GPR Reflection Profiles of Sedimentary Deposits in Lower Taylor Valley, Antarctica

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GPR Reflection Profiles of Sedimentary Deposits in Lower Taylor Valley, Antarctica Steven A. Arcone and Allan J. Delaney U.S. Army Engineer Research and Development Center Cold Regions Research and Engineering Laboratory Hanover, NH, USA Steven.

edu Jennifer Horsman Plymouth State University Plymouth, NH, USA Jen@jencarta.

glacial lakes such as dumped, ice-rafted debris. In either case, they appear to have formed at least 1 years before present when an ice sheet that spread across the Ross Sea intruded the valley.

1 GPR Reflection Profiles of Sedimentary Deposits in Lower Taylor Valley, Antarctica Steven A. Arcone and Allan J. Delaney U.S. Army Engineer Research and Development Center Cold Regions Research and Engineering Laboratory Hanover, NH, USA Michael Prentice U.S. Geological Survey Bloomington, IN, USA Jennifer Horsman Plymouth State University Plymouth, NH, USA Abstract - Landforms within Lower Taylor Valley of the Dry Valleys of Antarctica have been hypothesized to be either glacial deposits such as eskers and moraines, or deposits unique to polar glacial lakes such as dumped, ice-rafted debris. In either case, they appear to have formed at least 1 years before present when an ice sheet that spread across the Ross Sea intruded the valley. We discuss a few of many 85- and 3-MHz GPR profiles we obtained to test these landform hypotheses. The landforms lie near the 8-m elevation contour, which appears to us to define at least one persistent lake level. We interpret a terrace to be the surface of a delta because of foreset and bottomset beds, a stream channel to be an incised moraine because of inconsistent horizons, and a series of ridges to be outwash deposits because they cover prograding beds with cross-cutting stratification and lie on likely lacustrine deposits. We speculate that the features are glacial deposits but do not preclude the existence of a large lake. Keywords - GPR, Dry Valleys, Antarctica. I. INTRODUCTION The climate change of the present should be weighed against the climate changes during the last 2, years following the Late Glacial Maximum (LGM). Strong evidence for world-scale climate variability and change comes from the temperature records of the Greenland and Vostok ice cores. Locally, however, large anomalies exist, and a strong one is in the Dry Valleys of Antarctica, where a large freshwater lake (Lake Washburn) existed in Taylor Valley (Fig. 1, 2) from 9 to 2 b.p. during the late Wisconsinan. Lakes probably occurred in other valleys as well. There is no local ice-core record from which we can interpret a dramatic warming. Consequently, we seek geologic evidence to indicate lake extent and, consequently, the size of the Ross Ice lobe, a tongue of the West Antarctic Ice Sheet (WAIS) that crossed the Ross Sea and dammed the lake. A relatively large lake (Fig. 2) [1] implies relatively 77 3 N x Crescent Stream x Sloth Lake x Coral Ridge 5 1 km 77 4 Figure 1. Location of our sites in the Dry Valleys area. The Hjorth Hill site contains ice-rich strata but is not discussed. x Hjorth Hill warm temperatures, less intrusion, less ice sheet expansion, lower WAIS levels, more open ocean (decreased sea ice cover), greater alpine glacier activity, and a lake iceconveyer mechanism to explain the genesis of many deposits in Eastern Taylor Valley (ETV). A relatively small lake [2] implies the opposite conditions and the dominance of outwash deposits as the ice retreated. We attack this climate history problem via subsurface sedimentary characterization. Our objectives were to determine the structures beneath landforms that appear to have dual interpretations based on their morphology. We used GPR at pulse center frequencies of 85, 3, and 7 MHz (~ ground-loaded values) and chose to profile ridges, terraces, and channels, which could be interpreted as either lake, outwash or direct glacial deposits. We obtained several longitudinal and transverse (to possible stream and ice

flow direction) profiles of each feature.

Eastern Taylor Valley Km.5 1 2 Figure 3. Location of our sites on the main floor of Eastern Taylor Valley, and the 8-m elevation contour.

Two interpretations for the extent of Lake Washburn within Taylor Valley. In (a), the greater extent of the Ross Ice lobe would be evidenced by obliquely stratified ice-marginal outwash deposits.

SITES Figure 1 shows a map of ETV based on satellite imagery. This and other nearby valleys are ice-free because mountains to the west block the flow of the East Antarctic Ice Sheet (EAIS).

2 flow direction) profiles of each feature. We used these frequencies because of the convenience in hauling the antenna units and because of their proven ability to resolve strata and reach nearly 4 m deep in these areas [3, 4]. Eastern Taylor Valley Km Figure 3. Location of our sites on the main floor of Eastern Taylor Valley, and the 8-m elevation contour. Crescent Delta is in the smaller box, Sloth Lake ridges are in the larger box, and the arrow at right crosses the channel where it is next to Coral Ridge. ± Figure 2. Two interpretations for the extent of Lake Washburn within Taylor Valley. In (a), the greater extent of the Ross Ice lobe would be evidenced by obliquely stratified ice-marginal outwash deposits. In (b), the greater extent of the lake would be evidenced by flat-lying lake bottom deposits. The valley floor sites of Figure 1 lie within the larger depicted Ross Ice lobe. II. SITES Figure 1 shows a map of ETV based on satellite imagery. This and other nearby valleys are ice-free because mountains to the west block the flow of the East Antarctic Ice Sheet (EAIS). During the Neogene, EAIS was much larger and was probably responsible for the hundreds of meters of frozen sands and gravels that fill ETV to an unknown depth [5]. Commonwealth and Canada Glaciers flow south towards the valley and grew during warmer times when there was less sea ice cover. They are most likely frozen to the bottom and move about 1 m per year, but they have left visible moraines. Lakes Fryxell and Hoare are proglacial lakes with maximum depths of 19 and 34 m, respectively, and with non-marine salty water beneath a 3-m ice cover. There are other smaller lakes throughout the Dry Valleys. The sites we discuss are located on the valley floor (Fig. 3). The terraced Crescent Stream site is along an active drainage channel from the upland Crescent Glacier to the south. The Sloth Lake site is named for a nearby frozen pond and contains a peculiar set of ridges. Coral Ridge is 4 5 km long ridge that snakes across ETV, and is paralleled by a channel for several km across which we profiled several transects. All of these sites lie along the 8 m elevation contour. They have rough, gravelly to smooth sandy surfaces, which still accommodated our antennas. III. EQUIPMENT AND PROCEDURES We used a GSSI SIR3 16-bit control unit and GSSI Model 327 (85 MHz, a ground-loaded value) for our deep soundings (Fig. 4) to greater than 4 m and Model 513 (3 MHz, ground-loaded) to about 7 m. We also used the Model 311 (7 MHz, ground-loaded) to sound a few meters, the results of which have been reported elsewhere recently [6]. The pulse waveform has 1½ major half-cycles and a 35% 3-dB bandwidth. We recorded at time ranges of 1 ns and less. We hand-towed antennas at < 1 m/s with the polarization orthogonal to the transect directions. Transect distance marks were located with GPS. We recorded with range gain. We processed all profiles with RADAN (GSSI) software and included high- and low-pass filtering, background removal, elevation corrections, and, in some cases, single velocity time migration. Figure 4. Profiling at 85 MHz on Crescent Stream.

We profiled other similar features as high as 22 m asl [5]. These and other terraces have been surmised to be deltas [2].

3 Figure 5. Sandy terrace along Crescent Stream (top; Lake Fryxell is in the background) and a 4-MHz longitudinal radar profile. IV. RESULTS Crescent Stream The Crescent Stream channel is cut into several sandcovered mesas. The surface of the mesa (Fig. 5) lies 8 m above sea level. We profiled other similar features as high as 22 m asl [5]. These and other terraces have been surmised to be deltas [2]. Alternatively, they might be incised point-bar deposits in valley-side, alpine-glacier-sourced fluvial systems. The sigmoidally shaped foreset beds overlain by topset beds (Fig. 5) confirm the delta hypothesis. The lower tails of the beds merge congruently with a stronger reflecting horizon of bottomset beds. The tops of the foreset beds seem to be slightly eroded and superposed by a surfaceconformable layer of sediments. Typically, these topset beds are deposited subaerially. The depth scale in Figure 5 is based on a dielectric constant ε = 5.5, the determination of which is discussed next. The deepest events occur near 5-m distance and are ~6 m from the surface. The vertical exaggeration is 3.5, so the surface is much flatter than it appears. The delta confirms a lake in Taylor Valley but not the lateral extent of the lake. This might have been a narrow icemarginal lake with the Ross ice margin very near, or a much longer lake if the Ross ice front was 3 km to the east. From GPR only, we surmise that the lake level remained at 8o m for a significant length of time. Sloth Lake Cross-valley ridges Sloth Lake is a small frozen pond lying on a valley-side bench; we use the name for location. In this vicinity there are a series of cross-valley ridges (Fig. 6), with ridge axes oriented ~ NNW SSE. The glacio-lacustrine interpretation is that these were formed from sandy sediments falling through seasonal cracks in the lake ice cover. The sediment

is derived from debris that melted out onto the lake ice surface and deposited directly onto the lake floor when water level drops [1].

Consequently, the sediment of these ridges should be that which settled upon the lake floor lacustrine sediments.

The profiles show a more complex picture. Figure 7 shows two profiles recorded along the axis of a single ridge.

The near-surface bed is about 2 7 m thick; close examination suggests foreset bedding, but the resolution is poor.

4 is derived from debris that melted out onto the lake ice surface and deposited directly onto the lake floor when water level drops [1]. In this interpretation the debris may have existed within ablating glacial ice thrust onto the lake ice cover. Consequently, the sediment of these ridges should be that which settled upon the lake floor lacustrine sediments. A glacial interpretation might suggest that these ridges contain ice-marginal outwash and that their shape is a consequence of deposition constrained by ice walls. The profiles show a more complex picture. Figure 7 shows two profiles recorded along the axis of a single ridge. In the lower, 85-MHz profile, two relatively flat-lying beds are prominent, and flat bedding is suggested to at least 3 m deep (arrow). The near-surface bed is about 2 7 m thick; close examination suggests foreset bedding, but the resolution is poor. However, the 3-MHz top profile, recorded over the same transect, shows that the upper, flat-lying bed has a general slope of ~3 and appears scalloped, which may be bottom ripple marks. The prograding beds appear to be a superposition of two processes. The first was progradation caused by stream flow. The second generated scalloped, convexly oriented horizons. In the upper profile, they appear synchronous with scalloping in the more continuous, deeper horizon between 2 5 m. They also appear to cut across the progradation. Alternatively, they may only seem to cut the beds but, in fact, they separate distinct periods of deposition. çç çççççççççççç ççç ççççççççççççççççççççççççççççççççççççççç SL16 çççççççç çççç ç ç ç çç SL15 çççççççççççççççççççççç SL5 Meters N SL13 SL14 SL12 Figure 6. Top: Transects over ridges (left side) and over a sinuous ridge (right side) near Sloth Lake. The jagged stipling at upper left is the 8-m elevation contour. Bottom: The ridge system with our lines superimposed. NNW SSE Figure (top) and 85-MHz profiles of transect SL 16, along the axis of a single ridge near Sloth Lake. The arrow indicates the deepest visible horizon.

$ENE WNW Figure 8. 3-MHz profile along transect SL 15, which crosses two ridge axes (left), and detail of a single diffraction hyperbola found near the 9-m distance.$

5 ENE WNW Figure 8. 3-MHz profile along transect SL 15, which crosses two ridge axes (left), and detail of a single diffraction hyperbola found near the 9-m distance. Figure 8 shows another view of the convexly oriented scalloped horizons. The profile was recorded from ridge to ridge in an ENE WSW direction. Figures 7 and 8, therefore, show that the scalloping is three dimensional. There is also no appearance of progradation, so that the profile in Figure 7 was better aligned with the direction of deposition, which would be between NNW and NNE. Consequently, these features could have been generated from a lateral melt water channel associated with Ross ice. We note also that the scalloped horizons appear truncated, so that erosion may have formed the ridges. At right in Figure 8 is a section that contains the narrowest hyperbola we have found. It originates near the 9-m distance. We show it without correction for elevation for clarity, but the elevation change across the m spanned by this diffraction is < 1 m. We match the hyperbola with a theoretical diffraction for ε = 5.5. We measured ε of fine sand samples from the region with Fourier Transform time domain spectroscopy and found ε = 3.4 at an average volumetric water content (three samples) of 1.%. Correcting for the density of a gravel (~.15 porosity), eliminating water, filling the interstices with ice, and using the popular CRIM dielectric mixing formula for the real part elevates ε to 5.5, given an average mineral n = 2.45 (ε = 6.). The ε of feldspars vary from 5.7 to 7., with the higher values for the plagioclases, which are more common than orthoclases in the McMurdo volcanic rocks. Pyroxenes are about 8.5, and micas range from 6.2 to 8.. So, an average of 6. or higher is plausible for the mineral matter. We used ε = 5.5 for all of our profiles, given that all these sites contain erosion from similar rocks. Figure 9 shows the first 2 m of an 85-MHz profile recorded over larger ridges northeast of the cross-valley Figure MHz profile along transect SL 5, located north of the ridges, starting from the north and delineated by bars in Figure 6.

6 SE NW SE NW SL-13 2 SL m SL SW 6 NE Figure 1. 3-MHz profiles recorded along sinuous ridge transects in Figure 6. ridges (Fig. 6). The direct coupling has been removed to reveal the near-surface structure, yet there is no suggestion of the stratification we see in Figures 7 and 8. Consequently, it is not part of the cross-valley ridge system. However, we do see long sloping horizons at 1 15 m deep, just as in Figure 7. Sinuous Ridge A sinuous ridge with dark, blackish lithology occurs near Sloth Lake (Fig. 6). Hall et al. [1] interpreted it to be a deposit from a lake ice stream that cut headward into the lake ice in summer. This ridge would then be deposited through standing water which should have disaggregated fluvial sediments marking the former stream channels. In other words, a stream on the lake ice incised a channel in which sediment was deposited, and then the sedimentation diffused through the ice to the lake below. If so, we might expect that the stratigraphy of this ridge would be surface conformable because, as sediment rained down, it would have continually draped over the forming ridge. The three profiles of Figure 1 show good evidence of surface-conformable stratigraphy. The horizons at 3 5 m deep generally follow the surface elevation. However, both the NW slope of transect 14 and the NE slope of the longitudinal profile show nonconformable sections of prograding deposition not expected for sediments raining down from the ice cover. Therefore, this ridge might also be caused by ice-marginal fluvial deposition that formed an esker. Coral Ridge Coral Ridge snakes across the mouth of ETV in a generally north-to-south direction. It appears to define the threshold of the valley floor. The maximum lake interpretation would require the existence of flat-lying beds, while the more terrestrial glacial interpretation would suggest a till structure of little stratigraphy. The 85-MHz profile (Fig. 11) along the transect indicated in Figure 3 is 67 m long and transects a distinct channel. The profile shows no evidence of lacustrine-type sedimentation. The beds are not flat, extensive, or continuous. The incision of the stream channel is obvious, but continuation of horizons from one side to the other is not evident. The horizons exhibit various strengths, and there are numerous prominent diffractions. The uneven surface is typical of moraine, so we speculate that this part of Coral Ridge is, indeed, some kind of recessional moraine. Although tills may be well mixed on average, they can be partly stratified. V. DISCUSSION AND CONCLUSIONS We started with two propositions regarding the presence of an ice lobe (WAIS) and a proglacial lake in lower Taylor Valley. We sought subsurface information to support either. The Crescent Stream delta indicates a stable lake level at 8 m asl. Yet we do not know the lateral extent of the lake. Was it a narrow ice-marginal lake whose level was strongly controlled by ice extent? Or was it a long lake whose level might reflect regional climate? The equivalent elevations of the stratigraphic deposits at Sloth Lake is not consistent with a sedimentary origin of settling through standing water. Therefore, Sloth deposits should not be used to reconstruct the proglacial lake. The Sloth Lake GPR evidence indicates glaciofluvial sedimentation along an active ice margin at nearly the same elevation as the proglacial lake level. The ice-marginal channel system flowed from north to south. The sinuous ridge structure is more consistent with ice-marginal sedimentation than with lake sedimentation. Coral Ridge is most likely a terrestrial moraine, and not a glacio-lacustrine deposit.

7 Figure MHz profile across Coral Ridge. Consequently, we think that a large ice lobe advanced into and receded from lower Taylor Valley in contact with a proglacial lake. We speculate that the Ross ice lobe reached as far west as Lake Fryxell, and then the lake formed as it receded. Coral Ridge may have been a recessional moraine where the ice lobe stagnated upon retreat and supplied meltwater to the lake. ACKNOWLEDGMENTS This work was supported by the National Science Foundation, Office of Polar Programs. REFERENCES [1] Hall, B. L., Hendy, C. H., and Denton, G. H. 26. Lake-ice conveyor deposits: Geomorphology, sedimentology, and importance in reconstructing the glacial history of the Dry Valleys. Geomorphology, 75, [2] Stuiver, M., Denton, G. H., Hughes, T. J., and Fastook, J. L History of the marine ice sheet in West Antarctica during the last glaciation: A working hypothesis. In Denton, G. H., and Hughes, T. J. (Ed.), The Last Great Ice Sheets. New York: John Wiley and Sons. [3] Arcone, S. A., Delaney, A. J., and Prentice, M. E. 2. Stratigraphic profiling in the Antarctic Dry Valleys. Proc., 8th Int. Conf. on Ground-Penetrating Radar, Gold Coast, Australia, May 2. [4] Arcone, S. A., Prentice, M. L., and Delaney, A. J. 22. Stratigraphic profiling with groundpenetrating radar in permafrost: A review of possible analogs for Mars. J. Geophysical Research, 17(E11), doi: 1.129/22JE196. [5] Horsman, J. 28. Ground-penetrating radar studies of late Pleistocene-Holocene lacustrine deltas in Lower Taylor Valley, Antarctica: Implications for lake history. M.S. thesis. Plymouth, NH: Plymouth State University. [6] Arcone, S. A., Spikes, V. B., and Hamilton, G. S. 25. Stratigraphic variation in polar firn caused by differential accumulation and ice flow: Interpretation of a 4-MHz short-pulse radar profile from West Antarctica. J. Glaciology, 51(7), [7] Spikes, V. B., Hamilton, G. S., Arcone, S. A., Kaspari, S., and Mayewski, P. A. 24. Primary causes of variability in Antarctic accumulation rates. Annals of Glaciology, 39,

4. What type of glacier forms in a sloping valley between rock walls? a. firn glacier b. ice sheet c. cirque d. alpine glacier

4. What type of glacier forms in a sloping valley between rock walls? a. firn glacier b. ice sheet c. cirque d. alpine glacier Multiple Choice Questions 1. The term means the loss of snow and ice by evaporation and melting. a. sublimation b. ablation c. erosion d. abrasion 2. What condition must be met for a glacier to begin flowing