Basal ice sequences in Antarctic ice stream - Wiley Online Library

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, F03034, doi:10.1029/2009JF001430, 2010

Basal ice sequences in Antarctic ice stream: Exposure of past hydrologic conditions and a principal mode of sediment transfer Poul Christoffersen,1 Slawek Tulaczyk,2 and Alberto Behar3 Received 28 June 2009; revised 16 March 2010; accepted 12 April 2010; published 21 September 2010.

[1] The brightness distribution of sequentially extracted borehole camera imagery shows two distinct sequences of basal ice in Kamb Ice Stream. The upper sequence (7.3 m) comprises clear ice and layers with dispersed and stratified debris. The lower sequence (8.2 m) consists of accretion ice with alternating layers of stratified and solid debris. The two sequences have volumetric debris contents of about 5% and 20%, respectively. We infer that the upper sequence formed in a tributary where subglacial meltwater was abundant and that the lower sequence formed on the Siple Coast plain where basal freezing currently dominates the basal thermal regime. The basal ice layer contains the equivalent of 2.1 ± 0.4 m of frozen sediment. The high volume of sediment is a result of debris‐rich layers interpreted to have formed by accretion in the stagnant phase of ice stream on/off cycles. Fast ice streaming punctuated by entrainment of debris during stagnation episodes is consistent with inferred past flow of adjacent ice streams and geologic features in the Ross Sea. Our results show that sediment wedges near grounding lines may form by melt out of basal ice debris. This is different from previous studies where it was assumed that sediment transfer occurs through sediment deformation within layers of subglacial till. Cumulative freezing in recurring ice stream cycles shows that entrainment of sediment by basal freeze‐on is an important erosion mechanism and that high sediment fluxes can occur when ice streams override Coulomb‐plastic substrates. Citation: Christoffersen, P., S. Tulaczyk, and A. Behar (2010), Basal ice sequences in Antarctic ice stream: Exposure of past hydrologic conditions and a principal mode of sediment transfer, J. Geophys. Res., 115, F03034, doi:10.1029/2009JF001430.

1. Introduction [2] Understanding of sedimentary processes beneath ice streams is important because the extent of sedimentary basins appears to be a first‐order control on the complex flow of ice sheets [Anandakrishnan et al., 1998; Bamber et al., 2000; Bell et al., 1998; Studinger et al., 2001]. Marine sediments deposited at a time when the West Antarctic Ice Sheet was smaller than present or absent modulate the current dynamics of ice streams because they govern the texture and physical properties of sub‐ice‐stream tills [Tulaczyk et al., 1998]. These tills are commonly found in 5–10 m thick layers [Alley et al., 1986; Blankenship et al., 1986; Engelhardt et al., 1990; Kamb, 2001; King et al., 2004; Smith et al., 2007]. [3] Retreat of ice streams that once occupied continental margins has exposed glacially transported sediments in a variety of landforms that include drapes, wedges, and fans [Kellogg et al., 1979; Anderson et al., 1984; King et al., 1

Scott Polar Research Institute, University of Cambridge, Cambridge,

UK.

2 Earth & Planetary Sciences Department, University of California, Santa Cruz, California, USA. 3 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.

Copyright 2010 by the American Geophysical Union. 0148‐0227/10/2009JF001430

1991; Domack et al., 1999; Shipp et al., 1999; Ó Cofaigh et al., 2002; Dowdeswell et al., 2004; Ottesen et al., 2005; Evans et al., 2006; Mosola and Anderson, 2006; Laberg et al., 2009]. The accumulation of glacial sediments near ice stream grounding lines is an example of significant interaction between sedimentary processes and ice sheet stability. Grounding zone wedges, which are found in numerous places along the Antarctic continental margin [O’Brien et al., 1999; Howat and Domack, 2003; Mosola and Anderson, 2006; Ó Cofaigh et al., 2005; Heroy and Anderson, 2005] and elsewhere [King et al., 1991; Ottesen et al., 2005; Nygård et al., 2007; Laberg et al., 2009], may stabilize ice sheets against grounding line retreat in response to sea level rise on the order of up to several meters [Alley et al., 2007]. The stabilizing effect of sedimentation near grounding lines may be lost if the supply of sediment is terminated [Alley et al., 1989], and this suggests it is important to consider sediment transfer and till layer continuity when ice sheet stability is assessed in numerical models [Cuffey and Alley, 1996; Bougamont and Tulaczyk, 2003a; Dowdeswell et al., 2008]. [4] To account for landform formation, it is often assumed that transfer of sediment by ice streams occurs from deformation of the subglacial till layer [Alley et al., 1987; Anderson et al., 2002; Ó Cofaigh et al., 2002; Dowdeswell et al., 2006; Mosola and Anderson, 2006]. This assumption is, however, not consistent with the available observational constraints

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Figure 1. Subsection of the MODIS mosaic of Antarctica with inset showing location. Black crosses show approximate locations of borehole camera deployments: (right) on the southern branch of Kamb Ice Stream and (left) within a former shear margin. White dashed lines denote former shear margins that define two branches of the ice stream. Whillans Ice Stream (WIS) and Bindschadler Ice Stream (BIS) are visible in the upper right and lower left corner of the image. StS refers to the “sticky spot” that separates north and south branches of KIS. GrL shows location of grounding line. (Courtesy of the National Snow and Ice Data Center). from boreholes drilled to the bed of ice steams. The latter has revealed vertically limited extent of till deformation [Engelhardt and Kamb, 1998] and highly nonlinear basal properties, which are consistent with Coulomb‐plastic sediment behavior [Kamb, 1991; Tulaczyk et al., 2000a]. The limited vertical extent of till deformation beneath ice streams has appeared conceptually problematic because melting at the base of ice streams was assumed to limit the extent of which ice streams can acquire subglacial sediments by freezing. Ice temperature measurements and numerical modeling, however, show that basal freezing of several millimeters per year is widespread beneath the Siple Coast ice streams [Joughin et al., 2003, 2004a, 2004b; Vogel et al., 2003]. This result is corroborated by the identification of a 15 m thick basal ice layer in Kamb Ice Stream [Carsey et al., 2002; Vogel et al., 2005], which stopped 150 years ago [Shabtaie and Bentley, 1987; Retzlaff and Bentley, 1993]. The stagnation of the ice stream resulted in a relatively high rate of basal freezing (∼4 mm a−1) as latent heat of fusion replaced frictional heat in the basal thermal heat budget [Christoffersen and Tulaczyk, 2003a]. However, poststagnation accretion has produced less than 1 m of basal ice [Vogel et al., 2005]. The large vertical extent of prestagnation basal ice shows that subglacial accretion is extensive when ice flows from the interior of the ice sheet to the coastal ice plain. Christoffersen et al. [2006]

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used a numerical model to show that the configuration of debris in different types of basal ice depends on environmental conditions such as subglacial water availability and freezing rate. The stratigraphy and arrangement of facies within the basal ice layer as a whole therefore offer a window of opportunity to examine the long‐term interaction between the ice stream and its bed. [5] Here we use a sequential optical analysis of brightness in borehole camera imagery to estimate the vertical distribution of debris in the basal ice layer in Kamb Ice Stream. We show that the extent and character of debris is consistent with the interpretation that subglacial sediment is entrained when the ice stream shuts down and transferred when the ice stream is active. The image analysis reveals two different sequences of basal ice, and the transition between these are inferred to reflect a reduction in the availability of basal water when ice moves from an ice stream tributary to the shallower coastal ice plain. We estimate that the basal ice layer contains the equivalent of 2.1 m of sediment. The high volume of accreted debris in Kamb Ice Stream explains key geologic features in the Ross Sea. The results show that there may be a direct link between sediment transport rate and temporal character of ice stream on/off cycles.

2. Borehole Camera Observations From Kamb Ice Stream [6] A camera system developed at the Jet Propulsion Laboratory (JPL) was deployed in boreholes drilled near the UpC camp on Kamb Ice Stream during the austral summer of 2000–2001 [Carsey et al., 2002]. The borehole system recorded digital videos from down‐ and side‐looking cameras. Down‐looking imagery was obtained with fixed 4 mm lens and contained 768 × 494 pixels. Side‐looking imagery was captured with 720 × 480 pixels, and this camera had a zoom function [Carsey et al., 2002]. Three halogen bulbs provided the illumination. Video sequences were obtained in several boreholes including one drilled to the bed near a former shear margin and one drilled to the bed of the southern ice stream branch (Figure 1). The vertical extent of a basal ice layer at these respective locations is 10 and 15 m. Isotopic d 18O and d D records from ice sampled near the bed of the shear margin borehole show that a visible transition between opaque ice with bubbles (Figure 2a) and transparent ice with inclusions of debris (Figure 2b) marks the boundary between meteoric glacier ice and ice formed by subglacial accretion [Vogel, 2004]. Vogel et al. [2005] focused on the lowermost part of the basal ice layer at the shear‐margin location and used estimates of the current basal freeze‐on rate (4 ± 1 mm a−1) to show that stagnation of Kamb Ice Stream ∼150 years ago caused accretion of a 0.6 m thick layer of frozen debris (Figure 2c). The presence of clear accretion ice and a water‐filled cavity underneath the layer of frozen debris indicates that a subglacial water system is active at the drill site location. The vertical extent of clear accretion ice (Figure 2c), which appears to have formed from water in the subglacial cavity, indicates that the hydrologic system became active ∼60 years ago [Vogel et al., 2005]. Hydrologic processes beneath ice streams may trigger reactivation before thickening of ice reverts the basal thermal regime to melting. This could explain why flow switches of the Siple Coast ice streams occur on a centennial

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Figure 2. Down‐looking images from the shear margin borehole in Kamb Ice Stream. The approximate location of this borehole is shown in Figure 1. Shown here is (a) opaque bubbly glacier ice 13.3 m above the bed, (b) clear basal accretion ice with bands of debris 6.8 m above the bed, and (c) debris‐rich basal accretion ice 0.72 m above the bed. A transition from frozen sediment to clear basal accretion ice occurs 0.27 m above the bed. See text for details.

time scale [Hulbe and Fahnestock, 2007], which is about an order of magnitude faster than expected of the binge/purge model [MacAyeal, 1993]. 2.1. Optical Image Analysis [7] Optical imaging of dust, ash, and stratigraphy is a common technique in studies of snow and ice [Ram and Koenig, 1997; Bay et al., 2001; Bramall et al., 2005]. For instance, Ram and Koenig [1997] used light scattering to identify seasonal dust peaks in the Greenland Ice Sheet Project 2 borehole, and the optically established chronology was in good agreement with gas‐age dating of the ice core. Price et al. [2000] also used optical imaging to determine age versus depth of glacial ice but at the South Pole. Hawley et al. [2003] used a light‐emitting‐diode‐illuminated borehole camera system similar to the JPL system described above to record optical stratigraphy and to manually identify

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annual layers at Siple Dome. Their results also agreed well with conventional dating of an ice core. The remote sensing of peaks in scattering and absorption of light in boreholes is an attractive and complementary alternative to the study of ice cores, and it is relevant to the field of neutrino astrophysics [Ackermann et al., 2006]. The physical principles of light scattering for particle characterization are described by Jones [1999]. [8] To examine the form and character of debris in the basal ice layer in Kamb Ice Stream, we analyzed imagery from camera televiewing in a borehole drilled on the southern branch of Kamb Ice Stream (Figure 1). The spatial distribution of ice and debris was estimated from 300 images covering 0.04 × 0.06 m2 and extracted sequentially at 0.05 m vertical interval, starting 15 m above the ice base and finishing at the bed. We used the first evidence of debris in the 15 m thick bubble‐free basal ice layer as an indicator of the top of the accreted ice layer. This criterion was used because isotopic and sedimentary analysis of 3 m long ice core, collected in the neighboring shear margin borehole, show that transition from opaque meteoric glacier ice to clear ice formed by accretion coincide with entrainment of debris [Vogel, 2004; Vogel et al., 2005]. The 3‰ difference in d18O between meteoric ice and the upper 2 m of accreted ice, reported by Vogel et al. [2005], is consistent with (1) the different d18O compositions of accreted ice and water at the base of the Byrd ice core [Gow et al., 1979] and (2) theoretical estimates of d 18O enrichment of basal ice forming in the interior of ice sheets [Boulton and Spring, 1986]. Christoffersen and Tulaczyk [2003a] show that the isotopic composition of basal ice forming at several mm a−1 beneath Kamb Ice Stream should also be isotopically enriched because diffusion and advection processes counteract the impoverishment of heavy isotopes in the water adjacent to the freezing ice‐water interface. If the latter condition is not met, e.g., beneath interstream ice ridges where freeze‐on rates are small [Christoffersen and Tulaczyk, 2003a], accretion with no apparent fractionation may occur [Souchez et al., 2004]. [9] To estimate the content of debris, we examined the brightness distribution of side‐looking borehole camera imagery (Figure 3a). The optical properties of the accreted basal ice are governed by the arrangement of entrained particles because a large crystal size and absence of air bubbles make the accreted ice visually transparent. Using ENVI software, we produced image selections in black and white based on a threshold criteria, which is explained below. In the analyzed imagery, black pixels represent transparent accretion ice and white pixels show debris. The summation of white and black pixels was used to quantify the spatial arrangement of ice and debris in each image. This method is simpler than the object‐oriented segmentation method used in a previous study [Christoffersen et al., 2006]. We used the simpler approach because it is suitable for the available imagery and the purpose of study. [10] A complete automation of the process described above was not feasible due to variable rates of camera deployment and variable influence on brightness from the light source. Sequentially extracted images were therefore manually processed. We avoided brightness variations around image corners by extracting a representative segment from the center area of each image, and each image segment was

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imagery. To examine the influence of variations in the value of f, we calculated debris contents according to f ± c where c = [0, 10, 20, 30, 50] are values defining a widening range toward lower and upper bound estimates of the integer used to separate ice and debris. This sensitivity assessment is illustrated in Figure 3b, which shows that the calculation of debris changes by 8% if f ± 10 is used instead of f. For cmax = 50, the difference is ∼35%. The uncertainty of defining a correct value of f scales linearly with factor of 0.007c (R2 = 0.98). This indicates that a positive bias in the representation of debris in imagery may be as high as 35%–40%. Basal ice with dispersed debris, as shown in Figure 3b, is most prone to this error because relative abundance of clear ice can make debris visible at depth. Other types of basal ice, as discussed below, are less prone to this error because their structure and debris content is more easily represented in 2‐D. The examination of 300 images showed that the range of f where representation of debris is visually satisfactory was c ≤ 20. The overall uncertainty of estimating debris content from image brightness is therefore estimated to be around 20%. This level of uncertainty is acceptable given that the aim of this study is to produce a first‐order estimate of basal ice debris in Kamb Ice Stream. A more wide‐ranging approach, including, e.g., improved spectral analysis, edge‐enhancement, and/or object‐oriented segmentation, may be suitable for studies based on imagery and data from advanced borehole cameras and sensors [Hubbard et al., 2008].

Figure 3. (a) Brightness‐frequency distribution of different types of basal ice: (i) clear basal ice, (ii) dispersed basal ice, (iii) stratified basal ice, and (iv) solid particle. (b) Relative change in estimated debris content when threshold f in conversion of 8 bit imagery to black and white is scaled with c. See text for details. converted to black and white using a manually defined threshold value f. The value of f was an integer on the 256‐ value color palette where 0 and 255 represents black and white, respectively. The selection of values of f introduced uncertainty in the separation of transparent accretion ice and entrained debris. To elucidate this uncertainty, we examined the brightness distribution in images of different types of basal ice. These distributions were distinctly different. For instance, the mean brightness was 120 for a solid rock particle and 32 for transparent ice with no visible debris (Figure 3a). We used these values to guide the separation of ice and debris in the imagery. The most common value of f was around 80. [11] Visual assessments are common in optical imaging of ice [e.g., Hawley et al., 2003], but 2‐D representation of 3‐D structure can lead to overestimation of debris in borehole

2.2. Form and Character of Basal Ice [12] The conducted image analysis yields large variations in the arrangement of debris in the basal ice layer of Kamb Ice Stream (Figure 4). We have identified four main types of basal ice: (1) clear basal ice with little or no debris, (2) clear basal ice with debris in suspension, (3) basal ice with laminas or bands of frozen sediment, and (4) basal ice consisting of frozen sediment with pore ice. The basal ice types are the same as those reported by Christoffersen et al. [2006], and they are in good agreement with basal ice facies classification proposed by Hubbard et al. [2009] on the basis of an extensive review of basal ice literature. The main difference between our studies and that of Hubbard et al. is that our use of the term “stratified” includes facies with millimeter thick laminas and centimeter‐sized bands. [13] The sequential examination of optical properties in this study allows us to quantify the debris content throughout the basal ice layer. We estimate debris in clear basal ice amount to ∼0.2%, whereas dispersed, stratified, and solid basal ice have contents around 6% ± 3%, 22% ± 11%, and 44% ± 6%, respectively. The estimated debris content of the observed types of basal ice are consistent with basal ice sequences observed in the interior of ice sheets and along ice sheet margins [Gow et al., 1979; Herron and Langway, 1979; Boulton and Spring, 1986; Knight, 1994; Souchez et al., 2004; Hubbard et al., 2009]. [14] The basal ice layer can be divided into two almost equally thick sequences based on distribution of debris (Figure 4a). Using the basal ice classification by Hubbard et al. [2009], we define the upper 7.2 m as consisting of alternating layers of clear, dispersed, and stratified dispersed

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Figure 4. Results from brightness analysis of basal ice imaged in Kamb Ice Stream. (a) Stratigraphic log showing vertical distribution of debris in the Kamb Ice Stream basal ice layer. Crosses mark estimates derived from image analysis. Thick black line shows 0.15 m moving average. The vertical scale marks the borehole camera’s depth counter (1 = 1.03 m). Dashed lines represent layers consisting of clear (C), dispersed (D), stratified dispersed (StD), and stratified solid (StSo) facies. (b) Borehole camera image illustrates clear basal ice with dispersed debris 14 m above the bed. (c) The image segment illustrates a spatial arrangement of ice (black) and debris (white) within the white box shown in Figure 4b. Additional examples of borehole camera images and image sections show (d and e) clear ice and debris in stratified solid facies, (f and g) clots of debris in dispersed facies, and (h and i) clear basal ice immediately above the ice stream bed.

facies (Figures 4a–4c). The mean sediment contents in these facies are ∼0.2%, 3% ± 2%, and 12% ± 7% (Table 1). The lower 8.3 m consists of alternating layers of clear, dispersed, and stratified solid facies (Figures 4a, 4d−4i). The mean debris content of these facies are ∼0.2%, 7% ± 4%, and 27% ± 12% (Table 1). The average debris content in the upper 7.2 m amounts to ∼5%, whereas debris in the lower 8.3 m is ∼20% (Figure 4a). The basal ice layer imaged in the shear margin borehole consists of two similar sequences (Figure 2). Two‐sequence structure of basal ice is also observed in the Greenland Ice Sheet [Sugden et al., 1987; Knight, 1994]. By summating debris estimates from the 300 images, we estimate that Kamb Ice Stream has acquired the equivalent of 2.1 ± 0.4 m of sediment during its course from the interior of West Antarctica to the UpC drill site location.

2.3. Interpretation [15] The basal ice observed in the southern branch of Kamb Ice Stream contains six layers where the estimated debris content is >12%. The layer closest to the bed is 1.2 m Table 1. Facies and Estimated Debris Content of Basal Ice Sequences Observed in Kamb Ice Stream

Upper sequence Lower sequence

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Basal Ice Facies

Debris Content (vol%)

Debris Content (vol%)

Clear (C) Dispersed (D) Stratified dispersed (StD) Clear (C) Dispersed (D) Stratified solid (StSo)

∼0.2% 3 ± 2% 12 ± 7% ∼0.2% 7 ± 4% 27 ± 12%

∼5% ∼20%

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Figure 5. Side‐looking borehole camera images showing transition from (a) a layer of clean basal ice consisting of clear facies to (b–d) debris‐rich basal ice consisting of stratified solid facies. The progressive increase in debris from 2% in Figure 5a to 35% in Figure 5d occurs over 0.3 m, and enhanced debris‐entrainment is interpreted to be a result of ice stream shutdown in approximately 100–150 years. The sequence is shown from the bottom and up because up in image is down in borehole.

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thick. Borehole temperature profiles show that the ice base is currently freezing at a rate of ∼4 mm a−1 [Kamb, 2001] so we expect that the layer probably took about 300 years to form. This layer corresponds with the lowermost layer imaged in the borehole drilled near the former shear margin [Vogel et al., 2005], and this shows that sedimentary entrainment occurred over a relatively large spatial area. We interpret the entrainment process to be associated with the sequential stagnation of the ice stream, which started with narrowing and slowdown 350 years ago and culminated with shutdown 200 years later [Catania et al., 2006]. The vertical arrangement of debris shown in Figure 4a shows overlying (older) layers where the content of debris is similar to the layer produced by the recent stagnation event. This may be explained by (1) sequence repetition from thrusting and folding of the basal ice layer or (2) recurrence of stagnation events. The former mechanism is observed widely along terrestrial margins of ice sheets and valley glaciers [e.g., Fitzsimons, 1990; Hambrey et al., 1999; Larsen et al., 2010], but we favor the latter interpretation for a number of reasons. (1) Borehole measurements [Kamb, 2001] and geophysical inversions [Joughin et al., 2004a, 2004b] show widespread regional extent of weak sedimentary beds along the Siple Coast. The low strength of these beds (