Flow stripes, GPR stratigraphy and RADARSAT imagery

In a recent paper, Campbell and others (2008) discussed the lack of correlation between the deep (>100m) cross-flow folds in radar-detected stratigraphy and surface ‘flow stripes’. Their study focused on an area upstream of the now stagnant part of Kamb Ice Stream (KIS), West Antarctica, and they noted the lack of near-surface stratigraphy obtained by higher-frequency radar to help explain the apparent anomaly. Such data do exist and, as we illustrate below, show a good correlation between the flow-stripe image features and the near-surface stratigraphy. Figure 1 illustrates part of the RADARSAT Antarctic Mapping Mission (AMM) 125mmosaic and the position of two of the lines (AA’ and BB’ in figure 2 of Campbell and others, 2008). These lines were also studied in the fieldwork of Smith and others (2002) and Ng and Conway (2004). The groundpenetrating radar (GPR) data used by Smith and others (2002) in the study of buried crevasses in the shear zone of KIS did map near-surface stratigraphy. Lines LL’ and KK’ in their figure 3 were acquired in fieldwork carried out in the 1997/ 98 austral summer, and correspond to the deep radar lines AA’ and BB’ of Campbell and others (2008) and lines XX’ and YY’ of Ng and Conway (2004). The choice of LL’ and KK’ by Smith and others (2002) reflected two of the lines used in the pioneering work of Shabtaie and Bentley (1987). Details of the processing of the 80MHz Geophysical Survey Systems Inc. (GSSI) monopulse GPR are given by Smith and others (2002), together with the technique used to date particular layers in the stratigraphy. The depth of the firn layer deposited around 1951 ( 46 4 years prior to the fieldwork) is plotted in Figure 2 for the two test lines and compared with the average 5.3GHz radiometric variation. Radiometric smoothing of the RADARSAT data was performed primarily along the direction of the flow-stripe feature. First, the orientations of the flow stripes were estimated with respect to the two lines. Then averaging was applied with a smoothing function ( 250 800m) such that the extra averaging was applied in the along-flow-stripe direction. The correlation between the depth of the particular layer and the average radiometry in Figure 2 is remarkable. Some examples of these correlations have been presented at West Antarctic Ice Sheet meetings (Conway and others, http://neptune.gsfc.nasa.gov/wais/pastmeetings/PPT04/ Question3/Conway.ppt; Gray and others, http://neptune. gsfc.nasa.gov/wais/pastmeetings/PPT05/Gray.ppt; King and others, http://neptune.gsfc.nasa.gov/wais/pastmeetings/ abstracts04/King.htm), including results collected during the 2000/01 British Antarctic Survey–University of Texas radar experiment over parts of Bindschadler Ice Stream and a tributary of KIS. This result appears to be general for cold polar firn in West Antarctica and does not depend on the orientation of the test line with respect to the ice motion, or the magnitude of the ice velocity. However, if the strain rates are high enough for crevasse formation then this relationship breaks down. Figure 3 illustrates the near-surface stratigraphy studied by Smith and others (2002) (top curve) and the deep stratigraphy studied by Ng and Conway (2004). The weak correlation between the near-surface and deep stratigraphy confirms the conclusion of Campbell and others (2008) that there is a weak correlation between flow stripes and deep folds. This is puzzling because, as isochrones, the nearsurface stratigraphy should evolve smoothly into the deep stratigraphy. In Figure 3 the upper 300m of the deep radar results of line A’A are obscured by the illustration of the depth of the 1951 isochronal layer. Note that there is correlation between the bed and the stratigraphy (both deep and nearsurface) at the left edge of the figure where the ice moves over what appears to be a subglacial escarpment. Also, there is correspondence between the near-surface and deep folds in the region vertically above the ‘Bed’ label, but otherwise the correlation is poor. These results prompt two questions: why does the shape of cross-flow isochrones and in particular the positions of folds in the stratigraphy change with near-surface depth in a region such as the KIS? and what is the origin of the strong correlation between the RADARSAT image radiometry and near-surface stratigraphy? The first question has been discussed in depth by Campbell and others (2008), and we make only the following comments: Near-surface stratigraphy (to 10m) is controlled largely by accumulation which varies locally, depending primarily on surface slope and prevailing winds. But the surface slope is itself influenced by forces acting on the ice and firn, including those that reflect movement over the varied basal topography and other conditions. Also, with time (and depth), isochrone separation becomes more a function of the cumulative history of the strain than of the original accumulation. For example, if the firn in a region of creep or tributary flow undergoes a relatively modest horizontal strain of 10 a in the flow direction, and zero across-flow, then the associated vertical GPR stratigraphy will decrease by 10% in only 100 years. Remembering that it takes many hundreds of years to accumulate 100m of firn in this region of West Antarctica, and that deeper layers originated further and further upstream, the deeper folds reflect the accumulated strain history much more than the original spatial accumulation pattern. We believe this to be particularly true in this region where the forces, and the Gray and others: Correspondence