IMMEDIATE SCIENCE REPORT
K049 NZ ITASE – Holocene Climate Variability along the Victoria Land Coast
Antarctica New Zealand 2003/04
1 Popular Summary of Scientific Work Achieved
Unprecedented changes are occurring in the Earth's climate. The 1990's were the warmest decade in the last 2000 years and average global temperature is projected to rise between 1.4°C and 5.8°C by 2100 [ IPCC, 2001]. Although the scientific evidence of global warming is now widely regarded as incontrovertible, predicting regional impacts is proving more problematic. Especially, conclusions of the Southern Hemisphere record are limited by the sparseness of available proxy data at present [ Mann and Jones, 2003].
While meteorological records from instrumental and remote sensing data available display the large intercontinental climate variability, the data series are insufficient to infer trends or to understand the forcing, which renders prediction difficult [ Jones et al., 1999; Mann and Jones, 2003]. The long ice core records from the Antarctic interior and Greenland revolutionised our understanding of global climate and showed for the first time the occurrence of RCE (Rapid Climate Change Events) (for review e.g. Mayweski and White [2002]). To understand the drivers and consequences of climate change on timescales important to us humans, a new focus of ice core work is now moving to the acquisition of 'local' ice cores that overlap with and extend the instrumental records of the last 40 years back over the last several thousand years.
This has been a key motivation behind the US-led International Transantarctic Scientific Expedition (ITASE) of which New Zealand is a member. The NZ ITASE objective is to recover a series of ice cores from glaciers along a 14 degree latitudinal transect of the climatically sensitive Victoria Land coastline to establish the drivers and feedback mechanism of the Ross Sea climate variability. Furthermore, the ice core records will provide a baseline for climate change in the region that will contribute to the NZ-led multinational Latitudinal Gradient Project as well as providing a reference record for the NZ-led ANDRILL objective to obtain a high-resolution sedimentary archive of Ross Ice Shelf stability.
During the 2003/04 field season a site reconnaissance for two of the proposed sites, Evans Piedmont Glacier and Mt Erebus Saddle, has been carried out, using ground penetrating radar and snow properties analyses. The survey shows that both sites are excellent drilling locations with adequate ice thickness to potentially provide as much as 10,000 year long records. Furthermore, Victoria Lower Glacier, the first ice core site of this programme has been revisited to maintain the longer-term mass balance measurement time series, that commenced in 1999. Our 3 year results show that this glacier is ablating by about 12cm per year.
2 Proposed Programme
Seven key locations were identified for the NZ ITASE (International Transantarctic Scientific Expedition) programme. The analyses on the ice core from the first site, Victoria Lower Glacier in the McMurdo Dry Valleys, have almost been completed. For the 2003/04 field season we proposed to carry out a detailed reconnaissance and to recover intermediate length cores from sites 2 and 3: Evans Piedmont Glacier and Mt Erebus Saddle. Due to logistical constraints of Antarctica New Zealand the drilling was deferred to the 2004/05 season, but the site reconnaissance was conducted. Furthermore we continued our longer-term mass balance measurement time series at Victoria Lower Glacier.
Evans Piedmont Glacier Ice Core Record
To our knowledge the Evans Piedmont Glacier site is, with 380m asl, the lowest elevation site identified for drilling in Antarctica. As such it has the potential to provide a climate record with unprecedented sensitivity for tropospheric climate variability and hereby contributing directly to the discussion on the dominant driving force of Antarctic climate variability [ van den Broeke, 2000; Hall and Visbeck, 2002; Thompson and Solomon, 2002; Venegas, 2003].
The Evans Piedmont Glacier is located in the vicinity of the proposed ANDRILL site in 'Granite Harbour', which is expected to provide also a Holocene record. The comparison of the overlap between the two records provides us with the unusual opportunity to distinguish between the terrestrial and marine signal. This will add significantly to the discussion on the relative importance of the ACW (oceanic), SAM (atmospheric), and ENSO (both); their forcing and feedback mechanism. Furthermore, the sub-annual to decadal ice core record from Evans Piedmont Glacier can help to tune the marine Granite Harbour record and potentially provides a record of the final retreat of the Ross/McMurdo Ice Shelf.
The regional climate record contained in the ice will provide background information for the Latitudinal Gradient Project site 'Granite Harbour', especially for temperature, precipitation, sea-ice extent, storminess, seasonality, and snow accumulation. As mentioned before, this will help to determine if the current ecological system found has evolved under prevailing climate, or how much time the ecological system had to adjust to potential climate change in the recent past.
In collaboration with the US ITASE effort (a traverse from South Pole to Northern Victoria Land along the plateau side of the Transantarctic Mountains) we aim to provide continentality and elevation gradients, to compliment to our understanding of deep ice core records from the Antarctic interior. Furthermore, the importance of katabatic winds and the intrusion frequency of marine airmasses into the Antarctic interior through time can be established.
Mt. Erebus Saddle Ice Core Record
Mt Erebus Saddle lies in the pathway of the ENSO initiated katabatic surges across the Ross Ice Shelf [Cullather et al., 1996; Bromwich et al., 2000] and also of enhanced cyclonic activity from the Southern Ocean [Bertler et al., submitted]. Therefore, a Mt Erebus Saddle ice core record is likely to show the strongest ENSO influence of all proposed sites. The ENSO record (Southern Oscillation Index) is short and various proxies have been tentatively proposed to reconstruct ENSO variability [Adams et al., 2003; Tudhope and Collins, 2003]. The ice core record from Mt Erebus Saddle potentially provides a mean to reconstruct ENSO since its initiation in the early Holocene. Furthermore, the comparison between Mt Erebus and Mt Prior at Cape Hallett will provide a mean to distinguish ENSO driven climate variability from SAM and ACW forcing.
The semi-permanent Ross Ice Shelf polyna, just east of Ross Island, is also the result of these katabatic winds and is an important area for the production of sea-ice [King and Turner, 1997] and Antarctic Bottom Water [Bromwich et al., 1993]. The Mt Erebus ice core is likely to provide a record of the winter polyna activity through time, via the marine fingerprint in the ice chemistry.
Moreover, the Mt Erebus Saddle is located in the vicinity of the proposed ANDRILL location 'Windless Bight'. The sub-annual to decadal ice core record provides a high resolution Holocene record for the much longer but overlaying ANDRILL ice shelf and marine record.
Additionally, the site is only 37km from Scott Base and McMurdo Station and will provide a long-term perspective on climate variability and iceberg discharge, relevant for both, Evans Piedmont Glacier and Mt Erebus sites.
3 Scientific Endeavours and Achievements
During the 2003/04 season we visited three sites: Victoria Lower Glacier, Evans Piedmont Glacier, and Mount Erebus Saddle.
Victoria Lower Glacier
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Fig.2 Mass balance measurement device at Victoria Lower 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.2). 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.2) 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 4 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.
Evans Piedmont Glacier
Evans Piedmont Glacier is one of the three drilling targets planned for season 2004/05. The site reconnaissance included a GPS and GRP survey and closer investigation of the snow properties from a snow profile.
Ground Penetrating Radar (GPR)
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 (35 MHz, 200MHz, and 400MHz) 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 Slege, 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 Evans Piedmont Glacier camp.
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Fig.3: GPR/GPS set-up. A 400MHz antenna travels in front of the first skidoo. A second skidoo tails the set-up for crevasse safety.
Traverses totaling approximately 30km have been surveyed with GPR. The measurements show that the glacier thickness exceeds on average 150m (Fig.4) and is well over 200m deep at the identified drilling location. Excellent isochrone reflections are visible throughout the profile (Fig. 4), 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.
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Fig.4: Radar profile from Evans Piedmont Glacier showing bedrock topography, ice thickness and internal flow structures
Analyses of Snow Properties
A 4m deep snow pit was excavated to allow high resolution snow analysis. The snow profile was sampled with 1cm resolution for analysis on snow chemistry (Na, Ca, K, Mg, Cl, NO 3, SO 4, MS, Al, Fe, Si, Sr, Tr, Zn) and isotopic composition (δ 18O and δD), dust content and mineralogy (Fig.5). 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. Furthermore density and temperature of the snow pack was measured with 5cm resolution (Fig.5), and snow crystal structure was investigated. 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 analysis.
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Fig.5: Analysis of snow properties including high resolution snow sampling (left), and density and temperature measurements with 5cm resolution (right).
During our stay at Evans Piedmont Glacier two snow precipitation events occurred which provided the opportunity to sample hourly throughout the snowfall (Fig.6). This will allow us to fingerprint snow chemistry and isotopic composition with the meteorological situation as seen in the satellite image, enabling airmass trajectory reconstruction in ice core material from this site. While the first event was caused by mesoscale cyclonic activity and consisted of blowing and precipitating snow, the second event was caused by intrusion of local moist, marine airmass, leading to crystal growth at the snow surface.
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Fig.6: Two snowfall events at Evans Piedmont Glacier.
Mt Erebus Saddle
Mt Erebus Saddle is one of the three drilling targets planned for season 2004/05. Due to bad weather that shortened the field time available, we were not able to set up camp at this site. A one-day site reconnaissance included a GPS and GRP survey and initial investigation of the snow properties from a shallow snow pit and core. The results from this initial investigation suggest that the proposed drilling site has the necessary characteristics for a suitable drilling site.
Ground Penetrating Radar (GPR)
As bad weather prohibited a full camp set-up at Mt Erebus Saddle the GPR measurement was done by manhauling a lightweight version of our Evans Piedmont Glacier GPR setup (Fig.7). For this reason only one 35MHz antenna was used. An example of the raw data is shown in Fig.7. A strong bedrock reflection indicates that ice depth exceeds 200m. The prominent isochrones seen at about 50m are potentially tephra layers. The sparse coverage does not permit investigating spatial accumulation pattern. However, the persistence of isochrones throughout the profile, their horizontal direction and the total depth of the glacier at the proposed drilling site indicate promising characteristics for a potential ice core record from Mt Erebus Saddle.
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Fig.7: Manhaul of a lightweight GPR and high resolution GPS equipment (right). Radar profile from Mt Erebus Saddle showing bedrock topography, ice thickness and internal flow structures (left).
Analyses of Snow Properties
While high resolution snow sampling was not possible in the time available, investigation of a 2.50m firn core and a 1m deep snow pit indicates that Mt Erebus Saddle is a high accumulation site, likely in excess of 40cm water equivalent per year. For this reason Mt Erebus Saddle potentially contains a climate record of sub-seasonal resolution.
4 Publications
- Bertler, N., Barrett, P., Mayewski, P.: High resolution climate record from coastal Victoria Land.
- Bertler, N.: What causes the McMurdo Dry Valleys glacier to retreat?
- Bertler, N. & Watson, M.: Past and present mass balance and accumulation pattern as derived from ground penetrating radar
- Watson, M. & Bertler, N.: Dielectric properties of a marine influenced glacier as recorded in ground penetrating radar profile.
5 Acknowledgments
We would like to thank Antarctica New Zealand staff based in Christchurch and Scott Base for their enthusiastic and innovative support with our project, especially Lou Sanson, Dean Peterson, Paul Woodgate, Julian Tangaere, Keith Springer, and John Roberts. We are grateful for the professional assistance of Ewan Paterson in the field. The crevasse safety training with Rob McBrearty was highly appreciated. We are indebted to Helicopter NZ staff, Rob McPhail and Richard Desborough. This project is funded by Victoria University of Wellington, FrST and the Trans-Antarctic Association Fund.
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