The 2006/07 field season comprised objectives at Whitehall Glacier (WHG), Mt Erebus Saddle (MES), Windless Bight (WB), Victoria Lower Glacier (VLG), and Evans Piedmont Glacier (EPG). Malta Plateau was an alternative site to Whitehall Glacier. However, we found excellent conditions for ice core drilling at WHG and therefore did not visit Malta Plateau.

Fig. 1: Overview map of the Ross Sea region showing the location of the satellite images A and B. A) Locations of sites in the McMurdo Sound region, B) Location of sites in the Cape Hallett region. Satellite images are derived from MODIS

Test Drilling at Windless Bight

To test our drilling equipment before deploying to Whitehall Glacier, we conducted a test drill at Windless Bight (Fig.2). This is a convenient location, as it is close to Scott Base and in the vicinity to the ANDRILL drill. The shakedown went well, none of the equipment suffered from the transport. The recovered 20m firn core will contribute to a PhD thesis quantifying dust input into the McMurdo Sound and hereby contributing to the ANDRILL science effort.

Fig. 2: Drilling at Windless Bight

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

The scientific goal of the NZ ice core programme is to improve our understanding of the major Southern Hemisphere climate drivers causing high frequency climate variability. These are in particular the El Niño Southern Oscillation (ENSO), the Antarctic Oscillation, and the Antarctic Circumpolar Wave, as well as drivers and feedback mechanisms causing abrupt climate change. These climate drivers operate on relatively short time scales (sub-decadal) but also potentially respond to longer term forcing (centennial to millennial). It is therefore important to obtain high resolution (sub-annual) records that can reliably capture the high frequency variability of these drivers from sites that are particularly sensitive to their influence, and at the same time providing a long enough record to investigate superimposed longer-term trends. ITASE focuses on the last 200 years and where possible longer.

We have identified key locations at low elevation, coastal sites that are particularly climate sensitive, as they capture tropospheric climate variability and in general have a higher snow accumulation rate than sites from the Antarctic interior. This makes these sites ideal when investigating abrupt climate change. For this reason, the International Partnership of Ice Coring Sciences (IPICS) has identified an array of 2000-year long records from especially coastal sites as one of four priorities for ice core research in the next 20 years. Currently only NZ and Australia have worked on coastal sites.

For this field season our objective was to recover two intermediate depth ice cores from WHG and MES.

WHG is a small, East Antarctic Ice Sheet independent ice mass with an ice divide at 500 above sea level, just 12km of the coast. Due to its coastal, low elevation characteristics it is ideal for our NZ ITASE objective. In addition to this, an ideal site should satisfy the following: a) consistent annual precipitation (even if seasonal), b) limited summer melt c) limited wind erosion or snow accumulation through wind drift, d) a long enough record (for our purposes at least 200 years but preferably ≥ 2000 years), and e) undisturbed ice flow and smooth bedrock topography.

MES has an extremely high accumulation rates, exceeding by one order of magnitude the regional average. The drill site is located at an ice divide at 1600 above sea level, just 20km of the coast.

Site survey at Whitehall Glacier

Ground penetrating radar (GPR) measurements provide an image of the internal layering of a glacier and the topography of the ice-rock interface beneath. We applied low and high frequency radar pulses (8 MHz, 35 MHz, 200MHz, and 500MHz) to map the bedrock interface and internal flow structures in the glacier. Those features are identified through reflectors that result from changes in physical and chemical properties, such as dust layers or aerosol and density variations and are thought to represent isochrones (Morse et al., 1998; Vaughan et al., 1999). The choice of antenna frequency involves a trade-off between penetration depth and mapping resolution. The control units were mounted on a Nansen Sledge, pulling transmitter and transceiver antennae. The sledge also carried high precision GPS antenna, which is tied to the temporary GPS base station deployed at the WHG camp.

Traverses totaling approximately 80km have been surveyed with GPR. The measurements show that the glacier thickness exceeds 550m. Excellent isochrone reflections are visible in the top part of the profile, which will also be used to investigate geographical and chronological accumulation changes. Further post-processing will enhance the reflectors and will correct for surface topography. At MES a site survey was conducted during the 2003/04 field season.

Fig. 3 A) ASTER satellite image of Whitehall Glacier and vicinity. See Figure 1 for overview. Image from January 2005. Yellow flag indicates approximate location of proposed drilling site. Yellow arrows indicate approximate major flow lines. B) Digital elevation model. X/Y/Z grid in UTM 58 map units. Yellow grid indicate proposed ground penetrating radar survey lines (differential, 8, 35, 200, and 400 MHz)

High resolution snow pit sampling at Whitehall Glacier and Evans Piedmont Glacier

At WHG one 4m, one 2m, and two 1m deep snow sequences were sampled at the drilling site to allow high resolution snow analyses. 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 little material to run high resolution analyses.

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. In addition, snow density and temperature was also measured.

This information is important to 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 no melt layers were found. This is particularly surprising, considering the coastal and low elevation (400m asl) setting of this site.

In addition snow samples were also collected from EPG to extend the proxy record for collation with automatic weather station data from this site.

Fig. 4: Snow sampling at WHG

Automatic weather station maintenance and data retrieval

In 2004/05 we deployed an automatic weather station on EPG. The data permit the calculation of transfer functions between ice core proxies and meteorological parameters, such as temperature, precipitation, meso-scale atmospheric circulation pattern, katabatic winds, and seasonality of snow accumulation. In addition a new snow accumulation sensor and high precision snow temperature probes allow us to monitor snow accumulation rates, the potential influence of snow loss through sublimation, wind erosion or melt, and the quality of preservation of the meteorological signal in the snow. Furthermore, the data allow us to estimate the uncertainty of re-analysis data (NCEP/NCAR and ERA-40 data) in the region.

At EPG additionally one 1m snow sequence was sampled excavated to measure density and temperature of the snow pack and to study snow crystal structure and their geographical variability.

Fig. 5: Automatic weather station at EPG

Submergence Velocity Measurements at Victoria Lower and Evans Piedmont Glacier

The response time of a glacier to changes in accumulation or ablation is dependent on the size and thickness of the ice mass. In general, the response time of cold-based glaciers is positively correlated with the size of its ice mass, leading to long response times in Antarctica. For glaciers in the McMurdo Dry Valleys, with lengths on average of 5-10km and flow rates of 1 to 3 m/a, the response times are thought to range from 1,500a to 15,000a (Chinn, 1987; Chinn, 1998). Consequently, annual variations in surface elevation may only reflect changes in loss rates. As a result surface measurements of mass balance are difficult to interpret in terms of long-term mass balance (Hamilton & Whillans, 2000). This is especially the case in places like the McMurdo Dry Valleys where mass loss is thought to be predominately due to sublimation at ice cliffs and glacier surface caused by wind and solar radiation (Chinn, 1987; Chinn, 1998). For Victoria Lower Glacier (VLG), two mass balance measurements are available in the literature for 1983 and 1991 based on ice cliff characteristics and the motion of the glacier snout (Chinn, 1998). The measurements indicate that VLG was advancing 1.24m/a into Victoria Valley during this time period. However, the small number of observations (2) and the cliff's sensitivity to sublimation (contemporary surface ablation) result in a high uncertainty of longer term mass balance. To determine the longer-term mass balance of the glaciers, unaffected by annual surface variations, three 'coffee-can' or 'submergence velocity' devices (Hamilton et al., 1998; Hamilton & Whillans, 2000) were deployed at Victoria Lower Glacier in 1999/2000 and two at Evans Piedmont Glacier in 2004/05. These are annually re-measured to monitor mass balance changes.

Fig. 6: Submergence Velocity Measurements at VLG

Snow Accumulation at Mt Erebus Saddle

The topography of Mt Erebus Saddle promotes strong winds leading to significant compaction of the surface snow (~0.45 gcm⁻³). Furthermore, average snow accumulation lies in the range of 72 – 150 cm yr⁻¹ water equivalent (Fig.XX). This is more than one order of magnitude higher than the regional average (Bromwich, 1988; Bromwich et al., 1998; Bertler et al., 2004a; Bertler et al., 2004b) and provides ideal characteristics for a high resolution ice core gas record. To measure the accumulation rate at the drill site we deployed three snow stakes, which we hope will endure the high wind velocities and snow accumulation.

Fig. 7: Snow accumulation (cm) in the Southern McMurdo Sound region (after Bromwich)

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.

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

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)

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

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

where: H =rate of thickness change (myr⁻¹) b_m=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)

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

Firn-Ice Transition at MES and WHG

Density measurements on the recovered cores show the depth of bubble close off is reached at MES at about 60m depth while at WHG the firn- ice transition lies below the recovered 100m depth. This underpins the unique characteristics of the MES core. Currently, only the Australian Law Dome ice core has an similar shallow bubble close off depth. Once the annual snow accumulation for the site as been calculated, we can determine whether MES is also as rapid or potentially even faster than at Law Dome, providing the opportunity to measure an extremely high resolution greenhouse gas record.

Fig. 15: Density measurements on MES (blue) and WHG (pink) ice cores.

Automatic Weather Station Data from EPG

The weather station continuously recorded data since our last visit on 01 December 2005 until 04 Jan 2007, when we visited the station to download the data and for maintenance work. The recorded data for pressure, solar irradiation, air temperature, snow temperature, dew point, and snow accumulation are shown in below (Fig.16).

Fig. 16: EPG automatic weather station data for 01 Dec 2005 to 04 Jan 2007

As shown in Fig.16 temperature tracks overall solar irradiance. However, from May until early October abrupt temperature increases of up to 20K occurred. Some of these temperature excursions are accompanied by changes in barometric pressure suggesting that these warm events could be katabatic outflow from the McKay Glacier portal. Temperatures in the snow pack measured concurrently at 16 depth horizons from 0.135m (light blue) to 2.085m (red) show the decreasing influence of air temperature variability with depth. The snow temperatures have yet to be corrected for their change in depth, which increased by 60cm as shown in the snow accumulation graph below. The snow accumulation record shows that most of the precipitation occurred during six events of 5 to 10cm snow accumulation. The data show also that are no prolonged time periods of snow loss, except in the first 2-5 days after the snow precipitation page 13 event which is partly due to snow compaction. After this time period the snow surface remains stable. Overall, the data confirm EPG as an excellent ice core site. The snow pit data and submergence velocity measurements from EPG and VLG have yet to be processed.