Part Of Ice Stream Research Articles

Uncovering the hidden part of a large ice stream of the Laurentide Ice Sheet, northern Ontario, Canada

Abstract This investigation was prompted by an enigmatic ice-flow anomaly (Area A) on the Glacial Map of Canada which covers about 10 000 km 2 in the Hearst/Kapuskasing area of northeastern Ontario. It consists of streamlined landforms and striations indicative of a major ice flow toward 130° oriented at right angle to another toward 220°. Both are late glacial flows but long-lasting disagreement exists regarding their relative age. The analysis of aerial photographs and satellite images in conjunction with a detailed survey of bedrock cross-striated surfaces over an area of about 30 000 km 2 within and around Area A clearly indicate that the 130° flow preceded the 220° flow. The earlier conflicting interpretations within Area A are attributed mainly to the sporadic occurrence of relict striated surfaces formed by older southwestward (220°–240°) Wisconsinan ice flows that have locally escaped destruction by late glacial flows, with the result that the southwestward flows are older (Wisconsinan) at some sites and younger (late glacial 220°) at others relative to the 130° flow. When considered with other factors such as the maximum elevation reached by the youngest late glacial flow, these ice-flow relationships indicate that Area A is the outcropping southern part of a much larger ESE ice-flow system, which is probably related to a large fluted belt located to the north and that was identified as the Winisk Ice Stream. The distal part of the ice stream, except for Area A, escaped detection by remote sensing mapping methods because depositional and erosional features associated with it are masked by deposits laid down by the younger (220°, Cochrane) ice flow and/or by postglacial marine and organic deposits (or were destroyed by the younger ice flow). The only reliable indicators of the passage of the ice stream in this “buried” section are ESE relict striations crossed by SW striations. The advancing ice stream toward the ESE not only preceded the late Cochrane 220° flow but probably outlasted it, as suggested by the mapping of several thousand iceberg furrows in Quebec and Ontario, also directed toward the ESE and overprinted on flutes formed by the last glacial flow. If this interpretation is correct, this makes the Winisk Ice stream the largest terrestrial ice stream in the Hudson Bay basin. An alternative interpretation associates this fluted belt to the exposed western fringe of streamlined bedforms from a late Hudson lobe buried by younger sediments.

The morphology of ice streams A, B, and C, west Antarctica, and their environs

Airborne radar soundings of the ice sheet surface made in 1984–1985, together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams A, B, and C and much of the region around them. The flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking. Ice thicknesses were also measured along the flight lines but have not yet been mapped. The surfaces of active ice streams A and B exhibit a longitudinal ridgetrough topography of uncertain origin. Prominent surface valleys are associated with most, but not all, of their marginal shear zones. There is a deep subglacial trough beneath the grid northeastern side of ice stream A that connects to the subglacial trough beneath Reedy Glacier. The boundary between ice streams Bl and B2, the two tributaries of ice stream B, persists as a suture zone for 250 km downstream from their merger. Ice stream Bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream B until it disappears close to the grounding line. Between ice streams Bl and B2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice, which suggest to us that “rafts” of ice are being incorporated into the ice streams. The previously reported negative mass balance for the drainage system of ice stream B may arise from the expansion of the ice stream into the interior of the West Antarctic ice sheet. Crevasse bands at some places on ridge AB, together with high glacial velocities reported elsewhere, also suggest this expansion. Close to the Ross Ice Shelf, ice stream B widens into a nearly flat area where the mean surface slope is only 3.5 × 10−4, and the surface elevations are only a few tens of meters above hydrostatic equilibrium. Inactive ice stream C differs from active ice streams A and B in surficial and basal characteristics. No elongated ridges and troughs are observed; instead, the ice stream surface exhibits several terraces, including some maxima and minima in elevation. Radar sounding reveals areas where basal echoes are strong and steady, indicating subglacial water, alternating with areas of weaker echoes with short fading lengths. Calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick, depending on the electrical conductivity of the water. Due to reversals in the surface slope, the hydrologic potential gradient changes sign several times along ice stream C, which implies that subglacial water should collect in some locations.