Ice core drilling at Whitehall Glacier (WHG) and Mt Erebus Saddle (MES)

At WHG and MES 100m and 160m deep ice cores of excellent core quality were recovered using the ice core drill of the German Alfred Wegener Institute. Above the firn-ice transition clean suits, facial masks, and thin polyethylene gloves are used by the core processing crew to avoid contamination during core handling. Below the firn-ice transition, after gas bubble close off, the inner section of the core is protected from contamination, and more comfortable, warmer clothing can be worn.

Fig. 8: Drilling operation at WHG

Once the core is extruded from the core barrel the piece is fitted to the previous run and the recovery is measured and logged. The core is then cut into 1m long sequences. Before the pieces are sawed, a 2mm hole is drilled at the meter mark and the core temperature is measured. This measurement has to be done within 5min of core recovery, as ambient temperatures in the drilling tent can influence core temperature. Therefore, temperature is only measured if the core could be processed within 5min. The temperature is a direct measurement of glacier temperature and reflects in the upper 10m seasonal temperature fluctuations, at around 10m, average annual temperature, and below 15m the signal is a memory of major past temperature fluctuations.

Fig. 9: Core processing at MES

The 1m sections are then weight to calculate density and determine the depth of bubble close-off and firn/ice transition. The densification depends on annual temperature and snow accumulation. Warmer temperatures and higher snow accumulation lead to rapid densification. This is important, as it determines the age difference between the gas trapped in the bubbles and the ambient ice. The faster the bubble close-off is reached, the smaller the age difference and the smaller the dating error. While both sites reach in comparison to other sites bubble close-off very rapidly, the extraordinary setting of MES, makes it a site of special interest. Due to the prevailing high wind speeds snow density at the surface is much higher than at other ice core sites. This in combination with extremely high snow accumulation, and warm annual temperature, the gas bubble close-off is reached at the depth, that is likely unprecedented page 7 even in the high accumulation areas of Greenland. For this reason the gas record of this ice core could potentially provide the best dated, highest resolution CO₂ and methane record yet available.

Once these initial measurements on the core are conducted, the core is then packed into well insulating ice core boxes. Cuttings are used to cement the cores into the box for stability and to maintain core temperature, as the cuttings are recovered from the same depth as the core. Furthermore, small chips were used to study gas bubble properties, such as porosity, gas bubble size and geometry. This is especially interesting close to bedrock, as bubble geometry provides clues as to whether the ice is moving or is frozen to bedrock. The boxes are then stored in underground ice caves at temperatures below −20C before transport back to the Scott Base Science freezer.

Fig. 10: Ice core storage in the field in ice caves cut from the drilling trench

In addition, a total of six snow sequences were sampled (5 at WHG and 1 at EPG) with 1cm resolution for analysis on snow chemistry, isotopic composition, dust content and mineralogy. The snow sampling surface was cleaned with a pre-cleaned plastic spade, and subsequently with a sterile scalpel, at least 20cm horizontally into the snow to prevent sampling of contaminated snow. All tools, sampling equipment, and bottles were rinsed and soaked with ultra pure 18MΩ Millipore® water and dried in a class 100 clean room facility prior to fieldwork. A scalpel was used to collect 1cm thick samples. Sample were collected into sterile Nasco whirl-paks®. Tyvek® clean suits and dust free polyethylene gloves prevent contamination from sampling procedure.