Sedimentation on the continental rise west of the Antarctic Peninsula over the last three glacial cycles

The continental rise west of the Antarctic Peninsula includes a number of large sediment mounds interpreted as contourite drifts. Cores from six sediment drifts spanning some 650 km of the margin and 4° of latitude have been dated using chemical and isotopic tracers of palaeoproductivity and diatom biostratigraphy. Interglacial sedimentation rates range from 1.1 to 4.3 cm/ka. Glacial sedimentation rates range from 1.8 to 13.5 cm/ka, and decrease from proximal to distal sites on each drift. Late Quaternary sedimentation was cyclic, with brown, biogenic, burrowed mud containing ice-rafted debris (IRD) in interglacials and grey, barren, laminated mud in glacials. Foraminiferal intervals occur in interglacial stages 5 and 7 but not in the Holocene. Processes of terrigenous sediment supply during glacial stages differed; meltwater plumes were more important in stages 2–4, turbidity currents and ice-rafting in stage 6. The terrigenous component shows compositional changes along the margin, more marked in glacials. The major oxides Al2O3 and K2O are higher in the southwest, and CaO and TiO2 higher in the northeast. There is more smectite among the clay minerals in the northeast. Magnetic susceptibility varies along and between drifts. These changes reflect source variations along the margin. Interglacial sediments show less clear trends, and their IRD was derived from a wider area. Downslope processes were dominant in glacials, but alongslope processes may have attained equal importance in interglacials. The area contrasts with the East Antarctic continental slope in the SE Weddell Sea, where ice-rafting is the dominant process and where interglacial sedimentation rates are much higher than glacial. The differences in glacial setting and margin physiography can account for these contrasts

Living (stained) and dead foraminifera from the newly ice-free Larsen Ice Shelf, Weddell Sea, Antarctica: ecology and taphonomy

Within the past 7 years, the northern Larsen Ice Shelf has broken up so it is now possible to sample the sea floor that formerly lay beneath it. Box cores have yielded surface sediment samples (0–1 cm) that give information on living and dead foraminiferal assemblages. The living assemblages are of moderate diversity and four have >50% calcareous tests while five have >50% agglutinated tests. This is an area of high primary production and the standing crops of the benthic foraminiferal assemblages are high. All the dead assemblages are much enriched in agglutinated tests, often >90%. They give a time-averaged record of the past 7 ice-free years and several decades of ice cover. The loss of permanent ice cover (there is still seasonal ice cover) may have caused some response from the fauna, but it is likely that it was mainly changes in relative/absolute abundance of the existing fauna. The differences between the live and dead assemblages in the surface 1 cm are attributed mainly to taphonomic effects: dissolution of calcareous tests and loss of fragile agglutinated tests. Subsurface samples down to 5 cm show that dissolution of calcareous tests is widespread and there may be some loss of fragile agglutinated forms such as Reophax subdentaliniformis. For these reasons, in this area, it may be best to make palaeoecological interpretations on the agglutinated component of the fossil assemblages

High-resolution diatom stratigraphy of Quaternary sediments from the Scotia Sea

Upper Quaternary pelagic and hemipelagic sediments from the Scotia Sea and South Scotia Ridge range from diatom ooze to diatom-bearing mud. Diatom content increases northwards, and at most sites diatom-rich and diatom-poor sediments alternate downcore on a scale of metres. A local stratigraphy is based on relative abundance of six prominent diatom taxa:Eucampia antarctica, Rhizosolenia spp.,Thalassiosira spp.,Chaetoceros spores,Nitzschia kerguelensis and otherNitzschia species and one silicoflagellate species (Distephanus speculum). These stratigraphic units defined using diatoms are correlated with radiolarian abundance stratigraphy (Cycladophora davisiana) and with palaeomagnetic stratigraphy. Information from modern environments (phytoplankton and sediment trap studies) indicates that changes in diatom species composition are related to N-S movement of the winter ice edge and of the Antarctic Convergence

Sedimentation associated with Antarctic Peninsula ice shelves: implications for palaeoenvironmental reconstructions of glacimarine sediments

Recent disintegration of a number of Antarctic Peninsula ice shelves has given us a unique opportunity to investigate sub-ice-shelf sediments. We characterize three sediment facies associations of two Antarctic Peninsula ice shelves (Larsen-A and Larsen Inlet). Subglacial facies consist mainly of basal till (diamicton) with high shear strengths deposited under a grounded ice sheet. Progressive upward decrease in shear strength reflects a gradual decrease in the confining vertical effective pressure of grounded ice during till deposition. Proximal ice-shell glacimarine facies (diamicton, gravel-rich and sand-rich facies, gravelly mud, dropstone Mud and sands muds) ere deposited by sub-ice-shelf rain out, bottom current activity and sediment gravity flows following decoupling of grounded ice from the sea-floor, Distal, he-shell glacimarine and/or open marine facies comprise terrigenous and diatom-bearing bioturbated muds and gravelly muds that contain limited ice-rafted debris these accumulated after recession of the grounding line to the coast, with coarse-grained surface sediments possible documenting most recent ice-shelf break-up. Antarctic Peninsula ice-shelf sediments are more heterogeneous than ice-shelf facies deposited else's here in Antarctica. Cold. polar Antarctic ice shelves can be differentiated from temperate and sub-polar marine-terminated glacial sedimentary systems by the dominance of coarse-grained proximal. ice-shelf glacimarine facies and an absence of subaqueous outwash/meltwater sediment facies

Ice sheet retreat from the Antarctic Peninsula shelf

Side-scan sonar and sub-bottom acoustic profiler data and sediment cores reveal the processes that controlled sediment transport and deposition on the continental shelf of the Antarctic Peninsula Pacific margin off Anvers Island, during deglaciation over the last 11,000 years or more. Glacial flutes and striations mark the flow of low-profile ice streams draining the interior, across the middle and outer shelf. Most probably, ice sheets were grounded to the continental shelf edge along this margin during the last glacial maximum. Iceberg furrows overwrite the ice sheet record in areas between 500 and 350 m water depth, and reflect calving from a retreating ice shelf front. Cores show open marine sedimentation replacing diamicton deposition close to the grounding line during this retreat, which rapidly cleared the outer and middle shelf shortly before 11,000 years BP (from AMS14C dates on organic carbon). The shallower, scoured and largely sediment-free inner shelf cleared later, probably before 6000 years BP. Open marine sediments on the middle and outer shelf include a pelagic biogenic component and suspended sediment from modern glacier tongues, supplemented by resuspension of older sediment in shallow shelf regions (by currents and by grounded icebergs). Sedimentation is too slow to be able to fill in the concave-up profile of the continental shelf during a full interglacial, confirming the intense glacial-interglacial cyclicity of sedimentation on the continental slope inferred from seismic reflection profiles. The observed rapid deglaciation of the middle and outer shelf supports published numerical model results that the Antarctic Peninsula's narrow interior and broad continental shelf make the ice sheet sensitive to imposed eustatic sea-level change. A low-profile marine-based ice sheet over the continental shelf during glacial maximum would have made a major contribution to that sensitivity, in the early stages of deglaciation. It follows that the Antarctic Peninsula ice sheet, and probably most others, are not so sensitive today

Continental slope morphology and sedimentary processes at the mouth of an Antarctic palaeo-ice stream.

Continental-slope and shelf-edge morphology off Marguerite Bay, western Antarctic Peninsula, is investigated using swath-bathymetric data and parametric sub-bottom profiler records, together with sediment cores. Marguerite Bay has a well-defined cross-shelf trough, and a relatively steep continental slope. The slope beyond the trough mouth is convex in longitudinal profile, whereas to the north and south it is concave and reaches a maximum of 12degrees. There are no deep canyons cutting into the prograding outer shelf and slope. Instead, a series of gullies runs down the upper slope, reaching depths of >200 m south of the trough mouth but <120 m deep beyond the trough. The mid and lower slope appears to be relatively smooth and downslope sediment transfer is probably by small-scale slides, slumps and debris flows. The continental rise contains dendritic channels related to turbidity currents, and sediment drifts produced by southwest-flowing bottom currents from the fine-grained component of the turbidity currents. Elongate sedimentary bedforms indicate that a fast-flowing ice stream occupied the trough under full-glacial conditions, and transferred deforming subglacial till rapidly to the shelf edge. By contrast, on either side of the trough mouth, ice is inferred to have been slower-moving and probably cold-based, delivering little sediment to the upper slope. The steepness of the continental slope results in rapid downslope sediment transfer by debris flows, slumps and turbidity currents and accounts for the lack of a well-developed trough-mouth fan, which is typical of many lower-gradient glacier-influenced margins

Thickness and extent of the subglacial till layer beneath an Antarctic paleo-ice stream.

Fast-flowing ice streams and outlet glaciers currently account for as much as 90% of the discharge from the Antarctic and Greenland Ice Sheets. Although the deformation of subglacial material has been proposed as the mechanism for this rapid motion, such sediment is usually hidden under several kilometers of ice. Marine-geophysical records have allowed reconstruction of the three-dimensional thickness of the sedimentary bed beneath a large Antarctic paleo-ice stream for the first time. Fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength, unsorted sediment, averaging 4.6 m thick. Rapid motion of the paleo-ice stream was a result of subglacial deformation within this layer