Discharge of debris from ice at the margin of the Greenland ice sheet

Abstract

Sediment production at a terrestrial section of the ice-sheet margin inWest Greenland is dominated by debris released through the basal ice layer. The debris flux through the basal ice at the margin is estimated to be 12^45 m 3 m^1a^1. This is three orders of magnitude higher than that previously reported for East Antarctica, an order of magnitude higher than sites reported from in Norway, Iceland and Switzerland, but an order of magnitude lower than values previously reported from tidewater glaciers in Alaska and other high-rate environments such as surging glaciers. At our site, only negligible amounts of debris are released through englacial, supraglacial or subglacial sediment transfer. Glaciofluvial sediment production is highly localized, and long sections of the ice-sheet margin receive no sediment from glaciofluvial sources. These findings differ from those of studies at more temperate glacial settings where glaciofluvial routes are dominant and basal ice contributes only a minor percentage of the debris released at the margin. These data on debris flux through the terrestrial margin of an outlet glacier contribute to our limited knowledge of debris production from the Greenland ice sheet.

Changes in ice‐margin processes and sediment routing during ice‐sheet advance across a marginal moraine

“…At our site the traversing of those parts of the moraine that were entirely overtopped during our study period appears to have taken approximately 30 years, which allows us to estimate an average rate of sediment production by moraine destruction during this peak period of c. 45 m 3 /m/year, which is of a similar order of magnitude to the amount of sediment being transferred through the basal ice (Knight et al 2002). This suggests that, for our site, we can postulate that through the sequence of stages from A to F sediment transfer into the distal proglacial zone is initially limited by storage of sediment behind or within the moraine, achieves peak values during stages C-E when moraine destruction is combined with unimpeded release of basal ice debris, and returns to 'normal' values (defined by basal ice flux) at stage F. This very approximate estimate is illustrated in Fig.…”

“…The debris-rich basal ice layer is the dominant route for sediment transfer through the ice over long sections of the ice-sheet margin, with a debris flux of 12-45 m 3 /m/year (Knight et al 2002). The character of the basal ice layer varies around the ice margin but certain components are consistent throughout the area: a ubiquitous basal stratified facies averaging 2 m thickness and 39% debris by volume at base of the sequence; overlying this, a basal dispersed facies up to 20 m thick, containing 2% debris by volume, dominated by silt pellets up to 10 cm in diameter; debris bands a few centimetres thick but similar in composition to the stratified facies intercalated with the bottom part of the dispersed facies in many locations, making stacked sequences of bands sometimes in excess of 10 m. Fig.…”

“…Our measurements of basal ice were made by detailed logging of exposed sections combined with sampling and laboratory analysis of debris following procedures described by Knight et al (2002). The basal ice sequence was scrutinised along the entire margin, and detailed logs were measured and sampled at close intervals (metres or tens of metres depending on the variability along the margin).…”

“…Ice velocity and ablation were calculated from multi-year repeat theodolite surveys of markers emplaced on the glacier surface (Knight 1992). Particle size distributions of debris from each facies were determined by measuring large clasts individually in the field and by sieving and hydrometer analysis of bulk debris samples returned to the laboratory (see Knight (2002) for further details). Sed- iment concentration in meltwater breaching the moraine was measured by bottle-sampling and filtering but as this was carried out at only one site (a moraine breach at the western end of the morainedammed lake shown on Fig.…”

“…Maizels 1979;Warburton 1990), but less is known about sediment discharge from modern terrestrial ice-sheet margins that might be analogous to former mid-latitude ice sheets. Knight et al (2002) measured sediment production at our site, but did not address the problem of what happens to this sediment after it is released from the ice. In the same area Van Tatenhove (1995) demonstrated that the ice-margin moraines contain more sediment than can be accounted for by glacial sediment production during the periods of their formation.…”