De-icing progression of ice-cored moraines in a humid, subpolar climate, Kotlujokull, Iceland

Modes and rates of de-icing ice-cored moraines under humid subpolar conditi ons have been determined for two dead-ice fields of different age represent ing a mature and a final stage of collapse in the Kotlujokull terminus regi on, south Iceland. The melting processes considered are backwasting, define d as the lateral retreat of near-vertical ice walls, or steep, ice-cored sl opes, and downwasting, defined as the thinning of the ice core by melting a long the top and bottom surfaces. In the mature stage of collapse, where th ere is less than 1 m of sediment cover, major faces of ice exposed for rapi d melting are ordinarily significant and long-lived due to intense precipit ation. In this fully ice-cored terrain the predominating processes are back wasting of exposed ice walls and downwasting by bottom melt, each contribut ing to the annual surface lowering of about 0.5 m, or 35%; the total annual surface lowering (melt rate) due to all processes averages 1.4 m. In those parts of the fully ice-cored dead-ice field where ice-degradation is at a more advanced stage and free-faces of ice are ordinarily absent, the melt r ate averages 0.8 m. In the final stage of collapse, where the ice mass is d isintegrated into isolated dead-ice blocks buried beneath multiple resedime nted deposits of 1-3 m, the annual rate of surface lowering of ice cores ha s decreased to 0.3 m. Factors contributing to this are a thicker sediment c over, insulation by an increased amount of vegetation, and decreased amount of running water beneath the ice blocks. It is concluded that in the curre nt climate it takes around 50 years to melt down 40 m of stagnant dirty gla cier ice; about 20 years are required to disintegrate the ice into isolated dead-ice blocks, and another 30 years to entirely melt out the ice cores.