990; Dowsett et al., 1994, 1999; Haywood et al., 2002; Raymo et al., 2006; Naish & Wilson, 2009). A notable exception is the recent modeling study by Pollard and DeConto (2009), which suggests relatively minor change in East Antarctica over the last ~5 Ma despite considerable fluctuations of the West Antarctic Ice Sheet (WAIS).

Ultimately, deficiencies in our understanding of global Pliocene sea level preclude indirect assessment of the contribution – and, thus, stability – of the EAIS. Alongside the limited spatial and temporal resolution of the terrestrial Antarctic dataset and the incongruent scenarios implied by marine records from the continent, these shortcomings limit our knowledge of the long-term behavior of the EAIS and render it difficult/impossible to address such questions as, Was the configuration and thermal character of the EAIS different during the Pliocene from today? Did the EAIS undergo extensive melting or was it relatively insensitive to Pliocene warmth? Answers to these questions would allow us to delve into the fundamental sensitivity of the ice sheet to climate warming and underscore the pertinence of resolving more fully the geologic record of glaciation from Antarctica itself.

Potential Pliocene glacial-geologic records in the Transantarctic Mountains Previous work The TAM contain abundant glacial deposits corresponding to changes in ice configuration. EAIS outlet glaciers, such as Beardmore and Shackleton Glaciers (Figs. 1, 2), flow through the TAM and today discharge into the Ross Sea Embayment or, farther south, into the WAIS. Consequently, these outlet glaciers have undergone repeated fluctuations in both thickness and profile in response to ice-sheet variability both up-glacier in East Antarctica and down-glacier in West Antarctica (Mercer, 1968). To date, the geologic record of these glacial fluctuations has been used primarily to investigate Pleistocene behavior of the relatively dynamic WAIS (Bockheim et al., 1989; Denton et al., 1989; Orombelli et al., 1989; Ackert et al., 2007; Bromley et al., 2010, 2012; Todd et al., 2010). In contrast, the longer-term record of ice-sheet variability has been explored only minimally in the TAM, rendering our knowledge of the Pliocene EAIS incomplete.

Previous work on pre-Pleistocene glacial deposits has focused on three principal areas along the ~3000 km length of the TAM: southern Victoria Land, Beardmore-Hatherton Glaciers (central TAM), and Reedy Glacier (southern TAM). In the Quartermain Mountains of southern Victoria Land (Fig. 1), moraines of Taylor Glacier – an outlet of the EAIS – were mapped and dated as part of a larger investigation into landscape evolution in the Dry Valleys (see Denton et al., 1993) and provide some indication of Pliocene ice extent. In Arena Valley, for example, the age of the cold-based “Taylor IVb drift” is constrained by 40Ar/39Ar and surface-exposure ages to between 2.1 and 7.4 Ma (Marchant et al., 1993; Brook & Kurz, 1993; Brook et al., 1993). This unit, together with the 40Ar/39Ar-dated Thompson moraine (2.7–3 Ma; Wilch et al., 1993) in neighboring Taylor Valley, is interpreted as representing minor Pliocene expansion of the EAIS (Marchant et al., 1994). An older deposit – “Quartermain I drift” – representing an earlier incursion of East Antarctic ice into Arena Valley is only broadly constrained to between 4.4 and 11.6 Ma (Brown et al., 1991; Marchant et al., 1993) and thus might predate the Pliocene altogether. At lower elevation, lateral moraines in Wright Valley record minor expansions of alpine glaciers potentially dating to the Pliocene. Specifically, 40Ar/39Ar ages on associated volcanics indicate that advances

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occurred both prior to 3.7 Ma (“Alpine IV drift”) and after 3.5 Ma (“Alpine III drift”) (Hall et al., 1993).

Farther south, investigation of glacial deposits alongside Beardmore and Hatherton Glaciers (Fig. 1) revealed several broad groups representing cold-based deposition by East Antarctic ice flowing through the TAM (Mercer, 1972; Denton et al., 1989; Bockheim et al., 1989). These units are correlated between sites on the basis of position and geomorphology (Denton et al., 1989), thereby affording a more regional view of deposition in the central TAM. On the basis of this record, the oldest and most weathered units – “Dominion drift” (Beardmore Glacier) and “Isca drift” (Hatherton Glacier) – were interpreted as pre-Quaternary in age (Denton et al., 1989), a view that has since been verified by surface-exposure dating in the Dominion Range (Fig. 2), upper Beardmore Glacier. There, five 21Ne ages from Dominion moraines on Mercer Platform give an average exposure age of 1.9 Ma (Ackert, 2000), while eight 3He ages from the Oliver Platform moraine fall between 2 and 5.2 Ma (Ackert & Kurz, 2004). Although the impetus for that work was partly to demonstrate the antiquity of the underlying Sirius Group, the resulting chronology (i) provides one of the clearest indications yet of the potential for constraining long-term records of EAIS variability with cosmogenic nuclides and (ii) suggests Pliocene deposits are preserved in the upper reaches of the central TAM.

At the southern end of the TAM range, Bromley et al. (2010) reported at least five distinct cold-based till units alongside Reedy Glacier, an EAIS outlet glacier in the Wisconsin Range (Fig. 1). As in the central TAM, the distribution of landforms indicates deposition by north-flowing East Antarctic ice conforming to the current landscape. While 10Be surface-exposure ages indicate the lower three units (“Reedy III, B, and C drifts”) correspond to Pleistocene expansions of Reedy Glacier (Bromley et al., 2010; Todd et al., 2010), ages from higher, more deeply weathered tills suggest a much longer history of deposition. For example, one boulder on “Reedy D drift” gives an exposure age of ~2.5 Ma, while sixteen ages from the underlying “Reedy E drift” fall between 1.2 and 4.9 Ma (Bromley et al., 2010). Considering that typical Antarctic erosion rates would be sufficient to explain the observed spread in Reedy E 10Be concentrations, the drift unit is interpreted as being at least 5 Ma in age.

At present, the spatial and temporal resolution of the TAM glacial-geologic record is insufficient to assess with certainty Pliocene configuration of the EAIS. Specifically, the distribution of deposits pre-dating the late Pleistocene has been mapped at only a handful of sites, while chronologic constraint generally consists of minimum or broadly bracketing ages. Nonetheless, the existing dataset is fundamental to the further development of the terrestrial record of ice-sheet behavior in Antarctica. First, the abundance and extraordinary degree of preservation of glacial deposits documented in the TAM potentially allow for long-term reconstruction of ice-sheet configuration, possibly as far back as the mid Miocene. Second, published chronologic data suggest that at least part of the depositional record corresponds to the critical Pliocene period. Third, these surface-exposure studies have demonstrated the method’s efficacy in dating glacial landforms in Antarctica. In summary, the geologic evidence for pre-Pleistocene glaciation in the TAM, although fragmentary, underscores the potential wealth of information regarding past EAIS behavior during warmer-than-present climate conditions such as the Pliocene, and emphasizes the value in developing the terrestrial record further.

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Proposed Study Our goal is to reconstruct and date directly the timing of ice-surface fluctuations at sites

adjacent the EAIS, in order to assess questions concerning the timing, duration, and

magnitude of ice-sheet fluctuations during the Pliocene. As major outlets of the EAIS draining the Polar Plateau, the upper reaches of Beardmore and Shackleton Glaciers (Fig. 2) are influenced primarily by the EAIS (Denton et al., 1989). Isolated mountains located near the heads of Beardmore and Shackleton Glaciers, where they exit the EAIS, include the Dominion Range, Otway Massif, Roberts Massif, and Bennett Platform (Figs. 2, 3), as well as numerous smaller nunataks. These upland areas are characterized by cold polar-desert conditions and cold-based glaciation, and are kept largely ice free due to persistent katabatic winds.

The Dominion Range, Otway Massif, Roberts Massif, and Bennett Platform are ideally located to reconstruct past fluctuations of the EAIS. Not only do they occupy positions immediately adjacent or close to the ice sheet, these sites contain abundant landforms correlated to past glacial periods of ice expansion and/or stability (Fig. 4). As detailed above, previous work in the Dominion Range has documented as many as four generations of drift overlying older Sirius deposits (Mercer, 1972; Mayewski, 1975; Denton et al., 1989) and these have been directly correlated with tills farther north at Hatherton Glacier (Denton et al., 1989; Bockheim et al., 1989). However, by comparison to Reedy Glacier (Bromley et al., 2010) - also visited by Mercer (Mercer, 1968) - and Scott Glacier (Bromley et al., 2012), the number of glacial episodes represented by the deposits in the Dominion Range likely is much higher. While the existing surface-exposure dataset from the site (Ackert, 2000; Ackert & Kurz, 2004; see Previous Work) is of insufficient size to study the long-term glacial record in detail, these exposure ages, like those from Reedy Glacier (Bromley et al., 2010), raise the distinct possibility that the glacial-geologic record from the upper Beardmore Glacier spans at least the Pliocene.

Fig. 2. Field-site locations: 1 -

Roberts Massif and Bennett

Platform, upper Shackleton

Glacier; 2 - Otway Massif, upper

Mill Glacier (a tributary of

Beardmore Glacier); 3 -

Dominion Range), upper

Beardmore Glacier. (RIS - Ross

Ice Shelf).

Despite their prime location, relatively little prior glacial-geologic work has been conducted at Roberts Massif, Otway Massif, and Bennett Platform, with most attention having been paid to the Sirius Group (Mayewski, 1975;

Mayewski & Goldthwaite, 1986; Hambrey et al., 2003). Overlying those diamictons, Mayewski (1975) described erratics, bouldery moraines, and kame terraces and assigned

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Fig. 3. Oblique aerial views of proposed field sites, with prominent pre-late-Pleistocene moraines

highlighted: (A) view southeast toward Otway Massif; (B) North side of Roberts Massif, with

moraines of both Shackleton Glacier (arrows) and alpine glaciers visible; (C) Bennett Platform from

the south, with Shackleton Glacier to the right; (D) the Dominion Range from the north, with

Beardmore Glacier on the right. OP and MP denote Oliver and mercer platforms, respectively, sites

at which extensive pre-late-Pleistocene deposits have been documented (Denton et al., 1989; Ackert,

2000, Ackert & Kurz, 2004).

them to three broad groups: “high”, “medium”, and “low” drift. Similarly, Hambrey et al. (2003) reported well-preserved suites of bouldery moraines overlying Sirius Group tills at Bennett Platform and Roberts Massif. However, while these deposits have not been mapped in detail or dated, their preliminary descriptions, as well as the sites’ proximity to both the EAIS and well-mapped Dominion Range, suggest that they have great potential for advancing the terrestrial record of glaciation in the TAM. Moreover, the combination of excellent moraine preservation and local lithologies suitable for cosmogenic 10Be, 26Al, and 21Ne dating (Elliot et al., 1986; Hambrey et al., 2003) is a good indication that the glacial record can be constrained effectively with established surface-exposure techniques (see Geochronology section). Thus, the upper Beardmore and Shackleton glaciers represent an

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important opportunity to provide a long, directly dated record of ice-sheet fluctuation in East Antarctica.

Goals and Methods In order to establish the long-term record of ice-sheet behavior in East Antarctica, with particular emphasis on potential Pliocene variability, we propose to determine directly the

timing, duration, and magnitude of ice-sheet fluctuations in the upper Beardmore and

Shackleton Glaciers region. We will do this using an integrated glacial-geologic--exposure dating program. Specific goals of this study are to:

x Determine former glacier, and thus ice-sheet, extent by mapping the distribution, elevation, and morphology of moraines, drift sheets, erratics, and erosional features.

x Date the record of glaciation using cosmogenic-isotope methods. Details of how we will achieve these goals are given in the following sections. Glacial-geologic mapping of Plio-Pleistocene moraine sequences We will map the distribution, elevation, morphology, and geometry of moraines, drift sheets, erratics, and glacial erosional features on vertical aerial photographs (1:20,000 scale or better) and satellite imagery. Using hand-dug excavations, we will characterize the sedimentology and stratigraphy of each unit (i.e., Bromley et al., 2010). We also will note lithology and weathering characteristics of clasts (both internal and external) from the landforms, as well as all samples collected for surface-exposure dating. Elevations of moraines and other glacial landforms will be determined with differential GPS (see UNAVCO letter of collaboration). These data, combined with the dating work described below, will allow us to carefully separate and correlate drift units among different locations in the upper Beardmore and Shackleton Glaciers. Our previous work at Reedy and Scott Glaciers resulted in detailed geomorphic maps and the separation of at least six (Reedy Glacier) and ten (Scott Glacier) major drift units (Bromley et al., 2010, 2012). We will produce similar maps as part of the proposed project. Our primary sites will be Otway Massif, Roberts Massif, Bennett Platform, and the Dominion Range (Fig. 3). At the latter, we will build on the detailed mapping work of Denton et al. (1989) but will focus on the older, pre-late Pleistocene deposits (e.g., Fig. 4). At the other three sites, our work will constitute the first high-resolution glacial-geologic studies of those deposits.

The results of the glacial-geologic work will be implemented in two ways. First, we will use those data to reconstruct former surface elevations in the upper reaches of Beardmore and Shackleton Glaciers. This will allow us to constrain ice thickness for this sector of the EAIS under potentially different climate conditions (e.g., during Pliocene warmth), data that then can be compared to ice-sheet-model predictions. Second, the glacial-geologic mapping will lay the groundwork for surface-exposure dating. Careful mapping, correlation and characterization of weathering features is essential for successful exposure dating, particularly in Antarctica (i.e., Stone et al., 2003; Todd et al., 2010), where predominantly cold-based glaciation and repeated oscillation of glaciers can result in complicated drift stratigraphy and recycling of old material into young moraines. Therefore, producing detailed maps will be a main priority of this study.

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Geochronology We will constrain the age of pre-late-Pleistocene deposits sheets using cosmogenic surface-exposure dating (see Balco et al., 2011). The premise of this method is that glacially transported clasts originate from erosion at glacier beds, where they have not experienced geologically recent exposure to the cosmic ray flux, so contain negligible concentrations of cosmogenic nuclides such as 10Be, 26Al, and 21Ne. These clasts are transported to the ice margin, whereupon they are exposed to the cosmic-ray flux and accumulation of these nuclides begins. Thus, measurements of nuclide concentrations in glacially derived boulders provide an exposure age for the moraine. Although straightforward in principle, application of this technique in the present project is subject to several challenges inherent

to Antarctic glacial systems generally and also to the expected Pliocene age of the deposits. Here, we discuss these challenges, show that we have adequate resources and strategies to address them, and argue that our proposed approach will likely yield significant improvements in the chronology of pre-late-Pleistocene glaciation in the TAM. Fig. 4. Well-preserved moraines on Mercer Platform,

Dominion Range. The upper limit of Beardmore drift

(BD), dating to the LGM, and myriad older moraines

are clearly visible. The earliest cold-based deposits,

collectively termed Dominion drift (DD; Denton et

al., 1989), likely represent advances of many ages.

Previous ages for Dominion drift range from 2 to 5

Ma (Ackert, 2000; Ackert & Kurz, 2004), indicating

the potentially long record preserved at such sites. RG

- Rutkowski Gl.

Prior exposure. Prior exposure (“inheritance”) is related to the repeated exposure and ice burial of ice-marginal sites. Past studies determined that samples of erratics and bedrock from Antarctic nunataks covered by ice at the LGM contain much larger cosmogenic-nuclide inventories than could have been produced in the time available since post-LGM deglaciation (Stone et al., 2003; Sugden et al., 2005). In the case of bedrock, this occurs because much of the Antarctic ice sheet is cold-based, so subglacial erosion is negligible during periods of ice cover. Thus, nuclide concentrations in bedrock surfaces record the integrated effect of numerous periods of intermittent exposure. This can also be true for glacially transported clasts emplaced during one deglaciation, buried by cold-based ice, and re-exposed in subsequent ice-free periods. More relevant for our proposal, erratics can originate as supraglacial rather than subglacial debris, and therefore can contain significant cosmogenic-nuclide concentrations when entrained by ice. Similarly, erratics exposed at one site may be re-entrained and subsequently exposed at a different site.

Our strategy for addressing inheritance in supraglacially derived clasts and potential intermittent exposure of field sites relies on (i) guidance from previous studies on how to deal with this issue, and (ii) careful geologic and geomorphic mapping to clearly identify these conditions based on geologic criteria. Ultimately, our goal is to use geologic

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field observations to determine what happened, and then cosmogenic-nuclide measurements to determine when it happened. Numerous exposure-dating studies in Antarctica have shown that proportions of “fresh” erratics -- whose exposure ages correctly record the time they were emplaced by deglaciation -- and “pre-exposed” ones have a clear relationship to a site’s glaciological context. For example, in the Ford Ranges (Stone et al., 2003), where exposure-dating sites bordered the lower reaches of major glaciers draining the interior of Marie Byrd Land, the relative frequency of pre-exposed erratics was relatively low (88% of erratics analyzed had Holocene exposure ages). Conversely, in the Marble Hills (Todd et al., 2005), where the sampling site lay well inland and adjacent local ice draining the higher Ellsworth Mountains, only 38% of erratics had Holocene exposure ages. Guided by such studies, we will target sites at the margins of large glaciers flowing in deep troughs, thereby maximizing the chance that erratics originated in areas of active subglacial erosion.

We have selected our proposed field sites specifically because they display abundant moraines. These deposits occur in areas characterized by surface ablation, where englacial debris becomes concentrated at the ice margin. In contrast to other exposure-dating studies, which focused on bedrock nunataks mantled by thin drift, it is extremely unlikely that any ice sheet advances over our field sites were not accompanied by deposition of moraines or drift. Thus, although this leads to potentially complex stratigraphy, careful mapping and stratigraphic observations generally can reveal how many episodes of ice cover affected a potential sample site. This information, in turn, will guide our sampling strategy for dating to focus on sites with less potential ambiguity in interpreting measured nuclide concentrations. Additionally, we note that other geomorphic and sedimentological observations can help address prior exposure: for example, comparison of boulder lithologies with outcropping bedrock on nearby nunataks can identify samples that are most likely to be subglacially derived. Finally, we propose to exploit the relative speed and efficiency of cosmogenic 21Ne measurements in order to generate statistically representative age populations for specific landforms (discussed below), thereby helping to identify prior-exposed samples (outliers).

Landform degradation and surface erosion. Considering the antiquity of the landforms we aim to date, post-depositional disturbance by erosion or periglacial processes, as well as erosion of boulder surfaces, presents a challenge to accurate exposure-dating. For example, although surface erosion and weathering rates in Antarctica are the lowest on Earth (commonly 10-50 cm/Ma: Balco & Shuster, 2009), even 10 cm/Ma sustained over the durations of the Pliocene would have a significant effect on nuclide concentrations, leading to underestimation of true exposure age if left unaccounted. As discussed below, our strategy for addressing landform degradation and surface erosion relies on careful geomorphic observation and mapping, and also on more comprehensive analysis relative to previous studies. Observational and mapping criteria are well-established from previous studies and include consideration of surface stability and weathering characteristics of available lithologies. Novel sampling and analytical approaches that we will employ rely on (i) the increased speed, lower cost, and higher throughput of 21Ne measurements in quartz relative to 10Be or 26Al, and (ii) the fact that the extremely high nuclide concentrations characteristic of Pliocene-age deposits permit the use of multiple nuclides to gain data on both age and erosion rate of rock surfaces.

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In contrast to 10Be analysis, analysis of cosmogenic 21Ne in quartz requires relatively small aliquots (< 0.5 g) of the target mineral followed by ~15 minutes of heating under vacuum to extract Ne. Concentrations of Ne then are measured using a simple noble gas mass spectrometer rather than an accelerator mass spectrometer. Thus, sample preparation and analysis for 21Ne is much faster and cheaper. The BGC noble gas analytical system (see Facilities and Resources) is designed to make measurements quickly and cheaply for projects where large populations of ages would yield significant benefits. This is such a project. Currently, throughput of the BGC system is ~40 samples/week, so analysis of ca. 200 samples as proposed for this project is feasible (indeed, BGC completed 165 21Ne measurements for a recently completed project of similar scale (PLR-0838968, PIs Putkonen, Balco, and Schuster)). Generating tens, rather than a few (as in the case of 10Be alone), apparent exposure ages for each moraine should permit generation of statistically representative age distributions. This would (i) increase confidence in identifying outliers and (ii) permit comparison of observed distributions to those predicted by quantitative models (e.g., Applegate et al., 2009, 2010, 2012; Hallet & Putkonen, 1994). As our determination of landform stability will be critical for interpreting exposure ages, the generation of large data sets for each landform will greatly increase confidence in those interpretations. Additionally, the likely multi-million-year ages of the moraines are well-suited to 21Ne systematics: being stable, there is no loss of age sensitivity due to radioactive decay, as is the case for 10Be or 26Al at these ages. Additionally, high 21Ne concentrations mean that correction for U-Th-decay-produced 21Ne produced (Niedermann et al., 1993) adds minimal uncertainty to age estimates.

In addition to using 21Ne measurements to improve our understanding of exposure-age distributions, and thus landform stability, we will use the 21Ne/10Be nuclide pair to estimate both exposure age and erosion rate from rock surface samples. As pointed out by Lal (1991), a concentration of a single nuclide can be explained by infinite age/erosion-rate combinations. However, given concentrations of two nuclides with different half-lives, one can solve for age and erosion rate simultaneously. Gillespie and Bierman (1995) showed that, although true in theory, typical measurement uncertainties for commonly measured nuclides cause age/erosion-rate solutions to be poorly constrained in practice. However, the present study is an exception. Extremely high nuclide concentrations expected in our samples imply high-precision measurements (analytical uncertainties of ~2 % for 10Be and 2-3% for 21Ne), which in turn suggests that one can resolve unique solutions for age-erosion rate pairs. Figure 5 shows apparent exposure ages of 3-4.5 Ma for bedrock at Reedy Glacier based on 21Ne alone. However, paired 10Be-21Ne concentrations reveal the samples have been subjected to erosion rates of 5-10 cm/Ma and that their true exposure ages are 3.5-6 Ma. Applying this approach to the proposed study thus has the potential to advance our understanding both of former ice-sheet geometry and long-term erosion rates in the TAM.

Research plan and division of responsibilities PI Bromley will oversee all aspects of the investigation. He is an early career scientist bringing experience in Antarctic sedimentology and glacial-geologic mapping, and who is proficient in the collection and preparation of samples for surface-exposure dating. The focus of Bromley’s graduate work was reconstructing late-Cenozoic glacier fluctuations at Reedy Glacier (MS; Bromley et al., 2010) and Scott Glacier (PhD; Bromley et al., 2012), to assess the long-term evolution of the WAIS. He also has applied a coupled glacial-geologic--surface-exposure approach to late-Pleistocene climate records from the Peruvian Andes

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(Bromley et al., 2009, 2011a,b) and is leading similar investigations in Colombia, Scotland, and Maine. Co-PI Greg Balco is a glacial geologist and geochronologist specializing in cosmogenic-nuclide geochemistry, with six field seasons experience in Antarctica. He will (i) participate in fieldwork and (ii) and train and assist Bromley and his students in carrying out cosmogenic-nuclide measurements. Specifically, he will oversee the training and supervision of the graduate student in cosmogenic 21Ne measurements. The field party will comprise five people, including a UMaine graduate (MS) student who will carry out thesis fieldwork in Antarctica. We will be accompanied by two UMaine undergraduate field assistants (1 student per season) and a field safety expert. Fig. 5. Paired 10Be-21Ne

measurements in quartz from

bedrock samples at Quartz Hills,

Reedy Glacier. Heavy black lines

delimit the simple exposure region

(Lal, 1991; Granger, 2006). Lighter

lines are contours of exposure age

(solid) and surface erosion rate

(dashed). Asterisks indicate nuclide

concentrations have been

normalized to respective production

rates at each sample sites, allowing

comparison of data from multiple

sites on the same diagram.

Additionally, because 21Ne is stable,

the normalized 21Ne plotted as the

x-axis is the apparent exposure age of the sample. Ellipses are 68% measurement uncertainty

regions. Apparent 21Ne ages of these samples are 3-4.5 Ma. If these were true exposure ages, and

assuming no surface erosion, measured 10Be/21Ne ratios would lie on the simple-exposure line. In fact,

they are lower, consistent with surface erosion rates greater than zero. Thus, paired measurements

indicate exposure ages of 3.5-6 Ma at erosion rates of 4-12 cm/Ma, significantly older than apparent

exposure ages inferred from a single nuclide. If this approach is applied to boulders from a single

moraine, as proposed, we would expect to infer similar exposure ages, but a range of erosion rates;

this expectation provides an important test for internal consistency.

During year 1, fieldwork will focus on Ottway Massif and the Dominion Range. In year 2 we will work at Roberts Massif and Bennett Platform. The party will work together or in groups, as required, on both mapping and surface-exposure sampling. We will work out of fixed tent camps with one camp move per season. As noted in the Logistical Requirements, sites are accessible either by helicopter from the proposed Shackleton Glacier Camp or by fixed-wing aircraft from McMurdo. Following each field season, glacial-geologic data will be compiled and drafted into maps, and samples will be prepared for chronologic analysis. In year 1, Bromley will travel to the University of Washington to prepare the first batch of samples for 10Be/26Al measurement (see John Stone letter of support). Subsequent 10Be/26Al samples will be prepared at UMaine (see Facilities, Equipment, and Other Resources). Balco, who has expertise in 10Be/26Al sample preparation, will also assist Bromley in the streamlining and quality control of the UMaine lab. In years 1 and 2, the graduate student

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will travel to BGC for 21Ne measurements. As this is likely to be a significant part of the analytical effort, we anticipate the student spending 1-2 months per visit, during which they will also have the opportunity to interact with other BGC and UC-Berkeley students and researchers. In year 3 we will complete analyses, prepare publications, and present results. Potential Outcomes: Intellectual Merit The primary outcome of this project will be the chronologic constraint of pre-late-Pleistocene ice-sheet fluctuations in the central TAM. These data will be used to identify periods during which the EAIS was at least as extensive as today: if we find moraine sequences clearly Pliocene in age, it will (i) indicate that the ice sheet was present and (ii) provide concrete constraint on ice configuration at that time; if we find no deposits that could possibly date to the Pliocene, given uncertainties in the ages, it would suggest the EAIS was less extensive than present, potentially supporting the collapse hypothesis. Thus, our anticipated results would provide a terrestrial benchmark for EAIS geometry in long-term Antarctic ice-sheet models. Finally, this work will provide a crucial foundation for future efforts to constrain past ice-sheet configuration using evidence from below the present ice surface, since that approach first will require detailed resolution of those portions of the record preserved above the present surface. Potential Outcomes - Broader Impacts This investigation will continue a strong commitment to education and scientific training (see Biographical Sketches and Results from Prior NSF Support). Not only will the project be headed by a beginning investigator (Bromley), one graduate student will receive extensive field and analytical training, traveling to BGC for comprehensive training in measurement of 21Ne. Also, two undergraduate students will receive field experience and employment in sample preparation. Additionally, data are routinely incorporated into undergraduate and graduate courses in Quaternary and glacial geology, climate change, and isotope geochemistry at UMaine (taught and co-taught by Bromley). UMaine supports students from rural Maine, typically an economically disadvantaged area (Maine is within the lower 1/3 of states economically). This work also has significant broader impacts beyond the scientific fields. We are committed to K-12 outreach: In previous projects Bromley has engaged with students to discuss Antarctica and the impacts of climate change on the continent. Students follow our progress on a website where we post journal entries. As part of this project, we will continue this interactive engagement with Maine schoolchildren, with an emphasis on reaching out to those in economically challenged and educationally underserved areas. Bromley also incorporates his research into regular classes for the Maine Senior College, part of the University of Maine's Lifelong Learning initiative, and for the Maine School of Science and Mathematics (MSSM). Moreover, we recently created a public, interactive, web-based learning system and kiosk, based on a previous Antarctic research project (funded by the education division of NSF). As this is a modular system, new projects, such as proposed here, can be added easily. Results from prior NSF support Collaborative Research: Grounding-line retreat in the southern Ross Sea- Constraints from Scott Glacier. PIs B. Hall (UMaine), J. Stone (U. Washington), & H. Conway (U. Washington). $582,184 between two institutions. 8/1/07-07/31/11. This project supported

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the Ph.D. research of G. Bromley. We mapped and dated deposits adjacent Scott Glacier, TAM, in order to reconstruct LGM-present glacier history in the southern Ross Sea Embayment. This work resulted in four high-resolution geomorphic maps of key sites along the glacier and a detailed reconstruction of glacier profile at the last glacial maximum (LGM). Our results show a pattern of asymmetric thickening during the LGM, being greater at the glacier mouth than at the head, suggestive of varying thickness of the WAIS. A complete vertical 10Be transect at Mt. Rigby, at the current grounding line, indicates (i) that the peak was overrun by ice at the LGM, (ii) began to emerge ~12.5 ka, and (iii) that rates of post-LGM thinning were gradual until ~8 ka. The majority of thinning occurred between 9 and 4 ka. This record indicates that significant deglaciation of the southern Ross Sea was delayed until Holocene time, in keeping with existing models of grounding-line recession. Concurrently, we developed a recessional transect at Cox Peaks based on 14C dating of algae that lived in former ice-dammed ponds. Both the Mt. Rigby and Cox Peaks data suggest that Scott Gl. attained its present-day profile by ~2 ka. Intellectual Merit: The maps and LGM surface-profile produced as part of this project provide geologic constraint of ice thickness in West Antarctica, and have since been incorporated into modeled reconstructions of LGM Antarctic configuration. In conjunction with the deglacial chronology, these data provide compelling evidence against Antarctica being the likely source of meltwater pulse 1A. Broader Impacts

: This work supported two undergraduate and two graduate students, produced eight abstracts, and led to the following publications so far: Bromley et al. (2012); Hall et al. (in prep.); Stone et al. (in prep.). Additionally, results have been used as the basis for K-12 outreach by PI Hall in the central Maine area, and by G. Bromley in his continuing classes for the Maine Senior College and Maine School of Science and Mathematics.

Fig. 6. Glacial-geomorphic map of Cenozoic deposits Taylor Ridge, Scott Glacier (from Bromley et

al., 2012). Such maps form the fundamental basis for ice-sheet reconstruction and chronology, and

therefore will be produced as part of the proposed investigation.

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Collaborative Research: Last Glacial Maximum and deglaciation chronology for the Foundation Ice Stream and southeast Weddell Sea Embayment. PIs G. Balco (BGC), C. Todd (PLU), & H. Conway (U. Washington). $681,620 among three institutions. 8/15/2009 - 9/30/2013. This project involved fieldwork in the Pensacola Mountains. Its primary aim is to reconstruct LGM-present ice sheet change in the Weddell Sea sector by generating an exposure-age chronology of ice-thickness changes at nunataks adjacent the Foundation Ice Stream (FIS). We mapped glacial deposits and collected exposure-age data from the Schmidt Hills, near the present grounding line; the Williams Hills, ~70 km upstream; and the Thomas Hills, a further 100 km upstream. The Williams Hills results are relatively straightforward, showing an LGM ice-surface elevation >500 m above present, and 500 m of thinning between 11 and 5 ka.. The Schmidt Hills chronology is more complex; our extensive sampling and analytical campaign yielded only exposure ages > 0.2 Ma and no evidence for LGM ice cover. Field observations and glaciological constraints suggest this site was covered by cold-based ice, thereby preserving deposits of much older advances. Results from the Thomas Hills are partially complete and represent an intermediate scenario: a minority of Holocene-age erratics overly much older deposits and indicates at least 200 m of LGM thickening at this site. Intellectual merit: The exposure-age data so far generated by this project provide new information about LGM-to-present ice sheet change in the Weddell Sea region, an area where little such information is available and that contributes significant uncertainty in understanding the Antarctic contribution to past sea level change. Broader impacts:

During this project, co-PI Todd transitioned to a tenure-track position at PLU. Support was provided for U. Washington graduate student K. Huybers and, partially, for UMaine graduate student S. Campbell, as well as three PLU undergraduate capstone projects (two included field participation). All but one (Balco et al., 2012) of the publications and abstracts resulting from this project so far are student-authored (Vermeulen et al., 2011; Hegland et al., 2012a,b; Huybers et al., 2012; Campbell et al, 2013). Additional broader impacts included K-12 outreach in ME and WA.

Fig. 7. 10Be exposure ages in glacially

transported clasts from the Williams Hills,

Pensacola Mountains, show that the

Foundation Ice Stream was at least 500 m

thicker than present during the early

Holocene and thinned steadily between 10

and 5 ka. The present ice surface at the site

is near 450 m elevation. Color-coding and

associated names refer to individual

nunataks within the Williams Hills. Inset

shows location.

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