Site Selection

The WAIS Divide site location was chosen based on the following site requirements:

1. Relatively smooth bed topography and minimal horizontal ice flow.
2. Individually identifiable annual layers of at least 1cm thickness in ice 40,000 years old.
3. Ice-accumulation rate sufficient to reduce the age difference between the gases in the ice and the ice itself to less than 500 years.
4. Well-behaved stratigraphy to an age of at least 80,000 years.

High-resolution, grid-based airborne geophysical surveys of the Ross/Amundsen ice divide region were conducted during 1994-1996 and identified several sites with favorable surface topography, ice thickness, accumulation rate, and bedrock topography characteristics. On-ice site reconnaissance started in 1995/1996 with exploratory traverses into the area and the drilling of three shallow firn cores to confirm the preservation of climatic signals in the snow and ice. Ice flow modeling and temperature calculations were applied to the candidate sites to predict time scales and annual layer resolution. This was then followed by two seasons of ground-based geophysical surveys to further investigate the candidate sites.

The chosen WAIS Divide site is favored because:

1. bedrock topography is relatively smooth at km scales;
2. internal layers are flat and undisturbed;
3. location is ~24 km downsloap of the flow divide (but within 10 ice thicknesses of the divide) to insure that no divide migration has compromised the stratigraphy;
4. annual layers will be detectable (1 cm thick) to at least 40,000 years;
5. ice from deglacial period will not be brittle;
6. gas-age ice-age difference is ~200 years for the Holocene and ~300 to ~500 years for the last glacial period.

---

Site Details

Borehole Locations Hole Year Drilled Drill Used Diameter (cm) TopDepth (m) BottomDepth (m) Latitude, Longitude Altitude (ft) Remarks WDC06A 2007/08 DISC 12.2 114 580 79o28.058'S, 112o05.189'W ~5927 Deep core WDC06A 2006 4" drill 10 0 114 79o28.058'S, 112o05.189'W ~5927 Deep core WDC06B 2006 4" drill 10 0 130 79o28.048'S, 112o05.160'W ~5927 Allocation to be determined WDC05A 2005 4" drill 10 0 ~299 79o27.777'S, 112o07.506'W ~5927 Collected for gas analysis WDC05B 2005 3" Eclipse drill 7.5 0 ~75 79o27.775'S, 112o07.422'W ~5927 Hole drilled for firn gas sampling; core will be used for chemistry method development WDC05C 2005 3" Eclipse drill 7.5 0 ~80 79o27.777'S, 112o07.362'W ~5927 Hole drilled for firn gas sampling; core will be used for chemistry method development WDC05Q 2005 4" drill 10 0 ~130 79o28.052'S, 112o05.137'W ~5927 Drilled at the arch pit; used for chemistry and isotopes WDC05E 2005 4" drill 10 0 ~32 79o27.777'S, 112o07.506'W ~5927 Collected for firn structure studies

Site Characteristics Longitude: 112.085oW Latitude: 79.467oS Surface elevation: 1,759 m Distance from current flow divide: 24 km Current ice-accumulation rate: 22 cm/yr Current average annual surface temperature: -31oC Age at 90% of total depth1: 98-105 ka Age at 96% of total depth1: 175-236 ka Age by which annual layers have thinned to a thickness of 1 cm2: 45-46 ka Age of the ice in the depth range when the ice is brittle3: 1.8-9.4 ka Ice thickness: 3,465 m Ice age-gas age difference in Holocene: 200 years Ice age-gas age difference in the glacial period: 300-500 years Notes: 1 Calculated with the expected high and low geothermal heat flux. 2 Calculated with the expected high and low geothermal heat flux. This is approximately how far back the core can be dated by counting annual layers. 3 Core quality will be lower in this age range.