Constraints from Geologic Observations, Cosmogenic Nuclides and Ice Sheet Modeling

Earth and Planetary Science Letters 337–338 (2012) 243–251

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Earth and Planetary Science Letters

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Miocene to recent ice elevation variations from the interior of the West Antarctic ice sheet: Constraints from geologic observations, cosmogenic nuclides and ice sheet modeling

Sujoy Mukhopadhyay a,n, Robert P. Ackert Jr.a, Allen E. Pope a,b, David Pollard c, Robert M. DeConto d a Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA b Scott Polar Research Institute, University of Cambridge, Lensﬁeld Road, CB2 1ER, England c Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802, USA d Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA article info abstract

Article history: Observations of long-term West Antarctic Ice Sheet (WAIS) behavior can be used to test and constrain Received 10 January 2012 dynamic ice sheet models. Long-term observational constraints are however, rare. Here we present the Received in revised form ﬁrst constraints on long-term (Miocene–Holocene) WAIS elevation from the interior of the ice sheet 11 May 2012 near the WAIS divide. We use geologic observations and measurements of cosmogenic 21Ne and 10Be in Accepted 12 May 2012 bedrock surfaces to constrain WAIS elevation variations to 160 m above the present-day ice levels Editor: B. Marty o Available online 3 July 2012 since 7 Ma, and o110 m above present-day ice levels since 5.4 Ma. The cosmogenic nuclide data indicate that bedrock surfaces 35 m above the present-day ice levels had near continuous exposure Keywords: over the past 3.5 Ma, requiring average interior WAIS elevations to have been similar to, or lower than WAIS present, since the beginning of the Pliocene warm period. We use a continental ice sheet model to Pliocene simulate the history of ice cover at our sampling sites and thereby compute the expected concentration 21Ne 10Be of the cosmogenic nuclides. The ice sheet model indicates that during the past 5 Ma interior WAIS exposure dating elevations of 465 m above present-day ice levels at the Ohio Range occur only rarely during brief ice ice sheet model sheet highstands, consistent with the observed cosmogenic nuclide data. Furthermore, the model’s prediction that highstand elevations have increased on average since the Pliocene is in good agreement with the cosmogenic nuclide data that indicate the highest ice elevation over the past 5 Ma was reached during the highstand at 11 ka. Since the simulated cosmogenic nuclide concentrations derived from the model’s ice elevation history are in good agreement with our measurements, we suggest that the model’s prediction of more frequent collapsed-WAIS states and smaller WAIS volumes during the Pliocene are also correct. & 2012 Elsevier B.V. All rights reserved.

1. Introduction mapping can provide insights into past WAIS geometry on timescales ranging from millennial to millions of years. Such reconstruc- The West Antarctic Ice Sheet (WAIS) is the only marine-based ice tions during warmer intervals of Earth history can provide potential sheet and is susceptible to rapid collapse in response to increasing analogs for future WAIS behavior resulting from sea level changes air and sea temperatures (Jenkins et al., 2010; Joughin and Alley, and increased greenhouse gases. 2011; Rignot et al., 2008). Satellite-based altimetry and gravity data ProjectingthefuturefootprintoftheWAISinresponsetoglobal show rapid thinning and retreat of outlet glaciers in the Amundsen warming requires physical models of ice sheet behavior and geologic Sea sector over the past two decades (Rignot et al., 2008, 2011; evidence to test and calibrate these models (Ackert et al., 2011; Naish Wingham et al., 2009). Concern has been heightened by the et al., 2009; Pollard and DeConto, 2009). The conﬁdence in the acceleration of outlet glaciers on the Antarctic Peninsula following predictions of ice sheet models is strengthened when the models disintegration of their buttressing ice shelves (Rignot et al., 2008). accurately reproduce past WAIS behavior that is independently However, given the short observation period, it is not known if the constrained by observations. Reproducing WAIS behavior during the changes in the Amundsen Sea sector are unusual on century to Pliocene is of particular interest because it is the most recent period in millennial timescales. Cosmogenic nuclides, combined with ﬁeld which global temperatures were warmer than present and atmo-

spheric CO2 concentrations may have been slightly higher than current levels of 380 ppmv (Ravelo et al., 2004; Raymo et al., 1996). n Corresponding author. However, WAIS geometry and ice volumes during the Pliocene E-mail address: [email protected] (S. Mukhopadhyay). remain poorly constrained by observations. During the last glacial

0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2012.05.015 244 S. Mukhopadhyay et al. / Earth and Planetary Science Letters 337–338 (2012) 243–251 maximum, the WAIS did not approach equilibrium (Ackert et al., The escarpment exposes granitic basement overlain by sand- 1999; Price et al., 2007; Waddington et al., 2005). For example, while stones and tillites. The WAIS terminates along the base of the WAIS retreat in the Ross Sea section was underway by 14–15 ka escarpment with ablation zones in the lee of Discovery and (Clark et al., 2009), maximum ice elevations in interior WAIS was Darling Ridge, promontories that project northward from the reached during the early Holocene (11 ka) as a result of increased escarpment. The nunataks mark ridges extending northward snow accumulation (Ackert et al., 1999, 2007, 2011). Did increased beneath the WAIS from the escarpment. Since little ice flows snow accumulation lead to increased interior ice elevations during over the escarpment, ice levels along the base of escarpment are the Pliocene warm period? Or did the WAIS completely or partially determined by the regional WAIS elevation (Fig. 1). Glacial collapse because of higher rates of sub-ice-shelf ocean melting erratics on the ridges record higher WAIS elevations (Ackert resulting from warmer ocean temperatures (Pollard and DeConto, et al., 2007; Mercer, 1963). 2009, 2012a,b)? Discovery Ridge consists of a series of benches and rectilinear Observations from the ANDRILL sediment core recovered from slopes rising to the level of the escarpment (Figs. 1–3). The upper beneath the Ross Ice Shelf document frequent and extended bench on Discovery Ridge (Fig. 3a), 160 m above the adjacent occurrences of open marine conditions in the Ross embayment WAIS, is formed on a medium to coarse-grained, cross-bedded during the Pliocene, suggesting WAIS collapse was common (Naish sandstone unit with thick (up to 1 cm) weathering rinds. The et al., 2009). While the sediment core data document the dynamic preservation of sandstone tafoni, delicate rock weathering fea- nature of the WAIS and its sensitivity to small perturbations in the tures less than 10 cm in thickness, along the edge of the upper climate system, they do not directly constrain WAIS footprint and bench indicate that ice has not overrun the bench since their volume during the non-collapsed states in the Pliocene. However, formation. The lower bench on Discovery Ridge (Fig. 3b), 110 m additional geologic evidence, such as reconstruction of past Antarc- above the adjacent WAIS, consists of an exhumed peneplain tic ice sheet elevation using field mapping and cosmogenic nuclides, beveling the granitic basement and is overlain by a friable can be used to constrain past ice sheet thickness (Ackert et al., 2007, sandstone unit. A curving moraine, consisting of granite boulders 2011; Bentley et al., 2010; Lilly et al., 2010; Mackintosh et al., 2007; and cobbles, many with cavernous weathering on one or more Stone et al., 2003; Todd et al., 2010). Here, we present the first sides, follows the eastern edge of the bench and marks the last observational constraints on interior WAIS elevation over million WAIS highstand at 11 ka (Ackert et al., 2007). year timescales from the Ohio Range (851S; 1141W) in the Trans- Tuning Nunatak projects 65 m through the WAIS as it flows antarctic Mountains (TAM). We then compare our observations with westward around Darling Ridge (Figs. 1, 2 and 4a). The granitic a simulation of interior WAIS elevation history from a continental bedrock composing the nunatak is cavernously weathered with ice sheet model to evaluate the model’s prediction of ice elevation the areas between the weathering pits reduced to delicate history and to better understand WAIS history during the Pliocene. features only several centimeter thick (tafoni), particularly on the eastern side. However, clear evidence of cold-based glaciation exists; glacial erratics occur within the weathering pits and the 2. Samples more delicate tafoni have been broken off on the eastern side. The Bennett Nunataks outcrop within the large ablation area 2.1. Location and methods on the west (leeward) side of Daring Ridge rising 35 and 85 m above the surrounding ice (Figs. 1–2 and 4b and c). The nunataks The Ohio Range forms an escarpment at the southern end of are surrounded by large moats up to 40 m deep and are composed the TAM that separates the WAIS from the East Antarctic Ice Sheet of course-grained granites. The granites are cavernously weath- (EAIS) and is located near the present-day WAIS divide (Fig. 1). ered all the way from the Nunatak summits to the base of the

Fig. 1. (a) Map of West Antarctica showing the location of the Ohio Range. (b) Sketch map of the study area based on the USGS 1:250,000 Ohio Range topographic reconnaissance map. Blue ice ablation areas are lightly shaded. (c) Mosaic of oblique aerial photographs showing the WAIS margin along the Ohio Range Escarpment and the sample locations. S. Mukhopadhyay et al. / Earth and Planetary Science Letters 337–338 (2012) 243–251 245

Fig. 2. Location of bedrock samples showing elevation above adjacent WAIS surface (a) benches on Discovery Ridge, (b) the Tuning Nunatak, and (c and d) the Bennett Nunataks.

Fig. 3. Discovery Ridge showing sample locations on the upper and the lower bench. (a) View of the upper sandstone bench on Discovery Ridge. The sandstones show weathering rinds and there are no erratics, striations, or any other geologic evidence of glaciation on this bench. (b) View of the lower sandstone bench showing the 11 ka moraine on the outer limit of the bench. There is no evidence of glaciation in the inner part of the bench. moats. Like Tuning Nunatak, erratics are found from the summits concentration of stable 21Ne. Hence, samples that have undergone to ice edges of the Bennett Nunataks indicating that the nunataks prolonged cover fall below the steady-state curve in the complex have been ice covered. exposure field. Consequently, the paired 21Ne-10Be system (Fig. 5) To constrain long-term variations in WAIS elevation near the can be used to determine whether bedrock surfaces have been ice ice divide, we used cosmogenic 10Be and 21Ne measurements in covered for significant periods of time (Lal, 1991). For surfaces quartz from granite bedrock surfaces from the peaks of Bennett-I, that have undergone continuous cosmic ray irradiation, the Bennett-II, and Tuning Nunatak (Figs. 1, 2 and 4; Table 1). In paired 21Ne–10Be system allows both the exposure age and addition we sampled sandstones from the two benches cut into erosion rate to be determined (Lal, 1991). As a result, cosmogenic the Ohio Range Escarpment at Discovery Ridge (Figs. 1–3; nuclide data from bedrock surfaces at different elevations with Table 1). 21Ne is a stable nuclide while 10Be is radioactive, and respect to present ice levels provide constraints on past WAIS both nuclides are produced when bedrock surfaces are exposed to variability. cosmic radiation. In plots of 21Ne versus 10Be/21Ne, samples undergoing continuous exposure at a constant erosion rate 2.2. Ne measurements (defined as simple exposure) follow curved trajectories bounded by the steady-state erosion curve and the zero erosion rate curve. Approximately 200–250 mm sized quartz grains were leached

The steady-state curve deﬁnes the loci of points where the in a 4:1 HF:HNO3 mixture to dissolve at least 30% of the grains production of the cosmogenic nuclides are balanced by loss (440 mm thick rind). After thoroughly rinsing in deionized water, through erosion and radioactive decay. When bedrock surfaces samples were loaded into Nb packets. Neon measurements were are shielded from cosmic radiation by ice cover, the production of replicated in a subset of the samples. For these samples, chemi- cosmogenic nuclides ceases, which leads to a decrease in 10Be cally puriﬁed quartz grains from the Be preparation procedure concentrations through radioactive decay but does not affect the were picked free of visible inclusions under a binocular 246 S. Mukhopadhyay et al. / Earth and Planetary Science Letters 337–338 (2012) 243–251

Fig. 4. Sample location on the three nunataks. (a) Bedrock sample from the peak of the Tuning Nunatak showing staining and weathering along joints. Compared to the Bennett Nunataks, the bedrock surface is more fractured. (b) Bedrock surface sampled from the peak of the Bennett-I Nunatak showing dark brown stain and cavernous weathering along joints. (c) Bedrock surface sampled from the peak of the Bennett-II Nunatak. The bedrock surface is smooth, with a thick dark brown weathering rind and shallow pits. Bennett-I Nunatak is seen in the background. Note the deep moats around both nunataks. All three nunataks have erratics and evidence of quarrying indicating that they have been overrun by ice.

Table 1 Bedrock sample data from the Ohio range.

Sample Latitude S Longitude W Elevation Lithology Thickness Shielding factor (m) (cm)

Upper bench ODY-05–033 84143.7320 114113.1580 1728 Sandstone 3.0 0.999 ODY-05–034 84143.7320 114113.1580 1728 Sandstone 4.0 0.999 ODY-05–035 84143.7320 114113.1580 1728 Sandstone 3.5 0.999

Lower bench ODY-05–062 84143.6830 114113.5610 1680 Sandstone 6.0 0.994 ODY-05–063 84143.6830 114113.5610 1680 Sandstone 4.0 0.994

Nunataks OBN-05–027 84147.2300 116123.9580 1486 Granite 5.0 0.999 OBN-05-066 84146.7980 116128.2700 1451 Granite 5.0 0.999 OTN-05–116 84143.8800 115157.8860 1540 Granite 4.0 0.999

microscope before loading them into Nb packets. Gases evolved spectrometer tuning conditions. 40Arþþ/40Ar þ ratios were þþ þ from the samples during step-heating in a high-vacuum furnace 0.10870.005 and 0.06470.005 and the CO2 /CO2 ratios were were purified by sequential exposure to hot and cold SAES getters, 0.00870.001 and 0.00470.001 for the two batches, respectively. and Ne was separated from He by trapping Ne onto a cryogenic While the doubly-charged Ar and doubly-charged CO2 ratios cold-finger at 32 K. Neon was liberated by heating the cold-finger varied with tuning conditions, significant variations were not to 74 K and then inlet to the Nu Noblesse noble gas mass observed either as a function of the Ar, CO2,orH2 partial pressure spectrometer. The neon isotopes were measured simultaneously in the mass spectrometer (See Supplementary Figures S1 and S2 with three discrete dynode multipliers operating in pulse count- in Appendix). The neon isotopic compositions were determined ing mode. 21Ne was measured on the axial multiplier fitted with through sample-standard bracketing, with the standard being air. an energy filter, while 20Ne and 22Ne were measured on the low The 1s variation on air standards that spanned the range of and high mass multipliers, respectively (Mukhopadhyay, 2012; measured sample signals were 0.2–3.0% for 22Ne/20Ne ratios and Parai et al., 2009). Neon measurements were corrected for 0.5–4.5% for 21Ne/20Ne ratios. Procedural blank values were isobaric interferences from doubly-charged Ar and doubly- o2.8 105 atoms for 21Ne, o1% of the measured signal. The charged CO2. A liquid nitrogen trap in the mass spectrometer samples had Ne isotopic compositions statistically indistinguish- was used to keep Ar and CO2 backgrounds low. The measure- able from the air-cosmogenic mixing line (Figure S3 in Appendix) ments were made in two batches with different mass and thus, cosmogenic 21Ne concentrations were calculated S. Mukhopadhyay et al. / Earth and Planetary Science Letters 337–338 (2012) 243–251 247 assuming a two-component mixture of air and cosmogenic Ne. constant of 5.1 107 a1 (Nishiizumi et al., 2007). Combined Table 2 lists the cosmogenic 21Ne concentrations in the samples process and carrier blanks for 10Be were 1.070.6 105 atoms. and the complete step-heating data are presented in Supplemen- Table 2 lists the 10Be concentrations. tal Table S1 in Appendix.

2.3. 10Be measurements 3. Results and discussion

Techniques for the extraction of 10Be from quartz and its All but one sample plot in the region bounded by the zero- measurement by AMS are well established. Samples were pre- erosion and the steady-state curves (Fig. 5). Therefore, we present pared at Oregon State University following methods of Goehring both exposure age and erosions rate assuming continuous irra- et al. (2008). AMS measurements were carried out at the PRIME diation of the surfaces (Table 2). The oldest exposure ages occur lab AMS facility, Purdue University and are referenced to the on the upper bench of the Discovery Ridge (Fig. 3a) and range 1 07KNSTD isotope ratio standard and the associated 10Be decay from 6.8 to 7.9 Ma with erosion rates of 4.9–5.7 cm Ma (Table 2). The bedrock exposure ages from 50 m below on the lower Discovery Bench at 110 m above present ice levels (Fig. 3b) 0.65 also date to the Late Miocene, 5.4 Ma with erosion rates up to Upper Discovery Ridge 1 Lower Discovery Ridge 29 cm Ma (Table 2). Although the two samples on the lower Bennett-I Nunatak bench were collected at the same elevation and within a few Bennett-II Nunatak Tuning Nunatak meters of each other, one sample plots near the steady-state line 0.55 in the simple exposure field, while the other plots just below the steady-state line in the complex exposure field (Fig. 5). Given that the 10Be/21Ne ratios are identical, the most likely explanation for the differences in concentration is that the sample in the complex Zero-erosion 0.45 exposure line field recently lost 7 cm from the bedrock surface, consistent with the friable nature of this sandstone surface (See Supplemen- tary Figure S4 in Appendix). The exposure ages of the bedrock Steady-state line (spallation and muons) surfaces from the nunataks are younger than those from the 0.35 Discovery benches and range from 3.4 to 4.8 Ma with erosion rates ranging from 10.4 to 30.5 cm Ma1 (Table 2).

3.1. Long-term erosion rates 0.25 123456Long-term erosion rates of 10–250 cm Ma1 have been pre- viously reported for the Antarctic Dry Valleys (Balco and Shuster, 2009; Brown et al., 1991; Middleton et al., in review; Nishiizumi Fig. 5. Plot of the sample cosmogenic 21Ne concentrations vs. 10Be/21Ne. In order to compare samples from different elevations on the same plot, the 10Be and 21Ne et al., 1991; Schafer et al., 1999; Summerfield et al., 1999). These concentrations in each sample have been normalized by their respective sample- low erosion rates in the Dry Valleys have been generally used to specific production rates, indicated by the superscripted stars (Balco and Shuster, argue for a stable hyperarid polar environment since the Late 2009). The spallation and muon production rates have been scaled for altitudes in Miocene. The erosion rates calculated from the 10Be and 21Ne data Antarctica using the scheme implemented by Balco et al. (2008) and using a in interior WAIS at the Ohio Range are at the low end of the values 21Ne/10Be production ratio of 4.08 for Antarctica (Balco and Shuster, 2009). All of 1 our samples are consistent with a simple exposure history, suggesting that ice from the Dry Valleys (Table 2). Erosion rates of 10–30 cm Ma cover must be minimal. Error ellipses represent the 68% confidence interval. for the granitic nunataks (Table 2) are quite low. Furthermore,

Table 2 Exposure ages and erosion rates of bedrock samples from the Ohio Range, West Antarctica.

a 21 10 b b Sample name Location Elevation NeC Be Exposure age Erosion rates (m) ( 108 at. g1)( 107 at. g1) (Ma) (cm Ma1)

ODY-05-033 Discovery ridge upper bench 160 5.8770.09 4.2870.08 þ 0:11 þ 0:21 7:90:17 5:50:20 ODY-05-033 repc 5.9370.09 ODY-05-034 5.4170.08 4.2670.09 þ 0:1 þ 0:21 6:80:12 4:90:20 ODY-05-035 5.6370.09 4.2270.08 þ 0:13 þ 0:21 7:60:13 5:70:22 ODY-05-062d Discovery ridge lower bench 110 1.5570.04 2.0470.04 –– 2.0370.08 ODY-05-063 1.7970.04 2.3570.04 5.4 þ 0:7 28:70:64 ODY-05-063 repc,e 1.7670.04 2.3170.07

Nunatak surfaces OBN-05-027 Bennett-1 85 2.1470.08 2.5670.07 þ 2:5 þ 0:64 4:50:9 16:40:64 OBN-05-066 Bennett-II 35 2.1070.06 2.6870.07 þ 0:6 þ 0:02 3:40:5 10:40:02 OTU-05-116e Tuning 65 1.5070.05 2.0970.07 4.8 þ 1:02 30:50:91

a Elevation of bed rock surfaces are with respect to present day ice levels. b Exposure ages and erosion rates were calculated using the formulism presented by Lal (1991). The spallation and muon production rates were scaled using the scheme implemented by Balco et al. (2008) and using a 21Ne/10Be production ratio of 4.08 for Antarctica (Balco and Shuster, 2009). c Rep stands for replicate analyses. d Age and erosion rate was not calculated as the sample plots below the steady-state line although the error ellipse overlaps with the steady-state line. e Formal error bars are not presented as the error ellipse overlaps with the steady-state line (Fig. 5). 248 S. Mukhopadhyay et al. / Earth and Planetary Science Letters 337–338 (2012) 243–251 average erosion rates of 4.9–5.7 cm Ma1 for the sandstone bed- generate the cavernously weathered surfaces at erosion rates rock of the upper bench at Discovery Ridge are amongst the typical of Antarctica (Table 2), present-day ice levels at the Ohio lowest reported erosion rates from Antarctica. Such low erosion Range cannot be unusually low. Hence, there is no geologic and rates suggest a continuous polar environment in interior WAIS cosmogenic nuclide evidence that interior WAIS thickened for since 7 Ma and indicate that the Pliocene warm period in interior prolonged periods during the Pliocene in response to increased WAIS was not characterized by pulses of rapid erosion. snow accumulation. Rather, average WAIS elevations in the interior must have been at, or below, present-day ice levels since 3.2. Constraints on interior WAIS elevations from cosmogenic the Pliocene. Furthermore, the cosmogenic data and the geologic nuclides observations indicate that the highest ice elevation over the past 5 Ma in interior WAIS occurred during the last WAIS highstand at Theobservationthatthecosmogenicnuclidedata(10Be and 21Ne) 11 ka, which is consistent with increasing ice thickness since fall within the simple exposure field (Fig. 5) demonstrates that the the Pliocene. bedrock surfaces in the WAIS interior have undergone near continuous exposure since Miocene–Pliocene times. The upper bench lacks 3.3. Comparison of observed cosmogenic nuclide concentrations geologic evidence of glaciation (Figs. 2aand3a), consistent with the with predictions from a continental-scale ice sheet model cosmogenic data indicating long uninterrupted exposure. Thus, we infer WAIS levels have not been as high as 160 m above present-day We now compare the ice elevation constraints inferred from ice levels since 7 Ma indicating that interior WAIS did not thicken the cosmogenic nuclides with predictions of WAIS elevation at significantly during the warmer Pliocene climates in response to the Ohio Range over the last 5 Ma from a continental-scale ice- increased snow accumulation. sheet model (Fig. 6)(Pollard and DeConto, 2009, 2012a,b). This The cosmogenic nuclide data from the lower bench at Dis- model uses a hybrid combination of the scaled dynamical equa- covery ridge also indicate continuous irradiation of the surface tions for grounded/shearing and floating/shearing flows, and a since 5.4 Ma. We note that the sandstone samples were collected parameterization of ice flux across grounding lines that make 50 m distal from, and at the same elevation as the boulder long-term paleoclimatic applications feasible while still capturing moraine deposited on the outer margin of the bench during the realistic grounding-line migration. In the ice sheet simulation, last WAIS highstand at 11 ka (Fig. 3b; Ackert et al., 2007). climatic and oceanic forcing are parameterized mainly as a However, there are no erratics or any other evidence of glaciation function of the stacked benthic d18O record (Lisiecki and Raymo, on the bench beyond the 11 ka moraine or on the slope between 2005). The model version used here is similar to that presented in the upper and lower bench. The highstand at11 ka therefore, Pollard and DeConto (2009) but includes a new inversion scheme must have barely overtopped the lower bench. The geologic to deduce basal sliding coefficients and more physical parameter- evidence combined with the cosmogenic nuclide data suggest izations of ice-shelf calving and sub-ice oceanic melting. Large- that the moraine on the margin of the bench at 110 m above scale results and volumetric time series are essentially as in present-day ice levels represents the highest WAIS highstand at Pollard and DeConto (2009), although the basal sliding scheme the Ohio Range since 5.4 Ma. Hence, the cosmogenic nuclide data, notably improves ice thicknesses over parts of modern West indicating continuous exposure in the inner part of the bench, and Antarctica (Pollard and DeConto, 2012a, b)(Fig. 6). the glacial geology, further restrict WAIS thickening to o110 m The model’s simulations of Antarctic ice variations during the above present-day ice levels during the Pliocene warm period. Pliocene and Pleistocene agree well with ANDRILL core records In contrast to the benches on Discovery Ridge, the nunataks (Naish et al., 2009) and data on other periods including MIS 31 were likely overrun by the WAIS multiple times since erratics (DeConto et al., in press) and the last deglacial retreat (Ackert et al., with exposure ages 4125 ka occur on the nunataks (Ackert et al., 2011; Mackintosh et al., 2011; Pollard and DeConto, 2009). Overall, 2011). However, because the samples plot in the simple exposure the model behavior is consistent with limited knowledge of West field, the duration of cover has to be short in comparison to the Antarctic Ice Sheet collapses and regrowths during the last several total exposure time, suggesting that the nunataks were only ice million years, although the association of some collapse events with covered during brief WAIS highstands. In particular, the bedrock specific interglacials may be model dependent. surface from the peak of Bennett-II Nunatak is only 35 m above The nearest grid point to the Ohio Range in the ice sheet present-day ice levels and yet the cosmogenic nuclides indicate simulation is at 84.7941S and 117.4741W, 25 km away. In the that the surface must have been mostly exposed since the simulation, the long-term mean WAIS level at the Ohio Range is at Pliocene (Fig. 5). Constraints can be placed on the total duration or below the present-day levels since 5 Ma, and the predicted of cover by assuming a relatively simple exposure history, such as present-day ice elevation is remarkably close to observed ice continuous exposure followed by a single episode of cover and levels (Fig. 6). Simulated variations in WAIS elevation near the recent exposure during the last deglaciation. Under this scenario, Ohio Range are characterized by Milankovitch frequency changes and assuming an unrealistic zero erosion rate for the Bennett-II in ice elevation of up to 300 m superimposed on a longer-term bedrock surface, the initial exposure was 2.5 Myrs long followed trend of increasing WAIS elevations and total WAIS volume since by 160 kyrs of cover. In other words, the surface was covered the Pliocene (Fig. 6). Thus, the ice sheet simulation indicates that about 6% of the time. We note that assuming a zero erosion rate interior WAIS thickness, as well as total WAIS volume was lower yields the minimum duration of first exposure and the maximum during the Pliocene. Moreover, the durations of the WAIS high- duration of cover, so the conclusion of brief cover for the Bennett- stands above present-day ice levels are short, consistent with the II nunatak surface based on the cosmogenic nuclides is robust. cosmogenic data. Hence, like the geologic and cosmogenic nuclide More plausible exposure histories of the bedrock surfaces and the data, the ice sheet model does not support interior WAIS thicken- duration of cover will be presented when discussing the con- ing during the Pliocene warm period in response to increased tinental ice sheet model in Section 3.3. snow accumulation. In the ice sheet model, the Pliocene WAIS The geologic evidence from the Ohio Range also supports short volume is smaller as warmer ocean temperatures lead to greater duration of cover for the bedrock surfaces since the Pliocene. For rates of oceanic sub-ice-shelf melting. The reduced volume due to example, we observe cavernously weathered bedrock surfaces grounding line retreat outweighs any effects from increased exposed within moats around the nunatak that extends to 40 m snowfall during the Pliocene. Significantly, while the ANDRILL below the ice level. Since millions of years are required to data indicate more frequent occurrences of open ocean conditions S. Mukhopadhyay et al. / Earth and Planetary Science Letters 337–338 (2012) 243–251 249

1800

1700

1600

1500

1400

Ice Elevation1300 (m)

1200

32 ) 3 30 km 6 28

24 Ice volume (10 22

20 012345 Age (Ma)

Fig. 6. (a) Predicted ice surface elevation at the closest grid point (84.7941S and 117.4741W) to the Ohio Range over the past 5 Ma from a continental ice sheet model (Pollard and DeConto, 2009) using the new basal roughness distributions, ocean-melt and calving parameterizations. The black line represents a 50,000 year running mean, intended to highlight the long-term trend in elevation variations. The horizontal reference line corresponds to the elevation of the lower Bennett-II Nunatak with respect to the predicted present-day ice elevation of 1540 m. The present-day ice elevation at the Ohio Range is 1550 m, similar to the predicted present-day elevation from the ice sheet simulation. (b) Variation in total Antarctic ice volume over the past 5 Ma from the continental ice sheet model. Although collapsed WAIS volumes are similar, the ice volume of non-collapsed states increases markedly since the mid-Pliocene. in the Ross Sea during the Pliocene (Naish et al., 2009), and ice and the ice cover later in the exposure history results in the sheet modeling indicates more frequent collapse of the marine- nuclide trajectory moving into the complex exposure field based sectors of the WAIS during the Pliocene, the Ohio Range and (Fig. 7a). Thus, the predicted ice sheet elevations over the past other areas where the bed is above sea level remain glaciated 3.4 Ma are too high to fit the observed cosmogenic nuclide data during the past 5 Ma. for the Bennett-II surface even if a zero-erosion rate is assumed In the ice sheet simulation, a highstand of 100 m at the Ohio (Fig. 7a). Changing the age of first exposure and erosion rates does Range occurs during the last deglaciation at 11 ka, consistent with not improve the fit. If, however, the predicted elevations from the the geologic evidence (Ackert et al., 2007). Over the past 5 Ma, the ice sheet model are lowered on average by only 20–25 m, ice ice sheet model indicates intervals of ice elevation as high as the cover of the Bennett-II surface is reduced to 10% of the time and peak of the Bennett-I Nunatak, Z85 m above present-day ice a fit to the cosmogenic nuclide data is obtained with exposure levels, occur only during WAIS highstands associated with glacial ages ranging from 2.9 to 3.4 Ma and erosion rates from 1 to terminations. The Bennett-I nunatak is exposed 98% of the time 6cmMa1 (Fig. 7b). Hence, the cosmogenic nuclide data from the (Fig. 7a) and the typical duration of cover associated with the Bennett-II Nunatak require that, over the past 2.9-3.4 Ma, WAIS highstand is a few to approximately ten thousand years. Conse- elevations have not exceeded 35 m above present-day ice levels quently, predicted ice elevations imply only brief periods of cover for more than 10% of the time. A longer duration of ice cover for this nunatak. The cosmogenic nuclide concentrations of the makes it impossible for the Bennett-II surface to acquire the Bennett-I Nunatak calculated from the predicted ice elevation measured 21Ne and 10Be concentrations even with a zero erosion history fits the observed concentrations if the bedrock surface rate. Since the Bennett-II Nunatak peak is the lowest surface that was exposed at 4.2 Ma and erodes at a rate of 14.5 cm Ma1 we sampled, the Bennett-II cosmogenic nuclide data limits max- (Fig. 7a). The simulation predicts slightly more cover for the peak imum ice cover to elevations above 35 m to o10% of the time of the Tuning Nunatak at 65 m above present-day ice levels; the over the past 2.9-3.4 Ma. surface is exposed 96% of the time and the calculated cosmo- The prediction from the ice sheet simulation that the nunataks genic nuclide concentrations fit the observed values if the bedrock were overrun by ice multiple times during WAIS highstands is surface erodes at 23 cm Ma1 and is first exposed at 3.5 Ma, consistent with the glacial geologic evidence; exposure ages of significantly later than that obtained assuming continuous expo- erratics from the nunataks indicate multiple episodes of ice cover sure (Fig. 7a; Table 2). We note that the age of first exposure likely over the past 200 ka (Ackert et al., 2011). While the ice sheet reflects the last major re-surfacing event on the nunataks, such as model appears to marginally over-estimate mean ice elevations at a rockfalls or glacial quarrying, which brought rocks from below the Ohio Range, a 20–25 m misfit in predicted absolute ice the shielding depth (41 m) to the surface. elevation is remarkably small. The overall good correspondence In contrast to the Bennett-I, and Tuning Nunataks, the simula- between observed cosmogenic nuclide concentrations and that tion predicts significantly more ice cover for the peak of the predicted from the ice elevation history at the Ohio Range (Fig. 7) Bennett-II Nunatak, 24% of the time over the past 3.4 Ma. indicates that the model captures WAIS dynamics on both Importantly, 71% of the cover occurs during the past 1.5 Ma, Milankovitch (Ackert et al., 2011) and longer million-year 250 S. Mukhopadhyay et al. / Earth and Planetary Science Letters 337–338 (2012) 243–251

2012), and the ice-elevation history at the Ohio Range over the II past 200 ka (Ackert et al., 2011),wesuggestthatthebasic model results of smaller WAIS volume and more frequent collapse of the marine-based WAIS sectors during the Pliocene are also correct. In the future, this suggestion should be tested by similar model-data comparisons at other West Antarctic sites, in a more comprehensive framework of Latin-Hypercube 0.8 sampling (Stone et al., 2010) or Bayesian calibration (Briggs et al., 2011).

I 4. Conclusions 0.6 Long-term erosion rates in interior WAIS at the Ohio Range vary between 4.9 and 30.5 cm Ma1 and are amongst the lowest erosion rates determined in Antarctica. There is no evidence for 0.4 higher erosion rates associated with the Pliocene warm period, II suggesting that a continuous polar environment has persisted in the Ohio Range since the Late-Miocene. Geologic and cosmogenic nuclide data suggest that WAIS elevations at the Ohio Range remained lower than 160 m above present ice levels over the past 0.2 1 234 7 Ma and the highest ice level over the past 5.4 Ma was 110 m, reached during the last highstand at 11 ka. Over the past 2.9- 3.4 Ma, average ice elevation has been at or below present-day Fig. 7. (a) Cosmogenic nuclide trajectories for the summits of Tuning and Bennett- levels, and WAIS elevations exceeded 85 m above present ice I and Bennett-II Nunataks based on ice cover predicted by the ice sheet simulation. levels only during brief WAIS highstands. These results are The nuclide trajectories for the bedrock surfaces are calculated based on a consistent with a simulated ice elevation history from a con- prescribed erosion rate, production of stable (21Ne) and radioactive (10Be) nuclides during exposure and radioactive decay of 10Be during episodes of cover (Lal, 1991). tinental scale ice sheet model over the past 5 Ma. As a result, we Because calculated erosion rates are quite low and since cold-based glaciers can suggest that the main model result of more collapsed states of the erode surfaces through plucking (Cuffey et al., 2000), we assume bedrock surfaces marine-based WAIS sectors and smaller WAIS volume in the are eroded at the same rate when ice covered as during sub-aerial exposure. Pliocene transitioning to more glaciated states and larger ice Assuming a zero erosion rate during ice cover, however, does not change our volumes in the Pleistocene are also correct. results. The two free parameters in the calculation of the nuclide trajectories, age of first exposure (Exp) of the bedrock surface (in Ma) and erosion rates (in cm Ma1), were determined through a grid search. The superscripted values represent the range of values for which the nuclide trajectories were within the 1s Acknowledgments error ellipse of the measured 21Ne and 10Be data. Spallation and muon production rates for the samples were scaled as in Fig. 5. Zero-erosion line and steady state line as in Fig. 5. Note that the ice sheet simulation predicts too much cover for the Logistical support for the fieldwork was provided by Raytheon Bennett-II Nunatak. (b) Cosmogenic nuclide trajectory for Bennett-II Nunatak with Polar services, Air National Guard, and Ken Borak Air. SM and RPA absolute elevation predictions from the ice sheet simulation decreased by 25 m. specially acknowledge the help and support from Peter Braddock The small decrease in predicted ice elevations produces a fit to the observed and Aaron Putnam during the fieldwork in the Ohio Range. This cosmogenic nuclide data. work was funded by US National Foundation grants ANT- 0338271, ATM-0513402/0513421, ANT-034248, and ANT- 0424589. We thank Bernard Marty and an anonymous reviewer timescales. Therefore, we also suggest that the basic model result for comments that helped to improve the manuscript. indicating more collapsed-WAIS states and smaller WAIS volumes during the Pliocene, transitioning to more-glaciated and larger WAIS volumes in the Pleistocene, is correct. Appendix A. Supporting information While the predicted elevation variations at the Ohio Range over the past 5 Ma from the continental scale ice sheet model are Supplementary data associated with this article can be found consistent with the cosmogenic nuclide data, the predicted ice in the online version at http://dx.doi.org/10.1016/j.epsl.2012.05. elevation history is not a unique solution to the observed 015. concentrations of cosmogenic nuclides. Much simpler exposure histories are also likely to be consistent with the cosmogenic nuclide data. Furthermore, the geologic and cosmogenic nuclide data from the bedrock surfaces do not constrain WAIS elevation References fluctuations that are lower than the present ice surface and observations from a single locality do not necessarily validate a Ackert, R.P., Barclay, D.J., Borns, H.W., Calkin, P.E., Kurz, M.D., Fastook, J.L., Steig, model. Nonetheless, the geologic and cosmogenic nuclide data do E.J., 1999. 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