Dynamics of tidewater surge-type glaciers in northwest Svalbard Damien MANSELL, * Adrian LUCKMAN, Tavi MURRAY Department of Geography, College of Science, Swansea University, Swansea, UK E-mail: d.t.mansell@exeter.ac.uk ABSTRACT. The evolution of ice dynamics through surges of four tidewater-terminating glaciers in northwest Svalbard is investigated by remote sensing. A 20year time series of glacier surface flow speeds and frontal positions is presented covering the recent surges of Monacobreen, Comfortlessbreen, Blomstrandbreen and Fjortende Julibreen. Surface flow speeds were derived using feature tracking between pairs of ERS SAR and ALOS PALSAR images, while frontal positions were taken from the same imagery, as well as more frequent but lower-spatial-resolution Envisat Wide Swath Mode images. During all four surges, increased ice flow caused the tidewater margin to advance while the calving flux was initially reduced to near zero due to compressive stresses limiting crevasse propagation. As ice speed decreased, the terminus continued to advance, until the glacier’s speed had returned to its pre-surge flow rate. Only at this time did the terminus start to retreat and peak iceberg calving flux was established. We conclude that terminus advance closely tracks glacier speed-up, that there is little mass loss through calving during the most active phase of the surge, and that seasonal cycles of terminus positions diminish during the active surge phase. 1. INTRODUCTION Surge-type glaciers undergo non-steady ice flow whereby decadal-long quiescent periods of low activity and marginal retreat are punctuated by short-lived periods of rapid flow, where velocities increase by a factor of 10–1000 times (Meier and Post, 1969; Murray and others, 2003a). During quiescence, the glacier flows at a rate slower than the balance velocity, and a large store of ice is built up in the reservoir zone. During the active phase of the surge, glacier flow increases to a rate faster than the balance velocity, the glacier surface becomes heavily crevassed and the down- glacier transfer of ice mass takes place (Meier and Post, 1969). The glacier terminus may advance by several kilometres and, where the front is tidewater-terminating, the iceberg calving flux may also be greatly increased (Murray and others, 2003a). During the active phase of the surge, the increased ice flux to the tidewater ice cliff and the advance of the terminus into potentially deeper offshore water can increase the iceberg calving flux (Dowdeswell, 1989; Vieli and others, 2001). The tidewater terminus may advance by several kilometres as was exemplified by Bra ˚svellbreen, which advanced up to 20 km when it surged in 1936–38 (Schytt, 1969). When the increased ice flux of the surge is matched closely with the advance of the terminus such as in Perseibreen (Dowdeswell and Benham, 2003), iceberg calving may not be enhanced and little mass may be lost in this way. Despite the large population of Svalbard tidewater-terminating surge-type glaciers (Błaszczyk and others, 2009), and the limited understanding of this phenomenon, observations of tide- water surging glaciers remain scarce. As well as understanding calving losses during surges, determining the quiescent period duration is crucial for forecasting sea-level rise contributions of densely populated surge-type glacier regions such as Svalbard, North America, Iceland and the Pamirs. Overall cycle lengths in the archipelago are not well known since only five glaciers have been observed to surge more than once (Dowdeswell and others, 1991). By assessing the changes of four tidewater-terminating glaciers in northwest Svalbard surge phases, we investigate the ice dynamics, timing of surge initiation and termination and the general nature of Svalbard surges. Ice flux and retreat rates are combined to investigate the controls of ice- dynamic regimes on the calving rate and to better under- stand the complex relationship of evolving calving rates during a surge. Where glaciers have been observed to surge for the second time, we provide insight into the overall cycle lengths of these glaciers. 2. SVALBARD SURGE-TYPE GLACIERS Svalbard contains one of the largest glaciated areas in the Arctic (Hagen and others, 1993; Dowdeswell and Hambrey, 2002), covering 36 600 km 2 . Calving of icebergs makes up a substantial part of mass loss from the archipelago’s ice masses, estimated to be 4  1 km 3 a –1 , which includes a 1 km 3 a –1 contribution from the general retreat of Svalbard glaciers over the past 80 years (Dowdeswell and Hagen, 2004). Calving takes place from two main types of ice mass in Svalbard: tidewater-terminating glaciers, generally flow- ing within steep-sided fjords; and larger ice caps and outlet glaciers, which terminate in long sections of vertical ice cliffs (Błaszczyk and others, 2009). Svalbard calving glaciers can be further divided into surge-type and non-surge-type. Only 1% of the global glacier population is thought to be of surge type (Jiskoot and others, 2000), yet estimates for Svalbard surge-type glaciers vary between 13% (Jiskoot and others, 1998) and 54–90% (Lefauconnier and Hagen, 1991). Within this wide range, Błaszczyk and others (2009) estimate that of all the Svalbard tidewater-terminating glaciers, 43% could be surge-type. The fact that surge-type glaciers occur in clusters around the world, such as in Svalbard, implies that their distribution is a result of local variables, including geology, climate and topography (Clarke and others, 1986). The typical duration Journal of Glaciology, Vol. 58, No. 207, 2012 doi: 10.3189/2012JoG11J058 *Present address: Department of Geography, College of Life and Environ- mental Sciences, University of Exeter, Exeter, UK. 110