Englacial hydrology of Annette Plateau, a temperate alpine glacier, Southern Alps, New Zealand
The movement of water through temperate glaciers is important for understanding fundamental issues within glaciology. These include glacier induced floods, glacier dynamics and run-off prediction. Traditional englacial hydrology is thought to consist of interconnected tubular channels that merge down-glacier and drain through the glacier to the bed. However, englacial hydrology is much debated as the links between the glacier surface and bed are not well understood. Ground penetrating radar (GPR) is a geophysical tool that is well suited for studying glaciated areas. Recent ice coring attempts in New Zealand’s temperate alpine glaciers were not successful in coring to bedrock due to the interception of water at depth. This highlights the need for a better understanding of the englacial hydrology of temperate systems. This study investigates the englacial hydrology at Annette Plateau where on three occasions the interception of water has prevented successful coring to the glacier bed. Ground penetrating radar was used to conduct two high-resolution surveys on Annette Plateau in early spring 2011 and early summer 2011. Across-glacier profiles were acquired at 20 m spacing to enable tracking of englacial reflectors between profiles. Models of temperate englacial features were made to aid feature identification within radar profiles. Radar data is compared with density, stratigraphy and chemistry results from the 45 m ice core obtained at Annette Plateau in winter 2009. The early-summer survey indicates an increase in the glacier’s water content compared with the early-spring survey. Englacial reflectors show evidence of (a) spatially continuous englacial conduits, (b) the formation of a water table feature which shallows down glacier, and (c) detailed bedrock topography. Hydropotential surfaces, calculated for the water table and bedrock horizons, show the direction of water flow. Ice core chemistry shows a correlation between the depth of the water table and a significant hiatus indicated by tritium dating. We infer that an extensive water table has formed on an old melt surface where ice from approximately 1930-1991 has been removed. This water table responds to seasonal temperature changes and hydrological inputs.