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The Greenland Ice Sheet has experienced significant mass loss in recent years. A substantial component of this is attributable to the retreat of marine-terminating outlet glaciers, which lose mass through increases in calving, submarine melting and terrestrial meltwater discharge. In terms of iceberg production, Jakobshavn Isbræ is the largest marine-terminating glacier in Greenland, yet relatively little is known about its history before the first glacier margin observations in 1851. Two marine sediment cores obtained 15 and 19 km northwest from the mouth of Jakobshavn Isfjord were analysed to reconstruct the past behaviour of Jakobshavn Isbræ and to investigate the response of the glacier system to ocean forcing. These records provide long-term (~2000) context for assessing the significance of the rapid changes in glacier stability over the last century. The X-ray imagery and high-resolution grain size analysis from both cores reveal distinct multi-centennial-scale changes in the flux of iceberg-rafted debris (IRD) from Jakobshavn Isbræ. Foraminiferal analysis shows that variability in the relatively warm West Greenland Current (WGC) may have been an important driver of calving activity at Jakobshavn Isbræ. We find that iceberg rafting and WGC inflow were relatively high from onset of the record, at 60 BC, until AD 1100. Subsequently, the inflow of the WGC into Disko Bugt decreased. This was accompanied by a dramatic reduction in IRD from AD 1500 to 1850, which is attributed to the establishment of a floating ice tongue. We also show that ocean warming in the 20th century is part of a longer-term warming trend in the WGC which started at around AD 1700. Finally, these new records underline the complexity of glaciomarine sediments; IRD variability was driven by the inflow of the WGC but was also modulated by a complex interplay of air temperature, sea-ice coverage and ice margin proximity. [ABSTRACT FROM AUTHOR]

Observations over the past 2 decades show substantial ice loss associated with the speed-up of marine-terminating glaciers in Greenland. Here we use a regional three-dimensional outlet glacier model to simulate the behaviour of Jakobshavn Isbræ (JI) located in western Greenland. Our approach is to model and understand the recent behaviour of JI with a physical process-based model. Using atmospheric forcing and an ocean parametrization we tune our model to reproduce observed frontal changes of JI during 1990-2014. In our simulations, most of the JI retreat during 1990-2014 is driven by the ocean parametrization used and the glacier's subsequent response, which is largely governed by bed geometry. In general, the study shows significant progress in modelling the temporal variability of the flow at JI. Our results suggest that the overall variability in modelled horizontal velocities is a response to variations in terminus position. The model simulates two major accelerations that are consistent with observations of changes in glacier terminus. The first event occurred in 1998 and was triggered by a retreat of the front and moderate thinning of JI prior to 1998. The second event, which started in 2003 and peaked in the summer 2004, was triggered by the final break-up of the floating tongue. This break-up reduced the buttressing at the JI terminus that resulted in further thinning. As the terminus retreated over a reverse bed slope into deeper water, sustained high velocities over the last decade have been observed at JI. Our model provides evidence that the 1998 and 2003 flow accelerations are most likely initiated by the ocean parametrization used but JI's subsequent dynamic response was governed by its own bed geometry. We are unable to reproduce the observed 2010-2012 terminus retreat in our simulations. We attribute this limitation to either inaccuracies in basal topography or to misrepresentations of the climatic forcings that were applied. Nevertheless, the model is able to simulate the previously observed increase in mass loss through 2014. [ABSTRACT FROM AUTHOR]