A 2000-year record of ocean influence on Jakobshavn Isbræ calving activity, based on marine sediment cores

Abstract

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.

Sea Surface Temperature Variability on the SE‐Greenland Shelf (1796–2013 CE) and Its Influence on Thrym Glacier in Nørre Skjoldungesund

“…Whereas the SST record shows a clear change in variability around 1900 CE, the IRD record remains relatively constant in this period. This is in contrast with studies of the larger marine terminating glaciers, for example, Jakobshavn Isbræ (Wangner et al, ) and Helheim Glacier (Andresen et al, ), where increased IRD was related to observed glacier margin retreat and climate warming, suggesting a link between ocean forcing, iceberg production, and glacier retreat.…”

“…Whereas the SST record shows a clear change in variability around 1900 CE, the IRD record remains relatively constant in this period. This is in contrast with studies of the larger marine terminating glaciers, for example, Jakobshavn Isbrae (Wangner et al, 2018) and Helheim Glacier (Andresen et al,…”

“…In order to extend information on the calving behavior of Thrym Glacier back in time, the percentage of sand within the sediment core was measured. This parameter represents the amount of IRD, which is commonly used as an indicator for the iceberg rafting from marine‐terminating glaciers (Andresen et al, , ; Andrews et al, ; Andrews, ; Dowdeswell, ; Dyke et al, ; Syvitski & Shaw, ; Vermassen, Wangner, et al, ; Wangner et al, ). These studies indicate that iceberg rafting in fjords not only depends on the iceberg production but can also be modulated by a variety of processes including glacier margin type (shelf versus calving glacier), changes to transit time of icebergs down the fjord or changes to the distance between coring site and glacier margin.…”

“…The proxy ocean data are compared with the available instrumental oceanographic temperature observations and other proxy temperature records from offshore SE-Greenland. Sediment cores have additionally been used to investigate the link between ocean warming and glacier dynamics of some of the largest marine-terminating glaciers in Greenland, such as Jakobshavn Isbrae (Holland et al, 2008;Lloyd, 2006Lloyd, , 2011Andresen et al, 2011, Wangner et al, 2018, Helheim Glacier (Andresen et al, , 2013, and Kangerlussuaq glacier (Vermassen, Bjørk, et al, 2019). While the glacier geometry of these glaciers are characterized by deep subglacial troughs connected directly to the fjords, smaller glaciers terminating in shallower fjords such as Thrym Glacier, which terminates into Skjoldungen Fjord, usually have a limited ice-ocean interface.…”

Evaluating ice‐rafted debris as a proxy for glacier calving in Upernavik Isfjord, NW Greenland

“…Environmental factors such as sea‐ice conditions (frozen vs. unfrozen fjord) and ambient water temperature can depress or enhance the general ice‐rafting signal and therefore complicate its interpretation as an indicator of calving intensity. In addition, IRD variability has an inherent random component due to local effects such as differential iceberg transport pathways and variations in the sediment load and grain‐size distribution of icebergs (Andrews, ; Wangner et al , ). Our results confirm this high spatial variability since the IRD fluxes vary markedly between core sites with little to no correlation (Fig.…”