Glacimarine Sediments Research Articles

Morphology and Late Quaternary sedimentation on the North Faeroes slope and abyssal plain, North Atlantic

Whilst the interplay between tectonics, glaciation and oceanography is reasonably well understood on some high-latitude margins, relatively little is known about the nature of Quaternary processes affecting the continental margin north of the Faeroe Islands. We therefore present side-scan sonar (GLORIA and TOBI) and 3.5 kHz data in combination with a new regional bathymetry in order to characterise and analyse the margin's physiography and the sedimentary processes that have occurred in the area during the Late Quaternary. Seven acoustic facies are identified and interpreted, in combination with published core records. The margin is subject to differing sedimentation regimes over glacial/interglacial cycles, relating to the changing importance of contouritic, hemipelagic, glacimarine and mass-movement processes. Mass movement is much more extensive than previously thought, with a total sediment volume of >2000 km3, similar to that of the Storegga Slide (5500 km3) and may occur primarily at glacial terminations. The downslope limit of instabilities is associated with a series of channels, focusing the deposition of distal, turbiditic sediments into the regional deep beyond. Together with glacigenic debris flows from the western margin of the North Sea Fan, mass-movement processes are identified to have affected 52–70% of the North Faeroes margin area. In contrast, distal glacimarine sedimentation, alternating with hemipelagic and contouritic development, is found on the remaining 30–48% of the margin during glacials, and hemipelagic and contouritic sedimentation are active across 90% of the margin in interglacial conditions. Despite a lack of any local, direct glacial influence, these observations on the relative importance of sedimentation processes are comparable with other glacially influenced passive margins.

The Late Weichselian glaciation of the Franz Victoria Trough, northern Barents Sea: ice sheet extent and timing

High resolution seismic profiles (PARASOUND, 4 kHz) and three sediment cores from the Franz Victoria Trough and the adjacent continental slope were studied in order to constrain the timing and extent of the northern Svalbard/Barents Sea ice sheet during the Late Weichselian glaciation. Stacked debris flow lobes and layers of glacimarine diamicton on the lower continental slope indicate that large quantities of glacially derived sediments were deposited by the northern Svalbard/Barents Sea ice sheet directly onto the upper continental slope at approximately 23 14 C ka. A grounding-line advance to the shelf break is supported by the identification of diamicton, interpreted as till, in the seismic profile near the shelf break. After several ice sheet instabilities marked by significant input of ice rafted detritus to the continental margin, the disintegration of the northern Svalbard/Barents Sea ice sheet (Termination Ia) is indicated by a distinct pulse of ice rafted detritus at 15.4 14 C ka and the transition to an isotopically defined meltwater signal. The drastic change in sedimentary pattern on the upper continental slope, dated to about 13.4 14 C ka, is interpreted as grounding-line retreat from the shelf edge. A further stepwise retreat of the northern Svalbard/Barents Sea ice sheet is indicated by pulses of ice rafted detritus which appear to be contemporaneous with the onset of distinct ice rafting events in adjacent areas and pulses of glacimarine sedimentation in the southwestern Barents Sea. q 2000 Elsevier Science B.V. All rights reserved.

BRAZILIAN IRON FORMATIONS AND THEIR GEOLOGICAL SETTING

In the Quadrilatero Ferrifero banded iron formations (BIFs) are metamorphic jaspilites named itabirites that occur in the Paleoproterozoic platformal sequence of the Minas Supergroup (2.6 - 2.12 Ga). Several types of itabirites can be defined according to primary (sedimentary) and secondary features, acquired under the influence of metamorphic, deformational and weathering processes. A complex interaction of primary and secondary processes was responsible for the anomalous concentration of iron in high-grade deposits. In the Carajas Range in the central north of Para, BIF-related iron ores are weathered anchimetamorphic jaspilites with preserved primary textures and structures, making up approx. 18 billion tons of ore with 66 % Fe. The jaspilites are weakly deformed and extend over 2500 km 2 in the eastern portion of the region in the Archean Grao-Para Group (2.7-2.6 Ga) between metavolcanic rocks. The Urucum District in Mato Grosso do Sul, at the Brazilian - Bolivian border, displays the youngest Brazilian banded iron formation as flat- lying anchimetamorphic jaspilites of Proterozoic age (ca. 0.6 Ga). These are associated with fine glacial marine sediments and manganese layers that outcrop as inselbergs locally called Morrarias. Urucum jaspilites belong to the group of Rapitan-Type iron formations and can be distinguished from the Carajas jaspilites by the presence of glacial-clastic components and nodules of uncertain origin. Tropical weathering as well as primary concentration are the principal processes responsible for the high grade iron ores with more than 63% Fe and reserves of approximately only 100 million tons, that are very appreciated for direct reduction plants.

BRAZILIAN IRON FORMATIONS AND THEIR GEOLOGICAL SETTING

In the Quadrilátero Ferrífero banded iron formations (BIFs) are metamorphic jaspilites named itabirites that occur in the Paleoproterozoic platformal sequence of the Minas Supergroup (2.6 - 2.12 Ga). Several types of itabirites can be defined according to primary (sedimentary) and secondary features, acquired under the influence of metamorphic, deformational and weathering processes. A complex interaction of primary and secondary processes was responsible for the anomalous concentration of iron in high-grade deposits. In the Carajás Range in the central north of Pará, BIF–related iron ores are weathered anchimetamorphic jaspilites with preserved primary textures and structures, making up approx. 18 billion tons of ore with 66 % Fe. The jaspilites are weakly deformed and extend over 2500 km2 in the eastern portion of the region in the Archean Grão-Pará Group (2.7-2.6 Ga) between metavolcanic rocks. The Urucúm District in Mato Grosso do Sul, at the Brazilian - Bolivian border, displays the youngest Brazilian banded iron formation as flatlying anchimetamorphic jaspilites of Proterozoic age (ca. 0.6 Ga). These are associated with fine glacial marine sediments and manganese layers that outcrop as inselbergs locally called Morrarias. Urucúm jaspilites belong to the group of Rapitan-Type iron formations and can be distinguished from the Carajás jaspilites by the presence of glacial-clastic components and nodules of uncertain origin. Tropical weathering as well as primary concentration are the principal processes responsible for the high grade iron ores with more than 63% Fe and reserves of approximately only 100 million tons, that are very appreciated for direct reduction plants.

An origin for laminated glacimarine sediments through sea‐ice build‐up and suppressed iceberg rafting

Laminated glacimarine sediments are observed in visual core logs and x‐radiographs from Scoresby Sund and Nansen Fjord, east Greenland. They are mostly underlain and overlain by massive or stratified glacimarine diamicton (Dmm or Dms), which is a product of iceberg delivery of heterogeneous debris and, in Scoresby Sund, reworking by deep‐drafted iceberg keels. The laminated sediments are AMS radiocarbon dated to two cold periods since the last, Late Weichselian deglaciation: the Younger Dryas stadial (Milne Land Stadial in east Greenland) and the Little Ice Age. During cold climatic events, multiyear shorefast sea ice (‘sikussak’) formed in these fjords and trapped the icebergs. Fine‐grained, laminated muds (Fl) were deposited in Scoresby Sund when the flux of icebergs was suppressed, but turbid meltwater continued to provide some sediment flux to the fjord systems, varying through time to produce laminations. In Nansen Fjord, thinner and often massive mud layers (Fm) resulted from shorter intervals of sea‐ice cover with no ice rafting. Stratified diamicton layers (Dms), which alternate with mud deposition to produce a laminated unit, probably represent intervening times of more open conditions with iceberg rafting. In Scoresby Sund, foraminifera are either absent from the laminated unit or begin to appear towards the end of its deposition. The absence of both benthic and planktonic foraminifera also suggests that multiyear sea ice was covering the core sites. There is no evidence of macrofaunal activity, and bioturbation is absent from the laminated sediments. Satellite data show that multiyear shorefast sea ice is present in several areas of the high Arctic today, and this traps icebergs calved from interior ice‐cap drainage basins. Thus, the process of laminated glacimarine sediment formation is likely to be applicable to a number of areas of the modern and Quaternary Arctic.

Marine sedimentation at a calving glacier margin

A 2-month study at a subpolar, tidewater glacier was conducted at Anvers Island, Antarctic Peninsula (lat 64°S), to improve understanding of sedimentary processes and associated deposits in glacial marine settings. Data collected in the ice-proximal environment were (1) brash ice-size distribution and melt rates to assess sediment source and transport mechanisms; (2) conductivity-temperature-turbidity-depth (CTTD) profiles, water samples, and sediment-trap catches to study processes and patterns of sediment dispersal; (3) bottom cores and grabs to document recent glacial marine sedimentation; and (4) remotely operated vehicle video surveys of the ice terminus and sea bottom to obtain images of physical and biological processes. The water column is stratified and a halocline occurs at ∼20–30 m. Water temperature ranged from −1 to 1.6 °C, salinity was 32‰–34‰, and suspended solids were 3–5 mg/l well above the bottom. Ephemeral turbid horizons contain 8–15 mg/l suspended solids; rare surface plumes and a quasipermanent turbid layer produced by resuspension during calving events reach 25–35 mg/l. Meltwater drains from the glacier surface, but no subglacial meltwater was detected directly entering the sea. The most important sources of sediment appear to be the direct melting of the submerged ice front and melting of calved glacial ice. Most fragments of floating ice melt within 1 km of the ice front. Some sediment may be contributed as low-density (1.1 g/cm 3 ) algae-bound sediment pellets by meltwater streams draining the ablating ice surface. Sediment enters the stratified water column and is moved by tidal and wind-driven circulation. Marine sedimentation rates are low (∼3 mm/yr) relative to depth of bioturbation, resulting in unstratified sandy mud with dispersed clasts.

Seismic data from the Northern basin, Ross Sea, record extreme expansions of the East Antarctic Ice Sheet during the late Neogene

Approximately 3700 km of intermediate-resolution single-channel seismic data from the Northern basin were analyzed to investigate the late Neogene history of East Antarctic Ice Sheet (EAIS) grounding events on the shelf, and to evaluate how glacial unconformities on the shelf are manifested on the upper slope. The Northern basin was chosen as the site of this study because ice-sheet reconstructions show that the basin received sediment by ice emanating from East Antarctica . In addition, seismic correlations to DSDP Site 273 suggest that a relatively thick late Neogene section exists on the basin's outer shelf and upper slope. On the Northern basin shelf, glacial unconformities exhibiting broad, low-angle relief, topset truncation, and cross-cutting relationships reveal a dynamic history of expansions and contractions during which the EAIS was larger than present on at least eight occasions during the late Neogene. On the upper slope, the correlative conformities of the glacial unconformities are indistinct reflections within thick trough-mouth fan (TMF) depocenters at the mouths of Drygalsky and Joides basins. The glacial unconformities and correlative conformities define TMF sequences, and each TMF sequence contains several topset-truncated prograding-slope reflectors. We infer that the correlative conformities on the continental slope correspond to the interface between prograding glaciogenic deposits (glacial maximum) and diatomaceous glacial-marine sediments (glacial minimum). The seismic–stratigraphic analysis and regional mapping indicate that the upper slope does not contain a more complete late Neogene section than that which exists on the shelf. We infer that diatomaceous glacial-marine sediments on the slope may be relatively undisturbed, and hence may provide a means of dating the TMF sequences. It is hoped that these results will stimulate efforts to core the late Neogene section in the Northern basin TMFs to investigate how these EAIS expansions and contractions relate to other records of late Neogene climate and eustasy .

Rock-magnetic carriers of century-scale susceptibility cycles in glacial–marine sediments from the Palmer Deep, Antarctic Peninsula

A 4200 year long magnetic susceptibility profile from the Palmer Deep records century- and millennial-scale variability in paleoenvironmental conditions along the western Antarctic Peninsula. The susceptibility profile shows regularly spaced highs and lows over the last 3360 years B.P. then drops to uniformly low values. These features are formed by the variable dilution of terrigenous material with biogenic sediments and shifts in magnetic mineralogy. High-susceptibility intervals are characterized by multi-domain (MD) pure magnetite. Low-susceptibility intervals are characterized by pseudo-single-domain (PSD) magnetite and titanomagnetite. Prior to 3360 years B.P., the magnetic mineral assemblage is dominated by fine PSD titanium-rich titanomagnetite and superparamagnetic particles, suggesting a change in sediment provenance and/or sediment transport processes. The absence of MD magnetite suggests a cessation in locally derived ice-rated debris. This horizon represents the transition from the mid-Holocene climatic optimum to the Neoglacial period, marked by an increase in glacial sedimentation.

Icebergs and Iceberg Rafted Detritus (IRD) in the North Atlantic: Facts and Assumptions

Over the last decade considerable attention has been given to evidence for the iceberg rafting of sediments into the North Atlantic, especially during Heinrich (H) events (presumed to be associated with short-lived, abrupt intervals of increased iceberg and meltwater flux). However, our knowledge of the present sediment distribution within icebergs and the fate of this sediment during iceberg drift is rudimentary. Debris is usually concentrated in a 1 3 m layer at the base of a glacier ; if present in calved icebergs, this layer is rapidly melted within a few 10s to 100s of km from an ice margin. Ice rafted debris (IRD) is frequently characterized by the weight percentage or counts of mineral grains within certain size fractions (i.e. > 63, > 250 Fun, etc.) with an implicit assumption that the selection of the size limits does not bias the data nor their interpretation. However, studies of glacial and glacial marine sediments indicate there is a poor correlation between the weight percentage in different sand-size classes, moreover, the bulk of glacially derived sediments are typically silts and clays (< 63 Fun). In areas close to the probable sources of iceberg discharge during Heinrich events, the abrupt changes are poorly defined by the sand-size IRD fraction but are dramatically highlighted by the input of fine-grained (< 63 Fun) detrital carbonate. During the last glaciation, abrupt changes in IRD concentrations (IRD spikes) appear to be an important feature of the mid-latitude belt of the North Atlantic, whereas at higher latitudes, such as areas of the Labrador and Nordic seas, IRD input was more continuous and individual H-events cannot be identified solely by their IRD signal.

Cyclic sedimentation produced by fluctuations in meltwater discharge, tides and marine productivity in an Alaskan fjord

Glacimarine sediment deposited in the fjord adjacent to Muir Glacier in south‐eastern Alaska consists of rhythmically laminated muds, stratified sandy mud, sand and gravelly mud facies. Cyclicity is recorded by gravelly mud facies deposited during winter by ice‐rafting, black mud laminae formed by spring plankton blooms and variations in tidal rhythmite thickness and texture produced by the interaction of meltwater discharges and tidal currents in the macrotidal fjord. Regular cyclicity in laminae thickness is tested statistically by Fourier transform and can be attributed to a lunar tidal cycle control in the five cores collected up to 6 km from the sediment source. Cores close to the source can have additional laminae as a result of discharge fluctuations, and distal cores may lack full cycles because of variability in the plume path and attenuation with distance. Cyclic variations in sediment texture are recorded in magnetic susceptibility (MS) profiles of the cores. High MS values are produced by turbidite sand beds or by stratified sandy mud deposited by overflow plumes during peak summer meltwater discharge. Low values reflect muddy intervals deposited during periods of low meltwater discharge, such as during autumn and winter. Sediment accumulation rates measured by 210Pb dating range from 82 cm year–1, 2 km from the sediment source at the head of the fjord, to 16 cm year–1, 6 km away. These rates are within the same range as average sediment accumulation rates determined from cyclic seasonal markers within the cores. These data show that, with careful documentation, annual cycles of glacimarine sediment accumulation can be detected within marine cores. Cores collected from the distal portion of the basin were deposited during the transition of Muir Glacier from a tidewater terminus ending in deep water to a terrestrial glacier with an ice‐contact delta deposited in front of the terminus. This transition is recorded by a coarsening‐upward sedimentary sequence formed by turbidite sands originating from the prograding delta above fine‐grained, laminated basin fill deposited by turbid overflow plumes.

Clastic dykes in over-consolidated tills: evidence for subglacial hydrofracturing at Killiney Bay, eastern Ireland

A swarm of vertical gravel-filled dykes up to 6 m high and several decimetres wide, cut through an over-consolidated till at Killiney Bay. The dykes are rooted in a gravel layer and many display plumes of clastic debris ejected into the overlying sediments — `burst-out structures'. Such features have not previously been described. These clastic dykes are interpreted as the infillings of hydrofractures which formed when water pressures in the basal gravel layer exceeded the overburden pressure and tensile shear strength of the capping till. The burst-out structures extend up to 7 m from the tops of the dykes and provide strong evidence for forceful upward flow. Evidence suggests that the hydrofractures formed subglacially, probably during a minor re-advance. Their presence in Late Devensian (26–13 ka BP) tills with an Irish Sea provenance (`Irish Sea till') may have important implications for the subglacial hydrology of the last Irish Sea ice sheet. These hydrofractures cannot form in unconsolidated glacimarine sediment and their presence precludes a glacimarine origin for these deposits. They greatly affect the geotechnical properties of Irish Sea tills, in particular providing very permeable routes through otherwise impermeable layers, with important consequences in situations elsewhere in the Irish Sea basin, where they have been used as aquicludes in landfill and low-level nuclear waste disposal sites.

High-resolution Seismic Evidence for Multiple Glaciation across the Southwest Iceland Shelf

Geophysical and lithological data collected from the southwest margin of Iceland provide clear evidence of multiple glaciations across the shelf, many of which reached the shelf-slope break. These paleo-ice sheets delivered much glacial sediment to the slope facing Denmark Strait. A large moraine some 20 to 100 m in height lies at the shelf break due west of Látra Bank. The moraine may mark the Late Weichselian extent of the western Iceland ice, although a single date from a 50-cm gravity core collected from the seaward side of the moraine provided an age of 35,650 ± 560 14C BP. Retreat of this ice sheet appears to have been rapid, except for a minor pause when the ice sheet terminus approached the coastline. A marine reservoir-corrected basal date of 12,705 ± 85 14C BP was obtained from a 17-m giant piston core collected in Jökuldjúp (a shelf trough) which sampled glacial marine sediments close to underlying ice-contact sediment. Because this date is very close to the age of shells near the marine limit in West Iceland, some 100 km to the east-northeast, it suggests that this margin of the Iceland Ice Sheet retreated extremely rapidly during the Bølling/Allerød interstadial (13,000 to 11,000 14C BP).

Surficial Sediments along the Inner Continental Shelf of Maine

Through 10 years of support from the Minerals Management Service Association of American State Geologists' Continental Margins Program we have mapped along the Maine coast, seaward to the 100 m isobath. In all, 1,773 bottom sample stations were occupied, 3,358 km of side-scan sonar and 5,011 km of seismic reflection profiles were gathered. On the basis of these data, a surficial sediment map was created for the Maine inner continental shelf during the Year 8 project, and cores and seismic data were collected to evaluate sand thickness during Years 9 and 10. Sand covers only 8 % of the Maine shelf, and is concentrated seaward of beaches off southern Maine in water depths less than 60 m. Sand occurs in three depositional settings: (1) in shoreface deposits connected dynamically to contemporary beaches; (2) in submerged deltas associated with lower sea-level positions; and (3) in submerged lowstand shoreline positions between 50 and 60 m. Seismic profiles over the shoreface off Saco Bay, Wells Embayment, and off the Kennebec River mouth each imaged a wedge-shaped acoustic unit which tapered off between 20 and 30 m. Cores determined that this was sand that was underlain by a variable but thin (commonly 1 m) deposit of estuarine muddy sand and a thick deposit of glacial-marine mud. Off Saco Bay, more than 55 million m3 of sand exists in the shoreface, compared with about 22 million m3 on the adjacent beach and dunes. Seaward of the Kennebec River, a large delta deposited between 13 ka and the present time holds more than 300 million m3 of sand and gravel. The best sorted sand is on the surface nearshore, with increasing amounts of gravel offshore and mud beneath the surficial sand sheet. Bedforms indicate that the surficial sand is moved by waves to at least 55 m depth. Seaward of the Penobscot River, no significant sand or gravel was encountered. Muddy estuarine sediments overlie muddy glacial-marine sediment throughout the area offshore area of this river. No satisfactory explanation is offered for lack of a sandy delta seaward of Maine's largest river. Lowstand-shoreline deposits were cored in many places in Saco Bay and off the Kennebec River mouth. Datable materials from cores indicated that the lowstand occurred around 10.5 ka off the Kennebec. Cores did not penetrate glacial-marine sediment in the lowstand deposits, and seismic profiles were ambiguous about the vertical extent of sand in these units. For these reasons, no total thickness of sand was determined from the lowstand deposits, but given the area of the surficial sand, the volume is probably in the hundreds of millions of cubic meters.

Radiocarbon constraints on ice sheet advance and retreat in the Weddell Sea, Antarctica

We report here the results of a study aimed at providing radiometric age control on glacial events in the Weddell Sea during the late Quaternary. Sediment cores from the eastern continental shelf, where the East Antarctic ice sheet was grounded, have recovered glacial-marine sediments resting on tills and the latter deposits predate the isotope stage 2 last glacial maximum. Sediment cores from the continental slope and rise sampled a prominent ice-rafted debris layer, and radiocarbon ages indicate that this ice-rafting event took place prior to 26 000 yr B.P. Thus, the combined data indicate that significant deglaciation of the Weddell Sea continental shelf took place prior to the last glacial maximum. Our data also suggest that the ice masses that border the Weddell Sea are more extensive than they were during the previous glacial minimum.

Magnetic stratigraphy and sedimentology of Holocene glacial marine deposits in the Palmer Deep, Bellingshausen Sea, Antarctica: implications for climate change?

The Palmer Deep is a closed bathymetric depression on the Antarctic Peninsula continental shelf. It contains three separate sub-basins. These basins lie along a northeast–southwest axis with water depths ranging from >1400 m to the southwest (Basins II and III) to just over 1000 m to the northeast (Basin I). Six sediment piston cores were collected from the study region; these cores clearly demonstrate the varied sediment character for each basin. Sediments in Basin I are laminated and thinly bedded consisting of diatomaceous, pelagic/hemipelagic sediments, siliciclastic, terrigenous sediments, and ice rafted, hemipelagic sediments. In concurrence with other investigators, we propose that these laminations and thin beds represent climatically forced productivity cycles. Basin II and Basin III sediments alternate between pelagic/hemipelagic units and bio-siliceous mud turbidites. Correlations between cores are based on their remarkable magnetic susceptibility (MS) records which indicate alternating biogenic (low MS) and siliciclastic (high MS) dominated sedimentation; the bio-siliceous mud turbidites are characterized by intermediate to low MS values. Cores taken from within the main axis of the basins are expanded ultra-high resolution sections. A core collected on the sill between Basins II and III represents a condensed sediment section and may contain a complete Holocene record of changing paleoenvironments, one that records the transition from a glacial, ice shelf environment to an open marine, Holocene environment. A sharp drop in magnetic susceptibility at mid-core is a common sedimentological feature of each basin. Presently, we favor a climate change hypothesis for this magnetic lithostratigraphic transition which may reflect the termination of the Holocene Hypsithermal and a marked change in productivity dated ca. 2500 years BP.

Marine evidence for the last glacial advance across eastern Hudson Strait, eastern Canadian Arctic

Data from accelerator mass spectrometer radiocarbon dated sediment cores and Huntec high-resolution seismic profiles were used to investigate the age and origin of the sediments in the Eastern Basin of Hudson Strait. The data indicate that the ice-contact and glacial-marine sediments on the basin flanks and much of the upper sequence in the deep floor of the basin were produced during the Noble Inlet advance (8.9 to 8.4 ka), the last northward expansion of the Labrador Dome on to southeastern Baffin Island. On the northern flank of Eastern Basin one sequence of ice-contact sediments and glacial-marine deposits overlies bedrock; the glacial-marine sediments are transitional upslope to ice-contact sediments, and form at least two successive ice-sheet grounding zones. The earliest abundance peaks of benthic Foramininfera in glacial-marine sediments date ca. 8.6 and 8.4 ka, and correlate to sediments near the base of the 58-m-thick glacial-marine section in the deepest part of Eastern Basin. This correlation suggests that Noble Inlet ice was grounded throughout Eastern Basin during the early part of its advance. In later stages the thinning ice produced grounding zones on the basin flanks while glacial-marine sediments were deposited in the deep basin. © 1998 John Wiley & Sons, Ltd.

A middle pleistocene shallow marine interglacial sequence, inner silver pit, southern north sea: pollen and dinoflagellate cyst stratigraphy and sea-level history

The Sand Hole Formation is a Pleistocene seismic–stratigraphic unit of restricted distribution in the Inner Silver Pit, southern North Sea. Pollen and dinoflagellate cyst analyses of this formation, penetrated by British Geological Survey borehole 81/52A and three juxtaposed vibrocores, enables division of the formation into two sequences of contrasting palaeoclimatic affinity. The lower sequence contains poorly-preserved pollen assemblages of low concentration dominated by residual reworked elements, and is coincident with cold, shallow marine dinoflagellate cyst and foraminiferal assemblages. This sequence overlies till and glacimarine sediments of the Anglian/Elsterian Swarte Bank Formation, and is also interpreted as late Anglian/Elsterian in age. The upper sequence contains well-preserved, high concentration, pollen spectra recording the latter half of an interglacial stage with Hoxnian affinities. The strong terrigenous signal preserved in this sequence suggests a subtidal depositional setting within a region of freshwater influence; correlation of the vibrocores with the longer record preserved in borehole 81/52A indicates that interglacial sedimentation rates increase southwards towards a suggested terrigenous source in the Wash area. The first half of the interglacial cycle is missing, indicating that a hiatus is present between the two sequences preserved within the Sand Hole Formation, but the termination of the interglacial is clearly preserved. The spatial and temporal distribution of the sediments and bounding unconformities in the region are consistent in recording the transgressive ravinement surface, highstand systems tract and subsequent regression within an interglacial eustatic cycle. The character, age and elevation of the preserved facies is typical of a glacio-isostatically controlled emergence cycle and lends support to the notion that this temperate stage (oxygen isotope stage 9) immediately followed extensive glaciation during the late Anglian/Elsterian.

Sedimentary processes at the base of a West Antarctic ice stream; constraints from textural and compositional properties of subglacial debris

Samples of sediments from beneath Ice Stream B (at camp UpB), West Antarctica, provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub-ice-stream environment. Piston coring in holes bored by hot-water drilling yielded five 1-3 m long, undisturbed subglacial sediment cores. We analyzed granulometry, composition, and particle morphology in these cores. The UpB cores are composed of a clay-rich, unsorted diamicton containing rare marine diatoms. Sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments. The presence of reworked diatoms and their state of preservation, as well as the relative spatial homogeneity of this diamicton, suggest that the UpB cores sampled a several-meter-thick till layer and not in situ glacimarine sediments. The till does incorporate material recycled from the subjacent poorly indurated Tertiary glacimarine sediments of the Ross Sea sedimentary basin, which extends beneath this part of the West Antarctic Ice Sheet. We propose that the lack of significant comminution in the UpB till is ultimately due to its setting over these easily erodible, clay-rich source sediments. The resulting fine-grained till matrix inhibits glacial comminution, because it facilitates buildup of high pore-water pressures and hinders interparticle stress concentrations. Our observations are consistent with the conjecture that subglacial deformation of weak, fine-grained tills does not produce significant comminution of till debris (Elson 1988). Based on our findings, we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting, poorly indurated, fine-grained sediments. Since such weak till layers create a permissive condition for ice streaming, sub-glacial geology may have an indirect but strong control over the location, extent, and basal mechanics of ice streams.

Contrasting glacimarine sedimentary environments of two arctic fiords on Disko, West Greenland

Two fiords, Kangerluk and Akulliit, on the island of Disko, central West Greenland have very different sedimentary regimes caused by differences in the characteristics of their drainage basins and in the exposure of their waters to the open sea of Baffin Bay. Their insular location also results in much lower input of water and sediment than in nearby fiords on the mainland of Greenland that are affected by glaciers from the Greenland Ice Cap. Akulliit has a small drainage basin and low input of water and sediment; it is directly exposed to the higher energy wave environment of Baffin Bay. As a consequence, the mean thickness of Holocene glacimarine sediment assessed in acoustic records is less than 6 m in the inner and middle reaches of the fiord (mean Holocene accumulation, 0.4 mm/a) and about 11 m (1 mm/a) near the mouth. Dated core sediment indicates that rates are less than 0.5 mm/a in the outer fiord in the last 2 ka. The larger, more glacier-covered drainage basin of Kangerluk contributes much larger loads of sediment to a fiord environment protected from the open sea. As a result, Holocene glacimarine sediment is more than 80 m thick in the inner fiord (mean Holocene accumulation up to 5 mm/a), decreasing to 10–40 m (about 1 mm/a) in the middle reaches. Accumulation rates in Kangerluk are within the range of arctic fiords, including those of Baffin Island, but those in Akulliit are near the lower limit. The decrease in thickness distally in Kangerluk is typical of arctic fiords, but the opposite trend in Akulliit is probably due to erosion and re-distribution of sediment in the higher energy environment there.

Holocene glacimarine sedimentation and macrofossil palaeoecology in the Canadian High Arctic: environmental controls

Raised glacimarine sediments adjacent to Strand Bay, Axel Heiberg Island and on Fosheim Peninsula, Ellesmere Island, record sedimentation in environments proximal to tidewater glaciers and glacimarine deltas. The different styles of sedimentation during early Holocene deglaciation (ca. 8–10 ka BP) of these regions are of particular interest. In Strand Fiord, Axel Heiberg Island coarse-grained sediments deposited from high-density turbidity currents generated along the margin of tidewater glaciers, are overlain by laminated fine-grained sediments deposited by suspension settling in a shallow ice-proximal environment. The fining-upward sequence records the reduction in sediment supply as tidewater glaciers retreated onto land. In contrast, on Fosheim Peninsula, Ellesmere Island, laminated to massive fine-grained sediments were deposited from low-density turbidity currents and by suspension settling in a deep prodeltaic environment several kilometres from upland ice caps. Sediment texture coarsens-upward, reflecting increasing proximity to sediment sources as deltas prograded during early Holocene marine regression. Two macrofossil faunas, a Portlandia arctica assemblage and a Hiatella arctica-Mya truncata assemblage, occur in association with early Holocene sediments. Mid-Holocene sedimentation (ca. 4–8 ka BP) in both regions was characterized by the accumulation of laminated or massive fine-grained sediments deposited from low-density turbidity currents and by suspension settling in prodeltaic environments. Several macrofossil faunas are recorded in these sediments; a Portlandia arctica assemblage and an Astarte borealis-Mya truncata assemblage associated with laminated muds deposited in shallow water, and a deep-water Portlandia arctica assemblage associated with massive muds. Variations in sedimentation rates can be correlated with the summer-melt layer record of the Agassiz Ice Cap on Ellesmere Island. Early Holocene deglaciation coincided with a period of increased summer ablation (ca. 8.5–11.5 ka BP) that contributed to the massive influx of sediment into marine environments. The mid-Holocene ice core-record indicates a sharp decline in summer ablation (ca. 6.0–9.5 ka BP) resulting in the reduction of sediment flux to marine environments.