Evidence for seasonal subglacial outburst events at a polythermal glacier,Finsterwalderbreen, Svalbard

Bulk runoff and meteorological data suggest the occurrence of two meltwater outburst events at Finsterwalderbreen, Svalbard, during the 1995 and 1999 melt seasons. Increased bulk meltwater concentrations of Cl- during the out bursts indicate the release of snowmelt from storage. Bulk meltwater hydroc hernical data and suspended sediment concentrations suggest that this snowm elt accessed a chemical weathering environment characterized by high rock:w ater ratios and long rock-water contact times. This is consistent with a su bglacial origin. The trigger for both the 1995 and 1999 outbursts is believ ed to be high rates of surface meltwater production and the oversupply of m eltwater to areas of the glacier bed that were at the pressure melting poin t, but which were unconnected to the main subglacial drainage network. An i ncrease in subglacial water pressure to above the overburden pressure lead to the forcing of a hydrological connection between the expanding subglacia l reservoir and the ice-marginal channelized system. The purging of ice blo cks from the glacier during the outbursts may indicate the breach of an ice dam during connection. Although subglacial meltwater issued continually fr om the glacier terminus via a subglacial upwelling during both melt seasons , field observations showed outburst meltwaters were released solely via an ice-marginal channel. It is possible that outburst events are a seasonal p henomenon at this glacier and reflect the periodic drainage of meltwaters f rom the same subglacial reservoir from year to year. However, the location of this reservoir is uncertain. A 100 m high bedrock ridge traverses the gl acier 6.5 km from its terminus. The overdeepened area up-glacier from this is the most probable site for subglacial meltwater accumulation. Copyright (C) 2001 John Wiley & Sons, Ltd.