Ice core drilling at Evans Piedmont Glacier (EPG) and Mt Erebus Saddle (MES)

At EPG and MES 180m and 200m deep ice cores of excellent core quality were recovered. This was only possible due to the drilling expertise of Pyne, Kingan, and Kipfstuhl and high quality ice core drill of the German Alfred Wegener Insitute. Furthermore, the drilling tent designed by Pyne greatly improved drilling conditions and provided a clean room facility for core processing in the immediate vicinity of the drilling operation. This allowed the drilling crew to monitor directly changes in ice core properties and drilling performance and easy communication with the core processing team. Furthermore, it made the drilling operation largely weather independent.

The daily core recovery at EPG averaged at about 18m, ranging from about 10m to 30m, decreasing with increasing depth due to longer travel times. Core quality between 0 and 120m is excellent. A brittle zone between 120m and 140m provided good core quality, while between 140m and 160m core quality was again excellent, and fair between 160m and 180m. At MES core recovery averaged 22m, ranging from 16m to 45m. Core quality with depth displayed similar pattern to EPG.

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Above the firn-ice transition the recovered core consisted of snow and firn and therefore clean suits, facial masks, and thin polyethylene gloves are used by the core processing crew to avoid contamination during core handling (Fig.4a to 4c). 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.4d to 4e)

Once the core is extruded from the core barrel (Fig.4A), the piece is fitted to the previous run (Fig.4D) and the recovery is measured and logged (Fig.4E, B). The core is then cut into 1m long sequences (Fig.5A). Before the pieces are sawed, a 2mm hole is drilled at the meter mark and the core temperature is measured (Fig.5B). 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, such as the Last Glacial Maximum. Temperature at EPG remained relatively stable below 10m, indicating that the record represents the Holocene. However, the MES record showed an unusually high increase of temperature with depth, which is likely be caused by the geothermal gradient from the active volcano Mt Erebus. The temperature increase was about 2K over 100m.

The core then was packed in layflat and investigated on the light table for crystal structure, melt and dust/tephra layer occurrence (Fig.5D). At EPG several coarse grained dust layers are observed, evidence of large katabatic storms. Analyses of volume, grain size, and mineralogy will allow us to determine source region in the Transantarctic Mountains and to infer circulation patter and wind strength through time. At MES a total of six visible layers are observed (Fig. 5D). These are potentially tephra layers from pervious eruption and could provide a time line of Mt Erebus activity through the recent past. Samples are currently analyses at the Alfred Wegener Institute.

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 page 6 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 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. At MES we drilled within ~2m of bedrock. The lack of cloudy bands or elongated bubbles so close to bedrock indicates that the ice is not moving. This suggests that core was taken at the ice divide and furthermore, that it is frozen to bedrock. For this reason the ice drains in this region only through compaction, and hence could be up to 200,000 years old at the bottom of our core.