Ice-Ocean Interaction On Ronne Ice Shelf, Antarctica

Abstract

Detailed glaciological studies have been completed at 28 sites lying on an approximate flow line, extending 760 km across Ronne Ice Shelf. Parameters measured at each location include ice velocity, thickness, principal strain rates, surface elevation, temperature, and accumulation rate. The data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site, from the principles of mass and energy conservation. These calculations indicate basal melting in excess of 1 m yr 4 over the first 100 km of the flow line downstream of the grounding line, where the ice shelf is between 1200 m and 1600 m thick. The maximum melt rates in this region occur near the inland margin and exceed 4 m yr 4. Melting continues at a lesser rate over the next 200 km before freezing commences. Freezing then dominates up to the final 100 km before the ice front, resulting in the accumulation of a layer of basal sea ice up to 50 m thick. This is rapidly removed as melt rates increase to over 6 m yr 4 at the ice front. The cumulative effect of this pattem of basal accumulation and ablation is the wastage by melting of 85% of the mass discharged across the grounding line before it reaches the ice front. Supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections, observed during radio echo sounding of ice thickness. In the region where mass balance calculations suggest an accretion of basal sea ice, reflection coefficients are consistently low, ranging from -6 dB to -38 dB. Most of these weak reflections are believed to originate from the true base of the ice shelf, the additional energy loss being the result of increased attenuation of the radar signal within the saline layer. The derived pattem of basal melting and freezing is consistent with a simple model of sub-ice-shelf oceanic circulation, involving a deep thermohaline convection cell. Dense, saline water, which is formed over the continental shelf during winter when the sea surface freezes, drains into the deepest parts of the sub-ice-shelf cavity. At the inland margin, where this water mass comes into contact with the ice shelf, its temperature is 1 *C above the local pressure freezing point. Melting of ice results, producing a buoyant outflow of cold, relatively fresh water, along the ice shelf base. Basal freezing occurs towards the ice front, where the ascending water becomes supercooled. This circulation has important implications for the production of Antarctic Bottom Water and for the response of the ice shelf to driving stresses, through the temperature dependent viscosity of ice. thickness or surface area of the ice shelves would weaken their buttressing effect, leading to a reduction in the volume of grounded Published in 1991 by the American Geophysical Union. Paper number 90JC01952. ice and a rise in global sea level. However, the magnitude and speed of the ice sheet' s response to such changes, and the likelihood of a complete collapse of the West Ant...

Interactions between ice and ocean observed with phase-sensitive radar near an Antarctic ice-shelf grounding line

“…The agreement between these entirely independent determinations of compaction is remarkably good. Clearly, had we been unable to identify a reference layer deep enough to lie within solid ice, using a shallower reflector and an estimate of compaction beneath it based on the earlier observations of Jenkins and Doake (1991), i.e. the solid black line in Figure 11b, would have introduced additional errors of no more than a few centimetres per year to our estimates of basal melting.…”

“…The latter is complex (Fig. 1), with a sinuous grounding line and ice thickness ranging from <1600 to >2000 m. Ice velocities peak at around 400 m a -1 near the grounding line, and decrease steadily downstream (Stephenson and Doake, 1982;Jenkins and Doake, 1991). In the centre of the ice stream a ridge of grounded ice extends 15-20 km downstream.…”

“…The rate factor, A $ 10 -18 Pa -3 a -1 (Jenkins and Doake, 1991), the exponent n ¼ 3, and the effective strain rate is $10 -3 a -1 (Fig. 4a).…”

“…The underlying linear trend in the more complex model is -1.46 AE 0.10 m a -1 . This knowledge makes it possible to identify the weak return in Stephenson and Doake (1982) Ground-based 600 1.7 Jenkins and Doake (1991) Ground-based 12.5 1.2 AE 0.5 Corr and others (1996) Ground-based and airborne 400 1.5 AE 1.6…”

“…Jenkins and Doake (1991) measured an accumulation rate of 450 AE 65 kg m -2 a -1 at the location of site 1 and estimated a density-depth profile based on measurements of firn density in the upper 2-5 m at seven sites on the Ronne Ice Shelf. The compaction rate inferred from these data is also shown in Figure 12b.…”