Variations in valley glacier activity in the Transantarctic Mountains as indicated by associated flow bands in the Ross Ice Shelf*

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1 Sea Level, Ice, and Climatic Change (Proceedings of the Canberra Symposium, December 1979). IAHS Publ. no Variations in valley glacier activity in the Transantarctic Mountains as indicated by associated flow bands in the Ross Ice Shelf* C. R, BENTLEY Geophysical and Polar Research Center, University of Wisconsin, Madison, Wisconsin 53706, USA ABSTRACT Radar sounding records from the grid eastern Ross Ice Shelf show striking variations in basal reflectivity closely associated with the source of the ice. Ice from glaciers shows a strong basal echo, whereas on ice from between the glaciers the echo is weak or absent, presumably due to brine infiltration. The width and continuity of the strongly reflecting flow bands, as they are traced across the Ross Ice Shelf, provide information about the relative activity of the glaciers compared with that of the surrounding ice. Large changes in activity within the last 1500 years have been found. The cause is not certain, but the author believes it to be related to past oscillations of the glacier grounding lines resulting from changes in ice shelf thickness. A good correlation between glacier activity and the oxygen isotope records from ice cores at Dome C and Byrd stations strongly suggest some palaeoclimatic significance. The continuity of the glacier flow-band record precludes any major surges of the West Antarctic ice sheet in the 1500 years. Radar sounding records from the grid eastern sector of the Ross Ice Shelf show striking variations in basal reflectivity closely associated with the source of the ice. Ice from glaciers shows an echo that is at least intermittently strong, whereas ice from between glaciers reflects the radar wave weakly at best, presumably due to brine infiltration above the firn-ice boundary. There are not only large variations in reflection characteristics between glacial and interglacial bands, however, but also marked variations in the reflection characteristics within individual ice bands as they are traced downstream across the ice shelf from the glacier mouths. Here we examine in particular a group of valley glaciers that form four distinct bands between Beardmore and Nimrod glaciers, both of which are major East Antarctic outlet glaciers. This group was selected principally because of the large number of radar sounding tracks crossing their associated flow bands - about 25 when flights of the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS) are combined with those of the National Science Foundation/Scott Polar Research Institute/Technical University of Denmark cooperative programme (Fig. 1). * University of Wisconsin-Madison, Geophysical and Polar Research Center, Contribution no

2 248 C.R. Bentley JEG DS RG DOMEC BYRD Fig. 2 (a) "Quality" vs. residence 2 S time on the ice shelf for the bands corresponding to Jacobsen and Ekblad Fig. 1 Map of grid eastern portion of the Ross Ice glaciers (JEG), Davidson Glacier (DG), and Robb Glacier (RG). Black sections have high "quality", as defined in text, compared to white sections, (b) Oxygen isotope ratios vs. age in drill holes at Dome C (East Antarctica) and Byrd station (West Antarctica). Black sections correspond Shelf, showing the relationship of flight lines to the Transantarctic Mtn glaciers and the ice shelf flow band. The numbered and lettered lines denote NSF/SPRI/TUD and RIGGS flights, respectively. to relatively high SO values (algebraically), i.e. warmer temperatures; white sections to relatively low S 18 0 values. Two characteristics of the glacier bands were measured, the width and the reflection strength. In order to make an approximate correction for the downstream convergence of ice flow lines, the widths were measured as percentages of the total width of the band from the grid eastern edge of Beardmore Glacier ice to the grid eastern edge of Nimrod Glacier ice. No quantitative measure of the reflection strengths was available, so a qualitative assessment was made instead on a five-point scale (excellent, good, fair, poor, and non-existent, assigned numerical values of 4, 3, 2, 1 and O, respectively). Upon careful examination of the echograms, it became clear that there were, in three of the four bands, two sets of echoes, either one or both of which might be present in a particular location: the principal echo from the bottom of the ice shelf, and an echo from an internal reflector at a height of roughly 50 m above the ice-water boundary. Reflection strengths were assigned separately to the two sets of

3 Valley glacier activity in the Transantarctic Mtns 249 echoes, since, to a considerable degree, they vary independently of each other. Finally, a semi-quantitative "quality" factor was assigned to each reflector at each crossing of each glacier band by multiplying the relative width by the numerical reflection strength. A scale of residence time of the ice in the ice shelf, i.e. length of time since the ice at a particular spot left the corresponding glacier mouth, was developed using ice movement data from RIGGS. Velocities were first plotted for the outlet glacier bands because of the availability of velocities right at the mouths of two of them (Beardmore and Nimrod glaciers; Swithinbank, 1963). Plots of velocity vs. distance on the ice shelf downstream from the outlet glaciers were then interpolated for the valley glaciers, with correction factors near the mouths calculated for each glacier from the relative widths of the glacier mouths and the corresponding band on the nearest radar sounding track. The corrections, based on conservation of mass, are likely to lead to velocities near the glaciers that are too large rather than too small, since no account was made of a probable, although unmeasured, decrease in ice thickness between the glacier mouths and the first flight crossing. Because of the uncertainties near the glacier mouths, the residence times on the ice shelf calculated from the velocity vs. distance plots could easily be in error by loo years or more, probably on the low side. Plots of "quality" as a function of residence time for each of the valley glacier bands show pronounced downstream variations. The bottom echoes for all four glaciers have been relatively low in "quality" for the last 600 years. Prior to that there are peaks of high "quality" for each glacier lasting 50 to 150 years, but the correlation between bands is rather poor. For the internal reflector, however, the inter-band correlation is much better. All three bands show very similar patterns (Fig. 2(a)): high "quality" since about 300 years ago (the record begins at about 200 BP), between 500 and 800 BP, and more than 1150 BP (the record ends at 1350 BP), with periods of very low "quality" in between. These variations are not minor - "quality" factors along a particular band vary by an order of magnitude or more. Just what causes these striking variations is not at all clear. It seems inescapable that some sort of variation in the valley glaciers themselves has been occurring over the last 1500 years, but it is difficult to be more specific. The source of the internal reflections is not even known. The most likely possibilities are concentrated bottom crevasses and englacial moraine layers. Neither of these explanations is without difficulty: bottom crevasses elsewhere on the ice shelf seem always to show diffraction hyperbolas associated with the junction of the crevasses and the base of the ice, features that are largely absent in the bands in question. On the other hand, it is difficult to understand how moraine could regularly, but intermittently, be entrained to a height of 50 m above the glacier bed. A particularly striking feature of the record is the virtual disappearance of both basal and internal reflections associated with the glaciers about 500 years ago. This might be attributable

4 250 C.R. Bentley to rifting and brine infiltration above the firn-ice boundary, as is believed to occur between glacier bands, although there is no direct evidence of such an occurrence. However, most of the variations are not attributable to that mechanism, because the basal or near-basal echoes, even when weak, are clearly present. The most likely explanation in the author's opinion, is that past variations in the thickness of the ice shelf have caused advances and retreats in the grounding lines at the mouths of the glaciers, and that these advances and retreats, by some unknown mechanism, govern the strength of the internal reflector. This interpretation is strengthened by evidence presented by Jezek (1980) that a large shift in the grounding line of Crary Ice Rise, in the grid northwestern part of the Ross Ice Shelf, took place about 500 years ago. Changes in the width of the bands, however, while less coherent than the internal reflection changes, are nevertheless pronounced, suggesting that there were also major changes in the mass output of the glaciers. Evidence that the glacial phenomena are related to climatic changes comes from the oxygen isotope ratios in the ice cores from Byrd station (Johnsen et al., 1972) and Dome C (Lorius et al., 1979). The Dome C record shows a direct correspondence with all five of the "quality" zones in the internal reflections, with the more negative & O values corresponding to the low "quality" zones (Fig. 2(b)). At Byrd station, the four deeper zones all appear, although shifted perhaps years later; the record for the last 300 years does not exist in the Byrd station core. The amplitude of the ô 8 0 variations is between 1 and l^/oo at both stations, corresponding to a temperature change of 1 - lh. Although oxygen isotope ratios can change because of surface elevation changes, it is highly unlikely that the heights of the East and West Antarctic ice sheets have been oscillating in phase over a range of m with a period of 800 years. Thus a real atmospheric temperature change is indicated. Whether the temperature changes caused the glacial variations in some way, or both temperature and glacial changes reflect some larger-scale alterations in the Ross Ice Shelf and conceivably, in the regional sea ice cover, is an open question, although some model along the latter lines seems more likely to the author. However, the good (but not perfect) agreement between glacier band boundaries and the present flow lines as determined by the ice movement measurements precludes any major surges of West Antarctic ice within the last 1300 years or so, such as those proposed by Wilson (1978), since a surge would inevitably result in a large distortion of the historic flow bands. REFERENCES Jezek, K. C. (1980) Radar investigations of the Ross Ice Shelf, Antarctica. PhD Thesis, Geophysical and Polar Research Center, Univ. of Wisconsin, Madison, Wisconsin. Johnsen, S. J., Dansgaard, W., Clausen, H. B. & Langway, C. C, Jr (1972) Oxygen isotope profiles through the Antarctic and Greenland ice sheets. Nature 235,

5 Valley glacier activity in the Transantarctic Mtns 251 Lorius, C, Merlivat, L., Jouzel, J. & Pourchet, M. (1979) A 30 OOO-yr isotope climatic record from Antarctic ice. Nature 280, Swithinbank, C. (1963) Ice movement of valley glaciers flowing onto the Ross Ice Shelf, Antarctica. Science 141, Wilson, A. T. (1978) Past surges in the West Antarctic ice sheet and their climatological significance. In: Antarctic Glacial History and Palaeoenvironments (ed. by E. M. van Zinderen Bakker), Balkema, Rotterdam.

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