Rapid Ice Sheet Retreat Research Articles

Paleoceanographic changes in the Southern Ocean off Elephant Island since the last glacial period: Links between surface water productivity, nutrient utilization, bottom water currents, and ice-rafted debris

To understand past changes in ocean–cryosphere interactions in the Southern Ocean off the Antarctic Peninsula, multi-proxy analyses of three sediment cores located off Elephant Island were used to reconstruct changes in paleoproductivity, nutrient utilization, bottom current intensity, and iceberg calving since the last glacial period. The glacial period was characterized by low surface water productivity with high nutrient utilization, indicating surface water stratification. During the deglaciation, surface water productivity increased with decreasing nutrient utilization, implying that the increase is associated with increased nutrient supply from the subsurface water by enhancing Antarctic Circumpolar Current (ACC) influence as fronts migrate southward with warming. Abundant occurrence of grains >1 mm during the deglacial period indicates rapid ice sheet retreat with large-scale melting and calving. During the glacial period, however, coarse silt–fine sand-sized fraction represented ice-rafted debris (IRD). The different IRD grain size characteristics are thought to be related to the IRD source material characteristics. Regardless of IRD input, the running downcore correlation (5 to 9-point) between sortable silt mean grain size and percentage showed that sediments are well sorted by bottom current. However, the cross plot of them showed different temporal relationships. Sediments were sorted by the ACC and southwestward flowing bottom current. Along with southward migration of fronts and the ACC, southwestward flowing bottom current influence diminished, whereas the ACC influence increased particularly from 7 ka. Our results indicate that the sedimentary processes in the Scotia Sea largely depend on the regional interactions between the ocean and the cryosphere.

Ice-flow patterns and precise timing of ice sheet retreat across a dissected fjord landscape in western Norway

We reconstruct patterns of ice flow and retreat of the southwestern Scandinavian Ice Sheet, from 2900 field observations of glacial striae and elevation measurements of 60 ice-marginal-deltas from a high-resolution LiDAR DEM. During the Last Glacial Maximum, ice flow was towards the west across the entire area, including across several-hundred meter deep north-south oriented fjords. During deglaciation, ice flow adjusted to topography and the dominant flow direction switched towards the south-west. We use a shoreline diagram constructed from relative sea-level curves to establish the age of each delta, which allows us to constrain the timing of retreat with almost decadal precision. Rapid ice sheet retreat commenced at the onset of the Holocene at 11,600 cal years BP. Retreat rates were 160 m a−1 in the deepest fjords, 60–80 m a−1 in shallower fjords, and even slower for land-terminating margins. The fastest retreat rates, 240 m a−1 and 340 m a−1, were experienced in the largest fjords, Hardangerfjorden and Sognefjorden, which border the study area to the south and north. Crosscutting glacial striae indicate that calving bays developed during retreat along the widest fjords. The combination of complex fjord topography with fast ice-margin retreat by iceberg calving, led to isolation of ice remnants on islands and peninsulas, a process that accelerated the overall rate of deglaciation. Ice-margin retreat paused between 11,300–11,100 cal years BP, probably due to cooling during the Preboreal Oscillation.

Sea ice, ice‐rafting, and ocean climate across Denmark Strait during rapid deglaciation (∼16–12 cal ka BP) of the Iceland and East Greenland shelves

ABSTRACTA suite of cores from the North‐West Iceland and East Greenland shelves sampled fossiliferous or unfossiliferous basal glacial diamictons. Radiocarbon dates above the diamictons are similar on both shelves, but the value of the ocean reservoir correction, ΔR, is unknown. Deglaciation occurred either ∼16 or 14 cal ka BP depending on the choice of ΔR. The ice sheets were behind the present coastline by 12.2 cal ka BP. We examine seven cores that record the glacial/deglacial transition and present new data on the sea‐ice biomarkers IP25 and C25:2 from four of the cores plus data on ice‐rafted debris counts, grain‐size spectra, δ18O on the near‐surface planktonic foraminifera Neogloboquadrina pachyderma (s), foraminifera assemblages, and quartz percentage. IP25 concentrations are markedly higher for the East Greenland sites, while they are frequently below the limit of quantification off Iceland, observations that parallel the wt% quartz in the sediments. The δ18O N. pachyderma (s) data show a strong gradient across Denmark Strait with lighter δ18O values towards the East Greenland shelf indicative of a large freshwater flux. The presence of the chilled Atlantic Water benthic foraminifera, Cassidulina neoteretis indicates that rapid ice sheet retreat was associated with ocean forcing, combined with other factors.

Rapid ice sheet retreat triggered by ice stream debuttressing: Evidence from the North Sea

Using high-resolution bathymetric and shallow seismic data from the North Sea, we have mapped hitherto unknown glacial landforms that connect and resolve longstanding gaps in the Quaternary geological history of the basin. We use these data combined with published information and dates from sediment cores to reconstruct the extent of the Fennoscandian and British Ice Sheets (FIS and BIS) in the North Sea during the last phases of the last glacial stage. It is concluded that the BIS occupied a much larger part of the North Sea than previously suggested and that North Sea ice underwent a dramatic disintegration ~18,500 yr ago. This was triggered by grounding-line retreat of the Norwegian Channel Ice Stream, which debuttressed adjacent ice masses, and led to an unzipping of the BIS and FIS accompanied by drainage of a large ice-dammed lake. Our reconstruction of events provides an opportunity to improve understanding and modeling of the disintegration of marine-based ice sheets, and the complex interplay between ocean circulation and the cryosphere.

Pleistocene glacimarine sedimentation on the continental slope off Vancouver Island, British Columbia

Cordilleran Ice Sheet activity has had a significant influence on glacimarine sediment transport and deposition on the western Canadian continental slope resulting in distinct sediment facies. Here we present lithogenic sediment grain size distributions (grains > 250 µm g − 1 , % > 63 µm, % 10 to 60 µm, and % 2 to 4 µm) and mass accumulation rates (MAR) from a marine core (MD02-2496) collected ~ 50 km offshore of Vancouver Island, British Columbia. The lithogenic fraction of sediment deposited on the slope increased as grain size decreased and sediment sorting increased at ~ 30 ka when the Cordilleran Ice Sheet grew into southern British Columbia during the Fraser Glaciation. As the ice sheet advanced onto the continental shelf near core MD02-2496 between 19 and 17 ka, the proximal ice margin dramatically increased lithogenic sediment MARs. Glacimarine sedimentation at this time was associated with cyclic deposition of generally coarse, poorly sorted sediment. An ~ 80-year cyclicity suggests the influence of 1) outburst floods, 2) lunar or 3) solar cycles on tidewater outlet glacier discharge. Rapid ice sheet retreat at ~ 17 ka is indicated by ice rafted debris (IRD) deposition, followed by the deposition of fine, well-sorted silt suggestive of iceberg discharge followed by rock flour in a meltwater plume. Lithogenic sediment MARs decreased dramatically at the Bølling with a brief return to coarse, poorly sorted sediments during the Younger Dryas. Glacimarine sedimentation decreased by ~ 10 ka as the Cordilleran Ice Sheet extent diminished to the areal extent of modern ice fields.

Changes in ice dynamics and mass balance of the Antarctic ice sheet

The concept that the Antarctic ice sheet changes with eternal slowness has been challenged by recent observations from satellites. Pronounced regional warming in the Antarctic Peninsula triggered ice shelf collapse, which led to a 10-fold increase in glacier flow and rapid ice sheet retreat. This chain of events illustrated the vulnerability of ice shelves to climate warming and their buffering role on the mass balance of Antarctica. In West Antarctica, the Pine Island Bay sector is draining far more ice into the ocean than is stored upstream from snow accumulation. This sector could raise sea level by 1m and trigger widespread retreat of ice in West Antarctica. Pine Island Glacier accelerated 38% since 1975, and most of the speed up took place over the last decade. Its neighbour Thwaites Glacier is widening up and may double its width when its weakened eastern ice shelf breaks up. Widespread acceleration in this sector may be caused by glacier ungrounding from ice shelf melting by an ocean that has recently warmed by 0.3 degrees C. In contrast, glaciers buffered from oceanic change by large ice shelves have only small contributions to sea level. In East Antarctica, many glaciers are close to a state of mass balance, but sectors grounded well below sea level, such as Cook Ice Shelf, Ninnis/Mertz, Frost and Totten glaciers, are thinning and losing mass. Hence, East Antarctica is not immune to changes.

A comparison of models of glacial sedimentation along the eastern Canadian margin

The distribution and thickness of glacial and postglacial sediments of the eastern Gulf of Maine, Scotian Shelf, Grand Banks of Newfoundland, Labrador Shelf and Hudson Strait have been mapped using seismic reflection systems and lithological, chronological and biostratigraphic data from cores and seabed samples. Deposits of till, represented by incoherent seismic reflections, commonly occur as sheets but also as linear ridges, lenses and tongues. Discontinuous multiple till sequences, whose thicknesses range from 1–300 m occur across the shelves and pinch out in water depths of 500–700 m. Till thickness and distribution is in part controlled by erodability and geomorphology of the underlying substrate. Glaciomarine sediments up to 300 m thick, represented by medium to high intensity, parallel, continuous coherent seismic reflections occur as conformable to slightly ponded deposits. Glaciomarine sediments can interfinger, conformably overlie or abruptly terminate against the till surfaces. On the southern shelves, from the Gulf of Maine to northeast Newfoundland Shelf and also in eastern Hudson Strait, the glaciomarine sediments are typically interbedded with the tills at former ice-sheet buoyancy zones where much of the material has been derived from subglacial meltout. On the Labrador Shelf, subglacial meltout did not occur as the till and glaciomarine sediments are of different lithology and do not interfinger. From the Gulf of Maine to the Grand Banks of Newfoundland, acoustically weakly stratified, ponded postglacial muds were eroded from the shallow bank surfaces by a marine transgression and deposited in the basins of the shelf. On the Labrador Shelf, iceberg scouring has reworked the entire bank top surface into an ice-keel turbate up to 10 m thick and released the fines which now occupy the basins. The thick (mid-late Wisconsinan) multiple till sections and associated glaciomarine sediments in the basins of the southern shelves suggest the presence of an ice shelf for 20,000 years, beneath which large volumes of sediment were deposited. In contrast, the relatively thin (mid-late Wisconsinan) glacial sequence of the Labrador Shelf and the lack of subglacial deposits within the glaciomarine sequence may indicate either a rapid ice-sheet retreat for that area or an ice shelf largely devoid of englacial debris.