Predicted age‐depth scales for Siple Dome and Inland WAIS Ice Cores in west Antarctica

If an ice sheet is frozen to its bed, deep ice directly under a divide experiences low deviatoric stress and is relatively hard, because the rheology of polar ice is described by a power-law constitutive relation. In steady state, stratigraphic layers tend to form an arch (“Raymond bump”) in this region. However, when the basal ice can slide, the stresses are redistributed, and longitudinal extension due to sliding is associated with increased deviatoric stress in the deep ice under the divide. This increased deviatoric stress weakens the tendency to form a Raymond bump. To find a realistic spatial distribution of sliding under an ice divide, we incorporate a thin layer of viscous till in a finite-element plane-strain flow model. The resulting basal “sliding” velocity varies approximately linearly with distance from the ice divide. By varying the till viscosity, we can adjust the amount of basal motion. We find that the Raymond bump decays exponentially with the fraction of total ice flux carried by sliding: the arch is 50% smaller when the sliding flux is only 7% of the total ice flux. This implies that the possibility of a wet bed must be considered when inferring past ice-divide locations from radar internal layering.

Section: Discussionmentioning

confidence: 62%

Effects of basal sliding on isochrones and flow near an ice divide

Pettit¹,

Jacobson²,

Waddington³

2003

“…the time-scale and temporal resolution of any recovered record and potential for disruption of the oldest portions of that record. Our analyses extend those of Nereson and others (1996) by relying on new, detailed ice-sheet morphology measurements.…”

Section: Introductionmentioning

confidence: 63%

A site for deep ice coring in West Antarctica: results from aerogeophysical surveys and thermo-kinematic modeling

Morse¹,

Blankenship²,

Waddington

et al. 2002

The U.S. Science Plan for Deep Ice Coring in West Antarctica calls for two ice cores to be collected. the first of these cores, from Siple Dome, was completed during the 1997/98 field season. the second core is to be collected from a site near the divide that separates ice flowing to the Ross Sea and to the Amundsen Sea.Using high-resolution, grid-based aerogeophysical surveys of the Ross/Amundsen ice-divide region, we identify seven candidate sites and assess their suitability for deep coring. We apply ice-flow and temperature calculations to predict time-scales and annual-layer resolution, and to assess the potential for basal melting for several selected sites. We conclude that basal melting is likely for sites with very thick ice, as was observed at the Byrd core site. Nevertheless, these sites are most attractive for coring since they promise recovery of a long climate record with comparatively high time resolution during the last glacial period.

“…Siple Dome (SD) is the best example of ice that has been frozen to the bed for at least 100 kyr (Nereson and others, 1996), but, because of the temperature changes that occurred 10 kyr ago after the last glacial period, its temperature profile has not reached full steady state (Engelhardt, 2004). Because of its unique location at a stable dome site, the SD temperature profile had more time to approach steady state than all the other measurement sites that experienced continuous changes in boundary conditions due to flow.…”

Section: Basal Temperature Gradientsmentioning

confidence: 99%

Thermal regime and dynamics of the West Antarctic ice sheet

Engelhardt¹

2004

The temperature-depth profiles measured in 22 boreholes drilled on the West Antarctic ice sheet exhibit two distinctly different thermal states of its basal ice. The warm state shows on Siple Dome and on Whillans Ice Stream. A relatively colder state, found at the Unicorn, Kamb Ice Stream (former Ice Stream C) and Bindschadler Ice Stream (former Ice Stream D), has basal temperature gradients greater than 50 K km -1 . A large block of cold ice stranded and frozen to the bed at the Unicorn and simultaneously much warmer ice existing only a few kilometers across the Dragon shear margin in fastmoving Alley Ice Stream (former Ice Stream B2) poses a paradox. The relatively cold ice at the Unicorn must have come from a source different from the present Whillans Ice Stream catchment area. It is hypothesized that the Unicorn paradox was created by a super-surge. Also, the stagnant Siple Ice Stream, many relict shear margins, cold patches of ice at the Crary Ice Rise, ice rafts embedded in the Ross Ice Shelf, all point to a major event triggered either by an internal instability or by a subareal volcanic eruption. Most of these features appeared to have been formed about 500 years ago. Subsequent freeze-on of a 10-20 m thick basal layer of debris-laden ice and water loss caused a slowdown of ice streams and, in the case of Kamb Ice Stream, an almost complete stoppage.