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Victoria University Antarctic Research Expedition Science and Logistics Reports 2006-07: VUWAE 51

c. Methodology

page 6

c. Methodology

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

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

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

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.

The following parameters will be measured on the obtained ice cores and snow samples:
  • Major Cations, Anions, and Methylsulfonate

    Major ion concentrations are measured for cations (Na, K, Mg, Ca, NH3) using a Dionex™ Ion Chromatograph with Dionex CS12 column and 20 mM methanesulfonic acid eluent. Anion concentrations (Cl, NO3, SO4) are measured with a Dionex AS11 column, 6.0 mM NaOH eluent. For both measurements a 0.25 mL sample loop is used. Methylsulfonate (MS) content is measured using a Dionex AS11 column with 0.1 mM NaOH eluent and a 1.60 mL sample loop

  • Trace Elements and Cations

    Samples are analysed for trace elements and cations (Al, Ca, Cu, Fe, K, Mg, Mn, Na, P, S, Si, Sr, and Zn) using a Perkin-Elmer Optima 3000 XL axial inductively coupled plasma optical emission spectroscopy with a CETAC ultrasonic nebuliser (ICP-OES-USN at UMaine) and a Finnigan Thermo inductively coupled plasma mass spectrometer (ICP-MS at VUW) for all other trace elements and selected isotopic ratios.

  • Oxygen and Hydrogen Isotope Ratio

    Oxygen isotope ratios are measured using a CO2 dual-inlet system coupled to a Micromass Isoprime mass spectrometer at GNS Science. The sample is measured in the presence of a standard CO2 gas. Sample duplicates and standard measurements page 8 showed a precision of ±0.08‰. Samples are analysed for stable hydrogen isotope radios δD via Cr reduction with a continuous Helium flow Eurovector elemental analyser coupled to a Micromass Isoprime mass spectrometer. Sample duplicates and standard measurements showed a precision of ±0.6‰.

  • Gas analysis in ice core bubbles

    The gas extraction is carried out using an ice core gas extraction device. The obtained gas sampled will be measured on the two mass spectrometers Micromass (13CH4) and Mat252 (13CO2) at NIWA. Concentrations of CO2, CH4 and N2O will be measured by gas chromatography at NIWA.

  • Dust concentration and mineralogy

    500cm3 volume of snow/ice is filtered through Whatman quantative 2.5μm ashless filter paper. The filter is burnt in a Vulcan A-550 furnace at 500°C for 24 hours. The residue is weight with a AG204 Mettler Toledo analytical balance, and reweighed after 24hours to check for moisture absorbance during cooling. Mineralogy is determined by mounting the dust samples in glycerol gelatine for examination under an optical particles found in the dust are analysed in a JEOL 733 Electron Microprobe at VUW.

  • 32Si

    Cosmogenic 32Si has a half-life of about 140yr. Dr. Morgenstern at GNS has developed an improved method for radiometric detection of natural 32Si, and measures natural32Si by accelerator mass spectrometry (AMS). With AMS the necessary sample size is reduced by a factor of ca. 1000 to 1kg.

  • Tritium

    A radiometric detection method (TR = 0.03-0.04, Bq/kg = 0.004-0.005) is used to measure tritium concentration. This is achieved by combining a high degree of electrolytic enrichment with a low-level liquid scintillation spectrometer.

  • Dating the ice core

    To ensure a good chronology, a multi-proxy dating technique will be applied. The annual snow accumulation at those low elevation sites is expected to be at least 10cm water equivalent affording annual layer count ice core dating techniques using seasonal signals of the isotope and chemistry records. Furthermore, volcanic time markers in the sulphate records will be used to tie the counted ages to known volcanic eruptions found in Antarctica. Additionally, two new world-leading methods developed by Dr. Morgenstern at IGNS using high resolution tritium and 32Si will be used as independent age benchmarks. The high resolution gas bubble record will provide further age control through wiggle-matching with the extremely high resolution gas record from Mt Erebus Saddle (2004/05 season).

page 9

Automatic Weather Station at Evans Piedmont Glacier

During the 2004/05 field season an automatic weather station (AWS) has been established near the ice coring site. The AWS records several parameters to help characterise the meteorology and snow accumulation regime of the area (Fig.11).

Fig. 11: Automatic weather station after the system was dug out and placed a few meters to the south of the original site

Fig. 11: Automatic weather station after the system was dug out and placed a few meters to the south of the original site

Parameters measured as of 15 November 2004 are:
  • Air Temperature at 2.5 height
  • Snow accumulation, and air temperature at 1.5 m height
  • Dew point temperature at 2.5 m height
  • Solar radiation (incoming) at 2.5 m height
  • Snow temperatures (thermistor resistance) from 0.135 to 2.085 m depth in at 13.5 cm intervals

To these were added as of 01 December 2005

  • Barometric pressure
  • Wind speed (ultrasonic)
  • Wind direction (ultrasonic)

Ground Penetrating Radar

For mapping glacier flow structures and the glacier-bedrock interface a 'GSSI SIR 10 A and GSSI SIR 20A were used with a maximum time window of 10,000 ns. A 35MHz antennae-pair (Radarteam AB-SE-40), a 200MHz antennae-pair, and a single 500MHz antenna are pulled by a Nansen Sledge, which carries the control units, generator, and solar panels. A Trimble 5700 differential, kinematic GPS, provides absolute positioning of the GPR data and allows survey of the glacier surface topography. GPR and GPS measurements are taken in kinematic mode, every 5-15 seconds ≈ 10-30m.

Fig. 12: Photo showing Nansen sledge carrying GPR and crevasse rescue equipment (from Watson, 2007)

Fig. 12: Photo showing Nansen sledge carrying GPR and crevasse rescue equipment (from Watson, 2007)

page 10

Submergence Velocity Measurements at Victoria Lower and Evans Piedmont Glacier

During the 1999/2000 season three submergence velocity devices (Hamilton & Whillans, 2000) for mass balance measurements in the McMurdo Dry Valleys were installed. During the 2004/2005 season two submergence velocity devices have also been installed at EPG. 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.

High precision GPS measurements are used to determine absolute position of the tracking point during subsequent years. Trimble 5700 base station and rover unit were used to measure the absolute position of the tracking point of the mass balance devices. At Victoria Lower Glacier, the base station was deployed on a rocky platform at Staeffler Ridge <3km away from all mass balance sites. The proximity of the base station to the rover allowed the tracking points to be measured with a horizontal precision of <1mm and a vertical precision of <5mm. At Evans Piedmont Glacier base station data from the Cape Roberts permanent GPS/GLONASS and tide gauge observatory will be used. All GPS measurements are post-processed using precise orbits, which are published on-line at "http://igscb.jpl.nasa.gov/components/prods_cb.html". These data are corrected using GIPSY-OASIS II software and provide precise point positions by taking into account satellite orbit, Earth orientation, and clock solution from NASA Jet Propulsion Laboratory's independent analysis of globally distributed GPS receivers.

Fig. 13 Cartoon of the 'coffee can' submergence mass balance device (modified after Hamilton and Whillans 2000)and picture of coffee can device deployed at Victoria Lower Glacier

Fig. 13 Cartoon of the 'coffee can' submergence mass balance device (modified after Hamilton and Whillans 2000)and picture of coffee can device deployed at Victoria Lower Glacier

The rate of thickness change H, can then be calculated using (Hamilton et al., 1998):

where: H =rate of thickness change (myr−1) bm=accumulation rate (Mgm−2yr−1) ρ =density at marker depth to account for densification processes (Mgm−3) z =vertical component of ice velocity (upward is positive, myr−1) α =surface slope (radians) u =horizontal velocity (myr−1 with azimuth)

page 11

Snow Accumulation at Mt Erebus Saddle

To accommodate high accumulation rates of about 2m snow/year, 6m snow stakes that are anchored 2m into the ground were deployed. The three snow stakes, made of epoxy/carbon fibre, have been chosen for their flexibility to withstand high wind velocities in excess of 100knts.

Fig. 14: Three snow stakes placed in the vicinity of the 2004/05 drill hole casing at Mt Erebus Saddle

Fig. 14: Three snow stakes placed in the vicinity of the 2004/05 drill hole casing at Mt Erebus Saddle