AN ANNUALLY RESOLVED LITTLE ICE AGE RECORD OF SURGE PERIODICITY, ICEBERG CALVING, AND DYNAMIC TERMINUS FLUCTUATIONS OF LANGJöKULL, CENTRAL ICELAND

Reconstructions of glacier fluctuations in response to past climate variability and internal dynamic processes can improve projections of future glacier changes, including estimates of their meltwater contributions to runoff and sea-level rise. Varved sediments from Hvítárvatn, a large proglacial lake in central Iceland, record changing dimensions of the adjacent Langjökull ice cap that can be evaluated at annual resolution. Langjökull achieved its maximum Holocene extent during the Little Ice Age, between ~1800 and ~1930 AD, when two outlet glaciers, Norðurjökull and Suðurjökull, advanced into the lake and maintained active calving margins. Here we use a combination of multibeam bathymetric data, seismic profiles, and multiple sediment cores collected from throughout Hvítárvatn to reconstruct the dynamic terminus fluctuations of these glaciers. We find that while Norðurjökull advanced into the basin stably and remained at or near maximum extension for most of the 19^th century, Suðurjökull experienced a quasi-periodic series of eight surges between 1828 and 1930 AD. Each surge event resulted in fragmentation of the glacier terminus during advances of up to ~1.6 km that occurred in less than 2 years. Collapse of the expanded ice and re-establishment of the ice front at a near-shore grounding line occurred within 1 to 3 years of the surge. The surge periodicity was 14 ± 4 years (range: 10 to 20 years). This is the first firm evidence demonstrating surging behavior of Suðurjökull during the Little Ice Age.