Henrik Højmark Thomsen

Present and past climate control on fjord glaciations in Greenland: Implications for IRD‐deposition in the sea

Calving of icebergs is the dominant ablation mechanism for large outlet glaciers from the Greenland ice sheet except in northernmost Greenland where bottom melting from floating glaciers dominates. This difference is controlled by present climate conditions. Glacial geological evidence indicates that the transition between the associated types of fjord-glaciations moved north-south in response to past climate change. In cold periods, local melt-out of debris from the bottom of an increasing number of floating glaciers reduces the potential for iceberg transport of IRD. Thus, the marine IRD signal of Greenland origin is not a simple cold climate signal. Our findings are discussed in the context of the ongoing debate about the kind of ice transporting IRD - icebergs or sea ice.

Sea ice and the stability of north and northeast Greenland floating glaciers

Abstract The interaction between sea ice and glaciers has been studied for the floating tongue of Nioghalvfjerdsfjorden glacier, northeast Greenland (79°30’N, 22° W). Information from glacial geological studies, expedition reports, aerial photographs and satellite imagery is used to document the glacier front position and fast-ice conditions on millennial to decadal time-scales. The studies indicate that the stability of the floating glacier margin is dependent on the presence of a protecting fast-ice cover in front of the glacier. In periods with a permanent fast-ice cover, no calving occurs, but after fast-ice break-up the glacier responds with a large calving activity, whereby several years of accumulated glacier-ice flux suddenly breaks away. Climate-induced changes of sea-ice conditions in the Arctic Ocean with seasonal break-up of the near-shore fast ice could lead to disintegration of the floating glaciers. The present dominant mass loss by bottom melting would then to a large extent be taken over by grounding-line calving of icebergs. The local influx of fresh water from the north Greenland glaciers to the sea would be reduced and the local iceberg production would increase.

The recession of the Inland Ice margin during the Holocene climatic optimum in the Jakobshavn Isfjord area of West Greenland

Recent subsurface mapping of parts of the Greenland Inland Ice margin in the region of Jakobshavn Isbrae indicates that the fjord system in the period of at least 2700–4700 calendar yr B.P. was more ice free than at present, and that the front of the glacier was at least 15 km behind the present position. The 14C-datings of subfossils brought to the present ice margin fit with the climatic records from ice cores and confirm the favourable conditions for Greenlands first settlers, the Sarqaq people, who arrived in the region about 4000 yr ago to find hunting grounds 10–20% larger than the present.

The Greenland ice-sheet margin — A mine of ice for paleo-environmental studies

Abstract Stable isotope analysis of surface-ice samples from three locations on the margin of the Greenland ice sheet have provided δ 18 O records dating back into the Wisconsinan. The samples were collected in profiles transverse to the ice margin on a location about 40 km northeast of Jakobshavn, West Greenland, and at two locations in Warming Land, North Greenland. The Holocene/Wisconsinan transition, as detected by a large aburpt δ-shift, was found between 470 and more than 850 m from the ice edge. The magnitude of the δ-shift was about 8% and 12% in the West and North Greenland records, respectively. The West Greenland record, which is the more detailed, documents that some δ-variations in the Wisconsinan are apparently preserved in the ice margin isotopic records. By linking locations in the accumulation and ablation zones with the same observed δ-value, the surface ice of the ablation zone can be dated back to at least the Wisconsinan/Holocene transition, if the velocity of the ice motion is measured or estimated by ice flow modelling. This means that dated records of ice constituents (gas-composition, chemistry, content of microparticles, micro-meteorites, etc.) can be established by mining the easily accessible surface ice of the ablation zone.

Tidal movement of Nioghalvfjerdsfjorden glacier, northeast Greenland: observations and modelling

Abstract Nioghalvfjerdsfjorden glacier is a >60 km long and 20 km wide floating outlet glacier located at 79°30’ N, 22° W, draining a large area of the northeast Greenland ice sheet. Climate, mass-balance and dynamics studies were carried out on the glacier in three field seasons in 1996, 1997 and 1998. As part of this work, tidal-movement observations were carried out by simultaneous differential global positioning system (GPS) measurements at several locations distributed on the glacier surface. The GPS observations were performed continuously over several tidal cycles. At the same time, tiltmeter measurements were carried out in the grounding zones along the glacier margins and upstream, where the glacier leaves the main ice sheet A tide gauge installed in the sea immediately in front of the glacier front recorded the tide in the open sea during the field seasons. The observations show that the main part of the glacier tongue responds as a freely floating plate to the phase and amplitude of the local tide in the sea. However, kilometre-wide flexure zones exist along the marginal and upstream grounding lines. Attempts to model the observed tidal deflection and tilt patterns in the flexure zone by elastic-beam theory are unsuccessful, in contrast to previous findings by other investigators. The strongest disagreement between our measurements and results derived from elastic-beam theory is a significant variation of the phase of the tidal records with distance from the grounding line (most clearly displayed by the tilt records). We suggest that the viscous properties of glacier ice must be taken into account, and consequently that a viscoelastic-beam model must be used to adequately describe tidal bending of floating glaciers.

Calculated variations of annual ice ablation at the margin of the Greenland ice sheet, West Greenland, 1961-90

Annual ice ablation has been measured at three locations at the margin of the Greenland ice sheet : Nordbogletscher (for 6 years : 1977-78 to 1982-83), Qamanârssup sermia (8 years : 1979-80 to 1986-87) and at Paakitsup Akuliarusersua (7 years : 1982-83 to the present). As the data sets cover different periods, it is difficult to compare ablation directly between the three sites. However, measured series at each site can be extended over the last 30 years (1961-90) by simulations using climatic data, and the extended data series can be compared

Applied Glacier Research for Planning Hydro-Electric Power, Ilulissat/Jakobshavn, West Greenland

Glaciological investigations were carried out on the Greenland ice sheet to help develop plans for a hydro-electric power station to supply energy to Ilulissat/1akobshavn . The investigations required research on supraglacial and subglacial melt-water drainage in order to delineate water-drainage basins. This involved repeated detailed photogrammetric mapping, radio echo-sounding, hot-water drilling, and mathematical modelling of subglacial drainage. Repeated mappings combine to show an overall stability in the supraglacial drainage pattern, while model calculations for the subglacial conditions show a limited sensitivity in the predicted drainage areas to changes in hydraulic conditions. The investigations provide the basis for setting safer limits for planning hydro-electric power in the area, and give a general understanding of glacier hydrology for a continuous ice cover such as the Greenland ice sheet.

SIMULATION OF RUN - OFF FROM THE GREENLAND ICE SHEET FOR PLANNING HYDRO- ELECTRIC POWER. ILUlISSAT / JAKOBSHAVN. WEST GREENLAND

Simulations of run-off from the Greenland ice sheet were made as part of a feasibility study for provision of hydroelectric power for lIulissatfJakobshavn, West Greenland. The aims were to see if the available short series of run-off measurements are typical of those under present climatic conditions, and to assess possible changes in run-off likely to be caused by gross changes in drainage pattern on the ice sheet. Specific run-off was calculated from climatological data, whilst run-off volumes were calculated by integrating specific run-off over the area of the ice sheet. There have been substantial year-to-year variations in run-off, but the 6 year measurement period is reasonabl y re prese ntati ve of present climatic conditions. Run-off could be reduced by 21% as a result of changes in hydrauli c conditions on the ice sheet without this having a significant effect on the economy of the planned hydro-electric power station.

Using stable isotopes as natural tracers to delineate hydrological drainage basins on the Greenland ice-sheet margin

Abstract Stable isotope studies of surface ice and water samples collected in the ablation zone of the Greenland ice-sheet margin at Paakitsup Akliarusersua ∼40 km northeast of Ilulissat/Jakobshavn, West Greenland, show that there is a large system-atic spatial variation of the 18 O 16 O ratio (δ 18 O) over the ablation area. δ 18 O can, therefore, be used as a natural tracer to locate the origin of the melt-water runoff. A simple model is set up for simulating the δ 18 O variation in the melt-water discharge from the area. Simulations are made with a drainage basin obtained by modelling the subglacial drainage conditions, which requires costly and time-consuming mapping of the subglacial topography. The simulated δ-values agree well with observed δ-values of water samples from three pro-glacial lakes which receive the melt-water discharge. This suggests that the stable isotope method can be used to give an initial estimate of melt-water drainage-basin boundaries on the Greenland ice-sheet margin.