Post−surge geometry and thermal structure of Hørbyebreen, central Spitsbergen

Hørbyebreen surged in the 19^th or early 20^th century, as suggested by geomorphological evidences and looped medial moraines. In this study, we investigate its wide−spread geometry changes and geodetic mass balance with 1960 contour lines, 1990 and 2009 digital elevation models, in order to define the present−day state of the glacier. We also study its thermal structure from ground−penetrating radar data. Little is known about the glacier behaviour in the first part of the 20^th century, but from its surge maximum until 1960 it has been retreating and losing its area. In the period 1960-1990, fast frontal thinning (2-3ma⁻¹) and a slow mass build−up in the higher zones (~0.15 m a⁻¹) have been noted, resulting in generally negative mass balance (−0.40 ± 0.07 m w. eq. a⁻¹). In the last studied period 1990-2009, the glacier showed an acceleration of mass loss (−0.64 m ± 0.07 w. eq. a⁻¹) and no build−up was observed anymore. We conclude that Hørbyebreen system under present climate will not surge anymore and relate this behaviour to a considerable increase in summer temperature on Svalbard after 1990. Radar soundings indicate that the studied glacial system is polythermal, with temperate ice below 100-130 m depth. It has therefore not (or not yet) switched to cold−bedded, as has been suggested in previous works for some small Svalbard surge−type glaciers in a negative mass balance mode.