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)