During the 2004/05 season we visited four sites: Windless Bight, Evans Piedmont Glacier, Victoria Lower Glacier, and Mount Erebus Saddle.

Windless Bight

To test our drilling equipment before deploying to Evans Piedmont Glacier, we conducted a test drill at Windless Bight (Fig.1). This is a convenient location, as it is close to Scott Base and also a future drill site of ANDRILL. The shakedown went well, none of the equipment suffered from the transport. The recovered 21m firn core will be the basis for a MSc study to quantify dust input into the McMurdo Sound and hereby has the potential to contribute directly to the ANDRILL science effort.

Fig. 1: Test drilling at Windless Bight

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.

Fig. 2 A) and B) inside the drilling tent, C) outside the drilling tent, D) 180m deep borehole

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.

Fig. 3: A) Core barrel loading, B) record of core quality, C) control panel, D) extracting cuttings, E) core barrel extracted from drill barrel, F) discharging cuttings, G) adjustment of cutters and core catchers, H) drilling discussions, I) drilling at 165.5m depth.

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)

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Fig. 4: A) Firn core handling, B) firn core logging, C) core has been packed in layflat and is investigated over a light table, D) ice core fitting and marking, E) ice core logging, F) ice core packing in layflat and logged on light table.

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.

Fig. 5: A) core sectioning to 1m length, B) core temperature measurement, C) core packing and logging, D) visible layer, E) core weighing for density measurements, F) core storage of six 1m long core in ice core boxes, packed with cuttings

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.

Borehole Measurements

Once the drilling operation was completed borehole temperature and light penetration measurements were conducted. Borehole temperature provides a back-up measurement for the core temperature and verified that average annual temperature at EPG is −22°C and at MES – 25.6°C. The light penetration measurement was conducted to investigate if optical stimulated luminescence (OSL) could be used to date independently dust layers in the ice core record. Our measurements are encouraging. The OSL clock starts counting, when the sample is in complete darkness and is reset when exposed to light. We therefore wanted to determine the depth at which the OSL age would start counting. Our light measurements reveal that the light extinction curve is very sharp at both sites and darkness (<micro lux) is reached within 6m of the surface.

Analyses of Snow Properties

At EPG and MES a 4m and 2m deep snow sequence was sampled at the drilling site prior to drilling to allow high resolution snow analysis. The snow profile was sampled with 1cm resolution for analysis on snow chemistry (Na, Ca, K, Mg, Cl, NO3, SO4, MS, Al, Fe, Si, Sr, Tr, Zn) and isotopic composition (δ¹⁸O and δD), dust content and mineralogy (Fig.6). This is necessary as the top 4m are usually of very low density, providing too little material to run high resolution analyses. Due to the unusually dense snow at MES, only the upper 2m were sampled. The data are used to establish transfer functions between meteorological records and the snow/ice core record, for temperature, precipitation, airmass origin, wind strength and direction, storm frequency, etc. The high sampling resolution provides sub-annual resolution of the climate record. At EPG additionally three snow pits were excavated to measure density and temperature of the snow pack and to study snow crystal structure and their geographical variability (Fig.7). This information is important to page 7 calculate annual accumulation rates and to evaluate the potential of re-crystallisation in the snow pack. Our initial results suggest excellent characteristics for ice core analyses. Annual layers did not show any sign of inclination or erosion and only 2 fine melt layers (<2mm) were found. This is particularly surprising, considering the coastal and low elevation (380m asl) setting of this site. The snow profile temperature showed the winter temperature wave travelling downwards in the snow pack. At the bottom of the profile the temperature touched upon the winter wave, reaching –33.6°C with a decreasing tendency.

Fig. 6: A) 4m deep snow pit at EPG, B) high resolution snow sampling

The geographical variability of density, temperature and stratigraphy was small and within the limits of ±σ.

Fig. 7: A) Snow stratigraphy – annual layers are clearly visible, B) additional snow pit to measure geographic variability of snow density and temperature, C) double snow pit with snow wall in-between to study snow stratigraphy

Automatic Weather Station at Evans Piedmont Glacier

An automatic weather station has been established near the 2004/2005 ice coring site that will record several parameters to help characterise the snow accumulation regime of the local glacier area (Fig.8B).

Barometric pressure and wind speed/direction (ultrasonic) sensors are installed but were inoperative when the party left on 16 November.

The installation is expected to operate throughout the upcoming winter. The data will be downloaded from the site in 2005/06 and the installation maintained and the non operational sensors incorporated into the recording program.

Fig. 8: A) Schematic figure of submergence velocity device, B) submergence velocity device and weather station at EPG, C) base and battery seat for the weather station dug in, D) dimension of weather station base.

Submergence Velocity Measurements at Victoria Lower and Evans Piedmont Glacier

During the 1999/2000 season three submergence velocity devices [Hamilton and Whillans, 2000] for mass balance measurements in the McMurdo Dry Valleys were installed (Fig.9). During the 2004/2005 season two submergence velocity devices have also been installed at EPG (Fig.8A and B). This method is used to determine mass balance by comparing vertical velocity of a marker in firn or ice with long-term, average snow accumulation rates. The movement of the marker is the result of three motions: firn compaction, gravitational glacial flow, and changes in mass balance. The device (Fig.9) consists of a non-stretchable, stainless steel wire attached to a metal anchor that is heated and placed into a drilling hole drilled in firn (or ice). The anchor melts the bottom ice and freezes in. A wire is stretched tight and guided by a stainless steel tube from the top of the drilling hole. A rod is held in place using plywood that has been buried ~40cm into the snow to avoid melt around the darker wood surface. The top end of the wire has a loop and permanent marker, the tracking point. High precision GPS measurements are used to determine absolute position of the tracking point during subsequent years. Density measurements are made on the core recovered from the drilling. To calculate the surface slope in the direction of the glacier flow, the ice surface topography is surveyed using GPS in the vicinity the device. We revisited the three sites to measure current mass balance in continuation of the time series over the last 5 years. A GPS base station was deployed for the time of our visit at Staeffler Ridge. Our time series indicates a negative mass balance of about 12cm per year. At EPG we measured this year's position and mass balance data will be available from next year onwards.

Fig. 9 Mass balance measurement device at Victoria Lower Glacier