Glacimarine Sediments Research Articles

The variety and distribution of submarine glacial landforms and implications for ice-sheet reconstruction

Glacimarine processes affect about 20% of the global ocean today, and this area expanded considerably under cyclical full-glacial conditions during the Quaternary (Fig. 1) (Dowdeswell et al. 2016 b ). Many of the submarine landforms produced at the base and margin of past ice sheets remain well preserved on the seafloor in fjords and on high-latitude continental shelves after the retreat of the ice that produced them. These glacial landforms, protected from subaerial erosion and beneath wave-base and tidal currents in water that is often hundreds of metres deep, are gradually buried by both hemipelagic and glacimarine sedimentation; they may be preserved over long periods in the geological record if palaeo-continental shelves are not reworked by subsequent glacier advances or bottom currents (Dowdeswell et al. 2007). This means that, first, submarine glacial landforms can be observed at or close to the modern seafloor after retreat of the last great ice sheets from their most recent Quaternary maximum about 18–20 000 years ago using swath-bathymetric mapping systems and, secondly, buried glacial landforms may also be identified and examined within glacial-sedimentary sequences from Quaternary and earlier ice ages using seismic-reflection methods. Fig. 1. The global distribution of glaciers and ice sheets and the glacier-influenced, or glacimarine, environment. The approximate modern (yellow dotted line) and Quaternary full-glacial (yellow dashed line) limits of ice-rafting and ice-keel ploughing of the seafloor are shown (modified from Anderson 1983). GEBCO World Map: Gall projection. Numbered yellow dots refer to the locations of subsequent figures. The development of multibeam echo sounding over the past two decades, coupled with high-accuracy GPS positioning, has allowed morphological mapping of the seafloor at an unprecedented level of detail. In this paper, the variety of submarine glacial landforms observed in modern, Quaternary and more ancient sediments is described. Landforms produced subglacially, those formed at and beyond …

Fjord-mouth submarine moraines, SW Newfoundland

Arcuate submarine moraines at the mouths of fjords along the SW coast of Newfoundland are interfingered with glacimarine deposits. Based on radiocarbon dates from piston cores, it has been proposed that they formed when Late Wisconsinan glacier ice on the continental shelf of Atlantic Canada calved back into deep water, isolating the Newfoundland Ice Cap (NIC) c. 14 ka BP. In SW Newfoundland, drawdown from the interior of the ice cap was localized in fjords and ice margins were stabilized at fjord mouths. Similar submarine moraines are found off the west coast of Newfoundland, but do not occur at fjord mouths elsewhere. The coast of Newfoundland is indented by a series of very narrow fjords (average width 1 km) that are relatively shallow (average maximum depth 250 m) compared with those of the NE coast (cf. Shaw et al. 2000) (Fig. 1). The exception is Bay d'Espoir, the deepest fjord in Newfoundland, which attains a maximum depth of 790 m (Fig. 1c). The fjord entrances coincide with the boundary between crystalline basement rocks that predominate onshore and the younger Palaeozoic sedimentary rocks offshore (Fader et al. 1989). Fig. 1. ( a ) Submarine moraines off Bay de Loup and White Bear Bay. Acquisition …

Post-glacial sand drifts burying De Geer moraines on the continental shelf off North Norway

Glacial landforms are continuously modified by sedimentary processes after deglaciation. Examples of post-depositional modification occur on the continental shelf off Vesteralen, North Norway, an area that is thought to have been covered by a relatively slow-flowing or stagnant ice dome during the Last Glacial Maximum (LGM) (Ottesen et al. 2005). In this period, the Lofoten-Vesteralen archipelago would have been largely sheltered from fast-flowing ice streams of the Fennoscandian Ice Sheet draining NE and SE of the area. Multibeam bathymetry nevertheless reveals evidence of local glacial activity on the continental shelf offshore Vesteralen, including glacial lineations indicative of faster ice-flow in cross-shelf troughs (Elvenes 2009). After the ice retreated from the shelf, glacial and glacimarine deposits on the surrounding shallow banks were eroded and reworked by waves and currents. Later winnowing of shell material has led to deposition of bioclastic sand in sand fields (Boe et al. 2009). On the present-day seafloor, sandy sediment drifts are migrating over moraines and other glacial deposits, burying and obscuring the surficial signatures of glaciation. The narrow continental shelf extending about 20–40 km off the Vesteralen archipelago is characterized by many shallow banks (40–120 m deep) intersected by 100–500 m deep, steep-sided troughs. These troughs may be cross-shelf or coast-parallel, and several contain series of transverse, evenly spaced ridges …

Anthropogenic modification of a fjord: Bay of Islands, Newfoundland

The majority of Newfoundland fjords are relatively pristine. However, the Bay of Islands fjord, site of Corner Brook (Newfoundland's second-largest city with a population of 20 000), shows considerable anthropogenic modification. Submarine slides have been triggered by dock and road construction. Dredge spoil and a scuttled barge litter the seafloor. A large mound of wood chips extending into the Bay of Islands is waste from a paper mill. The Bay of Islands, western Newfoundland, is a fjord bounded at its seaward end by an arcuate submarine moraine (Fig. 1a) up to 100 m thick (Josenhans et al. 1990; Josenhans & Zevenhuizen 1993; Josenhans & Lehman 1999). The moraine formed after retreat of ice that formerly extended far out on the continental shelf (Shaw et al. 2006). The deepest areas of the fjord (295 m) are immediately landward of the shallow (30 m) moraine. The fjord has three principal arms, the largest of which (Humber Arm, Fig. 1a, c), averages 100 m deep and shallows steeply at its head where the Humber River enters. Humber Arm has many of the attributes of fjords elsewhere; it hosts thick glacimarine sediments (Fig. 1b) overlain by postglacial mud (Shaw et al. 2000 …

Grounding-zone wedges and mega-scale glacial lineations in Kveithola Trough, Barents Sea

Multibeam-bathymetric data from Kveithola Trough show a seafloor characterized by east–west-trending mega-scale glacial lineations (MSGLs) overprinted by transverse grounding-zone wedges (GZWs). GZWs are thought to form by deposition of subglacial till at temporarily stable ice-sheet marine termini between successive episodic retreats (Batchelor & Dowdeswell 2015). Sub-bottom profiles show that the present-day morphology of Kveithola Trough is largely inherited from the palaeo-seafloor topography of the GZWs, which is now draped by deglacial to early Holocene glacimarine sediment about 15 m thick. The ice stream that produced such subglacial morphology was flowing from east to west towards the shelf edge in Kveithola Trough during the Last Glacial Maximum (LGM). Its rapid retreat was probably associated with progressive lift-off and successive rapid melting of grounded ice induced by eustatic sea-level rise. The east–west-trending Kveithola Trough in the Barents Sea (Fig. 1) shows well-preserved subglacial landform morphologies (Rebesco et al. …

Stratified glacimarine basin-fills in West Greenland fjords

Acoustically stratified sediments that infill glacially eroded bedrock basins are common in fjords (Seramur et al. 1997; Gilbert et al. 1998; Hogan et al. 2012; Dowdeswell & Vasquez 2013). Internally, such stratified sediments are often characterized by relatively smooth reflections that parallel the seafloor. Sills between basins usually comprise bedrock, sometimes covered by a thin veneer of glacimarine sediment and/or till. Fjords in West Greenland, including Rink Fjord and Vaigat, contain bedrock basins infilled by acoustically stratified sediment (Fig. 1). The basin fills are a product of rain-out from turbid meltwater plumes, probably supplemented by iceberg-rafting and downslope resedimentation. Fig. 1. Multibeam bathymetry and acoustic sub-bottom profiles of submarine basins close to the coast of West Greenland. ( a ) Multibeam bathymetry from Rink Fjord, West Greenland, with water depths >1000 m. ( b ) 3.5 kHz sub-bottom profile from Rink Fjord showing stratified, conformable basin fill (Facies 2) between basement pinnacles (Facies 1). VE×96. ( c , d ) Multibeam bathymetry in Vaigat strait and a coastal basin, West Greenland, respectively; arrows indicate ice-flow directions during the last glacial based on the orientation of sculpted …

Tidal power generation in the glaciated, macrotidal Minas Scour Trough, Bay of Fundy, Atlantic Canada

The configuration of the Bay of Fundy results in large tides (up to 17 m) which give rise to strong currents (Fig. 1d). Harnessing these currents to generate electricity has been the focus of engineering schemes dating from 1910. Fundy tidal power generation has garnered attention periodically throughout the twentieth century (Gordon et al. 2014). The seabed and water column conditions in the Minas Scour Trough at the NE head of the Bay of Fundy (Fig. 1a, d) are now the subject of engineering studies to characterize the site for electricity generation. Test installations have been deployed on the seabed, the subsea cable to collect and transmit power to the electrical grid on land has been laid and future infrastructure instalments are planned. Fig. 1. Multibeam swath bathymetry and geophysical profile of the main Minas Scour Trough, Bay of Fundy. ( a ) Sun-illuminated multibeam-bathymetric image showing the tidal scour system developed in, from west to east, Minas Channel, Minas Passage and Minas Basin. Inset interpretation shows the extent of the scour trough. Acquisition system Kongsberg EM1000. Frequency 95 kHz. Grid-cell size 5 m. ( b ) Seismic-reflection profile in Minas Channel showing eroded glacimarine sediments and exposed bedrock cropping out in Minas Scour Trough. VE×67. Acquisition …

Paraglacial landscapes in St George's Bay, Newfoundland, Canada

The Late Wisconsinan submarine moraine in St George's Bay, Newfoundland, was exposed to erosional processes during and after an early Holocene −25 m lowstand of relative sea level. Primary morphology has been largely removed, resulting in a gently sloping platform veneered with mobile sand and gravel, terminating on the landward side in progradational ‘spillover’ lobes. Erosion of coastal Quaternary deposits has produced systems of gravel-dominated spits and barriers constructed upon subaqueous platforms that have buried glacimarine deposits and a lowstand delta. Paraglacial processes have therefore created a complex submarine terrain with few parallels in Atlantic Canada. A gently sloping platform-like shoal (average depth 20 m) extends across inner St George's Bay (Fig. 1a), enclosing several basins to the east (maximum depth 100 m). The platform surface comprises sand waves, areas of gravel ripples and patches of immobile, bouldery gravel with relief of 2–3 m (e.g. A in Fig. 1a). This mosaic of terrains is reflected in a backscatter image (Fig. 1b). As a result of wave action, sand and gravel have migrated to the NE, spilling into the adjacent basins to form lobate, prograding sediment wedges (e.g. B in …

Significant differences in late Quaternary bedrock erosion and transport: East versus West Greenland ∼70°N – evidence from the mineralogy of offshore glacial marine sediments

ABSTRACTWe use quantitative X‐ray diffraction to determine the mineralogy of late Quaternary marine sediments from the West and East Greenland shelves offshore from early Tertiary basalt outcrops. Despite the similar basalt outcrop area (60 000–70 000 km2), there are significant differences between East and West Greenland sediments in the fraction of minerals (e.g. pyroxene) sourced from the basalt outcrops. We demonstrate the differences in the mineralogy between East and West Greenland marine sediments on three scales: (1) modern day, (2) late Quaternary inputs and (3) detailed down‐core variations in 10 cores from the two margins. On the East Greenland Shelf (EGS), late Quaternary samples have an average quartz weight per cent of 6.2 ± 2.3 versus 12.8 ± 3.9 from the West Greenland Shelf (WGS), and 12.02 ± 4.8 versus 1.9 ± 2.3 wt% for pyroxene. K‐means clustering indicated only 9% of the samples did not fit a simple EGS vs. WGS dichotomy. Sediments from the EGS and WGS are also isotopically distinct, with the EGS having higher ϵNd (−18 to 4) than those from the WGS (ϵNd = −25 to −35). We attribute the striking dichotomy in sediment composition to fundamentally different long‐term Quaternary styles of glaciation on the two basalt outcrops.

Ice sheet extension to the Celtic Sea shelf edge at the Last Glacial Maximum

Previous reconstructions of the British–Irish Ice Sheet (BIIS) envisage ice streaming from the Irish Sea to the Celtic Sea at the Last Glacial Maximum, to a limit on the mid-shelf of the Irish-UK sectors. We present evidence from sediment cores and geophysical profiles that the BIIS extended 150 km farther seaward to reach the continental shelf edge. Three cores recently acquired from the flank of outer Cockburn Bank, a shelf-crossing sediment ridge, terminated in an eroded glacigenic layer including two facies: overconsolidated stratified diamicts; and finely-bedded muddy sand containing micro- and macrofossil species of cold water affinities. We interpret these facies to result from subglacial deformation and glacimarine deposition from turbid meltwater plumes. A date of 24,265 ± 195 cal BP on a chipped but unabraded mollusc valve in the glacimarine sediments indicates withdrawal of a tidewater ice sheet margin from the shelf edge by this time, consistent with evidence from deep-sea cores for ice-rafted debris peaks of Celtic Sea provenance between 25.5 and 23.4 ka BP. Together with terrestrial evidence, this supports rapid (ca 2 ka) purging of the BIIS by an ice stream that advanced from the Irish Sea to the shelf edge and collapsed back during Heinrich event 2.

Reconstruction of ice-sheet changes in the Antarctic Peninsula since the Last Glacial Maximum

This paper compiles and reviews marine and terrestrial data constraining the dimensions and configuration of the Antarctic Peninsula Ice Sheet (APIS) from the Last Glacial Maximum (LGM) through deglaciation to the present day. These data are used to reconstruct grounding-line retreat in 5 ka time-steps from 25 ka BP to present. Glacial landforms and subglacial tills on the eastern and western Antarctic Peninsula (AP) shelf indicate that the APIS was grounded to the outer shelf/shelf edge at the LGM and contained a series of fast-flowing ice streams that drained along cross-shelf bathymetric troughs. The ice sheet was grounded at the shelf edge until ∼20 cal ka BP. Chronological control on retreat is provided by radiocarbon dates on glacimarine sediments from the shelf troughs and on lacustrine and terrestrial organic remains, as well as cosmogenic nuclide dates on erratics and ice moulded bedrock. Retreat in the east was underway by about 18 cal ka BP. The earliest dates on recession in the west are from Bransfield Basin where recession was underway by 17.5 cal ka BP. Ice streams were active during deglaciation at least until the ice sheet had pulled back to the mid-shelf. The timing of initial retreat decreased progressively southwards along the western AP shelf; the large ice stream in Marguerite Trough may have remained grounded at the shelf edge until about 14 cal ka BP, although terrestrial cosmogenic nuclide ages indicate that thinning had commenced by 18 ka BP. Between 15 and 10 cal ka BP the APIS underwent significant recession along the western AP margin, although retreat between individual troughs was asynchronous. Ice in Marguerite Trough may have still been grounded on the mid-shelf at 10 cal ka BP. In the Larsen-A region the transition from grounded to floating ice was established by 10.7–10.6 cal ka BP. The APIS had retreated towards its present configuration in the western AP by the mid-Holocene but on the eastern peninsula may have approached its present configuration several thousand years earlier, by the start of the Holocene. Mid to late-Holocene retreat was diachronous with stillstands, re-advances and changes in ice-shelf configuration being recorded in most places. Subglacial topography exerted a major control on grounding-line retreat with grounding-zone wedges, and thus by inference slow-downs or stillstands in the retreat of the grounding line, occurring in some cases on reverse bed slopes.

Geomorphic and stratigraphic criteria used to date the Råda Landslide, Västra Götaland, Sweden

A high-resolution digital elevation model has been used to locate a large and previously unknown landslide in glacial-marine sediments in southwest Sweden. Interpretation of the geomorphology and stratigraphy allow for bracketing of paleo-sea level to between 68 and 71 m above sea level when the landslide occurred. Existing sea-level curves suggest that this elevation would have emerged above sea level, and the landslide would have occurred, about 10 800 calendar years before the present. We suggest that the presence of a confined aquifer beneath the area that slid created a weakness in the slope upon which it eventually failed.

Meltwater intensive glacial retreat in polar environments and investigation of associated sediments: example from Pine Island Bay, West Antarctica

Modern Pine Island and Thwaites Glaciers, which both drain into Pine Island Bay, are among the fastest changing portions of the cryosphere and the least stable ice streams in Antarctica. Here we show that the uppermost sediment unit in Pine Island Bay was deposited from a meltwater plume, a plumite, during the late stages of ice sheet retreat ∼7–8.6 k cal yr BP and argue that this deposit records episodes of meltwater intensive sedimentation.The plumite is a hydraulically sorted, glacially sourced, draping deposit that overlies proximal glacimarine sediments and thickens towards the modern grounding line. The uppermost sediment unit is interpreted as a product of non-steady-state processes in which low background sedimentation in large bedrock-carved basins alternates with episodic purging of sediment-laden water from these basins. The inner part of Pine Island Bay contains several basins that are linked by channels with a storage capacity on the order of 70 km3 of stagnant water and significant sediment storage capacity. Purging of these basins is caused by changes in hydraulic potential and glacial reorganization. The sediment mobilized by these processes is found here to total 120 km3.This study demonstrates that episodes of meltwater-intensive sedimentation in Pine Island Bay occurred at least three times in the Holocene. The most recent episode coincides with rapid retreat of the grounding line in historical time and has an order of magnitude greater flux relative to the entire unit. We note that the final phase of ice stream retreat in Marguerite Bay was marked by a similar sedimentary event and suggest that the modern Thwaites Glacier is poised for an analogous meltwater-intensive phase of retreat.

Late Quaternary ice flow in a West Greenland fjord and cross-shelf trough system: submarine landforms from Rink Isbrae to Uummannaq shelf and slope

Sea-floor landforms and acoustic-stratigraphic records allow interpretation of the past form and flow of a westward-draining ice stream of the Greenland Ice Sheet, Rink Isbrae. The Late Pliocene–Pleistocene glacial package is several hundred metres thick and down-laps onto an upper Miocene horizon. Several acoustic facies are mapped from sub-bottom profiler records of the 400 km-long Uummannaq fjord-shelf-slope system. An acoustically stratified facies covers much of the fjord and trough floor, interpreted as glacimarine sediment from rain-out of fine-grained debris in turbid meltwater. Beneath this facies is a semi-transparent deformation-till unit, which includes buried streamlined landforms. Landform distribution in the Uummannaq system is used to reconstruct past ice extent and flow directions. The presence of streamlined landforms (mega-scale glacial lineations, drumlins, crag-and-tails) shows that an ice stream advanced through the fjord system to fill Uummannaq Trough, reaching the shelf edge at the Last Glacial Maximum. Beyond the trough there is a major fan built mainly of glacigenic debris flows. Turbidity-current channels were not observed on Uummannaq Fan, contrasting with well-developed channels on Disko Fan, 300 km to the south. Ice retreat had begun by 14.8 cal. ka ago. Grounding-zone wedges (GZW) in Uummannaq Trough imply that retreat was episodic, punctuated by several still-stands. Ice retreat between GZWs may have been relatively rapid. There is little sedimentary evidence for still-stands in the inner fjords, except for a major moraine ridge marking a Little Ice Age maximum position. On the shallow banks either side of Uummannaq Trough, iceberg ploughing has reworked any morphological evidence of earlier ice-sheet activity.

Modern rates of glacial sediment accumulation along a 15° S-N transect in fjords from the Antarctic Peninsula to southern Chile

[1] Rates of glacial erosion in temperate climates rank among the highest worldwide, and the sedimentary products of such erosion record climatic and tectonic signals in many glaciated settings, as well as temporal changes in glacier behavior. Glacial sediment yields are expected to decrease with increasing latitude because decreased temperature and meltwater production reduce glacial sliding, erosion, and sediment transfer; however, this expectation lacks a solid supportive database. Herein we present modern 210Pb-derived sediment accumulation rates on decadal to century time scales for 12 fjords spanning 15° of latitude from the Antarctic Peninsula to southern Chile and interpret the results in light of glacimarine sediment accumulation worldwide. 210Pb records from the Antarctic Peninsula show surprisingly steady sediment accumulation throughout the past century at rates of 1–7 mm yr−1, despite rapid warming and glacial retreat. Cores from the South Shetland Islands reveal accelerated sediment accumulation over the past few decades, likely due to changes in the thermal state of the glaciers in this region, which straddles the boundary between subpolar and temperate conditions. In Patagonia and Tierra del Fuego, sediment accumulates faster (11–24 mm yr−1), and previously collected seismic profiles show that rates reach meters per year close to the glacier termini. This increase in sediment accumulation rates with decreasing latitude reflects the gradient from subpolar to temperate climates and is consistent with glacial erosion being much faster in the temperate climate of southern Chile than in the polar climate of the Antarctic Peninsula.

Provenance, volcanic record, and tectonic setting of the Paleozoic Ventania Fold Belt and the Claromecó Foreland Basin: Implications on sedimentation and volcanism along the southwestern Gondwana margin

This study focuses on the provenance, volcanic record, and tectonic setting of the Paleozoic Ventania System, a geologic province which comprises the Cambro-Devonian Ventania Fold Belt and the adjoining Permo-Carboniferous Claromecó Foreland Basin, located inboard the deformation front. The Ventania Fold Belt is formed of the Curamalal and Ventana groups, which are composed mainly of mature quartzites that were unconformably deposited on igneous and metamorphic basement. The Pillahuincó Group is exposed as part of the Claromecó Basin and it has lithological and structural features totally distinct from the lowermost groups. This group is composed of immature arkoses and subarkoses with intercalated tuff horizons, unconformably overlaying the quartzites and associated with glacial-marine deposits of the lower Late Carboniferous to Early Permian section. The petrography, as well as major and trace elements (including rare earth elements) support that the Ventania quartzites were derived from cratonic sources and deposited in a passive margin environment. For the Pillahuincó Group, we suggest a transition between rocks derived from and deposited in a passive margin environment to those with geochemical and petrographical signatures indicative of an active continental margin provenance. LA-MC-ICP-MS analysis performed on euhedral and prismatic zircon grains of the tuffs revealed an age of 284 ± 15 Ma. The geochemical fingerprints and geochronological data of the tuffs found in the Claromecó Basin support the presence of an active and widespread Lower Permian pyroclastic activity in southwestern Gondwana, which is interpreted as part of the Choiyoi Volcanic Province in Argentina and Chile.

LGM ice sheet extent in the Weddell Sea: evidence for diachronous behavior of Antarctic Ice Sheets

Inaccessibility due to harsh weather conditions and perennial sea ice has resulted in the Weddell Sea being one of the more under-studied regions of Antarctica. Re-examination of existing cores and unpublished multibeam bathymetric data provide new insight into the LGM configuration of the ice sheet in the region. Sediment cores from the eastern continental shelf, where the East Antarctic Ice Sheet (EAIS) drains, display a retreat stratigraphy with distal glacimarine sediments overlying proximal glacimarine sediments and till. The distinction between distal and proximal glacimarine sediments is based on degrees of marine influence based on grain-size distributions and foraminiferal abundance and diversity. Glacimarine sediments yielded AMS radiocarbon ages that span the past 30,476 cal yr BP, indicating that retreat of the EAIS from the continental shelf occurred prior to the LGM. Multibeam swath bathymetry data show mega-scale glacial lineations in two troughs on the southern continental shelf, which indicates the existence of two ice streams draining the West Antarctic Ice Sheet (WAIS). More subtle lineations within Filchner Trough imply more sluggish flow across bedrock and older strata within this trough. Although there are no radiocarbon ages to absolutely constrain the timing of this grounding event on the southern continental shelf, we interpret the lineations as LGM age based on their pristine nature. This interpretation is consistent with results from other sectors of the West Antarctic continental shelf where compelling evidence for significant WAIS advance during the LGM exists. In the Weddell Sea, this apparent diachronous behavior of the EAIS and WAIS is consistent with results from onshore studies.

Post-LGM deglaciation in Pine Island Bay, West Antarctica

To date, understanding of ice sheet retreat within Pine Island Bay (PIB) following the Last Glacial Maximum (LGM) was based on seven radiocarbon dates and only fragmentary seafloor geomorphic evidence. During the austral summer 2009–2010, restricted sea ice cover allowed for the collection of 27 sediment cores from the outer PIB trough region. Combining these cores with data from prior cruises, over 133 cores have been used to conduct a detailed sedimentological facies analysis. These results, augmented by 23 new radiocarbon dates, are used to reconstruct the post-LGM deglacial history of PIB.Our results record a clear retreat stratigraphy in PIB composed of, from top to base; terrigenous sandy silt (distal glacimarine), pebbly sandy mud (ice-proximal glacimarine), and till. Initial retreat from the outer-continental shelf began shortly after the LGM and before 16.4 k cal yr BP, as a likely response to rising sea level. Bedforms in outer PIB document episodic retreat in the form of back-stepping grounding zone wedges and are associated with proximal glacimarine sediments. A sub-ice shelf facies is observed in central PIB and spans ∼12.3–10.6 k cal yr BP. It is possible that widespread impingement of warm water onto the continental shelf caused an abrupt and widespread change from sub-ice shelf sedimentation to distal glacimarine sedimentation dominated by widespread dispersal of terrigenous silt between 7.8 and 7.0 k cal yr BP. The final phase of retreat ended before ∼1.3 k cal yr BP, when the grounding line migrated to a location near the current ice margin.

Productivity and sedimentary δ15N variability for the last 17,000 years along the northern Gulf of Alaska continental slope

Biogenic opal, organic carbon, organic matter stable isotope, and trace metal data from a well‐dated, high‐resolution jumbo piston core (EW0408–85JC; 59° 33.3′N, 144° 9.21′W, 682 m water depth) recovered from the northern Gulf of Alaska continental slope reveal changes in productivity and nutrient utilization over the last 17,000 years. Maximum values of opal concentration (∼10%) occur during the deglacial Bølling‐Allerød (B‐A) interval and earliest Holocene (11.2 to 10.8 cal ka BP), moderate values (∼6%) occur during the Younger Dryas (13.0 to 11.2 cal ka BP) and Holocene, and minimum values (∼3.5%) occur during the Late Glacial Interval (LGI). When converted to opal mass accumulation rates, the highest values (∼5000 g cm−2 kyr−1) occur during the LGI prior to 16.7 cal ka BP, which points to a strong influence by LGI glacimarine sedimentation regimes. Similar patterns are also observed in total organic carbon and cadmium paleoproductivity proxies. Mid‐Holocene peaks in the terrestrial organic matter fraction at 5.5, 4.7, 3.5, and 1.2 cal ka BP indicate periods of enhanced delivery of glaciomarine sediments by the Alaska Coastal Current. The B‐A and earliest Holocene intervals are laminated, and enrichments of redox‐sensitive elements suggest dysoxic‐to‐anoxic conditions in the water column. The laminations are also associated with mildly enriched sedimentary δ15N ratios, indicating a link between productivity, nitrogen cycle dynamics, and sedimentary anoxia. After applying a correction for terrestrial δ15N contributions based on end‐member mixing models of terrestrial and marine organic matter, the resulting B‐A marine δ15N (6.3 ± 0.4 ‰) ratios are consistent with either mild denitrification, or increased nitrate utilization. These findings can be explained by increased micronutrient (Fe) availability during episodes of rapid rising sea level that released iron from the previously subaerial coastal plain; iron input from enhanced terrestrial runoff; and/or the intermittent presence of seasonal sea ice resulting from altered ocean/atmospheric circulation during the B‐A in the Gulf of Alaska.

Центрические диатомовые (Biddulphiales, Hemiaulales, Rhizosoleniales, Chaetocerotales, Bacillariophyta) из неогеновых отложений массива Фишер (горы Принс-Чарльз, Восточная Антарктида)

Detailed data on morphology and taxonomy of 11 species of centric diatoms from the Neogene glacial-marine sediments of the Fisher Massif (Prince Charles Mountains, East Antarctica) are presented. Two new species Dicladia antarctica Gogorev et Pushina sp. nov. and Trigonium antarcticum Gogorev et Pushina sp. nov. are described.