Interaction between subsurface ocean waters and calving of Jakobshavn Isbrae during the late Holocene

It is becoming increasingly clear that the cryosphere is responding quickly to human induced temperature change. However, prediction of the contribution of the Greenland ice sheet to global sea level rise is complicated by lack of knowledge about mechanisms behind ice sheet change. In particular ice streams and their interaction with components of the atmospheric and oceanic climate system needs further investigations in order to make realistic models of future sea level rise. In this study we have reconstructed the Late Holocene climate history of iceberg calving from the Jakobshavn Isbrae tidewater outlet glacier in west Greenland, which is the most prolific exporter of Greenland ice into the ocean. By analysing the foraminiferal content and the lithology in a sediment record from Vaigat Strait in Disco Bay we conclude that during the past c. 5000 years there have been a number of centennial long episodes with warming of the West Greenland Current. These episodes are accompanied by increased iceberg rafting and we link them to increased iceberg calving in relation to warming of subsurface waters in the West Greenland Current. Between 5200 and 3800 cal yr BP the data are interpreted to document the end of a late Holocene Thermal Maximum in West Greenland. An increased response of the iceberg rafting during warm pulses of WGC in the time period 3800 to 2000 cal yr BP is interpreted as the result of a westward/seaward advance of the glacier margin in relation to onset of Neoglaciation. After 2000 cal yr BP the relatively marked responses of iceberg rafting to warm water incursions may document the development of the floating glacier tongue further out into the ice fjord, making it even more sensitive towards warm subsurface waters. Our data may imply that the 1997 acceleration of the Jakobshavn Isbrae is comparable to a recurrent phenomenon characterising the Holocene on multidecadal to centennial time-scales. The modern acceleration was triggered by substantial warming of the North Atlantic sub polar gyre from the mid-1990s onwards with a maximum reached in 2003. Recent changes in the NAO index and a comparison of our West Greenland record with a record from the Eastern North Atlantic indicates that a NAO-like anomaly pattern may have been operating during most of the late Holocene.