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12th International Conference on Ground Penetrating Radar, June 16-19, 2008, Birmingham, UK

GPR Investigations of Subglacially Deformed Fossil TreeHorizons: Bering Glacier, Alaska

Matthew J. Burke a*, P. Jay Fleisher b, John Woodward a, Palmer K. Bailey c, Eric Natel d and Andrew J. Russell e

a School of Applied Sciences, Northumbria University, Newcastle-upon-Tyne, NE1 8ST, UKb Earth Sciences Department, SUNY-Oneonta, Oneonta, NY, 13820-4015, USA

c Kenai Peninsula College, Anchor Point, AK, 99556-9160, USAd Research and Development/Legal, Eastman Kodak, Rochester, NY 14650, USA

e School of Geography, Politics and Sociology, University of Newcastle, Newcastle-upon-Tyne, NE1 7RU, UKEmail [email protected]


Abstract – The eastern forefield of Bering Glacier encom-passes a lake and island complex overridden by glacier surge advances in 1965-1967 and 1993-1995. Multiple ‘fossil’ tree horizons (still in growth position) have been identified within outwash deposits in proglacial stratigraphic sections. Those at the near surface demonstrate deformation and shearing indirectly linked to the stresses applied by overriding glacier advance. This project aims to assess the suitability of ground-penetrating radar (GPR) as a tool for detecting con-tinuous till and diamicton surfaces associated with the defor-mation layer, as well as to identify sheared and deformed trees. In 2006, a grid of six 100 m common offset GPR lines were collected on Bentwood Island using a PulseEKKO Pro 1100 system at a nominal frequency of 200 MHz. A maxi-mum penetration depth of approximately 11 m within mate-rials composed of diamicton, sand, gravel and some silts and clays, has been achieved. Two sub horizontal reflections have been identified at depth that can be traced across the grid. These are interpreted to represent the upper and lower boundaries of sand and gravel units, which are bounded by till and peat at the upper and lower contact, respectively. Between the prominent reflections, a number of stacked hy-perbolae or point source reflections can be identified, indi-cating the presence of sheared trees, which are still upright within the substrate. GPR observations of the spatial conti-nuity of deformation units on Bentwood Island coupled with observations on other nearby islands, suggests subglacial sediment deformation was widespread during advances of Bering Glacier.

Keywords - Bering Glacier, surge, forefield, subglacial sedi-ment deformation, ground-penetrating radar.

I. INTRODUCTIONMany ice masses overlie a bed of soft sediments [2; 3; 5] that may deform due to overriding ice [6; 8], enhancing glacier flow for ice streams and valley glaciers [1; 4; 7; 10]. Recent work at Bering Glacier, Alaska has identified the importance of subglacial sediment deformation during recent surges [7]. Surging is a phenomenon during which the glacier undergoes a relatively short period (a few months to a few years) of abruptly accelerated movement, with velocities of up to tens of metres per day [8]. Fol-lowing this phase of rapid motion the glacier undergoes a comparatively long period (decades to hundreds of years) of passive flow [11]. Bering Glacier, Alaska is the largest surge-type Glacier in the world (Figure 1) [6; 7]. The most recent surge occurred in 1993-1995, culminating in a total advance of up to 9 km [9] and was associated with deformation of pre-existing subglacial material [7].

Figure 1. a) the state of Alaska, with the location of the Bering Glacier being indicated. b) Map of the Bering glacier, with the box indicating the location of inset c). c) Map of the eastern sector forefield. The lo-cations of the ‘Grove’ section and the GPR survey site are shown on Bentwood Island (BWI) and the location of Weeping Peat Island (WPI) is also indicated. The solid line margin of Bering Glacier indi-cates its position in 1993 (pre-surge), whilst the dashed line represents the 1995 margin (post-surge) (Figure after [7]).

In the eastern sector forefield, a lake/island complex cov-ers an area of approximately 100 km2. The strata on the is-lands is dominated by units of sand and gravel outwash (materials deposited at the near ice-margin directly by glacier meltwater) interspersed by thin units of till (<0.5 m thick) and peat layers representing growth horizons formed during quiescent surge conditions, when the glacier margin retreated from the area. On Weeping Peat Island buried trees that have been sheared and deformed at a common elevation, have been identified between the till and peat [7]. The tree stumps, still in growth position within sand and gravel outwash, have been sheared at an elevation corresponding to the till (poorly-sorted fine ma-terial directly deposited or re-worked by ice), are rooted in the gravel and show deformation patterns parallel to glacier flow direction [7]. A similar deformation layer, in-cluding sheared and deformed trees buried within outwash can be identified in sections on Bentwood Island (Figure 2).


Understanding the spatial extent of these deformation sur-faces will help constrain controls on glacier flow dynam-ics during ice advance into the forefield of the Bering Glacier. GPR is considered an appropriate technique for mapping the spatial extent of these horizons. This paper aims to assess the suitability of GPR for detecting continu-ous till surfaces associated with subglacial tree deforma-tion as well as sheared and deformed tree stumps in out-wash sediments. This will allow analysis of the extent of subglacial deformation surfaces beyond exposed strati-graphic sections located on Weeping Peat Island [7].

II. METHODOLOGYSubsurface data were gathered to the south of the ‘grove’ sections on Bentwood Island (Figure 1). In June 2006, six 100 m common offset (CO) GPR lines were collected as an interlinked grid with a line spacing of 50 m, using a PulseEkko Pro 1100 GPR system at a nominal frequency of 200 MHz. During CO data collection the antennae were kept at a constant separation of 1 m and data were collected along the transect at increments of 0.5 m. In ad-dition, two common mid-point (CMP) surveys were con-ducted to calculate a subsurface radar wave velocity (0.079 m/ns) for migration and conversion of two-way

travel-time to depth. All lines were surveyed for topo-graphic variation using an engineering level. GPR profiles have been processed within REFLEXW [12]. Profiles were statically corrected, ‘dewowed’, bandpass filtered and migrated. A background removal and a gain function were also applied prior to topographic correction.

Figure 2. Photographs showing the exposures of ‘grovemower’ till at Bentwood Island ‘Grove’ section (see Figure 1c). a) Photograph of fluted terrain in the area of GPR data collection on Bentwood Island. b) Section showing the repeated occurrence of multiple till surfaces and deformed trees at depth (arrows), within outwash, but below diamicton (non-genetic term for very poorly sorted material). Colluvium refers to sediment derived from the section face, which has collected at the section base due to gravitational processes. c) Buried trees and occurrence of exposed till (circled), which corresponds to the depth at which the trees have been sheared and deformed. The arrow indicates the location of d). d) Exposed sheared trees, the location of which are indicated by the arrow on c). It can be seen how the trees are rooted in a peat horizon, above which outwash has been deposited. The person in the image is holding a 2 m ruler for scale.


III. RESULTS AND DISCUSSION

3.1 Description and quantification of GPR lineBentwood Island sediments demonstrate a complex archi-tecture consequent of a dynamic proglacial environment (Figures 3-4). Figure 3b-d shows a profile collected ap-proximately parallel and closest to a section on the west-ern side of Bentwood Island. Here, two relatively continu-ous sub-horizontal reflections (SHR1 and SHR2) can be identified at maximum depths of ~5 m and ~9 m, respec-tively (Figure 3). This reflection horizon is sub-parallel to the ground surface, relatively flat, and continuous de-spite some irregularities and lateral variance in the ampli-tude of the reflections (Figure 3b-c). Furthermore, this re-flection horizon can be traced perpendicularly away from the ‘grove’ section, as can the presence of buried trees (Figure 3e-f). SHR1 and SHR2 can be traced continu-ously across the entire grid (Figure 4), and are sub-parallel to the ground surface. They are at their greatest elevation at the south-eastern area of the grid and consequently dip from southeast-northwest and south-north, which is largely parallel to ice flow direction. In un-migrated section (Figure 3b & e) vertically stacked hyperbolic diffractions can be discerned between SHR1 and SHR2 (Figure 3b & e). The occurrence of stacked diffraction patterns are interpreted to be created by strong reflectors associated with the presence of buried trees. The associated reflection pattern, however, is variable

(Figure 3 (top right)) and consequently the likelihood of a reflector being a tree can be quantified: Strong – In unmigrated section, strong, stacked dif-

fractions are identified, which collapse to a point-source when migrated. These represent 26% of iden-tified reflections, representing instances when the GPR line runs directly over buried trees.

Moderate – In unmigrated section, medium strength stacked diffractions can be discerned. These reflec-tions make up 50% of the buried tree stumps identi-fied within the grid. It is likely that the reflections are weaker when the tree trunks are smaller, and are not hyperbolic when the trunks are offline to the survey, or when the trunks are not vertical.

Weak – In unmigrated section, weak and irregular dif-fractions are identified, which collapse into weak, dis-continuous reflections when migrated. Unlike for strong and moderate signals, no clear stacked diffrac-tions are produced making such reflection sets diffi-cult to identify. These have the lowest frequency making up 24% of identified tree stumps across the grid. The source of such diffraction sets is likely to be fallen trees, shorter or more angled stumps, or source objects some distance from the GPR antennae.

A total of thirty-eight potential trees have been identified within the grid, though this will be a minimum estimate of the total number present in this area, as the likelihood of

Figure 4. Interpretation of processed GPR lines collected in the grid on Bentwood Island (Figure 1c). The key corresponds to the shading scheme in Figure 3. Both SHR1 and SHR2 can be traced across the entire grid. Between these sub-horizontal reflections buried trees can be identified, none of which can be found above SHR1 or below SHR2.

Figure 3. Lines X1 (b-d) and Y2 (e-g) subsequent to processing and interpretation. The table (top right) is a key showing the signal types associated with the presence of buried trees within GPR line. Examples of the reflection types are given for un-migrated (UM) and migrated (M) data. a) Section showing sheared trees, which are rooted in peat (dashed line) and sheared at elevations (circled) corresponding to ‘Grovemower’ till. Although the surface slope appears to be opposite to that of line X1 (b-d), this is due to the orientation of the photograph being oblique to the section face. Line X1 is positioned ~20 m away from the section. b) X1 processed without the application of a migration algorithm. c) X1 processed, including the application of a migration algorithm. d) Interpretation of line X1. The reflection amplitude is indicated as either strong (solid line) or weak (dashed line). The reflections labeled SHR1 and SHR2 are indicated by the dashed line on b) and c). e) Y2 processed without the application of a migration algorithm. f) Y2 processed, including the application of a migration algorithm. g) Interpretation of line Y2. The reflection amplitude is indicated as either strong (solid line) or weak (dashed line). The reflections labeled SHR1 and SHR2 are indicated by the dashed line on e) and f).


identifying a tree varies depending upon its position rela-tive to the line.

3.2 Interpretation and wider implicationsThe stratigraphy of eastern forefield islands is composed of the products from a complex, dynamic history of surg-ing and more-sustained ice advance [13]. Composite sec-tions include buried trees, sand and gravel outwash, as well as deformation tills and peat horizons [13; 7]. SHR1 is interpreted as a relatively continuous unit of ‘Grove-mower’ till, corresponding to the depth at which this can be identified in sections on Bentwood Island. Materials above SHR1 are dominantly composed of crudely sorted dimaicton, with vague stratification. SHR2 is interpreted to be associated with a continuous unit of peat that devel-oped following an earlier phase of glacial retreat and con-comitant to the growth of spruce and alder.

Diffractions found between the till and peat units are inter-preted as fossil trees, killed by burial within outwash and lake sediments, largely remaining in growth position. Glacier advance, associated with subglacial sediment de-formation, sheared and deformed the buried trees. The identification of a deformation layer here and on other is-lands suggests that subglacial sediment deformation is widespread during phases of Bering Glacier advance [7].

The repeated presence of these units (Figure 2b) is testa-ment to the cyclic nature of Bering Glacier advance. Dur-ing periods of glacier retreat trees can re-colonize the newly exposed islands in the forefield of Bering Glacier. Subsequently, these trees become buried within outwash and lake sediments, probably associated with drainage re-organizations linked to fluctuating lake levels [13]. As the ice re-advanced, these trees are sheared and glacier flow is associated with subglacial sediment deformation, due to saturation of the substrate [5; 10]. This generates a new till at the ice-bed interface and deforms the buried trees in the direction of glacier flow [7]. The cyclic occurrence of glacier advance associated with surging and/or normal flow has created an island complex composed of at least four deformation units [7]. GPR is a useful tool in imag-ing the geospatial extent of buried and deformed trees at the near surface and may be utilized to reconstruct the ex-tent of recent Bering Glacier fluctuations.

IV. CONCLUSIONS

GPR has been successfully applied to the identification of deformation structure associated with advance of the Bering Glacier. Within the GPR surveys, two sub-hori-zontal reflections have been identified, between which stacked diffractions are often present. The upper and lower reflections are interpreted as till and peat horizons, respectively, whilst the hyperbolic diffractions are associ-ated with sheared and deformed trees. Whilst till defor-mation was not directly observed, the continuous presence of a deformation layer across the grid on Bentwood Island,

coupled with the presence of similar units on other eastern sector islands, is testament to the potential widespread oc-currence of subsole deformation during glacial advance. During glacier recession trees colonize the newly exposed forefield, but are subsequently buried in outwash and lake sediments. As the glacier re-advances it shears the buried trees, which remain in growth position and creates a till at the ice-bed interface due to subsole deformation. Sedi-ment deformation at depth results in deformation of the buried trees, the direction of which is parallel to ice flow. This study expands the work of Fleisher and co-workers [7] who identify multiple deformation units in sections on Weeping Peat Island. GPR is a useful tool that may be ap-plied to further studies of subglacial deformation at Bering Glacier.

ACKNOWLEDGEMENTSWe acknowledge fieldwork support from Northumbria University (MJB) and the Earthwatch Institute (MJB, PJF, PKB and EN). We thank Earthwatch volunteers for assis-tance in the field, as well as the Prince William Sound Science center and the Chugach National Forest Office for logistical assistance. We would like to thank the reviewer for constructive comments, which have improved the manuscript.

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