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Internal drainage system of Aldegondabreen, Spitsbergen, according to speleological studies

The systems of internal drainage of glaciers have been studied mainly by indirect methods. In order to reveal the structure of the internal drainage network inside Aldegondabreen, moulins and glacial caves were investigated by speleological methods in 2001–2021, which was accompanied by a semi-instrumental topographic survey in the cavities. This allowed us to see the change in the glacial cavities over time. There are three types of moulins in Aldegondadreen: active, dead and healed ones. We visited active and dead moulins. The depth of the entrance pits in the moulins varies from 52 to 65 m (moulin group No 1), from 70 to 75 (moulin group No 2) and from 45 to 60 m (moulin group No 3). The depth of moulins is equal to the thickness of the cold ice layer. Using the structure of the moulins, we show that the water from moulin group No 1 flows to the right marginal part of the glacier tongue. The water from moulin groups No 2 and No 3 flows to the left margin part of the glacier tongue, which is confirmed by the mapping of healed moulins locations. We find that the number of active and dead moulins has been decreasing since 2001, while the number of healed moulins has increased. We attribute this to a decrease in the thickness of the temperate ice layer at the base of the glacier due to climate change. Many moulins have narrow meanders at the lower part of the entrance pits, which usually finish by siphons. None of the moulins reaches the glacier bed, their lower parts are usually located in clean transparent ice. The lifetime of the moulins usually does not exceed 6 years. Our study of the caves on the glacier tongue revealed that they can be englacial or subglacial, and they originate along sub-horizontal thrusts located in the ice. We assume that the moulins reach the slip planes along thrusts close to the glacier bed. The water from the moulins flows along these slip planes as a film in early summer and turns into channels in mid- or late summer. The presence of thrusts in the ice depths can explain the development of internal drainage systems in glaciers (regardless of their size), outbursts of glacial lakes, surges and the formation of eskers. Clastic material for eskers formation can penetrate into a cave channel from the contact areas of the thrusts with uplifts on the bed. The results obtained can help in the interpretation of the available geophysical data for this glacier.